Race condition within _bt_findinsertloc()? (new page split code)

--- configure.in.orig	2014-05-28 08:29:09.146829394 -0400
+++ configure.in	2014-05-28 16:45:42.406505235 -0400
@@ -1771,6 +1771,12 @@ operating system;  use --disable-thread-
 fi
 fi
 
+#  test whether this system has both the librt-style async io and the gcc atomic compare_and_swap
+#      and test operation  of the latter.
+PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP
+if test x"$pgac_cv_aio_atomic_builtin_comp_swap" = x"yes"; then
+      AC_DEFINE(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP, 1, [Define to select librt-style async io and the gcc atomic compare_and_swap.])
+fi
 
 # Select semaphore implementation type.
 if test "$PORTNAME" != "win32"; then
--- contrib/pg_stat_statements/pg_stat_statements--1.3.sql.orig	2014-05-28 08:50:32.110559768 -0400
+++ contrib/pg_stat_statements/pg_stat_statements--1.3.sql	2014-05-28 16:45:42.570505896 -0400
@@ -0,0 +1,52 @@
+/* contrib/pg_stat_statements/pg_stat_statements--1.3.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION pg_stat_statements VERSION '1.3'" to load this file. \quit
+
+-- Register functions.
+CREATE FUNCTION pg_stat_statements_reset()
+RETURNS void
+AS 'MODULE_PATHNAME'
+LANGUAGE C;
+
+CREATE FUNCTION pg_stat_statements(IN showtext boolean,
+    OUT userid oid,
+    OUT dbid oid,
+    OUT queryid bigint,
+    OUT query text,
+    OUT calls int8,
+    OUT total_time float8,
+    OUT rows int8,
+    OUT shared_blks_hit int8,
+    OUT shared_blks_read int8,
+    OUT shared_blks_dirtied int8,
+    OUT shared_blks_written int8,
+    OUT local_blks_hit int8,
+    OUT local_blks_read int8,
+    OUT local_blks_dirtied int8,
+    OUT local_blks_written int8,
+    OUT temp_blks_read int8,
+    OUT temp_blks_written int8,
+    OUT blk_read_time float8,
+    OUT blk_write_time float8
+   , OUT aio_read_noneed int8
+   , OUT aio_read_discrd int8
+   , OUT aio_read_forgot int8
+   , OUT aio_read_noblok int8
+   , OUT aio_read_failed int8
+   , OUT aio_read_wasted int8
+   , OUT aio_read_waited int8
+   , OUT aio_read_ontime int8
+)
+RETURNS SETOF record
+AS 'MODULE_PATHNAME', 'pg_stat_statements_1_3'
+LANGUAGE C STRICT VOLATILE;
+
+-- Register a view on the function for ease of use.
+CREATE VIEW pg_stat_statements AS
+  SELECT * FROM pg_stat_statements(true);
+
+GRANT SELECT ON pg_stat_statements TO PUBLIC;
+
+-- Don't want this to be available to non-superusers.
+REVOKE ALL ON FUNCTION pg_stat_statements_reset() FROM PUBLIC;
--- contrib/pg_stat_statements/Makefile.orig	2014-05-28 08:29:09.166829383 -0400
+++ contrib/pg_stat_statements/Makefile	2014-05-28 16:45:42.590505977 -0400
@@ -4,7 +4,8 @@ MODULE_big = pg_stat_statements
 OBJS = pg_stat_statements.o
 
 EXTENSION = pg_stat_statements
-DATA = pg_stat_statements--1.2.sql pg_stat_statements--1.1--1.2.sql \
+DATA = pg_stat_statements--1.3.sql pg_stat_statements--1.2--1.3.sql \
+	pg_stat_statements--1.2.sql pg_stat_statements--1.1--1.2.sql \
 	pg_stat_statements--1.0--1.1.sql pg_stat_statements--unpackaged--1.0.sql
 
 ifdef USE_PGXS
--- contrib/pg_stat_statements/pg_stat_statements.c.orig	2014-05-28 08:29:09.166829383 -0400
+++ contrib/pg_stat_statements/pg_stat_statements.c	2014-05-28 16:45:42.630506138 -0400
@@ -117,6 +117,7 @@ typedef enum pgssVersion
 	PGSS_V1_0 = 0,
 	PGSS_V1_1,
 	PGSS_V1_2
+	,PGSS_V1_3
 } pgssVersion;
 
 /*
@@ -148,6 +149,16 @@ typedef struct Counters
 	int64		local_blks_written;		/* # of local disk blocks written */
 	int64		temp_blks_read; /* # of temp blocks read */
 	int64		temp_blks_written;		/* # of temp blocks written */
+
+	int64		aio_read_noneed;		/* # of prefetches for which no need for prefetch as block already in buffer pool  */
+	int64		aio_read_discrd;		/* # of prefetches for which buffer not subsequently read and therefore discarded  */
+	int64		aio_read_forgot;		/* # of prefetches for which buffer not subsequently read and then forgotten about */
+	int64		aio_read_noblok;		/* # of prefetches for which no available BufferAiocb  control block               */
+	int64		aio_read_failed;		/* # of aio reads for which aio itself failed or the read failed with an errno     */
+	int64		aio_read_wasted;		/* # of aio reads for which in-progress aio cancelled and disk block not used      */
+	int64		aio_read_waited;		/* # of aio reads for which disk block used but had to wait for it                 */
+	int64		aio_read_ontime;		/* # of aio reads for which disk block used and ready on time when requested       */
+
 	double		blk_read_time;	/* time spent reading, in msec */
 	double		blk_write_time; /* time spent writing, in msec */
 	double		usage;			/* usage factor */
@@ -274,7 +285,7 @@ void		_PG_init(void);
 void		_PG_fini(void);
 
 PG_FUNCTION_INFO_V1(pg_stat_statements_reset);
-PG_FUNCTION_INFO_V1(pg_stat_statements_1_2);
+PG_FUNCTION_INFO_V1(pg_stat_statements_1_3);
 PG_FUNCTION_INFO_V1(pg_stat_statements);
 
 static void pgss_shmem_startup(void);
@@ -1026,7 +1037,25 @@ pgss_ProcessUtility(Node *parsetree, con
 		bufusage.temp_blks_read =
 			pgBufferUsage.temp_blks_read - bufusage_start.temp_blks_read;
 		bufusage.temp_blks_written =
-			pgBufferUsage.temp_blks_written - bufusage_start.temp_blks_written;
+			pgBufferUsage.temp_blks_written - bufusage.temp_blks_written;
+
+		bufusage.aio_read_noneed =
+			pgBufferUsage.aio_read_noneed - bufusage.aio_read_noneed;
+		bufusage.aio_read_discrd =
+			pgBufferUsage.aio_read_discrd - bufusage.aio_read_discrd;
+		bufusage.aio_read_forgot =
+			pgBufferUsage.aio_read_forgot - bufusage.aio_read_forgot;
+		bufusage.aio_read_noblok =
+			pgBufferUsage.aio_read_noblok - bufusage.aio_read_noblok;
+		bufusage.aio_read_failed =
+			pgBufferUsage.aio_read_failed - bufusage.aio_read_failed;
+		bufusage.aio_read_wasted =
+			pgBufferUsage.aio_read_wasted - bufusage.aio_read_wasted;
+		bufusage.aio_read_waited =
+			pgBufferUsage.aio_read_waited - bufusage.aio_read_waited;
+		bufusage.aio_read_ontime =
+			pgBufferUsage.aio_read_ontime - bufusage.aio_read_ontime;
+
 		bufusage.blk_read_time = pgBufferUsage.blk_read_time;
 		INSTR_TIME_SUBTRACT(bufusage.blk_read_time, bufusage_start.blk_read_time);
 		bufusage.blk_write_time = pgBufferUsage.blk_write_time;
@@ -1041,6 +1070,7 @@ pgss_ProcessUtility(Node *parsetree, con
 				   rows,
 				   &bufusage,
 				   NULL);
+
 	}
 	else
 	{
@@ -1224,6 +1254,16 @@ pgss_store(const char *query, uint32 que
 		e->counters.local_blks_written += bufusage->local_blks_written;
 		e->counters.temp_blks_read += bufusage->temp_blks_read;
 		e->counters.temp_blks_written += bufusage->temp_blks_written;
+
+		e->counters.aio_read_noneed     += bufusage->aio_read_noneed;
+		e->counters.aio_read_discrd     += bufusage->aio_read_discrd;
+		e->counters.aio_read_forgot     += bufusage->aio_read_forgot;
+		e->counters.aio_read_noblok     += bufusage->aio_read_noblok;
+		e->counters.aio_read_failed     += bufusage->aio_read_failed;
+		e->counters.aio_read_wasted     += bufusage->aio_read_wasted;
+		e->counters.aio_read_waited     += bufusage->aio_read_waited;
+		e->counters.aio_read_ontime     += bufusage->aio_read_ontime;
+
 		e->counters.blk_read_time += INSTR_TIME_GET_MILLISEC(bufusage->blk_read_time);
 		e->counters.blk_write_time += INSTR_TIME_GET_MILLISEC(bufusage->blk_write_time);
 		e->counters.usage += USAGE_EXEC(total_time);
@@ -1257,7 +1297,8 @@ pg_stat_statements_reset(PG_FUNCTION_ARG
 #define PG_STAT_STATEMENTS_COLS_V1_0	14
 #define PG_STAT_STATEMENTS_COLS_V1_1	18
 #define PG_STAT_STATEMENTS_COLS_V1_2	19
-#define PG_STAT_STATEMENTS_COLS			19		/* maximum of above */
+#define PG_STAT_STATEMENTS_COLS_V1_3	27
+#define PG_STAT_STATEMENTS_COLS			27		/* maximum of above */
 
 /*
  * Retrieve statement statistics.
@@ -1270,6 +1311,16 @@ pg_stat_statements_reset(PG_FUNCTION_ARG
  * function.  Unfortunately we weren't bright enough to do that for 1.1.
  */
 Datum
+pg_stat_statements_1_3(PG_FUNCTION_ARGS)
+{
+	bool		showtext = PG_GETARG_BOOL(0);
+
+	pg_stat_statements_internal(fcinfo, PGSS_V1_3, showtext);
+
+	return (Datum) 0;
+}
+
+Datum
 pg_stat_statements_1_2(PG_FUNCTION_ARGS)
 {
 	bool		showtext = PG_GETARG_BOOL(0);
@@ -1358,6 +1409,10 @@ pg_stat_statements_internal(FunctionCall
 			if (api_version != PGSS_V1_2)
 				elog(ERROR, "incorrect number of output arguments");
 			break;
+		case PG_STAT_STATEMENTS_COLS_V1_3:
+			if (api_version != PGSS_V1_3)
+				elog(ERROR, "incorrect number of output arguments");
+			break;
 		default:
 			elog(ERROR, "incorrect number of output arguments");
 	}
@@ -1534,11 +1589,24 @@ pg_stat_statements_internal(FunctionCall
 		{
 			values[i++] = Float8GetDatumFast(tmp.blk_read_time);
 			values[i++] = Float8GetDatumFast(tmp.blk_write_time);
+
+			if (api_version >= PGSS_V1_3)
+			{
+				values[i++] = Int64GetDatumFast(tmp.aio_read_noneed);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_discrd);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_forgot);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_noblok);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_failed);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_wasted);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_waited);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_ontime);
+			}
 		}
 
 		Assert(i == (api_version == PGSS_V1_0 ? PG_STAT_STATEMENTS_COLS_V1_0 :
 					 api_version == PGSS_V1_1 ? PG_STAT_STATEMENTS_COLS_V1_1 :
 					 api_version == PGSS_V1_2 ? PG_STAT_STATEMENTS_COLS_V1_2 :
+					 api_version == PGSS_V1_3 ? PG_STAT_STATEMENTS_COLS_V1_3 :
 					 -1 /* fail if you forget to update this assert */ ));
 
 		tuplestore_putvalues(tupstore, tupdesc, values, nulls);
--- contrib/pg_stat_statements/pg_stat_statements--1.2--1.3.sql.orig	2014-05-28 08:50:32.110559768 -0400
+++ contrib/pg_stat_statements/pg_stat_statements--1.2--1.3.sql	2014-05-28 16:45:42.658506251 -0400
@@ -0,0 +1,51 @@
+/* contrib/pg_stat_statements/pg_stat_statements--1.2--1.3.sql */
+
+-- complain if script is sourced in psql, rather than via ALTER EXTENSION
+\echo Use "ALTER EXTENSION pg_stat_statements UPDATE TO '1.3'" to load this file. \quit
+
+/* First we have to remove them from the extension */
+ALTER EXTENSION pg_stat_statements DROP VIEW pg_stat_statements;
+ALTER EXTENSION pg_stat_statements DROP FUNCTION pg_stat_statements();
+
+/* Then we can drop them */
+DROP VIEW pg_stat_statements;
+DROP FUNCTION pg_stat_statements();
+
+/* Now redefine */
+CREATE FUNCTION pg_stat_statements(IN showtext boolean,
+    OUT userid oid,
+    OUT dbid oid,
+    OUT queryid bigint,
+    OUT query text,
+    OUT calls int8,
+    OUT total_time float8,
+    OUT rows int8,
+    OUT shared_blks_hit int8,
+    OUT shared_blks_read int8,
+    OUT shared_blks_dirtied int8,
+    OUT shared_blks_written int8,
+    OUT local_blks_hit int8,
+    OUT local_blks_read int8,
+    OUT local_blks_dirtied int8,
+    OUT local_blks_written int8,
+    OUT temp_blks_read int8,
+    OUT temp_blks_written int8,
+    OUT blk_read_time float8,
+    OUT blk_write_time float8
+   , OUT aio_read_noneed int8
+   , OUT aio_read_discrd int8
+   , OUT aio_read_forgot int8
+   , OUT aio_read_noblok int8
+   , OUT aio_read_failed int8
+   , OUT aio_read_wasted int8
+   , OUT aio_read_waited int8
+   , OUT aio_read_ontime int8
+)
+RETURNS SETOF record
+AS 'MODULE_PATHNAME', 'pg_stat_statements_1_3'
+LANGUAGE C STRICT VOLATILE;
+
+CREATE VIEW pg_stat_statements AS
+  SELECT * FROM pg_stat_statements(true);
+
+GRANT SELECT ON pg_stat_statements TO PUBLIC;
--- postgresql-prefetching-asyncio.README.orig	2014-05-28 09:18:59.386318460 -0400
+++ postgresql-prefetching-asyncio.README	2014-05-28 16:45:42.702506429 -0400
@@ -0,0 +1,542 @@
+Postgresql  --   Extended Prefetching using Asynchronous IO
+============================================================
+
+Postgresql currently (9.3.4) provides a limited prefetching capability
+using posix_fadvise to give hints to the Operating System kernel
+about which pages it expects to read in the near future.
+This capability is used only during the heap-scan phase of bitmap-index scans.
+It is controlled via the effective_io_concurrency configuration parameter.
+
+This capability is now extended in two ways :
+   .   use asynchronous IO into Postgresql shared buffers as an
+       alternative to posix_fadvise
+   .   Implement prefetching in other types of scan :
+            .  non-bitmap (i.e. simple) index scans - index pages
+                     currently only for B-tree indexes.
+                    (developed by Claudio Freire <klaussfreire(at)gmail(dot)com>)
+            .  non-bitmap (i.e. simple) index scans - heap pages
+                          currently only for B-tree indexes.
+            .  simple heap scans
+
+Posix asynchronous IO is chosen as the function library for asynchronous IO,
+since this is well supported and also fits very well with the model of
+the prefetching process,  particularly as regards checking for completion
+of an asynchronous read.    On linux,   Posix asynchronous IO is provided
+in the librt library.    librt uses independently-schedulable threads to
+achieve the asynchronicity,   rather than kernel functionality.
+
+In this implementation,  use of asynchronous IO is limited to prefetching
+while performing one of the three types of scan
+            .  B-tree bitmap index scan - heap pages    (as already exists)
+            .  B-tree non-bitmap (i.e. simple) index scans - index and heap pages
+            .  simple heap scans
+on permanent relations.   It is not used on temporary tables nor for writes.
+
+The advantages of Posix asynchronous IO into shared buffers
+compared to posix_fadvise are :
+   .   Beneficial for non-sequential access patterns as well as sequential
+   .   No restriction on the kinds of IO which can be used
+       (other kinds of asynchronous IO impose restrictions such as
+        buffer alignment,  use of non-buffered IO).
+   .   Does not interfere with standard linux kernel read-ahead functionality.
+       (It has been stated in 
+ www.postgresql.org/message-id/CAGTBQpbu2M=-M7NUr6DWr0K8gUVmXVhwKohB-Cnj7kYS1AhH4A@mail.gmail.com
+       that :
+          "the kernel stops doing read-ahead when a call to posix_fadvise comes.
+           I noticed the performance hit, and checked the kernel's code.
+           It effectively changes the prediction mode from sequential to fadvise,
+           negating the (assumed) kernel's prefetch logic")
+   .   When the read request is issued after a prefetch has completed,
+       no delay associated with a kernel call to copy the page from
+       kernel page buffers into the Postgresql shared buffer,
+       since it is already there.
+       Also,   in a memory-constrained environment,   there is a greater
+       probability that the prefetched page will "stick" in memory
+       since the linux kernel victimizes the filesystem page cache in preference
+       to swapping out user process pages.
+   .   Statistics on prefetch success can be gathered (see "Statistics" below)
+       which helps the administrator to tune the prefetching settings.
+
+These benefits are most likely to be obtained in a system whose usage profile
+(e.g. from iostat)  shows:
+     .   high IO wait from mostly-read activity
+     .   disk access pattern is not entirely sequential
+         (so kernel readahead can't predict it but postgresql can)
+     .   sufficient spare idle CPU to run the librt pthreads
+         or,  stated another way,    the CPU subsystem is relatively powerful
+         compared to the disk subsystem.
+In such ideal conditions,  and with a workload with plenty of index scans,
+around 10% - 20% improvement in throughput has been achieved.
+In an admittedly extreme environment measured by this author,    with a workload
+consisting of 8 client applications each running similar complex queries
+(same query structure but different predicates and constants),
+including 2 Bitmap Index Scans and 17 non-bitmap index scans,
+on a dual-core Intel laptop (4 hyperthreads) with the database on a single
+USB3-attached 500GB disk drive, and no part of the database in filesystem buffers
+initially,  (filesystem freshly mounted),  comparing unpatched build
+using posix_fadvise with effective_io_concurrency 4 against same build patched
+with async IO and effective_io_concurrency 4 and max_async_io_prefetchers 32,
+elapse time repeatably improved from around 640-670 seconds to around 530-550 seconds,
+a 17% - 18% improvement. 
+
+The disadvantages of Posix asynchronous IO compared to posix_fadvise are:
+     .   probably higher CPU utilization:
+         Firstly, the extra work performed by the librt threads adds CPU
+         overhead, and secondly, if the asynchronous prefetching is effective,
+         then it will deliver better (greater) overlap of CPU with IO, which
+         will reduce elapsed times and hence increase CPU utilization percentage
+         still more (during that shorter elapsed time).
+     .   more context switching,  because of the additional threads.
+
+
+Statistics:
+___________
+
+A number of additional statistics relating to effectiveness of asynchronous IO
+are provided as an extension of the existing pg_stat_statements loadable module.
+Refer to the appendix "Additional Supplied Modules" in the current
+PostgreSQL Documentation for details of this module.
+
+The following additional statistics are provided for asynchronous IO prefetching:
+
+    . aio_read_noneed  :   number of prefetches for which no need for prefetch as block already in buffer pool
+    . aio_read_discrd  :   number of prefetches for which buffer not subsequently read and therefore discarded
+    . aio_read_forgot  :   number of prefetches for which buffer not subsequently read and then forgotten about
+    . aio_read_noblok  :   number of prefetches for which no available BufferAiocb  control block
+    . aio_read_failed  :   number of aio reads for which aio itself failed or the read failed with an errno
+    . aio_read_wasted  :   number of aio reads for which in-progress aio cancelled and disk block not used
+    . aio_read_waited  :   number of aio reads for which disk block used but had to wait for it
+    . aio_read_ontime  :   number of aio reads for which disk block used and ready on time when requested
+
+Some of these are (hopefully) self-explanatory.    Some additional notes:
+
+    . aio_read_discrd and aio_read_forgot  :
+                    prefetch was wasted work since the buffer was not subsequently read
+                    The discrd case indicates that the scanner realized this and discarded the buffer,
+                    whereas the forgot case indicates that the scanner did not realize it,
+                    which should not normally occur.
+                    A high number in either suggests lowering effective_io_concurrency.
+
+    . aio_read_noblok  :   
+                    Any significant number in relation to all the other numbers indicates that
+                    max_async_io_prefetchers should be increased.
+
+    . aio_read_waited  :
+                    The page was prefetched but the asynchronous read had not completed by the time the
+                    scanner requested to read it.     causes extra overhead in waiting and indicates
+                    prefetching is not providing much if any benefit.
+                    The disk subsystem may be underpowered/overloaded in relation to the available CPU power.
+
+    . aio_read_ontime  :
+                    The page was prefetched and the asynchronous read had completed by the time the
+                    scanner requested to read it.     Optimal behaviour.      If this number if large
+                    in relation to all the other numbers except (possibly) aio_read_noneed,
+                    then prefetching is working well.
+
+To create the extension with support for these additional statistics, use the following syntax:
+     CREATE EXTENSION pg_stat_statements VERSION '1.3'
+or,  if you run the new code against an existing database which already has the extension
+( see installation and migration below ),  you can 
+     ALTER EXTENSION pg_stat_statements UPDATE TO '1.3'
+
+A suggested set of commands for displaying these statistics might be :
+
+ /*  OPTIONALLY */ DROP extension pg_stat_statements;
+                   CREATE extension pg_stat_statements VERSION '1.3';
+ /*  run your workload   */
+                  select userid , dbid , substring(query from 1 for 24) , calls , total_time , rows , shared_blks_read , blk_read_time , blk_write_time \
+                    , aio_read_noneed , aio_read_noblok , aio_read_failed , aio_read_wasted , aio_read_waited , aio_read_ontime , aio_read_forgot       \
+                      from pg_stat_statements where shared_blks_read > 0;
+
+
+Installation and Build Configuration:
+_____________________________________
+
+1. First -  a prerequsite:
+#  as well as requiring all the usual package build tools such as gcc , make etc,
+#  as described in the instructions for building postgresql,
+#  the following is required :
+    gnu autoconf at version 2.69 :
+# run the following command
+autoconf -V
+# it *must* return
+autoconf (GNU Autoconf) 2.69
+
+2. If you don't have it or it is a different version,
+then you must obtain version 2.69 (which is the current version)
+from your distribution provider or from the gnu software download site.
+
+3. Also you must have the source tree for postgresql version 9.4 (development version).
+#   all the following commands assume your current working directory is the top of the source tree.
+
+4. cd to top of source tree :
+#   check it appears to be a postgresql source tree
+ls -ld configure.in src
+#   should show both the file and the directory
+grep PostgreSQL COPYRIGHT
+#   should show PostgreSQL Database Management System
+
+5. Apply the patch :
+patch -b -p0 -i <patch_file_path>
+#   should report no errors, 43 files patched (see list at bottom of this README)
+#   and all hunks applied
+#  check the patch was appplied to configure.in
+ls -ld configure.in.orig configure.in
+#   should show both files
+
+6. Rebuild the configure script with the patched configure.in :
+mv configure configure.orig;
+autoconf configure.in >configure;echo "rc= $? from autoconf"; chmod +x configure;
+ls -lrt configure.orig configure;
+
+7. run the new configure script :
+#   if you have run configure before,
+#   then you may first want to save existing config.status and config.log if they exist,
+#   and then specify same configure flags and options as you specified before.
+#   the patch does not alter or extend the set of configure options
+#   if unsure,   run ./configure --help
+#   if still unsure,   run ./configure
+./configure <other configure options as desired>
+
+
+
+8. now check that configure decided that this environment supports asynchronous IO :
+grep USE_AIO_ATOMIC_BUILTIN_COMP_SWAP src/include/pg_config.h
+#  it should show
+#define USE_AIO_ATOMIC_BUILTIN_COMP_SWAP 1
+#  if not,  apparently your environment does not support asynch IO  -
+#  the config.log will show how it came to that conclusion,
+#  also check for :
+#    . a librt.so somewhere in the loader's library path (probably under /lib , /lib64 , or /usr)
+#    . your gcc must support the atomic compare_and_swap __sync_bool_compare_and_swap built-in function
+#  do not proceed without this define being set.
+
+9. do you want to use the new code on an existing cluster
+   that was created using the same code base but without the patch?
+   If so then run this nasty-looking command :
+   (cut-and-paste it into a terminal window or a shell-script file)
+   Otherwise continue to step 10.
+   see Migration note below for explanation.
+###############################################################################################
+   fl=src/Makefile.global; typeset -i bkx=0; while [[ $bkx < 200 ]]; do {
+       bkfl="${fl}.bak${bkx}"; if [[ -a ${bkfl} ]]; then ((bkx=bkx+1)); else break; fi;
+   }; done;
+   if [[ -a ${bkfl} ]]; then echo "sorry cannot find a backup name for $fl";
+   elif [[ -a $fl ]]; then {
+       mv $fl $bkfl && {
+          sed -e "/^CFLAGS =/ s/\$/ -DAVOID_CATALOG_MIGRATION_FOR_ASYNCIO/" $bkfl > $fl;
+          str="diff -w $bkfl $fl";echo "$str"; eval "$str";
+       };
+   };
+   else echo "ooopppss $fl is missing";
+   fi;
+###############################################################################################
+# it should report something like
+diff -w Makefile.global.bak0 Makefile.global
+222c222
+< CFLAGS = XXXX
+---
+> CFLAGS = XXXX -DAVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+#   where XXXX is some set of flags
+
+
+10. now run the rest of the build process as usual  -
+    follow instructions in file INSTALL if that file exists,
+    else e.g. run
+make && make install
+
+If the build fails with the following error:
+undefined reference to `aio_init'
+Then edit the following file
+src/include/pg_config_manual.h
+and add the following line at the bottom:
+
+#define DONT_HAVE_AIO_INIT
+
+and then run
+make clean && make && make install
+See notes to section Runtime Configuration below for more information on this.
+
+
+
+Migration , Runtime Configuration, and Use:
+___________________________________________
+
+
+Database Migration:
+___________________
+
+The new prefetching code for non-bitmap index scans introduces a new btree-index
+function named btpeeknexttuple.    The correct way to add such a function involves
+also adding it to the catalog as an internal function in pg_proc.
+However,  this results in the new built code considering an existing database to be
+incompatible,  i.e requiring backup on the old code and restore on the new.
+This is normal behaviour for migration to a new version of postgresql,  and is
+also a valid way of migrating a database for use with this asynchronous IO feature,
+but in this case it may be inconvenient.
+
+As an alternative,    the new code may be compiled with the macro define
+AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+which does what it says by not altering the catalog.   The patched build can then
+be run against an existing database cluster initdb'd using the unpatched build.
+
+There are no known ill-effects of so doing,  but :
+     .  in any case,  it is strongly suggested to make a backup of any precious database
+        before accessing it with a patched build
+     .  be aware that if this asynchronous IO feature is eventually released as part of postgresql,
+        migration will probably be required anyway.
+
+This option to avoid catalog migration is intended as a convenience for a quick test,
+and also makes it easier to obtain performance comparisons on the same database.
+
+
+
+Runtime Configuration:
+______________________
+
+One new configuration parameter settable in postgresql.conf and
+in any other way as described in the postgresql documentation :
+
+max_async_io_prefetchers
+  Maximum number of background processes concurrently using asynchronous
+  librt threads to prefetch pages into shared memory buffers
+
+This number can be thought of as the maximum number
+of librt threads concurrently active,   each working on a list of
+from 1 to target_prefetch_pages pages ( see notes 1 and 2 ).
+
+In practice,    this number simply controls how many prefetch requests in total
+may be active concurrently :
+        max_async_io_prefetchers * target_prefetch_pages ( see note 1)
+
+default is max_connections/6
+and recall that the default for max_connections is 100
+
+
+note 1  a number based on effective_io_concurrency and approximately n * ln(n)
+        where n is effective_io_concurrency
+
+note 2  Provided that the gnu extension to Posix AIO which provides the
+aio_init() function is present,   then aio_init() is called
+to set the librt maximum number of threads to max_async_io_prefetchers,
+and to set the maximum number of concurrent aio read requests to the product of
+        max_async_io_prefetchers * target_prefetch_pages
+
+
+As well as this regular configuration parameter,
+there are several other parameters that can be set via environment variable.
+The reason why they are environment vars rather than regular configuration parameters
+is that it is not expected that they should need to be set,   but they may be useful :
+                variable name         values                  default        meaning
+   PG_TRY_PREFETCHING_FOR_BITMAP      [Y|N]                    Y         whether to prefetch bitmap heap scans
+   PG_TRY_PREFETCHING_FOR_ISCAN       [Y|N|integer[,[N|Y]]]   256,N      whether to prefetch  non-bitmap index scans
+                                                                    also numeric size of list of prefetched blocks
+                                                                    also whether to prefetch forward-sequential-pattern index pages
+   PG_TRY_PREFETCHING_FOR_BTREE       [Y|N]                    Y         whether to prefetch heap pages in non-bitmap index scans
+   PG_TRY_PREFETCHING_FOR_HEAP        [Y|N]                    N         whether to prefetch relation (un-indexed) heap scans
+
+
+The setting for PG_TRY_PREFETCHING_FOR_ISCAN is a litle complicated.
+It can be set to Y or N to control prefetching of  non-bitmap index scans;
+But in addition it can be set to an integer,   which both implies Y
+and also sets the size of a list used to remember prefetched but unread heap pages.
+This list is an optimization used to avoid re-prefetching and maximise the potential
+set of prefetchable blocks indexed by one index page.
+And if set to an integer,  this integer may be followed by either ,Y or ,N
+to specify to prefetch index pages which are being accessed forward-sequentially.
+It has been found that prefetching is not of great benefit for this access pattern,
+and so it is not the default,  but also does no harm (provided sufficient CPU capacity).
+
+
+
+Usage :
+______
+
+
+There are no changes in usage other than as noted under Configuration and Statistics.
+However,   in order to assess benefit from this feature,   it will be useful to
+understand the query access plans of your workload using EXPLAIN.    Before doing that,
+make sure that statistics are up to date using ANALYZE.
+
+
+
+Internals:
+__________
+
+
+Internal changes span two areas and the interface between them :
+
+ .  buffer manager layer
+ .  programming interface for scanner to call buffer manager
+ .  scanner layer
+
+ .  buffer manager layer
+    ____________________
+
+    changes comprise :
+       .   allocating,  pinning , unpinning buffers
+            this is complex and discussed briefly below in "Buffer Management"
+       .   acquiring and releasing a BufferAiocb, the control block
+            associated with a single aio_read,  and checking for its completion
+            a new file,  backend/storage/buffer/buf_async.c, provides three new functions,
+                  BufStartAsync        BufReleaseAsync            BufCheckAsync
+            which handle this.
+       .   calling librt asynch io functions
+            this follows the example of all other filesystem interfaces
+            and is straightforward.    
+            two new functions are provided in fd.c:
+                   FileStartaio        FileCompleteaio
+            and corresponding interfaces in smgr.c
+
+ .  programming interface for scanner to call buffer manager
+    ________________________________________________________
+       . calling interface for existing function PrefetchBuffer is modified :
+           .  one new argument,   BufferAccessStrategy strategy
+           .  now returns an int return code which indicates :
+                     whether pin count on buffer has been increased by 1
+                     whether block was already present in a buffer
+       .  new function DiscardBuffer
+           .  discard buffer used for a previously prefetched page
+                 which scanner decides it does not want to read.
+           .  same arguments as for PrefetchBuffer except for omission of BufferAccessStrategy
+           .  note - this is different from the existing function ReleaseBuffer
+                     in that ReleaseBuffer takes a buffer_descriptor as argument
+                     for a buffer which has been read, but has similar purpose.
+
+ .  scanner layer
+    _____________
+        common to all scanners is that the scanner which wishes to prefetch must do two things:
+          .  decide which pages to prefetch and call PrefetchBuffer to prefetch them
+                 nodeBitmapHeapscan already does this (but note one extra argument on PrefetchBuffer)
+          .  remember which pages it has prefetched in some list (actual or conceptual,  e.g. a page range),
+                 removing each page from this list if and when it subsequently reads the page.
+          .  at end of scan,  call DiscardBuffer for every remembered (i.e. prefetched not unread) page
+       how this list of prefetched pages is implemented varies for each of the three scanners and four scan types:
+            .  bitmap index scan - heap pages
+            .  non-bitmap (i.e. simple) index scans - index pages
+            .  non-bitmap (i.e. simple) index scans - heap pages
+            .  simple heap scans
+       The consequences of forgetting to call DiscardBuffer on a prefetched but unread page are:
+            .   counted in aio_read_forgot  (see "Statistics" above)
+            .   may incur an annoying but harmless warning in the pg_log "Buffer Leak ... "
+                  (the buffer is released at commit)
+       This does sometimes happen ...
+     
+
+
+Buffer Management
+_________________
+
+With async io,   PrefetchBuffer must allocate and pin a buffer,  which is relatively straightforward,
+but also every other part of buffer manager must know about the possibility that a buffer may be in
+a state of async_io_in_progress state and be prepared to determine the possible completion.
+That is,  one backend BK1 may start the io but another BK2 may try to read it before BK1 does.
+Posix Asynchronous IO provides a means for waiting on this or another task's read if in progress,
+namely aio_suspend(),  which this extension uses.    Therefore,  although StartBufferIO and TerminateBufferIO
+are called as part of asynchronous prefetching,   their role is limited to maintaining the buffer descriptor flags,
+and they do not track the asynchronous IO itself.    Instead,   asynchronous IOs are tracked in
+a separate set of shared control blocks,  the BufferAiocb list -
+refer to     include/storage/buf_internals.h
+Checking asynchronous io status is handled in  backend/storage/buffer/buf_async.c BufCheckAsync function.
+Read the commentary for this function for more details.
+
+Pinning and unpinning of buffers is the most complex aspect of asynch io prefetching,
+and the logic is spread throughout BufStartAsync , BufCheckAsync , and many functions in bufmgr.c.
+When a backend BK2 requests ReadBuffer of a page for which asynch read is in progress,
+buffer manager has to determine which backend BK1 pinned this buffer during previous PrefetchBuffer,
+and for example must not be re-pinned a second time if BK2 is BK1.
+Information concerning which backend initiated the prefetch is held in the BufferAiocb.
+
+The trickiest case concerns the scenario in which :
+   .  BK1 initiates prefetch and acquires a pin
+   .  BK2 possibly waits for completion and then reads the buffer,  and perhaps later on
+         releases it by ReleaseBuffer.
+   .  Since the asynchronous IO is no longer in progress,     there is no longer any
+         BufferAiocb associated with it.    Yet buffer manager must remember that BK1 holds a
+         "prefetch" pin, i.e. a pin which must not be repeated if and when BK1 finally issues ReadBuffer.
+   .  The solution to this problem is to invent the concept of a "banked" pin,
+      which is a pin obtained when prefetch was issued,   identied as in "banked" status only if and when
+      the associated asynchronous IO terminates,  and redeemable by the next use by same task,
+      either by ReadBuffer or DiscardBuffer.
+      The pid of the backend which holds a banked pin on a buffer (there can be at most one such backend)
+      is stored in the buffer descriptor.
+      This is done without increasing size of the buffer descriptor,  which is important since
+      there may be a very large number of these.     This does overload the relevant field in the descriptor.
+      Refer to include/storage/buf_internals.h for more details
+      and search for BM_AIO_PREFETCH_PIN_BANKED in storage/buffer/bufmgr.c and  backend/storage/buffer/buf_async.c
+
+______________________________________________________________________________
+The following 43 files are changed in this feature (output of the patch command) :
+
+patching file configure.in
+patching file contrib/pg_stat_statements/pg_stat_statements--1.3.sql
+patching file contrib/pg_stat_statements/Makefile
+patching file contrib/pg_stat_statements/pg_stat_statements.c
+patching file contrib/pg_stat_statements/pg_stat_statements--1.2--1.3.sql
+patching file config/c-library.m4
+patching file src/backend/postmaster/postmaster.c
+patching file src/backend/executor/nodeBitmapHeapscan.c
+patching file src/backend/executor/nodeIndexscan.c
+patching file src/backend/executor/instrument.c
+patching file src/backend/storage/buffer/Makefile
+patching file src/backend/storage/buffer/bufmgr.c
+patching file src/backend/storage/buffer/buf_async.c
+patching file src/backend/storage/buffer/buf_init.c
+patching file src/backend/storage/smgr/md.c
+patching file src/backend/storage/smgr/smgr.c
+patching file src/backend/storage/file/fd.c
+patching file src/backend/storage/lmgr/proc.c
+patching file src/backend/access/heap/heapam.c
+patching file src/backend/access/heap/syncscan.c
+patching file src/backend/access/index/indexam.c
+patching file src/backend/access/index/genam.c
+patching file src/backend/access/nbtree/nbtsearch.c
+patching file src/backend/access/nbtree/nbtinsert.c
+patching file src/backend/access/nbtree/nbtpage.c
+patching file src/backend/access/nbtree/nbtree.c
+patching file src/backend/nodes/tidbitmap.c
+patching file src/backend/utils/misc/guc.c
+patching file src/backend/utils/mmgr/aset.c
+patching file src/include/executor/instrument.h
+patching file src/include/storage/bufmgr.h
+patching file src/include/storage/smgr.h
+patching file src/include/storage/fd.h
+patching file src/include/storage/buf_internals.h
+patching file src/include/catalog/pg_am.h
+patching file src/include/catalog/pg_proc.h
+patching file src/include/pg_config_manual.h
+patching file src/include/access/nbtree.h
+patching file src/include/access/heapam.h
+patching file src/include/access/relscan.h
+patching file src/include/nodes/tidbitmap.h
+patching file src/include/utils/rel.h
+patching file src/include/pg_config.h.in
+
+
+Future Possibilities:
+____________________
+
+There are several possible extensions of this feature :
+   .   Extend prefetching of index scans to types of index
+       other than B-tree.
+       This should be fairly straightforward,  but requires some
+       good base of benchmarkable workloads to prove the value.
+   .   Investigate why asynchronous IO prefetching does not greatly
+       improve sequential relation heap scans and possibly find how to
+       achieve a benefit.
+   .   Build knowledge of asycnhronous IO prefetching into the
+       Query Planner costing.
+       This is far from straightforward.    The Postgresql Query Planner's
+       costing model is based on resource consumption rather than elapsed time.
+       Use of asynchronous IO prefetching is intended to improve elapsed time
+       as the expense of (probably) higher resource consumption.
+       Although Costing understands about the reduced cost of reading buffered
+       blocks, it does not take asynchronicity or overlap of CPU with disk
+       into account.  A naive approach might be to try to tweak the Query
+       Planner's Cost Constant configuration parameters
+       such as seq_page_cost , random_page_cost
+       but this is hazardous as explained in the Documentation.
+
+
+
+John Lumby,  johnlumby(at)hotmail(dot)com
--- config/c-library.m4.orig	2014-05-28 08:29:09.142829396 -0400
+++ config/c-library.m4	2014-05-28 16:45:42.746506606 -0400
@@ -367,3 +367,50 @@ if test "$pgac_cv_type_locale_t" = 'yes
   AC_DEFINE(LOCALE_T_IN_XLOCALE, 1,
             [Define to 1 if `locale_t' requires <xlocale.h>.])
 fi])])# PGAC_HEADER_XLOCALE
+
+
+# PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP
+# ---------------------------------------
+# test whether this system has both the librt-style async io and the gcc atomic compare_and_swap
+#      and test operation  of the latter.
+#
+AC_DEFUN([PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP],
+[AC_MSG_CHECKING([whether have both librt-style async io and the gcc atomic compare_and_swap])
+AC_CACHE_VAL(pgac_cv_aio_atomic_builtin_comp_swap,
+pgac_save_LIBS=$LIBS
+LIBS=" -lrt $pgac_save_LIBS"
+[AC_TRY_RUN([#include <stdio.h>
+#include <unistd.h>
+#include "aio.h"
+
+int main (int argc, char *argv[])
+ {
+   int rc;
+   struct aiocb volatile * first_aiocb;
+   struct aiocb volatile * second_aiocb;
+   struct aiocb volatile * my_aiocbp = (struct aiocb *)20000008;
+
+   first_aiocb = (struct aiocb *)20000008;
+   second_aiocb = (struct aiocb *)40000008;
+
+   /*  return zero as success if two comp-swaps both worked as expected -
+   **  first compares equal and swaps,  second compares unequal
+   */
+   rc = (__sync_bool_compare_and_swap (&my_aiocbp, first_aiocb, second_aiocb));
+   if (rc) {
+      rc = (__sync_bool_compare_and_swap (&my_aiocbp, first_aiocb, second_aiocb));
+   } else {
+      rc = -1;
+   }
+
+   return rc;
+}],
+[pgac_cv_aio_atomic_builtin_comp_swap=yes],
+[pgac_cv_aio_atomic_builtin_comp_swap=no],
+[pgac_cv_aio_atomic_builtin_comp_swap=cross])
+])dnl AC_CACHE_VAL
+AC_MSG_RESULT([$pgac_cv_aio_atomic_builtin_comp_swap])
+if test x"$pgac_cv_aio_atomic_builtin_comp_swap" != x"yes"; then
+LIBS=$pgac_save_LIBS
+fi
+])# PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP
--- src/backend/postmaster/postmaster.c.orig	2014-05-28 08:29:09.322829301 -0400
+++ src/backend/postmaster/postmaster.c	2014-05-28 16:45:42.814506880 -0400
@@ -123,6 +123,11 @@
 #include "storage/spin.h"
 #endif
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+void ReportFreeBAiocbs(void);
+int CountInuseBAiocbs(void);
+extern int hwmBufferAiocbs;         /*  high water mark of in-use  BufferAiocbs in pool           */
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * Possible types of a backend. Beyond being the possible bkend_type values in
@@ -1493,9 +1498,15 @@ ServerLoop(void)
 	fd_set		readmask;
 	int			nSockets;
 	time_t		now,
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                           count_baiocb_time,
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 				last_touch_time;
 
 	last_touch_time = time(NULL);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        count_baiocb_time = time(NULL);
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 	nSockets = initMasks(&readmask);
 
@@ -1654,6 +1665,19 @@ ServerLoop(void)
 			last_touch_time = now;
 		}
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                /*   maintain the hwm of used baiocbs every 10 seconds  */
+		if ((now - count_baiocb_time) >= 10)
+		{
+                        int inuseBufferAiocbs;         /*  current in-use  BufferAiocbs in pool */
+                        inuseBufferAiocbs = CountInuseBAiocbs();
+                        if (inuseBufferAiocbs > hwmBufferAiocbs) {
+			    hwmBufferAiocbs = inuseBufferAiocbs;
+			}
+			count_baiocb_time = now;
+		}
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 		/*
 		 * If we already sent SIGQUIT to children and they are slow to shut
 		 * down, it's time to send them SIGKILL.  This doesn't happen
@@ -3444,6 +3468,9 @@ PostmasterStateMachine(void)
 						signal_child(PgStatPID, SIGQUIT);
 				}
 			}
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                        ReportFreeBAiocbs();
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 		}
 	}
 
--- src/backend/executor/nodeBitmapHeapscan.c.orig	2014-05-28 08:29:09.270829328 -0400
+++ src/backend/executor/nodeBitmapHeapscan.c	2014-05-28 16:45:42.834506961 -0400
@@ -34,6 +34,8 @@
  *		ExecEndBitmapHeapScan		releases all storage.
  */
 #include "postgres.h"
+#include <sys/types.h> 
+#include <unistd.h>
 
 #include "access/relscan.h"
 #include "access/transam.h"
@@ -47,6 +49,10 @@
 #include "utils/snapmgr.h"
 #include "utils/tqual.h"
 
+#ifdef USE_PREFETCH
+extern unsigned int prefetch_dbOid; /*  database oid of relations on which prefetching to be done - 0 means all */
+extern unsigned int prefetch_bitmap_scans; /*  boolean whether to prefetch bitmap heap scans         */
+#endif   /* USE_PREFETCH */
 
 static TupleTableSlot *BitmapHeapNext(BitmapHeapScanState *node);
 static void bitgetpage(HeapScanDesc scan, TBMIterateResult *tbmres);
@@ -111,10 +117,21 @@ BitmapHeapNext(BitmapHeapScanState *node
 		node->tbmres = tbmres = NULL;
 
 #ifdef USE_PREFETCH
-		if (target_prefetch_pages > 0)
-		{
+		if (    prefetch_bitmap_scans
+                     && (target_prefetch_pages > 0)
+                     && (    (   (prefetch_dbOid > 0)
+                               && (prefetch_dbOid == scan->rs_rd->rd_node.dbNode)
+                             )
+                         ||  (prefetch_dbOid == 0)
+                        )
+                        /* sufficient number of blocks - at least twice the target_prefetch_pages */
+                     && (scan->rs_nblocks > (2*target_prefetch_pages))
+                   ) {
 			node->prefetch_iterator = prefetch_iterator = tbm_begin_iterate(tbm);
 			node->prefetch_pages = 0;
+                        if (prefetch_iterator) {
+                          tbm_zero(prefetch_iterator);  /* zero list of prefetched and unread blocknos */
+                        }
 			node->prefetch_target = -1;
 		}
 #endif   /* USE_PREFETCH */
@@ -138,12 +155,14 @@ BitmapHeapNext(BitmapHeapScanState *node
 			}
 
 #ifdef USE_PREFETCH
+                        if (prefetch_iterator) {
 			if (node->prefetch_pages > 0)
 			{
 				/* The main iterator has closed the distance by one page */
 				node->prefetch_pages--;
+                                tbm_subtract(prefetch_iterator, tbmres->blockno); /* remove this blockno from list of prefetched and unread blocknos */
 			}
-			else if (prefetch_iterator)
+                            else
 			{
 				/* Do not let the prefetch iterator get behind the main one */
 				TBMIterateResult *tbmpre = tbm_iterate(prefetch_iterator);
@@ -151,6 +170,7 @@ BitmapHeapNext(BitmapHeapScanState *node
 				if (tbmpre == NULL || tbmpre->blockno != tbmres->blockno)
 					elog(ERROR, "prefetch and main iterators are out of sync");
 			}
+                        }
 #endif   /* USE_PREFETCH */
 
 			/*
@@ -239,16 +259,26 @@ BitmapHeapNext(BitmapHeapScanState *node
 			while (node->prefetch_pages < node->prefetch_target)
 			{
 				TBMIterateResult *tbmpre = tbm_iterate(prefetch_iterator);
+                                int             PrefetchBufferRc; /*  return value from PrefetchBuffer  - refer to bufmgr.h */
+
 
 				if (tbmpre == NULL)
 				{
 					/* No more pages to prefetch */
-					tbm_end_iterate(prefetch_iterator);
-					node->prefetch_iterator = prefetch_iterator = NULL;
+                                        /* let ExecEndBitmapHeapScan terminate the prefetch_iterator
+				        **	tbm_end_iterate(prefetch_iterator);
+					**      node->prefetch_iterator = NULL;
+                                        */
+                                        prefetch_iterator = NULL;
 					break;
 				}
 				node->prefetch_pages++;
-				PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, tbmpre->blockno);
+				PrefetchBufferRc = PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, tbmpre->blockno , 0);
+                                /*  add this blockno to list of prefetched and unread blocknos
+                                **  if pin count did not increase then indicate so in the Unread_Pfetched list
+                                */
+                                tbm_add(prefetch_iterator
+                                   ,( (PrefetchBufferRc & PREFTCHRC_BUF_PIN_INCREASED) ? tbmpre->blockno : InvalidBlockNumber ) );
 			}
 		}
 #endif   /* USE_PREFETCH */
@@ -482,12 +512,31 @@ ExecEndBitmapHeapScan(BitmapHeapScanStat
 {
 	Relation	relation;
 	HeapScanDesc scanDesc;
+	TBMIterator *prefetch_iterator;
 
 	/*
 	 * extract information from the node
 	 */
 	relation = node->ss.ss_currentRelation;
 	scanDesc = node->ss.ss_currentScanDesc;
+	prefetch_iterator = node->prefetch_iterator;
+
+#ifdef USE_PREFETCH
+        /*  before any other cleanup,  discard any prefetched but unread buffers  */
+        if (prefetch_iterator != NULL) {
+            TBMIterateResult *tbmpre = tbm_locate_IterateResult(prefetch_iterator);
+            BlockNumber *Unread_Pfetched_base = tbmpre->Unread_Pfetched_base;
+            unsigned int Unread_Pfetched_next = tbmpre->Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+            unsigned int Unread_Pfetched_count = tbmpre->Unread_Pfetched_count;
+
+            while ((Unread_Pfetched_count--) > 0) {
+                DiscardBuffer( scanDesc->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                Unread_Pfetched_next++;
+                if (Unread_Pfetched_next >= target_prefetch_pages)
+                    Unread_Pfetched_next = 0;
+            }
+        }
+#endif   /* USE_PREFETCH */
 
 	/*
 	 * Free the exprcontext
--- src/backend/executor/nodeIndexscan.c.orig	2014-05-28 08:29:09.270829328 -0400
+++ src/backend/executor/nodeIndexscan.c	2014-05-28 16:45:42.858507057 -0400
@@ -35,8 +35,13 @@
 #include "utils/rel.h"
 
 
+
 static TupleTableSlot *IndexNext(IndexScanState *node);
 
+#ifdef USE_PREFETCH
+extern unsigned int prefetch_dbOid; /*  database oid of relations on which prefetching to be done - 0 means all */
+extern unsigned int prefetch_index_scans; /*  boolean whether to prefetch bitmap heap scans         */
+#endif   /* USE_PREFETCH */
 
 /* ----------------------------------------------------------------
  *		IndexNext
@@ -418,7 +423,12 @@ ExecEndIndexScan(IndexScanState *node)
 	 * close the index relation (no-op if we didn't open it)
 	 */
 	if (indexScanDesc)
+        {
 		index_endscan(indexScanDesc);
+
+        /*  note  -  at this point all scan controlblock resources have been freed by IndexScanEnd called by index_endscan */
+
+        }
 	if (indexRelationDesc)
 		index_close(indexRelationDesc, NoLock);
 
@@ -609,6 +619,33 @@ ExecInitIndexScan(IndexScan *node, EStat
 											   indexstate->iss_NumScanKeys,
 											 indexstate->iss_NumOrderByKeys);
 
+#ifdef USE_PREFETCH
+        /*  initialize prefetching   */
+                indexstate->iss_ScanDesc->pfch_index_page_list =  (struct pfch_index_pagelist*)0;
+                indexstate->iss_ScanDesc->pfch_block_item_list = (struct pfch_block_item*)0;
+		if (    prefetch_index_scans
+			 && (target_prefetch_pages > 0)
+			 &&	(!RelationUsesLocalBuffers(indexstate->iss_ScanDesc->heapRelation)) /* I think this must always be true for an indexed heap ? */
+			 && (    (   (prefetch_dbOid > 0)
+					   && (prefetch_dbOid == indexstate->iss_ScanDesc->heapRelation->rd_node.dbNode)
+					 )
+				 ||  (prefetch_dbOid == 0)
+				)
+		   ) {
+			indexstate->iss_ScanDesc->pfch_index_page_list = palloc( sizeof(struct pfch_index_pagelist) + ( (target_prefetch_pages-1) * sizeof(struct pfch_index_item) ) );
+			indexstate->iss_ScanDesc->pfch_block_item_list = palloc( prefetch_index_scans * sizeof(struct pfch_block_item) );
+			if (     ( (struct pfch_index_pagelist*)0 != indexstate->iss_ScanDesc->pfch_index_page_list )
+                  && ( (struct pfch_block_item*)0 != indexstate->iss_ScanDesc->pfch_block_item_list )
+               ) {
+                          indexstate->iss_ScanDesc->pfch_used = 0;
+                          indexstate->iss_ScanDesc->pfch_next = prefetch_index_scans; /* ensure first entry is at index 0 */
+                          indexstate->iss_ScanDesc->pfch_index_page_list->pfch_index_pagelist_next = (struct pfch_index_pagelist*)0;
+                          indexstate->iss_ScanDesc->pfch_index_page_list->pfch_index_item_count = 0;
+                          indexstate->iss_ScanDesc->do_prefetch = 1;
+            }
+		}
+#endif   /* USE_PREFETCH */
+
 	/*
 	 * If no run-time keys to calculate, go ahead and pass the scankeys to the
 	 * index AM.
--- src/backend/executor/instrument.c.orig	2014-05-28 08:29:09.266829330 -0400
+++ src/backend/executor/instrument.c	2014-05-28 16:45:42.882507154 -0400
@@ -41,6 +41,14 @@ InstrAlloc(int n, int instrument_options
 		{
 			instr[i].need_bufusage = need_buffers;
 			instr[i].need_timer = need_timer;
+			instr[i].bufusage_start.aio_read_noneed = 0;
+			instr[i].bufusage_start.aio_read_discrd = 0;
+			instr[i].bufusage_start.aio_read_forgot = 0;
+			instr[i].bufusage_start.aio_read_noblok = 0;
+			instr[i].bufusage_start.aio_read_failed = 0;
+			instr[i].bufusage_start.aio_read_wasted = 0;
+			instr[i].bufusage_start.aio_read_waited = 0;
+			instr[i].bufusage_start.aio_read_ontime = 0;
 		}
 	}
 
@@ -143,6 +151,16 @@ BufferUsageAccumDiff(BufferUsage *dst,
 	dst->local_blks_written += add->local_blks_written - sub->local_blks_written;
 	dst->temp_blks_read += add->temp_blks_read - sub->temp_blks_read;
 	dst->temp_blks_written += add->temp_blks_written - sub->temp_blks_written;
+
+	dst->aio_read_noneed       += add->aio_read_noneed - sub->aio_read_noneed;
+	dst->aio_read_discrd       += add->aio_read_discrd - sub->aio_read_discrd;
+	dst->aio_read_forgot       += add->aio_read_forgot - sub->aio_read_forgot;
+	dst->aio_read_noblok       += add->aio_read_noblok - sub->aio_read_noblok;
+	dst->aio_read_failed       += add->aio_read_failed - sub->aio_read_failed;
+	dst->aio_read_wasted       += add->aio_read_wasted - sub->aio_read_wasted;
+	dst->aio_read_waited       += add->aio_read_waited - sub->aio_read_waited;
+	dst->aio_read_ontime       += add->aio_read_ontime - sub->aio_read_ontime;
+
 	INSTR_TIME_ACCUM_DIFF(dst->blk_read_time,
 						  add->blk_read_time, sub->blk_read_time);
 	INSTR_TIME_ACCUM_DIFF(dst->blk_write_time,
--- src/backend/storage/buffer/Makefile.orig	2014-05-28 08:29:09.330829297 -0400
+++ src/backend/storage/buffer/Makefile	2014-05-28 16:45:42.942507396 -0400
@@ -12,6 +12,6 @@ subdir = src/backend/storage/buffer
 top_builddir = ../../../..
 include $(top_builddir)/src/Makefile.global
 
-OBJS = buf_table.o buf_init.o bufmgr.o freelist.o localbuf.o
+OBJS = buf_table.o buf_init.o bufmgr.o freelist.o localbuf.o buf_async.o
 
 include $(top_srcdir)/src/backend/common.mk
--- src/backend/storage/buffer/bufmgr.c.orig	2014-05-28 08:29:09.334829294 -0400
+++ src/backend/storage/buffer/bufmgr.c	2014-05-28 16:45:42.978507541 -0400
@@ -29,7 +29,7 @@
  *		buf_table.c -- manages the buffer lookup table
  */
 #include "postgres.h"
-
+#include <sys/types.h> 
 #include <sys/file.h>
 #include <unistd.h>
 
@@ -50,7 +50,6 @@
 #include "utils/resowner_private.h"
 #include "utils/timestamp.h"
 
-
 /* Note: these two macros only work on shared buffers, not local ones! */
 #define BufHdrGetBlock(bufHdr)	((Block) (BufferBlocks + ((Size) (bufHdr)->buf_id) * BLCKSZ))
 #define BufferGetLSN(bufHdr)	(PageGetLSN(BufHdrGetBlock(bufHdr)))
@@ -63,6 +62,8 @@
 #define BUF_WRITTEN				0x01
 #define BUF_REUSABLE			0x02
 
+extern volatile struct BAiocbAnchor *BAiocbAnchr;  /*  anchor for all control blocks pertaining to aio  */
+
 #define DROP_RELS_BSEARCH_THRESHOLD		20
 
 /* GUC variables */
@@ -78,26 +79,33 @@ bool		track_io_timing = false;
  */
 int			target_prefetch_pages = 0;
 
-/* local state for StartBufferIO and related functions */
+/* local state for StartBufferIO and related functions
+**  but ONLY for synchronous IO  -  not altered for aio
+*/
 static volatile BufferDesc *InProgressBuf = NULL;
 static bool IsForInput;
+pid_t this_backend_pid = 0;    /*    pid of this backend */
 
 /* local state for LockBufferForCleanup */
 static volatile BufferDesc *PinCountWaitBuf = NULL;
 
-
-static Buffer ReadBuffer_common(SMgrRelation reln, char relpersistence,
+extern int
+BufStartAsync(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy);
+extern int
+BufCheckAsync(SMgrRelation caller_smgr, Relation caller_reln, volatile BufferDesc *buf_desc, int intention
+        ,BufferAccessStrategy strategy , int index_for_aio , bool spinLockHeld , LWLockId PartitionLock );
+Buffer ReadBuffer_common(SMgrRelation reln, char relpersistence,
 				  ForkNumber forkNum, BlockNumber blockNum,
 				  ReadBufferMode mode, BufferAccessStrategy strategy,
-				  bool *hit);
-static bool PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy);
-static void PinBuffer_Locked(volatile BufferDesc *buf);
-static void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner);
+				  bool *hit , int index_for_aio);
+bool PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy);
+void PinBuffer_Locked(volatile BufferDesc *buf);
+void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner);
 static void BufferSync(int flags);
 static int	SyncOneBuffer(int buf_id, bool skip_recently_used);
 static void WaitIO(volatile BufferDesc *buf);
-static bool StartBufferIO(volatile BufferDesc *buf, bool forInput);
-static void TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
+static bool StartBufferIO(volatile BufferDesc *buf, bool forInput , int index_for_aio );
+void TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
 				  int set_flag_bits);
 static void shared_buffer_write_error_callback(void *arg);
 static void local_buffer_write_error_callback(void *arg);
@@ -106,24 +114,66 @@ static volatile BufferDesc *BufferAlloc(
 			ForkNumber forkNum,
 			BlockNumber blockNum,
 			BufferAccessStrategy strategy,
-			bool *foundPtr);
+			int *foundPtr , int index_for_aio );
 static void FlushBuffer(volatile BufferDesc *buf, SMgrRelation reln);
 static void AtProcExit_Buffers(int code, Datum arg);
 static int	rnode_comparator(const void *p1, const void *p2);
 
+BlockNumber BlocknumOfBuffer(Buffer buffer);
+bool BlocknotinBuffer(Buffer buffer, Relation relation, BlockNumber blockNum);
 
 /*
  * PrefetchBuffer -- initiate asynchronous read of a block of a relation
  *
- * This is named by analogy to ReadBuffer but doesn't actually allocate a
- * buffer.  Instead it tries to ensure that a future ReadBuffer for the given
- * block will not be delayed by the I/O.  Prefetching is optional.
+ * This is named by analogy to ReadBuffer but allocates a buffer only if using asynchronous I/O.
+ * Its purpose  is to try to ensure that a future ReadBuffer for the given block
+ * will not be delayed by the I/O.  Prefetching is optional.
  * No-op if prefetching isn't compiled in.
- */
-void
-PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
-{
+ *
+ * Originally the prefetch simply called posix_fadvise() to recommend read-ahead into kernel page cache.
+ * Extended to provide an alternative of issuing an asynchronous aio_read() to read into a buffer.
+ * This extension has an implication on how this bufmgr component manages concurrent requests
+ * for the same disk block.
+ *
+ * Synchronous IO (read()) does not provide a means for waiting on another task's read if in progress,
+ * and bufmgr implements its own scheme in StartBufferIO, WaitIO, and TerminateBufferIO.
+ *
+ * Asynchronous IO (aio_read()) provides a means for waiting on this or another task's read if in progress,
+ * namely aio_suspend(),  which this extension uses.    Therefore,  although StartBufferIO and TerminateBufferIO
+ * are called as part of asynchronous prefetching,   their role is limited to maintaining the buffer desc flags,
+ * and they do not track the asynchronous IO itself.    Instead,   asynchronous IOs are tracked in 
+ * a separate set of shared control blocks,  the BufferAiocb list -
+ *   refer to     include/storage/buf_internals.h and storage/buffer/buf_init.c
+ *
+ * Another implication of asynchronous IO concerns buffer pinning.
+ * The buffer used for the prefetch is pinned before aio_read is issued.
+ * It is expected that the same task (and possibly others) will later ask to read the page
+ * and eventually release and unpin the buffer.
+ * However,  if the task which issued the aio_read later decides not to read the page,
+ * and return code indicates delta_pin_count > 0 (see below)
+ * it *must* instead issue a DiscardBuffer() (see function later in this file)
+ * so that its pin is released.
+ * Therefore,  each client which uses the PrefetchBuffer service must either always read all
+ * prefetched pages,  or keep track of prefetched pages and discard unread ones at end of scan.
+ *
+ * return code:   is an int bitmask defined in bufmgr.h
+        PREFTCHRC_BUF_PIN_INCREASED 0x01      pin count on buffer has been increased by 1
+        PREFTCHRC_BLK_ALREADY_PRESENT 0x02    block was already present in a buffer
+ *
+ * PREFTCHRC_BLK_ALREADY_PRESENT is a hint to caller that the prefetch may be unnecessary
+ */
+int
+PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy)
+{
+	Buffer		buf_id; /* indicates buffer containing the requested block  */
+        int             PrefetchBufferRc = 0; /*  return value as described above  */
+        int             PinCountOnEntry = 0;  /*  pin count on entry           */
+        int             PinCountdelta = 0;    /*  pin count delta increase     */
+
+
 #ifdef USE_PREFETCH
+
+	buf_id = -1;
 	Assert(RelationIsValid(reln));
 	Assert(BlockNumberIsValid(blockNum));
 
@@ -145,8 +195,13 @@ PrefetchBuffer(Relation reln, ForkNumber
 	{
 		BufferTag	newTag;		/* identity of requested block */
 		uint32		newHash;	/* hash value for newTag */
+        int         BufStartAsyncrc = -1;  /*  retcode from BufStartAsync :
+                                                       **        0 if started successfully (which implies buffer was newly pinned )
+                                                       **       -1 if failed for some reason
+                                                       **        1+PrivateRefCount if we found desired buffer in buffer pool
+                                                       **  and we set it likewise if we find buffer in buffer pool
+                                                       */
 		LWLock	   *newPartitionLock;	/* buffer partition lock for it */
-		int			buf_id;
 
 		/* create a tag so we can lookup the buffer */
 		INIT_BUFFERTAG(newTag, reln->rd_smgr->smgr_rnode.node,
@@ -158,28 +213,119 @@ PrefetchBuffer(Relation reln, ForkNumber
 
 		/* see if the block is in the buffer pool already */
 		LWLockAcquire(newPartitionLock, LW_SHARED);
-		buf_id = BufTableLookup(&newTag, newHash);
+		buf_id = (Buffer)BufTableLookup(&newTag, newHash);
+                if (buf_id >= 0) {
+                    PinCountOnEntry = PrivateRefCount[buf_id];  /*  pin count on entry               */
+                    BufStartAsyncrc = 1 + PinCountOnEntry; /* indicate this backends pin count - see above comment */
+                    PrefetchBufferRc = PREFTCHRC_BLK_ALREADY_PRESENT;       /* indicate buffer present */
+                } else {
+                    PrefetchBufferRc = 0;                                   /* indicate buffer not present */
+                }
 		LWLockRelease(newPartitionLock);
 
+     not_in_buffers:
 		/* If not in buffers, initiate prefetch */
-		if (buf_id < 0)
+		if (buf_id < 0) {
+                    /*    try using async aio_read with a buffer */
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                    BufStartAsyncrc = BufStartAsync( reln, forkNum, blockNum , strategy );
+                    if (BufStartAsyncrc < 0) {
+                            pgBufferUsage.aio_read_noblok++;
+                    }
+#else /* not USE_AIO_ATOMIC_BUILTIN_COMP_SWAP so try the alternative that does not read the block into a postgresql buffer */
 			smgrprefetch(reln->rd_smgr, forkNum, blockNum);
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+		}
 
-		/*
-		 * If the block *is* in buffers, we do nothing.  This is not really
-		 * ideal: the block might be just about to be evicted, which would be
-		 * stupid since we know we are going to need it soon.  But the only
-		 * easy answer is to bump the usage_count, which does not seem like a
-		 * great solution: when the caller does ultimately touch the block,
-		 * usage_count would get bumped again, resulting in too much
-		 * favoritism for blocks that are involved in a prefetch sequence. A
-		 * real fix would involve some additional per-buffer state, and it's
-		 * not clear that there's enough of a problem to justify that.
+        if (  (buf_id >= 0) || (BufStartAsyncrc >= 1)  ) {
+                        /* The block *is* in buffers.    */
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                        pgBufferUsage.aio_read_noneed++;
+#ifndef USE_PREFETCH_BUT_DONT_PIN_ALREADY_PRESENT /* jury is out on whether the following wins but it ought to ...  */
+                        /*
+                        ** If this backend already had pinned it,
+                        ** or another backend had banked a pin on it,
+                        ** or there is an IO in progress,
+                        ** or it is not marked valid,
+                        ** then do nothing.
+                        ** Otherwise pin it and mark the buffer's pin as banked by this backend.
+                        ** Note  -  it may or not be pinned by another backend -
+                        **          it is ok for us to bank a pin on it
+                        **          *provided* the other backend did not bank its pin.
+                        **          The reason for this is that the banked-pin indicator is global -
+                        **          it can identify at most one process.
+                        */
+                        /* pgBufferUsage.aio_read_wasted++;  overload counter - only for debugging */
+                        if (BufStartAsyncrc == 1) {            /*   not pinned by me  */
+                              /*  pgBufferUsage.aio_read_wasted++;  overload counter - only for debugging */
+                              /*  note   -   all we can say with certainty is that the buffer is not pinned by me
+                              **             we cannot be sure that it is still in buffer pool
+                              **             so must go through the entire locking and searching all over again ...
 		 */
+                            LWLockAcquire(newPartitionLock, LW_SHARED);
+                            buf_id = (Buffer)BufTableLookup(&newTag, newHash);
+                            /* If in buffers, proceed */
+                            if (buf_id >= 0) {
+                                /*  since the block is now present,
+                                **  save the current pin count to ensure final delta is calculated correctly
+                                */
+                                PinCountOnEntry = PrivateRefCount[buf_id];  /*  pin count on entry               */
+                                if ( PinCountOnEntry == 0) { /*  paranoid check it's still not pinned by me */
+                                    volatile        BufferDesc *buf_desc;
+
+                                    buf_desc = &BufferDescriptors[buf_id];        /* found buffer descriptor */
+                                    LockBufHdr(buf_desc);
+                                    if (    (buf_desc->flags & BM_VALID)           /* buffer is valid        */
+                                         && (!(buf_desc->flags & (BM_IO_IN_PROGRESS|BM_AIO_IN_PROGRESS|BM_AIO_PREFETCH_PIN_BANKED))) /* buffer is not any of ... */
+                                       ) {
+                                        buf_desc->flags |= BM_AIO_PREFETCH_PIN_BANKED; /* bank the pin for next use by this task */
+                                        /*  note  - we can call PinBuffer_Locked with the BM_AIO_PREFETCH_PIN_BANKED flag set because it is not yet pinned by me */
+                                        buf_desc->freeNext = -(this_backend_pid);       /* remember which pid banked it */
+                                        /*  pgBufferUsage.aio_read_wasted--;      overload counter - not wasted after all - only for debugging */
+
+                                        /* Make sure we will have room to remember the buffer pin */
+                                        ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
+
+                                        PinBuffer_Locked(buf_desc);
 	}
-#endif   /* USE_PREFETCH */
+                                    else {
+                                        UnlockBufHdr(buf_desc);
+                                    }
+                                }
+                            }
+                            LWLockRelease(newPartitionLock);
+                            /*  although unlikely,  maybe it was evicted while we were puttering about  */
+                            if (buf_id < 0) {
+                                pgBufferUsage.aio_read_noneed--;   /*   back out the accounting */
+                                goto not_in_buffers;               /*   and try again           */
+                            }
+                        }
+#endif /*  USE_PREFETCH_BUT_DONT_PIN_ALREADY_PRESENT */
+
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+		}
+
+		if (buf_id >= 0) {
+			PinCountdelta = PrivateRefCount[buf_id] - PinCountOnEntry;  /*  pin count delta increase    */
+			if (  (PinCountdelta < 0) || (PinCountdelta > 1) ) {
+				  elog(ERROR,
+						 "PrefetchBuffer #%d : incremented pin count by %d on bufdesc %p refcount %u localpins %d\n"
+								  ,(buf_id+1) , PinCountdelta , &BufferDescriptors[buf_id] ,BufferDescriptors[buf_id].refcount , PrivateRefCount[buf_id]);
 }
+		} else
+		if (BufStartAsyncrc == 0) {  /* aio started successfully (which implies buffer was newly pinned ) */
+			PinCountdelta = 1;
+		}
+
+		/*  set final PrefetchBufferRc according to previous value */
+		PrefetchBufferRc |= PinCountdelta;  /* set the PREFTCHRC_BUF_PIN_INCREASED bit */
+	}
+
+#endif   /* USE_PREFETCH */
 
+	return PrefetchBufferRc; /*  return value as described above */
+}
 
 /*
  * ReadBuffer -- a shorthand for ReadBufferExtended, for reading from main
@@ -252,7 +398,7 @@ ReadBufferExtended(Relation reln, ForkNu
 	 */
 	pgstat_count_buffer_read(reln);
 	buf = ReadBuffer_common(reln->rd_smgr, reln->rd_rel->relpersistence,
-							forkNum, blockNum, mode, strategy, &hit);
+							forkNum, blockNum, mode, strategy, &hit , 0);
 	if (hit)
 		pgstat_count_buffer_hit(reln);
 	return buf;
@@ -280,7 +426,7 @@ ReadBufferWithoutRelcache(RelFileNode rn
 	Assert(InRecovery);
 
 	return ReadBuffer_common(smgr, RELPERSISTENCE_PERMANENT, forkNum, blockNum,
-							 mode, strategy, &hit);
+							 mode, strategy, &hit , 0);
 }
 
 
@@ -288,15 +434,18 @@ ReadBufferWithoutRelcache(RelFileNode rn
  * ReadBuffer_common -- common logic for all ReadBuffer variants
  *
  * *hit is set to true if the request was satisfied from shared buffer cache.
+ * index_for_aio ,  if -ve , is negative of ( index of the aiocb in the BufferAiocbs array + 3 )
+ *     which is passed through to StartBufferIO
  */
-static Buffer
+Buffer
 ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 				  BlockNumber blockNum, ReadBufferMode mode,
-				  BufferAccessStrategy strategy, bool *hit)
+				  BufferAccessStrategy strategy, bool *hit , int index_for_aio )
 {
 	volatile BufferDesc *bufHdr;
 	Block		bufBlock;
 	bool		found;
+        int             allocrc;             /*  retcode from BufferAlloc */
 	bool		isExtend;
 	bool		isLocalBuf = SmgrIsTemp(smgr);
 
@@ -328,16 +477,40 @@ ReadBuffer_common(SMgrRelation smgr, cha
 	}
 	else
 	{
+                allocrc = mode; /* pass mode to BufferAlloc since it must not wait for async io if RBM_NOREAD_FOR_PREFETCH */
 		/*
 		 * lookup the buffer.  IO_IN_PROGRESS is set if the requested block is
 		 * not currently in memory.
 		 */
 		bufHdr = BufferAlloc(smgr, relpersistence, forkNum, blockNum,
-							 strategy, &found);
-		if (found)
-			pgBufferUsage.shared_blks_hit++;
+							 strategy, &allocrc , index_for_aio );
+		if (allocrc < 0) {
+                    if (mode == RBM_ZERO_ON_ERROR || zero_damaged_pages)
+                    {
+                        ereport(WARNING,
+                                (errcode(ERRCODE_DATA_CORRUPTED),
+                                 errmsg("invalid page header in block %u of relation %s; zeroing out page",
+                                        blockNum,
+                                        relpath(smgr->smgr_rnode, forkNum))));
+                        bufBlock = BufHdrGetBlock(bufHdr);
+                        MemSet((char *) bufBlock, 0, BLCKSZ);
+                    }
 		else
+                      ereport(ERROR,
+                              (errcode(ERRCODE_DATA_CORRUPTED),
+                               errmsg("invalid page header in block %u of relation %s",
+                                      blockNum,
+                                      relpath(smgr->smgr_rnode, forkNum))));
+                        found = true;
+                }
+		else if (allocrc > 0) {
+			pgBufferUsage.shared_blks_hit++;
+                        found = true;
+                }
+		else {
 			pgBufferUsage.shared_blks_read++;
+                        found = false;
+                }
 	}
 
 	/* At this point we do NOT hold any locks. */
@@ -410,7 +583,7 @@ ReadBuffer_common(SMgrRelation smgr, cha
 				Assert(bufHdr->flags & BM_VALID);
 				bufHdr->flags &= ~BM_VALID;
 				UnlockBufHdr(bufHdr);
-			} while (!StartBufferIO(bufHdr, true));
+			} while (!StartBufferIO(bufHdr, true, 0));
 		}
 	}
 
@@ -430,6 +603,7 @@ ReadBuffer_common(SMgrRelation smgr, cha
 
 	bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);
 
+        if (mode != RBM_NOREAD_FOR_PREFETCH) {
 	if (isExtend)
 	{
 		/* new buffers are zero-filled */
@@ -499,6 +673,7 @@ ReadBuffer_common(SMgrRelation smgr, cha
 	VacuumPageMiss++;
 	if (VacuumCostActive)
 		VacuumCostBalance += VacuumCostPageMiss;
+	}
 
 	TRACE_POSTGRESQL_BUFFER_READ_DONE(forkNum, blockNum,
 									  smgr->smgr_rnode.node.spcNode,
@@ -520,21 +695,39 @@ ReadBuffer_common(SMgrRelation smgr, cha
  * the default strategy.  The selected buffer's usage_count is advanced when
  * using the default strategy, but otherwise possibly not (see PinBuffer).
  *
- * The returned buffer is pinned and is already marked as holding the
- * desired page.  If it already did have the desired page, *foundPtr is
- * set TRUE.  Otherwise, *foundPtr is set FALSE and the buffer is marked
+ * index_for_aio is an input parm which,  if non-zero,  identifies a BufferAiocb
+ * acquired by caller and to be used for any StartBufferIO performed by this routine.
+ * In this case,  if block not found in buffer pool and we allocate a new buffer,
+ * then we must maintain the spinlock on the buffer and pass it back to caller.
+ *
+ * foundPtr is input and output :
+ *  . input   -  indicates the read-buffer mode  ( see bufmgr.h )
+ *  . output  -  indicates the status of the buffer - see below
+ *
+ * Except for the case of RBM_NOREAD_FOR_PREFETCH and buffer is found,
+ * the returned buffer is pinned and is already marked as holding the
+ * desired page.
+ *  If it already did have the desired page and page content is valid,
+ *  *foundPtr is set to 1
+ *  If it already did have the desired page and mode is RBM_NOREAD_FOR_PREFETCH
+ *    and StartBufferIO returned false
+ *    (meaning it could not initialise the buffer for aio)
+ *  *foundPtr is set to 2
+ *  If it already did have the desired page but page content is invalid,
+ *  *foundPtr is set to -1
+ *   this can happen only if the buffer was read by an async read
+ *   and the aio is still in progress or pinned by the issuer of the startaio.
+ *  Otherwise, *foundPtr is set to 0 and the buffer is marked
  * as IO_IN_PROGRESS; ReadBuffer will now need to do I/O to fill it.
  *
- * *foundPtr is actually redundant with the buffer's BM_VALID flag, but
- * we keep it for simplicity in ReadBuffer.
- *
- * No locks are held either at entry or exit.
+ * No locks are held either at entry or exit EXCEPT for case noted above
+ * of passing an empty buffer back to async io caller ( index_for_aio set ).
  */
 static volatile BufferDesc *
 BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 			BlockNumber blockNum,
 			BufferAccessStrategy strategy,
-			bool *foundPtr)
+			int *foundPtr , int index_for_aio )
 {
 	BufferTag	newTag;			/* identity of requested block */
 	uint32		newHash;		/* hash value for newTag */
@@ -546,6 +739,13 @@ BufferAlloc(SMgrRelation smgr, char relp
 	int			buf_id;
 	volatile BufferDesc *buf;
 	bool		valid;
+        int             IntentionBufferrc;      /* retcode from BufCheckAsync */
+        bool            StartBufferIOrc;        /* retcode from StartBufferIO */
+        ReadBufferMode mode;
+
+
+        mode = *foundPtr;
+        *foundPtr = 0;
 
 	/* create a tag so we can lookup the buffer */
 	INIT_BUFFERTAG(newTag, smgr->smgr_rnode.node, forkNum, blockNum);
@@ -560,21 +760,53 @@ BufferAlloc(SMgrRelation smgr, char relp
 	if (buf_id >= 0)
 	{
 		/*
-		 * Found it.  Now, pin the buffer so no one can steal it from the
-		 * buffer pool, and check to see if the correct data has been loaded
-		 * into the buffer.
+		 * Found it.
 		 */
+                *foundPtr = 1;
 		buf = &BufferDescriptors[buf_id];
 
-		valid = PinBuffer(buf, strategy);
-
-		/* Can release the mapping lock as soon as we've pinned it */
+                /*   If prefetch mode,  then return immediately indicating found,
+                **   and NOTE in this case only,  we did not pin buffer.
+                **   In theory we might try to check whether the buffer is valid,  io in progress,  etc
+                **   but in practice it is simpler to abandon the prefetch if the buffer exists
+                */
+                if (mode == RBM_NOREAD_FOR_PREFETCH) {
+                    /* release the mapping lock and return    */
 		LWLockRelease(newPartitionLock);
+                } else {
+                    /*   note that the current request is for same tag as the one associated with the aio -
+                    **   so simply complete the aio and we have our buffer.
+                    **         If an aio was started on this buffer,
+                    **         check complete and wait for it if not.
+                    **         And,  if aio had been started,  then the task
+                    **         which issued the start aio already pinned it for this read,
+                    **         so if that task was me and the aio was successful,
+                    **         pass the current pin to this read without dropping and re-acquiring.
+                    **         this is all done by BufCheckAsync
+                    */
+                    IntentionBufferrc = BufCheckAsync(smgr , 0 , buf, BUF_INTENTION_WANT , strategy , index_for_aio , false , newPartitionLock );
 
-		*foundPtr = TRUE;
+                    /*       check to see if the correct data has been loaded into the buffer.  */
+                    valid = (IntentionBufferrc == BUF_INTENT_RC_VALID);
 
-		if (!valid)
-		{
+                    /*  check for serious IO errors   */
+                    if (!valid) {
+                        if (    (IntentionBufferrc != BUF_INTENT_RC_INVALID_NO_AIO)
+                             && (IntentionBufferrc != BUF_INTENT_RC_INVALID_AIO)
+                           ) {
+                            *foundPtr = -1;  /*  inform caller of serious error */
+                        }
+                        else
+                        if (IntentionBufferrc == BUF_INTENT_RC_INVALID_AIO) {
+                            goto proceed_with_not_found;  /*  yes,  I know,  a goto ... think of it as a break out of the if */
+                        }
+                     }
+
+                    /* BufCheckAsync pinned the buffer            */
+                    /* so can now release the mapping lock               */
+                    LWLockRelease(newPartitionLock);
+
+                    if (!valid) {
 			/*
 			 * We can only get here if (a) someone else is still reading in
 			 * the page, or (b) a previous read attempt failed.  We have to
@@ -582,19 +814,21 @@ BufferAlloc(SMgrRelation smgr, char relp
 			 * own read attempt if the page is still not BM_VALID.
 			 * StartBufferIO does it all.
 			 */
-			if (StartBufferIO(buf, true))
+                                if (StartBufferIO(buf, true, index_for_aio))
 			{
 				/*
 				 * If we get here, previous attempts to read the buffer must
 				 * have failed ... but we shall bravely try again.
 				 */
-				*foundPtr = FALSE;
+                                        *foundPtr = 0;
+                                }
 			}
 		}
 
 		return buf;
 	}
 
+  proceed_with_not_found:
 	/*
 	 * Didn't find it in the buffer pool.  We'll have to initialize a new
 	 * buffer.  Remember to unlock the mapping lock while doing the work.
@@ -619,8 +853,10 @@ BufferAlloc(SMgrRelation smgr, char relp
 		/* Must copy buffer flags while we still hold the spinlock */
 		oldFlags = buf->flags;
 
-		/* Pin the buffer and then release the buffer spinlock */
-		PinBuffer_Locked(buf);
+                /*         If an aio was started on this buffer,
+                **         check complete and cancel it if not.
+                */
+                BufCheckAsync(smgr , 0 , buf, BUF_INTENTION_REJECT_OBTAIN_PIN , 0 , index_for_aio, true , 0 );
 
 		/* Now it's safe to release the freelist lock */
 		if (lock_held)
@@ -791,13 +1027,18 @@ BufferAlloc(SMgrRelation smgr, char relp
 				 * then set up our own read attempt if the page is still not
 				 * BM_VALID.  StartBufferIO does it all.
 				 */
-				if (StartBufferIO(buf, true))
+                                StartBufferIOrc = StartBufferIO(buf, true , index_for_aio); /* retcode from StartBufferIO */
+				if (StartBufferIOrc)
 				{
 					/*
 					 * If we get here, previous attempts to read the buffer
 					 * must have failed ... but we shall bravely try again.
 					 */
-					*foundPtr = FALSE;
+					*foundPtr = 0;
+                                } else
+                                if (mode == RBM_NOREAD_FOR_PREFETCH) {
+					UnpinBuffer(buf, true); /* must unpin if RBM_NOREAD_FOR_PREFETCH and found */
+                                        *foundPtr = 2;  /*  inform BufStartAsync that buffer must not be used */
 				}
 			}
 
@@ -860,10 +1101,17 @@ BufferAlloc(SMgrRelation smgr, char relp
 	 * lock.  If StartBufferIO returns false, then someone else managed to
 	 * read it before we did, so there's nothing left for BufferAlloc() to do.
 	 */
-	if (StartBufferIO(buf, true))
-		*foundPtr = FALSE;
-	else
-		*foundPtr = TRUE;
+        StartBufferIOrc = StartBufferIO(buf, true , index_for_aio); /* retcode from StartBufferIO */
+        if (StartBufferIOrc) {
+		*foundPtr = 0;
+        } else {
+                if (mode == RBM_NOREAD_FOR_PREFETCH) {
+                    UnpinBuffer(buf, true); /* must unpin if RBM_NOREAD_FOR_PREFETCH and found */
+                    *foundPtr = 2;  /*  inform BufStartAsync that buffer must not be used */
+                } else {
+                    *foundPtr = 1;
+                }
+        }
 
 	return buf;
 }
@@ -970,6 +1218,10 @@ retry:
 	/*
 	 * Insert the buffer at the head of the list of free buffers.
 	 */
+        /*   avoid confusing freelist with strange-looking freeNext */
+        if (buf->freeNext <= FREENEXT_BAIOCB_ORIGIN) { /* means was used for aiocb index */
+            buf->freeNext = FREENEXT_NOT_IN_LIST;
+        }
 	StrategyFreeBuffer(buf);
 }
 
@@ -1022,6 +1274,56 @@ MarkBufferDirty(Buffer buffer)
 	UnlockBufHdr(bufHdr);
 }
 
+/*  return the blocknum of the block in a buffer if it is valid
+**  if a shared buffer,  it must be pinned
+*/
+BlockNumber
+BlocknumOfBuffer(Buffer buffer)
+{
+	volatile BufferDesc *bufHdr;
+        BlockNumber rc = 0;
+
+	if (BufferIsValid(buffer)) {
+            if  (BufferIsLocal(buffer)) {
+                bufHdr = &LocalBufferDescriptors[-buffer - 1];
+            } else {
+                bufHdr = &BufferDescriptors[buffer - 1];
+            }
+
+            rc = bufHdr->tag.blockNum;
+        }
+
+        return rc;
+}
+
+/*  report whether specified buffer contains same or different block
+**  if a shared buffer,  it must be pinned
+*/
+bool
+BlocknotinBuffer(Buffer buffer,
+					 Relation relation,
+					 BlockNumber blockNum)
+{
+	volatile BufferDesc *bufHdr;
+        bool rc = false;
+
+	if (BufferIsValid(buffer)) {
+            if  (BufferIsLocal(buffer)) {
+                bufHdr = &LocalBufferDescriptors[-buffer - 1];
+            } else {
+                bufHdr = &BufferDescriptors[buffer - 1];
+            }
+
+            rc =
+                  (   (bufHdr->tag.blockNum != blockNum)
+                   || (!(RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) ))
+                   || (bufHdr->tag.forkNum != MAIN_FORKNUM)
+                  );
+        }
+
+        return rc;
+}
+
 /*
  * ReleaseAndReadBuffer -- combine ReleaseBuffer() and ReadBuffer()
  *
@@ -1040,18 +1342,18 @@ ReleaseAndReadBuffer(Buffer buffer,
 					 Relation relation,
 					 BlockNumber blockNum)
 {
-	ForkNumber	forkNum = MAIN_FORKNUM;
 	volatile BufferDesc *bufHdr;
+        bool isDifferentBlock;   /*       requesting different block from that already in buffer ? */
 
 	if (BufferIsValid(buffer))
 	{
+	    /* if a shared buff, we have pin, so it's ok to examine tag without spinlock */
+            isDifferentBlock = BlocknotinBuffer(buffer,relation,blockNum); /*  requesting different block from that already in buffer ? */
 		if (BufferIsLocal(buffer))
 		{
 			Assert(LocalRefCount[-buffer - 1] > 0);
 			bufHdr = &LocalBufferDescriptors[-buffer - 1];
-			if (bufHdr->tag.blockNum == blockNum &&
-				RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
-				bufHdr->tag.forkNum == forkNum)
+			if (!isDifferentBlock)
 				return buffer;
 			ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
 			LocalRefCount[-buffer - 1]--;
@@ -1060,12 +1362,12 @@ ReleaseAndReadBuffer(Buffer buffer,
 		{
 			Assert(PrivateRefCount[buffer - 1] > 0);
 			bufHdr = &BufferDescriptors[buffer - 1];
-			/* we have pin, so it's ok to examine tag without spinlock */
-			if (bufHdr->tag.blockNum == blockNum &&
-				RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
-				bufHdr->tag.forkNum == forkNum)
+                        BufCheckAsync(0 , relation , bufHdr , ( isDifferentBlock ? BUF_INTENTION_REJECT_FORGET
+                                                                                 : BUF_INTENTION_REJECT_KEEP_PIN )
+                                                            , 0 , 0 , false , 0 ); /* end any IO and maybe unpin */
+			if (!isDifferentBlock) {
 				return buffer;
-			UnpinBuffer(bufHdr, true);
+                        }
 		}
 	}
 
@@ -1090,11 +1392,12 @@ ReleaseAndReadBuffer(Buffer buffer,
  * Returns TRUE if buffer is BM_VALID, else FALSE.  This provision allows
  * some callers to avoid an extra spinlock cycle.
  */
-static bool
+bool
 PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy)
 {
 	int			b = buf->buf_id;
 	bool		result;
+        bool       pin_already_banked_by_me = 0;  /* buffer is already pinned by me and redeemable */
 
 	if (PrivateRefCount[b] == 0)
 	{
@@ -1116,12 +1419,34 @@ PinBuffer(volatile BufferDesc *buf, Buff
 	else
 	{
 		/* If we previously pinned the buffer, it must surely be valid */
+                /* Errr  -   is that really true  ???    I don't think so  :
+                ** what if I pin,  try an IO,  in progress,  then mistakenly pin again
 		result = true;
+                */
+		LockBufHdr(buf);
+                pin_already_banked_by_me = (    (PrivateRefCount[b] > 0) && (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                                             && (   (    (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                                                     : (-(buf->freeNext))  )  == this_backend_pid )
+                                           );
+                if (pin_already_banked_by_me) {
+                        elog(LOG, "PinBuffer : on buffer %d with banked pin rel=%s, blockNum=%u, with flags %X refcount=%u"
+                       ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                       ,buf->tag.blockNum, buf->flags, buf->refcount);
+                    buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;
+                    if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                        buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
 	}
+                }
+		result = (buf->flags & BM_VALID) != 0;
+		UnlockBufHdr(buf);
+	}
+
+        if (!pin_already_banked_by_me) {
 	PrivateRefCount[b]++;
 	Assert(PrivateRefCount[b] > 0);
 	ResourceOwnerRememberBuffer(CurrentResourceOwner,
 								BufferDescriptorGetBuffer(buf));
+        }
 	return result;
 }
 
@@ -1138,19 +1463,36 @@ PinBuffer(volatile BufferDesc *buf, Buff
  * to save a spin lock/unlock cycle, because we need to pin a buffer before
  * its state can change under us.
  */
-static void
+void
 PinBuffer_Locked(volatile BufferDesc *buf)
 {
 	int			b = buf->buf_id;
+        bool       pin_already_banked_by_me;  /* buffer is already pinned by me and redeemable */
 
-	if (PrivateRefCount[b] == 0)
+        pin_already_banked_by_me = (    (PrivateRefCount[b] > 0) && (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                                     && (   (    (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                                             : (-(buf->freeNext))  )  == this_backend_pid )
+                             );
+	if (PrivateRefCount[b] == 0) {
 		buf->refcount++;
+        }
+        if (pin_already_banked_by_me) {
+                        elog(LOG, "PinBuffer_Locked : on buffer %d with banked pin rel=%s, blockNum=%u, with flags %X refcount=%u"
+                       ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                       ,buf->tag.blockNum, buf->flags, buf->refcount);
+            buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;
+                    if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                        buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+                    }
+        }
 	UnlockBufHdr(buf);
+        if (!pin_already_banked_by_me) {
 	PrivateRefCount[b]++;
 	Assert(PrivateRefCount[b] > 0);
 	ResourceOwnerRememberBuffer(CurrentResourceOwner,
 								BufferDescriptorGetBuffer(buf));
 }
+}
 
 /*
  * UnpinBuffer -- make buffer available for replacement.
@@ -1160,29 +1502,68 @@ PinBuffer_Locked(volatile BufferDesc *bu
  * Most but not all callers want CurrentResourceOwner to be adjusted.
  * Those that don't should pass fixOwner = FALSE.
  */
-static void
+void
 UnpinBuffer(volatile BufferDesc *buf, bool fixOwner)
 {
+
 	int			b = buf->buf_id;
+        bool       pin_already_banked_by_me;  /* buffer is already pinned by me and redeemable */
 
-	if (fixOwner)
+	if (fixOwner) {
 		ResourceOwnerForgetBuffer(CurrentResourceOwner,
 								  BufferDescriptorGetBuffer(buf));
+        }
 
 	Assert(PrivateRefCount[b] > 0);
 	PrivateRefCount[b]--;
 	if (PrivateRefCount[b] == 0)
 	{
+
 		/* I'd better not still hold any locks on the buffer */
 		Assert(!LWLockHeldByMe(buf->content_lock));
 		Assert(!LWLockHeldByMe(buf->io_in_progress_lock));
 
 		LockBufHdr(buf);
 
+		/* this backend has released last pin - buffer should not have pin banked by me
+                ** and if AIO in progress then there should be another backend pin
+                */
+                pin_already_banked_by_me = (       (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                                             &&    (   (    (buf->flags & BM_AIO_IN_PROGRESS)
+                                                         ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                         : (-(buf->freeNext))
+                                                       )  == this_backend_pid )
+                                           );
+                if (pin_already_banked_by_me) {
+                        /*  this is a strange situation  -  caller had a banked pin (which callers are supposed not to know about)
+                        **                                  but either discovered it had it or has over-counted how many pins it has
+                        */
+                        buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;   /*   redeem the pin although it is now of no use since about to release */
+                        if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                            buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+                        }
+
+                        /*     temporarily suppress logging error to avoid performance degradation -
+                        **     either this task really does not need the buffer in which case the error is harmless
+                        **     or a more severe error will be detected later (possibly immediately below)
+                        elog(LOG, "UnpinBuffer :  released last this-backend pin on buffer %d rel=%s, blockNum=%u, but had banked pin flags %X refcount=%u"
+                       ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                       ,buf->tag.blockNum, buf->flags, buf->refcount);
+                        */
+                }
+
 		/* Decrement the shared reference count */
 		Assert(buf->refcount > 0);
 		buf->refcount--;
 
+                if (   (buf->refcount == 0) && (buf->flags & BM_AIO_IN_PROGRESS)  ) {
+
+                        elog(ERROR, "UnpinBuffer :  released last any-backend pin on buffer %d rel=%s, blockNum=%u, but AIO in progress flags %X refcount=%u"
+                            ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                            ,buf->tag.blockNum, buf->flags, buf->refcount);
+                }
+
+
 		/* Support LockBufferForCleanup() */
 		if ((buf->flags & BM_PIN_COUNT_WAITER) &&
 			buf->refcount == 1)
@@ -1657,6 +2038,7 @@ SyncOneBuffer(int buf_id, bool skip_rece
 	volatile BufferDesc *bufHdr = &BufferDescriptors[buf_id];
 	int			result = 0;
 
+
 	/*
 	 * Check whether buffer needs writing.
 	 *
@@ -1789,6 +2171,8 @@ PrintBufferLeakWarning(Buffer buffer)
 	char	   *path;
 	BackendId	backend;
 
+
+
 	Assert(BufferIsValid(buffer));
 	if (BufferIsLocal(buffer))
 	{
@@ -1799,12 +2183,28 @@ PrintBufferLeakWarning(Buffer buffer)
 	else
 	{
 		buf = &BufferDescriptors[buffer - 1];
+#ifdef USE_PREFETCH
+                /*   If reason that this buffer is pinned
+                **   is that it was prefetched with async_io
+                **   and never read or discarded, then omit the
+                **   warning,  because this is expected in some
+                **   cases when a scan is closed abnormally.
+                **   Note that the buffer will be released soon by our caller.
+                */
+                if (buf->flags & BM_AIO_PREFETCH_PIN_BANKED) {
+                    pgBufferUsage.aio_read_forgot++; /* account for it */
+                    return;
+                }
+#endif /*  USE_PREFETCH */
 		loccount = PrivateRefCount[buffer - 1];
 		backend = InvalidBackendId;
 	}
 
+/* #if defined(DO_ISCAN_DISCARD_BUFFER) && defined(DO_HEAPSCAN_DISCARD_BUFFER) not yet working safely 130530 */
 	/* theoretically we should lock the bufhdr here */
 	path = relpathbackend(buf->tag.rnode, backend, buf->tag.forkNum);
+
+
 	elog(WARNING,
 		 "buffer refcount leak: [%03d] "
 		 "(rel=%s, blockNum=%u, flags=0x%x, refcount=%u %d)",
@@ -1812,6 +2212,7 @@ PrintBufferLeakWarning(Buffer buffer)
 		 buf->tag.blockNum, buf->flags,
 		 buf->refcount, loccount);
 	pfree(path);
+/* #endif defined(DO_ISCAN_DISCARD_BUFFER) && defined(DO_HEAPSCAN_DISCARD_BUFFER) not yet working safely 130530 */
 }
 
 /*
@@ -1928,7 +2329,7 @@ FlushBuffer(volatile BufferDesc *buf, SM
 	 * false, then someone else flushed the buffer before we could, so we need
 	 * not do anything.
 	 */
-	if (!StartBufferIO(buf, false))
+	if (!StartBufferIO(buf, false, 0))
 		return;
 
 	/* Setup error traceback support for ereport() */
@@ -2512,6 +2913,70 @@ FlushDatabaseBuffers(Oid dbid)
 	}
 }
 
+#ifdef USE_PREFETCH
+/*
+ * DiscardBuffer -- discard shared buffer used for a previously
+ *                  prefetched but unread block of a relation
+ *
+ * If the buffer is found and pinned with a banked pin,  then :
+ *      .  if AIO in progress, terminate AIO without waiting
+ *      .  if AIO had already completed successfully,
+ *         then mark buffer valid (in case someone else wants it)
+ *      .  redeem the banked pin and unpin it.
+ *
+ * This function is similar in purpose to ReleaseBuffer (below)
+ * but sufficiently different that it is a separate function.
+ * Two important differences are :
+ *   .   caller identifies buffer by blocknumber,  not buffer number
+ *   .   we unpin buffer *only* if the pin is banked,
+ *                      *never* if pinned but not banked.
+ *       This is essential as caller may perform a sequence of
+ *  SCAN1   . PrefetchBuffer   (and remember block was prefetched)
+ *  SCAN2   . ReadBuffer       (but fails to connect this read to the prefetch by SCAN1)
+ *  SCAN1   . DiscardBuffer    (SCAN1 terminates early)
+ *  SCAN2   . access tuples in buffer
+ *        Clearly the Discard *must not* unpin the buffer since SCAN2 needs it!
+ *
+ *
+ * caller may pass InvalidBlockNumber as blockNum to mean do nothing
+ */
+void
+DiscardBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
+{
+        BufferTag	newTag;		   /* identity of requested block */
+        uint32		newHash;	   /* hash value for newTag */
+        LWLockId	newPartitionLock;  /* buffer partition lock for it */
+        Buffer		buf_id;
+        volatile        BufferDesc *buf_desc;
+
+    if (!SmgrIsTemp(reln->rd_smgr)) {
+	Assert(RelationIsValid(reln));
+	if (BlockNumberIsValid(blockNum)) {
+
+            /* create a tag so we can lookup the buffer */
+            INIT_BUFFERTAG(newTag, reln->rd_smgr->smgr_rnode.node,
+                                       forkNum, blockNum);
+
+            /* determine its hash code and partition lock ID */
+            newHash = BufTableHashCode(&newTag);
+            newPartitionLock = BufMappingPartitionLock(newHash);
+
+            /* see if the block is in the buffer pool already */
+            LWLockAcquire(newPartitionLock, LW_SHARED);
+            buf_id = BufTableLookup(&newTag, newHash);
+            LWLockRelease(newPartitionLock);
+
+            /* If in buffers, proceed */
+            if (buf_id >= 0) {
+                buf_desc = &BufferDescriptors[buf_id];        /* found buffer descriptor */
+                BufCheckAsync(0 , reln, buf_desc , BUF_INTENTION_REJECT_UNBANK , 0 , 0 , false , 0); /* end the IO and unpin if banked */
+                pgBufferUsage.aio_read_discrd++; /* account for it */
+            }
+        }
+    }
+}
+#endif   /* USE_PREFETCH */
+
 /*
  * ReleaseBuffer -- release the pin on a buffer
  */
@@ -2520,26 +2985,23 @@ ReleaseBuffer(Buffer buffer)
 {
 	volatile BufferDesc *bufHdr;
 
+
 	if (!BufferIsValid(buffer))
 		elog(ERROR, "bad buffer ID: %d", buffer);
 
-	ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
 
 	if (BufferIsLocal(buffer))
 	{
+                ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
 		Assert(LocalRefCount[-buffer - 1] > 0);
 		LocalRefCount[-buffer - 1]--;
 		return;
 	}
-
-	bufHdr = &BufferDescriptors[buffer - 1];
-
-	Assert(PrivateRefCount[buffer - 1] > 0);
-
-	if (PrivateRefCount[buffer - 1] > 1)
-		PrivateRefCount[buffer - 1]--;
 	else
-		UnpinBuffer(bufHdr, false);
+        {
+                bufHdr = &BufferDescriptors[buffer - 1];
+                BufCheckAsync(0 , 0 , bufHdr , BUF_INTENTION_REJECT_NOADJUST , 0 , 0 , false , 0 );
+        }
 }
 
 /*
@@ -2565,14 +3027,41 @@ UnlockReleaseBuffer(Buffer buffer)
 void
 IncrBufferRefCount(Buffer buffer)
 {
+        bool       pin_already_banked_by_me = false;  /* buffer is already pinned by me and redeemable */
+        volatile BufferDesc *buf;                     /* descriptor for a shared buffer */
+
 	Assert(BufferIsPinned(buffer));
+
+        if (!(BufferIsLocal(buffer))) {
+                buf = &BufferDescriptors[buffer - 1];
+		LockBufHdr(buf);
+                pin_already_banked_by_me =
+                      (    (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                        && (   (    (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                                      : (-(buf->freeNext))  )  == this_backend_pid )
+                      );
+        }
+
+        if (!pin_already_banked_by_me) {
 	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
 	ResourceOwnerRememberBuffer(CurrentResourceOwner, buffer);
+        }
+
 	if (BufferIsLocal(buffer))
 		LocalRefCount[-buffer - 1]++;
-	else
+	else {
+                if (pin_already_banked_by_me) {
+                    buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;
+                    if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                        buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+                    }
+                }
+		UnlockBufHdr(buf);
+                if (!pin_already_banked_by_me) {
 		PrivateRefCount[buffer - 1]++;
 }
+        }
+}
 
 /*
  * MarkBufferDirtyHint
@@ -2994,61 +3483,138 @@ WaitIO(volatile BufferDesc *buf)
  *
  * In some scenarios there are race conditions in which multiple backends
  * could attempt the same I/O operation concurrently.  If someone else
- * has already started I/O on this buffer then we will block on the
+ * has already started synchronous I/O on this buffer then we will block on the
  * io_in_progress lock until he's done.
  *
+ * if an async io is in progress and we are doing synchronous io,
+ * then readbuffer uses call to smgrcompleteaio to wait,
+ * and so we treat this request as if no io in progress
+ *
  * Input operations are only attempted on buffers that are not BM_VALID,
  * and output operations only on buffers that are BM_VALID and BM_DIRTY,
  * so we can always tell if the work is already done.
  *
+ * index_for_aio is an input parm which,  if non-zero,  identifies a BufferAiocb
+ * acquired by caller and to be attached to the buffer header for use with async io
+ *
  * Returns TRUE if we successfully marked the buffer as I/O busy,
  * FALSE if someone else already did the work.
  */
 static bool
-StartBufferIO(volatile BufferDesc *buf, bool forInput)
+StartBufferIO(volatile BufferDesc *buf, bool forInput , int index_for_aio )
 {
+#ifdef USE_PREFETCH
+        struct BufferAiocb volatile * BAiocb = (struct BufferAiocb*)0;  /*  BufferAiocb for use with aio */
+#endif   /* USE_PREFETCH */
+ 
+        if (!index_for_aio)
 	Assert(!InProgressBuf);
 
 	for (;;)
 	{
+                if (!index_for_aio) {
 		/*
 		 * Grab the io_in_progress lock so that other processes can wait for
 		 * me to finish the I/O.
 		 */
 		LWLockAcquire(buf->io_in_progress_lock, LW_EXCLUSIVE);
+                }
 
 		LockBufHdr(buf);
 
-		if (!(buf->flags & BM_IO_IN_PROGRESS))
+                /*     the following test is intended to distinguish between :
+                **      .   buffer which : 
+                **           .     has io in progress
+                **             AND is not associated with a current aio
+                **      .   not the above
+                **     Here,  "recent" means an aio marked by buf->freeNext <= FREENEXT_BAIOCB_ORIGIN but no longer in progress -
+                **          this situation arises when the aio has just been cancelled and this process now wishes to recycle the buffer.
+                **          In this case,  the first such would-be recycler (i.e. me) must :
+                **             . avoid waiting for the cancelled aio to complete
+                **             . if not myself doing async read, then assume responsibility for posting other future readbuffers.
+                */
+		if (    (buf->flags & BM_AIO_IN_PROGRESS)
+                     || (!(buf->flags & BM_IO_IN_PROGRESS))
+                   )
 			break;
 
 		/*
-		 * The only way BM_IO_IN_PROGRESS could be set when the io_in_progress
+		 * The only way BM_IO_IN_PROGRESS without AIO in progress could be set when the io_in_progress
 		 * lock isn't held is if the process doing the I/O is recovering from
 		 * an error (see AbortBufferIO).  If that's the case, we must wait for
 		 * him to get unwedged.
 		 */
 		UnlockBufHdr(buf);
+                if (!index_for_aio) {
 		LWLockRelease(buf->io_in_progress_lock);
+                }
 		WaitIO(buf);
 	}
 
-	/* Once we get here, there is definitely no I/O active on this buffer */
-
+#ifdef USE_PREFETCH
+	/* Once we get here, there is definitely no synchronous I/O active on this buffer
+        ** but if being asked to attach a BufferAiocb to the buf header,
+        ** then we must also check if there is any async io currently
+        ** in progress or pinned started by a different task.
+        */
+        if (index_for_aio) {
+            BAiocb = (BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext);
+            if (    (buf->flags & (BM_AIO_IN_PROGRESS|BM_AIO_PREFETCH_PIN_BANKED))
+                 && (buf->freeNext <= FREENEXT_BAIOCB_ORIGIN)
+                 && (BAiocb->pidOfAio != this_backend_pid)
+               ) {
+                    /* someone else already doing async I/O */
+                    UnlockBufHdr(buf);
+                    return false;
+            }
+	}
+#endif   /* USE_PREFETCH */
 	if (forInput ? (buf->flags & BM_VALID) : !(buf->flags & BM_DIRTY))
 	{
 		/* someone else already did the I/O */
 		UnlockBufHdr(buf);
+                if (!index_for_aio) {
 		LWLockRelease(buf->io_in_progress_lock);
+                }
 		return false;
 	}
 
 	buf->flags |= BM_IO_IN_PROGRESS;
 
+#ifdef USE_PREFETCH
+        if (index_for_aio) {
+            BAiocb = (BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - index_for_aio);
+            /*   insist that no other buffer is using this BufferAiocb for async IO */
+            if (BAiocb->BAiocbbufh == (struct sbufdesc *)0) {
+                BAiocb->BAiocbbufh = buf;
+            }
+            if (BAiocb->BAiocbbufh != buf) {
+                               ereport(ERROR,
+                                    (errcode(ERRCODE_INTERNAL_ERROR),
+                                     errmsg("AIO control block %p to be used by %p already in use by %p"
+                                              ,BAiocb ,buf , BAiocb->BAiocbbufh)));
+            }
+            /*   note - there is no need to register self as an dependent of BAiocb
+            **   as we shall not unlock buf_desc before we free the BAiocb
+            */
+
+            buf->flags |= BM_AIO_IN_PROGRESS;
+            buf->freeNext = index_for_aio;
+            /*  at this point,  this buffer appears to have an in-progress aio_read,
+            **  and any other task which is able to look inside the buffer might try waiting on that aio -
+            **  except we have not yet issued the aio!   So we must keep the buffer header locked
+            **  from here all the way back to the BufStartAsync caller
+            */
+        } else {
+#endif   /* USE_PREFETCH */
+
 	UnlockBufHdr(buf);
 
 	InProgressBuf = buf;
 	IsForInput = forInput;
+#ifdef USE_PREFETCH
+        }
+#endif   /* USE_PREFETCH */
 
 	return true;
 }
@@ -3058,7 +3624,7 @@ StartBufferIO(volatile BufferDesc *buf,
  *	(Assumptions)
  *	My process is executing IO for the buffer
  *	BM_IO_IN_PROGRESS bit is set for the buffer
- *	We hold the buffer's io_in_progress lock
+ *	if no async IO in progress,  then We hold the buffer's io_in_progress lock
  *	The buffer is Pinned
  *
  * If clear_dirty is TRUE and BM_JUST_DIRTIED is not set, we clear the
@@ -3070,26 +3636,32 @@ StartBufferIO(volatile BufferDesc *buf,
  * BM_IO_ERROR in a failure case.  For successful completion it could
  * be 0, or BM_VALID if we just finished reading in the page.
  */
-static void
+void
 TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
 				  int set_flag_bits)
 {
-	Assert(buf == InProgressBuf);
+        int flags_on_entry;
 
 	LockBufHdr(buf);
 
+        flags_on_entry = buf->flags;
+
+        if (! (flags_on_entry & BM_AIO_IN_PROGRESS) )
+            Assert( buf == InProgressBuf );
+
 	Assert(buf->flags & BM_IO_IN_PROGRESS);
-	buf->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR);
+	buf->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR | BM_AIO_IN_PROGRESS);
 	if (clear_dirty && !(buf->flags & BM_JUST_DIRTIED))
 		buf->flags &= ~(BM_DIRTY | BM_CHECKPOINT_NEEDED);
 	buf->flags |= set_flag_bits;
 
 	UnlockBufHdr(buf);
 
+        if (! (flags_on_entry & BM_AIO_IN_PROGRESS) ) {
 	InProgressBuf = NULL;
-
 	LWLockRelease(buf->io_in_progress_lock);
 }
+}
 
 /*
  * AbortBufferIO: Clean up any active buffer I/O after an error.
--- src/backend/storage/buffer/buf_async.c.orig	2014-05-28 08:50:32.446571884 -0400
+++ src/backend/storage/buffer/buf_async.c	2014-05-28 16:45:43.014507687 -0400
@@ -0,0 +1,920 @@
+/*-------------------------------------------------------------------------
+ *
+ * buf_async.c
+ *	  buffer manager asynchronous disk read routines
+ *
+ * Portions Copyright (c) 1996-2012, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/storage/buffer/buf_async.c
+ *
+ *-------------------------------------------------------------------------
+ */
+/*
+ * Principal entry points:
+ *
+ * BufStartAsync() -- start an asynchronous read of a block into a buffer and
+ *	 pin it so that no one can destroy it while this process is using it.
+ *
+ * BufCheckAsync() -- check completion of an asynchronous read
+ *       and either claim buffer or discard it
+ *
+ * Private helper
+ *
+ * BufReleaseAsync() -- release the BAiocb resources used for an asynchronous read
+ *
+ * See also these files:
+ *		bufmgr.c -- main buffer manager functions
+ *		buf_init.c -- initialisation of resources
+ */
+#include "postgres.h"
+#include <sys/types.h> 
+#include <sys/file.h>
+#include <unistd.h>
+#include <sched.h>
+
+#include "catalog/catalog.h"
+#include "common/relpath.h"
+#include "executor/instrument.h"
+#include "miscadmin.h"
+#include "pg_trace.h"
+#include "pgstat.h"
+#include "postmaster/bgwriter.h"
+#include "storage/buf_internals.h"
+#include "storage/bufmgr.h"
+#include "storage/ipc.h"
+#include "storage/proc.h"
+#include "storage/smgr.h"
+#include "storage/standby.h"
+#include "utils/rel.h"
+#include "utils/resowner_private.h"
+
+/*
+ * GUC parameters
+ */
+int      max_async_io_prefetchers; /* Maximum number of backends using asynchronous librt threads to read pages into our buffers */
+
+/* Note: these two macros only work on shared buffers, not local ones! */
+#define BufHdrGetBlock(bufHdr)	((Block) (BufferBlocks + ((Size) (bufHdr)->buf_id) * BLCKSZ))
+#define BufferGetLSN(bufHdr)	(PageGetLSN(BufHdrGetBlock(bufHdr)))
+
+extern volatile struct BAiocbAnchor *BAiocbAnchr;  /*  anchor for all control blocks pertaining to aio  */
+extern int numBufferAiocbs;        /*  total number of  BufferAiocbs in pool  */
+extern int maxGetBAiocbTries;      /*   max times we will try to get a free BufferAiocb */
+extern int maxRelBAiocbTries;      /*   max times we will try to release a BufferAiocb back to freelist */
+extern pid_t this_backend_pid;     /*   pid of this backend */
+
+extern bool PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy);
+extern void PinBuffer_Locked(volatile BufferDesc *buf);
+extern Buffer ReadBuffer_common(SMgrRelation reln, char relpersistence,
+				  ForkNumber forkNum, BlockNumber blockNum,
+				  ReadBufferMode mode, BufferAccessStrategy strategy,
+				  bool *hit , int index_for_aio);
+extern void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner);
+extern void TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
+				  int set_flag_bits);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+int BufStartAsync(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy);
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+int BufCheckAsync(SMgrRelation caller_smgr, Relation caller_reln, volatile BufferDesc *buf_desc
+  ,int intention , BufferAccessStrategy strategy , int index_for_aio , bool spinLockHeld , LWLockId PartitionLock );
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+
+static struct BufferAiocb volatile * cachedBAiocb = (struct BufferAiocb*)0;  /*  one cached BufferAiocb for use with aio */
+
+#ifdef USE_PREFETCH
+/*  BufReleaseAsync releases a BufferAiocb and returns 0 if successful else non-zero
+**  it *must* be called :
+**     EITHER with a valid  BAiocb->BAiocbbufh -> buf_desc
+**            and that buf_desc must be spin-locked
+**     OR     with BAiocb->BAiocbbufh == 0
+*/
+static int
+BufReleaseAsync(struct BufferAiocb volatile * BAiocb)
+{
+        int    LockTries;         /*  max times we will try to release the BufferAiocb */
+        volatile struct BufferAiocb *BufferAiocbs;
+        struct BufferAiocb volatile * oldFreeBAiocb;  /*  old value of FreeBAiocbs */
+
+        int failed = 1; /* by end of this function, non-zero  will indicate if we failed to return the BAiocb */
+
+
+        if (    ( BAiocb == (struct BufferAiocb*)0 )
+             || ( BAiocb == (struct BufferAiocb*)BAIOCB_OCCUPIED )
+             || ( ((unsigned long)BAiocb) & 0x1 )
+           ) {
+                          elog(ERROR,
+                                 "AIO control block corruption on release of aiocb %p - invalid BAiocb"
+                                          ,BAiocb);
+        }
+        else 
+        if (   (0 == BAiocb->BAiocbDependentCount)     /*  no dependents  */
+            && ((struct BufferAiocb*)BAIOCB_OCCUPIED == BAiocb->BAiocbnext)  /*  not already on freelist */
+           ) {
+
+            if ((struct sbufdesc*)0 != BAiocb->BAiocbbufh) { /*  if a buffer was attached */
+                volatile        BufferDesc *buf_desc = BAiocb->BAiocbbufh;
+
+                /*  spinlock held so instead of TerminateBufferIO(buf, false , 0); ... */
+                if (buf_desc->flags & BM_AIO_PREFETCH_PIN_BANKED) { /* if a pid banked the pin */
+                    buf_desc->freeNext = -(BAiocb->pidOfAio);       /* then remember which pid */
+                }
+                else if (buf_desc->freeNext <= FREENEXT_BAIOCB_ORIGIN) {
+                    buf_desc->freeNext = FREENEXT_NOT_IN_LIST;   /* disconnect BufferAiocb from buf_desc */
+                }
+                buf_desc->flags &= ~BM_AIO_IN_PROGRESS;
+            }
+            
+            BAiocb->BAiocbbufh = (struct sbufdesc *)0; /* disconnect buf_desc from BufferAiocb */
+            BAiocb->pidOfAio = 0;                      /*  clean */
+            LockTries = maxRelBAiocbTries;         /*  max times we will try to release the BufferAiocb */
+            do {
+                register long long int dividend , remainder;
+
+                /*      retrieve old value of FreeBAiocbs  */
+                BAiocb->BAiocbnext = oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs;  /*  old value of FreeBAiocbs */
+
+                /*  this is a volatile value unprotected by any lock,  so must validate it;
+                **  safest is to verify that it is identical to one of the BufferAiocbs
+                **  to do so,  verify by direct division that its address offset from first control block 
+                **  is an integral multiple of the control block size
+                **  that lies within the range [ 0 , (numBufferAiocbs-1) ]
+                */
+                BufferAiocbs = BAiocbAnchr->BufferAiocbs;
+                remainder = ((long long int)oldFreeBAiocb - (long long int)BufferAiocbs)
+                            % (long long int)(sizeof(struct BufferAiocb));
+                failed = (int)remainder;
+                if (!failed) {
+                    dividend = ((long long int)oldFreeBAiocb - (long long int)BufferAiocbs)
+                               / (long long int)(sizeof(struct BufferAiocb));
+                     failed = ( (dividend < 0 ) || ( dividend >= numBufferAiocbs) );
+                     if (!failed) {
+                         if (__sync_bool_compare_and_swap (&(BAiocbAnchr->FreeBAiocbs), oldFreeBAiocb, BAiocb)) {
+                            LockTries = 0;   /*  end the do loop  */
+
+                            goto cheering;   /*  cant simply break because then failed would be set incorrectly */
+                         }
+                    }
+                }
+                /*  if we reach here, we have failed and failed is set to -1 */
+
+       cheering: ;
+
+                if ( LockTries > 1 ) {
+                    sched_yield();    /*   yield to another process,  (hopefully a backend) */
+                }
+            } while  (LockTries-- > 0);
+
+            if (failed) {
+#ifdef LOG_RELBAIOCB_DEPLETION
+                          elog(LOG,
+                                "BufReleaseAsync:AIO control block %p unreleased after tries= %d\n"
+                                       ,BAiocb,maxRelBAiocbTries);
+#endif  /* LOG_RELBAIOCB_DEPLETION */
+            }
+
+        }
+              else
+                          elog(LOG,
+                                "BufReleaseAsync:AIO control block %p either has dependents= %d or is already on freelist %p or has no buf_header %p\n"
+                                       ,BAiocb , BAiocb->BAiocbDependentCount , BAiocb->BAiocbnext , BAiocb->BAiocbbufh);
+        return failed;
+}
+
+/*  try using asynchronous aio_read to prefetch into a buffer
+**  return code :
+**        0 if started successfully
+**       -1 if failed for some reason
+**        1+PrivateRefCount if we found desired buffer in buffer pool
+**
+**  There is a harmless race condition here :
+**  two different backends may both arrive here simultaneously
+**  to prefetch the same buffer.    This is not unlikely when a syncscan is in progress.
+**  .  One will acquire the buffer and issue the smgrstartaio
+**  .  Other will find the buffer on return from  ReadBuffer_common with hit = true
+**  Only the first task has a pin on the buffer since ReadBuffer_common knows not to get a pin
+**  on a found buffer in prefetch mode.
+**  Therefore  -   the second task must simply abandon the prefetch if it finds the buffer in the buffer pool.
+**
+**  if we fail to acquire a BAiocb because of concurrent theft from freelist by other backend,
+**  retry up to maxGetBAiocbTries times provided that there actually was at least one BAiocb on the freelist.
+*/
+int
+BufStartAsync(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy) {
+
+        int retcode = -1;
+        struct BufferAiocb volatile * BAiocb = (struct BufferAiocb*)0;  /*  BufferAiocb for use with aio */
+        int  smgrstartaio_rc = -1;           /*  retcode from smgrstartaio */
+        bool do_unpin_buffer = false;        /* unpin must be deferred until after buffer descriptor is unlocked */
+        Buffer		buf_id;
+        bool		hit = false;
+        volatile        BufferDesc *buf_desc = (BufferDesc *)0;
+
+        int    LockTries;         /*  max times we will try to get a free BufferAiocb */
+
+        struct BufferAiocb volatile * oldFreeBAiocb;  /*  old value of FreeBAiocbs */
+        struct BufferAiocb volatile * newFreeBAiocb;  /*  new value of FreeBAiocbs */
+
+
+    /*  return immediately if no async io resources */
+    if (numBufferAiocbs > 0) {
+        buf_id = (Buffer)0;
+
+        if ( (struct BAiocbAnchor *)0 != BAiocbAnchr ) {
+
+            volatile struct BufferAiocb *BufferAiocbs;
+
+            if ((struct BufferAiocb*)0 != cachedBAiocb) {  /* any cached BufferAiocb ? */
+                BAiocb = cachedBAiocb;                     /* yes  use it  */
+                cachedBAiocb = BAiocb->BAiocbnext;
+                BAiocb->BAiocbnext = (struct BufferAiocb*)BAIOCB_OCCUPIED; /* mark as not on freelist */
+                BAiocb->BAiocbDependentCount = 0;  /* no dependent yet */
+                BAiocb->BAiocbbufh = (struct sbufdesc*)0;
+                BAiocb->pidOfAio = 0;
+            } else {
+
+                LockTries = maxGetBAiocbTries;         /*  max times we will try to get a free BufferAiocb */
+                do {
+                    register long long int dividend = -1 , remainder;
+                    /*  check if we have a free BufferAiocb */
+
+                    oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs;  /*  old value of FreeBAiocbs */
+
+                    /*  check if we have a free BufferAiocb */
+
+                    /*  BAiocbAnchr->FreeBAiocbs is a volatile value unprotected by any lock,
+                    **  and use of compare-and-swap to add and remove items from the list has
+                    **  two potential pitfalls,    both relating to the fact that we must
+                    **  access data de-referenced from this pointer before the compare-and-swap.
+                    **  1)  The value we load may be corrupt,  e.g. mixture of bytes from
+                    **      two different values,   so must validate it;
+                    **      safest is to verify that it is identical to one of the BufferAiocbs.
+                    **      to do so,  verify by direct division that its address offset from
+                    **      first control block is an integral multiple of the control block size
+                    **      that lies within the range [ 0 , (numBufferAiocbs-1) ]
+                    **      Thus we completely prevent this pitfall.
+                    **  2)  The content of the item's next pointer may have changed between the
+                    **      time we de-reference it and the time of the compare-and-swap.
+                    **      Thus even though the compare-and-swap succeeds,   we might set the
+                    **      new head of the freelist to an invalid value  (either a free item
+                    **      that is not the first in the free chain  -  resulting only in
+                    **      loss of the orphaned free items, or,  much worse, an in-use item).
+                    **      In practice this is extremely unlikely because of the implied huge delay
+                    **      in this window interval in this (current) process.    Here are two scenarios:
+                    **      legend:
+                    **         P0  -  this (current) process,  P1,  P2 , ... other processes
+                    **         content of freelist shown as BAiocbAnchr->FreeBAiocbs -> first item -> 2nd item ...
+                    **         @[X] means address of X
+                    **         |      timeline of window of exposure to problems
+                    **      successive lines in chronological order                                       content of freelist
+                    **        2.1    P0           access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I0]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **               P0          access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I1]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1        access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I0]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1       access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I1]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1        swap-remove I0,  place I1 at head of list                F -> I1 -> I2 -> I3 ...
+                    **         |           P2     access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I1]  F -> I1 -> I2 -> I3 ...
+                    **         |           P2    access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I2]  F -> I1 -> I2 -> I3 ...
+                    **         |           P2     swap-remove I1,  place I2 at head of list                F -> I2 -> I3 ...
+                    **         |        P1       complete aio,  replace I0 at head of list                 F -> I0 -> I2 -> I3 ...
+                    **               P0        swap-remove I0,  place I1 at head of list                   F -> I1 IS IN USE !! CORRUPT !!
+                    **               compare-and-swap succeeded but the value of newFreeBAiocb was stale
+                    **               and had become in-use during the window.
+                    **        2.2    P0           access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I0]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **               P0          access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I1]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1        access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I0]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1       access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I1]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1        swap-remove I0,  place I1 at head of list                F -> I1 -> I2 -> I3 ...
+                    **         |           P2     access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I1]  F -> I1 -> I2 -> I3 ...
+                    **         |           P2    access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I2]  F -> I1 -> I2 -> I3 ...
+                    **         |           P2     swap-remove I1,  place I2 at head of list                F -> I2 -> I3 ...
+                    **         |              P3  access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I2]  F -> I2 -> I3 ...
+                    **         |              P3 access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I3]  F -> I2 -> I3 ...
+                    **         |              P3  swap-remove I2,  place I3 at head of list                F -> I3 ...
+                    **         |           P2    complete aio,  replace I1 at head of list                 F -> I1 -> I3 ...
+                    **         |              P3 complete aio,  replace I2 at head of list                 F -> I2 -> I1 -> I3 ...
+                    **         |        P1       complete aio,  replace I0 at head of list                 F -> I0 -> I2 -> I1 -> I3 ...
+                    **               P0        swap-remove I0,  place I1 at head of list                   F -> I1 -> I3 ... ! I2 is orphaned !
+                    **               compare-and-swap succeeded but the value of newFreeBAiocb was stale
+                    **               and had moved further down the free list during the window.
+                    **      Unfortunately, we cannot prevent this pitfall but we can detect it (after the fact),
+                    **      by checking that the next pointer of the item we have just removed for our use still points to the same item.
+                    **      This test is not subject to any timing or uncertainty since :
+                    **       .  The fact that the compare-and-swap succeeded implies that the item we removed
+                    **          was defintely on the freelist (at the head) when it was removed,
+                    **          and therefore cannot be in use,  and therefore its next pointer is no longer volatile.
+                    **       .  Although pointers of the anchor and items on the freelist are volatile,
+                    **          the addresses of items never change -  they are in an allocated array and never move.
+                    **      E.g. in the above two scenarios,   the test is that I0.next still -> I1,
+                    **      and this is true if and only if the second item on the freelist is
+                    **      still the same at the end of the window as it was at the start of the window.
+                    **      Note that we do not insist that it did not change during the window,
+                    **           only that it is still the correct new head of freelist.
+                    **      If this test fails,  we abort immediately as the subsystem is damaged and cannot be repaired.
+                    **      Note that at least one aio must have been issued *and* completed during the window
+                    **           for this to occur,  and since the window is just one single machine instruction,
+                    **           it is very unlikely in practice.
+                    */
+                    BufferAiocbs = BAiocbAnchr->BufferAiocbs;
+                    remainder = ((long long int)oldFreeBAiocb - (long long int)BufferAiocbs)
+                                % (long long int)(sizeof(struct BufferAiocb));
+                    if (remainder == 0) {
+                        dividend = ((long long int)oldFreeBAiocb - (long long int)BufferAiocbs)
+                                    / (long long int)(sizeof(struct BufferAiocb));
+                    }
+                    if (    (remainder == 0)
+                         && ( (dividend >= 0 ) && ( dividend < numBufferAiocbs) )
+                       )
+                    {
+                        newFreeBAiocb = oldFreeBAiocb->BAiocbnext; /*  tentative new value is second on free list */
+                        /*   Here we are in the exposure window referred to in the above comments,
+                        **   so moving along rapidly ...
+                        */
+                        if (__sync_bool_compare_and_swap (&(BAiocbAnchr->FreeBAiocbs), oldFreeBAiocb, newFreeBAiocb)) {   /*  did we get it ? */
+                                /*   We have successfully swapped head of freelist pointed to by oldFreeBAiocb off the list;
+                                **   Here we check that the item we just placed at head of freelist, pointed to by newFreeBAiocb,
+                                **   is the right one
+                                **
+                                **   also check that the BAiocb we have acquired was not in use
+                                **   i.e. that scenario 2.1 above did not occur just before our compare-and-swap
+                                **   The test is that the BAiocb is not in use.
+                                **
+                                **  in one hypothetical case,
+                                **  we can be certain that there is no corruption -
+                                **  the case where newFreeBAiocb == 0 and oldFreeBAiocb->BAiocbnext != BAIOCB_OCCUPIED -
+                                **  i.e. we have set the freelist to empty but we have a baiocb chained from ours.
+                                **  in this case our comp_swap removed all BAiocbs from the list (including ours)
+                                **  so the others chained from ours are either orphaned (no harm done)
+                                **  or in use by another backend and will eventually be returned (fine).
+                                */
+                                if ((struct BufferAiocb *)0 == newFreeBAiocb) {
+                                    if ((struct BufferAiocb *)BAIOCB_OCCUPIED == oldFreeBAiocb->BAiocbnext) {
+                                        goto baiocb_corruption;
+                                    } else if ((struct BufferAiocb *)0 != oldFreeBAiocb->BAiocbnext) {
+                                      elog(LOG,
+                                         "AIO control block inconsistency on acquiring aiocb %p - its next free %p may be orphaned (no corruption has occurred)"
+                                         	,oldFreeBAiocb , oldFreeBAiocb->BAiocbnext);
+                                    }
+                                } else {
+                                    /*  case of newFreeBAiocb not null  -  so must check more carefully ... */
+                                    remainder = ((long long int)newFreeBAiocb - (long long int)BufferAiocbs)
+                                                % (long long int)(sizeof(struct BufferAiocb));
+                                    dividend = ((long long int)newFreeBAiocb - (long long int)BufferAiocbs)
+                                                / (long long int)(sizeof(struct BufferAiocb));
+
+                                    if (    (newFreeBAiocb != oldFreeBAiocb->BAiocbnext)
+                                         || (remainder != 0)
+                                         || ( (dividend < 0 ) || ( dividend >= numBufferAiocbs) )
+                                       ) {
+                                        goto baiocb_corruption;
+                                    }
+                                }
+                                BAiocb = oldFreeBAiocb;
+                                BAiocb->BAiocbnext = (struct BufferAiocb*)BAIOCB_OCCUPIED; /* mark as not on freelist */
+                                BAiocb->BAiocbDependentCount = 0;  /* no dependent yet */
+                                BAiocb->BAiocbbufh = (struct sbufdesc*)0;
+                                BAiocb->pidOfAio = 0;
+
+                                LockTries = 0;   /*  end the do loop  */
+
+                        }
+                    }
+
+                    if ( LockTries > 1 ) {
+                        sched_yield();    /*   yield to another process,  (hopefully a backend) */
+                    }
+                } while (     ((struct BufferAiocb*)0 == BAiocb)            /*  did not get a BAiocb    */
+                           && ((struct BufferAiocb*)0 != oldFreeBAiocb)     /*  there was a free BAiocb */
+                           && (LockTries-- > 0)                             /*  told to retry           */
+                        );
+            }
+        }
+
+        if ( BAiocb != (struct BufferAiocb*)0 ) {
+            /*  try an async io    */
+            BAiocb->BAiocbthis.aio_fildes = -1; /* necessary to ensure any thief realizes aio not yet started */
+            BAiocb->pidOfAio = this_backend_pid;
+
+            /*  now try to acquire a buffer :
+            **  note -   ReadBuffer_common returns hit=true if the block is found in the buffer pool,
+            **            in which case there is no need to prefetch.
+            **  otherwise ReadBuffer_common pins returned buffer and calls StartBufferIO
+            **           and StartBufferIO :
+            **      . sets buf_desc->freeNext to negative of ( index of the aiocb in the BufferAiocbs array + 3 )
+            **      . sets  BAiocb->BAiocbbufh -> buf_desc
+            **           and in this case the buffer spinlock is held.
+            **           This is essential as no other task must issue any intention with respect
+            **           to the buffer until we have started the aio_read.
+            **  Also note that ReadBuffer_common handles enlarging the ResourceOwner buffer list as needed
+            **       so I dont need to do that
+            */
+            buf_id = ReadBuffer_common(reln->rd_smgr, reln->rd_rel->relpersistence,
+                                        forkNum, blockNum
+                                       ,RBM_NOREAD_FOR_PREFETCH  /*  tells ReadBuffer not to do any read,  just alloc buf */
+                                       ,strategy , &hit , (FREENEXT_BAIOCB_ORIGIN - (BAiocb - (BAiocbAnchr->BufferAiocbs))));
+            buf_desc = &BufferDescriptors[buf_id-1];    /* find buffer descriptor */
+
+            /*  normally hit will be false as presumably it was not in the pool
+            **  when our caller looked - but it could be there now ...
+            */
+            if (hit) {
+                   /*   see earlier comments  -  we must abandon the prefetch */
+                   retcode = 1 + PrivateRefCount[buf_id];
+                   BAiocb->BAiocbbufh = (struct sbufdesc *)0; /* indicator that we must release the BAiocb before exiting */
+            } else
+            if (  (buf_id > 0) && ((BufferDesc *)0 != buf_desc) && (buf_desc == BAiocb->BAiocbbufh)  ) {
+                   /*   buff descriptor header lock should be held.
+                   **   However,  just to be safe ,   now validate that
+                   **   we are still the owner and no other task already stole it.
+                   */
+
+                   buf_desc->flags &= ~BM_AIO_PREFETCH_PIN_BANKED; /* ensure no banked pin */
+                   /*  there should not be any other pid waiting on this buffer
+                   **  so check both of BM_VALID and BM_PIN_COUNT_WAITER are not set
+                   */
+                   if (    ( !(buf_desc->flags & (BM_VALID|BM_PIN_COUNT_WAITER) ) )
+                        && (buf_desc->flags & BM_AIO_IN_PROGRESS)
+                        && ((FREENEXT_BAIOCB_ORIGIN - (BAiocb - (BAiocbAnchr->BufferAiocbs))) == buf_desc->freeNext) /* it is still mine */
+                        && (-1 == BAiocb->BAiocbthis.aio_fildes)   /* no thief stole it */
+                        && (0 == BAiocb->BAiocbDependentCount)     /* no dependent */
+                     ) {
+                        /*   we have an empty buffer for our use */
+
+                        BAiocb->BAiocbthis.aio_buf = (void *)(BufHdrGetBlock(buf_desc)); /* Location of actual buffer.  */
+
+                        /*   note - there is no need to register self as an dependent of BAiocb
+                        **   as we shall not unlock buf_desc before we free the BAiocb
+                        */
+
+                        /*   smgrstartaio retcode is returned in smgrstartaio_rc -
+                        **   it indicates whether started or not
+                        */
+                        smgrstartaio(reln->rd_smgr, forkNum, blockNum , (char *)&(BAiocb->BAiocbthis) , &smgrstartaio_rc );
+
+                        if (smgrstartaio_rc == 0) {
+                            retcode = 0;
+                            buf_desc->flags |= BM_AIO_PREFETCH_PIN_BANKED; /* bank the pin for next use by this task */
+                            /* we did not register register self as an dependent of BAiocb so no need to unregister  */
+                        } else {
+                            /*  failed - return the BufferAiocb to the free list  */
+                            do_unpin_buffer = true;    /* unpin must be deferred until after buffer descriptor is unlocked */
+                            /*  spinlock held so instead of TerminateBufferIO(buf_desc, false , 0); ... */
+                            buf_desc->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR | BM_AIO_IN_PROGRESS | BM_AIO_PREFETCH_PIN_BANKED | BM_VALID);
+                            /* we did not register register self as an dependent of BAiocb so no need to unregister  */
+
+                            /*  return the BufferAiocb to the free list  */
+                            if (
+                                   BufReleaseAsync(BAiocb)
+                               ) {         /*  failed ? */
+                                BAiocb->BAiocbnext = cachedBAiocb;    /* then ...       */
+                                cachedBAiocb = BAiocb;                /*  ... cache it  */
+                            }
+
+                            pgBufferUsage.aio_read_failed++;
+                            smgrstartaio_rc = 1;  /*   to distinguish from aio not even attempted */
+                        }
+                   }
+                   else {
+                            /*  buffer was stolen or in use by other task - return the BufferAiocb to the free list  */
+                            do_unpin_buffer = true;    /* unpin must be deferred until after buffer descriptor is unlocked */
+                   }
+
+                   UnlockBufHdr(buf_desc);
+                   if (do_unpin_buffer) {
+                        if (smgrstartaio_rc >= 0) { /*  if  aio was attempted */
+                            TerminateBufferIO(buf_desc, false , 0);
+                        }
+                        UnpinBuffer(buf_desc, true);
+                   }
+            }
+            else {
+                BAiocb->BAiocbbufh = (struct sbufdesc *)0; /* indicator that we must release the BAiocb before exiting */
+            }
+
+            if ((struct sbufdesc*)0 == BAiocb->BAiocbbufh) { /*  we did not associate a buffer */
+                                                             /*  so return the BufferAiocb to the free list  */
+                            if (
+                                   BufReleaseAsync(BAiocb)
+                               ) {         /*  failed ? */
+                                BAiocb->BAiocbnext = cachedBAiocb;    /* then ...       */
+                                cachedBAiocb = BAiocb;                /*  ... cache it  */
+                            }
+            }
+        }
+    }
+
+    return retcode;
+
+    baiocb_corruption:;
+         elog(PANIC,
+              "AIO control block corruption on acquiring aiocb %p - its next free %p conflicts with new freelist pointer %p which may be invalid (corruption may have occurred)"
+                                                    ,oldFreeBAiocb , oldFreeBAiocb->BAiocbnext , newFreeBAiocb);
+}
+#endif   /* USE_PREFETCH */
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+/*
+ * BufCheckAsync --      act upon caller's intention regarding a shared buffer,
+ *                       primarily in connection with any async io in progress on the buffer.
+ *     class  subvalue   intention has two main classes and some subvalues within those :
+ *      +ve      1            .   want    <=>  caller wants the buffer,
+ *                                             wait for in-progress aio and then always pin
+ *      -ve                   .   reject  <=>  caller does not want the buffer,
+ *                                             if there are no dependents,  then cancel the aio
+ *              -1, -2 , -3 , ... (see below)        and then optionally unpin
+ *                             Used when there may have been a previous fetch or prefetch.
+ *
+ * buffer is assumed to be an existing member of the shared buffer pool
+ *    as returned by BufTableLookup.
+ * if AIO in progress,  then :
+ *      .  terminate AIO, waiting for completion if +ve intention, else without waiting
+ *      .  if the AIO had already completed successfully,   then mark buffer valid
+ *      .  pin/unpin as requested
+ *
+ * +ve intention indicates that buffer must be pinned :
+ *   if the strategy parameter is null,  then use the PinBuffer_Locked optimization
+ *   to pin and unlock in one operation.   But always update buffer usage count.
+ *
+ * -ve intention indicates whether and how to unpin :
+ *   BUF_INTENTION_REJECT_KEEP_PIN 	-1   pin already held, do not unpin, (caller wants to keep it)
+ *   BUF_INTENTION_REJECT_OBTAIN_PIN	-2   obtain pin,  caller wants it for same buffer
+ *   BUF_INTENTION_REJECT_FORGET	-3   unpin and tell resource owner to forget
+ *   BUF_INTENTION_REJECT_NOADJUST	-4   unpin and call ResourceOwnerForgetBuffer myself
+ *                                           instead of telling UnpinBuffer to adjust CurrentResource owner
+ *                                           (quirky simulation of ReleaseBuffer logic)
+ *   BUF_INTENTION_REJECT_UNBANK   	-5   unpin only if pin banked by caller
+ *   The behaviour for the -ve case is based on that of ReleaseBuffer, adding handling of async io.
+ *
+ * pin/unpin action must take account of whether this backend hold a "disposable" pin on the particular buffer.
+ * A "disposable" pin is a pin acquired by buffer manager without caller knowing, such as :
+ *      when required to safeguard an async AIO  -  pin can be held across multiple bufmgr calls
+ *      when required to safeguard waiting for an async AIO  -  pin acquired and released within this function
+ * if a disposable pin is held,   then :
+ *      if a new pin is requested,  the disposable pin must be retained (redeemed) and any flags relating to it unset
+ *      if an unpin is requested,   then :
+ *              if    either no AIO in progress or this backend did not initiate the AIO
+ *              then  the disposable pin must be dropped (redeemed) and any flags relating to it unset
+ *              else  log warning and do nothing
+ *  i.e. in either case,   there is no longer a disposable pin after this function has completed.
+ *       Note that if    intention is BUF_INTENTION_REJECT_UNBANK,
+ *                 then caller expects there to be a disposable banked pin
+ *                      and if there isn't one,  we do nothing
+ *                 for all other intentions,  if there is no disposable pin,   we pin/unpin normally.
+ *
+ * index_for_aio indicates the BAiocb to be used for next aio (see PrefetchBuffer)
+ * BufFreelistLockHeld indicates whether freelistlock is held
+ * spinLockHeld indicates whether buffer header spinlock is held
+ * PartitionLock is the  buffer partition lock to be used
+ *
+ * return code (meaningful ONLY if intention is +ve) indicates validity of buffer :
+ *  -1    buffer is invalid and failed PageHeaderIsValid check
+ *   0    buffer is not valid
+ *   1    buffer is valid
+ *   2    buffer is valid but tag changed  -  (so content does not match the relation block that caller expects)
+ */
+int
+BufCheckAsync(SMgrRelation caller_smgr, Relation caller_reln, BufferDesc volatile * buf_desc, int intention , BufferAccessStrategy strategy , int index_for_aio , bool spinLockHeld , LWLockId PartitionLock )
+{
+
+        int retcode = BUF_INTENT_RC_INVALID_NO_AIO;
+        bool valid = false;
+
+	BufferTag origTag = buf_desc->tag;	/* original identity of selected buffer */
+
+#ifdef USE_PREFETCH
+        int smgrcompleteaio_rc;          /*  retcode from smgrcompleteaio */
+        SMgrRelation smgr = caller_smgr;
+        int aio_successful = -1;         /*  did the aio_read succeed ?  -1 = no aio,  0 unsuccessful , 1 successful */
+	BufFlags	flags_on_entry;  /*  for debugging  -  can be printed in gdb */
+        int    		freeNext_on_entry;  /*  for debugging  -  can be printed in gdb */
+        int    		BAiocbDependentCount_after_aio_finished = -1;  /*  for debugging  -  can be printed in gdb */
+        bool       disposable_pin = false;            /* this backend had a disposable pin on entry or pins the buffer while waiting for aio_read to complete */
+        bool       pin_already_banked_by_me;          /* buffer is already pinned by me and redeemable */
+        int local_intention;
+#endif   /* USE_PREFETCH */
+
+
+
+#ifdef USE_PREFETCH
+            if (!spinLockHeld) {
+                /*  lock buffer header    */
+                LockBufHdr(buf_desc);
+            }
+
+	    flags_on_entry = buf_desc->flags;
+	    freeNext_on_entry = buf_desc->freeNext;
+            pin_already_banked_by_me =
+                      (    (flags_on_entry & BM_AIO_PREFETCH_PIN_BANKED)
+                        && (   (    (flags_on_entry & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - freeNext_on_entry))->pidOfAio )
+                                                                      : (-(freeNext_on_entry))  )  == this_backend_pid )
+                      );
+
+            if (pin_already_banked_by_me) {
+                if (PrivateRefCount[buf_desc->buf_id] == 0) {  /*  but do we actually have a pin ?? */
+                    /*   this is an anomalous situation   -  somehow our disposable pin was lost without us noticing
+                    **   if AIO is in progress and we started it,
+                    **   then this is disastrous  -   two backends might both issue IO on same buffer
+                    **   otherwise,  it is harmless,  and simply means we have no disposable pin,
+                    **               but we must update flags to "notice" the fact now
+                    */
+                    if (flags_on_entry & BM_AIO_IN_PROGRESS) {
+                            elog(ERROR, "BufCheckAsync : AIO control block issuer of aio_read lost pin with BM_AIO_IN_PROGRESS on buffer %d rel=%s, blockNum=%u, flags 0x%X refcount=%u intention= %d"
+                                ,buf_desc->buf_id,relpathbackend(buf_desc->tag.rnode, InvalidBackendId, buf_desc->tag.forkNum)
+                           ,buf_desc->tag.blockNum, flags_on_entry, buf_desc->refcount ,intention);
+                    } else {
+                            elog(LOG, "BufCheckAsync : AIO control block issuer of aio_read lost pin on buffer %d rel=%s, blockNum=%u, with flags 0x%X refcount=%u intention= %d"
+                               ,buf_desc->buf_id,relpathbackend(buf_desc->tag.rnode, InvalidBackendId, buf_desc->tag.forkNum)
+                               ,buf_desc->tag.blockNum, flags_on_entry, buf_desc->refcount ,intention);
+							buf_desc->flags &= ~BM_AIO_PREFETCH_PIN_BANKED; /*   redeem the banked pin  */
+							/*   since AIO not in progress,  disconnect the buffer from banked pin */
+							buf_desc->freeNext = FREENEXT_NOT_IN_LIST;      /*   and forget the bank client */
+							pin_already_banked_by_me = false;
+                    }
+                } else {
+                    disposable_pin = true;
+                }
+            }
+
+            /*  the case of BUF_INTENTION_REJECT_UNBANK is handled specially :
+            **    if this backend has a banked pin,  then proceed just as for BUF_INTENTION_REJECT_FORGET
+            **    else the call is a no-op  --  unlock buf header and return immediately
+            */
+            local_intention = intention;
+            if (intention == BUF_INTENTION_REJECT_UNBANK) {
+                if (pin_already_banked_by_me) {
+                    local_intention = BUF_INTENTION_REJECT_FORGET;
+                } else {
+                    goto unlock_buf_header;  /*  code following the unlock will do nothing since local_intention still set to BUF_INTENTION_REJECT_UNBANK */
+                }
+            }
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+            /*       we do not expect that BM_AIO_IN_PROGRESS is set without freeNext identifying the BAiocb */
+            if ( (buf_desc->flags & BM_AIO_IN_PROGRESS) && (buf_desc->freeNext == FREENEXT_NOT_IN_LIST) ) {
+
+					elog(ERROR, "BufCheckAsync : found BM_AIO_IN_PROGRESS without a BAiocb on buffer %d rel=%s, blockNum=%u, flags %X refcount=%u"
+						,buf_desc->buf_id,relpathbackend(buf_desc->tag.rnode, InvalidBackendId, buf_desc->tag.forkNum)
+						,buf_desc->tag.blockNum, buf_desc->flags, buf_desc->refcount);
+                }
+            /*     check whether aio in progress  */
+            if  (    ( (struct BAiocbAnchor *)0 != BAiocbAnchr )
+                  && (buf_desc->flags & BM_AIO_IN_PROGRESS)
+                  && (buf_desc->freeNext <= FREENEXT_BAIOCB_ORIGIN)                       /*  has a valid BAiocb */
+                  && ((FREENEXT_BAIOCB_ORIGIN - buf_desc->freeNext) < numBufferAiocbs)    /*  double-check */
+                ) {        /* this is aio   */
+                    struct BufferAiocb volatile * BAiocb = (BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf_desc->freeNext); /*  BufferAiocb associated with this aio */
+                    if ((struct BufferAiocb*)BAIOCB_OCCUPIED == BAiocb->BAiocbnext) { /*  ensure BAiocb is occupied */
+                        aio_successful = 0;       /*  tentatively the aio_read did not succeed   */
+                        retcode = BUF_INTENT_RC_INVALID_AIO;
+
+                        if (smgr == NULL) {
+                            if (caller_reln == NULL) {
+                                smgr = smgropen(buf_desc->tag.rnode, InvalidBackendId);
+                            } else {
+                                smgr = caller_reln->rd_smgr;
+                            }
+                        }
+
+                        /*  assert that this AIO is not using the same BufferAiocb as the one caller asked us to use */
+                        if ((index_for_aio < 0) && (index_for_aio == buf_desc->freeNext)) {
+                                   ereport(ERROR,
+                                        (errcode(ERRCODE_INTERNAL_ERROR),
+                                         errmsg("AIO control block index %d to be used by %p already in use by %p"
+                                                  ,index_for_aio, buf_desc, BAiocb->BAiocbbufh)));
+                        }
+
+                        /*    Call smgrcompleteaio only if either we want buffer or there are no dependents.
+                        **    In the other case of reject and there are dependents,
+                        **    then one of them will do it.
+                        */
+                        if (   (local_intention > 0) || (0 == BAiocb->BAiocbDependentCount)  ) {
+                            if (local_intention > 0) {
+                                /*  wait for in-progress aio and then pin
+                                **  OR  if I did not issue the aio and do not have a pin
+                                **  then pin now before waiting to ensure the buffer does not become unpinned while I wait
+                                **  we may potentially wait for io to complete
+                                **  so release buf header lock so that others may also wait here
+                                */
+                                BAiocb->BAiocbDependentCount++; /* register self as dependent  */
+                                if (PrivateRefCount[buf_desc->buf_id] == 0) {   /* if this buffer not pinned by me */
+                                    disposable_pin = true; /* this backend has pinned the buffer while waiting for aio_read to complete */
+                                    PinBuffer_Locked(buf_desc);
+                                } else {
+                                    UnlockBufHdr(buf_desc);
+                                }
+                                LWLockRelease(PartitionLock);
+
+                                smgrcompleteaio_rc = 1;   /*  tell smgrcompleteaio to wait  */
+                            } else {
+                                smgrcompleteaio_rc = 0;   /*  tell smgrcompleteaio to cancel */
+                            }
+
+                            smgrcompleteaio( smgr , (char *)&(BAiocb->BAiocbthis) , &smgrcompleteaio_rc );
+                            if ( (smgrcompleteaio_rc == 0) || (smgrcompleteaio_rc == 1) ) {
+                                  aio_successful = 1;
+                            }
+
+                            /*   statistics  */
+                            if (local_intention > 0) {
+                                if (smgrcompleteaio_rc == 0) {
+                                    /*    completed successfully and did not have to wait  */
+                                    pgBufferUsage.aio_read_ontime++;
+                                } else if (smgrcompleteaio_rc == 1) {
+                                    /*    completed successfully and did have to wait  */
+                                    pgBufferUsage.aio_read_waited++;
+                                } else {
+                                    /*  bad news   -   read failed and so buffer not usable
+                                    **  the buffer is still pinned so unpin and proceed with "not found" case
+                                    */
+                                    pgBufferUsage.aio_read_failed++;
+                                }
+
+                                /*  regain locks and handle the validity of the buffer and intention regarding it    */
+                                LWLockAcquire(PartitionLock, LW_SHARED);
+                                LockBufHdr(buf_desc);
+                                BAiocb->BAiocbDependentCount--; /* unregister self as dependent  */
+                            } else {
+                                    pgBufferUsage.aio_read_wasted++;  /*   regardless of whether aio_successful */
+                            }
+
+
+                            if (local_intention > 0) {
+                                /*  verify the buffer is still ours and has same identity
+                                **  There is one slightly tricky point here -
+                                **  if there are other dependents,   then each of them will perform this same check
+                                **  this is unavoidable as the correct setting of retcode and the BM_VALID flag
+                                **  is required by each dependent,  so we may not leave it to the last one to do it.
+                                **  It should not do any harm and easier to let them all do it than try to avoid.
+                                */
+                                if ((FREENEXT_BAIOCB_ORIGIN - (BAiocb - (BAiocbAnchr->BufferAiocbs))) == buf_desc->freeNext) { /* it is still mine */
+
+                                    if (aio_successful) {
+                                        /* validate page header.   If valid,  then mark the buffer as valid */
+                                        if (PageIsVerified((Page)(BufHdrGetBlock(buf_desc)) , ((BAiocb->BAiocbthis).aio_offset/BLCKSZ))) {
+                                            buf_desc->flags |= BM_VALID;
+                                            if (BUFFERTAGS_EQUAL(origTag , buf_desc->tag)) {
+                                                retcode = BUF_INTENT_RC_VALID;
+                                            } else {
+                                                retcode = BUF_INTENT_RC_CHANGED_TAG;
+                                            }
+                                        } else {
+                                            retcode = BUF_INTENT_RC_BADPAGE;
+                                        }
+                                    }
+                                }
+                            }
+
+                            BAiocbDependentCount_after_aio_finished = BAiocb->BAiocbDependentCount;
+
+                            /*  if no dependents,   then disconnect the BAiocb and update buffer header */
+                            if (BAiocbDependentCount_after_aio_finished == 0 ) {
+
+
+                                /*  return the BufferAiocb to the free list  */
+                                buf_desc->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR | BM_AIO_IN_PROGRESS);
+                                if (
+                                           BufReleaseAsync(BAiocb)
+                                   ) {        /*  failed ? */
+                                    BAiocb->BAiocbnext = cachedBAiocb;   /* then ...       */
+                                    cachedBAiocb = BAiocb;               /*  ... cache it  */
+                                }
+                            }
+
+                        }
+                    }
+            }
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+            /*  note whether buffer is valid before unlocking spinlock */
+            valid = ((buf_desc->flags & BM_VALID) != 0);
+
+            /*  if there was a disposable pin on entry to this function (i.e. marked in buffer flags)
+            **  then unmark it  -  refer to prologue comments talking about :
+            **    if a disposable pin is held,   then :
+            **     ...
+            **    i.e. in either case,   there is no longer a disposable pin after this function has completed.
+            */
+            if (pin_already_banked_by_me) {
+                        buf_desc->flags &= ~BM_AIO_PREFETCH_PIN_BANKED; /*   redeem the banked pin  */
+                        /*   if AIO not in progress,  then disconnect the buffer from BAiocb and/or banked pin */
+                        if (!(buf_desc->flags & BM_AIO_IN_PROGRESS)) {
+                            buf_desc->freeNext = FREENEXT_NOT_IN_LIST;      /*   and forget the bank client */
+                        }
+                        /********** for debugging   *****************
+                        else elog(LOG, "BufCheckAsync : found BM_AIO_IN_PROGRESS when redeeming banked pin on buffer %d rel=%s, blockNum=%u, flags %X refcount=%u"
+                       ,buf_desc->buf_id,relpathbackend(buf_desc->tag.rnode, InvalidBackendId, buf_desc->tag.forkNum)
+                       ,buf_desc->tag.blockNum, buf_desc->flags, buf_desc->refcount);
+                        ********** for debugging     *****************/
+            }
+
+            /*  If we are to obtain new pin, then use pin optimization  -  pin and unlock.
+            **  However,   if the caller is the same backend who issued the aio_read,
+            **  then he ought to have obtained the pin at that time and must not acquire
+            **  a "second" one since this is logically the same read -  he would have obtained
+            **  a single pin if using synchronous read and we emulate that behaviour.
+            **  Its important to understand that the caller is not aware that he already obtained a pin -
+            **  because calling PrefetchBuffer did not imply a pin -
+            **  so we must track that via the pidOfAio field in the BAiocb.
+            **  And to add one further complication :
+            **      we assume that although PrefetchBuffer pinned the buffer,
+            **      it did not increment the usage count.
+            **      (because it called PinBuffer_Locked which does not do that)
+            **      so in this case,   we must increment the usage count without double-pinning.
+            **      yes its ugly  -  and theres a goto!
+            */
+            if (   (local_intention > 0)
+                || (local_intention == BUF_INTENTION_REJECT_OBTAIN_PIN)
+               ) {
+
+                /* Make sure we will have room to remember the buffer pin */
+                ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
+
+                /*    here we really want a version of PinBuffer_Locked which updates usage count ... */
+                if (   (PrivateRefCount[buf_desc->buf_id] == 0) /*   if this buffer not previously pinned by me */
+                    || pin_already_banked_by_me                 /*   or I had a disposable pin on entry */
+                   ) {
+                    if (strategy == NULL)
+                    {
+                            if (buf_desc->usage_count < BM_MAX_USAGE_COUNT)
+                                    buf_desc->usage_count++;
+                    }
+                    else
+                    {
+                            if (buf_desc->usage_count == 0)
+                                    buf_desc->usage_count = 1;
+                    }
+		}
+
+                /*  now pin buffer unless we have a disposable */
+                if (!disposable_pin) { /* this backend neither banked pin for aio nor pinned the buffer while waiting for aio_read to complete */
+                    PinBuffer_Locked(buf_desc);
+                    goto unlocked_it;
+                }
+                else
+                /*    if this task previously issued the aio or pinned the buffer while waiting for aio_read to complete
+                **       and aio was unsuccessful,    then release the pin
+                */
+                if (     disposable_pin
+                      && (aio_successful == 0)       /*  aio_read failed ? */
+                   ) {
+                           UnpinBuffer(buf_desc, true);
+                }
+            }
+
+    unlock_buf_header:
+            UnlockBufHdr(buf_desc);
+    unlocked_it:
+#endif   /* USE_PREFETCH */
+
+            /*   now do any requested pin (if not done immediately above) or unpin/forget  */
+            if (local_intention == BUF_INTENTION_REJECT_KEEP_PIN) {
+            /*   the caller is supposed to hold a pin already so there should be nothing to do ... */
+                if (PrivateRefCount[buf_desc->buf_id] == 0) {
+                    elog(LOG, "request to keep pin on unpinned buffer %d",buf_desc->buf_id);
+
+                    valid = PinBuffer(buf_desc, strategy);
+                }
+            }
+            else
+            if (   (   (local_intention == BUF_INTENTION_REJECT_FORGET)
+                    || (local_intention == BUF_INTENTION_REJECT_NOADJUST)
+                   )
+                && (PrivateRefCount[buf_desc->buf_id] > 0) /*   if this buffer was previously pinned by me ... */
+               )  {
+
+                    if (local_intention == BUF_INTENTION_REJECT_FORGET) {
+                        UnpinBuffer(buf_desc, true); /*  ... then release the pin                   */
+                    } else
+                    if (local_intention == BUF_INTENTION_REJECT_NOADJUST) {
+                        /*   following code moved from ReleaseBuffer :
+                        **   not sure why we can't simply UnpinBuffer(buf_desc, true)
+                        **   but better leave it the way it was
+                        */
+                        ResourceOwnerForgetBuffer(CurrentResourceOwner, BufferDescriptorGetBuffer(buf_desc));
+                        if (PrivateRefCount[buf_desc->buf_id] > 1) {
+                            PrivateRefCount[buf_desc->buf_id]--;
+                        } else {
+                            UnpinBuffer(buf_desc, false);
+                        }
+                    }
+            }
+
+            /*    if retcode has not been set to one of the unusual conditions
+            **        namely failed header validity or tag changed
+            **    then the setting of valid takes precedence
+            **    over whatever retcode may be currently set to.
+            */
+            if ( ( (retcode == BUF_INTENT_RC_INVALID_NO_AIO) || (retcode == BUF_INTENT_RC_INVALID_AIO) ) && valid) {
+                   retcode = BUF_INTENT_RC_VALID;
+            } else
+            if ((retcode == BUF_INTENT_RC_VALID) && (!valid)) {
+                   if (aio_successful == -1) { /* aio not attempted */
+                       retcode = BUF_INTENT_RC_INVALID_NO_AIO;
+                   } else {
+                       retcode = BUF_INTENT_RC_INVALID_AIO;
+                   }
+            }
+
+            return retcode;
+}
--- src/backend/storage/buffer/buf_init.c.orig	2014-05-28 08:29:09.330829297 -0400
+++ src/backend/storage/buffer/buf_init.c	2014-05-28 16:45:43.038507784 -0400
@@ -13,15 +13,89 @@
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
+#include <sys/types.h> 
+#include <unistd.h>
 
 #include "storage/bufmgr.h"
 #include "storage/buf_internals.h"
-
+#include <stdlib.h> /* for getenv() */
+#include <errno.h> /* for strtoul() */
 
 BufferDesc *BufferDescriptors;
 char	   *BufferBlocks;
-int32	   *PrivateRefCount;
+int32	   *PrivateRefCount;       /*  array of counts per buffer of how many times this task has pinned this buffer */
+
+volatile struct BAiocbAnchor *BAiocbAnchr = (struct BAiocbAnchor *)0;  /*  anchor for all control blocks pertaining to aio  */
 
+int CountInuseBAiocbs(void);     /*  keep compiler happy */
+void ReportFreeBAiocbs(void);    /*  keep compiler happy */
+
+extern int	MaxConnections;  /*  max number of client connections which postmaster will allow  */
+int numBufferAiocbs = 0;         /*  total number of  BufferAiocbs in pool (0 <=> no async io) */
+int hwmBufferAiocbs = 0;         /*  high water mark of in-use  BufferAiocbs in pool
+                                 **  (not required to be accurate, kindly maintained for us somehow by postmaster)
+                                 */
+
+#ifdef USE_PREFETCH
+unsigned int prefetch_dbOid = 0; /*  database oid of relations on which prefetching to be done - 0 means all */
+unsigned int prefetch_bitmap_scans = 1; /*  boolean whether to prefetch bitmap heap scans        */
+unsigned int prefetch_heap_scans = 0;   /*  boolean whether to prefetch non-bitmap heap scans    */
+unsigned int prefetch_sequential_index_scans = 0;  /*  boolean whether to prefetch sequential-access non-bitmap index scans  */
+unsigned int prefetch_index_scans = 256;  /*  boolean whether to prefetch non-bitmap index scans also numeric size of pfch_list  */
+unsigned int prefetch_btree_heaps = 1;  /*  boolean whether to prefetch heap pages in _bt_next for non-bitmap index scans   */
+#endif /* USE_PREFETCH */
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+int maxGetBAiocbTries = 1;       /*  max times we will try to get a free BufferAiocb */
+int maxRelBAiocbTries = 1;       /*  max times we will try to release a BufferAiocb back to freelist */
+
+/*  locking protocol for manipulating the BufferAiocbs and FreeBAiocbs list :
+**    1.    ownership of a BufferAiocb :
+**          to gain ownership of a BufferAiocb, a task must
+**          EITHER    remove it from FreeBAiocbs (it is now temporary owner and no other task can find it)
+**                    if decision is to attach it to a buffer descriptor header, then
+**                       .   lock the buffer descriptor header
+**                       .   check  NOT flags & BM_AIO_IN_PROGRESS
+**                       .   attach to buffer descriptor header
+**                       .   increment the BufferAiocb.dependent_count
+**                       .   unlock the buffer descriptor header
+**                     and ownership scope is from lock to unlock
+***          OR        locate it by dereferencing the pointer in a buffer descriptor,
+**                    in which case :
+**                       .   lock the buffer descriptor header
+**                       .   check  flags & BM_AIO_IN_PROGRESS
+**                       .   increment the BufferAiocb.dependent_count
+**                       .   if decision is to return to FreeBAiocbs,
+**                           then   (with buffer descriptor header still locked)
+**                                  .  turn off BM_AIO_IN_PROGRESS
+**                       .   IF        the BufferAiocb.dependent_count == 1 (I am sole dependent)
+**                       .   THEN
+**                       .       .  decrement the BufferAiocb.dependent_count
+**                               .  return to FreeBAiocbs (see below)
+**                       .   unlock the buffer descriptor header
+**                     and ownership scope is from lock to either return to FreeBAiocbs or unlock
+**    2.    adding and removing from FreeBAiocbs :
+**      two alternative methods - controlled by conditional macro definition LOCK_BAIOCB_FOR_GET_REL
+**       2.1 LOCK_BAIOCB_FOR_GET_REL is defined - use a lock
+**          .   lock BufFreelistLock exclusive
+**          .   add / remove from FreeBAiocbs
+**          .   unlock BufFreelistLock exclusive
+**          advantage of this method  -  never fails to add or remove
+**       2.2  LOCK_BAIOCB_FOR_GET_REL is not defined - use compare_and_swap
+**          .   retrieve the current Freelist pointer and validate
+**          .   compare_and_swap on/off the FreeBAiocb list
+**          .   unlock BufFreelistLock exclusive
+**          advantage of this method  -  never waits
+**          to avoid losing a FreeBAiocbs,   save it in a process-local cache and reuse
+*/
+#else   /*  not USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+        /*   this dummy structure is to ensure that references to these fields in other bufmgr runtime code
+        **   that is not conditionally ifdefd on USE_AIO_ATOMIC_BUILTIN_COMP_SWAP compiles and runs correctly
+        */
+        struct BufferAiocb dummy_BAiocbAnchr = { (struct BufferAiocb*)0 , (struct BufferAiocb*)0 };
+int maxGetBAiocbTries = -1;       /*  max times we will try to get a free BufferAiocb */
+int maxRelBAiocbTries = -1;       /*  max times we will try to release a BufferAiocb back to freelist */
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * Data Structures:
@@ -73,7 +147,14 @@ void
 InitBufferPool(void)
 {
 	bool		foundBufs,
-				foundDescs;
+				foundDescs
+                        , foundAiocbs
+          ;
+#if defined(USE_PREFETCH) || defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP)
+        char *envvarpointer = (char *)0;  /*  might point to an environment variable string */
+        char *charptr;
+#endif /* USE_PREFETCH */
+
 
 	BufferDescriptors = (BufferDesc *)
 		ShmemInitStruct("Buffer Descriptors",
@@ -83,6 +164,142 @@ InitBufferPool(void)
 		ShmemInitStruct("Buffer Blocks",
 						NBuffers * (Size) BLCKSZ, &foundBufs);
 
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        BAiocbAnchr = (struct BAiocbAnchor *)0; /*  anchor for all control blocks pertaining to aio  */
+        if (max_async_io_prefetchers < 0) {  /*  negative value indicates to initialize to something sensible during buf_init */
+            max_async_io_prefetchers = MaxConnections/6;  /*  default allows for average of MaxConnections/6 concurrent prefetchers  - reasonable ??? */
+        }
+
+        if ((target_prefetch_pages > 0) && (max_async_io_prefetchers > 0)) {
+            int ix;
+            volatile struct BufferAiocb *BufferAiocbs;
+            volatile struct BufferAiocb * volatile FreeBAiocbs;
+
+            numBufferAiocbs = (target_prefetch_pages*max_async_io_prefetchers);  /*  target_prefetch_pages per prefetcher */
+            BAiocbAnchr = (struct BAiocbAnchor *)
+		ShmemInitStruct("Buffer Aiocbs",
+                          sizeof(struct BAiocbAnchor) + (numBufferAiocbs * sizeof(struct BufferAiocb)), &foundAiocbs);
+            if (BAiocbAnchr) {
+                BufferAiocbs = BAiocbAnchr->BufferAiocbs = (struct BufferAiocb*)(((char *)BAiocbAnchr) + sizeof(struct BAiocbAnchor));
+                FreeBAiocbs = (struct BufferAiocb*)0;
+                for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                    (BufferAiocbs+ix)->BAiocbnext = FreeBAiocbs;   /*  init the free list,  last one -> 0  */
+                    (BufferAiocbs+ix)->BAiocbbufh = (struct sbufdesc*)0;
+                    (BufferAiocbs+ix)->BAiocbDependentCount = 0;
+                    (BufferAiocbs+ix)->pidOfAio = 0;
+                    FreeBAiocbs = (BufferAiocbs+ix);
+
+                }
+                BAiocbAnchr->FreeBAiocbs = FreeBAiocbs;
+                envvarpointer = getenv("PG_MAX_GET_BAIOCB_TRIES");
+                if (    (envvarpointer != (char *)0)
+                     && (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   )
+                   ) {
+                      maxGetBAiocbTries = strtol(envvarpointer, 0, 10);
+                }
+                envvarpointer = getenv("PG_MAX_REL_BAIOCB_TRIES");
+                if (    (envvarpointer != (char *)0)
+                     && (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   )
+                   ) {
+                      maxRelBAiocbTries = strtol(envvarpointer, 0, 10);
+                }
+
+                /*   init the aio subsystem max number of threads and max number of requests
+                **   max number of threads   <-->  max_async_io_prefetchers
+                **   max number of requests  <-->  numBufferAiocbs = (target_prefetch_pages*max_async_io_prefetchers)
+                **   there is no return code so we just hope.
+                */
+                smgrinitaio(max_async_io_prefetchers , numBufferAiocbs);
+
+            }
+        }
+#else   /*  not USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+        /*   this dummy structure is to ensure that references to these fields in other bufmgr runtime code
+        **   that is not conditionally ifdefd on USE_AIO_ATOMIC_BUILTIN_COMP_SWAP compiles and runs correctly
+        */
+        BAiocbAnchr = &dummy_BAiocbAnchr;
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+
+#ifdef USE_PREFETCH
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_BITMAP");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_bitmap_scans = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+                 prefetch_bitmap_scans = 1;
+             }
+        }
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_ISCAN");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_index_scans = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+                 prefetch_index_scans = 1;
+             } else
+             if (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   ) {
+                 prefetch_index_scans = strtol(envvarpointer, &charptr, 10);
+                 if (charptr && (',' == *charptr)) {   /*  optional sequential prefetch in index scans */
+					 charptr++;        /*   following the comma ... */
+					 if ( ('Y' == *charptr) || ('y' == *charptr) || ('1' == *charptr) ) {
+                         prefetch_sequential_index_scans = 1;
+					 }
+				 }
+             }
+             /*  if prefeching for ISCAN,  then we require size of pfch_list to be at least target_prefetch_pages */
+             if (   (prefetch_index_scans > 0)
+                 && (prefetch_index_scans < target_prefetch_pages)
+                ) {
+                 prefetch_index_scans = target_prefetch_pages;
+             }
+        }
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_BTREE");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_btree_heaps = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+                 prefetch_btree_heaps = 1;
+             }
+        }
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_HEAP");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_heap_scans = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+              prefetch_heap_scans = 1;
+             }
+        }
+        envvarpointer = getenv("PG_PREFETCH_DBOID");
+        if (    (envvarpointer != (char *)0)
+             && (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   )
+           ) {
+              errno = 0;   /*  required in order to distinguish error from 0 */
+              prefetch_dbOid = (unsigned int)strtoul((const char *)envvarpointer, 0, 10);
+              if (errno) {
+                  prefetch_dbOid = 0;
+              }
+        }
+        elog(LOG, "prefetching initialised with target_prefetch_pages= %d "
+                  ", max_async_io_prefetchers= %d implying aio concurrency= %d "
+                  ", prefetching_for_bitmap= %s "
+                  ", prefetching_for_heap= %s "
+                  ", prefetching_for_iscan= %d with sequential_index_page_prefetching= %s "
+                  ", prefetching_for_btree= %s"
+                   ,target_prefetch_pages ,max_async_io_prefetchers ,numBufferAiocbs
+                   ,(prefetch_bitmap_scans ? "Y" : "N")
+                   ,(prefetch_heap_scans ? "Y" : "N")
+                   ,prefetch_index_scans
+                   ,(prefetch_sequential_index_scans ? "Y" : "N")
+                   ,(prefetch_btree_heaps ? "Y" : "N")
+            );
+#endif /* USE_PREFETCH */
+
+
 	if (foundDescs || foundBufs)
 	{
 		/* both should be present or neither */
@@ -176,3 +393,80 @@ BufferShmemSize(void)
 
 	return size;
 }
+
+/*     imprecise count of number of in-use BAiocbs at any time
+ *     we scan the array read-only without latching so are subject to unstable result
+ *     (but since the array is in well-known contiguous storage,
+ *     we are not subject to segmentation violation)
+ *     This function may be called at any time and just does its best
+ *     return the count of what we counted.
+ */
+int
+CountInuseBAiocbs(void)
+{
+        volatile struct BufferAiocb *BAiocb;
+        int count = 0;
+        int ix;
+
+
+        if (BAiocbAnchr != (struct BAiocbAnchor *)0 ) {
+            BAiocb = BAiocbAnchr->BufferAiocbs;             /*   start of list */
+
+            for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                    if ((struct BufferAiocb*)BAIOCB_OCCUPIED == (BAiocb+ix)->BAiocbnext) {   /* not on freelist ? */
+                        count++;
+                    }
+            }
+        }
+        return count;
+}
+
+/*
+ * report how many free BAiocbs at shutdown
+ * DO NOT call this while backends are actively working!!
+ * this report is useful when compare_and_swap method used (see above)
+ * as it can be used to deduce how many BAiocbs were in process-local caches -
+ * (original_number_on_freelist_at_startup - this_reported_number_at_shutdown)
+ */
+void
+ReportFreeBAiocbs(void)
+{
+        volatile struct BufferAiocb *BAiocb;
+        volatile struct BufferAiocb *BufferAiocbs;
+        int count = 0;
+        int fx , ix;
+
+
+        if (BAiocbAnchr != (struct BAiocbAnchor *)0 ) {
+            BAiocb = BAiocbAnchr->FreeBAiocbs;             /*   start of free list */
+            BufferAiocbs = BAiocbAnchr->BufferAiocbs;
+
+            for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                    (BufferAiocbs+ix)->BAiocbDependentCount = 0;  /* use this as marker for finding it on freelist */
+            }
+            for (fx = (numBufferAiocbs-1);  ( (fx>=0) &&  ( BAiocb != (struct BufferAiocb*)0 ) );  fx--) {
+                    
+                    /*  check if it is a valid BufferAiocb */
+                    for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                        if ((BufferAiocbs+ix) == BAiocb) { /*  is it this one ? */
+                             break;
+                        }
+                    }
+                    if (ix >= 0) {
+                        if (BAiocb->BAiocbDependentCount) {   /* seen it already ? */
+                            elog(LOG, "ReportFreeBAiocbs closed cycle on AIO control block freelist %p"
+                                          ,BAiocb);
+                            fx = 0; /* give up at this point */
+                        }
+                        BAiocb->BAiocbDependentCount = 1;  /* use this as marker for finding it on freelist */
+                        count++;
+                        BAiocb = BAiocb->BAiocbnext;
+                    } else {
+                        elog(LOG, "ReportFreeBAiocbs invalid item on AIO control block freelist %p"
+                                          ,BAiocb);
+                        fx = 0; /* give up at this point */
+                    }
+            }
+        }
+        elog(LOG, "ReportFreeBAiocbs AIO control block list : poolsize= %d  in-use-hwm= %d  final-free= %d" ,numBufferAiocbs , hwmBufferAiocbs , count);
+}
--- src/backend/storage/smgr/md.c.orig	2014-05-28 08:29:09.338829292 -0400
+++ src/backend/storage/smgr/md.c	2014-05-28 16:45:43.070507912 -0400
@@ -647,6 +647,62 @@ mdprefetch(SMgrRelation reln, ForkNumber
 }
 
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*
+ *	mdinitaio() --  init the aio subsystem max number of threads and max number of requests
+ */
+void
+mdinitaio(int max_aio_threads, int max_aio_num)
+{
+     FileInitaio( max_aio_threads, max_aio_num );
+}
+
+/*
+ *	mdstartaio() -- start aio read of the specified block of a relation
+ */
+void
+mdstartaio(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum , char *aiocbp , int *retcode )
+{
+#ifdef USE_PREFETCH
+	off_t		seekpos;
+	MdfdVec    *v;
+        int local_retcode;
+
+	v = _mdfd_getseg(reln, forknum, blocknum, false, EXTENSION_FAIL);
+
+	seekpos = (off_t) BLCKSZ *(blocknum % ((BlockNumber) RELSEG_SIZE));
+
+	Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+
+	local_retcode = FileStartaio(v->mdfd_vfd, seekpos, BLCKSZ , aiocbp);
+	if (retcode) {
+            *retcode = local_retcode;
+        }
+#endif   /* USE_PREFETCH */
+}
+
+
+/*
+ *	mdcompleteaio() -- complete aio read of the specified block of a relation
+ *      on entry, *inoutcode should indicate :
+ *           .  non-0  <=>   check if complete and wait if not
+ *           .  0      <=>   cancel io immediately
+ */
+void
+mdcompleteaio( char *aiocbp , int *inoutcode )
+{
+#ifdef USE_PREFETCH
+        int local_retcode;
+
+	local_retcode = FileCompleteaio(aiocbp, (inoutcode ? *inoutcode : 0));
+	if (inoutcode) {
+            *inoutcode = local_retcode;
+        }
+#endif   /* USE_PREFETCH */
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+
 /*
  *	mdread() -- Read the specified block from a relation.
  */
--- src/backend/storage/smgr/smgr.c.orig	2014-05-28 08:29:09.338829292 -0400
+++ src/backend/storage/smgr/smgr.c	2014-05-28 16:45:43.094508008 -0400
@@ -49,6 +49,12 @@ typedef struct f_smgr
 						 BlockNumber blocknum, char *buffer, bool skipFsync);
 	void		(*smgr_prefetch) (SMgrRelation reln, ForkNumber forknum,
 											  BlockNumber blocknum);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+	void		(*smgr_initaio) (int max_aio_threads, int max_aio_num);
+	void		(*smgr_startaio) (SMgrRelation reln, ForkNumber forknum,
+											  BlockNumber blocknum , char *aiocbp , int *retcode );
+	void		(*smgr_completeaio) ( char *aiocbp , int *inoutcode );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 	void		(*smgr_read) (SMgrRelation reln, ForkNumber forknum,
 										  BlockNumber blocknum, char *buffer);
 	void		(*smgr_write) (SMgrRelation reln, ForkNumber forknum,
@@ -66,7 +72,11 @@ typedef struct f_smgr
 static const f_smgr smgrsw[] = {
 	/* magnetic disk */
 	{mdinit, NULL, mdclose, mdcreate, mdexists, mdunlink, mdextend,
-		mdprefetch, mdread, mdwrite, mdnblocks, mdtruncate, mdimmedsync,
+		mdprefetch
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                        ,mdinitaio, mdstartaio, mdcompleteaio
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+              , mdread, mdwrite, mdnblocks, mdtruncate, mdimmedsync,
 		mdpreckpt, mdsync, mdpostckpt
 	}
 };
@@ -612,6 +622,35 @@ smgrprefetch(SMgrRelation reln, ForkNumb
 	(*(smgrsw[reln->smgr_which].smgr_prefetch)) (reln, forknum, blocknum);
 }
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*
+ *	smgrinitaio() -- initialize the aio subsystem max number of threads and max number of requests
+ */
+void
+smgrinitaio(int max_aio_threads, int max_aio_num)
+{
+	(*(smgrsw[0].smgr_initaio)) ( max_aio_threads, max_aio_num );
+}
+
+/*
+ *	smgrstartaio() -- Initiate aio read of the specified block of a relation.
+ */
+void
+smgrstartaio(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum , char *aiocbp , int *retcode )
+{
+	(*(smgrsw[reln->smgr_which].smgr_startaio)) (reln, forknum, blocknum , aiocbp , retcode );
+}
+
+/*
+ *	smgrcompleteaio() -- Complete aio read of the specified block of a relation.
+ */
+void
+smgrcompleteaio(SMgrRelation reln,  char *aiocbp , int *inoutcode )
+{
+	(*(smgrsw[reln->smgr_which].smgr_completeaio)) ( aiocbp , inoutcode );
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 /*
  *	smgrread() -- read a particular block from a relation into the supplied
  *				  buffer.
--- src/backend/storage/file/fd.c.orig	2014-05-28 08:29:09.334829294 -0400
+++ src/backend/storage/file/fd.c	2014-05-28 16:45:43.122508122 -0400
@@ -77,6 +77,9 @@
 #include "utils/guc.h"
 #include "utils/resowner_private.h"
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+#include "aio.h"
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * We must leave some file descriptors free for system(), the dynamic loader,
@@ -1239,6 +1242,10 @@ FileClose(File file)
  * We could add an implementation using libaio in the future; but note that
  * this API is inappropriate for libaio, which wants to have a buffer provided
  * to read into.
+ * Also note that a new, different implementation of asynchronous prefetch
+ * using librt,  not libaio,  is provided by the two functions following this one,
+ * FileStartaio and FileCompleteaio.   These also require to have a buffer provided
+ * to read into,  which the new async_io support provides.
  */
 int
 FilePrefetch(File file, off_t offset, int amount)
@@ -1266,6 +1273,139 @@ FilePrefetch(File file, off_t offset, in
 #endif
 }
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*
+ * FileInitaio - initialize the aio subsystem max number of threads and max number of requests
+ *  input parms
+ *  max_aio_threads;    maximum number of threads
+ *  max_aio_num;        maximum number of concurrent aio read requests
+ *
+ *  on linux, man page for the librt implemenation of aio_init() says :
+ *         This function is a GNU extension.
+ *  If your posix aio does not have it,   then add the following line to 
+ *        src/include/pg_config_manual.h
+ *    #define DONT_HAVE_AIO_INIT
+ *  to render it as a no-op
+ */
+void
+FileInitaio(int max_aio_threads, int max_aio_num )
+{
+#ifndef DONT_HAVE_AIO_INIT
+    struct aioinit aioinit_struct;  /*  structure to pass to aio_init */
+
+    aioinit_struct.aio_threads = max_aio_threads; /*     maximum number of threads */
+    aioinit_struct.aio_num = max_aio_num;         /*     maximum number of concurrent aio read requests */
+    aioinit_struct.aio_idle_time = 1;             /*     we dont want to alter this but aio_init does not ignore it so set to the default */
+    aio_init(&aioinit_struct);
+#endif  /* ndef DONT_HAVE_AIO_INIT */
+    return;
+}
+
+/*
+ * FileStartaio - initiate asynchronous read of a given range of the file.
+ * The logical seek position is unaffected.
+ *
+ * use standard posix aio (librt)
+ *  ASSUME   BufferAiocb.aio_buf already set to -> buffer by caller
+ *  return 0 if successfully started,  else non-zero
+ */
+int
+FileStartaio(File file, off_t offset, int amount , char *aiocbp )
+{
+	int	returnCode;
+        struct aiocb *my_aiocbp = (struct aiocb *)aiocbp;
+
+	Assert(FileIsValid(file));
+
+	DO_DB(elog(LOG, "FileStartaio: %d (%s) " INT64_FORMAT " %d",
+			   file, VfdCache[file].fileName,
+			   (int64) offset, amount));
+
+	returnCode = FileAccess(file);
+	if (returnCode >= 0) {
+
+            my_aiocbp->aio_fildes = VfdCache[file].fd;
+            my_aiocbp->aio_lio_opcode = LIO_READ;
+            my_aiocbp->aio_nbytes = amount;
+            my_aiocbp->aio_offset = offset;
+            returnCode = aio_read(my_aiocbp);
+        }
+
+	return returnCode;
+}
+
+/*
+ * FileCompleteaio - complete asynchronous aio read
+ * normal_wait indicates whether to cancel or wait -
+ *                 0 <=> cancel
+ *                 1 <=> wait
+ *
+ * use standard posix aio (librt)
+ *  return 0 if successfull and did not have to wait,
+ *         1 if successfull and had to wait,
+ *    else x'ff'
+ */
+int
+FileCompleteaio( char *aiocbp , int normal_wait )
+{
+	int	returnCode;
+	int	aio_errno;
+        struct aiocb *my_aiocbp = (struct aiocb *)aiocbp;
+        const struct aiocb const*cblist[1];
+        int fd;
+        struct timespec my_timeout = { 0 , 10000 };
+        int max_polls;
+
+        fd = my_aiocbp->aio_fildes;
+        cblist[0] = my_aiocbp;
+        returnCode = aio_errno = aio_error(my_aiocbp);
+        /* note that aio_error returns 0 if op already completed successfully */
+
+        /*  first handle normal case of waiting for op to complete  */
+        if (normal_wait) {
+            while (aio_errno == EINPROGRESS) {
+                max_polls = 256;
+                my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                returnCode = aio_suspend(cblist , 1 , &my_timeout);
+                while ((returnCode < 0) && (EAGAIN == errno) && (max_polls-- > 0)) {
+                    my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                    returnCode = aio_suspend(cblist , 1 , &my_timeout);
+                }
+                returnCode = aio_errno = aio_error(my_aiocbp);
+                /*  now return_code is from aio_error  */
+                if (returnCode == 0) {
+                    returnCode = 1;    /*  successful but had to wait */
+                }
+            }
+            if (aio_errno) {
+                elog(LOG, "FileCompleteaio: %d %d", fd, returnCode);
+                returnCode = 0xff; /*  unsuccessful */
+            }
+        } else {
+            if (aio_errno == EINPROGRESS) {
+                do {
+                        max_polls = 256;
+                        my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                        returnCode = aio_cancel(fd, my_aiocbp);
+                        while ((returnCode == AIO_NOTCANCELED) && (max_polls-- > 0)) {
+                            my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                            returnCode = aio_cancel(fd, my_aiocbp);
+                        }
+                    returnCode = aio_errno = aio_error(my_aiocbp);
+                } while (aio_errno == EINPROGRESS);
+                returnCode = 0xff; /*  unsuccessful */
+            }
+            if (returnCode != 0)
+                returnCode = 0xff; /*  unsuccessful */
+        }
+
+	DO_DB(elog(LOG, "FileCompleteaio: %d %d",
+			   fd, returnCode));
+
+	return returnCode;
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 int
 FileRead(File file, char *buffer, int amount)
 {
--- src/backend/storage/lmgr/proc.c.orig	2014-05-28 08:29:09.338829292 -0400
+++ src/backend/storage/lmgr/proc.c	2014-05-28 16:45:43.146508219 -0400
@@ -52,6 +52,7 @@
 #include "utils/timeout.h"
 #include "utils/timestamp.h"
 
+extern pid_t this_backend_pid;     /*   pid of this backend */
 
 /* GUC variables */
 int			DeadlockTimeout = 1000;
@@ -361,6 +362,7 @@ InitProcess(void)
 	MyPgXact->xid = InvalidTransactionId;
 	MyPgXact->xmin = InvalidTransactionId;
 	MyProc->pid = MyProcPid;
+        this_backend_pid = getpid();    /*    pid of this backend */
 	/* backendId, databaseId and roleId will be filled in later */
 	MyProc->backendId = InvalidBackendId;
 	MyProc->databaseId = InvalidOid;
--- src/backend/access/heap/heapam.c.orig	2014-05-28 08:29:09.242829343 -0400
+++ src/backend/access/heap/heapam.c	2014-05-28 16:45:43.202508444 -0400
@@ -71,6 +71,28 @@
 #include "utils/syscache.h"
 #include "utils/tqual.h"
 
+#ifdef USE_PREFETCH
+#include <sys/types.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+#include "executor/instrument.h"
+
+extern unsigned int prefetch_dbOid; /*  database oid of relations on which prefetching to be done - 0 means all */
+extern unsigned int prefetch_heap_scans; /*  boolean whether to prefetch non-bitmap heap scans         */
+
+/*  special values for scan->rs_prefetch_target indicating as follows :               */
+#define PREFETCH_MAYBE 0xffffffff      /*   prefetch permitted but not yet in effect  */
+#define PREFETCH_DISABLED 0xfffffffe   /*   prefetch disabled and not permitted       */
+/*  PREFETCH_WRAP_POINT indicates a pretcher who has reached the point where the scan would wrap -
+**  at this point the prefetcher runs on the spot until scan catches up.
+**  This *must* be < maximum valid setting of target_prefetch_pages aka effective_io_concurrency.
+*/
+#define PREFETCH_WRAP_POINT 0x0fffffff
+
+#endif   /* USE_PREFETCH */
+
 
 /* GUC variable */
 bool		synchronize_seqscans = true;
@@ -115,6 +137,8 @@ static XLogRecPtr log_heap_new_cid(Relat
 static HeapTuple ExtractReplicaIdentity(Relation rel, HeapTuple tup, bool key_modified,
 					   bool *copy);
 
+static void heap_unread_add(HeapScanDesc scan, BlockNumber blockno);
+static void heap_unread_subtract(HeapScanDesc scan, BlockNumber blockno);
 
 /*
  * Each tuple lock mode has a corresponding heavyweight lock, and one or two
@@ -292,9 +316,148 @@ initscan(HeapScanDesc scan, ScanKey key,
 	 * Currently, we don't have a stats counter for bitmap heap scans (but the
 	 * underlying bitmap index scans will be counted).
 	 */
-	if (!scan->rs_bitmapscan)
+#ifdef USE_PREFETCH
+        /*    by default,  no prefetching on any scan  */
+        scan->rs_prefetch_target = PREFETCH_DISABLED;  /*  tentatively disable  */
+        scan->rs_pfchblock = 0; /*  scanner will reset this to be ahead of scan */
+        scan->rs_Unread_Pfetched_base = (BlockNumber *)0; /*  list of prefetched but unread blocknos */
+        scan->rs_Unread_Pfetched_next = 0; /*  next unread blockno */
+        scan->rs_Unread_Pfetched_count = 0; /* number of valid unread blocknos */
+#endif   /* USE_PREFETCH */
+	if (!scan->rs_bitmapscan) {
+
 		pgstat_count_heap_scan(scan->rs_rd);
+#ifdef USE_PREFETCH
+                /*    bitmap scans do their own prefetching -
+                **    for others,  set up prefetching now
+                */
+                if (    prefetch_heap_scans
+                     && (target_prefetch_pages > 0)
+                     &&	(!RelationUsesLocalBuffers(scan->rs_rd))
+                   ) {
+                      /*   prefetch_dbOid may be set to a database Oid to specify only prefetch in that db */
+                      if (    (   (prefetch_dbOid > 0)
+                               && (prefetch_dbOid == scan->rs_rd->rd_node.dbNode)
+                              )
+                          ||  (prefetch_dbOid == 0)
+                         ) {
+                          scan->rs_prefetch_target = PREFETCH_MAYBE;    /*  permitted but let the scan decide */
+                      }
+                      else {
+                      }
+                }
+#endif   /* USE_PREFETCH */
+        }
+}
+
+/* add this blockno to list of prefetched and unread blocknos
+** use the one identified by the (next+count|modulo circumference) index if it is unused,
+** else search for the first available slot if there is one,
+** else error.
+*/
+static void
+heap_unread_add(HeapScanDesc scan, BlockNumber blockno)
+{
+      BlockNumber *available_P;   /*  where to store new blockno */
+      unsigned int Unread_Pfetched_index = scan->rs_Unread_Pfetched_next
+                                         + scan->rs_Unread_Pfetched_count; /* index of next unused slot */
+
+      /*  caller is not supposed to pass InvalidBlockNumber but check anyway */
+      if (blockno != InvalidBlockNumber) {
+
+		  /*  ensure there is some room somewhere   */
+		  if (scan->rs_Unread_Pfetched_count < target_prefetch_pages) {
+
+			  /*  try the "next+count" one */
+			  if (Unread_Pfetched_index >= target_prefetch_pages) {
+				  Unread_Pfetched_index -= target_prefetch_pages;  /* modulo circumference */
+			  }
+			  available_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);   /*  where to store new blockno */
+			  if (*available_P == InvalidBlockNumber) { /* unused */
+				  goto store_blockno;
+			  } else {
+                  /*  slow-search the entire list  */
+				  for (Unread_Pfetched_index = 0; Unread_Pfetched_index < target_prefetch_pages; Unread_Pfetched_index++) {
+					  available_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);   /*  where to store new blockno */
+					  if (*available_P == InvalidBlockNumber) { /* unused */
+                          /*  before storing this blockno,
+                          **  since the next pointer did not locate an unused slot,
+                          **  set it to one which is more likely to be so for the next time
+                          */
+                          scan->rs_Unread_Pfetched_next = Unread_Pfetched_index;
+						  goto store_blockno;
+					  }
+				  }
+			  }
+		  }
+
+          /*  if we reach here,    either there was no available slot
+          **  or we thought there was one and didn't find any  --
+          */
+  		  ereport(ERROR,
+			  (errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("heap_unread_add overflowed list cannot add blockno %d", blockno)));
+
+  		  return;
+      }
+
+    store_blockno:
+      *available_P = blockno;
+      scan->rs_Unread_Pfetched_count++;  /*  update count */
+
+    return;
+}
+
+/* remove specified blockno from list of prefetched and unread blocknos.
+/* Usually this will be found at the rs_Unread_Pfetched_next item -
+** else search for it.    If not found,   inore it  -  no error results.
+*/
+static void
+heap_unread_subtract(HeapScanDesc scan, BlockNumber blockno)
+{
+      unsigned int Unread_Pfetched_index = scan->rs_Unread_Pfetched_next; /* index of next unread blockno */
+      BlockNumber *candidate_P;   /*  location of callers blockno - maybe */
+      BlockNumber nextUnreadPfetched;
+
+      /*  caller is not supposed to pass InvalidBlockNumber but check anyway */
+      if (    (blockno != InvalidBlockNumber)
+		   && ( scan->rs_Unread_Pfetched_count > 0 )   /*  if the list is not empty  */
+         ) {
+
+			  /*  take modulo of the circumference.
+			  **  actually rs_Unread_Pfetched_next should never exceed the circumference but check anyway.
+			  */
+			  if (Unread_Pfetched_index >= target_prefetch_pages) {
+				  Unread_Pfetched_index  -= target_prefetch_pages;
 }
+			  candidate_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);
+			  nextUnreadPfetched = *candidate_P;
+
+			  if ( nextUnreadPfetched == blockno ) {
+				  goto remove_blockno;
+			  } else {
+                  /*  slow-search the entire list  */
+				  for (Unread_Pfetched_index = 0; Unread_Pfetched_index < target_prefetch_pages; Unread_Pfetched_index++) {
+					  candidate_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);   /*  where to store new blockno */
+					  if (*candidate_P == blockno) { /* unused */
+						  goto remove_blockno;
+					  }
+				  }
+			  }
+
+    remove_blockno:
+			  *candidate_P = InvalidBlockNumber;
+
+			  scan->rs_Unread_Pfetched_next = (Unread_Pfetched_index+1);  /*  update next pfchd unread */
+			  if (scan->rs_Unread_Pfetched_next >= target_prefetch_pages) {
+					  scan->rs_Unread_Pfetched_next = 0;
+			  }
+			  scan->rs_Unread_Pfetched_count--;  /*  update count */
+	  }
+
+      return;
+}
+
 
 /*
  * heapgetpage - subroutine for heapgettup()
@@ -304,7 +467,7 @@ initscan(HeapScanDesc scan, ScanKey key,
  * which tuples on the page are visible.
  */
 static void
-heapgetpage(HeapScanDesc scan, BlockNumber page)
+heapgetpage(HeapScanDesc scan, BlockNumber page , BlockNumber prefetchHWM)
 {
 	Buffer		buffer;
 	Snapshot	snapshot;
@@ -314,6 +477,10 @@ heapgetpage(HeapScanDesc scan, BlockNumb
 	OffsetNumber lineoff;
 	ItemId		lpp;
 	bool		all_visible;
+#ifdef USE_PREFETCH
+	int             PrefetchBufferRc;  /*   indicates whether requested prefetch block already in a buffer and if pin count on buffer has been increased */
+#endif   /* USE_PREFETCH */
+
 
 	Assert(page < scan->rs_nblocks);
 
@@ -336,6 +503,98 @@ heapgetpage(HeapScanDesc scan, BlockNumb
 									   RBM_NORMAL, scan->rs_strategy);
 	scan->rs_cblock = page;
 
+#ifdef USE_PREFETCH
+
+        heap_unread_subtract(scan, page);
+
+        /*    maybe prefetch some pages  starting with rs_pfchblock */
+        if (scan->rs_prefetch_target >= 0) {       /*   prefetching enabled on this scan ?                         */
+            int next_block_to_be_read = (page+1);  /*   next block to be read = lowest possible prefetchable block */
+            int num_to_pfch_this_time;             /*   eventually holds the number of blocks to prefetch now      */
+            int prefetchable_range;                /*   size of the area ahead of the current prefetch position    */
+
+            /*  check if prefetcher reached wrap point and the scan has now wrapped */
+            if (  (page == 0) && (scan->rs_prefetch_target == PREFETCH_WRAP_POINT)  ) {
+                scan->rs_prefetch_target = 1;
+                scan->rs_pfchblock = next_block_to_be_read;
+            } else
+            if (scan->rs_pfchblock < next_block_to_be_read) {
+                scan->rs_pfchblock = next_block_to_be_read; /* next block to be prefetched must be ahead of one we just read */
+            }
+
+            /* now we know where we would start prefetching -
+            ** next question   -  if this is a sync scan,  ensure we do not prefetch behind the HWM
+            ** debatable whether to require strict inequality or >=  -   >= works better in practice
+            */
+            if ( (!scan->rs_syncscan) || (scan->rs_pfchblock >= prefetchHWM) ) {
+
+                /* now we know where we will start prefetching -
+                ** next question   -  how many?
+                ** apply two limits :
+                **  1.   target prefetch distance
+                **  2.   number of available blocks ahead of us
+                */
+
+                /*  1.   target prefetch distance   */
+                num_to_pfch_this_time = next_block_to_be_read + scan->rs_prefetch_target; /* page beyond prefetch target */
+                num_to_pfch_this_time -= scan->rs_pfchblock;                              /*  convert to offset        */
+
+                /*   first do prefetching up to our current limit  ...
+                **   highest page number that a scan (pre)-fetches is scan->rs_nblocks-1
+                **   note  -  prefetcher does not wrap a prefetch range -
+                **            instead just stop and then start again if and when main scan wraps
+                */
+                if (scan->rs_pfchblock <= scan->rs_startblock) {  /*  if on second leg towards startblock */
+                    prefetchable_range = ((int)(scan->rs_startblock) - (int)(scan->rs_pfchblock));
+                }
+                else {                                            /*     on first leg towards nblocks     */
+                    prefetchable_range = ((int)(scan->rs_nblocks) - (int)(scan->rs_pfchblock));
+                }
+                if (prefetchable_range > 0) {           /*  if theres a range to prefetch */
+
+                    /*  2.   number of available blocks ahead of us        */
+                    if (num_to_pfch_this_time > prefetchable_range) {
+                        num_to_pfch_this_time = prefetchable_range;
+                    }
+                    while (num_to_pfch_this_time-- > 0) {
+                        PrefetchBufferRc = PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, scan->rs_pfchblock, scan->rs_strategy);
+                        /*  if pin acquired on buffer,  then remember in case of future Discard */
+                        if (PrefetchBufferRc & PREFTCHRC_BUF_PIN_INCREASED) {
+                            heap_unread_add(scan, scan->rs_pfchblock);
+						}
+                        scan->rs_pfchblock++;
+                        /*  if syncscan and requested block was already in buffer pool,
+                        **  this suggests that another scanner is ahead of us and we should advance
+                        */
+                        if ( (scan->rs_syncscan) && (PrefetchBufferRc & PREFTCHRC_BLK_ALREADY_PRESENT) ) {
+                            scan->rs_pfchblock++;
+                            num_to_pfch_this_time--;
+                        }
+                    }
+                }
+                else {
+                    /*   we must not modify scan->rs_pfchblock here
+                    **   because it is needed for possible DiscardBuffer at end of scan  ...
+                    **   ... instead ...
+                    */
+                    scan->rs_prefetch_target = PREFETCH_WRAP_POINT;  /*   mark this prefetcher as waiting to wrap */
+                }
+
+                /*   ...  then adjust prefetching limit : by doubling on each iteration */
+                if (scan->rs_prefetch_target == 0) {
+                    scan->rs_prefetch_target = 1;
+                }
+                else {
+                    scan->rs_prefetch_target *= 2;
+                    if (scan->rs_prefetch_target > target_prefetch_pages) {
+                        scan->rs_prefetch_target = target_prefetch_pages;
+                    }
+                }
+            }
+        }
+#endif   /* USE_PREFETCH */
+
+
 	if (!scan->rs_pageatatime)
 		return;
 
@@ -452,6 +711,8 @@ heapgettup(HeapScanDesc scan,
 	OffsetNumber lineoff;
 	int			linesleft;
 	ItemId		lpp;
+    BlockNumber  prefetchHWM = 0;   /*  HWM of prefetch BlockNum for all participants in same sync scan */
+    int          ix;
 
 	/*
 	 * calculate next starting lineoff, given scan direction
@@ -470,7 +731,25 @@ heapgettup(HeapScanDesc scan,
 				return;
 			}
 			page = scan->rs_startblock; /* first page */
-			heapgetpage(scan, page);
+#ifdef USE_PREFETCH
+                        /*  decide if we shall do prefetching :  only if :
+                        **   .  prefetching enabled for this scan
+                        **   .  not a bitmap scan (which do their own)
+                        **   .  sufficient number of blocks - at least twice the target_prefetch_pages
+                        */
+                        if (    (!scan->rs_bitmapscan)
+                             && (scan->rs_prefetch_target >= PREFETCH_MAYBE)
+                             && (scan->rs_nblocks > (2*target_prefetch_pages))
+                           )  {
+                                scan->rs_prefetch_target = 1;  /*  do prefetching on forward non-bitmap scan */
+                                scan->rs_Unread_Pfetched_base = (BlockNumber *)palloc(sizeof(BlockNumber)*target_prefetch_pages);
+                                /*  Initialise the list */
+                                for (ix = 0; ix < target_prefetch_pages; ix++) {
+									*(scan->rs_Unread_Pfetched_base + ix) = InvalidBlockNumber;
+								}
+                        }
+#endif   /* USE_PREFETCH */
+			heapgetpage(scan, page, prefetchHWM);
 			lineoff = FirstOffsetNumber;		/* first offnum */
 			scan->rs_inited = true;
 		}
@@ -516,7 +795,7 @@ heapgettup(HeapScanDesc scan,
 				page = scan->rs_startblock - 1;
 			else
 				page = scan->rs_nblocks - 1;
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 		}
 		else
 		{
@@ -557,7 +836,7 @@ heapgettup(HeapScanDesc scan,
 
 		page = ItemPointerGetBlockNumber(&(tuple->t_self));
 		if (page != scan->rs_cblock)
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 
 		/* Since the tuple was previously fetched, needn't lock page here */
 		dp = (Page) BufferGetPage(scan->rs_cbuf);
@@ -660,8 +939,10 @@ heapgettup(HeapScanDesc scan,
 			 * a little bit backwards on every invocation, which is confusing.
 			 * We don't guarantee any specific ordering in general, though.
 			 */
-			if (scan->rs_syncscan)
-				ss_report_location(scan->rs_rd, page);
+			if (scan->rs_syncscan) {
+                                prefetchHWM = scan->rs_pfchblock;
+				ss_report_location(scan->rs_rd, page, &prefetchHWM);
+                        }
 		}
 
 		/*
@@ -671,6 +952,22 @@ heapgettup(HeapScanDesc scan,
 		{
 			if (BufferIsValid(scan->rs_cbuf))
 				ReleaseBuffer(scan->rs_cbuf);
+#ifdef USE_PREFETCH
+                        if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                            /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                            while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+                        }
+#endif   /* USE_PREFETCH */
 			scan->rs_cbuf = InvalidBuffer;
 			scan->rs_cblock = InvalidBlockNumber;
 			tuple->t_data = NULL;
@@ -678,7 +975,7 @@ heapgettup(HeapScanDesc scan,
 			return;
 		}
 
-		heapgetpage(scan, page);
+		heapgetpage(scan, page, prefetchHWM);
 
 		LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);
 
@@ -727,6 +1024,8 @@ heapgettup_pagemode(HeapScanDesc scan,
 	OffsetNumber lineoff;
 	int			linesleft;
 	ItemId		lpp;
+    BlockNumber  prefetchHWM = 0;   /*  HWM of prefetch BlockNum for all participants in same sync scan */
+    int          ix;
 
 	/*
 	 * calculate next starting lineindex, given scan direction
@@ -745,7 +1044,25 @@ heapgettup_pagemode(HeapScanDesc scan,
 				return;
 			}
 			page = scan->rs_startblock; /* first page */
-			heapgetpage(scan, page);
+#ifdef USE_PREFETCH
+                        /*  decide if we shall do prefetching :  only if :
+                        **   .  prefetching enabled for this scan
+                        **   .  not a bitmap scan (which do their own)
+                        **   .  sufficient number of blocks - at least twice the target_prefetch_pages
+                        */
+                        if (    (!scan->rs_bitmapscan)
+                             && (scan->rs_prefetch_target >= PREFETCH_MAYBE)
+                             && (scan->rs_nblocks > (2*target_prefetch_pages))
+                           )  {
+                                scan->rs_prefetch_target = 1;  /*  do prefetching on forward non-bitmap scan */
+                                scan->rs_Unread_Pfetched_base = (BlockNumber *)palloc(sizeof(BlockNumber)*target_prefetch_pages);
+                                /*  Initialise the list */
+                                for (ix = 0; ix < target_prefetch_pages; ix++) {
+									*(scan->rs_Unread_Pfetched_base + ix) = InvalidBlockNumber;
+								}
+                        }
+#endif   /* USE_PREFETCH */
+			heapgetpage(scan, page, prefetchHWM);
 			lineindex = 0;
 			scan->rs_inited = true;
 		}
@@ -788,7 +1105,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 				page = scan->rs_startblock - 1;
 			else
 				page = scan->rs_nblocks - 1;
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 		}
 		else
 		{
@@ -826,7 +1143,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 
 		page = ItemPointerGetBlockNumber(&(tuple->t_self));
 		if (page != scan->rs_cblock)
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 
 		/* Since the tuple was previously fetched, needn't lock page here */
 		dp = (Page) BufferGetPage(scan->rs_cbuf);
@@ -921,8 +1238,10 @@ heapgettup_pagemode(HeapScanDesc scan,
 			 * a little bit backwards on every invocation, which is confusing.
 			 * We don't guarantee any specific ordering in general, though.
 			 */
-			if (scan->rs_syncscan)
-				ss_report_location(scan->rs_rd, page);
+			if (scan->rs_syncscan) {
+                                prefetchHWM = scan->rs_pfchblock;
+				ss_report_location(scan->rs_rd, page, &prefetchHWM);
+                        }
 		}
 
 		/*
@@ -932,6 +1251,22 @@ heapgettup_pagemode(HeapScanDesc scan,
 		{
 			if (BufferIsValid(scan->rs_cbuf))
 				ReleaseBuffer(scan->rs_cbuf);
+#ifdef USE_PREFETCH
+                        if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                            /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                             while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+                        }
+#endif   /* USE_PREFETCH */
 			scan->rs_cbuf = InvalidBuffer;
 			scan->rs_cblock = InvalidBlockNumber;
 			tuple->t_data = NULL;
@@ -939,7 +1274,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 			return;
 		}
 
-		heapgetpage(scan, page);
+		heapgetpage(scan, page, prefetchHWM);
 
 		dp = (Page) BufferGetPage(scan->rs_cbuf);
 		lines = scan->rs_ntuples;
@@ -1394,6 +1729,23 @@ void
 heap_rescan(HeapScanDesc scan,
 			ScanKey key)
 {
+
+#ifdef USE_PREFETCH
+         if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                       /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                            while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+         }
+#endif   /* USE_PREFETCH */
 	/*
 	 * unpin scan buffers
 	 */
@@ -1418,6 +1770,23 @@ heap_endscan(HeapScanDesc scan)
 {
 	/* Note: no locking manipulations needed */
 
+
+#ifdef USE_PREFETCH
+         if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                       /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                            while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+         }
+#endif   /* USE_PREFETCH */
 	/*
 	 * unpin scan buffers
 	 */
@@ -1435,6 +1804,10 @@ heap_endscan(HeapScanDesc scan)
 	if (scan->rs_strategy != NULL)
 		FreeAccessStrategy(scan->rs_strategy);
 
+    if (scan->rs_Unread_Pfetched_base) {
+        pfree(scan->rs_Unread_Pfetched_base);
+    }
+
 	if (scan->rs_temp_snap)
 		UnregisterSnapshot(scan->rs_snapshot);
 
@@ -1464,7 +1837,6 @@ heap_endscan(HeapScanDesc scan)
 #define HEAPDEBUG_3
 #endif   /* !defined(HEAPDEBUGALL) */
 
-
 HeapTuple
 heap_getnext(HeapScanDesc scan, ScanDirection direction)
 {
@@ -6347,6 +6719,25 @@ heap_markpos(HeapScanDesc scan)
 void
 heap_restrpos(HeapScanDesc scan)
 {
+
+
+#ifdef USE_PREFETCH
+                        if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                            /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                             while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+                        }
+#endif   /* USE_PREFETCH */
+
 	/* XXX no amrestrpos checking that ammarkpos called */
 
 	if (!ItemPointerIsValid(&scan->rs_mctid))
--- src/backend/access/heap/syncscan.c.orig	2014-05-28 08:29:09.242829343 -0400
+++ src/backend/access/heap/syncscan.c	2014-05-28 16:45:43.254508653 -0400
@@ -90,6 +90,7 @@ typedef struct ss_scan_location_t
 {
 	RelFileNode relfilenode;	/* identity of a relation */
 	BlockNumber location;		/* last-reported location in the relation */
+	BlockNumber prefetchHWM;	/* high-water-mark of prefetched Blocknum */
 } ss_scan_location_t;
 
 typedef struct ss_lru_item_t
@@ -113,7 +114,7 @@ static ss_scan_locations_t *scan_locatio
 
 /* prototypes for internal functions */
 static BlockNumber ss_search(RelFileNode relfilenode,
-		  BlockNumber location, bool set);
+		  BlockNumber location, bool set , BlockNumber *prefetchHWMp);
 
 
 /*
@@ -160,6 +161,7 @@ SyncScanShmemInit(void)
 			item->location.relfilenode.dbNode = InvalidOid;
 			item->location.relfilenode.relNode = InvalidOid;
 			item->location.location = InvalidBlockNumber;
+			item->location.prefetchHWM = InvalidBlockNumber;
 
 			item->prev = (i > 0) ?
 				(&scan_locations->items[i - 1]) : NULL;
@@ -185,7 +187,7 @@ SyncScanShmemInit(void)
  * data structure.
  */
 static BlockNumber
-ss_search(RelFileNode relfilenode, BlockNumber location, bool set)
+ss_search(RelFileNode relfilenode, BlockNumber location, bool set , BlockNumber *prefetchHWMp)
 {
 	ss_lru_item_t *item;
 
@@ -206,6 +208,22 @@ ss_search(RelFileNode relfilenode, Block
 			{
 				item->location.relfilenode = relfilenode;
 				item->location.location = location;
+                                /*  if prefetch information requested,
+                                **  then reconcile and either update or report back the new HWM.
+                                */
+                                if (prefetchHWMp)
+                                {
+                                    if (   (item->location.prefetchHWM == InvalidBlockNumber)
+                                        || (item->location.prefetchHWM < *prefetchHWMp)
+                                       )
+                                    {
+                                      item->location.prefetchHWM = *prefetchHWMp;
+                                    }
+                                    else
+                                    {
+                                      *prefetchHWMp = item->location.prefetchHWM;
+                                    }
+                                }
 			}
 			else if (set)
 				item->location.location = location;
@@ -252,7 +270,7 @@ ss_get_location(Relation rel, BlockNumbe
 	BlockNumber startloc;
 
 	LWLockAcquire(SyncScanLock, LW_EXCLUSIVE);
-	startloc = ss_search(rel->rd_node, 0, false);
+	startloc = ss_search(rel->rd_node, 0, false , 0);
 	LWLockRelease(SyncScanLock);
 
 	/*
@@ -282,7 +300,7 @@ ss_get_location(Relation rel, BlockNumbe
  * same relfilenode.
  */
 void
-ss_report_location(Relation rel, BlockNumber location)
+ss_report_location(Relation rel, BlockNumber location , BlockNumber *prefetchHWMp)
 {
 #ifdef TRACE_SYNCSCAN
 	if (trace_syncscan)
@@ -306,7 +324,7 @@ ss_report_location(Relation rel, BlockNu
 	{
 		if (LWLockConditionalAcquire(SyncScanLock, LW_EXCLUSIVE))
 		{
-			(void) ss_search(rel->rd_node, location, true);
+			(void) ss_search(rel->rd_node, location, true , prefetchHWMp);
 			LWLockRelease(SyncScanLock);
 		}
 #ifdef TRACE_SYNCSCAN
--- src/backend/access/index/indexam.c.orig	2014-05-28 08:29:09.242829343 -0400
+++ src/backend/access/index/indexam.c	2014-05-28 16:45:43.298508831 -0400
@@ -79,6 +79,55 @@
 #include "utils/tqual.h"
 
 
+#ifdef USE_PREFETCH
+bool BlocknotinBuffer(Buffer buffer, Relation relation, BlockNumber blockNum);
+BlockNumber BlocknumOfBuffer(Buffer buffer);
+void index_mark_or_evict_block(IndexScanDesc scan , BlockNumber blocknumber , int markit);
+
+extern unsigned int prefetch_index_scans; /*  boolean whether to prefetch bitmap heap scans         */
+
+/*  if specified block number is present in the prefetch array,
+**  then either mark it as not to be discarded or evict it according to input param
+*/
+void index_mark_or_evict_block(IndexScanDesc scan , BlockNumber blocknumber , int markit)
+{
+        unsigned short int pfchx , pfchy , pfchz; /*  indexes in BlockIdData array   */
+
+        if (    scan->do_prefetch
+             && ((struct pfch_block_item*)0 != scan->pfch_block_item_list)
+             /* no need to check for scan->pfch_next < prefetch_index_scans
+             ** since we will do nothing if scan->pfch_used == 0
+             */
+           ) {
+            /*  search the prefetch list to find if the block is a member */
+            for (pfchx = 0; pfchx < scan->pfch_used; pfchx++) {
+                if (BlockIdGetBlockNumber(&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)) == blocknumber) {
+                      if (markit) {
+						      /*  mark it as not to be discarded */
+						      ((scan->pfch_block_item_list)+pfchx)->pfch_discard &= ~PREFTCHRC_BUF_PIN_INCREASED;
+					  } else {
+							  /*  shuffle all following the evictee to the left
+							  **  and update next pointer if its element moves
+							  */
+							  pfchy = (scan->pfch_used - 1); /*  current rightmost */
+							  scan->pfch_used = pfchy;
+
+							  while (pfchy > pfchx) {
+								  pfchz = pfchx + 1;
+								  BlockIdCopy((&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)), (&(((scan->pfch_block_item_list)+pfchz)->pfch_blockid)));
+								  ((scan->pfch_block_item_list)+pfchx)->pfch_discard = ((scan->pfch_block_item_list)+pfchz)->pfch_discard;
+								  if (scan->pfch_next == pfchz) {
+									  scan->pfch_next = pfchx;
+								  }
+								  pfchx = pfchz; /* advance */
+							  }
+                      }
+                }
+            }
+        }
+}
+#endif /* USE_PREFETCH */
+
 /* ----------------------------------------------------------------
  *					macros used in index_ routines
  *
@@ -253,6 +302,11 @@ index_beginscan(Relation heapRelation,
 	 */
 	scan->heapRelation = heapRelation;
 	scan->xs_snapshot = snapshot;
+#ifdef USE_PREFETCH
+	scan->do_prefetch = 0;             /*  no prefetching by default  */
+        scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+	scan->pfch_block_item_list = (struct pfch_block_item*)0;
+#endif   /* USE_PREFETCH */
 
 	return scan;
 }
@@ -277,6 +331,11 @@ index_beginscan_bitmap(Relation indexRel
 	 * up by RelationGetIndexScan.
 	 */
 	scan->xs_snapshot = snapshot;
+#ifdef USE_PREFETCH
+	scan->do_prefetch = 0;             /*  no prefetching by default  */
+        scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+	scan->pfch_block_item_list = (struct pfch_block_item*)0;
+#endif   /* USE_PREFETCH */
 
 	return scan;
 }
@@ -311,6 +370,9 @@ index_beginscan_internal(Relation indexR
 									  Int32GetDatum(nkeys),
 									  Int32GetDatum(norderbys)));
 
+	scan->heap_tids_seen = 0;
+	scan->heap_tids_fetched = 0;
+	
 	return scan;
 }
 
@@ -342,6 +404,12 @@ index_rescan(IndexScanDesc scan,
 	/* Release any held pin on a heap page */
 	if (BufferIsValid(scan->xs_cbuf))
 	{
+#ifdef USE_PREFETCH
+                /*   if specified block number is present in the prefetch array,  then evict it */
+                if (scan->do_prefetch) {
+                      index_mark_or_evict_block(scan , BlocknumOfBuffer(scan->xs_cbuf) , 0);
+                }
+#endif   /* USE_PREFETCH */
 		ReleaseBuffer(scan->xs_cbuf);
 		scan->xs_cbuf = InvalidBuffer;
 	}
@@ -373,10 +441,30 @@ index_endscan(IndexScanDesc scan)
 	/* Release any held pin on a heap page */
 	if (BufferIsValid(scan->xs_cbuf))
 	{
+#ifdef USE_PREFETCH
+                /*   if specified block number is present in the prefetch array,  then evict it */
+                if (scan->do_prefetch) {
+                      index_mark_or_evict_block(scan , BlocknumOfBuffer(scan->xs_cbuf) , 0);
+                }
+#endif   /* USE_PREFETCH */
 		ReleaseBuffer(scan->xs_cbuf);
 		scan->xs_cbuf = InvalidBuffer;
 	}
 
+#ifdef USE_PREFETCH
+        /*   discard prefetched but unread buffers */
+        if (    scan->do_prefetch
+             && ((struct pfch_block_item*)0 != scan->pfch_block_item_list)
+           ) {
+            unsigned short int pfchx; /*  index in BlockIdData array   */
+                for (pfchx = 0; pfchx < scan->pfch_used; pfchx++) {
+                  if (((scan->pfch_block_item_list)+pfchx)->pfch_discard) {
+                      DiscardBuffer(scan->heapRelation, MAIN_FORKNUM, BlockIdGetBlockNumber(&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)));
+                  }
+                }
+        }
+#endif   /* USE_PREFETCH */
+
 	/* End the AM's scan */
 	FunctionCall1(procedure, PointerGetDatum(scan));
 
@@ -472,6 +560,12 @@ index_getnext_tid(IndexScanDesc scan, Sc
 		/* ... but first, release any held pin on a heap page */
 		if (BufferIsValid(scan->xs_cbuf))
 		{
+#ifdef USE_PREFETCH
+                    /*   if specified block number is present in the prefetch array,  then evict it */
+                    if (scan->do_prefetch) {
+                          index_mark_or_evict_block(scan , BlocknumOfBuffer(scan->xs_cbuf) , 0);
+                    }
+#endif   /* USE_PREFETCH */
 			ReleaseBuffer(scan->xs_cbuf);
 			scan->xs_cbuf = InvalidBuffer;
 		}
@@ -479,6 +573,11 @@ index_getnext_tid(IndexScanDesc scan, Sc
 	}
 
 	pgstat_count_index_tuples(scan->indexRelation, 1);
+	if (scan->heap_tids_seen++ >= (~0)) {
+		/* Avoid integer overflow */
+		scan->heap_tids_seen = 1;
+		scan->heap_tids_fetched = 0;
+	}
 
 	/* Return the TID of the tuple we found. */
 	return &scan->xs_ctup.t_self;
@@ -502,6 +601,10 @@ index_getnext_tid(IndexScanDesc scan, Sc
  * enough information to do it efficiently in the general case.
  * ----------------
  */
+#if defined(USE_PREFETCH) && defined(AVOID_CATALOG_MIGRATION_FOR_ASYNCIO)
+extern Datum btpeeknexttuple(IndexScanDesc scan);
+#endif /* USE_PREFETCH */
+
 HeapTuple
 index_fetch_heap(IndexScanDesc scan)
 {
@@ -509,16 +612,105 @@ index_fetch_heap(IndexScanDesc scan)
 	bool		all_dead = false;
 	bool		got_heap_tuple;
 
+
+
 	/* We can skip the buffer-switching logic if we're in mid-HOT chain. */
 	if (!scan->xs_continue_hot)
 	{
 		/* Switch to correct buffer if we don't have it already */
 		Buffer		prev_buf = scan->xs_cbuf;
 
+#ifdef USE_PREFETCH
+
+                /*   If the old block is different from new block, then evict old
+                **   block from prefetched array.   It is arguable we should leave it
+                **   in the array because it's likely to remain in the buffer pool
+                **   for a while,  but in that case , if we encounter the block
+                **   again,  prefetching it again does no harm.
+                **   (and note that,  if it's not pinned,  prefetching it will try to
+                **   pin it since prefetch tries to bank a pin for a buffer in the buffer pool).
+                **   therefore it should usually win.
+                */
+                if (    scan->do_prefetch
+                     && ( BufferIsValid(prev_buf) )
+                     && (BlocknotinBuffer(prev_buf,scan->heapRelation,ItemPointerGetBlockNumber(tid)))
+                     && (scan->pfch_next < prefetch_index_scans)  /* ensure there is an entry */
+                        ) {
+                          index_mark_or_evict_block(scan , BlocknumOfBuffer(prev_buf) , 0);
+                }
+
+#endif   /* USE_PREFETCH  */
 		scan->xs_cbuf = ReleaseAndReadBuffer(scan->xs_cbuf,
 											 scan->heapRelation,
 											 ItemPointerGetBlockNumber(tid));
 
+                /*   If the new block had been prefetched and pinned,
+                **   then mark that it no longer requires to be discarded.
+                **   Of course,  we don't evict the entry,
+                **   because we want to remember that it was recently prefetched.
+                */
+                index_mark_or_evict_block(scan , BlocknumOfBuffer(prev_buf) , 1);
+
+				scan->heap_tids_fetched++;
+
+#ifdef USE_PREFETCH
+                /*  try prefetching next data block
+                **    (next meaning one containing TIDs from matching keys
+                **     in same index page and different from any block
+                **     we previously prefetched and listed in prefetched array)
+                */
+                {
+                    FmgrInfo   *procedure;
+                    bool	found;             /*  did we find the "next" heap tid in current index page */
+                    int         PrefetchBufferRc;  /*  indicates whether requested prefetch block already in a buffer and if pin count on buffer has been increased */
+
+                    if (scan->do_prefetch) {
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+                        procedure = &scan->indexRelation->rd_aminfo->ampeeknexttuple; /* is incorrect but avoids adding function to catalog */
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                        GET_SCAN_PROCEDURE(ampeeknexttuple); /* is correct but requires adding function to catalog */
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+
+                        if (    procedure          /* does the index access method support peektuple? */
+#ifndef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+                             && procedure->fn_addr /* procedure->fn_addr is non-null only if in catalog */
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                           ) {
+                            int iterations = 1;      /*  how many iterations of prefetching shall we try  -
+                                                     **  if used entries in prefetch list is < target_prefetch_pages
+                                                     **  then 2,  else 1
+                                                     **  this should result in gradually and smoothly increasing up to target_prefetch_pages
+                                                     */
+                            /*  note we trust InitIndexScan verified this scan is forwards only and so set that */
+                            if (scan->pfch_used < target_prefetch_pages) {
+                                iterations = 2;
+                            }
+                            do {
+                                found =  DatumGetBool(
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+                                                         btpeeknexttuple(scan)     /*  pass scan as direct parameter since cant use fmgr because not in catalog */
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                                                         FunctionCall1(procedure, PointerGetDatum(scan)) /* use fmgr to call it because in catalog  */
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                                                     );
+                                if (found) {
+                                    /*    btpeeknexttuple set pfch_next to point to the item in block_item_list to be prefetched */
+                                    PrefetchBufferRc = PrefetchBuffer(scan->heapRelation, MAIN_FORKNUM, BlockIdGetBlockNumber((&((scan->pfch_block_item_list + scan->pfch_next))->pfch_blockid)) , 0);
+                                    /* elog(LOG,"index_fetch_heap prefetched rel %u blockNum %u"
+                                       ,scan->heapRelation->rd_node.relNode ,BlockIdGetBlockNumber(scan->pfch_block_item_list + scan->pfch_next));
+                                    */
+
+                                    /*  if pin acquired on buffer,  then remember in case of future Discard */
+                                    (scan->pfch_block_item_list + scan->pfch_next)->pfch_discard = (PrefetchBufferRc & PREFTCHRC_BUF_PIN_INCREASED);
+
+
+                                }
+                            } while (--iterations > 0);
+                        }
+                    }
+                }
+#endif   /* USE_PREFETCH */
+
 		/*
 		 * Prune page, but only if we weren't already on this page
 		 */
--- src/backend/access/index/genam.c.orig	2014-05-28 08:29:09.242829343 -0400
+++ src/backend/access/index/genam.c	2014-05-28 16:45:43.322508927 -0400
@@ -77,6 +77,12 @@ RelationGetIndexScan(Relation indexRelat
 
 	scan = (IndexScanDesc) palloc(sizeof(IndexScanDescData));
 
+#ifdef USE_PREFETCH
+        scan->do_prefetch = 0;             /*  no prefetching by default  */
+        scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+        scan->pfch_block_item_list = (struct pfch_block_item*)0;
+#endif   /* USE_PREFETCH */
+
 	scan->heapRelation = NULL;	/* may be set later */
 	scan->indexRelation = indexRelation;
 	scan->xs_snapshot = InvalidSnapshot;		/* caller must initialize this */
@@ -139,6 +145,19 @@ RelationGetIndexScan(Relation indexRelat
 void
 IndexScanEnd(IndexScanDesc scan)
 {
+#ifdef USE_PREFETCH
+	if (scan->do_prefetch) {
+		if ( (struct pfch_block_item*)0 != scan->pfch_block_item_list ) {
+			pfree(scan->pfch_block_item_list);
+			scan->pfch_block_item_list = (struct pfch_block_item*)0;
+		}
+		if ( (struct pfch_index_pagelist*)0 != scan->pfch_index_page_list ) {
+			pfree(scan->pfch_index_page_list);
+			scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+		}
+	}
+#endif   /* USE_PREFETCH */
+
 	if (scan->keyData != NULL)
 		pfree(scan->keyData);
 	if (scan->orderByData != NULL)
--- src/backend/access/nbtree/nbtsearch.c.orig	2014-05-28 08:29:09.242829343 -0400
+++ src/backend/access/nbtree/nbtsearch.c	2014-05-28 16:45:43.350509042 -0400
@@ -23,13 +23,16 @@
 #include "utils/lsyscache.h"
 #include "utils/rel.h"
 
+extern unsigned int prefetch_btree_heaps;  /*  boolean whether to prefetch heap pages in _bt_next for non-bitmap index scans   */
+extern unsigned int prefetch_sequential_index_scans;  /*  boolean whether to prefetch sequential-access non-bitmap index scans  */
 
 static bool _bt_readpage(IndexScanDesc scan, ScanDirection dir,
 			 OffsetNumber offnum);
 static void _bt_saveitem(BTScanOpaque so, int itemIndex,
 			 OffsetNumber offnum, IndexTuple itup);
-static bool _bt_steppage(IndexScanDesc scan, ScanDirection dir);
-static Buffer _bt_walk_left(Relation rel, Buffer buf);
+static bool _bt_steppage(IndexScanDesc scan, ScanDirection dir, 
+			 bool prefetch);
+static Buffer _bt_walk_left(IndexScanDesc scan, Relation rel, Buffer buf);
 static bool _bt_endpoint(IndexScanDesc scan, ScanDirection dir);
 
 
@@ -226,7 +229,7 @@ _bt_moveright(Relation rel,
 				_bt_relbuf(rel, buf);
 
 			/* re-acquire the lock in the right mode, and re-check */
-			buf = _bt_getbuf(rel, blkno, access);
+			buf = _bt_getbuf(rel, blkno, access , (struct pfch_index_pagelist*)0);
 			continue;
 		}
 
@@ -1005,7 +1008,7 @@ _bt_first(IndexScanDesc scan, ScanDirect
 		 * There's no actually-matching data on this page.  Try to advance to
 		 * the next page.  Return false if there's no matching data at all.
 		 */
-		if (!_bt_steppage(scan, dir))
+		if (!_bt_steppage(scan, dir, false))
 			return false;
 	}
 
@@ -1040,6 +1043,8 @@ _bt_next(IndexScanDesc scan, ScanDirecti
 {
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
 	BTScanPosItem *currItem;
+	BlockNumber prevblkno = ItemPointerGetBlockNumber(
+		&scan->xs_ctup.t_self);
 
 	/*
 	 * Advance to next tuple on current page; or if there's no more, try to
@@ -1052,11 +1057,53 @@ _bt_next(IndexScanDesc scan, ScanDirecti
 			/* We must acquire lock before applying _bt_steppage */
 			Assert(BufferIsValid(so->currPos.buf));
 			LockBuffer(so->currPos.buf, BT_READ);
-			if (!_bt_steppage(scan, dir))
+			if (!_bt_steppage(scan, dir, target_prefetch_pages > 0))
 				return false;
 			/* Drop the lock, but not pin, on the new page */
 			LockBuffer(so->currPos.buf, BUFFER_LOCK_UNLOCK);
 		}
+		
+		if (prefetch_btree_heaps && (target_prefetch_pages > 0)) {
+			BlockNumber currblkno = ItemPointerGetBlockNumber(
+				&so->currPos.items[so->currPos.itemIndex].heapTid);
+		
+			if (currblkno != prevblkno) {
+				if (so->prefetchBlockCount > 0)
+					so->prefetchBlockCount--;
+			
+				/* If we have heap fetch frequency stats, and it's above ~94%,
+				 * initiate heap prefetches */
+				if (so->currPos.moreRight
+					&& scan->heap_tids_seen > 256 
+					&& ( (scan->heap_tids_seen - scan->heap_tids_seen/16)
+						 <= scan->heap_tids_fetched ) )
+				{
+					bool nonsequential = false;
+				
+					if (so->prefetchItemIndex <= so->currPos.itemIndex)
+						so->prefetchItemIndex = so->currPos.itemIndex + 1;
+					while (    (so->prefetchItemIndex <= so->currPos.lastItem)
+							&& (so->prefetchBlockCount < target_prefetch_pages) )
+					{
+						ItemPointer tid = &so->currPos.items[so->prefetchItemIndex++].heapTid;
+						BlockNumber blkno = ItemPointerGetBlockNumber(tid);
+						if (blkno != so->lastHeapPrefetchBlkno) {  /*  if not a repetition of previous block */
+							/*   start prefetch on next page, providing :
+                            **   EITHER  .  we're reading non-sequentially previously or for this block
+                            **   OR      .  user explicitly specified to prefetch for sequential pattern
+							**   as it may be counterproductive otherwise
+                            */
+							nonsequential = (nonsequential || blkno != (so->lastHeapPrefetchBlkno+1));
+							if (prefetch_sequential_index_scans || nonsequential) {
+								_bt_prefetchbuf(scan->heapRelation, blkno , &scan->pfch_index_page_list );
+							}
+							so->lastHeapPrefetchBlkno = blkno;
+							so->prefetchBlockCount++;
+						}
+					}
+				}
+			}
+		}
 	}
 	else
 	{
@@ -1065,11 +1112,53 @@ _bt_next(IndexScanDesc scan, ScanDirecti
 			/* We must acquire lock before applying _bt_steppage */
 			Assert(BufferIsValid(so->currPos.buf));
 			LockBuffer(so->currPos.buf, BT_READ);
-			if (!_bt_steppage(scan, dir))
+			if (!_bt_steppage(scan, dir, target_prefetch_pages > 0))
 				return false;
 			/* Drop the lock, but not pin, on the new page */
 			LockBuffer(so->currPos.buf, BUFFER_LOCK_UNLOCK);
 		}
+		
+		if (prefetch_btree_heaps && (target_prefetch_pages > 0)) {
+			BlockNumber currblkno = ItemPointerGetBlockNumber(
+				&so->currPos.items[so->currPos.itemIndex].heapTid);
+		
+			if (currblkno != prevblkno) {
+				if (so->prefetchBlockCount > 0)
+					so->prefetchBlockCount--;
+			
+				/* If we have heap fetch frequency stats, and it's above ~94%,
+				 * initiate heap prefetches */
+				if (so->currPos.moreLeft
+					&& scan->heap_tids_seen > 256 
+					&& ( (scan->heap_tids_seen - scan->heap_tids_seen/16)
+						 <= scan->heap_tids_fetched ) )
+				{
+					bool nonsequential = false;
+			
+					if (so->prefetchItemIndex >= so->currPos.itemIndex)
+						so->prefetchItemIndex = so->currPos.itemIndex - 1;
+					while (    (so->prefetchItemIndex >= so->currPos.firstItem)
+							&& (so->prefetchBlockCount < target_prefetch_pages) )
+					{
+						ItemPointer tid = &so->currPos.items[so->prefetchItemIndex--].heapTid;
+						BlockNumber blkno = ItemPointerGetBlockNumber(tid);
+						if (blkno != so->lastHeapPrefetchBlkno) {  /*  if not a repetition of previous block */
+							/*   start prefetch on next page, providing :
+                            **   EITHER  .  we're reading non-sequentially previously or for this block
+                            **   OR      .  user explicitly specified to prefetch for sequential pattern
+							**   as it may be counterproductive otherwise
+                            */
+							nonsequential = (nonsequential || blkno != (so->lastHeapPrefetchBlkno+1));
+							if (prefetch_sequential_index_scans || nonsequential) {
+								_bt_prefetchbuf(scan->heapRelation, blkno , &scan->pfch_index_page_list );
+							}
+							so->lastHeapPrefetchBlkno = blkno;
+							so->prefetchBlockCount++;
+						}
+					}
+				}
+			}
+		}
 	}
 
 	/* OK, itemIndex says what to return */
@@ -1119,9 +1208,11 @@ _bt_readpage(IndexScanDesc scan, ScanDir
 	/*
 	 * we must save the page's right-link while scanning it; this tells us
 	 * where to step right to after we're done with these items.  There is no
-	 * corresponding need for the left-link, since splits always go right.
+	 * corresponding need for the left-link, since splits always go right,
+	 * but we need it for back-sequential scan detection.
 	 */
 	so->currPos.nextPage = opaque->btpo_next;
+	so->currPos.prevPage = opaque->btpo_prev;
 
 	/* initialize tuple workspace to empty */
 	so->currPos.nextTupleOffset = 0;
@@ -1156,6 +1247,7 @@ _bt_readpage(IndexScanDesc scan, ScanDir
 		so->currPos.firstItem = 0;
 		so->currPos.lastItem = itemIndex - 1;
 		so->currPos.itemIndex = 0;
+		so->prefetchItemIndex = 0;
 	}
 	else
 	{
@@ -1187,6 +1279,7 @@ _bt_readpage(IndexScanDesc scan, ScanDir
 		so->currPos.firstItem = itemIndex;
 		so->currPos.lastItem = MaxIndexTuplesPerPage - 1;
 		so->currPos.itemIndex = MaxIndexTuplesPerPage - 1;
+		so->prefetchItemIndex = MaxIndexTuplesPerPage - 1;
 	}
 
 	return (so->currPos.firstItem <= so->currPos.lastItem);
@@ -1224,7 +1317,7 @@ _bt_saveitem(BTScanOpaque so, int itemIn
  * locks and pins, set so->currPos.buf to InvalidBuffer, and return FALSE.
  */
 static bool
-_bt_steppage(IndexScanDesc scan, ScanDirection dir)
+_bt_steppage(IndexScanDesc scan, ScanDirection dir, bool prefetch)
 {
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
 	Relation	rel;
@@ -1278,7 +1371,7 @@ _bt_steppage(IndexScanDesc scan, ScanDir
 			/* check for interrupts while we're not holding any buffer lock */
 			CHECK_FOR_INTERRUPTS();
 			/* step right one page */
-			so->currPos.buf = _bt_getbuf(rel, blkno, BT_READ);
+			so->currPos.buf = _bt_getbuf(rel, blkno, BT_READ , scan->pfch_index_page_list);
 			/* check for deleted page */
 			page = BufferGetPage(so->currPos.buf);
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
@@ -1287,9 +1380,20 @@ _bt_steppage(IndexScanDesc scan, ScanDir
 				PredicateLockPage(rel, blkno, scan->xs_snapshot);
 				/* see if there are any matches on this page */
 				/* note that this will clear moreRight if we can stop */
-				if (_bt_readpage(scan, dir, P_FIRSTDATAKEY(opaque)))
+				if (_bt_readpage(scan, dir, P_FIRSTDATAKEY(opaque))) {
+					if (    prefetch && so->currPos.moreRight
+						/*   start prefetch on next page, providing :
+						**   EITHER  .  we're reading non-sequentially for this block
+						**   OR      .  user explicitly specified to prefetch for sequential pattern
+						**   as it may be counterproductive otherwise
+						*/
+						&& (prefetch_sequential_index_scans || opaque->btpo_next != (blkno+1))
+                       ) {
+ 						  _bt_prefetchbuf(rel, opaque->btpo_next , &scan->pfch_index_page_list);
+					}
 					break;
 			}
+			}
 			/* nope, keep going */
 			blkno = opaque->btpo_next;
 		}
@@ -1317,7 +1421,7 @@ _bt_steppage(IndexScanDesc scan, ScanDir
 			}
 
 			/* Step to next physical page */
-			so->currPos.buf = _bt_walk_left(rel, so->currPos.buf);
+			so->currPos.buf = _bt_walk_left(scan , rel, so->currPos.buf);
 
 			/* if we're physically at end of index, return failure */
 			if (so->currPos.buf == InvalidBuffer)
@@ -1332,14 +1436,58 @@ _bt_steppage(IndexScanDesc scan, ScanDir
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 			if (!P_IGNORE(opaque))
 			{
+				/* We must rely on the previously saved prevPage link! */
+				BlockNumber blkno = so->currPos.prevPage;
+				
 				PredicateLockPage(rel, BufferGetBlockNumber(so->currPos.buf), scan->xs_snapshot);
 				/* see if there are any matches on this page */
 				/* note that this will clear moreLeft if we can stop */
-				if (_bt_readpage(scan, dir, PageGetMaxOffsetNumber(page)))
+				if (_bt_readpage(scan, dir, PageGetMaxOffsetNumber(page))) {
+					if (prefetch && so->currPos.moreLeft) {
+						/* detect back-sequential runs and increase prefetch window blindly 
+						 * downwards 2 blocks at a time. This only works in our favor
+						 * for index-only scans, by merging read requests at the kernel,
+						 * so we want to inflate target_prefetch_pages since merged 
+						 * back-sequential requests are about as expensive as a single one 
+						 */
+						if (scan->xs_want_itup && blkno > 0 && opaque->btpo_prev == (blkno-1)) {
+							BlockNumber backPos;
+							unsigned int back_prefetch_pages = target_prefetch_pages * 16;
+							if (back_prefetch_pages > 64)
+								back_prefetch_pages = 64;
+							
+							if (so->backSeqRun == 0)
+								backPos = (blkno-1);
+							else
+								backPos = so->backSeqPos;
+							so->backSeqRun++;
+							
+							if (backPos > 0 && (blkno - backPos) <= back_prefetch_pages) {
+								_bt_prefetchbuf(rel, backPos-- , &scan->pfch_index_page_list);
+								/* don't start back-seq prefetch too early */
+								if (so->backSeqRun >= back_prefetch_pages
+										&& backPos > 0 
+										&& (blkno - backPos) <= back_prefetch_pages)
+								{
+									_bt_prefetchbuf(rel, backPos-- , &scan->pfch_index_page_list);
+								}
+							}
+							
+							so->backSeqPos = backPos;
+						} else {
+							/* start prefetch on next page */
+							if (so->backSeqRun != 0) {
+								if (opaque->btpo_prev > blkno || opaque->btpo_prev < so->backSeqPos)
+									so->backSeqRun = 0;
+							}
+							_bt_prefetchbuf(rel, opaque->btpo_prev , &scan->pfch_index_page_list);
+						}
+					}
 					break;
 			}
 		}
 	}
+	}
 
 	return true;
 }
@@ -1359,7 +1507,7 @@ _bt_steppage(IndexScanDesc scan, ScanDir
  * again if it's important.
  */
 static Buffer
-_bt_walk_left(Relation rel, Buffer buf)
+_bt_walk_left(IndexScanDesc scan, Relation rel, Buffer buf)
 {
 	Page		page;
 	BTPageOpaque opaque;
@@ -1387,7 +1535,7 @@ _bt_walk_left(Relation rel, Buffer buf)
 		_bt_relbuf(rel, buf);
 		/* check for interrupts while we're not holding any buffer lock */
 		CHECK_FOR_INTERRUPTS();
-		buf = _bt_getbuf(rel, blkno, BT_READ);
+		buf = _bt_getbuf(rel, blkno, BT_READ , scan->pfch_index_page_list );
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
@@ -1631,7 +1779,7 @@ _bt_endpoint(IndexScanDesc scan, ScanDir
 		 * There's no actually-matching data on this page.  Try to advance to
 		 * the next page.  Return false if there's no matching data at all.
 		 */
-		if (!_bt_steppage(scan, dir))
+		if (!_bt_steppage(scan, dir, false))
 			return false;
 	}
 
--- src/backend/access/nbtree/nbtinsert.c.orig	2014-05-28 08:29:09.242829343 -0400
+++ src/backend/access/nbtree/nbtinsert.c	2014-05-28 16:45:43.394509218 -0400
@@ -793,7 +793,7 @@ _bt_insertonpg(Relation rel,
 		{
 			Assert(!P_ISLEAF(lpageop));
 
-			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE , (struct pfch_index_pagelist*)0);
 			metapg = BufferGetPage(metabuf);
 			metad = BTPageGetMeta(metapg);
 
@@ -972,7 +972,7 @@ _bt_split(Relation rel, Buffer buf, Buff
 	bool		isleaf;
 
 	/* Acquire a new page to split into */
-	rbuf = _bt_getbuf(rel, P_NEW, BT_WRITE);
+	rbuf = _bt_getbuf(rel, P_NEW, BT_WRITE , (struct pfch_index_pagelist*)0);
 
 	/*
 	 * origpage is the original page to be split.  leftpage is a temporary
@@ -1175,7 +1175,7 @@ _bt_split(Relation rel, Buffer buf, Buff
 
 	if (!P_RIGHTMOST(oopaque))
 	{
-		sbuf = _bt_getbuf(rel, oopaque->btpo_next, BT_WRITE);
+		sbuf = _bt_getbuf(rel, oopaque->btpo_next, BT_WRITE , (struct pfch_index_pagelist*)0);
 		spage = BufferGetPage(sbuf);
 		sopaque = (BTPageOpaque) PageGetSpecialPointer(spage);
 		if (sopaque->btpo_prev != origpagenumber)
@@ -1817,7 +1817,7 @@ _bt_finish_split(Relation rel, Buffer lb
 	Assert(P_INCOMPLETE_SPLIT(lpageop));
 
 	/* Lock right sibling, the one missing the downlink */
-	rbuf = _bt_getbuf(rel, lpageop->btpo_next, BT_WRITE);
+	rbuf = _bt_getbuf(rel, lpageop->btpo_next, BT_WRITE , (struct pfch_index_pagelist*)0);
 	rpage = BufferGetPage(rbuf);
 	rpageop = (BTPageOpaque) PageGetSpecialPointer(rpage);
 
@@ -1829,7 +1829,7 @@ _bt_finish_split(Relation rel, Buffer lb
 		BTMetaPageData *metad;
 
 		/* acquire lock on the metapage */
-		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE , (struct pfch_index_pagelist*)0);
 		metapg = BufferGetPage(metabuf);
 		metad = BTPageGetMeta(metapg);
 
@@ -1877,7 +1877,7 @@ _bt_getstackbuf(Relation rel, BTStack st
 		Page		page;
 		BTPageOpaque opaque;
 
-		buf = _bt_getbuf(rel, blkno, access);
+		buf = _bt_getbuf(rel, blkno, access , (struct pfch_index_pagelist*)0);
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
@@ -2008,12 +2008,12 @@ _bt_newroot(Relation rel, Buffer lbuf, B
 	lopaque = (BTPageOpaque) PageGetSpecialPointer(lpage);
 
 	/* get a new root page */
-	rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE);
+	rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE , (struct pfch_index_pagelist*)0);
 	rootpage = BufferGetPage(rootbuf);
 	rootblknum = BufferGetBlockNumber(rootbuf);
 
 	/* acquire lock on the metapage */
-	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE , (struct pfch_index_pagelist*)0);
 	metapg = BufferGetPage(metabuf);
 	metad = BTPageGetMeta(metapg);
 
--- src/backend/access/nbtree/nbtpage.c.orig	2014-05-28 08:29:09.242829343 -0400
+++ src/backend/access/nbtree/nbtpage.c	2014-05-28 16:45:43.426509347 -0400
@@ -127,7 +127,7 @@ _bt_getroot(Relation rel, int access)
 		Assert(rootblkno != P_NONE);
 		rootlevel = metad->btm_fastlevel;
 
-		rootbuf = _bt_getbuf(rel, rootblkno, BT_READ);
+		rootbuf = _bt_getbuf(rel, rootblkno, BT_READ , (struct pfch_index_pagelist*)0);
 		rootpage = BufferGetPage(rootbuf);
 		rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);
 
@@ -153,7 +153,7 @@ _bt_getroot(Relation rel, int access)
 		rel->rd_amcache = NULL;
 	}
 
-	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
+	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ , (struct pfch_index_pagelist*)0);
 	metapg = BufferGetPage(metabuf);
 	metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
 	metad = BTPageGetMeta(metapg);
@@ -209,7 +209,7 @@ _bt_getroot(Relation rel, int access)
 		 * the new root page.  Since this is the first page in the tree, it's
 		 * a leaf as well as the root.
 		 */
-		rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE);
+		rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE , (struct pfch_index_pagelist*)0);
 		rootblkno = BufferGetBlockNumber(rootbuf);
 		rootpage = BufferGetPage(rootbuf);
 		rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);
@@ -350,7 +350,7 @@ _bt_gettrueroot(Relation rel)
 		pfree(rel->rd_amcache);
 	rel->rd_amcache = NULL;
 
-	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
+	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ , (struct pfch_index_pagelist*)0);
 	metapg = BufferGetPage(metabuf);
 	metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
 	metad = BTPageGetMeta(metapg);
@@ -436,7 +436,7 @@ _bt_getrootheight(Relation rel)
 		Page		metapg;
 		BTPageOpaque metaopaque;
 
-		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
+		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ , (struct pfch_index_pagelist*)0);
 		metapg = BufferGetPage(metabuf);
 		metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
 		metad = BTPageGetMeta(metapg);
@@ -562,6 +562,170 @@ _bt_log_reuse_page(Relation rel, BlockNu
 }
 
 /*
+ *	_bt_prefetchbuf() -- Prefetch a buffer by block number
+ *                           and keep track of prefetched and unread blocknums in pagelist.
+ *   input parms  :
+ *       rel and blockno identify block to be prefetched as usual
+ *       pfch_index_page_list_P points to the pointer anchoring the head of the index page list
+ *             Since the pagelist is a kind of optimization,
+ *             handle palloc failure by quietly omitting the keeping track.
+ */
+void
+_bt_prefetchbuf(Relation rel, BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P)
+{
+
+    int rc = 0;
+    struct pfch_index_pagelist* pfch_index_plp;      /*  pointer to current chunk */
+    struct pfch_index_item* found_item = 0;
+
+		if ((struct pfch_index_pagelist**)0 == pfch_index_page_list_P) {
+			pfch_index_plp = (struct pfch_index_pagelist*)0;
+		} else {
+			pfch_index_plp = *pfch_index_page_list_P;
+		}
+
+    	if (blkno != P_NEW && blkno != P_NONE)
+    	{
+            /* prefetch an existing block of the relation
+            ** but first,  check it has not recently already been prefetched and not yet read
+            */
+            found_item = _bt_find_block(blkno , pfch_index_plp);
+			if ((struct pfch_index_item*)0 == found_item) {  /*  not found */
+
+		        rc = PrefetchBuffer(rel, MAIN_FORKNUM, blkno , 0);
+
+                /*  add the pagenum to the list ,  indicating its discard status
+                **  since it's only an optimization,  ignore failure such as exceeded allowed space
+				*/
+                _bt_add_block( blkno , pfch_index_page_list_P , (uint32)(rc & PREFTCHRC_BUF_PIN_INCREASED));
+
+            }
+	    }
+        return;
+}
+
+/*   _bt_find_block finds the item referencing specified Block in index page list if present
+**   and returns the pointer to the pfch_index_item if found,  or null if not
+*/
+struct pfch_index_item*
+_bt_find_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list)
+{
+
+    struct pfch_index_item* found_item = 0;
+    struct pfch_index_pagelist* pfch_index_plp;      /*  pointer to current chunk */
+    int ix, tx;
+
+		pfch_index_plp = pfch_index_page_list;
+
+		while (     (struct pfch_index_pagelist*)0 != pfch_index_plp
+                &&  ( (struct pfch_index_item*)0 == found_item)
+              ) {
+			ix = 0;
+			tx = pfch_index_plp->pfch_index_item_count;
+			while (     (ix < tx)
+                    &&  ( (struct pfch_index_item*)0 == found_item)
+                  ) {
+				if (pfch_index_plp->pfch_indexid[ix].pfch_blocknum == blkno) {
+					found_item = &pfch_index_plp->pfch_indexid[ix];
+				}
+                ix++;
+			}
+			pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+		}
+
+     return found_item;
+}
+
+/*   _bt_add_block adds the specified Block to the index page list
+**   and returns 0 if successful,  non-zero if not
+*/
+int
+_bt_add_block(BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P , uint32 discard_status)
+{
+    int rc = 1;
+    int ix;
+    struct pfch_index_pagelist* pfch_index_plp;      /*  pointer to current chunk */
+    struct pfch_index_pagelist* pfch_index_page_list_anchor; /*  pointer to first chunk if any  */
+	/*  allow expansion of pagelist to 16 chunks
+	**  which accommodates backwards-sequential index scans
+	**  where the scanner increases target_prefetch_pages by a factor of up to 16
+	**   see code in _bt_steppage
+	**  note - this creates an undesirable weak dependency on this number in _bt_steppage,
+	**         but :
+	**           there is no disaster if the numbers disagree  -  just sub-optimal use of the list
+	**           to implement a proper interface would require that chunks have a variable size
+	**           which would require an extra size variable in each chunk
+	*/
+	int num_chunks = 16;
+
+		if ((struct pfch_index_pagelist**)0 == pfch_index_page_list_P) {
+			pfch_index_page_list_anchor = (struct pfch_index_pagelist*)0;
+		} else {
+			pfch_index_page_list_anchor = *pfch_index_page_list_P;
+		}
+		pfch_index_plp = pfch_index_page_list_anchor;       /* pointer to current chunk */
+
+		while ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+			ix = pfch_index_plp->pfch_index_item_count;
+			if (ix < target_prefetch_pages) {
+				pfch_index_plp->pfch_indexid[ix].pfch_blocknum = blkno;
+				pfch_index_plp->pfch_indexid[ix].pfch_discard = discard_status;
+				pfch_index_plp->pfch_index_item_count = (ix+1);
+                rc = 0;
+				goto stored_pagenum;
+			}
+			pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+			num_chunks--;              /*  keep track of number of chunks */
+		}
+
+		/*   we did not find any free space in existing chunks -
+		**   create new chunk if within our limit and we have a pfch_index_page_list
+		*/
+		if ( (num_chunks > 0) && ((struct pfch_index_pagelist*)0 != pfch_index_page_list_anchor) ) {
+			pfch_index_plp = (struct pfch_index_pagelist*)palloc( sizeof(struct pfch_index_pagelist) + ( (target_prefetch_pages-1) * sizeof(struct pfch_index_item) ) );
+			if ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+				pfch_index_plp->pfch_index_pagelist_next = pfch_index_page_list_anchor;  /* old head of list is next after this */
+				pfch_index_plp->pfch_indexid[0].pfch_blocknum = blkno;
+				pfch_index_plp->pfch_indexid[0].pfch_discard = discard_status;
+				pfch_index_plp->pfch_index_item_count = 1;
+				pfch_index_page_list_P = &pfch_index_plp;   /*  new head of list is new chunk */
+                rc = 0;
+			}
+		}
+
+    stored_pagenum:;
+     return rc;
+}
+
+/*  _bt_subtract_block removes a block from the prefetched-but-unread pagelist if present */
+void
+_bt_subtract_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list)
+{
+    struct pfch_index_pagelist* pfch_index_plp = pfch_index_page_list;
+	if ( (blkno != P_NEW) && (blkno != P_NONE) ) {
+            int ix , jx;
+                while ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+                    ix = pfch_index_plp->pfch_index_item_count;
+                    while (ix-- > 0) {
+                        if (pfch_index_plp->pfch_indexid[ix].pfch_blocknum == blkno) {
+                            /*   move the last item to the curent (now deleted) position and decrement count */
+                            jx = (pfch_index_plp->pfch_index_item_count-1); /*  index of last item ... */
+                            if (jx > ix) {                                  /*  ... is not the current one so move is required */
+                                pfch_index_plp->pfch_indexid[ix].pfch_blocknum = pfch_index_plp->pfch_indexid[jx].pfch_blocknum;
+                                pfch_index_plp->pfch_indexid[ix].pfch_discard = pfch_index_plp->pfch_indexid[jx].pfch_discard;
+                                ix = jx;
+                            }
+                            pfch_index_plp->pfch_index_item_count = ix;
+                            goto done_subtract;
+                        }
+                    }
+                    pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+                }
+        }
+    done_subtract:  return;
+}
+
+/*
  *	_bt_getbuf() -- Get a buffer by block number for read or write.
  *
  *		blkno == P_NEW means to get an unallocated index page.  The page
@@ -573,7 +737,7 @@ _bt_log_reuse_page(Relation rel, BlockNu
  *		_bt_checkpage to sanity-check the page (except in P_NEW case).
  */
 Buffer
-_bt_getbuf(Relation rel, BlockNumber blkno, int access)
+_bt_getbuf(Relation rel, BlockNumber blkno, int access , struct pfch_index_pagelist* pfch_index_page_list)
 {
 	Buffer		buf;
 
@@ -581,6 +745,10 @@ _bt_getbuf(Relation rel, BlockNumber blk
 	{
 		/* Read an existing block of the relation */
 		buf = ReadBuffer(rel, blkno);
+
+        /*  if the block is in the prefetched-but-unread pagelist,  remove it */
+        _bt_subtract_block( blkno , pfch_index_page_list);
+
 		LockBuffer(buf, access);
 		_bt_checkpage(rel, buf);
 	}
@@ -702,6 +870,10 @@ _bt_getbuf(Relation rel, BlockNumber blk
  * bufmgr when one would do.  However, now it's mainly just a notational
  * convenience.  The only case where it saves work over _bt_relbuf/_bt_getbuf
  * is when the target page is the same one already in the buffer.
+ *
+ * if prefetching of index pages is changed to use this function,
+ * then it should be extended to take the index_page_list as parameter
+ * and call_bt_subtract_block in the same way that _bt_getbuf does.
  */
 Buffer
 _bt_relandgetbuf(Relation rel, Buffer obuf, BlockNumber blkno, int access)
@@ -712,6 +884,7 @@ _bt_relandgetbuf(Relation rel, Buffer ob
 	if (BufferIsValid(obuf))
 		LockBuffer(obuf, BUFFER_LOCK_UNLOCK);
 	buf = ReleaseAndReadBuffer(obuf, rel, blkno);
+
 	LockBuffer(buf, access);
 	_bt_checkpage(rel, buf);
 	return buf;
@@ -965,7 +1138,7 @@ _bt_is_page_halfdead(Relation rel, Block
 	BTPageOpaque opaque;
 	bool		result;
 
-	buf = _bt_getbuf(rel, blk, BT_READ);
+	buf = _bt_getbuf(rel, blk, BT_READ , (struct pfch_index_pagelist*)0);
 	page = BufferGetPage(buf);
 	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
@@ -1069,7 +1242,7 @@ _bt_lock_branch_parent(Relation rel, Blo
 				Page		lpage;
 				BTPageOpaque lopaque;
 
-				lbuf = _bt_getbuf(rel, leftsib, BT_READ);
+				lbuf = _bt_getbuf(rel, leftsib, BT_READ, (struct pfch_index_pagelist*)0);
 				lpage = BufferGetPage(lbuf);
 				lopaque = (BTPageOpaque) PageGetSpecialPointer(lpage);
 
@@ -1265,7 +1438,7 @@ _bt_pagedel(Relation rel, Buffer buf)
 					BTPageOpaque lopaque;
 					Page		lpage;
 
-					lbuf = _bt_getbuf(rel, leftsib, BT_READ);
+					lbuf = _bt_getbuf(rel, leftsib, BT_READ, (struct pfch_index_pagelist*)0);
 					lpage = BufferGetPage(lbuf);
 					lopaque = (BTPageOpaque) PageGetSpecialPointer(lpage);
 					/*
@@ -1340,7 +1513,7 @@ _bt_pagedel(Relation rel, Buffer buf)
 		if (!rightsib_empty)
 			break;
 
-		buf = _bt_getbuf(rel, rightsib, BT_WRITE);
+		buf = _bt_getbuf(rel, rightsib, BT_WRITE, (struct pfch_index_pagelist*)0);
 	}
 
 	return ndeleted;
@@ -1593,7 +1766,7 @@ _bt_unlink_halfdead_page(Relation rel, B
 		target = topblkno;
 
 		/* fetch the block number of the topmost parent's left sibling */
-		buf = _bt_getbuf(rel, topblkno, BT_READ);
+		buf = _bt_getbuf(rel, topblkno, BT_READ, (struct pfch_index_pagelist*)0);
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		leftsib = opaque->btpo_prev;
@@ -1632,7 +1805,7 @@ _bt_unlink_halfdead_page(Relation rel, B
 		LockBuffer(leafbuf, BT_WRITE);
 	if (leftsib != P_NONE)
 	{
-		lbuf = _bt_getbuf(rel, leftsib, BT_WRITE);
+		lbuf = _bt_getbuf(rel, leftsib, BT_WRITE , (struct pfch_index_pagelist*)0);
 		page = BufferGetPage(lbuf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		while (P_ISDELETED(opaque) || opaque->btpo_next != target)
@@ -1646,7 +1819,7 @@ _bt_unlink_halfdead_page(Relation rel, B
 					 RelationGetRelationName(rel));
 				return false;
 			}
-			lbuf = _bt_getbuf(rel, leftsib, BT_WRITE);
+			lbuf = _bt_getbuf(rel, leftsib, BT_WRITE , (struct pfch_index_pagelist*)0);
 			page = BufferGetPage(lbuf);
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		}
@@ -1701,7 +1874,7 @@ _bt_unlink_halfdead_page(Relation rel, B
 	 * And next write-lock the (current) right sibling.
 	 */
 	rightsib = opaque->btpo_next;
-	rbuf = _bt_getbuf(rel, rightsib, BT_WRITE);
+	rbuf = _bt_getbuf(rel, rightsib, BT_WRITE , (struct pfch_index_pagelist*)0);
 	page = BufferGetPage(rbuf);
 	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 	if (opaque->btpo_prev != target)
@@ -1731,7 +1904,7 @@ _bt_unlink_halfdead_page(Relation rel, B
 		if (P_RIGHTMOST(opaque))
 		{
 			/* rightsib will be the only one left on the level */
-			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE , (struct pfch_index_pagelist*)0);
 			metapg = BufferGetPage(metabuf);
 			metad = BTPageGetMeta(metapg);
 
--- src/backend/access/nbtree/nbtree.c.orig	2014-05-28 08:29:09.242829343 -0400
+++ src/backend/access/nbtree/nbtree.c	2014-05-28 16:45:43.450509443 -0400
@@ -30,6 +30,18 @@
 #include "tcop/tcopprot.h"
 #include "utils/memutils.h"
 
+#ifdef USE_PREFETCH
+extern unsigned int prefetch_index_scans; /* boolean whether to prefetch non-bitmap index scans also numeric size of pfch_list */
+#endif   /* USE_PREFETCH */
+
+Datum
+btpeeknexttuple(
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+        IndexScanDesc scan
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+        PG_FUNCTION_ARGS
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+);
 
 /* Working state for btbuild and its callback */
 typedef struct
@@ -332,6 +344,74 @@ btgettuple(PG_FUNCTION_ARGS)
 }
 
 /*
+ *	btpeeknexttuple() -- peek at the next tuple different from any blocknum in pfch_block_item_list
+ *                           without reading a new index page
+ *                       and without causing any side-effects such as altering values in control blocks
+ *               if found,     store blocknum in next element of pfch_block_item_list
+ */
+Datum
+btpeeknexttuple(
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+        IndexScanDesc scan
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+        PG_FUNCTION_ARGS
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+)
+{
+#ifndef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+    IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+	BTScanOpaque so = (BTScanOpaque) scan->opaque;
+	bool		res = false;
+	int		itemIndex;		/* current index in items[] */
+
+        /*
+         * If we've already initialized this scan, we can just advance it in
+         * the appropriate direction.  If we haven't done so yet, bail out
+         */
+        if ( BTScanPosIsValid(so->currPos) ) {
+
+            itemIndex = so->currPos.itemIndex+1;    /*   next item */
+
+            /* This loop handles advancing till we find different data block or end of index page */
+            while (itemIndex <= so->currPos.lastItem) {
+                unsigned short int pfchx; /*  index in BlockIdData array   */
+                for (pfchx = 0; pfchx < scan->pfch_used; pfchx++) {
+                        if (BlockIdEquals((&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)) , &(so->currPos.items[itemIndex].heapTid.ip_blkid))) {
+                             goto block_match;
+                        }
+                }
+
+                /*  if we reach here,  no block in list matched this item  */
+                res = true;
+                /*   set item in prefetch list
+                **   prefer unused entry if there is one,  else overwrite
+                */
+                if (scan->pfch_used < prefetch_index_scans) {
+                    scan->pfch_next = scan->pfch_used;
+                } else {
+                    scan->pfch_next++;
+                    if (scan->pfch_next >= prefetch_index_scans) {
+                        scan->pfch_next = 0;
+                    }
+                }
+
+                BlockIdCopy((&((scan->pfch_block_item_list + scan->pfch_next)->pfch_blockid)) , &(so->currPos.items[itemIndex].heapTid.ip_blkid));
+                if (scan->pfch_used <= scan->pfch_next) {
+                     scan->pfch_used = (scan->pfch_next + 1);
+                }
+
+                goto peek_complete;
+
+      block_match:         itemIndex++;
+            }
+	}
+
+ peek_complete:
+	PG_RETURN_BOOL(res);
+}
+
+/*
  * btgetbitmap() -- gets all matching tuples, and adds them to a bitmap
  */
 Datum
@@ -425,6 +505,12 @@ btbeginscan(PG_FUNCTION_ARGS)
 	so->killedItems = NULL;		/* until needed */
 	so->numKilled = 0;
 
+	so->backSeqRun = 0;
+	so->backSeqPos = 0;
+	so->prefetchItemIndex = 0;
+	so->lastHeapPrefetchBlkno = P_NONE;
+	so->prefetchBlockCount = 0;
+	
 	/*
 	 * We don't know yet whether the scan will be index-only, so we do not
 	 * allocate the tuple workspace arrays until btrescan.  However, we set up
@@ -516,6 +602,23 @@ btendscan(PG_FUNCTION_ARGS)
 {
 	IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
+        struct pfch_index_pagelist* pfch_index_plp;
+        int ix;
+
+#ifdef USE_PREFETCH
+
+	/* discard all prefetched but unread index pages listed in the pagelist */
+        pfch_index_plp = scan->pfch_index_page_list;
+        while ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+                    ix = pfch_index_plp->pfch_index_item_count;
+                    while (ix-- > 0) {
+                        if (pfch_index_plp->pfch_indexid[ix].pfch_discard) {
+                            DiscardBuffer( scan->indexRelation , MAIN_FORKNUM , pfch_index_plp->pfch_indexid[ix].pfch_blocknum);
+                        }
+                    }
+                    pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+        }
+#endif /* USE_PREFETCH */
 
 	/* we aren't holding any read locks, but gotta drop the pins */
 	if (BTScanPosIsValid(so->currPos))
--- src/backend/nodes/tidbitmap.c.orig	2014-05-28 08:29:09.278829325 -0400
+++ src/backend/nodes/tidbitmap.c	2014-05-28 16:45:43.474509540 -0400
@@ -44,6 +44,9 @@
 #include "nodes/bitmapset.h"
 #include "nodes/tidbitmap.h"
 #include "utils/hsearch.h"
+#ifdef USE_PREFETCH
+extern int	target_prefetch_pages;
+#endif   /* USE_PREFETCH */
 
 /*
  * The maximum number of tuples per page is not large (typically 256 with
@@ -572,7 +575,12 @@ tbm_begin_iterate(TIDBitmap *tbm)
 	 * needs of the TBMIterateResult sub-struct.
 	 */
 	iterator = (TBMIterator *) palloc(sizeof(TBMIterator) +
-								 MAX_TUPLES_PER_PAGE * sizeof(OffsetNumber));
+								 MAX_TUPLES_PER_PAGE * sizeof(OffsetNumber)
+#ifdef USE_PREFETCH
+                                          		      /*  space for remembering every prefetched but unread blockno */
+                                          		      +  (target_prefetch_pages * sizeof(BlockNumber))
+#endif   /* USE_PREFETCH */
+                                         );
 	iterator->tbm = tbm;
 
 	/*
@@ -1020,3 +1028,68 @@ tbm_comparator(const void *left, const v
 		return 1;
 	return 0;
 }
+
+void
+tbm_zero(TBMIterator *iterator) /* zero list of prefetched and unread blocknos */
+{
+      /* locate the list of prefetched but unread blocknos immediately following the array of offsets
+      ** and note that tbm_begin_iterate allocates space for (1 + MAX_TUPLES_PER_PAGE) offsets  -
+      ** 1 included in struct TBMIterator and MAX_TUPLES_PER_PAGE additional
+      */
+      iterator->output.Unread_Pfetched_base = ((BlockNumber *)(&(iterator->output.offsets[MAX_TUPLES_PER_PAGE+1])));
+      iterator->output.Unread_Pfetched_next = iterator->output.Unread_Pfetched_count = 0;
+}
+
+void
+tbm_add(TBMIterator *iterator, BlockNumber blockno) /* add this blockno to list of prefetched and unread blocknos */
+{
+      unsigned int Unread_Pfetched_index = iterator->output.Unread_Pfetched_next + iterator->output.Unread_Pfetched_count++;
+
+      if (iterator->output.Unread_Pfetched_count > target_prefetch_pages) {
+		ereport(ERROR,
+			(errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("tbm_add overflowed list cannot add blockno %d", blockno)));
+      }
+
+      if (Unread_Pfetched_index >= target_prefetch_pages)
+          Unread_Pfetched_index -= target_prefetch_pages;
+      *(iterator->output.Unread_Pfetched_base + Unread_Pfetched_index) = blockno;
+}
+
+void
+tbm_subtract(TBMIterator *iterator, BlockNumber blockno) /* remove this blockno from list of prefetched and unread blocknos */
+{
+      unsigned int Unread_Pfetched_index = iterator->output.Unread_Pfetched_next++;
+      BlockNumber nextUnreadPfetched;
+
+      /*    make a weak check that the next blockno is the one to be removed,
+      **    although actually in case of disagreement,   we ignore callers blockno and remove next anyway,
+      **    which is really what caller wants
+      */
+      if ( iterator->output.Unread_Pfetched_count == 0 ) {
+		ereport(ERROR,
+			(errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("tbm_subtract empty list cannot subtract blockno %d", blockno)));
+      }
+
+      if (Unread_Pfetched_index >= target_prefetch_pages)
+          Unread_Pfetched_index = 0;
+      nextUnreadPfetched = *(iterator->output.Unread_Pfetched_base + Unread_Pfetched_index);
+      if (   ( nextUnreadPfetched != blockno ) 
+          && ( nextUnreadPfetched != InvalidBlockNumber ) /* dont report it if the block in the list was InvalidBlockNumber */
+         ) {
+		ereport(NOTICE,
+			(errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("tbm_subtract will subtract blockno %d not %d",
+					nextUnreadPfetched, blockno)));
+      }
+      if (iterator->output.Unread_Pfetched_next >= target_prefetch_pages)
+          iterator->output.Unread_Pfetched_next = 0;
+      iterator->output.Unread_Pfetched_count--;
+}
+
+TBMIterateResult *
+tbm_locate_IterateResult(TBMIterator *iterator)
+{
+   return &(iterator->output);
+}
--- src/backend/utils/misc/guc.c.orig	2014-05-28 08:29:09.406829256 -0400
+++ src/backend/utils/misc/guc.c	2014-05-28 16:45:43.550509846 -0400
@@ -2264,6 +2264,25 @@ static struct config_int ConfigureNamesI
 	},
 
 	{
+		{"max_async_io_prefetchers",
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+			PGC_USERSET,
+#else
+			PGC_INTERNAL,
+#endif
+			RESOURCES_ASYNCHRONOUS,
+			gettext_noop("Maximum number of background processes concurrently using asynchronous librt threads to prefetch pages into shared memory buffers."),
+		},
+		&max_async_io_prefetchers,
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+	        -1, 0, 8192,      /*  boot val -1 indicates to initialize to something sensible during buf_init */
+#else
+		0, 0, 0,
+#endif
+		NULL, NULL, NULL
+	},
+
+	{
 		{"max_worker_processes",
 			PGC_POSTMASTER,
 			RESOURCES_ASYNCHRONOUS,
--- src/backend/utils/mmgr/aset.c.orig	2014-05-28 08:29:09.406829256 -0400
+++ src/backend/utils/mmgr/aset.c	2014-05-28 16:45:43.610510088 -0400
@@ -733,6 +733,48 @@ AllocSetAlloc(MemoryContext context, Siz
 	 */
 	fidx = AllocSetFreeIndex(size);
 	chunk = set->freelist[fidx];
+#ifdef MEMORY_CONTEXT_CHECKING
+        /*    an instance of segfault caused by a rogue value in set->freelist[fidx]
+        **    has been seen - check for it using crude sanity check based on neighbours :
+        **    if at least one is sufficiently close, then pass,  else fail
+        */
+        if (chunk != 0) {
+            int frx, nrx; /*  frx is index,  nrx is index of failing neighbour for errmsg */
+            for (nrx = -1, frx = 0; (frx < ALLOCSET_NUM_FREELISTS); frx++) {
+                if (   (frx != fidx)     /*  not the chosen one */
+                    && ( ( (unsigned long)(set->freelist[frx]) ) != 0 ) /* not empty */
+                   ) {
+                    if (   ( (unsigned long)chunk < ( ( (unsigned long)(set->freelist[frx]) ) / 2 ) )
+                        && (  ( (unsigned long)(set->freelist[frx]) ) < 0x4000000  )
+               /***     || ( (unsigned long)chunk > ( ( (unsigned long)(set->freelist[frx]) ) * 2 ) )  ***/
+                       ) {
+                       nrx = frx;
+                    } else {
+                       nrx = -1;
+                       break;
+                    }
+                }
+            }
+
+            if (nrx >= 0) {
+                     /*  this must be a rogue value  -  put this list in the garbage */
+                     /*  build message but be careful to avoid recursively triggering same fault */
+                     set->freelist[fidx] = NULL;   /*  mark this list empty */
+                     elog(WARNING, "detected rogue value %p in freelist index %d compared with neighbour %p whose chunksize %d"
+				 , chunk , fidx , set->freelist[nrx] , set->freelist[nrx]->size);
+                     chunk = NULL;
+            }
+        }
+#else /* if not MEMORY_CONTEXT_CHECKING make very simple-minded check*/
+        if ( (chunk != 0) && ( (unsigned long)chunk <  0x40000 ) ) {
+                     /*  this must be a rogue value  -  put this list in the garbage */
+                     /*  build message but be careful to avoid recursively triggering same fault */
+                     set->freelist[fidx] = NULL;   /*  mark this list empty */
+                     elog(WARNING, "detected rogue value %p in freelist index %d"
+				 , chunk , fidx);
+                     chunk = NULL;
+        }
+#endif
 	if (chunk != NULL)
 	{
 		Assert(chunk->size >= size);
--- src/include/executor/instrument.h.orig	2014-05-28 08:29:09.454829232 -0400
+++ src/include/executor/instrument.h	2014-05-28 16:45:43.798510846 -0400
@@ -28,8 +28,18 @@ typedef struct BufferUsage
 	long		local_blks_written;		/* # of local disk blocks written */
 	long		temp_blks_read; /* # of temp blocks read */
 	long		temp_blks_written;		/* # of temp blocks written */
+
 	instr_time	blk_read_time;	/* time spent reading */
 	instr_time	blk_write_time; /* time spent writing */
+
+	long		aio_read_noneed;		/* # of prefetches for which no need for prefetch as block already in buffer pool */
+	long		aio_read_discrd;		/* # of prefetches for which no need for prefetch as block already in buffer pool */
+	long		aio_read_forgot;		/* # of prefetches for which no need for prefetch as block already in buffer pool */
+	long		aio_read_noblok;		/* # of prefetches for which no available BufferAiocb */
+	long		aio_read_failed;		/* # of aio reads for which aio itself failed or the read failed with an errno */
+	long		aio_read_wasted;		/* # of aio reads for which disk block not used */
+	long		aio_read_waited;		/* # of aio reads for which disk block used but had to wait for it */
+	long		aio_read_ontime;		/* # of aio reads for which disk block used and ready on time when requested */
 } BufferUsage;
 
 /* Flag bits included in InstrAlloc's instrument_options bitmask */
--- src/include/storage/bufmgr.h.orig	2014-05-28 08:29:09.462829227 -0400
+++ src/include/storage/bufmgr.h	2014-05-28 16:45:43.830510976 -0400
@@ -41,6 +41,7 @@ typedef enum
 	RBM_ZERO_ON_ERROR,			/* Read, but return an all-zeros page on error */
 	RBM_NORMAL_NO_LOG			/* Don't log page as invalid during WAL
 								 * replay; otherwise same as RBM_NORMAL */
+       ,RBM_NOREAD_FOR_PREFETCH   /* Don't read from disk, don't zero buffer, find buffer only */
 } ReadBufferMode;
 
 /* in globals.c ... this duplicates miscadmin.h */
@@ -57,6 +58,9 @@ extern int	target_prefetch_pages;
 extern PGDLLIMPORT char *BufferBlocks;
 extern PGDLLIMPORT int32 *PrivateRefCount;
 
+/*  in buf_async.c  */;
+extern int      max_async_io_prefetchers; /* Maximum number of backends using asynchronous librt threads to read pages into our buffers */
+
 /* in localbuf.c */
 extern PGDLLIMPORT int NLocBuffer;
 extern PGDLLIMPORT Block *LocalBufferBlockPointers;
@@ -159,9 +163,15 @@ extern PGDLLIMPORT int32 *LocalRefCount;
 #define BufferGetPage(buffer) ((Page)BufferGetBlock(buffer))
 
 /*
- * prototypes for functions in bufmgr.c
+ * prototypes for external functions in bufmgr.c and buf_async.c
  */
-extern void PrefetchBuffer(Relation reln, ForkNumber forkNum,
+extern int PrefetchBuffer(Relation reln, ForkNumber forkNum,
+			   BlockNumber blockNum , BufferAccessStrategy strategy);
+/*   return code  is an int bitmask : */
+#define PREFTCHRC_BUF_PIN_INCREASED 0x01    /*  pin count on buffer has been increased by 1 */
+#define PREFTCHRC_BLK_ALREADY_PRESENT 0x02  /*  block was already present in a buffer       */
+
+extern void DiscardBuffer(Relation reln, ForkNumber forkNum,
 			   BlockNumber blockNum);
 extern Buffer ReadBuffer(Relation reln, BlockNumber blockNum);
 extern Buffer ReadBufferExtended(Relation reln, ForkNumber forkNum,
--- src/include/storage/smgr.h.orig	2014-05-28 08:29:09.462829227 -0400
+++ src/include/storage/smgr.h	2014-05-28 16:45:43.854511072 -0400
@@ -92,6 +92,12 @@ extern void smgrextend(SMgrRelation reln
 		   BlockNumber blocknum, char *buffer, bool skipFsync);
 extern void smgrprefetch(SMgrRelation reln, ForkNumber forknum,
 			 BlockNumber blocknum);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern void smgrinitaio(int max_aio_threads, int max_aio_num);
+extern void smgrstartaio(SMgrRelation reln, ForkNumber forknum,
+			 BlockNumber blocknum , char *aiocbp , int *retcode);
+extern void smgrcompleteaio( SMgrRelation reln, char *aiocbp , int *inoutcode );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 extern void smgrread(SMgrRelation reln, ForkNumber forknum,
 		 BlockNumber blocknum, char *buffer);
 extern void smgrwrite(SMgrRelation reln, ForkNumber forknum,
@@ -118,6 +124,11 @@ extern void mdextend(SMgrRelation reln,
 		 BlockNumber blocknum, char *buffer, bool skipFsync);
 extern void mdprefetch(SMgrRelation reln, ForkNumber forknum,
 		   BlockNumber blocknum);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern void mdinitaio(int max_aio_threads, int max_aio_num);
+extern void mdstartaio(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum , char *aiocbp , int *retcode );
+extern void mdcompleteaio( char *aiocbp , int *inoutcode );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 extern void mdread(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 	   char *buffer);
 extern void mdwrite(SMgrRelation reln, ForkNumber forknum,
--- src/include/storage/fd.h.orig	2014-05-28 08:29:09.462829227 -0400
+++ src/include/storage/fd.h	2014-05-28 16:45:43.882511185 -0400
@@ -69,6 +69,11 @@ extern File PathNameOpenFile(FileName fi
 extern File OpenTemporaryFile(bool interXact);
 extern void FileClose(File file);
 extern int	FilePrefetch(File file, off_t offset, int amount);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern void FileInitaio(int max_aio_threads, int max_aio_num );
+extern int  FileStartaio(File file, off_t offset, int amount , char *aiocbp);
+extern int  FileCompleteaio( char *aiocbp , int normal_wait );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 extern int	FileRead(File file, char *buffer, int amount);
 extern int	FileWrite(File file, char *buffer, int amount);
 extern int	FileSync(File file);
--- src/include/storage/buf_internals.h.orig	2014-05-28 08:29:09.462829227 -0400
+++ src/include/storage/buf_internals.h	2014-05-28 16:45:43.906511281 -0400
@@ -22,7 +22,9 @@
 #include "storage/smgr.h"
 #include "storage/spin.h"
 #include "utils/relcache.h"
-
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+#include "aio.h"
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * Flags for buffer descriptors
@@ -38,8 +40,23 @@
 #define BM_JUST_DIRTIED			(1 << 5)		/* dirtied since write started */
 #define BM_PIN_COUNT_WAITER		(1 << 6)		/* have waiter for sole pin */
 #define BM_CHECKPOINT_NEEDED	(1 << 7)		/* must write for checkpoint */
-#define BM_PERMANENT			(1 << 8)		/* permanent relation (not
-												 * unlogged) */
+#define BM_PERMANENT			(1 << 8)	/* permanent relation (not unlogged) */
+#define BM_AIO_IN_PROGRESS		(1 << 9)	/* aio in progress    */
+#define BM_AIO_PREFETCH_PIN_BANKED	(1 << 10)	/* pinned when prefetch issued
+                                                        ** and this pin is banked - i.e.
+                                                        ** redeemable by the next use by same task
+                                                        ** note that for any one buffer, a pin can be banked
+                                                        **      by at most one process globally,
+                                                        **      that is,   only one process may bank a pin on the buffer
+                                                        **                 and it may do so only once (may not be stacked)
+                                                        */
+
+/*********
+for asynchronous aio-read prefetching, two golden rules concerning buffer pinning and buffer-header flags must be observed:
+  R1.  a buffer marked as BM_AIO_IN_PROGRESS must be pinned by at least one backend
+  R2.  a buffer marked as BM_AIO_PREFETCH_PIN_BANKED must be pinned by the backend identified by
+               (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio : (-(buf->freeNext))
+*********/
 
 typedef bits16 BufFlags;
 
@@ -140,17 +157,83 @@ typedef struct sbufdesc
 	BufFlags	flags;			/* see bit definitions above */
 	uint16		usage_count;	/* usage counter for clock sweep code */
 	unsigned	refcount;		/* # of backends holding pins on buffer */
-	int			wait_backend_pid;		/* backend PID of pin-count waiter */
+	int		wait_backend_pid;	/*  if     flags & BM_PIN_COUNT_WAITER
+                                                **  then   backend PID of pin-count waiter
+                                                **  else   not set
+                                                */
 
 	slock_t		buf_hdr_lock;	/* protects the above fields */
 
 	int			buf_id;			/* buffer's index number (from 0) */
-	int			freeNext;		/* link in freelist chain */
+        int    	volatile	freeNext;	/* overloaded and much-abused field :
+                                                ** EITHER
+                                                **     if     >= 0
+                                                **     then   link in freelist chain
+                                                **  OR
+                                                **     if     <  0
+                                                **     then    EITHER
+                                                **             if     flags & BM_AIO_IN_PROGRESS
+                                                **             then   negative of (the index of the aiocb in the BufferAiocbs array + 3)
+                                                **             else   if flags & BM_AIO_PREFETCH_PIN_BANKED
+                                                **             then   -(pid of task that issued aio_read and pinned buffer)
+                                                **             else   one of the special values -1 or -2 listed below
+                                                */
 
 	LWLock	   *io_in_progress_lock;	/* to wait for I/O to complete */
 	LWLock	   *content_lock;	/* to lock access to buffer contents */
 } BufferDesc;
 
+/*  structures for control blocks for our implementation of async io */
+
+/*  if USE_AIO_ATOMIC_BUILTIN_COMP_SWAP is not defined,  the following struct is not put into use at runtime
+**  but it is easier to let the compiler find the definition but hide the reference to aiocb
+**  which is the only type it would not understand
+*/
+
+struct BufferAiocb {
+       struct BufferAiocb volatile * volatile BAiocbnext;  /*    next free entry or value of BAIOCB_OCCUPIED means in use  */
+       struct sbufdesc    volatile * volatile BAiocbbufh;  /*    there can be at most one BufferDesc marked BM_AIO_IN_PROGRESS
+                                                           **    and using this BufferAiocb -
+                                                           **    if there is one, BAiocbbufh points to it, else BAiocbbufh is zero
+                                                           **    NOTE  BAiocbbufh should be zero for every BufferAiocb on the free list
+                                                           */
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+       struct aiocb       volatile            BAiocbthis;  /*    the aio library's control block for one async io */
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+       int                volatile  BAiocbDependentCount;  /*    count of tasks who depend on this BufferAiocb
+                                                           **    in the sense that they are waiting for io completion.
+                                                           **    only a Dependent may move the BufferAiocb onto the freelist
+                                                           **    and only when that Dependent is the *only* Dependent (count == 1)
+                                                           **    BAiocbDependentCount is protected by bufferheader spinlock
+                                                           **    and must be updated only when that spinlock is held
+                                                           */
+       pid_t              volatile  pidOfAio;              /*    pid of backend who issued an aio_read using this BAiocb -
+                                                           **    this backend must have pinned the associated buffer.
+                                                           */
+};
+
+#define BAIOCB_OCCUPIED 0x75f1        /*  distinct indicator of a BufferAiocb.BAiocbnext that is NOT on free list */
+#define BAIOCB_FREE 0x7b9d            /*  distinct indicator of a BufferAiocb.BAiocbbufh that IS     on free list */
+
+struct BAiocbAnchor {                 /*  anchor for all control blocks pertaining to aio  */
+       volatile struct BufferAiocb* BufferAiocbs;          /*  aiocbs ...                   */
+       volatile struct BufferAiocb* volatile FreeBAiocbs; /* ... and their free list   */
+};
+
+/*   values for BufCheckAsync input and retcode */
+#define BUF_INTENTION_WANT 		 1  /* wants the buffer, wait for in-progress aio and then pin */
+#define BUF_INTENTION_REJECT_KEEP_PIN 	-1  /* pin already held, do not unpin */
+#define BUF_INTENTION_REJECT_OBTAIN_PIN	-2  /* obtain pin,  caller wants it for same buffer */
+#define BUF_INTENTION_REJECT_FORGET	-3  /* unpin and tell resource owner to forget */
+#define BUF_INTENTION_REJECT_NOADJUST	-4  /* unpin and call ResourceOwnerForgetBuffer */
+#define BUF_INTENTION_REJECT_UNBANK   	-5  /* unpin only if pin banked by caller */
+
+#define BUF_INTENT_RC_CHANGED_TAG	-5
+#define BUF_INTENT_RC_BADPAGE		-4
+#define BUF_INTENT_RC_INVALID_AIO	-3    /*  invalid and aio was in progress */
+#define BUF_INTENT_RC_INVALID_NO_AIO	-1    /*  invalid and no aio was in progress */
+#define BUF_INTENT_RC_VALID		 1
+
 #define BufferDescriptorGetBuffer(bdesc) ((bdesc)->buf_id + 1)
 
 /*
@@ -159,6 +242,7 @@ typedef struct sbufdesc
  */
 #define FREENEXT_END_OF_LIST	(-1)
 #define FREENEXT_NOT_IN_LIST	(-2)
+#define FREENEXT_BAIOCB_ORIGIN	(-3)
 
 /*
  * Macros for acquiring/releasing a shared buffer header's spinlock.
--- src/include/catalog/pg_am.h.orig	2014-05-28 08:29:09.446829236 -0400
+++ src/include/catalog/pg_am.h	2014-05-28 16:45:43.926511362 -0400
@@ -67,6 +67,7 @@ CATALOG(pg_am,2601)
 	regproc		amcanreturn;	/* can indexscan return IndexTuples? */
 	regproc		amcostestimate; /* estimate cost of an indexscan */
 	regproc		amoptions;		/* parse AM-specific parameters */
+	regproc		ampeeknexttuple;	/* peek at the next tuple different from any blocknum in pfch_list without reading a new index page */
 } FormData_pg_am;
 
 /* ----------------
@@ -117,19 +118,19 @@ typedef FormData_pg_am *Form_pg_am;
  * ----------------
  */
 
-DATA(insert OID = 403 (  btree		5 2 t f t t t t t t f t t 0 btinsert btbeginscan btgettuple btgetbitmap btrescan btendscan btmarkpos btrestrpos btbuild btbuildempty btbulkdelete btvacuumcleanup btcanreturn btcostestimate btoptions ));
+DATA(insert OID = 403 (  btree		5 2 t f t t t t t t f t t 0 btinsert btbeginscan btgettuple btgetbitmap btrescan btendscan btmarkpos btrestrpos btbuild btbuildempty btbulkdelete btvacuumcleanup btcanreturn btcostestimate btoptions btpeeknexttuple ));
 DESCR("b-tree index access method");
 #define BTREE_AM_OID 403
-DATA(insert OID = 405 (  hash		1 1 f f t f f f f f f f f 23 hashinsert hashbeginscan hashgettuple hashgetbitmap hashrescan hashendscan hashmarkpos hashrestrpos hashbuild hashbuildempty hashbulkdelete hashvacuumcleanup - hashcostestimate hashoptions ));
+DATA(insert OID = 405 (  hash		1 1 f f t f f f f f f f f 23 hashinsert hashbeginscan hashgettuple hashgetbitmap hashrescan hashendscan hashmarkpos hashrestrpos hashbuild hashbuildempty hashbulkdelete hashvacuumcleanup - hashcostestimate hashoptions - ));
 DESCR("hash index access method");
 #define HASH_AM_OID 405
-DATA(insert OID = 783 (  gist		0 8 f t f f t t f t t t f 0 gistinsert gistbeginscan gistgettuple gistgetbitmap gistrescan gistendscan gistmarkpos gistrestrpos gistbuild gistbuildempty gistbulkdelete gistvacuumcleanup - gistcostestimate gistoptions ));
+DATA(insert OID = 783 (  gist		0 8 f t f f t t f t t t f 0 gistinsert gistbeginscan gistgettuple gistgetbitmap gistrescan gistendscan gistmarkpos gistrestrpos gistbuild gistbuildempty gistbulkdelete gistvacuumcleanup - gistcostestimate gistoptions - ));
 DESCR("GiST index access method");
 #define GIST_AM_OID 783
-DATA(insert OID = 2742 (  gin		0 6 f f f f t t f f t f f 0 gininsert ginbeginscan - gingetbitmap ginrescan ginendscan ginmarkpos ginrestrpos ginbuild ginbuildempty ginbulkdelete ginvacuumcleanup - gincostestimate ginoptions ));
+DATA(insert OID = 2742 (  gin		0 6 f f f f t t f f t f f 0 gininsert ginbeginscan - gingetbitmap ginrescan ginendscan ginmarkpos ginrestrpos ginbuild ginbuildempty ginbulkdelete ginvacuumcleanup - gincostestimate ginoptions - ));
 DESCR("GIN index access method");
 #define GIN_AM_OID 2742
-DATA(insert OID = 4000 (  spgist	0 5 f f f f f t f t f f f 0 spginsert spgbeginscan spggettuple spggetbitmap spgrescan spgendscan spgmarkpos spgrestrpos spgbuild spgbuildempty spgbulkdelete spgvacuumcleanup spgcanreturn spgcostestimate spgoptions ));
+DATA(insert OID = 4000 (  spgist	0 5 f f f f f t f t f f f 0 spginsert spgbeginscan spggettuple spggetbitmap spgrescan spgendscan spgmarkpos spgrestrpos spgbuild spgbuildempty spgbulkdelete spgvacuumcleanup spgcanreturn spgcostestimate spgoptions - ));
 DESCR("SP-GiST index access method");
 #define SPGIST_AM_OID 4000
 
--- src/include/catalog/pg_proc.h.orig	2014-05-28 08:29:09.450829234 -0400
+++ src/include/catalog/pg_proc.h	2014-05-28 16:45:43.966511524 -0400
@@ -536,6 +536,12 @@ DESCR("convert float4 to int4");
 
 DATA(insert OID = 330 (  btgettuple		   PGNSP PGUID 12 1 0 0 0 f f f f t f v 2 0 16 "2281 2281" _null_ _null_ _null_ _null_	btgettuple _null_ _null_ _null_ ));
 DESCR("btree(internal)");
+
+#ifndef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+DATA(insert OID = 3251 (  btpeeknexttuple	   PGNSP PGUID 12 1 0 0 0 f f f f t f v 1 0 16 "2281" _null_ _null_ _null_ _null_ btpeeknexttuple _null_ _null_ _null_ ));
+DESCR("btree(internal)");
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+
 DATA(insert OID = 636 (  btgetbitmap	   PGNSP PGUID 12 1 0 0 0 f f f f t f v 2 0 20 "2281 2281" _null_ _null_ _null_ _null_	btgetbitmap _null_ _null_ _null_ ));
 DESCR("btree(internal)");
 DATA(insert OID = 331 (  btinsert		   PGNSP PGUID 12 1 0 0 0 f f f f t f v 6 0 16 "2281 2281 2281 2281 2281 2281" _null_ _null_ _null_ _null_	btinsert _null_ _null_ _null_ ));
--- src/include/pg_config_manual.h.orig	2014-05-28 08:29:09.458829229 -0400
+++ src/include/pg_config_manual.h	2014-05-28 16:45:43.994511636 -0400
@@ -138,9 +138,11 @@
 /*
  * USE_PREFETCH code should be compiled only if we have a way to implement
  * prefetching.  (This is decoupled from USE_POSIX_FADVISE because there
- * might in future be support for alternative low-level prefetch APIs.)
+ * might in future be support for alternative low-level prefetch APIs  --
+ * -- update October 2013  -- now there is such a new prefetch capability --
+ *   async_io into postgres buffers  -   configuration parameter max_async_io_threads)
  */
-#ifdef USE_POSIX_FADVISE
+#if defined(USE_POSIX_FADVISE) || defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP)
 #define USE_PREFETCH
 #endif
 
--- src/include/access/nbtree.h.orig	2014-05-28 08:29:09.442829238 -0400
+++ src/include/access/nbtree.h	2014-05-28 16:45:44.022511749 -0400
@@ -19,6 +19,7 @@
 #include "access/sdir.h"
 #include "access/xlog.h"
 #include "access/xlogutils.h"
+#include "access/relscan.h"
 #include "catalog/pg_index.h"
 
 /* There's room for a 16-bit vacuum cycle ID in BTPageOpaqueData */
@@ -524,6 +525,7 @@ typedef struct BTScanPosData
 	Buffer		buf;			/* if valid, the buffer is pinned */
 
 	BlockNumber nextPage;		/* page's right link when we scanned it */
+	BlockNumber prevPage;		/* page's left link when we scanned it */
 
 	/*
 	 * moreLeft and moreRight track whether we think there may be matching
@@ -603,6 +605,15 @@ typedef struct BTScanOpaqueData
 	 */
 	int			markItemIndex;	/* itemIndex, or -1 if not valid */
 
+	/* prefetch logic state */
+	unsigned int	backSeqRun;	/* number of back-sequential pages in a run */
+	BlockNumber		backSeqPos;	/* blkid last prefetched in back-sequential 
+				          		   runs */
+	BlockNumber		lastHeapPrefetchBlkno;	/* blkid last prefetched from heap */
+	int				prefetchItemIndex; /* item index within currPos last
+					                      fetched by heap prefetch */
+	int				prefetchBlockCount; /* number of prefetched heap blocks */
+	
 	/* keep these last in struct for efficiency */
 	BTScanPosData currPos;		/* current position data */
 	BTScanPosData markPos;		/* marked position, if any */
@@ -655,7 +666,11 @@ extern Buffer _bt_getroot(Relation rel,
 extern Buffer _bt_gettrueroot(Relation rel);
 extern int	_bt_getrootheight(Relation rel);
 extern void _bt_checkpage(Relation rel, Buffer buf);
-extern Buffer _bt_getbuf(Relation rel, BlockNumber blkno, int access);
+extern Buffer _bt_getbuf(Relation rel, BlockNumber blkno, int access , struct pfch_index_pagelist* pfch_index_page_list);
+extern void _bt_prefetchbuf(Relation rel, BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P);
+extern struct pfch_index_item* _bt_find_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list);
+extern int _bt_add_block(BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P , uint32 discard_status);
+extern void _bt_subtract_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list);
 extern Buffer _bt_relandgetbuf(Relation rel, Buffer obuf,
 				 BlockNumber blkno, int access);
 extern void _bt_relbuf(Relation rel, Buffer buf);
--- src/include/access/heapam.h.orig	2014-05-28 08:29:09.442829238 -0400
+++ src/include/access/heapam.h	2014-05-28 16:45:44.046511845 -0400
@@ -175,7 +175,7 @@ extern void heap_page_prune_execute(Buff
 extern void heap_get_root_tuples(Page page, OffsetNumber *root_offsets);
 
 /* in heap/syncscan.c */
-extern void ss_report_location(Relation rel, BlockNumber location);
+extern void ss_report_location(Relation rel, BlockNumber location , BlockNumber *prefetchHWMp);
 extern BlockNumber ss_get_location(Relation rel, BlockNumber relnblocks);
 extern void SyncScanShmemInit(void);
 extern Size SyncScanShmemSize(void);
--- src/include/access/relscan.h.orig	2014-05-28 08:29:09.446829236 -0400
+++ src/include/access/relscan.h	2014-05-28 16:45:44.066511925 -0400
@@ -44,6 +44,24 @@ typedef struct HeapScanDescData
 	bool		rs_inited;		/* false = scan not init'd yet */
 	HeapTupleData rs_ctup;		/* current tuple in scan, if any */
 	BlockNumber rs_cblock;		/* current block # in scan, if any */
+#ifdef USE_PREFETCH
+	int	    rs_prefetch_target; /* target distance (numblocks) for prefetch to reach beyond main scan */
+	BlockNumber rs_pfchblock;	/* next block # to be prefetched in scan, if any */
+
+        /*   Unread_Pfetched is a "mostly" circular list of recently prefetched blocknos of size target_prefetch_pages
+        **   the index of the first unread block is held in Unread_Pfetched_next
+        **         and is advanced when a block is read
+        **   the count of number of unread blocks is in Unread_Pfetched_count (and this subset can wrap around)
+        **   "mostly" means that there may be gaps caused by storing entries for blocks which do not need to be discarded -
+        **   these are indicated by blockno = InvalidBlockNumber,  and these slots are reused when found.
+        */
+        BlockNumber *rs_Unread_Pfetched_base;   /*  where the list of prefetched but unread blocknos starts */
+        unsigned int rs_Unread_Pfetched_next;   /*  where the next unread blockno probably is relative to start --
+                                                **  this is only a hint which may be temporarily stale.
+                                                */
+        unsigned int rs_Unread_Pfetched_count;  /*  number of valid unread blocknos in list */
+#endif   /* USE_PREFETCH */
+
 	Buffer		rs_cbuf;		/* current buffer in scan, if any */
 	/* NB: if rs_cbuf is not InvalidBuffer, we hold a pin on that buffer */
 	ItemPointerData rs_mctid;	/* marked scan position, if any */
@@ -55,6 +73,27 @@ typedef struct HeapScanDescData
 	OffsetNumber rs_vistuples[MaxHeapTuplesPerPage];	/* their offsets */
 }	HeapScanDescData;
 
+/* pfch_index_items track prefetched and unread index pages -   chunks of blocknumbers are chained in singly-linked list from scan->pfch_index_item_list */
+struct pfch_index_item {                              /*  index-relation BlockIds which we will/have prefetched */
+       BlockNumber         pfch_blocknum;    /* Blocknum which we will/have prefetched */
+       uint32              pfch_discard;     /* whether block is to be discarded when scan is closed */
+};
+
+struct pfch_block_item {
+       struct BlockIdData  pfch_blockid;     /* BlockId which we will/have prefetched */
+       uint32              pfch_discard;     /* whether block is to be discarded when scan is closed */
+};
+
+/* pfch_index_page_items track prefetched and unread index pages -
+** chunks of blocknumbers are chained backwards (newest first,  oldest last)
+** in singly-linked list from scan->pfch_index_item_list
+*/
+struct pfch_index_pagelist {                          /*  index-relation BlockIds which we will/have prefetched */
+       struct pfch_index_pagelist* pfch_index_pagelist_next;  /*  pointer to next chunk if any */
+       unsigned int    pfch_index_item_count;         /*  number of used entries in this chunk */
+       struct pfch_index_item pfch_indexid[1];        /*  in-line list of Blocknums which we will/have prefetched and whether to be discarded */
+};
+
 /*
  * We use the same IndexScanDescData structure for both amgettuple-based
  * and amgetbitmap-based index scans.  Some fields are only relevant in
@@ -75,8 +114,15 @@ typedef struct IndexScanDescData
 	/* signaling to index AM about killing index tuples */
 	bool		kill_prior_tuple;		/* last-returned tuple is dead */
 	bool		ignore_killed_tuples;	/* do not return killed entries */
-	bool		xactStartedInRecovery;	/* prevents killing/seeing killed
-										 * tuples */
+	bool		xactStartedInRecovery;	/* prevents killing/seeing killed tuples */
+										 
+#ifdef USE_PREFETCH
+        struct pfch_index_pagelist* pfch_index_page_list;  /* array of index-relation BlockIds which we will/have prefetched */
+        struct pfch_block_item* pfch_block_item_list;   /* array of heap-relation BlockIds which we will/have prefetched */
+        unsigned short int     pfch_used;	/*  number of used elements in BlockIdData array   */
+        unsigned short int     pfch_next;	/*  next element for prefetch in BlockIdData array */
+	int             do_prefetch;    /*  should I prefetch ? */
+#endif   /* USE_PREFETCH */
 
 	/* index access method's private state */
 	void	   *opaque;			/* access-method-specific info */
@@ -91,6 +137,10 @@ typedef struct IndexScanDescData
 	/* NB: if xs_cbuf is not InvalidBuffer, we hold a pin on that buffer */
 	bool		xs_recheck;		/* T means scan keys must be rechecked */
 
+	/* heap fetch statistics for read-ahead logic */
+	unsigned int heap_tids_seen;
+	unsigned int heap_tids_fetched;
+
 	/* state data for traversing HOT chains in index_getnext */
 	bool		xs_continue_hot;	/* T if must keep walking HOT chain */
 }	IndexScanDescData;
--- src/include/nodes/tidbitmap.h.orig	2014-05-28 08:29:09.458829229 -0400
+++ src/include/nodes/tidbitmap.h	2014-05-28 16:45:44.106512088 -0400
@@ -41,6 +41,16 @@ typedef struct
 	int			ntuples;		/* -1 indicates lossy result */
 	bool		recheck;		/* should the tuples be rechecked? */
 	/* Note: recheck is always true if ntuples < 0 */
+#ifdef USE_PREFETCH
+        /*   Unread_Pfetched is a circular list of recently prefetched blocknos of size target_prefetch_pages
+        **   the index of the first unread block is held in Unread_Pfetched_next
+        **         and is advanced when a block is read
+        **   the count of number of unread blocks is in Unread_Pfetched_count (and this subset can wrap around)
+        */
+        BlockNumber *Unread_Pfetched_base;   /*  where the list of prefetched but unread blocknos starts */
+        unsigned int Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+        unsigned int Unread_Pfetched_count;  /*  number of valid unread blocknos in list */
+#endif   /* USE_PREFETCH */
 	OffsetNumber offsets[1];	/* VARIABLE LENGTH ARRAY */
 } TBMIterateResult;				/* VARIABLE LENGTH STRUCT */
 
@@ -62,5 +72,8 @@ extern bool tbm_is_empty(const TIDBitmap
 extern TBMIterator *tbm_begin_iterate(TIDBitmap *tbm);
 extern TBMIterateResult *tbm_iterate(TBMIterator *iterator);
 extern void tbm_end_iterate(TBMIterator *iterator);
-
+extern void tbm_zero(TBMIterator *iterator); /* zero list of prefetched and unread blocknos */
+extern void tbm_add(TBMIterator *iterator, BlockNumber blockno); /* add this blockno to list of prefetched and unread blocknos */
+extern void tbm_subtract(TBMIterator *iterator, BlockNumber blockno); /* remove this blockno from list of prefetched and unread blocknos */
+extern TBMIterateResult *tbm_locate_IterateResult(TBMIterator *iterator); /* locate the TBMIterateResult of an iterator */
 #endif   /* TIDBITMAP_H */
--- src/include/utils/rel.h.orig	2014-05-28 08:29:09.466829225 -0400
+++ src/include/utils/rel.h	2014-05-28 16:45:44.134512200 -0400
@@ -61,6 +61,7 @@ typedef struct RelationAmInfo
 	FmgrInfo	ammarkpos;
 	FmgrInfo	amrestrpos;
 	FmgrInfo	amcanreturn;
+	FmgrInfo	ampeeknexttuple;	/* peek at the next tuple different from any blocknum in pfch_list without reading a new index page */
 } RelationAmInfo;
 
 
--- src/include/pg_config.h.in.orig	2014-05-28 08:29:09.458829229 -0400
+++ src/include/pg_config.h.in	2014-05-28 16:45:44.150512266 -0400
@@ -1,4 +1,4 @@
-/* src/include/pg_config.h.in.  Generated from configure.in by autoheader.  */
+/* src/include/pg_config.h.in.  Generated from - by autoheader.  */
 
 /* Define to the type of arg 1 of 'accept' */
 #undef ACCEPT_TYPE_ARG1
@@ -748,6 +748,10 @@
 /* Define to the appropriate snprintf format for unsigned 64-bit ints. */
 #undef UINT64_FORMAT
 
+/* Define to select librt-style async io and the gcc atomic compare_and_swap.
+   */
+#undef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+
 /* Define to 1 to build with assertion checks. (--enable-cassert) */
 #undef USE_ASSERT_CHECKING
 

diff --git a/config/c-library.m4 b/config/c-library.m4
index 8f45010..9aee904 100644
--- a/config/c-library.m4
+++ b/config/c-library.m4
@@ -367,3 +367,50 @@ if test "$pgac_cv_type_locale_t" = 'yes (in xlocale.h)'; then
   AC_DEFINE(LOCALE_T_IN_XLOCALE, 1,
             [Define to 1 if `locale_t' requires <xlocale.h>.])
 fi])])# PGAC_HEADER_XLOCALE
+
+
+# PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP
+# ---------------------------------------
+# test whether this system has both the librt-style async io and the gcc atomic compare_and_swap
+#      and test operation  of the latter.
+#
+AC_DEFUN([PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP],
+[AC_MSG_CHECKING([whether have both librt-style async io and the gcc atomic compare_and_swap])
+AC_CACHE_VAL(pgac_cv_aio_atomic_builtin_comp_swap,
+pgac_save_LIBS=$LIBS
+LIBS=" -lrt $pgac_save_LIBS"
+[AC_TRY_RUN([#include <stdio.h>
+#include <unistd.h>
+#include "aio.h"
+
+int main (int argc, char *argv[])
+ {
+   int rc;
+   struct aiocb volatile * first_aiocb;
+   struct aiocb volatile * second_aiocb;
+   struct aiocb volatile * my_aiocbp = (struct aiocb *)20000008;
+
+   first_aiocb = (struct aiocb *)20000008;
+   second_aiocb = (struct aiocb *)40000008;
+
+   /*  return zero as success if two comp-swaps both worked as expected -
+   **  first compares equal and swaps,  second compares unequal
+   */
+   rc = (__sync_bool_compare_and_swap (&my_aiocbp, first_aiocb, second_aiocb));
+   if (rc) {
+      rc = (__sync_bool_compare_and_swap (&my_aiocbp, first_aiocb, second_aiocb));
+   } else {
+      rc = -1;
+   }
+
+   return rc;
+}],
+[pgac_cv_aio_atomic_builtin_comp_swap=yes],
+[pgac_cv_aio_atomic_builtin_comp_swap=no],
+[pgac_cv_aio_atomic_builtin_comp_swap=cross])
+])dnl AC_CACHE_VAL
+AC_MSG_RESULT([$pgac_cv_aio_atomic_builtin_comp_swap])
+if test x"$pgac_cv_aio_atomic_builtin_comp_swap" != x"yes"; then
+LIBS=$pgac_save_LIBS
+fi
+])# PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP
diff --git a/configure b/configure
index 3663e50..5ad28a1 100755
--- a/configure
+++ b/configure
@@ -13840,6 +13840,69 @@ operating system;  use --disable-thread-safety to disable thread safety." "$LINE
 fi
 fi
 
+#  test whether this system has both the librt-style async io and the gcc atomic compare_and_swap
+#      and test operation  of the latter.
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether have both librt-style async io and the gcc atomic compare_and_swap" >&5
+$as_echo_n "checking whether have both librt-style async io and the gcc atomic compare_and_swap... " >&6; }
+if ${pgac_cv_aio_atomic_builtin_comp_swap+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  pgac_save_LIBS=$LIBS
+LIBS=" -lrt $pgac_save_LIBS"
+if test "$cross_compiling" = yes; then :
+  pgac_cv_aio_atomic_builtin_comp_swap=cross
+else
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include <stdio.h>
+#include <unistd.h>
+#include "aio.h"
+
+int main (int argc, char *argv[])
+ {
+   int rc;
+   struct aiocb volatile * first_aiocb;
+   struct aiocb volatile * second_aiocb;
+   struct aiocb volatile * my_aiocbp = (struct aiocb *)20000008;
+
+   first_aiocb = (struct aiocb *)20000008;
+   second_aiocb = (struct aiocb *)40000008;
+
+   /*  return zero as success if two comp-swaps both worked as expected -
+   **  first compares equal and swaps,  second compares unequal
+   */
+   rc = (__sync_bool_compare_and_swap (&my_aiocbp, first_aiocb, second_aiocb));
+   if (rc) {
+      rc = (__sync_bool_compare_and_swap (&my_aiocbp, first_aiocb, second_aiocb));
+   } else {
+      rc = -1;
+   }
+
+   return rc;
+}
+_ACEOF
+if ac_fn_c_try_run "$LINENO"; then :
+  pgac_cv_aio_atomic_builtin_comp_swap=yes
+else
+  pgac_cv_aio_atomic_builtin_comp_swap=no
+fi
+rm -f core *.core core.conftest.* gmon.out bb.out conftest$ac_exeext \
+  conftest.$ac_objext conftest.beam conftest.$ac_ext
+fi
+
+
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv_aio_atomic_builtin_comp_swap" >&5
+$as_echo "$pgac_cv_aio_atomic_builtin_comp_swap" >&6; }
+if test x"$pgac_cv_aio_atomic_builtin_comp_swap" != x"yes"; then
+LIBS=$pgac_save_LIBS
+fi
+
+if test x"$pgac_cv_aio_atomic_builtin_comp_swap" = x"yes"; then
+
+$as_echo "#define USE_AIO_ATOMIC_BUILTIN_COMP_SWAP 1" >>confdefs.h
+
+fi
 
 # Select semaphore implementation type.
 if test "$PORTNAME" != "win32"; then
diff --git a/configure.in b/configure.in
index 80df1d7..b0876bd 100644
--- a/configure.in
+++ b/configure.in
@@ -1771,6 +1771,12 @@ operating system;  use --disable-thread-safety to disable thread safety.])
 fi
 fi
 
+#  test whether this system has both the librt-style async io and the gcc atomic compare_and_swap
+#      and test operation  of the latter.
+PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP
+if test x"$pgac_cv_aio_atomic_builtin_comp_swap" = x"yes"; then
+      AC_DEFINE(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP, 1, [Define to select librt-style async io and the gcc atomic compare_and_swap.])
+fi
 
 # Select semaphore implementation type.
 if test "$PORTNAME" != "win32"; then
diff --git a/contrib/pg_prewarm/pg_prewarm.c b/contrib/pg_prewarm/pg_prewarm.c
index df20e88..b52772f 100644
--- a/contrib/pg_prewarm/pg_prewarm.c
+++ b/contrib/pg_prewarm/pg_prewarm.c
@@ -159,7 +159,7 @@ pg_prewarm(PG_FUNCTION_ARGS)
 		 */
 		for (block = first_block; block <= last_block; ++block)
 		{
-			PrefetchBuffer(rel, forkNumber, block);
+			PrefetchBuffer(rel, forkNumber, block, 0);
 			++blocks_done;
 		}
 #else
diff --git a/contrib/pg_stat_statements/Makefile b/contrib/pg_stat_statements/Makefile
index 95a2767..ae52fa5 100644
--- a/contrib/pg_stat_statements/Makefile
+++ b/contrib/pg_stat_statements/Makefile
@@ -4,7 +4,8 @@ MODULE_big = pg_stat_statements
 OBJS = pg_stat_statements.o
 
 EXTENSION = pg_stat_statements
-DATA = pg_stat_statements--1.2.sql pg_stat_statements--1.1--1.2.sql \
+DATA = pg_stat_statements--1.3.sql pg_stat_statements--1.2--1.3.sql \
+	pg_stat_statements--1.2.sql pg_stat_statements--1.1--1.2.sql \
 	pg_stat_statements--1.0--1.1.sql pg_stat_statements--unpackaged--1.0.sql
 
 ifdef USE_PGXS
diff --git a/contrib/pg_stat_statements/pg_stat_statements.c b/contrib/pg_stat_statements/pg_stat_statements.c
index 32d16cc..b4bfb5a 100644
--- a/contrib/pg_stat_statements/pg_stat_statements.c
+++ b/contrib/pg_stat_statements/pg_stat_statements.c
@@ -117,6 +117,7 @@ typedef enum pgssVersion
 	PGSS_V1_0 = 0,
 	PGSS_V1_1,
 	PGSS_V1_2
+	,PGSS_V1_3
 } pgssVersion;
 
 /*
@@ -148,6 +149,16 @@ typedef struct Counters
 	int64		local_blks_written;		/* # of local disk blocks written */
 	int64		temp_blks_read; /* # of temp blocks read */
 	int64		temp_blks_written;		/* # of temp blocks written */
+
+	int64		aio_read_noneed;		/* # of prefetches for which no need for prefetch as block already in buffer pool  */
+	int64		aio_read_discrd;		/* # of prefetches for which buffer not subsequently read and therefore discarded  */
+	int64		aio_read_forgot;		/* # of prefetches for which buffer not subsequently read and then forgotten about */
+	int64		aio_read_noblok;		/* # of prefetches for which no available BufferAiocb  control block               */
+	int64		aio_read_failed;		/* # of aio reads for which aio itself failed or the read failed with an errno     */
+	int64		aio_read_wasted;		/* # of aio reads for which in-progress aio cancelled and disk block not used      */
+	int64		aio_read_waited;		/* # of aio reads for which disk block used but had to wait for it                 */
+	int64		aio_read_ontime;		/* # of aio reads for which disk block used and ready on time when requested       */
+
 	double		blk_read_time;	/* time spent reading, in msec */
 	double		blk_write_time; /* time spent writing, in msec */
 	double		usage;			/* usage factor */
@@ -274,7 +285,7 @@ void		_PG_init(void);
 void		_PG_fini(void);
 
 PG_FUNCTION_INFO_V1(pg_stat_statements_reset);
-PG_FUNCTION_INFO_V1(pg_stat_statements_1_2);
+PG_FUNCTION_INFO_V1(pg_stat_statements_1_3);
 PG_FUNCTION_INFO_V1(pg_stat_statements);
 
 static void pgss_shmem_startup(void);
@@ -1026,7 +1037,25 @@ pgss_ProcessUtility(Node *parsetree, const char *queryString,
 		bufusage.temp_blks_read =
 			pgBufferUsage.temp_blks_read - bufusage_start.temp_blks_read;
 		bufusage.temp_blks_written =
-			pgBufferUsage.temp_blks_written - bufusage_start.temp_blks_written;
+			pgBufferUsage.temp_blks_written - bufusage.temp_blks_written;
+
+		bufusage.aio_read_noneed =
+			pgBufferUsage.aio_read_noneed - bufusage.aio_read_noneed;
+		bufusage.aio_read_discrd =
+			pgBufferUsage.aio_read_discrd - bufusage.aio_read_discrd;
+		bufusage.aio_read_forgot =
+			pgBufferUsage.aio_read_forgot - bufusage.aio_read_forgot;
+		bufusage.aio_read_noblok =
+			pgBufferUsage.aio_read_noblok - bufusage.aio_read_noblok;
+		bufusage.aio_read_failed =
+			pgBufferUsage.aio_read_failed - bufusage.aio_read_failed;
+		bufusage.aio_read_wasted =
+			pgBufferUsage.aio_read_wasted - bufusage.aio_read_wasted;
+		bufusage.aio_read_waited =
+			pgBufferUsage.aio_read_waited - bufusage.aio_read_waited;
+		bufusage.aio_read_ontime =
+			pgBufferUsage.aio_read_ontime - bufusage.aio_read_ontime;
+
 		bufusage.blk_read_time = pgBufferUsage.blk_read_time;
 		INSTR_TIME_SUBTRACT(bufusage.blk_read_time, bufusage_start.blk_read_time);
 		bufusage.blk_write_time = pgBufferUsage.blk_write_time;
@@ -1041,6 +1070,7 @@ pgss_ProcessUtility(Node *parsetree, const char *queryString,
 				   rows,
 				   &bufusage,
 				   NULL);
+
 	}
 	else
 	{
@@ -1224,6 +1254,16 @@ pgss_store(const char *query, uint32 queryId,
 		e->counters.local_blks_written += bufusage->local_blks_written;
 		e->counters.temp_blks_read += bufusage->temp_blks_read;
 		e->counters.temp_blks_written += bufusage->temp_blks_written;
+
+		e->counters.aio_read_noneed     += bufusage->aio_read_noneed;
+		e->counters.aio_read_discrd     += bufusage->aio_read_discrd;
+		e->counters.aio_read_forgot     += bufusage->aio_read_forgot;
+		e->counters.aio_read_noblok     += bufusage->aio_read_noblok;
+		e->counters.aio_read_failed     += bufusage->aio_read_failed;
+		e->counters.aio_read_wasted     += bufusage->aio_read_wasted;
+		e->counters.aio_read_waited     += bufusage->aio_read_waited;
+		e->counters.aio_read_ontime     += bufusage->aio_read_ontime;
+
 		e->counters.blk_read_time += INSTR_TIME_GET_MILLISEC(bufusage->blk_read_time);
 		e->counters.blk_write_time += INSTR_TIME_GET_MILLISEC(bufusage->blk_write_time);
 		e->counters.usage += USAGE_EXEC(total_time);
@@ -1257,7 +1297,8 @@ pg_stat_statements_reset(PG_FUNCTION_ARGS)
 #define PG_STAT_STATEMENTS_COLS_V1_0	14
 #define PG_STAT_STATEMENTS_COLS_V1_1	18
 #define PG_STAT_STATEMENTS_COLS_V1_2	19
-#define PG_STAT_STATEMENTS_COLS			19		/* maximum of above */
+#define PG_STAT_STATEMENTS_COLS_V1_3	27
+#define PG_STAT_STATEMENTS_COLS			27		/* maximum of above */
 
 /*
  * Retrieve statement statistics.
@@ -1270,6 +1311,16 @@ pg_stat_statements_reset(PG_FUNCTION_ARGS)
  * function.  Unfortunately we weren't bright enough to do that for 1.1.
  */
 Datum
+pg_stat_statements_1_3(PG_FUNCTION_ARGS)
+{
+	bool		showtext = PG_GETARG_BOOL(0);
+
+	pg_stat_statements_internal(fcinfo, PGSS_V1_3, showtext);
+
+	return (Datum) 0;
+}
+
+Datum
 pg_stat_statements_1_2(PG_FUNCTION_ARGS)
 {
 	bool		showtext = PG_GETARG_BOOL(0);
@@ -1358,6 +1409,10 @@ pg_stat_statements_internal(FunctionCallInfo fcinfo,
 			if (api_version != PGSS_V1_2)
 				elog(ERROR, "incorrect number of output arguments");
 			break;
+		case PG_STAT_STATEMENTS_COLS_V1_3:
+			if (api_version != PGSS_V1_3)
+				elog(ERROR, "incorrect number of output arguments");
+			break;
 		default:
 			elog(ERROR, "incorrect number of output arguments");
 	}
@@ -1534,11 +1589,24 @@ pg_stat_statements_internal(FunctionCallInfo fcinfo,
 		{
 			values[i++] = Float8GetDatumFast(tmp.blk_read_time);
 			values[i++] = Float8GetDatumFast(tmp.blk_write_time);
+
+			if (api_version >= PGSS_V1_3)
+			{
+				values[i++] = Int64GetDatumFast(tmp.aio_read_noneed);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_discrd);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_forgot);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_noblok);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_failed);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_wasted);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_waited);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_ontime);
+			}
 		}
 
 		Assert(i == (api_version == PGSS_V1_0 ? PG_STAT_STATEMENTS_COLS_V1_0 :
 					 api_version == PGSS_V1_1 ? PG_STAT_STATEMENTS_COLS_V1_1 :
 					 api_version == PGSS_V1_2 ? PG_STAT_STATEMENTS_COLS_V1_2 :
+					 api_version == PGSS_V1_3 ? PG_STAT_STATEMENTS_COLS_V1_3 :
 					 -1 /* fail if you forget to update this assert */ ));
 
 		tuplestore_putvalues(tupstore, tupdesc, values, nulls);
diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index b77c32c..1d43917 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -71,6 +71,28 @@
 #include "utils/syscache.h"
 #include "utils/tqual.h"
 
+#ifdef USE_PREFETCH
+#include <sys/types.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+#include "executor/instrument.h"
+
+extern unsigned int prefetch_dbOid; /*  database oid of relations on which prefetching to be done - 0 means all */
+extern unsigned int prefetch_heap_scans; /*  boolean whether to prefetch non-bitmap heap scans         */
+
+/*  special values for scan->rs_prefetch_target indicating as follows :               */
+#define PREFETCH_MAYBE 0xffffffff      /*   prefetch permitted but not yet in effect  */
+#define PREFETCH_DISABLED 0xfffffffe   /*   prefetch disabled and not permitted       */
+/*  PREFETCH_WRAP_POINT indicates a pretcher who has reached the point where the scan would wrap -
+**  at this point the prefetcher runs on the spot until scan catches up.
+**  This *must* be < maximum valid setting of target_prefetch_pages aka effective_io_concurrency.
+*/
+#define PREFETCH_WRAP_POINT 0x0fffffff
+
+#endif   /* USE_PREFETCH */
+
 
 /* GUC variable */
 bool		synchronize_seqscans = true;
@@ -115,6 +137,8 @@ static XLogRecPtr log_heap_new_cid(Relation relation, HeapTuple tup);
 static HeapTuple ExtractReplicaIdentity(Relation rel, HeapTuple tup, bool key_modified,
 					   bool *copy);
 
+static void heap_unread_add(HeapScanDesc scan, BlockNumber blockno);
+static void heap_unread_subtract(HeapScanDesc scan, BlockNumber blockno);
 
 /*
  * Each tuple lock mode has a corresponding heavyweight lock, and one or two
@@ -292,10 +316,149 @@ initscan(HeapScanDesc scan, ScanKey key, bool is_rescan)
 	 * Currently, we don't have a stats counter for bitmap heap scans (but the
 	 * underlying bitmap index scans will be counted).
 	 */
-	if (!scan->rs_bitmapscan)
+#ifdef USE_PREFETCH
+        /*    by default,  no prefetching on any scan  */
+        scan->rs_prefetch_target = PREFETCH_DISABLED;  /*  tentatively disable  */
+        scan->rs_pfchblock = 0; /*  scanner will reset this to be ahead of scan */
+        scan->rs_Unread_Pfetched_base = (BlockNumber *)0; /*  list of prefetched but unread blocknos */
+        scan->rs_Unread_Pfetched_next = 0; /*  next unread blockno */
+        scan->rs_Unread_Pfetched_count = 0; /* number of valid unread blocknos */
+#endif   /* USE_PREFETCH */
+	if (!scan->rs_bitmapscan) {
+
 		pgstat_count_heap_scan(scan->rs_rd);
+#ifdef USE_PREFETCH
+                /*    bitmap scans do their own prefetching -
+                **    for others,  set up prefetching now
+                */
+                if (    prefetch_heap_scans
+                     && (target_prefetch_pages > 0)
+                     &&	(!RelationUsesLocalBuffers(scan->rs_rd))
+                   ) {
+                      /*   prefetch_dbOid may be set to a database Oid to specify only prefetch in that db */
+                      if (    (   (prefetch_dbOid > 0)
+                               && (prefetch_dbOid == scan->rs_rd->rd_node.dbNode)
+                              )
+                          ||  (prefetch_dbOid == 0)
+                         ) {
+                          scan->rs_prefetch_target = PREFETCH_MAYBE;    /*  permitted but let the scan decide */
+                      }
+                      else {
+                      }
+                }
+#endif   /* USE_PREFETCH */
+        }
+}
+
+/* add this blockno to list of prefetched and unread blocknos
+** use the one identified by the (next+count|modulo circumference) index if it is unused,
+** else search for the first available slot if there is one,
+** else error.
+*/
+static void
+heap_unread_add(HeapScanDesc scan, BlockNumber blockno)
+{
+      BlockNumber *available_P;   /*  where to store new blockno */
+      unsigned int Unread_Pfetched_index = scan->rs_Unread_Pfetched_next
+                                         + scan->rs_Unread_Pfetched_count; /* index of next unused slot */
+
+      /*  caller is not supposed to pass InvalidBlockNumber but check anyway */
+      if (blockno != InvalidBlockNumber) {
+
+		  /*  ensure there is some room somewhere   */
+		  if (scan->rs_Unread_Pfetched_count < target_prefetch_pages) {
+
+			  /*  try the "next+count" one */
+			  if (Unread_Pfetched_index >= target_prefetch_pages) {
+				  Unread_Pfetched_index -= target_prefetch_pages;  /* modulo circumference */
+			  }
+			  available_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);   /*  where to store new blockno */
+			  if (*available_P == InvalidBlockNumber) { /* unused */
+				  goto store_blockno;
+			  } else {
+                  /*  slow-search the entire list  */
+				  for (Unread_Pfetched_index = 0; Unread_Pfetched_index < target_prefetch_pages; Unread_Pfetched_index++) {
+					  available_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);   /*  where to store new blockno */
+					  if (*available_P == InvalidBlockNumber) { /* unused */
+                          /*  before storing this blockno,
+                          **  since the next pointer did not locate an unused slot,
+                          **  set it to one which is more likely to be so for the next time
+                          */
+                          scan->rs_Unread_Pfetched_next = Unread_Pfetched_index;
+						  goto store_blockno;
+					  }
+				  }
+			  }
+		  }
+
+          /*  if we reach here,    either there was no available slot
+          **  or we thought there was one and didn't find any  --
+          */
+  		  ereport(ERROR,
+			  (errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("heap_unread_add overflowed list cannot add blockno %d", blockno)));
+
+  		  return;
+      }
+
+    store_blockno:
+      *available_P = blockno;
+      scan->rs_Unread_Pfetched_count++;  /*  update count */
+
+    return;
+}
+
+/* remove specified blockno from list of prefetched and unread blocknos.
+/* Usually this will be found at the rs_Unread_Pfetched_next item -
+** else search for it.    If not found,   inore it  -  no error results.
+*/
+static void
+heap_unread_subtract(HeapScanDesc scan, BlockNumber blockno)
+{
+      unsigned int Unread_Pfetched_index = scan->rs_Unread_Pfetched_next; /* index of next unread blockno */
+      BlockNumber *candidate_P;   /*  location of callers blockno - maybe */
+      BlockNumber nextUnreadPfetched;
+
+      /*  caller is not supposed to pass InvalidBlockNumber but check anyway */
+      if (    (blockno != InvalidBlockNumber)
+		   && ( scan->rs_Unread_Pfetched_count > 0 )   /*  if the list is not empty  */
+         ) {
+
+			  /*  take modulo of the circumference.
+			  **  actually rs_Unread_Pfetched_next should never exceed the circumference but check anyway.
+			  */
+			  if (Unread_Pfetched_index >= target_prefetch_pages) {
+				  Unread_Pfetched_index  -= target_prefetch_pages;
+}
+			  candidate_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);
+			  nextUnreadPfetched = *candidate_P;
+
+			  if ( nextUnreadPfetched == blockno ) {
+				  goto remove_blockno;
+			  } else {
+                  /*  slow-search the entire list  */
+				  for (Unread_Pfetched_index = 0; Unread_Pfetched_index < target_prefetch_pages; Unread_Pfetched_index++) {
+					  candidate_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);   /*  where to store new blockno */
+					  if (*candidate_P == blockno) { /* unused */
+						  goto remove_blockno;
+					  }
+				  }
+			  }
+
+    remove_blockno:
+			  *candidate_P = InvalidBlockNumber;
+
+			  scan->rs_Unread_Pfetched_next = (Unread_Pfetched_index+1);  /*  update next pfchd unread */
+			  if (scan->rs_Unread_Pfetched_next >= target_prefetch_pages) {
+					  scan->rs_Unread_Pfetched_next = 0;
+			  }
+			  scan->rs_Unread_Pfetched_count--;  /*  update count */
+	  }
+
+      return;
 }
 
+
 /*
  * heapgetpage - subroutine for heapgettup()
  *
@@ -304,7 +467,7 @@ initscan(HeapScanDesc scan, ScanKey key, bool is_rescan)
  * which tuples on the page are visible.
  */
 static void
-heapgetpage(HeapScanDesc scan, BlockNumber page)
+heapgetpage(HeapScanDesc scan, BlockNumber page , BlockNumber prefetchHWM)
 {
 	Buffer		buffer;
 	Snapshot	snapshot;
@@ -314,6 +477,10 @@ heapgetpage(HeapScanDesc scan, BlockNumber page)
 	OffsetNumber lineoff;
 	ItemId		lpp;
 	bool		all_visible;
+#ifdef USE_PREFETCH
+	int             PrefetchBufferRc;  /*   indicates whether requested prefetch block already in a buffer and if pin count on buffer has been increased */
+#endif   /* USE_PREFETCH */
+
 
 	Assert(page < scan->rs_nblocks);
 
@@ -336,6 +503,98 @@ heapgetpage(HeapScanDesc scan, BlockNumber page)
 									   RBM_NORMAL, scan->rs_strategy);
 	scan->rs_cblock = page;
 
+#ifdef USE_PREFETCH
+
+        heap_unread_subtract(scan, page);
+
+        /*    maybe prefetch some pages  starting with rs_pfchblock */
+        if (scan->rs_prefetch_target >= 0) {       /*   prefetching enabled on this scan ?                         */
+            int next_block_to_be_read = (page+1);  /*   next block to be read = lowest possible prefetchable block */
+            int num_to_pfch_this_time;             /*   eventually holds the number of blocks to prefetch now      */
+            int prefetchable_range;                /*   size of the area ahead of the current prefetch position    */
+
+            /*  check if prefetcher reached wrap point and the scan has now wrapped */
+            if (  (page == 0) && (scan->rs_prefetch_target == PREFETCH_WRAP_POINT)  ) {
+                scan->rs_prefetch_target = 1;
+                scan->rs_pfchblock = next_block_to_be_read;
+            } else
+            if (scan->rs_pfchblock < next_block_to_be_read) {
+                scan->rs_pfchblock = next_block_to_be_read; /* next block to be prefetched must be ahead of one we just read */
+            }
+
+            /* now we know where we would start prefetching -
+            ** next question   -  if this is a sync scan,  ensure we do not prefetch behind the HWM
+            ** debatable whether to require strict inequality or >=  -   >= works better in practice
+            */
+            if ( (!scan->rs_syncscan) || (scan->rs_pfchblock >= prefetchHWM) ) {
+
+                /* now we know where we will start prefetching -
+                ** next question   -  how many?
+                ** apply two limits :
+                **  1.   target prefetch distance
+                **  2.   number of available blocks ahead of us
+                */
+
+                /*  1.   target prefetch distance   */
+                num_to_pfch_this_time = next_block_to_be_read + scan->rs_prefetch_target; /* page beyond prefetch target */
+                num_to_pfch_this_time -= scan->rs_pfchblock;                              /*  convert to offset        */
+
+                /*   first do prefetching up to our current limit  ...
+                **   highest page number that a scan (pre)-fetches is scan->rs_nblocks-1
+                **   note  -  prefetcher does not wrap a prefetch range -
+                **            instead just stop and then start again if and when main scan wraps
+                */
+                if (scan->rs_pfchblock <= scan->rs_startblock) {  /*  if on second leg towards startblock */
+                    prefetchable_range = ((int)(scan->rs_startblock) - (int)(scan->rs_pfchblock));
+                }
+                else {                                            /*     on first leg towards nblocks     */
+                    prefetchable_range = ((int)(scan->rs_nblocks) - (int)(scan->rs_pfchblock));
+                }
+                if (prefetchable_range > 0) {           /*  if theres a range to prefetch */
+
+                    /*  2.   number of available blocks ahead of us        */
+                    if (num_to_pfch_this_time > prefetchable_range) {
+                        num_to_pfch_this_time = prefetchable_range;
+                    }
+                    while (num_to_pfch_this_time-- > 0) {
+                        PrefetchBufferRc = PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, scan->rs_pfchblock, scan->rs_strategy);
+                        /*  if pin acquired on buffer,  then remember in case of future Discard */
+                        if (PrefetchBufferRc & PREFTCHRC_BUF_PIN_INCREASED) {
+                            heap_unread_add(scan, scan->rs_pfchblock);
+						}
+                        scan->rs_pfchblock++;
+                        /*  if syncscan and requested block was already in buffer pool,
+                        **  this suggests that another scanner is ahead of us and we should advance
+                        */
+                        if ( (scan->rs_syncscan) && (PrefetchBufferRc & PREFTCHRC_BLK_ALREADY_PRESENT) ) {
+                            scan->rs_pfchblock++;
+                            num_to_pfch_this_time--;
+                        }
+                    }
+                }
+                else {
+                    /*   we must not modify scan->rs_pfchblock here
+                    **   because it is needed for possible DiscardBuffer at end of scan  ...
+                    **   ... instead ...
+                    */
+                    scan->rs_prefetch_target = PREFETCH_WRAP_POINT;  /*   mark this prefetcher as waiting to wrap */
+                }
+
+                /*   ...  then adjust prefetching limit : by doubling on each iteration */
+                if (scan->rs_prefetch_target == 0) {
+                    scan->rs_prefetch_target = 1;
+                }
+                else {
+                    scan->rs_prefetch_target *= 2;
+                    if (scan->rs_prefetch_target > target_prefetch_pages) {
+                        scan->rs_prefetch_target = target_prefetch_pages;
+                    }
+                }
+            }
+        }
+#endif   /* USE_PREFETCH */
+
+
 	if (!scan->rs_pageatatime)
 		return;
 
@@ -452,6 +711,8 @@ heapgettup(HeapScanDesc scan,
 	OffsetNumber lineoff;
 	int			linesleft;
 	ItemId		lpp;
+    BlockNumber  prefetchHWM = 0;   /*  HWM of prefetch BlockNum for all participants in same sync scan */
+    int          ix;
 
 	/*
 	 * calculate next starting lineoff, given scan direction
@@ -470,7 +731,25 @@ heapgettup(HeapScanDesc scan,
 				return;
 			}
 			page = scan->rs_startblock; /* first page */
-			heapgetpage(scan, page);
+#ifdef USE_PREFETCH
+                        /*  decide if we shall do prefetching :  only if :
+                        **   .  prefetching enabled for this scan
+                        **   .  not a bitmap scan (which do their own)
+                        **   .  sufficient number of blocks - at least twice the target_prefetch_pages
+                        */
+                        if (    (!scan->rs_bitmapscan)
+                             && (scan->rs_prefetch_target >= PREFETCH_MAYBE)
+                             && (scan->rs_nblocks > (2*target_prefetch_pages))
+                           )  {
+                                scan->rs_prefetch_target = 1;  /*  do prefetching on forward non-bitmap scan */
+                                scan->rs_Unread_Pfetched_base = (BlockNumber *)palloc(sizeof(BlockNumber)*target_prefetch_pages);
+                                /*  Initialise the list */
+                                for (ix = 0; ix < target_prefetch_pages; ix++) {
+									*(scan->rs_Unread_Pfetched_base + ix) = InvalidBlockNumber;
+								}
+                        }
+#endif   /* USE_PREFETCH */
+			heapgetpage(scan, page, prefetchHWM);
 			lineoff = FirstOffsetNumber;		/* first offnum */
 			scan->rs_inited = true;
 		}
@@ -516,7 +795,7 @@ heapgettup(HeapScanDesc scan,
 				page = scan->rs_startblock - 1;
 			else
 				page = scan->rs_nblocks - 1;
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 		}
 		else
 		{
@@ -557,7 +836,7 @@ heapgettup(HeapScanDesc scan,
 
 		page = ItemPointerGetBlockNumber(&(tuple->t_self));
 		if (page != scan->rs_cblock)
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 
 		/* Since the tuple was previously fetched, needn't lock page here */
 		dp = (Page) BufferGetPage(scan->rs_cbuf);
@@ -660,8 +939,10 @@ heapgettup(HeapScanDesc scan,
 			 * a little bit backwards on every invocation, which is confusing.
 			 * We don't guarantee any specific ordering in general, though.
 			 */
-			if (scan->rs_syncscan)
-				ss_report_location(scan->rs_rd, page);
+			if (scan->rs_syncscan) {
+                                prefetchHWM = scan->rs_pfchblock;
+				ss_report_location(scan->rs_rd, page, &prefetchHWM);
+                        }
 		}
 
 		/*
@@ -671,6 +952,22 @@ heapgettup(HeapScanDesc scan,
 		{
 			if (BufferIsValid(scan->rs_cbuf))
 				ReleaseBuffer(scan->rs_cbuf);
+#ifdef USE_PREFETCH
+                        if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                            /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                            while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+                        }
+#endif   /* USE_PREFETCH */
 			scan->rs_cbuf = InvalidBuffer;
 			scan->rs_cblock = InvalidBlockNumber;
 			tuple->t_data = NULL;
@@ -678,7 +975,7 @@ heapgettup(HeapScanDesc scan,
 			return;
 		}
 
-		heapgetpage(scan, page);
+		heapgetpage(scan, page, prefetchHWM);
 
 		LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);
 
@@ -727,6 +1024,8 @@ heapgettup_pagemode(HeapScanDesc scan,
 	OffsetNumber lineoff;
 	int			linesleft;
 	ItemId		lpp;
+    BlockNumber  prefetchHWM = 0;   /*  HWM of prefetch BlockNum for all participants in same sync scan */
+    int          ix;
 
 	/*
 	 * calculate next starting lineindex, given scan direction
@@ -745,7 +1044,25 @@ heapgettup_pagemode(HeapScanDesc scan,
 				return;
 			}
 			page = scan->rs_startblock; /* first page */
-			heapgetpage(scan, page);
+#ifdef USE_PREFETCH
+                        /*  decide if we shall do prefetching :  only if :
+                        **   .  prefetching enabled for this scan
+                        **   .  not a bitmap scan (which do their own)
+                        **   .  sufficient number of blocks - at least twice the target_prefetch_pages
+                        */
+                        if (    (!scan->rs_bitmapscan)
+                             && (scan->rs_prefetch_target >= PREFETCH_MAYBE)
+                             && (scan->rs_nblocks > (2*target_prefetch_pages))
+                           )  {
+                                scan->rs_prefetch_target = 1;  /*  do prefetching on forward non-bitmap scan */
+                                scan->rs_Unread_Pfetched_base = (BlockNumber *)palloc(sizeof(BlockNumber)*target_prefetch_pages);
+                                /*  Initialise the list */
+                                for (ix = 0; ix < target_prefetch_pages; ix++) {
+									*(scan->rs_Unread_Pfetched_base + ix) = InvalidBlockNumber;
+								}
+                        }
+#endif   /* USE_PREFETCH */
+			heapgetpage(scan, page, prefetchHWM);
 			lineindex = 0;
 			scan->rs_inited = true;
 		}
@@ -788,7 +1105,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 				page = scan->rs_startblock - 1;
 			else
 				page = scan->rs_nblocks - 1;
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 		}
 		else
 		{
@@ -826,7 +1143,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 
 		page = ItemPointerGetBlockNumber(&(tuple->t_self));
 		if (page != scan->rs_cblock)
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 
 		/* Since the tuple was previously fetched, needn't lock page here */
 		dp = (Page) BufferGetPage(scan->rs_cbuf);
@@ -921,8 +1238,10 @@ heapgettup_pagemode(HeapScanDesc scan,
 			 * a little bit backwards on every invocation, which is confusing.
 			 * We don't guarantee any specific ordering in general, though.
 			 */
-			if (scan->rs_syncscan)
-				ss_report_location(scan->rs_rd, page);
+			if (scan->rs_syncscan) {
+                                prefetchHWM = scan->rs_pfchblock;
+				ss_report_location(scan->rs_rd, page, &prefetchHWM);
+                        }
 		}
 
 		/*
@@ -932,6 +1251,22 @@ heapgettup_pagemode(HeapScanDesc scan,
 		{
 			if (BufferIsValid(scan->rs_cbuf))
 				ReleaseBuffer(scan->rs_cbuf);
+#ifdef USE_PREFETCH
+                        if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                            /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                             while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+                        }
+#endif   /* USE_PREFETCH */
 			scan->rs_cbuf = InvalidBuffer;
 			scan->rs_cblock = InvalidBlockNumber;
 			tuple->t_data = NULL;
@@ -939,7 +1274,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 			return;
 		}
 
-		heapgetpage(scan, page);
+		heapgetpage(scan, page, prefetchHWM);
 
 		dp = (Page) BufferGetPage(scan->rs_cbuf);
 		lines = scan->rs_ntuples;
@@ -1394,6 +1729,23 @@ void
 heap_rescan(HeapScanDesc scan,
 			ScanKey key)
 {
+
+#ifdef USE_PREFETCH
+         if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                       /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                            while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+         }
+#endif   /* USE_PREFETCH */
 	/*
 	 * unpin scan buffers
 	 */
@@ -1418,6 +1770,23 @@ heap_endscan(HeapScanDesc scan)
 {
 	/* Note: no locking manipulations needed */
 
+
+#ifdef USE_PREFETCH
+         if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                       /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                            while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+         }
+#endif   /* USE_PREFETCH */
 	/*
 	 * unpin scan buffers
 	 */
@@ -1435,6 +1804,10 @@ heap_endscan(HeapScanDesc scan)
 	if (scan->rs_strategy != NULL)
 		FreeAccessStrategy(scan->rs_strategy);
 
+    if (scan->rs_Unread_Pfetched_base) {
+        pfree(scan->rs_Unread_Pfetched_base);
+    }
+
 	if (scan->rs_temp_snap)
 		UnregisterSnapshot(scan->rs_snapshot);
 
@@ -1464,7 +1837,6 @@ heap_endscan(HeapScanDesc scan)
 #define HEAPDEBUG_3
 #endif   /* !defined(HEAPDEBUGALL) */
 
-
 HeapTuple
 heap_getnext(HeapScanDesc scan, ScanDirection direction)
 {
@@ -6347,6 +6719,25 @@ heap_markpos(HeapScanDesc scan)
 void
 heap_restrpos(HeapScanDesc scan)
 {
+
+
+#ifdef USE_PREFETCH
+                        if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                            /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                             while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+                        }
+#endif   /* USE_PREFETCH */
+
 	/* XXX no amrestrpos checking that ammarkpos called */
 
 	if (!ItemPointerIsValid(&scan->rs_mctid))
diff --git a/src/backend/access/heap/syncscan.c b/src/backend/access/heap/syncscan.c
index 7ea1ead..59e5255 100644
--- a/src/backend/access/heap/syncscan.c
+++ b/src/backend/access/heap/syncscan.c
@@ -90,6 +90,7 @@ typedef struct ss_scan_location_t
 {
 	RelFileNode relfilenode;	/* identity of a relation */
 	BlockNumber location;		/* last-reported location in the relation */
+	BlockNumber prefetchHWM;	/* high-water-mark of prefetched Blocknum */
 } ss_scan_location_t;
 
 typedef struct ss_lru_item_t
@@ -113,7 +114,7 @@ static ss_scan_locations_t *scan_locations;
 
 /* prototypes for internal functions */
 static BlockNumber ss_search(RelFileNode relfilenode,
-		  BlockNumber location, bool set);
+		  BlockNumber location, bool set , BlockNumber *prefetchHWMp);
 
 
 /*
@@ -160,6 +161,7 @@ SyncScanShmemInit(void)
 			item->location.relfilenode.dbNode = InvalidOid;
 			item->location.relfilenode.relNode = InvalidOid;
 			item->location.location = InvalidBlockNumber;
+			item->location.prefetchHWM = InvalidBlockNumber;
 
 			item->prev = (i > 0) ?
 				(&scan_locations->items[i - 1]) : NULL;
@@ -185,7 +187,7 @@ SyncScanShmemInit(void)
  * data structure.
  */
 static BlockNumber
-ss_search(RelFileNode relfilenode, BlockNumber location, bool set)
+ss_search(RelFileNode relfilenode, BlockNumber location, bool set , BlockNumber *prefetchHWMp)
 {
 	ss_lru_item_t *item;
 
@@ -206,6 +208,22 @@ ss_search(RelFileNode relfilenode, BlockNumber location, bool set)
 			{
 				item->location.relfilenode = relfilenode;
 				item->location.location = location;
+                                /*  if prefetch information requested,
+                                **  then reconcile and either update or report back the new HWM.
+                                */
+                                if (prefetchHWMp)
+                                {
+                                    if (   (item->location.prefetchHWM == InvalidBlockNumber)
+                                        || (item->location.prefetchHWM < *prefetchHWMp)
+                                       )
+                                    {
+                                      item->location.prefetchHWM = *prefetchHWMp;
+                                    }
+                                    else
+                                    {
+                                      *prefetchHWMp = item->location.prefetchHWM;
+                                    }
+                                }
 			}
 			else if (set)
 				item->location.location = location;
@@ -252,7 +270,7 @@ ss_get_location(Relation rel, BlockNumber relnblocks)
 	BlockNumber startloc;
 
 	LWLockAcquire(SyncScanLock, LW_EXCLUSIVE);
-	startloc = ss_search(rel->rd_node, 0, false);
+	startloc = ss_search(rel->rd_node, 0, false , 0);
 	LWLockRelease(SyncScanLock);
 
 	/*
@@ -282,7 +300,7 @@ ss_get_location(Relation rel, BlockNumber relnblocks)
  * same relfilenode.
  */
 void
-ss_report_location(Relation rel, BlockNumber location)
+ss_report_location(Relation rel, BlockNumber location , BlockNumber *prefetchHWMp)
 {
 #ifdef TRACE_SYNCSCAN
 	if (trace_syncscan)
@@ -306,7 +324,7 @@ ss_report_location(Relation rel, BlockNumber location)
 	{
 		if (LWLockConditionalAcquire(SyncScanLock, LW_EXCLUSIVE))
 		{
-			(void) ss_search(rel->rd_node, location, true);
+			(void) ss_search(rel->rd_node, location, true , prefetchHWMp);
 			LWLockRelease(SyncScanLock);
 		}
 #ifdef TRACE_SYNCSCAN
diff --git a/src/backend/access/index/genam.c b/src/backend/access/index/genam.c
index 850008b..53af580 100644
--- a/src/backend/access/index/genam.c
+++ b/src/backend/access/index/genam.c
@@ -77,6 +77,12 @@ RelationGetIndexScan(Relation indexRelation, int nkeys, int norderbys)
 
 	scan = (IndexScanDesc) palloc(sizeof(IndexScanDescData));
 
+#ifdef USE_PREFETCH
+        scan->do_prefetch = 0;             /*  no prefetching by default  */
+        scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+        scan->pfch_block_item_list = (struct pfch_block_item*)0;
+#endif   /* USE_PREFETCH */
+
 	scan->heapRelation = NULL;	/* may be set later */
 	scan->indexRelation = indexRelation;
 	scan->xs_snapshot = InvalidSnapshot;		/* caller must initialize this */
@@ -139,6 +145,19 @@ RelationGetIndexScan(Relation indexRelation, int nkeys, int norderbys)
 void
 IndexScanEnd(IndexScanDesc scan)
 {
+#ifdef USE_PREFETCH
+	if (scan->do_prefetch) {
+		if ( (struct pfch_block_item*)0 != scan->pfch_block_item_list ) {
+			pfree(scan->pfch_block_item_list);
+			scan->pfch_block_item_list = (struct pfch_block_item*)0;
+		}
+		if ( (struct pfch_index_pagelist*)0 != scan->pfch_index_page_list ) {
+			pfree(scan->pfch_index_page_list);
+			scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+		}
+	}
+#endif   /* USE_PREFETCH */
+
 	if (scan->keyData != NULL)
 		pfree(scan->keyData);
 	if (scan->orderByData != NULL)
diff --git a/src/backend/access/index/indexam.c b/src/backend/access/index/indexam.c
index 53cf96f..94f716c 100644
--- a/src/backend/access/index/indexam.c
+++ b/src/backend/access/index/indexam.c
@@ -79,6 +79,55 @@
 #include "utils/tqual.h"
 
 
+#ifdef USE_PREFETCH
+bool BlocknotinBuffer(Buffer buffer, Relation relation, BlockNumber blockNum);
+BlockNumber BlocknumOfBuffer(Buffer buffer);
+void index_mark_or_evict_block(IndexScanDesc scan , BlockNumber blocknumber , int markit);
+
+extern unsigned int prefetch_index_scans; /*  boolean whether to prefetch bitmap heap scans         */
+
+/*  if specified block number is present in the prefetch array,
+**  then either mark it as not to be discarded or evict it according to input param
+*/
+void index_mark_or_evict_block(IndexScanDesc scan , BlockNumber blocknumber , int markit)
+{
+        unsigned short int pfchx , pfchy , pfchz; /*  indexes in BlockIdData array   */
+
+        if (    scan->do_prefetch
+             && ((struct pfch_block_item*)0 != scan->pfch_block_item_list)
+             /* no need to check for scan->pfch_next < prefetch_index_scans
+             ** since we will do nothing if scan->pfch_used == 0
+             */
+           ) {
+            /*  search the prefetch list to find if the block is a member */
+            for (pfchx = 0; pfchx < scan->pfch_used; pfchx++) {
+                if (BlockIdGetBlockNumber(&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)) == blocknumber) {
+                      if (markit) {
+						      /*  mark it as not to be discarded */
+						      ((scan->pfch_block_item_list)+pfchx)->pfch_discard &= ~PREFTCHRC_BUF_PIN_INCREASED;
+					  } else {
+							  /*  shuffle all following the evictee to the left
+							  **  and update next pointer if its element moves
+							  */
+							  pfchy = (scan->pfch_used - 1); /*  current rightmost */
+							  scan->pfch_used = pfchy;
+
+							  while (pfchy > pfchx) {
+								  pfchz = pfchx + 1;
+								  BlockIdCopy((&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)), (&(((scan->pfch_block_item_list)+pfchz)->pfch_blockid)));
+								  ((scan->pfch_block_item_list)+pfchx)->pfch_discard = ((scan->pfch_block_item_list)+pfchz)->pfch_discard;
+								  if (scan->pfch_next == pfchz) {
+									  scan->pfch_next = pfchx;
+								  }
+								  pfchx = pfchz; /* advance */
+							  }
+                      }
+                }
+            }
+        }
+}
+#endif /* USE_PREFETCH */
+
 /* ----------------------------------------------------------------
  *					macros used in index_ routines
  *
@@ -253,6 +302,11 @@ index_beginscan(Relation heapRelation,
 	 */
 	scan->heapRelation = heapRelation;
 	scan->xs_snapshot = snapshot;
+#ifdef USE_PREFETCH
+	scan->do_prefetch = 0;             /*  no prefetching by default  */
+        scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+	scan->pfch_block_item_list = (struct pfch_block_item*)0;
+#endif   /* USE_PREFETCH */
 
 	return scan;
 }
@@ -277,6 +331,11 @@ index_beginscan_bitmap(Relation indexRelation,
 	 * up by RelationGetIndexScan.
 	 */
 	scan->xs_snapshot = snapshot;
+#ifdef USE_PREFETCH
+	scan->do_prefetch = 0;             /*  no prefetching by default  */
+        scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+	scan->pfch_block_item_list = (struct pfch_block_item*)0;
+#endif   /* USE_PREFETCH */
 
 	return scan;
 }
@@ -311,6 +370,9 @@ index_beginscan_internal(Relation indexRelation,
 									  Int32GetDatum(nkeys),
 									  Int32GetDatum(norderbys)));
 
+	scan->heap_tids_seen = 0;
+	scan->heap_tids_fetched = 0;
+	
 	return scan;
 }
 
@@ -342,6 +404,12 @@ index_rescan(IndexScanDesc scan,
 	/* Release any held pin on a heap page */
 	if (BufferIsValid(scan->xs_cbuf))
 	{
+#ifdef USE_PREFETCH
+                /*   if specified block number is present in the prefetch array,  then evict it */
+                if (scan->do_prefetch) {
+                      index_mark_or_evict_block(scan , BlocknumOfBuffer(scan->xs_cbuf) , 0);
+                }
+#endif   /* USE_PREFETCH */
 		ReleaseBuffer(scan->xs_cbuf);
 		scan->xs_cbuf = InvalidBuffer;
 	}
@@ -373,10 +441,30 @@ index_endscan(IndexScanDesc scan)
 	/* Release any held pin on a heap page */
 	if (BufferIsValid(scan->xs_cbuf))
 	{
+#ifdef USE_PREFETCH
+                /*   if specified block number is present in the prefetch array,  then evict it */
+                if (scan->do_prefetch) {
+                      index_mark_or_evict_block(scan , BlocknumOfBuffer(scan->xs_cbuf) , 0);
+                }
+#endif   /* USE_PREFETCH */
 		ReleaseBuffer(scan->xs_cbuf);
 		scan->xs_cbuf = InvalidBuffer;
 	}
 
+#ifdef USE_PREFETCH
+        /*   discard prefetched but unread buffers */
+        if (    scan->do_prefetch
+             && ((struct pfch_block_item*)0 != scan->pfch_block_item_list)
+           ) {
+            unsigned short int pfchx; /*  index in BlockIdData array   */
+                for (pfchx = 0; pfchx < scan->pfch_used; pfchx++) {
+                  if (((scan->pfch_block_item_list)+pfchx)->pfch_discard) {
+                      DiscardBuffer(scan->heapRelation, MAIN_FORKNUM, BlockIdGetBlockNumber(&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)));
+                  }
+                }
+        }
+#endif   /* USE_PREFETCH */
+
 	/* End the AM's scan */
 	FunctionCall1(procedure, PointerGetDatum(scan));
 
@@ -472,6 +560,12 @@ index_getnext_tid(IndexScanDesc scan, ScanDirection direction)
 		/* ... but first, release any held pin on a heap page */
 		if (BufferIsValid(scan->xs_cbuf))
 		{
+#ifdef USE_PREFETCH
+                    /*   if specified block number is present in the prefetch array,  then evict it */
+                    if (scan->do_prefetch) {
+                          index_mark_or_evict_block(scan , BlocknumOfBuffer(scan->xs_cbuf) , 0);
+                    }
+#endif   /* USE_PREFETCH */
 			ReleaseBuffer(scan->xs_cbuf);
 			scan->xs_cbuf = InvalidBuffer;
 		}
@@ -479,6 +573,11 @@ index_getnext_tid(IndexScanDesc scan, ScanDirection direction)
 	}
 
 	pgstat_count_index_tuples(scan->indexRelation, 1);
+	if (scan->heap_tids_seen++ >= (~0)) {
+		/* Avoid integer overflow */
+		scan->heap_tids_seen = 1;
+		scan->heap_tids_fetched = 0;
+	}
 
 	/* Return the TID of the tuple we found. */
 	return &scan->xs_ctup.t_self;
@@ -502,6 +601,10 @@ index_getnext_tid(IndexScanDesc scan, ScanDirection direction)
  * enough information to do it efficiently in the general case.
  * ----------------
  */
+#if defined(USE_PREFETCH) && defined(AVOID_CATALOG_MIGRATION_FOR_ASYNCIO)
+extern Datum btpeeknexttuple(IndexScanDesc scan);
+#endif /* USE_PREFETCH */
+
 HeapTuple
 index_fetch_heap(IndexScanDesc scan)
 {
@@ -509,16 +612,109 @@ index_fetch_heap(IndexScanDesc scan)
 	bool		all_dead = false;
 	bool		got_heap_tuple;
 
+
+
 	/* We can skip the buffer-switching logic if we're in mid-HOT chain. */
 	if (!scan->xs_continue_hot)
 	{
 		/* Switch to correct buffer if we don't have it already */
 		Buffer		prev_buf = scan->xs_cbuf;
 
+#ifdef USE_PREFETCH
+
+                /*   If the old block is different from new block, then evict old
+                **   block from prefetched array.   It is arguable we should leave it
+                **   in the array because it's likely to remain in the buffer pool
+                **   for a while,  but in that case , if we encounter the block
+                **   again,  prefetching it again does no harm.
+                **   (and note that,  if it's not pinned,  prefetching it will try to
+                **   pin it since prefetch tries to bank a pin for a buffer in the buffer pool).
+                **   therefore it should usually win.
+                */
+                if (    scan->do_prefetch
+                     && ( BufferIsValid(prev_buf) )
+                     && (BlocknotinBuffer(prev_buf,scan->heapRelation,ItemPointerGetBlockNumber(tid)))
+                     && (scan->pfch_next < prefetch_index_scans)  /* ensure there is an entry */
+                        ) {
+                          index_mark_or_evict_block(scan , BlocknumOfBuffer(prev_buf) , 0);
+                }
+
+#endif   /* USE_PREFETCH  */
 		scan->xs_cbuf = ReleaseAndReadBuffer(scan->xs_cbuf,
 											 scan->heapRelation,
 											 ItemPointerGetBlockNumber(tid));
 
+                /*   If the new block had been prefetched and pinned,
+                **   then mark that it no longer requires to be discarded.
+                **   Of course,  we don't evict the entry,
+                **   because we want to remember that it was recently prefetched.
+                */
+                index_mark_or_evict_block(scan , BlocknumOfBuffer(prev_buf) , 1);
+
+				scan->heap_tids_fetched++;
+
+#ifdef USE_PREFETCH
+                /*  try prefetching next data block
+                **    (next meaning one containing TIDs from matching keys
+                **     in same index page and different from any block
+                **     we previously prefetched and listed in prefetched array)
+                */
+                {
+                    FmgrInfo   *procedure;
+                    bool	found;             /*  did we find the "next" heap tid in current index page */
+                    int         PrefetchBufferRc;  /*  indicates whether requested prefetch block already in a buffer and if pin count on buffer has been increased */
+
+                    if (scan->do_prefetch) {
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+                        procedure = &scan->indexRelation->rd_aminfo->ampeeknexttuple; /* is incorrect but avoids adding function to catalog */
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                        if (RegProcedureIsValid(scan->indexRelation->rd_am->ampeeknexttuple)) {
+                            GET_SCAN_PROCEDURE(ampeeknexttuple); /* is correct but requires adding function to catalog */
+                        } else {
+                            procedure = 0;
+                        }
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+
+                        if (    procedure          /* does the index access method support peektuple? */
+#ifndef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+                             && procedure->fn_addr /* procedure->fn_addr is non-null only if in catalog */
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                           ) {
+                            int iterations = 1;      /*  how many iterations of prefetching shall we try  -
+                                                     **  if used entries in prefetch list is < target_prefetch_pages
+                                                     **  then 2,  else 1
+                                                     **  this should result in gradually and smoothly increasing up to target_prefetch_pages
+                                                     */
+                            /*  note we trust InitIndexScan verified this scan is forwards only and so set that */
+                            if (scan->pfch_used < target_prefetch_pages) {
+                                iterations = 2;
+                            }
+                            do {
+                                found =  DatumGetBool(
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+                                                         btpeeknexttuple(scan)     /*  pass scan as direct parameter since cant use fmgr because not in catalog */
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                                                         FunctionCall1(procedure, PointerGetDatum(scan)) /* use fmgr to call it because in catalog  */
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                                                     );
+                                if (found) {
+                                    /*    btpeeknexttuple set pfch_next to point to the item in block_item_list to be prefetched */
+                                    PrefetchBufferRc = PrefetchBuffer(scan->heapRelation, MAIN_FORKNUM, BlockIdGetBlockNumber((&((scan->pfch_block_item_list + scan->pfch_next))->pfch_blockid)) , 0);
+                                    /* elog(LOG,"index_fetch_heap prefetched rel %u blockNum %u"
+                                       ,scan->heapRelation->rd_node.relNode ,BlockIdGetBlockNumber(scan->pfch_block_item_list + scan->pfch_next));
+                                    */
+
+                                    /*  if pin acquired on buffer,  then remember in case of future Discard */
+                                    (scan->pfch_block_item_list + scan->pfch_next)->pfch_discard = (PrefetchBufferRc & PREFTCHRC_BUF_PIN_INCREASED);
+
+
+                                }
+                            } while (--iterations > 0);
+                        }
+                    }
+                }
+#endif   /* USE_PREFETCH */
+
 		/*
 		 * Prune page, but only if we weren't already on this page
 		 */
diff --git a/src/backend/access/nbtree/nbtinsert.c b/src/backend/access/nbtree/nbtinsert.c
index ecee5ac..e32a79f 100644
--- a/src/backend/access/nbtree/nbtinsert.c
+++ b/src/backend/access/nbtree/nbtinsert.c
@@ -793,7 +793,7 @@ _bt_insertonpg(Relation rel,
 		{
 			Assert(!P_ISLEAF(lpageop));
 
-			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE , (struct pfch_index_pagelist*)0);
 			metapg = BufferGetPage(metabuf);
 			metad = BTPageGetMeta(metapg);
 
@@ -972,7 +972,7 @@ _bt_split(Relation rel, Buffer buf, Buffer cbuf, OffsetNumber firstright,
 	bool		isleaf;
 
 	/* Acquire a new page to split into */
-	rbuf = _bt_getbuf(rel, P_NEW, BT_WRITE);
+	rbuf = _bt_getbuf(rel, P_NEW, BT_WRITE , (struct pfch_index_pagelist*)0);
 
 	/*
 	 * origpage is the original page to be split.  leftpage is a temporary
@@ -1175,7 +1175,7 @@ _bt_split(Relation rel, Buffer buf, Buffer cbuf, OffsetNumber firstright,
 
 	if (!P_RIGHTMOST(oopaque))
 	{
-		sbuf = _bt_getbuf(rel, oopaque->btpo_next, BT_WRITE);
+		sbuf = _bt_getbuf(rel, oopaque->btpo_next, BT_WRITE , (struct pfch_index_pagelist*)0);
 		spage = BufferGetPage(sbuf);
 		sopaque = (BTPageOpaque) PageGetSpecialPointer(spage);
 		if (sopaque->btpo_prev != origpagenumber)
@@ -1817,7 +1817,7 @@ _bt_finish_split(Relation rel, Buffer lbuf, BTStack stack)
 	Assert(P_INCOMPLETE_SPLIT(lpageop));
 
 	/* Lock right sibling, the one missing the downlink */
-	rbuf = _bt_getbuf(rel, lpageop->btpo_next, BT_WRITE);
+	rbuf = _bt_getbuf(rel, lpageop->btpo_next, BT_WRITE , (struct pfch_index_pagelist*)0);
 	rpage = BufferGetPage(rbuf);
 	rpageop = (BTPageOpaque) PageGetSpecialPointer(rpage);
 
@@ -1829,7 +1829,7 @@ _bt_finish_split(Relation rel, Buffer lbuf, BTStack stack)
 		BTMetaPageData *metad;
 
 		/* acquire lock on the metapage */
-		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE , (struct pfch_index_pagelist*)0);
 		metapg = BufferGetPage(metabuf);
 		metad = BTPageGetMeta(metapg);
 
@@ -1877,7 +1877,7 @@ _bt_getstackbuf(Relation rel, BTStack stack, int access)
 		Page		page;
 		BTPageOpaque opaque;
 
-		buf = _bt_getbuf(rel, blkno, access);
+		buf = _bt_getbuf(rel, blkno, access , (struct pfch_index_pagelist*)0);
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
@@ -2008,12 +2008,12 @@ _bt_newroot(Relation rel, Buffer lbuf, Buffer rbuf)
 	lopaque = (BTPageOpaque) PageGetSpecialPointer(lpage);
 
 	/* get a new root page */
-	rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE);
+	rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE , (struct pfch_index_pagelist*)0);
 	rootpage = BufferGetPage(rootbuf);
 	rootblknum = BufferGetBlockNumber(rootbuf);
 
 	/* acquire lock on the metapage */
-	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE , (struct pfch_index_pagelist*)0);
 	metapg = BufferGetPage(metabuf);
 	metad = BTPageGetMeta(metapg);
 
diff --git a/src/backend/access/nbtree/nbtpage.c b/src/backend/access/nbtree/nbtpage.c
index bab5a49..afa0263 100644
--- a/src/backend/access/nbtree/nbtpage.c
+++ b/src/backend/access/nbtree/nbtpage.c
@@ -127,7 +127,7 @@ _bt_getroot(Relation rel, int access)
 		Assert(rootblkno != P_NONE);
 		rootlevel = metad->btm_fastlevel;
 
-		rootbuf = _bt_getbuf(rel, rootblkno, BT_READ);
+		rootbuf = _bt_getbuf(rel, rootblkno, BT_READ , (struct pfch_index_pagelist*)0);
 		rootpage = BufferGetPage(rootbuf);
 		rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);
 
@@ -153,7 +153,7 @@ _bt_getroot(Relation rel, int access)
 		rel->rd_amcache = NULL;
 	}
 
-	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
+	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ , (struct pfch_index_pagelist*)0);
 	metapg = BufferGetPage(metabuf);
 	metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
 	metad = BTPageGetMeta(metapg);
@@ -209,7 +209,7 @@ _bt_getroot(Relation rel, int access)
 		 * the new root page.  Since this is the first page in the tree, it's
 		 * a leaf as well as the root.
 		 */
-		rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE);
+		rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE , (struct pfch_index_pagelist*)0);
 		rootblkno = BufferGetBlockNumber(rootbuf);
 		rootpage = BufferGetPage(rootbuf);
 		rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);
@@ -350,7 +350,7 @@ _bt_gettrueroot(Relation rel)
 		pfree(rel->rd_amcache);
 	rel->rd_amcache = NULL;
 
-	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
+	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ , (struct pfch_index_pagelist*)0);
 	metapg = BufferGetPage(metabuf);
 	metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
 	metad = BTPageGetMeta(metapg);
@@ -436,7 +436,7 @@ _bt_getrootheight(Relation rel)
 		Page		metapg;
 		BTPageOpaque metaopaque;
 
-		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
+		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ , (struct pfch_index_pagelist*)0);
 		metapg = BufferGetPage(metabuf);
 		metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
 		metad = BTPageGetMeta(metapg);
@@ -562,6 +562,170 @@ _bt_log_reuse_page(Relation rel, BlockNumber blkno, TransactionId latestRemovedX
 }
 
 /*
+ *	_bt_prefetchbuf() -- Prefetch a buffer by block number
+ *                           and keep track of prefetched and unread blocknums in pagelist.
+ *   input parms  :
+ *       rel and blockno identify block to be prefetched as usual
+ *       pfch_index_page_list_P points to the pointer anchoring the head of the index page list
+ *             Since the pagelist is a kind of optimization,
+ *             handle palloc failure by quietly omitting the keeping track.
+ */
+void
+_bt_prefetchbuf(Relation rel, BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P)
+{
+
+    int rc = 0;
+    struct pfch_index_pagelist* pfch_index_plp;      /*  pointer to current chunk */
+    struct pfch_index_item* found_item = 0;
+
+		if ((struct pfch_index_pagelist**)0 == pfch_index_page_list_P) {
+			pfch_index_plp = (struct pfch_index_pagelist*)0;
+		} else {
+			pfch_index_plp = *pfch_index_page_list_P;
+		}
+
+    	if (blkno != P_NEW && blkno != P_NONE)
+    	{
+            /* prefetch an existing block of the relation
+            ** but first,  check it has not recently already been prefetched and not yet read
+            */
+            found_item = _bt_find_block(blkno , pfch_index_plp);
+			if ((struct pfch_index_item*)0 == found_item) {  /*  not found */
+
+		        rc = PrefetchBuffer(rel, MAIN_FORKNUM, blkno , 0);
+
+                /*  add the pagenum to the list ,  indicating its discard status
+                **  since it's only an optimization,  ignore failure such as exceeded allowed space
+				*/
+                _bt_add_block( blkno , pfch_index_page_list_P , (uint32)(rc & PREFTCHRC_BUF_PIN_INCREASED));
+
+            }
+	    }
+        return;
+}
+
+/*   _bt_find_block finds the item referencing specified Block in index page list if present
+**   and returns the pointer to the pfch_index_item if found,  or null if not
+*/
+struct pfch_index_item*
+_bt_find_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list)
+{
+
+    struct pfch_index_item* found_item = 0;
+    struct pfch_index_pagelist* pfch_index_plp;      /*  pointer to current chunk */
+    int ix, tx;
+
+		pfch_index_plp = pfch_index_page_list;
+
+		while (     (struct pfch_index_pagelist*)0 != pfch_index_plp
+                &&  ( (struct pfch_index_item*)0 == found_item)
+              ) {
+			ix = 0;
+			tx = pfch_index_plp->pfch_index_item_count;
+			while (     (ix < tx)
+                    &&  ( (struct pfch_index_item*)0 == found_item)
+                  ) {
+				if (pfch_index_plp->pfch_indexid[ix].pfch_blocknum == blkno) {
+					found_item = &pfch_index_plp->pfch_indexid[ix];
+				}
+                ix++;
+			}
+			pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+		}
+
+     return found_item;
+}
+
+/*   _bt_add_block adds the specified Block to the index page list
+**   and returns 0 if successful,  non-zero if not
+*/
+int
+_bt_add_block(BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P , uint32 discard_status)
+{
+    int rc = 1;
+    int ix;
+    struct pfch_index_pagelist* pfch_index_plp;      /*  pointer to current chunk */
+    struct pfch_index_pagelist* pfch_index_page_list_anchor; /*  pointer to first chunk if any  */
+	/*  allow expansion of pagelist to 16 chunks
+	**  which accommodates backwards-sequential index scans
+	**  where the scanner increases target_prefetch_pages by a factor of up to 16
+	**   see code in _bt_steppage
+	**  note - this creates an undesirable weak dependency on this number in _bt_steppage,
+	**         but :
+	**           there is no disaster if the numbers disagree  -  just sub-optimal use of the list
+	**           to implement a proper interface would require that chunks have a variable size
+	**           which would require an extra size variable in each chunk
+	*/
+	int num_chunks = 16;
+
+		if ((struct pfch_index_pagelist**)0 == pfch_index_page_list_P) {
+			pfch_index_page_list_anchor = (struct pfch_index_pagelist*)0;
+		} else {
+			pfch_index_page_list_anchor = *pfch_index_page_list_P;
+		}
+		pfch_index_plp = pfch_index_page_list_anchor;       /* pointer to current chunk */
+
+		while ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+			ix = pfch_index_plp->pfch_index_item_count;
+			if (ix < target_prefetch_pages) {
+				pfch_index_plp->pfch_indexid[ix].pfch_blocknum = blkno;
+				pfch_index_plp->pfch_indexid[ix].pfch_discard = discard_status;
+				pfch_index_plp->pfch_index_item_count = (ix+1);
+                rc = 0;
+				goto stored_pagenum;
+			}
+			pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+			num_chunks--;              /*  keep track of number of chunks */
+		}
+
+		/*   we did not find any free space in existing chunks -
+		**   create new chunk if within our limit and we have a pfch_index_page_list
+		*/
+		if ( (num_chunks > 0) && ((struct pfch_index_pagelist*)0 != pfch_index_page_list_anchor) ) {
+			pfch_index_plp = (struct pfch_index_pagelist*)palloc( sizeof(struct pfch_index_pagelist) + ( (target_prefetch_pages-1) * sizeof(struct pfch_index_item) ) );
+			if ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+				pfch_index_plp->pfch_index_pagelist_next = pfch_index_page_list_anchor;  /* old head of list is next after this */
+				pfch_index_plp->pfch_indexid[0].pfch_blocknum = blkno;
+				pfch_index_plp->pfch_indexid[0].pfch_discard = discard_status;
+				pfch_index_plp->pfch_index_item_count = 1;
+				pfch_index_page_list_P = &pfch_index_plp;   /*  new head of list is new chunk */
+                rc = 0;
+			}
+		}
+
+    stored_pagenum:;
+     return rc;
+}
+
+/*  _bt_subtract_block removes a block from the prefetched-but-unread pagelist if present */
+void
+_bt_subtract_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list)
+{
+    struct pfch_index_pagelist* pfch_index_plp = pfch_index_page_list;
+	if ( (blkno != P_NEW) && (blkno != P_NONE) ) {
+            int ix , jx;
+                while ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+                    ix = pfch_index_plp->pfch_index_item_count;
+                    while (ix-- > 0) {
+                        if (pfch_index_plp->pfch_indexid[ix].pfch_blocknum == blkno) {
+                            /*   move the last item to the curent (now deleted) position and decrement count */
+                            jx = (pfch_index_plp->pfch_index_item_count-1); /*  index of last item ... */
+                            if (jx > ix) {                                  /*  ... is not the current one so move is required */
+                                pfch_index_plp->pfch_indexid[ix].pfch_blocknum = pfch_index_plp->pfch_indexid[jx].pfch_blocknum;
+                                pfch_index_plp->pfch_indexid[ix].pfch_discard = pfch_index_plp->pfch_indexid[jx].pfch_discard;
+                                ix = jx;
+                            }
+                            pfch_index_plp->pfch_index_item_count = ix;
+                            goto done_subtract;
+                        }
+                    }
+                    pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+                }
+        }
+    done_subtract:  return;
+}
+
+/*
  *	_bt_getbuf() -- Get a buffer by block number for read or write.
  *
  *		blkno == P_NEW means to get an unallocated index page.  The page
@@ -573,7 +737,7 @@ _bt_log_reuse_page(Relation rel, BlockNumber blkno, TransactionId latestRemovedX
  *		_bt_checkpage to sanity-check the page (except in P_NEW case).
  */
 Buffer
-_bt_getbuf(Relation rel, BlockNumber blkno, int access)
+_bt_getbuf(Relation rel, BlockNumber blkno, int access , struct pfch_index_pagelist* pfch_index_page_list)
 {
 	Buffer		buf;
 
@@ -581,6 +745,10 @@ _bt_getbuf(Relation rel, BlockNumber blkno, int access)
 	{
 		/* Read an existing block of the relation */
 		buf = ReadBuffer(rel, blkno);
+
+        /*  if the block is in the prefetched-but-unread pagelist,  remove it */
+        _bt_subtract_block( blkno , pfch_index_page_list);
+
 		LockBuffer(buf, access);
 		_bt_checkpage(rel, buf);
 	}
@@ -702,6 +870,10 @@ _bt_getbuf(Relation rel, BlockNumber blkno, int access)
  * bufmgr when one would do.  However, now it's mainly just a notational
  * convenience.  The only case where it saves work over _bt_relbuf/_bt_getbuf
  * is when the target page is the same one already in the buffer.
+ *
+ * if prefetching of index pages is changed to use this function,
+ * then it should be extended to take the index_page_list as parameter
+ * and call_bt_subtract_block in the same way that _bt_getbuf does.
  */
 Buffer
 _bt_relandgetbuf(Relation rel, Buffer obuf, BlockNumber blkno, int access)
@@ -712,6 +884,7 @@ _bt_relandgetbuf(Relation rel, Buffer obuf, BlockNumber blkno, int access)
 	if (BufferIsValid(obuf))
 		LockBuffer(obuf, BUFFER_LOCK_UNLOCK);
 	buf = ReleaseAndReadBuffer(obuf, rel, blkno);
+
 	LockBuffer(buf, access);
 	_bt_checkpage(rel, buf);
 	return buf;
@@ -965,7 +1138,7 @@ _bt_is_page_halfdead(Relation rel, BlockNumber blk)
 	BTPageOpaque opaque;
 	bool		result;
 
-	buf = _bt_getbuf(rel, blk, BT_READ);
+	buf = _bt_getbuf(rel, blk, BT_READ , (struct pfch_index_pagelist*)0);
 	page = BufferGetPage(buf);
 	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
@@ -1069,7 +1242,7 @@ _bt_lock_branch_parent(Relation rel, BlockNumber child, BTStack stack,
 				Page		lpage;
 				BTPageOpaque lopaque;
 
-				lbuf = _bt_getbuf(rel, leftsib, BT_READ);
+				lbuf = _bt_getbuf(rel, leftsib, BT_READ, (struct pfch_index_pagelist*)0);
 				lpage = BufferGetPage(lbuf);
 				lopaque = (BTPageOpaque) PageGetSpecialPointer(lpage);
 
@@ -1265,7 +1438,7 @@ _bt_pagedel(Relation rel, Buffer buf)
 					BTPageOpaque lopaque;
 					Page		lpage;
 
-					lbuf = _bt_getbuf(rel, leftsib, BT_READ);
+					lbuf = _bt_getbuf(rel, leftsib, BT_READ, (struct pfch_index_pagelist*)0);
 					lpage = BufferGetPage(lbuf);
 					lopaque = (BTPageOpaque) PageGetSpecialPointer(lpage);
 					/*
@@ -1340,7 +1513,7 @@ _bt_pagedel(Relation rel, Buffer buf)
 		if (!rightsib_empty)
 			break;
 
-		buf = _bt_getbuf(rel, rightsib, BT_WRITE);
+		buf = _bt_getbuf(rel, rightsib, BT_WRITE, (struct pfch_index_pagelist*)0);
 	}
 
 	return ndeleted;
@@ -1593,7 +1766,7 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, bool *rightsib_empty)
 		target = topblkno;
 
 		/* fetch the block number of the topmost parent's left sibling */
-		buf = _bt_getbuf(rel, topblkno, BT_READ);
+		buf = _bt_getbuf(rel, topblkno, BT_READ, (struct pfch_index_pagelist*)0);
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		leftsib = opaque->btpo_prev;
@@ -1632,7 +1805,7 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, bool *rightsib_empty)
 		LockBuffer(leafbuf, BT_WRITE);
 	if (leftsib != P_NONE)
 	{
-		lbuf = _bt_getbuf(rel, leftsib, BT_WRITE);
+		lbuf = _bt_getbuf(rel, leftsib, BT_WRITE , (struct pfch_index_pagelist*)0);
 		page = BufferGetPage(lbuf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		while (P_ISDELETED(opaque) || opaque->btpo_next != target)
@@ -1646,7 +1819,7 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, bool *rightsib_empty)
 					 RelationGetRelationName(rel));
 				return false;
 			}
-			lbuf = _bt_getbuf(rel, leftsib, BT_WRITE);
+			lbuf = _bt_getbuf(rel, leftsib, BT_WRITE , (struct pfch_index_pagelist*)0);
 			page = BufferGetPage(lbuf);
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		}
@@ -1701,7 +1874,7 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, bool *rightsib_empty)
 	 * And next write-lock the (current) right sibling.
 	 */
 	rightsib = opaque->btpo_next;
-	rbuf = _bt_getbuf(rel, rightsib, BT_WRITE);
+	rbuf = _bt_getbuf(rel, rightsib, BT_WRITE , (struct pfch_index_pagelist*)0);
 	page = BufferGetPage(rbuf);
 	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 	if (opaque->btpo_prev != target)
@@ -1731,7 +1904,7 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, bool *rightsib_empty)
 		if (P_RIGHTMOST(opaque))
 		{
 			/* rightsib will be the only one left on the level */
-			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE , (struct pfch_index_pagelist*)0);
 			metapg = BufferGetPage(metabuf);
 			metad = BTPageGetMeta(metapg);
 
diff --git a/src/backend/access/nbtree/nbtree.c b/src/backend/access/nbtree/nbtree.c
index 36dc6c2..ceb5af9 100644
--- a/src/backend/access/nbtree/nbtree.c
+++ b/src/backend/access/nbtree/nbtree.c
@@ -30,6 +30,18 @@
 #include "tcop/tcopprot.h"
 #include "utils/memutils.h"
 
+#ifdef USE_PREFETCH
+extern unsigned int prefetch_index_scans; /* boolean whether to prefetch non-bitmap index scans also numeric size of pfch_list */
+#endif   /* USE_PREFETCH */
+
+Datum
+btpeeknexttuple(
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+        IndexScanDesc scan
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+        PG_FUNCTION_ARGS
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+);
 
 /* Working state for btbuild and its callback */
 typedef struct
@@ -332,6 +344,74 @@ btgettuple(PG_FUNCTION_ARGS)
 }
 
 /*
+ *	btpeeknexttuple() -- peek at the next tuple different from any blocknum in pfch_block_item_list
+ *                           without reading a new index page
+ *                       and without causing any side-effects such as altering values in control blocks
+ *               if found,     store blocknum in next element of pfch_block_item_list
+ */
+Datum
+btpeeknexttuple(
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+        IndexScanDesc scan
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+        PG_FUNCTION_ARGS
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+)
+{
+#ifndef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+    IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+	BTScanOpaque so = (BTScanOpaque) scan->opaque;
+	bool		res = false;
+	int		itemIndex;		/* current index in items[] */
+
+        /*
+         * If we've already initialized this scan, we can just advance it in
+         * the appropriate direction.  If we haven't done so yet, bail out
+         */
+        if ( BTScanPosIsValid(so->currPos) ) {
+
+            itemIndex = so->currPos.itemIndex+1;    /*   next item */
+
+            /* This loop handles advancing till we find different data block or end of index page */
+            while (itemIndex <= so->currPos.lastItem) {
+                unsigned short int pfchx; /*  index in BlockIdData array   */
+                for (pfchx = 0; pfchx < scan->pfch_used; pfchx++) {
+                        if (BlockIdEquals((&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)) , &(so->currPos.items[itemIndex].heapTid.ip_blkid))) {
+                             goto block_match;
+                        }
+                }
+
+                /*  if we reach here,  no block in list matched this item  */
+                res = true;
+                /*   set item in prefetch list
+                **   prefer unused entry if there is one,  else overwrite
+                */
+                if (scan->pfch_used < prefetch_index_scans) {
+                    scan->pfch_next = scan->pfch_used;
+                } else {
+                    scan->pfch_next++;
+                    if (scan->pfch_next >= prefetch_index_scans) {
+                        scan->pfch_next = 0;
+                    }
+                }
+
+                BlockIdCopy((&((scan->pfch_block_item_list + scan->pfch_next)->pfch_blockid)) , &(so->currPos.items[itemIndex].heapTid.ip_blkid));
+                if (scan->pfch_used <= scan->pfch_next) {
+                     scan->pfch_used = (scan->pfch_next + 1);
+                }
+
+                goto peek_complete;
+
+      block_match:         itemIndex++;
+            }
+	}
+
+ peek_complete:
+	PG_RETURN_BOOL(res);
+}
+
+/*
  * btgetbitmap() -- gets all matching tuples, and adds them to a bitmap
  */
 Datum
@@ -425,6 +505,12 @@ btbeginscan(PG_FUNCTION_ARGS)
 	so->killedItems = NULL;		/* until needed */
 	so->numKilled = 0;
 
+	so->backSeqRun = 0;
+	so->backSeqPos = 0;
+	so->prefetchItemIndex = 0;
+	so->lastHeapPrefetchBlkno = P_NONE;
+	so->prefetchBlockCount = 0;
+	
 	/*
 	 * We don't know yet whether the scan will be index-only, so we do not
 	 * allocate the tuple workspace arrays until btrescan.  However, we set up
@@ -516,6 +602,23 @@ btendscan(PG_FUNCTION_ARGS)
 {
 	IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
+        struct pfch_index_pagelist* pfch_index_plp;
+        int ix;
+
+#ifdef USE_PREFETCH
+
+	/* discard all prefetched but unread index pages listed in the pagelist */
+        pfch_index_plp = scan->pfch_index_page_list;
+        while ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+                    ix = pfch_index_plp->pfch_index_item_count;
+                    while (ix-- > 0) {
+                        if (pfch_index_plp->pfch_indexid[ix].pfch_discard) {
+                            DiscardBuffer( scan->indexRelation , MAIN_FORKNUM , pfch_index_plp->pfch_indexid[ix].pfch_blocknum);
+                        }
+                    }
+                    pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+        }
+#endif /* USE_PREFETCH */
 
 	/* we aren't holding any read locks, but gotta drop the pins */
 	if (BTScanPosIsValid(so->currPos))
diff --git a/src/backend/access/nbtree/nbtsearch.c b/src/backend/access/nbtree/nbtsearch.c
index 203b969..bee1f12 100644
--- a/src/backend/access/nbtree/nbtsearch.c
+++ b/src/backend/access/nbtree/nbtsearch.c
@@ -23,13 +23,16 @@
 #include "utils/lsyscache.h"
 #include "utils/rel.h"
 
+extern unsigned int prefetch_btree_heaps;  /*  boolean whether to prefetch heap pages in _bt_next for non-bitmap index scans   */
+extern unsigned int prefetch_sequential_index_scans;  /*  boolean whether to prefetch sequential-access non-bitmap index scans  */
 
 static bool _bt_readpage(IndexScanDesc scan, ScanDirection dir,
 			 OffsetNumber offnum);
 static void _bt_saveitem(BTScanOpaque so, int itemIndex,
 			 OffsetNumber offnum, IndexTuple itup);
-static bool _bt_steppage(IndexScanDesc scan, ScanDirection dir);
-static Buffer _bt_walk_left(Relation rel, Buffer buf);
+static bool _bt_steppage(IndexScanDesc scan, ScanDirection dir, 
+			 bool prefetch);
+static Buffer _bt_walk_left(IndexScanDesc scan, Relation rel, Buffer buf);
 static bool _bt_endpoint(IndexScanDesc scan, ScanDirection dir);
 
 
@@ -226,7 +229,7 @@ _bt_moveright(Relation rel,
 				_bt_relbuf(rel, buf);
 
 			/* re-acquire the lock in the right mode, and re-check */
-			buf = _bt_getbuf(rel, blkno, access);
+			buf = _bt_getbuf(rel, blkno, access , (struct pfch_index_pagelist*)0);
 			continue;
 		}
 
@@ -1005,7 +1008,7 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 		 * There's no actually-matching data on this page.  Try to advance to
 		 * the next page.  Return false if there's no matching data at all.
 		 */
-		if (!_bt_steppage(scan, dir))
+		if (!_bt_steppage(scan, dir, false))
 			return false;
 	}
 
@@ -1040,6 +1043,8 @@ _bt_next(IndexScanDesc scan, ScanDirection dir)
 {
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
 	BTScanPosItem *currItem;
+	BlockNumber prevblkno = ItemPointerGetBlockNumber(
+		&scan->xs_ctup.t_self);
 
 	/*
 	 * Advance to next tuple on current page; or if there's no more, try to
@@ -1052,11 +1057,53 @@ _bt_next(IndexScanDesc scan, ScanDirection dir)
 			/* We must acquire lock before applying _bt_steppage */
 			Assert(BufferIsValid(so->currPos.buf));
 			LockBuffer(so->currPos.buf, BT_READ);
-			if (!_bt_steppage(scan, dir))
+			if (!_bt_steppage(scan, dir, target_prefetch_pages > 0))
 				return false;
 			/* Drop the lock, but not pin, on the new page */
 			LockBuffer(so->currPos.buf, BUFFER_LOCK_UNLOCK);
 		}
+		
+		if (prefetch_btree_heaps && (target_prefetch_pages > 0)) {
+			BlockNumber currblkno = ItemPointerGetBlockNumber(
+				&so->currPos.items[so->currPos.itemIndex].heapTid);
+		
+			if (currblkno != prevblkno) {
+				if (so->prefetchBlockCount > 0)
+					so->prefetchBlockCount--;
+			
+				/* If we have heap fetch frequency stats, and it's above ~94%,
+				 * initiate heap prefetches */
+				if (so->currPos.moreRight
+					&& scan->heap_tids_seen > 256 
+					&& ( (scan->heap_tids_seen - scan->heap_tids_seen/16)
+						 <= scan->heap_tids_fetched ) )
+				{
+					bool nonsequential = false;
+				
+					if (so->prefetchItemIndex <= so->currPos.itemIndex)
+						so->prefetchItemIndex = so->currPos.itemIndex + 1;
+					while (    (so->prefetchItemIndex <= so->currPos.lastItem)
+							&& (so->prefetchBlockCount < target_prefetch_pages) )
+					{
+						ItemPointer tid = &so->currPos.items[so->prefetchItemIndex++].heapTid;
+						BlockNumber blkno = ItemPointerGetBlockNumber(tid);
+						if (blkno != so->lastHeapPrefetchBlkno) {  /*  if not a repetition of previous block */
+							/*   start prefetch on next page, providing :
+                            **   EITHER  .  we're reading non-sequentially previously or for this block
+                            **   OR      .  user explicitly specified to prefetch for sequential pattern
+							**   as it may be counterproductive otherwise
+                            */
+							nonsequential = (nonsequential || blkno != (so->lastHeapPrefetchBlkno+1));
+							if (prefetch_sequential_index_scans || nonsequential) {
+								_bt_prefetchbuf(scan->heapRelation, blkno , &scan->pfch_index_page_list );
+							}
+							so->lastHeapPrefetchBlkno = blkno;
+							so->prefetchBlockCount++;
+						}
+					}
+				}
+			}
+		}
 	}
 	else
 	{
@@ -1065,11 +1112,53 @@ _bt_next(IndexScanDesc scan, ScanDirection dir)
 			/* We must acquire lock before applying _bt_steppage */
 			Assert(BufferIsValid(so->currPos.buf));
 			LockBuffer(so->currPos.buf, BT_READ);
-			if (!_bt_steppage(scan, dir))
+			if (!_bt_steppage(scan, dir, target_prefetch_pages > 0))
 				return false;
 			/* Drop the lock, but not pin, on the new page */
 			LockBuffer(so->currPos.buf, BUFFER_LOCK_UNLOCK);
 		}
+		
+		if (prefetch_btree_heaps && (target_prefetch_pages > 0)) {
+			BlockNumber currblkno = ItemPointerGetBlockNumber(
+				&so->currPos.items[so->currPos.itemIndex].heapTid);
+		
+			if (currblkno != prevblkno) {
+				if (so->prefetchBlockCount > 0)
+					so->prefetchBlockCount--;
+			
+				/* If we have heap fetch frequency stats, and it's above ~94%,
+				 * initiate heap prefetches */
+				if (so->currPos.moreLeft
+					&& scan->heap_tids_seen > 256 
+					&& ( (scan->heap_tids_seen - scan->heap_tids_seen/16)
+						 <= scan->heap_tids_fetched ) )
+				{
+					bool nonsequential = false;
+			
+					if (so->prefetchItemIndex >= so->currPos.itemIndex)
+						so->prefetchItemIndex = so->currPos.itemIndex - 1;
+					while (    (so->prefetchItemIndex >= so->currPos.firstItem)
+							&& (so->prefetchBlockCount < target_prefetch_pages) )
+					{
+						ItemPointer tid = &so->currPos.items[so->prefetchItemIndex--].heapTid;
+						BlockNumber blkno = ItemPointerGetBlockNumber(tid);
+						if (blkno != so->lastHeapPrefetchBlkno) {  /*  if not a repetition of previous block */
+							/*   start prefetch on next page, providing :
+                            **   EITHER  .  we're reading non-sequentially previously or for this block
+                            **   OR      .  user explicitly specified to prefetch for sequential pattern
+							**   as it may be counterproductive otherwise
+                            */
+							nonsequential = (nonsequential || blkno != (so->lastHeapPrefetchBlkno+1));
+							if (prefetch_sequential_index_scans || nonsequential) {
+								_bt_prefetchbuf(scan->heapRelation, blkno , &scan->pfch_index_page_list );
+							}
+							so->lastHeapPrefetchBlkno = blkno;
+							so->prefetchBlockCount++;
+						}
+					}
+				}
+			}
+		}
 	}
 
 	/* OK, itemIndex says what to return */
@@ -1119,9 +1208,11 @@ _bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum)
 	/*
 	 * we must save the page's right-link while scanning it; this tells us
 	 * where to step right to after we're done with these items.  There is no
-	 * corresponding need for the left-link, since splits always go right.
+	 * corresponding need for the left-link, since splits always go right,
+	 * but we need it for back-sequential scan detection.
 	 */
 	so->currPos.nextPage = opaque->btpo_next;
+	so->currPos.prevPage = opaque->btpo_prev;
 
 	/* initialize tuple workspace to empty */
 	so->currPos.nextTupleOffset = 0;
@@ -1156,6 +1247,7 @@ _bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum)
 		so->currPos.firstItem = 0;
 		so->currPos.lastItem = itemIndex - 1;
 		so->currPos.itemIndex = 0;
+		so->prefetchItemIndex = 0;
 	}
 	else
 	{
@@ -1187,6 +1279,7 @@ _bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum)
 		so->currPos.firstItem = itemIndex;
 		so->currPos.lastItem = MaxIndexTuplesPerPage - 1;
 		so->currPos.itemIndex = MaxIndexTuplesPerPage - 1;
+		so->prefetchItemIndex = MaxIndexTuplesPerPage - 1;
 	}
 
 	return (so->currPos.firstItem <= so->currPos.lastItem);
@@ -1224,7 +1317,7 @@ _bt_saveitem(BTScanOpaque so, int itemIndex,
  * locks and pins, set so->currPos.buf to InvalidBuffer, and return FALSE.
  */
 static bool
-_bt_steppage(IndexScanDesc scan, ScanDirection dir)
+_bt_steppage(IndexScanDesc scan, ScanDirection dir, bool prefetch)
 {
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
 	Relation	rel;
@@ -1278,7 +1371,7 @@ _bt_steppage(IndexScanDesc scan, ScanDirection dir)
 			/* check for interrupts while we're not holding any buffer lock */
 			CHECK_FOR_INTERRUPTS();
 			/* step right one page */
-			so->currPos.buf = _bt_getbuf(rel, blkno, BT_READ);
+			so->currPos.buf = _bt_getbuf(rel, blkno, BT_READ , scan->pfch_index_page_list);
 			/* check for deleted page */
 			page = BufferGetPage(so->currPos.buf);
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
@@ -1287,9 +1380,20 @@ _bt_steppage(IndexScanDesc scan, ScanDirection dir)
 				PredicateLockPage(rel, blkno, scan->xs_snapshot);
 				/* see if there are any matches on this page */
 				/* note that this will clear moreRight if we can stop */
-				if (_bt_readpage(scan, dir, P_FIRSTDATAKEY(opaque)))
+				if (_bt_readpage(scan, dir, P_FIRSTDATAKEY(opaque))) {
+					if (    prefetch && so->currPos.moreRight
+						/*   start prefetch on next page, providing :
+						**   EITHER  .  we're reading non-sequentially for this block
+						**   OR      .  user explicitly specified to prefetch for sequential pattern
+						**   as it may be counterproductive otherwise
+						*/
+						&& (prefetch_sequential_index_scans || opaque->btpo_next != (blkno+1))
+                       ) {
+ 						  _bt_prefetchbuf(rel, opaque->btpo_next , &scan->pfch_index_page_list);
+					}
 					break;
 			}
+			}
 			/* nope, keep going */
 			blkno = opaque->btpo_next;
 		}
@@ -1317,7 +1421,7 @@ _bt_steppage(IndexScanDesc scan, ScanDirection dir)
 			}
 
 			/* Step to next physical page */
-			so->currPos.buf = _bt_walk_left(rel, so->currPos.buf);
+			so->currPos.buf = _bt_walk_left(scan , rel, so->currPos.buf);
 
 			/* if we're physically at end of index, return failure */
 			if (so->currPos.buf == InvalidBuffer)
@@ -1332,14 +1436,58 @@ _bt_steppage(IndexScanDesc scan, ScanDirection dir)
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 			if (!P_IGNORE(opaque))
 			{
+				/* We must rely on the previously saved prevPage link! */
+				BlockNumber blkno = so->currPos.prevPage;
+				
 				PredicateLockPage(rel, BufferGetBlockNumber(so->currPos.buf), scan->xs_snapshot);
 				/* see if there are any matches on this page */
 				/* note that this will clear moreLeft if we can stop */
-				if (_bt_readpage(scan, dir, PageGetMaxOffsetNumber(page)))
+				if (_bt_readpage(scan, dir, PageGetMaxOffsetNumber(page))) {
+					if (prefetch && so->currPos.moreLeft) {
+						/* detect back-sequential runs and increase prefetch window blindly 
+						 * downwards 2 blocks at a time. This only works in our favor
+						 * for index-only scans, by merging read requests at the kernel,
+						 * so we want to inflate target_prefetch_pages since merged 
+						 * back-sequential requests are about as expensive as a single one 
+						 */
+						if (scan->xs_want_itup && blkno > 0 && opaque->btpo_prev == (blkno-1)) {
+							BlockNumber backPos;
+							unsigned int back_prefetch_pages = target_prefetch_pages * 16;
+							if (back_prefetch_pages > 64)
+								back_prefetch_pages = 64;
+							
+							if (so->backSeqRun == 0)
+								backPos = (blkno-1);
+							else
+								backPos = so->backSeqPos;
+							so->backSeqRun++;
+							
+							if (backPos > 0 && (blkno - backPos) <= back_prefetch_pages) {
+								_bt_prefetchbuf(rel, backPos-- , &scan->pfch_index_page_list);
+								/* don't start back-seq prefetch too early */
+								if (so->backSeqRun >= back_prefetch_pages
+										&& backPos > 0 
+										&& (blkno - backPos) <= back_prefetch_pages)
+								{
+									_bt_prefetchbuf(rel, backPos-- , &scan->pfch_index_page_list);
+								}
+							}
+							
+							so->backSeqPos = backPos;
+						} else {
+							/* start prefetch on next page */
+							if (so->backSeqRun != 0) {
+								if (opaque->btpo_prev > blkno || opaque->btpo_prev < so->backSeqPos)
+									so->backSeqRun = 0;
+							}
+							_bt_prefetchbuf(rel, opaque->btpo_prev , &scan->pfch_index_page_list);
+						}
+					}
 					break;
 			}
 		}
 	}
+	}
 
 	return true;
 }
@@ -1359,7 +1507,7 @@ _bt_steppage(IndexScanDesc scan, ScanDirection dir)
  * again if it's important.
  */
 static Buffer
-_bt_walk_left(Relation rel, Buffer buf)
+_bt_walk_left(IndexScanDesc scan, Relation rel, Buffer buf)
 {
 	Page		page;
 	BTPageOpaque opaque;
@@ -1387,7 +1535,7 @@ _bt_walk_left(Relation rel, Buffer buf)
 		_bt_relbuf(rel, buf);
 		/* check for interrupts while we're not holding any buffer lock */
 		CHECK_FOR_INTERRUPTS();
-		buf = _bt_getbuf(rel, blkno, BT_READ);
+		buf = _bt_getbuf(rel, blkno, BT_READ , scan->pfch_index_page_list );
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
@@ -1631,7 +1779,7 @@ _bt_endpoint(IndexScanDesc scan, ScanDirection dir)
 		 * There's no actually-matching data on this page.  Try to advance to
 		 * the next page.  Return false if there's no matching data at all.
 		 */
-		if (!_bt_steppage(scan, dir))
+		if (!_bt_steppage(scan, dir, false))
 			return false;
 	}
 
diff --git a/src/backend/executor/instrument.c b/src/backend/executor/instrument.c
index a5118a4..c8c6fee 100644
--- a/src/backend/executor/instrument.c
+++ b/src/backend/executor/instrument.c
@@ -41,6 +41,14 @@ InstrAlloc(int n, int instrument_options)
 		{
 			instr[i].need_bufusage = need_buffers;
 			instr[i].need_timer = need_timer;
+			instr[i].bufusage_start.aio_read_noneed = 0;
+			instr[i].bufusage_start.aio_read_discrd = 0;
+			instr[i].bufusage_start.aio_read_forgot = 0;
+			instr[i].bufusage_start.aio_read_noblok = 0;
+			instr[i].bufusage_start.aio_read_failed = 0;
+			instr[i].bufusage_start.aio_read_wasted = 0;
+			instr[i].bufusage_start.aio_read_waited = 0;
+			instr[i].bufusage_start.aio_read_ontime = 0;
 		}
 	}
 
@@ -143,6 +151,16 @@ BufferUsageAccumDiff(BufferUsage *dst,
 	dst->local_blks_written += add->local_blks_written - sub->local_blks_written;
 	dst->temp_blks_read += add->temp_blks_read - sub->temp_blks_read;
 	dst->temp_blks_written += add->temp_blks_written - sub->temp_blks_written;
+
+	dst->aio_read_noneed       += add->aio_read_noneed - sub->aio_read_noneed;
+	dst->aio_read_discrd       += add->aio_read_discrd - sub->aio_read_discrd;
+	dst->aio_read_forgot       += add->aio_read_forgot - sub->aio_read_forgot;
+	dst->aio_read_noblok       += add->aio_read_noblok - sub->aio_read_noblok;
+	dst->aio_read_failed       += add->aio_read_failed - sub->aio_read_failed;
+	dst->aio_read_wasted       += add->aio_read_wasted - sub->aio_read_wasted;
+	dst->aio_read_waited       += add->aio_read_waited - sub->aio_read_waited;
+	dst->aio_read_ontime       += add->aio_read_ontime - sub->aio_read_ontime;
+
 	INSTR_TIME_ACCUM_DIFF(dst->blk_read_time,
 						  add->blk_read_time, sub->blk_read_time);
 	INSTR_TIME_ACCUM_DIFF(dst->blk_write_time,
diff --git a/src/backend/executor/nodeBitmapHeapscan.c b/src/backend/executor/nodeBitmapHeapscan.c
index 9b1e975..f5e6fd8 100644
--- a/src/backend/executor/nodeBitmapHeapscan.c
+++ b/src/backend/executor/nodeBitmapHeapscan.c
@@ -34,6 +34,8 @@
  *		ExecEndBitmapHeapScan		releases all storage.
  */
 #include "postgres.h"
+#include <sys/types.h> 
+#include <unistd.h>
 
 #include "access/relscan.h"
 #include "access/transam.h"
@@ -47,6 +49,10 @@
 #include "utils/snapmgr.h"
 #include "utils/tqual.h"
 
+#ifdef USE_PREFETCH
+extern unsigned int prefetch_dbOid; /*  database oid of relations on which prefetching to be done - 0 means all */
+extern unsigned int prefetch_bitmap_scans; /*  boolean whether to prefetch bitmap heap scans         */
+#endif   /* USE_PREFETCH */
 
 static TupleTableSlot *BitmapHeapNext(BitmapHeapScanState *node);
 static void bitgetpage(HeapScanDesc scan, TBMIterateResult *tbmres);
@@ -111,10 +117,21 @@ BitmapHeapNext(BitmapHeapScanState *node)
 		node->tbmres = tbmres = NULL;
 
 #ifdef USE_PREFETCH
-		if (target_prefetch_pages > 0)
-		{
+		if (    prefetch_bitmap_scans
+                     && (target_prefetch_pages > 0)
+                     && (    (   (prefetch_dbOid > 0)
+                               && (prefetch_dbOid == scan->rs_rd->rd_node.dbNode)
+                             )
+                         ||  (prefetch_dbOid == 0)
+                        )
+                        /* sufficient number of blocks - at least twice the target_prefetch_pages */
+                     && (scan->rs_nblocks > (2*target_prefetch_pages))
+                   ) {
 			node->prefetch_iterator = prefetch_iterator = tbm_begin_iterate(tbm);
 			node->prefetch_pages = 0;
+                        if (prefetch_iterator) {
+                          tbm_zero(prefetch_iterator);  /* zero list of prefetched and unread blocknos */
+                        }
 			node->prefetch_target = -1;
 		}
 #endif   /* USE_PREFETCH */
@@ -138,12 +155,14 @@ BitmapHeapNext(BitmapHeapScanState *node)
 			}
 
 #ifdef USE_PREFETCH
+                        if (prefetch_iterator) {
 			if (node->prefetch_pages > 0)
 			{
 				/* The main iterator has closed the distance by one page */
 				node->prefetch_pages--;
+                                tbm_subtract(prefetch_iterator, tbmres->blockno); /* remove this blockno from list of prefetched and unread blocknos */
 			}
-			else if (prefetch_iterator)
+                            else
 			{
 				/* Do not let the prefetch iterator get behind the main one */
 				TBMIterateResult *tbmpre = tbm_iterate(prefetch_iterator);
@@ -151,6 +170,7 @@ BitmapHeapNext(BitmapHeapScanState *node)
 				if (tbmpre == NULL || tbmpre->blockno != tbmres->blockno)
 					elog(ERROR, "prefetch and main iterators are out of sync");
 			}
+                        }
 #endif   /* USE_PREFETCH */
 
 			/*
@@ -239,16 +259,26 @@ BitmapHeapNext(BitmapHeapScanState *node)
 			while (node->prefetch_pages < node->prefetch_target)
 			{
 				TBMIterateResult *tbmpre = tbm_iterate(prefetch_iterator);
+                                int             PrefetchBufferRc; /*  return value from PrefetchBuffer  - refer to bufmgr.h */
+
 
 				if (tbmpre == NULL)
 				{
 					/* No more pages to prefetch */
-					tbm_end_iterate(prefetch_iterator);
-					node->prefetch_iterator = prefetch_iterator = NULL;
+                                        /* let ExecEndBitmapHeapScan terminate the prefetch_iterator
+				        **	tbm_end_iterate(prefetch_iterator);
+					**      node->prefetch_iterator = NULL;
+                                        */
+                                        prefetch_iterator = NULL;
 					break;
 				}
 				node->prefetch_pages++;
-				PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, tbmpre->blockno);
+				PrefetchBufferRc = PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, tbmpre->blockno , 0);
+                                /*  add this blockno to list of prefetched and unread blocknos
+                                **  if pin count did not increase then indicate so in the Unread_Pfetched list
+                                */
+                                tbm_add(prefetch_iterator
+                                   ,( (PrefetchBufferRc & PREFTCHRC_BUF_PIN_INCREASED) ? tbmpre->blockno : InvalidBlockNumber ) );
 			}
 		}
 #endif   /* USE_PREFETCH */
@@ -482,12 +512,31 @@ ExecEndBitmapHeapScan(BitmapHeapScanState *node)
 {
 	Relation	relation;
 	HeapScanDesc scanDesc;
+	TBMIterator *prefetch_iterator;
 
 	/*
 	 * extract information from the node
 	 */
 	relation = node->ss.ss_currentRelation;
 	scanDesc = node->ss.ss_currentScanDesc;
+	prefetch_iterator = node->prefetch_iterator;
+
+#ifdef USE_PREFETCH
+        /*  before any other cleanup,  discard any prefetched but unread buffers  */
+        if (prefetch_iterator != NULL) {
+            TBMIterateResult *tbmpre = tbm_locate_IterateResult(prefetch_iterator);
+            BlockNumber *Unread_Pfetched_base = tbmpre->Unread_Pfetched_base;
+            unsigned int Unread_Pfetched_next = tbmpre->Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+            unsigned int Unread_Pfetched_count = tbmpre->Unread_Pfetched_count;
+
+            while ((Unread_Pfetched_count--) > 0) {
+                DiscardBuffer( scanDesc->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                Unread_Pfetched_next++;
+                if (Unread_Pfetched_next >= target_prefetch_pages)
+                    Unread_Pfetched_next = 0;
+            }
+        }
+#endif   /* USE_PREFETCH */
 
 	/*
 	 * Free the exprcontext
diff --git a/src/backend/executor/nodeIndexscan.c b/src/backend/executor/nodeIndexscan.c
index 2b89dc6..c4fec3b 100644
--- a/src/backend/executor/nodeIndexscan.c
+++ b/src/backend/executor/nodeIndexscan.c
@@ -35,8 +35,13 @@
 #include "utils/rel.h"
 
 
+
 static TupleTableSlot *IndexNext(IndexScanState *node);
 
+#ifdef USE_PREFETCH
+extern unsigned int prefetch_dbOid; /*  database oid of relations on which prefetching to be done - 0 means all */
+extern unsigned int prefetch_index_scans; /*  boolean whether to prefetch bitmap heap scans         */
+#endif   /* USE_PREFETCH */
 
 /* ----------------------------------------------------------------
  *		IndexNext
@@ -418,7 +423,12 @@ ExecEndIndexScan(IndexScanState *node)
 	 * close the index relation (no-op if we didn't open it)
 	 */
 	if (indexScanDesc)
+        {
 		index_endscan(indexScanDesc);
+
+        /*  note  -  at this point all scan controlblock resources have been freed by IndexScanEnd called by index_endscan */
+
+        }
 	if (indexRelationDesc)
 		index_close(indexRelationDesc, NoLock);
 
@@ -609,6 +619,33 @@ ExecInitIndexScan(IndexScan *node, EState *estate, int eflags)
 											   indexstate->iss_NumScanKeys,
 											 indexstate->iss_NumOrderByKeys);
 
+#ifdef USE_PREFETCH
+        /*  initialize prefetching   */
+                indexstate->iss_ScanDesc->pfch_index_page_list =  (struct pfch_index_pagelist*)0;
+                indexstate->iss_ScanDesc->pfch_block_item_list = (struct pfch_block_item*)0;
+		if (    prefetch_index_scans
+			 && (target_prefetch_pages > 0)
+			 &&	(!RelationUsesLocalBuffers(indexstate->iss_ScanDesc->heapRelation)) /* I think this must always be true for an indexed heap ? */
+			 && (    (   (prefetch_dbOid > 0)
+					   && (prefetch_dbOid == indexstate->iss_ScanDesc->heapRelation->rd_node.dbNode)
+					 )
+				 ||  (prefetch_dbOid == 0)
+				)
+		   ) {
+			indexstate->iss_ScanDesc->pfch_index_page_list = palloc( sizeof(struct pfch_index_pagelist) + ( (target_prefetch_pages-1) * sizeof(struct pfch_index_item) ) );
+			indexstate->iss_ScanDesc->pfch_block_item_list = palloc( prefetch_index_scans * sizeof(struct pfch_block_item) );
+			if (     ( (struct pfch_index_pagelist*)0 != indexstate->iss_ScanDesc->pfch_index_page_list )
+                  && ( (struct pfch_block_item*)0 != indexstate->iss_ScanDesc->pfch_block_item_list )
+               ) {
+                          indexstate->iss_ScanDesc->pfch_used = 0;
+                          indexstate->iss_ScanDesc->pfch_next = prefetch_index_scans; /* ensure first entry is at index 0 */
+                          indexstate->iss_ScanDesc->pfch_index_page_list->pfch_index_pagelist_next = (struct pfch_index_pagelist*)0;
+                          indexstate->iss_ScanDesc->pfch_index_page_list->pfch_index_item_count = 0;
+                          indexstate->iss_ScanDesc->do_prefetch = 1;
+            }
+		}
+#endif   /* USE_PREFETCH */
+
 	/*
 	 * If no run-time keys to calculate, go ahead and pass the scankeys to the
 	 * index AM.
diff --git a/src/backend/nodes/tidbitmap.c b/src/backend/nodes/tidbitmap.c
index a880c81..1e34d6a 100644
--- a/src/backend/nodes/tidbitmap.c
+++ b/src/backend/nodes/tidbitmap.c
@@ -44,6 +44,9 @@
 #include "nodes/bitmapset.h"
 #include "nodes/tidbitmap.h"
 #include "utils/hsearch.h"
+#ifdef USE_PREFETCH
+extern int	target_prefetch_pages;
+#endif   /* USE_PREFETCH */
 
 /*
  * The maximum number of tuples per page is not large (typically 256 with
@@ -572,7 +575,12 @@ tbm_begin_iterate(TIDBitmap *tbm)
 	 * needs of the TBMIterateResult sub-struct.
 	 */
 	iterator = (TBMIterator *) palloc(sizeof(TBMIterator) +
-								 MAX_TUPLES_PER_PAGE * sizeof(OffsetNumber));
+								 MAX_TUPLES_PER_PAGE * sizeof(OffsetNumber)
+#ifdef USE_PREFETCH
+                                          		      /*  space for remembering every prefetched but unread blockno */
+                                          		      +  (target_prefetch_pages * sizeof(BlockNumber))
+#endif   /* USE_PREFETCH */
+                                         );
 	iterator->tbm = tbm;
 
 	/*
@@ -1020,3 +1028,68 @@ tbm_comparator(const void *left, const void *right)
 		return 1;
 	return 0;
 }
+
+void
+tbm_zero(TBMIterator *iterator) /* zero list of prefetched and unread blocknos */
+{
+      /* locate the list of prefetched but unread blocknos immediately following the array of offsets
+      ** and note that tbm_begin_iterate allocates space for (1 + MAX_TUPLES_PER_PAGE) offsets  -
+      ** 1 included in struct TBMIterator and MAX_TUPLES_PER_PAGE additional
+      */
+      iterator->output.Unread_Pfetched_base = ((BlockNumber *)(&(iterator->output.offsets[MAX_TUPLES_PER_PAGE+1])));
+      iterator->output.Unread_Pfetched_next = iterator->output.Unread_Pfetched_count = 0;
+}
+
+void
+tbm_add(TBMIterator *iterator, BlockNumber blockno) /* add this blockno to list of prefetched and unread blocknos */
+{
+      unsigned int Unread_Pfetched_index = iterator->output.Unread_Pfetched_next + iterator->output.Unread_Pfetched_count++;
+
+      if (iterator->output.Unread_Pfetched_count > target_prefetch_pages) {
+		ereport(ERROR,
+			(errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("tbm_add overflowed list cannot add blockno %d", blockno)));
+      }
+
+      if (Unread_Pfetched_index >= target_prefetch_pages)
+          Unread_Pfetched_index -= target_prefetch_pages;
+      *(iterator->output.Unread_Pfetched_base + Unread_Pfetched_index) = blockno;
+}
+
+void
+tbm_subtract(TBMIterator *iterator, BlockNumber blockno) /* remove this blockno from list of prefetched and unread blocknos */
+{
+      unsigned int Unread_Pfetched_index = iterator->output.Unread_Pfetched_next++;
+      BlockNumber nextUnreadPfetched;
+
+      /*    make a weak check that the next blockno is the one to be removed,
+      **    although actually in case of disagreement,   we ignore callers blockno and remove next anyway,
+      **    which is really what caller wants
+      */
+      if ( iterator->output.Unread_Pfetched_count == 0 ) {
+		ereport(ERROR,
+			(errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("tbm_subtract empty list cannot subtract blockno %d", blockno)));
+      }
+
+      if (Unread_Pfetched_index >= target_prefetch_pages)
+          Unread_Pfetched_index = 0;
+      nextUnreadPfetched = *(iterator->output.Unread_Pfetched_base + Unread_Pfetched_index);
+      if (   ( nextUnreadPfetched != blockno ) 
+          && ( nextUnreadPfetched != InvalidBlockNumber ) /* dont report it if the block in the list was InvalidBlockNumber */
+         ) {
+		ereport(NOTICE,
+			(errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("tbm_subtract will subtract blockno %d not %d",
+					nextUnreadPfetched, blockno)));
+      }
+      if (iterator->output.Unread_Pfetched_next >= target_prefetch_pages)
+          iterator->output.Unread_Pfetched_next = 0;
+      iterator->output.Unread_Pfetched_count--;
+}
+
+TBMIterateResult *
+tbm_locate_IterateResult(TBMIterator *iterator)
+{
+   return &(iterator->output);
+}
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index a5d5c2d..a16467d 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -123,6 +123,11 @@
 #include "storage/spin.h"
 #endif
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+void ReportFreeBAiocbs(void);
+int CountInuseBAiocbs(void);
+extern int hwmBufferAiocbs;         /*  high water mark of in-use  BufferAiocbs in pool           */
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * Possible types of a backend. Beyond being the possible bkend_type values in
@@ -1493,9 +1498,15 @@ ServerLoop(void)
 	fd_set		readmask;
 	int			nSockets;
 	time_t		now,
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                           count_baiocb_time,
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 				last_touch_time;
 
 	last_touch_time = time(NULL);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        count_baiocb_time = time(NULL);
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 	nSockets = initMasks(&readmask);
 
@@ -1654,6 +1665,19 @@ ServerLoop(void)
 			last_touch_time = now;
 		}
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                /*   maintain the hwm of used baiocbs every 10 seconds  */
+		if ((now - count_baiocb_time) >= 10)
+		{
+                        int inuseBufferAiocbs;         /*  current in-use  BufferAiocbs in pool */
+                        inuseBufferAiocbs = CountInuseBAiocbs();
+                        if (inuseBufferAiocbs > hwmBufferAiocbs) {
+			    hwmBufferAiocbs = inuseBufferAiocbs;
+			}
+			count_baiocb_time = now;
+		}
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 		/*
 		 * If we already sent SIGQUIT to children and they are slow to shut
 		 * down, it's time to send them SIGKILL.  This doesn't happen
@@ -3444,6 +3468,9 @@ PostmasterStateMachine(void)
 						signal_child(PgStatPID, SIGQUIT);
 				}
 			}
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                        ReportFreeBAiocbs();
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 		}
 	}
 
diff --git a/src/backend/storage/buffer/Makefile b/src/backend/storage/buffer/Makefile
index 2c10fba..96aa9e0 100644
--- a/src/backend/storage/buffer/Makefile
+++ b/src/backend/storage/buffer/Makefile
@@ -12,6 +12,6 @@ subdir = src/backend/storage/buffer
 top_builddir = ../../../..
 include $(top_builddir)/src/Makefile.global
 
-OBJS = buf_table.o buf_init.o bufmgr.o freelist.o localbuf.o
+OBJS = buf_table.o buf_init.o bufmgr.o freelist.o localbuf.o buf_async.o
 
 include $(top_srcdir)/src/backend/common.mk
diff --git a/src/backend/storage/buffer/buf_init.c b/src/backend/storage/buffer/buf_init.c
index e03394c..658764d 100644
--- a/src/backend/storage/buffer/buf_init.c
+++ b/src/backend/storage/buffer/buf_init.c
@@ -13,15 +13,89 @@
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
+#include <sys/types.h> 
+#include <unistd.h>
 
 #include "storage/bufmgr.h"
 #include "storage/buf_internals.h"
-
+#include <stdlib.h> /* for getenv() */
+#include <errno.h> /* for strtoul() */
 
 BufferDesc *BufferDescriptors;
 char	   *BufferBlocks;
-int32	   *PrivateRefCount;
+int32	   *PrivateRefCount;       /*  array of counts per buffer of how many times this task has pinned this buffer */
+
+volatile struct BAiocbAnchor *BAiocbAnchr = (struct BAiocbAnchor *)0;  /*  anchor for all control blocks pertaining to aio  */
+
+int CountInuseBAiocbs(void);     /*  keep compiler happy */
+void ReportFreeBAiocbs(void);    /*  keep compiler happy */
+
+extern int	MaxConnections;  /*  max number of client connections which postmaster will allow  */
+int numBufferAiocbs = 0;         /*  total number of  BufferAiocbs in pool (0 <=> no async io) */
+int hwmBufferAiocbs = 0;         /*  high water mark of in-use  BufferAiocbs in pool
+                                 **  (not required to be accurate, kindly maintained for us somehow by postmaster)
+                                 */
+
+#ifdef USE_PREFETCH
+unsigned int prefetch_dbOid = 0; /*  database oid of relations on which prefetching to be done - 0 means all */
+unsigned int prefetch_bitmap_scans = 1; /*  boolean whether to prefetch bitmap heap scans        */
+unsigned int prefetch_heap_scans = 0;   /*  boolean whether to prefetch non-bitmap heap scans    */
+unsigned int prefetch_sequential_index_scans = 0;  /*  boolean whether to prefetch sequential-access non-bitmap index scans  */
+unsigned int prefetch_index_scans = 256;  /*  boolean whether to prefetch non-bitmap index scans also numeric size of pfch_list  */
+unsigned int prefetch_btree_heaps = 1;  /*  boolean whether to prefetch heap pages in _bt_next for non-bitmap index scans   */
+#endif /* USE_PREFETCH */
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+int maxGetBAiocbTries = 1;       /*  max times we will try to get a free BufferAiocb */
+int maxRelBAiocbTries = 1;       /*  max times we will try to release a BufferAiocb back to freelist */
+
+/*  locking protocol for manipulating the BufferAiocbs and FreeBAiocbs list :
+**    1.    ownership of a BufferAiocb :
+**          to gain ownership of a BufferAiocb, a task must
+**          EITHER    remove it from FreeBAiocbs (it is now temporary owner and no other task can find it)
+**                    if decision is to attach it to a buffer descriptor header, then
+**                       .   lock the buffer descriptor header
+**                       .   check  NOT flags & BM_AIO_IN_PROGRESS
+**                       .   attach to buffer descriptor header
+**                       .   increment the BufferAiocb.dependent_count
+**                       .   unlock the buffer descriptor header
+**                     and ownership scope is from lock to unlock
+***          OR        locate it by dereferencing the pointer in a buffer descriptor,
+**                    in which case :
+**                       .   lock the buffer descriptor header
+**                       .   check  flags & BM_AIO_IN_PROGRESS
+**                       .   increment the BufferAiocb.dependent_count
+**                       .   if decision is to return to FreeBAiocbs,
+**                           then   (with buffer descriptor header still locked)
+**                                  .  turn off BM_AIO_IN_PROGRESS
+**                       .   IF        the BufferAiocb.dependent_count == 1 (I am sole dependent)
+**                       .   THEN
+**                       .       .  decrement the BufferAiocb.dependent_count
+**                               .  return to FreeBAiocbs (see below)
+**                       .   unlock the buffer descriptor header
+**                     and ownership scope is from lock to either return to FreeBAiocbs or unlock
+**    2.    adding and removing from FreeBAiocbs :
+**      two alternative methods - controlled by conditional macro definition LOCK_BAIOCB_FOR_GET_REL
+**       2.1 LOCK_BAIOCB_FOR_GET_REL is defined - use a lock
+**          .   lock BufFreelistLock exclusive
+**          .   add / remove from FreeBAiocbs
+**          .   unlock BufFreelistLock exclusive
+**          advantage of this method  -  never fails to add or remove
+**       2.2  LOCK_BAIOCB_FOR_GET_REL is not defined - use compare_and_swap
+**          .   retrieve the current Freelist pointer and validate
+**          .   compare_and_swap on/off the FreeBAiocb list
+**          .   unlock BufFreelistLock exclusive
+**          advantage of this method  -  never waits
+**          to avoid losing a FreeBAiocbs,   save it in a process-local cache and reuse
+*/
+#else   /*  not USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+        /*   this dummy structure is to ensure that references to these fields in other bufmgr runtime code
+        **   that is not conditionally ifdefd on USE_AIO_ATOMIC_BUILTIN_COMP_SWAP compiles and runs correctly
+        */
+        struct BufferAiocb dummy_BAiocbAnchr = { (struct BufferAiocb*)0 , (struct BufferAiocb*)0 };
+int maxGetBAiocbTries = -1;       /*  max times we will try to get a free BufferAiocb */
+int maxRelBAiocbTries = -1;       /*  max times we will try to release a BufferAiocb back to freelist */
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * Data Structures:
@@ -73,7 +147,14 @@ void
 InitBufferPool(void)
 {
 	bool		foundBufs,
-				foundDescs;
+				foundDescs
+                        , foundAiocbs
+          ;
+#if defined(USE_PREFETCH) || defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP)
+        char *envvarpointer = (char *)0;  /*  might point to an environment variable string */
+        char *charptr;
+#endif /* USE_PREFETCH */
+
 
 	BufferDescriptors = (BufferDesc *)
 		ShmemInitStruct("Buffer Descriptors",
@@ -83,6 +164,142 @@ InitBufferPool(void)
 		ShmemInitStruct("Buffer Blocks",
 						NBuffers * (Size) BLCKSZ, &foundBufs);
 
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        BAiocbAnchr = (struct BAiocbAnchor *)0; /*  anchor for all control blocks pertaining to aio  */
+        if (max_async_io_prefetchers < 0) {  /*  negative value indicates to initialize to something sensible during buf_init */
+            max_async_io_prefetchers = MaxConnections/6;  /*  default allows for average of MaxConnections/6 concurrent prefetchers  - reasonable ??? */
+        }
+
+        if ((target_prefetch_pages > 0) && (max_async_io_prefetchers > 0)) {
+            int ix;
+            volatile struct BufferAiocb *BufferAiocbs;
+            volatile struct BufferAiocb * volatile FreeBAiocbs;
+
+            numBufferAiocbs = (target_prefetch_pages*max_async_io_prefetchers);  /*  target_prefetch_pages per prefetcher */
+            BAiocbAnchr = (struct BAiocbAnchor *)
+		ShmemInitStruct("Buffer Aiocbs",
+                          sizeof(struct BAiocbAnchor) + (numBufferAiocbs * sizeof(struct BufferAiocb)), &foundAiocbs);
+            if (BAiocbAnchr) {
+                BufferAiocbs = BAiocbAnchr->BufferAiocbs = (struct BufferAiocb*)(((char *)BAiocbAnchr) + sizeof(struct BAiocbAnchor));
+                FreeBAiocbs = (struct BufferAiocb*)0;
+                for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                    (BufferAiocbs+ix)->BAiocbnext = FreeBAiocbs;   /*  init the free list,  last one -> 0  */
+                    (BufferAiocbs+ix)->BAiocbbufh = (struct sbufdesc*)0;
+                    (BufferAiocbs+ix)->BAiocbDependentCount = 0;
+                    (BufferAiocbs+ix)->pidOfAio = 0;
+                    FreeBAiocbs = (BufferAiocbs+ix);
+
+                }
+                BAiocbAnchr->FreeBAiocbs = FreeBAiocbs;
+                envvarpointer = getenv("PG_MAX_GET_BAIOCB_TRIES");
+                if (    (envvarpointer != (char *)0)
+                     && (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   )
+                   ) {
+                      maxGetBAiocbTries = strtol(envvarpointer, 0, 10);
+                }
+                envvarpointer = getenv("PG_MAX_REL_BAIOCB_TRIES");
+                if (    (envvarpointer != (char *)0)
+                     && (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   )
+                   ) {
+                      maxRelBAiocbTries = strtol(envvarpointer, 0, 10);
+                }
+
+                /*   init the aio subsystem max number of threads and max number of requests
+                **   max number of threads   <-->  max_async_io_prefetchers
+                **   max number of requests  <-->  numBufferAiocbs = (target_prefetch_pages*max_async_io_prefetchers)
+                **   there is no return code so we just hope.
+                */
+                smgrinitaio(max_async_io_prefetchers , numBufferAiocbs);
+
+            }
+        }
+#else   /*  not USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+        /*   this dummy structure is to ensure that references to these fields in other bufmgr runtime code
+        **   that is not conditionally ifdefd on USE_AIO_ATOMIC_BUILTIN_COMP_SWAP compiles and runs correctly
+        */
+        BAiocbAnchr = &dummy_BAiocbAnchr;
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+
+#ifdef USE_PREFETCH
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_BITMAP");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_bitmap_scans = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+                 prefetch_bitmap_scans = 1;
+             }
+        }
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_ISCAN");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_index_scans = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+                 prefetch_index_scans = 1;
+             } else
+             if (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   ) {
+                 prefetch_index_scans = strtol(envvarpointer, &charptr, 10);
+                 if (charptr && (',' == *charptr)) {   /*  optional sequential prefetch in index scans */
+					 charptr++;        /*   following the comma ... */
+					 if ( ('Y' == *charptr) || ('y' == *charptr) || ('1' == *charptr) ) {
+                         prefetch_sequential_index_scans = 1;
+					 }
+				 }
+             }
+             /*  if prefeching for ISCAN,  then we require size of pfch_list to be at least target_prefetch_pages */
+             if (   (prefetch_index_scans > 0)
+                 && (prefetch_index_scans < target_prefetch_pages)
+                ) {
+                 prefetch_index_scans = target_prefetch_pages;
+             }
+        }
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_BTREE");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_btree_heaps = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+                 prefetch_btree_heaps = 1;
+             }
+        }
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_HEAP");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_heap_scans = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+              prefetch_heap_scans = 1;
+             }
+        }
+        envvarpointer = getenv("PG_PREFETCH_DBOID");
+        if (    (envvarpointer != (char *)0)
+             && (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   )
+           ) {
+              errno = 0;   /*  required in order to distinguish error from 0 */
+              prefetch_dbOid = (unsigned int)strtoul((const char *)envvarpointer, 0, 10);
+              if (errno) {
+                  prefetch_dbOid = 0;
+              }
+        }
+        elog(LOG, "prefetching initialised with target_prefetch_pages= %d "
+                  ", max_async_io_prefetchers= %d implying aio concurrency= %d "
+                  ", prefetching_for_bitmap= %s "
+                  ", prefetching_for_heap= %s "
+                  ", prefetching_for_iscan= %d with sequential_index_page_prefetching= %s "
+                  ", prefetching_for_btree= %s"
+                   ,target_prefetch_pages ,max_async_io_prefetchers ,numBufferAiocbs
+                   ,(prefetch_bitmap_scans ? "Y" : "N")
+                   ,(prefetch_heap_scans ? "Y" : "N")
+                   ,prefetch_index_scans
+                   ,(prefetch_sequential_index_scans ? "Y" : "N")
+                   ,(prefetch_btree_heaps ? "Y" : "N")
+            );
+#endif /* USE_PREFETCH */
+
+
 	if (foundDescs || foundBufs)
 	{
 		/* both should be present or neither */
@@ -176,3 +393,80 @@ BufferShmemSize(void)
 
 	return size;
 }
+
+/*     imprecise count of number of in-use BAiocbs at any time
+ *     we scan the array read-only without latching so are subject to unstable result
+ *     (but since the array is in well-known contiguous storage,
+ *     we are not subject to segmentation violation)
+ *     This function may be called at any time and just does its best
+ *     return the count of what we counted.
+ */
+int
+CountInuseBAiocbs(void)
+{
+        volatile struct BufferAiocb *BAiocb;
+        int count = 0;
+        int ix;
+
+
+        if (BAiocbAnchr != (struct BAiocbAnchor *)0 ) {
+            BAiocb = BAiocbAnchr->BufferAiocbs;             /*   start of list */
+
+            for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                    if ((struct BufferAiocb*)BAIOCB_OCCUPIED == (BAiocb+ix)->BAiocbnext) {   /* not on freelist ? */
+                        count++;
+                    }
+            }
+        }
+        return count;
+}
+
+/*
+ * report how many free BAiocbs at shutdown
+ * DO NOT call this while backends are actively working!!
+ * this report is useful when compare_and_swap method used (see above)
+ * as it can be used to deduce how many BAiocbs were in process-local caches -
+ * (original_number_on_freelist_at_startup - this_reported_number_at_shutdown)
+ */
+void
+ReportFreeBAiocbs(void)
+{
+        volatile struct BufferAiocb *BAiocb;
+        volatile struct BufferAiocb *BufferAiocbs;
+        int count = 0;
+        int fx , ix;
+
+
+        if (BAiocbAnchr != (struct BAiocbAnchor *)0 ) {
+            BAiocb = BAiocbAnchr->FreeBAiocbs;             /*   start of free list */
+            BufferAiocbs = BAiocbAnchr->BufferAiocbs;
+
+            for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                    (BufferAiocbs+ix)->BAiocbDependentCount = 0;  /* use this as marker for finding it on freelist */
+            }
+            for (fx = (numBufferAiocbs-1);  ( (fx>=0) &&  ( BAiocb != (struct BufferAiocb*)0 ) );  fx--) {
+                    
+                    /*  check if it is a valid BufferAiocb */
+                    for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                        if ((BufferAiocbs+ix) == BAiocb) { /*  is it this one ? */
+                             break;
+                        }
+                    }
+                    if (ix >= 0) {
+                        if (BAiocb->BAiocbDependentCount) {   /* seen it already ? */
+                            elog(LOG, "ReportFreeBAiocbs closed cycle on AIO control block freelist %p"
+                                          ,BAiocb);
+                            fx = 0; /* give up at this point */
+                        }
+                        BAiocb->BAiocbDependentCount = 1;  /* use this as marker for finding it on freelist */
+                        count++;
+                        BAiocb = BAiocb->BAiocbnext;
+                    } else {
+                        elog(LOG, "ReportFreeBAiocbs invalid item on AIO control block freelist %p"
+                                          ,BAiocb);
+                        fx = 0; /* give up at this point */
+                    }
+            }
+        }
+        elog(LOG, "ReportFreeBAiocbs AIO control block list : poolsize= %d  in-use-hwm= %d  final-free= %d" ,numBufferAiocbs , hwmBufferAiocbs , count);
+}
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index c070278..e4ef7f9 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -29,7 +29,7 @@
  *		buf_table.c -- manages the buffer lookup table
  */
 #include "postgres.h"
-
+#include <sys/types.h> 
 #include <sys/file.h>
 #include <unistd.h>
 
@@ -50,7 +50,6 @@
 #include "utils/resowner_private.h"
 #include "utils/timestamp.h"
 
-
 /* Note: these two macros only work on shared buffers, not local ones! */
 #define BufHdrGetBlock(bufHdr)	((Block) (BufferBlocks + ((Size) (bufHdr)->buf_id) * BLCKSZ))
 #define BufferGetLSN(bufHdr)	(PageGetLSN(BufHdrGetBlock(bufHdr)))
@@ -63,6 +62,8 @@
 #define BUF_WRITTEN				0x01
 #define BUF_REUSABLE			0x02
 
+extern volatile struct BAiocbAnchor *BAiocbAnchr;  /*  anchor for all control blocks pertaining to aio  */
+
 #define DROP_RELS_BSEARCH_THRESHOLD		20
 
 /* GUC variables */
@@ -78,26 +79,33 @@ bool		track_io_timing = false;
  */
 int			target_prefetch_pages = 0;
 
-/* local state for StartBufferIO and related functions */
+/* local state for StartBufferIO and related functions
+**  but ONLY for synchronous IO  -  not altered for aio
+*/
 static volatile BufferDesc *InProgressBuf = NULL;
 static bool IsForInput;
+pid_t this_backend_pid = 0;    /*    pid of this backend */
 
 /* local state for LockBufferForCleanup */
 static volatile BufferDesc *PinCountWaitBuf = NULL;
 
-
-static Buffer ReadBuffer_common(SMgrRelation reln, char relpersistence,
+extern int
+BufStartAsync(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy);
+extern int
+BufCheckAsync(SMgrRelation caller_smgr, Relation caller_reln, volatile BufferDesc *buf_desc, int intention
+        ,BufferAccessStrategy strategy , int index_for_aio , bool spinLockHeld , LWLockId PartitionLock );
+Buffer ReadBuffer_common(SMgrRelation reln, char relpersistence,
 				  ForkNumber forkNum, BlockNumber blockNum,
 				  ReadBufferMode mode, BufferAccessStrategy strategy,
-				  bool *hit);
-static bool PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy);
-static void PinBuffer_Locked(volatile BufferDesc *buf);
-static void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner);
+				  bool *hit , int index_for_aio);
+bool PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy);
+void PinBuffer_Locked(volatile BufferDesc *buf);
+void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner);
 static void BufferSync(int flags);
 static int	SyncOneBuffer(int buf_id, bool skip_recently_used);
 static void WaitIO(volatile BufferDesc *buf);
-static bool StartBufferIO(volatile BufferDesc *buf, bool forInput);
-static void TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
+static bool StartBufferIO(volatile BufferDesc *buf, bool forInput , int index_for_aio );
+void TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
 				  int set_flag_bits);
 static void shared_buffer_write_error_callback(void *arg);
 static void local_buffer_write_error_callback(void *arg);
@@ -106,24 +114,66 @@ static volatile BufferDesc *BufferAlloc(SMgrRelation smgr,
 			ForkNumber forkNum,
 			BlockNumber blockNum,
 			BufferAccessStrategy strategy,
-			bool *foundPtr);
+			int *foundPtr , int index_for_aio );
 static void FlushBuffer(volatile BufferDesc *buf, SMgrRelation reln);
 static void AtProcExit_Buffers(int code, Datum arg);
 static int	rnode_comparator(const void *p1, const void *p2);
 
+BlockNumber BlocknumOfBuffer(Buffer buffer);
+bool BlocknotinBuffer(Buffer buffer, Relation relation, BlockNumber blockNum);
 
 /*
  * PrefetchBuffer -- initiate asynchronous read of a block of a relation
  *
- * This is named by analogy to ReadBuffer but doesn't actually allocate a
- * buffer.  Instead it tries to ensure that a future ReadBuffer for the given
- * block will not be delayed by the I/O.  Prefetching is optional.
+ * This is named by analogy to ReadBuffer but allocates a buffer only if using asynchronous I/O.
+ * Its purpose  is to try to ensure that a future ReadBuffer for the given block
+ * will not be delayed by the I/O.  Prefetching is optional.
  * No-op if prefetching isn't compiled in.
+ *
+ * Originally the prefetch simply called posix_fadvise() to recommend read-ahead into kernel page cache.
+ * Extended to provide an alternative of issuing an asynchronous aio_read() to read into a buffer.
+ * This extension has an implication on how this bufmgr component manages concurrent requests
+ * for the same disk block.
+ *
+ * Synchronous IO (read()) does not provide a means for waiting on another task's read if in progress,
+ * and bufmgr implements its own scheme in StartBufferIO, WaitIO, and TerminateBufferIO.
+ *
+ * Asynchronous IO (aio_read()) provides a means for waiting on this or another task's read if in progress,
+ * namely aio_suspend(),  which this extension uses.    Therefore,  although StartBufferIO and TerminateBufferIO
+ * are called as part of asynchronous prefetching,   their role is limited to maintaining the buffer desc flags,
+ * and they do not track the asynchronous IO itself.    Instead,   asynchronous IOs are tracked in 
+ * a separate set of shared control blocks,  the BufferAiocb list -
+ *   refer to     include/storage/buf_internals.h and storage/buffer/buf_init.c
+ *
+ * Another implication of asynchronous IO concerns buffer pinning.
+ * The buffer used for the prefetch is pinned before aio_read is issued.
+ * It is expected that the same task (and possibly others) will later ask to read the page
+ * and eventually release and unpin the buffer.
+ * However,  if the task which issued the aio_read later decides not to read the page,
+ * and return code indicates delta_pin_count > 0 (see below)
+ * it *must* instead issue a DiscardBuffer() (see function later in this file)
+ * so that its pin is released.
+ * Therefore,  each client which uses the PrefetchBuffer service must either always read all
+ * prefetched pages,  or keep track of prefetched pages and discard unread ones at end of scan.
+ *
+ * return code:   is an int bitmask defined in bufmgr.h
+        PREFTCHRC_BUF_PIN_INCREASED 0x01      pin count on buffer has been increased by 1
+        PREFTCHRC_BLK_ALREADY_PRESENT 0x02    block was already present in a buffer
+ *
+ * PREFTCHRC_BLK_ALREADY_PRESENT is a hint to caller that the prefetch may be unnecessary
  */
-void
-PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
+int
+PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy)
 {
+	Buffer		buf_id; /* indicates buffer containing the requested block  */
+        int             PrefetchBufferRc = 0; /*  return value as described above  */
+        int             PinCountOnEntry = 0;  /*  pin count on entry           */
+        int             PinCountdelta = 0;    /*  pin count delta increase     */
+
+
 #ifdef USE_PREFETCH
+
+	buf_id = -1;
 	Assert(RelationIsValid(reln));
 	Assert(BlockNumberIsValid(blockNum));
 
@@ -145,8 +195,13 @@ PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
 	{
 		BufferTag	newTag;		/* identity of requested block */
 		uint32		newHash;	/* hash value for newTag */
+        int         BufStartAsyncrc = -1;  /*  retcode from BufStartAsync :
+                                                       **        0 if started successfully (which implies buffer was newly pinned )
+                                                       **       -1 if failed for some reason
+                                                       **        1+PrivateRefCount if we found desired buffer in buffer pool
+                                                       **  and we set it likewise if we find buffer in buffer pool
+                                                       */
 		LWLock	   *newPartitionLock;	/* buffer partition lock for it */
-		int			buf_id;
 
 		/* create a tag so we can lookup the buffer */
 		INIT_BUFFERTAG(newTag, reln->rd_smgr->smgr_rnode.node,
@@ -158,28 +213,119 @@ PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
 
 		/* see if the block is in the buffer pool already */
 		LWLockAcquire(newPartitionLock, LW_SHARED);
-		buf_id = BufTableLookup(&newTag, newHash);
+		buf_id = (Buffer)BufTableLookup(&newTag, newHash);
+                if (buf_id >= 0) {
+                    PinCountOnEntry = PrivateRefCount[buf_id];  /*  pin count on entry               */
+                    BufStartAsyncrc = 1 + PinCountOnEntry; /* indicate this backends pin count - see above comment */
+                    PrefetchBufferRc = PREFTCHRC_BLK_ALREADY_PRESENT;       /* indicate buffer present */
+                } else {
+                    PrefetchBufferRc = 0;                                   /* indicate buffer not present */
+                }
 		LWLockRelease(newPartitionLock);
 
+     not_in_buffers:
 		/* If not in buffers, initiate prefetch */
-		if (buf_id < 0)
+		if (buf_id < 0) {
+                    /*    try using async aio_read with a buffer */
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                    BufStartAsyncrc = BufStartAsync( reln, forkNum, blockNum , strategy );
+                    if (BufStartAsyncrc < 0) {
+                            pgBufferUsage.aio_read_noblok++;
+                    }
+#else /* not USE_AIO_ATOMIC_BUILTIN_COMP_SWAP so try the alternative that does not read the block into a postgresql buffer */
 			smgrprefetch(reln->rd_smgr, forkNum, blockNum);
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+		}
 
-		/*
-		 * If the block *is* in buffers, we do nothing.  This is not really
-		 * ideal: the block might be just about to be evicted, which would be
-		 * stupid since we know we are going to need it soon.  But the only
-		 * easy answer is to bump the usage_count, which does not seem like a
-		 * great solution: when the caller does ultimately touch the block,
-		 * usage_count would get bumped again, resulting in too much
-		 * favoritism for blocks that are involved in a prefetch sequence. A
-		 * real fix would involve some additional per-buffer state, and it's
-		 * not clear that there's enough of a problem to justify that.
+        if (  (buf_id >= 0) || (BufStartAsyncrc >= 1)  ) {
+                        /* The block *is* in buffers.    */
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                        pgBufferUsage.aio_read_noneed++;
+#ifndef USE_PREFETCH_BUT_DONT_PIN_ALREADY_PRESENT /* jury is out on whether the following wins but it ought to ...  */
+                        /*
+                        ** If this backend already had pinned it,
+                        ** or another backend had banked a pin on it,
+                        ** or there is an IO in progress,
+                        ** or it is not marked valid,
+                        ** then do nothing.
+                        ** Otherwise pin it and mark the buffer's pin as banked by this backend.
+                        ** Note  -  it may or not be pinned by another backend -
+                        **          it is ok for us to bank a pin on it
+                        **          *provided* the other backend did not bank its pin.
+                        **          The reason for this is that the banked-pin indicator is global -
+                        **          it can identify at most one process.
+                        */
+                        /* pgBufferUsage.aio_read_wasted++;  overload counter - only for debugging */
+                        if (BufStartAsyncrc == 1) {            /*   not pinned by me  */
+                              /*  pgBufferUsage.aio_read_wasted++;  overload counter - only for debugging */
+                              /*  note   -   all we can say with certainty is that the buffer is not pinned by me
+                              **             we cannot be sure that it is still in buffer pool
+                              **             so must go through the entire locking and searching all over again ...
 		 */
+                            LWLockAcquire(newPartitionLock, LW_SHARED);
+                            buf_id = (Buffer)BufTableLookup(&newTag, newHash);
+                            /* If in buffers, proceed */
+                            if (buf_id >= 0) {
+                                /*  since the block is now present,
+                                **  save the current pin count to ensure final delta is calculated correctly
+                                */
+                                PinCountOnEntry = PrivateRefCount[buf_id];  /*  pin count on entry               */
+                                if ( PinCountOnEntry == 0) { /*  paranoid check it's still not pinned by me */
+                                    volatile        BufferDesc *buf_desc;
+
+                                    buf_desc = &BufferDescriptors[buf_id];        /* found buffer descriptor */
+                                    LockBufHdr(buf_desc);
+                                    if (    (buf_desc->flags & BM_VALID)           /* buffer is valid        */
+                                         && (!(buf_desc->flags & (BM_IO_IN_PROGRESS|BM_AIO_IN_PROGRESS|BM_AIO_PREFETCH_PIN_BANKED))) /* buffer is not any of ... */
+                                       ) {
+                                        buf_desc->flags |= BM_AIO_PREFETCH_PIN_BANKED; /* bank the pin for next use by this task */
+                                        /*  note  - we can call PinBuffer_Locked with the BM_AIO_PREFETCH_PIN_BANKED flag set because it is not yet pinned by me */
+                                        buf_desc->freeNext = -(this_backend_pid);       /* remember which pid banked it */
+                                        /*  pgBufferUsage.aio_read_wasted--;      overload counter - not wasted after all - only for debugging */
+
+                                        /* Make sure we will have room to remember the buffer pin */
+                                        ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
+
+                                        PinBuffer_Locked(buf_desc);
 	}
-#endif   /* USE_PREFETCH */
+                                    else {
+                                        UnlockBufHdr(buf_desc);
+                                    }
+                                }
+                            }
+                            LWLockRelease(newPartitionLock);
+                            /*  although unlikely,  maybe it was evicted while we were puttering about  */
+                            if (buf_id < 0) {
+                                pgBufferUsage.aio_read_noneed--;   /*   back out the accounting */
+                                goto not_in_buffers;               /*   and try again           */
+                            }
+                        }
+#endif /*  USE_PREFETCH_BUT_DONT_PIN_ALREADY_PRESENT */
+
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+		}
+
+		if (buf_id >= 0) {
+			PinCountdelta = PrivateRefCount[buf_id] - PinCountOnEntry;  /*  pin count delta increase    */
+			if (  (PinCountdelta < 0) || (PinCountdelta > 1) ) {
+				  elog(ERROR,
+						 "PrefetchBuffer #%d : incremented pin count by %d on bufdesc %p refcount %u localpins %d\n"
+								  ,(buf_id+1) , PinCountdelta , &BufferDescriptors[buf_id] ,BufferDescriptors[buf_id].refcount , PrivateRefCount[buf_id]);
 }
+		} else
+		if (BufStartAsyncrc == 0) {  /* aio started successfully (which implies buffer was newly pinned ) */
+			PinCountdelta = 1;
+		}
+
+		/*  set final PrefetchBufferRc according to previous value */
+		PrefetchBufferRc |= PinCountdelta;  /* set the PREFTCHRC_BUF_PIN_INCREASED bit */
+	}
+
+#endif   /* USE_PREFETCH */
 
+	return PrefetchBufferRc; /*  return value as described above */
+}
 
 /*
  * ReadBuffer -- a shorthand for ReadBufferExtended, for reading from main
@@ -252,7 +398,7 @@ ReadBufferExtended(Relation reln, ForkNumber forkNum, BlockNumber blockNum,
 	 */
 	pgstat_count_buffer_read(reln);
 	buf = ReadBuffer_common(reln->rd_smgr, reln->rd_rel->relpersistence,
-							forkNum, blockNum, mode, strategy, &hit);
+							forkNum, blockNum, mode, strategy, &hit , 0);
 	if (hit)
 		pgstat_count_buffer_hit(reln);
 	return buf;
@@ -280,7 +426,7 @@ ReadBufferWithoutRelcache(RelFileNode rnode, ForkNumber forkNum,
 	Assert(InRecovery);
 
 	return ReadBuffer_common(smgr, RELPERSISTENCE_PERMANENT, forkNum, blockNum,
-							 mode, strategy, &hit);
+							 mode, strategy, &hit , 0);
 }
 
 
@@ -288,15 +434,18 @@ ReadBufferWithoutRelcache(RelFileNode rnode, ForkNumber forkNum,
  * ReadBuffer_common -- common logic for all ReadBuffer variants
  *
  * *hit is set to true if the request was satisfied from shared buffer cache.
+ * index_for_aio ,  if -ve , is negative of ( index of the aiocb in the BufferAiocbs array + 3 )
+ *     which is passed through to StartBufferIO
  */
-static Buffer
+Buffer
 ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 				  BlockNumber blockNum, ReadBufferMode mode,
-				  BufferAccessStrategy strategy, bool *hit)
+				  BufferAccessStrategy strategy, bool *hit , int index_for_aio )
 {
 	volatile BufferDesc *bufHdr;
 	Block		bufBlock;
 	bool		found;
+        int             allocrc;             /*  retcode from BufferAlloc */
 	bool		isExtend;
 	bool		isLocalBuf = SmgrIsTemp(smgr);
 
@@ -328,16 +477,40 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 	}
 	else
 	{
+                allocrc = mode; /* pass mode to BufferAlloc since it must not wait for async io if RBM_NOREAD_FOR_PREFETCH */
 		/*
 		 * lookup the buffer.  IO_IN_PROGRESS is set if the requested block is
 		 * not currently in memory.
 		 */
 		bufHdr = BufferAlloc(smgr, relpersistence, forkNum, blockNum,
-							 strategy, &found);
-		if (found)
-			pgBufferUsage.shared_blks_hit++;
+							 strategy, &allocrc , index_for_aio );
+		if (allocrc < 0) {
+                    if (mode == RBM_ZERO_ON_ERROR || zero_damaged_pages)
+                    {
+                        ereport(WARNING,
+                                (errcode(ERRCODE_DATA_CORRUPTED),
+                                 errmsg("invalid page header in block %u of relation %s; zeroing out page",
+                                        blockNum,
+                                        relpath(smgr->smgr_rnode, forkNum))));
+                        bufBlock = BufHdrGetBlock(bufHdr);
+                        MemSet((char *) bufBlock, 0, BLCKSZ);
+                    }
 		else
+                      ereport(ERROR,
+                              (errcode(ERRCODE_DATA_CORRUPTED),
+                               errmsg("invalid page header in block %u of relation %s",
+                                      blockNum,
+                                      relpath(smgr->smgr_rnode, forkNum))));
+                        found = true;
+                }
+		else if (allocrc > 0) {
+			pgBufferUsage.shared_blks_hit++;
+                        found = true;
+                }
+		else {
 			pgBufferUsage.shared_blks_read++;
+                        found = false;
+                }
 	}
 
 	/* At this point we do NOT hold any locks. */
@@ -410,7 +583,7 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 				Assert(bufHdr->flags & BM_VALID);
 				bufHdr->flags &= ~BM_VALID;
 				UnlockBufHdr(bufHdr);
-			} while (!StartBufferIO(bufHdr, true));
+			} while (!StartBufferIO(bufHdr, true, 0));
 		}
 	}
 
@@ -430,6 +603,7 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 
 	bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);
 
+        if (mode != RBM_NOREAD_FOR_PREFETCH) {
 	if (isExtend)
 	{
 		/* new buffers are zero-filled */
@@ -499,6 +673,7 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 	VacuumPageMiss++;
 	if (VacuumCostActive)
 		VacuumCostBalance += VacuumCostPageMiss;
+	}
 
 	TRACE_POSTGRESQL_BUFFER_READ_DONE(forkNum, blockNum,
 									  smgr->smgr_rnode.node.spcNode,
@@ -520,21 +695,39 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
  * the default strategy.  The selected buffer's usage_count is advanced when
  * using the default strategy, but otherwise possibly not (see PinBuffer).
  *
- * The returned buffer is pinned and is already marked as holding the
- * desired page.  If it already did have the desired page, *foundPtr is
- * set TRUE.  Otherwise, *foundPtr is set FALSE and the buffer is marked
+ * index_for_aio is an input parm which,  if non-zero,  identifies a BufferAiocb
+ * acquired by caller and to be used for any StartBufferIO performed by this routine.
+ * In this case,  if block not found in buffer pool and we allocate a new buffer,
+ * then we must maintain the spinlock on the buffer and pass it back to caller.
+ *
+ * foundPtr is input and output :
+ *  . input   -  indicates the read-buffer mode  ( see bufmgr.h )
+ *  . output  -  indicates the status of the buffer - see below
+ *
+ * Except for the case of RBM_NOREAD_FOR_PREFETCH and buffer is found,
+ * the returned buffer is pinned and is already marked as holding the
+ * desired page.
+ *  If it already did have the desired page and page content is valid,
+ *  *foundPtr is set to 1
+ *  If it already did have the desired page and mode is RBM_NOREAD_FOR_PREFETCH
+ *    and StartBufferIO returned false
+ *    (meaning it could not initialise the buffer for aio)
+ *  *foundPtr is set to 2
+ *  If it already did have the desired page but page content is invalid,
+ *  *foundPtr is set to -1
+ *   this can happen only if the buffer was read by an async read
+ *   and the aio is still in progress or pinned by the issuer of the startaio.
+ *  Otherwise, *foundPtr is set to 0 and the buffer is marked
  * as IO_IN_PROGRESS; ReadBuffer will now need to do I/O to fill it.
  *
- * *foundPtr is actually redundant with the buffer's BM_VALID flag, but
- * we keep it for simplicity in ReadBuffer.
- *
- * No locks are held either at entry or exit.
+ * No locks are held either at entry or exit EXCEPT for case noted above
+ * of passing an empty buffer back to async io caller ( index_for_aio set ).
  */
 static volatile BufferDesc *
 BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 			BlockNumber blockNum,
 			BufferAccessStrategy strategy,
-			bool *foundPtr)
+			int *foundPtr , int index_for_aio )
 {
 	BufferTag	newTag;			/* identity of requested block */
 	uint32		newHash;		/* hash value for newTag */
@@ -546,6 +739,13 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 	int			buf_id;
 	volatile BufferDesc *buf;
 	bool		valid;
+        int             IntentionBufferrc;      /* retcode from BufCheckAsync */
+        bool            StartBufferIOrc;        /* retcode from StartBufferIO */
+        ReadBufferMode mode;
+
+
+        mode = *foundPtr;
+        *foundPtr = 0;
 
 	/* create a tag so we can lookup the buffer */
 	INIT_BUFFERTAG(newTag, smgr->smgr_rnode.node, forkNum, blockNum);
@@ -560,21 +760,53 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 	if (buf_id >= 0)
 	{
 		/*
-		 * Found it.  Now, pin the buffer so no one can steal it from the
-		 * buffer pool, and check to see if the correct data has been loaded
-		 * into the buffer.
+		 * Found it.
 		 */
+                *foundPtr = 1;
 		buf = &BufferDescriptors[buf_id];
 
-		valid = PinBuffer(buf, strategy);
-
-		/* Can release the mapping lock as soon as we've pinned it */
+                /*   If prefetch mode,  then return immediately indicating found,
+                **   and NOTE in this case only,  we did not pin buffer.
+                **   In theory we might try to check whether the buffer is valid,  io in progress,  etc
+                **   but in practice it is simpler to abandon the prefetch if the buffer exists
+                */
+                if (mode == RBM_NOREAD_FOR_PREFETCH) {
+                    /* release the mapping lock and return    */
 		LWLockRelease(newPartitionLock);
-
-		*foundPtr = TRUE;
-
-		if (!valid)
-		{
+                } else {
+                    /*   note that the current request is for same tag as the one associated with the aio -
+                    **   so simply complete the aio and we have our buffer.
+                    **         If an aio was started on this buffer,
+                    **         check complete and wait for it if not.
+                    **         And,  if aio had been started,  then the task
+                    **         which issued the start aio already pinned it for this read,
+                    **         so if that task was me and the aio was successful,
+                    **         pass the current pin to this read without dropping and re-acquiring.
+                    **         this is all done by BufCheckAsync
+                    */
+                    IntentionBufferrc = BufCheckAsync(smgr , 0 , buf, BUF_INTENTION_WANT , strategy , index_for_aio , false , newPartitionLock );
+
+                    /*       check to see if the correct data has been loaded into the buffer.  */
+                    valid = (IntentionBufferrc == BUF_INTENT_RC_VALID);
+
+                    /*  check for serious IO errors   */
+                    if (!valid) {
+                        if (    (IntentionBufferrc != BUF_INTENT_RC_INVALID_NO_AIO)
+                             && (IntentionBufferrc != BUF_INTENT_RC_INVALID_AIO)
+                           ) {
+                            *foundPtr = -1;  /*  inform caller of serious error */
+                        }
+                        else
+                        if (IntentionBufferrc == BUF_INTENT_RC_INVALID_AIO) {
+                            goto proceed_with_not_found;  /*  yes,  I know,  a goto ... think of it as a break out of the if */
+                        }
+                     }
+
+                    /* BufCheckAsync pinned the buffer            */
+                    /* so can now release the mapping lock               */
+                    LWLockRelease(newPartitionLock);
+
+                    if (!valid) {
 			/*
 			 * We can only get here if (a) someone else is still reading in
 			 * the page, or (b) a previous read attempt failed.  We have to
@@ -582,19 +814,21 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 			 * own read attempt if the page is still not BM_VALID.
 			 * StartBufferIO does it all.
 			 */
-			if (StartBufferIO(buf, true))
+                                if (StartBufferIO(buf, true, index_for_aio))
 			{
 				/*
 				 * If we get here, previous attempts to read the buffer must
 				 * have failed ... but we shall bravely try again.
 				 */
-				*foundPtr = FALSE;
+                                        *foundPtr = 0;
+                                }
 			}
 		}
 
 		return buf;
 	}
 
+  proceed_with_not_found:
 	/*
 	 * Didn't find it in the buffer pool.  We'll have to initialize a new
 	 * buffer.  Remember to unlock the mapping lock while doing the work.
@@ -619,8 +853,10 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 		/* Must copy buffer flags while we still hold the spinlock */
 		oldFlags = buf->flags;
 
-		/* Pin the buffer and then release the buffer spinlock */
-		PinBuffer_Locked(buf);
+                /*         If an aio was started on this buffer,
+                **         check complete and cancel it if not.
+                */
+                BufCheckAsync(smgr , 0 , buf, BUF_INTENTION_REJECT_OBTAIN_PIN , 0 , index_for_aio, true , 0 );
 
 		/* Now it's safe to release the freelist lock */
 		if (lock_held)
@@ -791,13 +1027,18 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 				 * then set up our own read attempt if the page is still not
 				 * BM_VALID.  StartBufferIO does it all.
 				 */
-				if (StartBufferIO(buf, true))
+                                StartBufferIOrc = StartBufferIO(buf, true , index_for_aio); /* retcode from StartBufferIO */
+				if (StartBufferIOrc)
 				{
 					/*
 					 * If we get here, previous attempts to read the buffer
 					 * must have failed ... but we shall bravely try again.
 					 */
-					*foundPtr = FALSE;
+					*foundPtr = 0;
+                                } else
+                                if (mode == RBM_NOREAD_FOR_PREFETCH) {
+					UnpinBuffer(buf, true); /* must unpin if RBM_NOREAD_FOR_PREFETCH and found */
+                                        *foundPtr = 2;  /*  inform BufStartAsync that buffer must not be used */
 				}
 			}
 
@@ -860,10 +1101,17 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 	 * lock.  If StartBufferIO returns false, then someone else managed to
 	 * read it before we did, so there's nothing left for BufferAlloc() to do.
 	 */
-	if (StartBufferIO(buf, true))
-		*foundPtr = FALSE;
-	else
-		*foundPtr = TRUE;
+        StartBufferIOrc = StartBufferIO(buf, true , index_for_aio); /* retcode from StartBufferIO */
+        if (StartBufferIOrc) {
+		*foundPtr = 0;
+        } else {
+                if (mode == RBM_NOREAD_FOR_PREFETCH) {
+                    UnpinBuffer(buf, true); /* must unpin if RBM_NOREAD_FOR_PREFETCH and found */
+                    *foundPtr = 2;  /*  inform BufStartAsync that buffer must not be used */
+                } else {
+                    *foundPtr = 1;
+                }
+        }
 
 	return buf;
 }
@@ -970,6 +1218,10 @@ retry:
 	/*
 	 * Insert the buffer at the head of the list of free buffers.
 	 */
+        /*   avoid confusing freelist with strange-looking freeNext */
+        if (buf->freeNext <= FREENEXT_BAIOCB_ORIGIN) { /* means was used for aiocb index */
+            buf->freeNext = FREENEXT_NOT_IN_LIST;
+        }
 	StrategyFreeBuffer(buf);
 }
 
@@ -1022,6 +1274,56 @@ MarkBufferDirty(Buffer buffer)
 	UnlockBufHdr(bufHdr);
 }
 
+/*  return the blocknum of the block in a buffer if it is valid
+**  if a shared buffer,  it must be pinned
+*/
+BlockNumber
+BlocknumOfBuffer(Buffer buffer)
+{
+	volatile BufferDesc *bufHdr;
+        BlockNumber rc = 0;
+
+	if (BufferIsValid(buffer)) {
+            if  (BufferIsLocal(buffer)) {
+                bufHdr = &LocalBufferDescriptors[-buffer - 1];
+            } else {
+                bufHdr = &BufferDescriptors[buffer - 1];
+            }
+
+            rc = bufHdr->tag.blockNum;
+        }
+
+        return rc;
+}
+
+/*  report whether specified buffer contains same or different block
+**  if a shared buffer,  it must be pinned
+*/
+bool
+BlocknotinBuffer(Buffer buffer,
+					 Relation relation,
+					 BlockNumber blockNum)
+{
+	volatile BufferDesc *bufHdr;
+        bool rc = false;
+
+	if (BufferIsValid(buffer)) {
+            if  (BufferIsLocal(buffer)) {
+                bufHdr = &LocalBufferDescriptors[-buffer - 1];
+            } else {
+                bufHdr = &BufferDescriptors[buffer - 1];
+            }
+
+            rc =
+                  (   (bufHdr->tag.blockNum != blockNum)
+                   || (!(RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) ))
+                   || (bufHdr->tag.forkNum != MAIN_FORKNUM)
+                  );
+        }
+
+        return rc;
+}
+
 /*
  * ReleaseAndReadBuffer -- combine ReleaseBuffer() and ReadBuffer()
  *
@@ -1040,18 +1342,18 @@ ReleaseAndReadBuffer(Buffer buffer,
 					 Relation relation,
 					 BlockNumber blockNum)
 {
-	ForkNumber	forkNum = MAIN_FORKNUM;
 	volatile BufferDesc *bufHdr;
+        bool isDifferentBlock;   /*       requesting different block from that already in buffer ? */
 
 	if (BufferIsValid(buffer))
 	{
+	    /* if a shared buff, we have pin, so it's ok to examine tag without spinlock */
+            isDifferentBlock = BlocknotinBuffer(buffer,relation,blockNum); /*  requesting different block from that already in buffer ? */
 		if (BufferIsLocal(buffer))
 		{
 			Assert(LocalRefCount[-buffer - 1] > 0);
 			bufHdr = &LocalBufferDescriptors[-buffer - 1];
-			if (bufHdr->tag.blockNum == blockNum &&
-				RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
-				bufHdr->tag.forkNum == forkNum)
+			if (!isDifferentBlock)
 				return buffer;
 			ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
 			LocalRefCount[-buffer - 1]--;
@@ -1060,12 +1362,12 @@ ReleaseAndReadBuffer(Buffer buffer,
 		{
 			Assert(PrivateRefCount[buffer - 1] > 0);
 			bufHdr = &BufferDescriptors[buffer - 1];
-			/* we have pin, so it's ok to examine tag without spinlock */
-			if (bufHdr->tag.blockNum == blockNum &&
-				RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
-				bufHdr->tag.forkNum == forkNum)
+                        BufCheckAsync(0 , relation , bufHdr , ( isDifferentBlock ? BUF_INTENTION_REJECT_FORGET
+                                                                                 : BUF_INTENTION_REJECT_KEEP_PIN )
+                                                            , 0 , 0 , false , 0 ); /* end any IO and maybe unpin */
+			if (!isDifferentBlock) {
 				return buffer;
-			UnpinBuffer(bufHdr, true);
+                        }
 		}
 	}
 
@@ -1090,11 +1392,12 @@ ReleaseAndReadBuffer(Buffer buffer,
  * Returns TRUE if buffer is BM_VALID, else FALSE.  This provision allows
  * some callers to avoid an extra spinlock cycle.
  */
-static bool
+bool
 PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy)
 {
 	int			b = buf->buf_id;
 	bool		result;
+        bool       pin_already_banked_by_me = 0;  /* buffer is already pinned by me and redeemable */
 
 	if (PrivateRefCount[b] == 0)
 	{
@@ -1116,12 +1419,34 @@ PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy)
 	else
 	{
 		/* If we previously pinned the buffer, it must surely be valid */
+                /* Errr  -   is that really true  ???    I don't think so  :
+                ** what if I pin,  try an IO,  in progress,  then mistakenly pin again
 		result = true;
+                */
+		LockBufHdr(buf);
+                pin_already_banked_by_me = (    (PrivateRefCount[b] > 0) && (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                                             && (   (    (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                                                     : (-(buf->freeNext))  )  == this_backend_pid )
+                                           );
+                if (pin_already_banked_by_me) {
+                        elog(LOG, "PinBuffer : on buffer %d with banked pin rel=%s, blockNum=%u, with flags %X refcount=%u"
+                       ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                       ,buf->tag.blockNum, buf->flags, buf->refcount);
+                    buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;
+                    if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                        buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+	}
+                }
+		result = (buf->flags & BM_VALID) != 0;
+		UnlockBufHdr(buf);
 	}
+
+        if (!pin_already_banked_by_me) {
 	PrivateRefCount[b]++;
 	Assert(PrivateRefCount[b] > 0);
 	ResourceOwnerRememberBuffer(CurrentResourceOwner,
 								BufferDescriptorGetBuffer(buf));
+        }
 	return result;
 }
 
@@ -1138,19 +1463,36 @@ PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy)
  * to save a spin lock/unlock cycle, because we need to pin a buffer before
  * its state can change under us.
  */
-static void
+void
 PinBuffer_Locked(volatile BufferDesc *buf)
 {
 	int			b = buf->buf_id;
+        bool       pin_already_banked_by_me;  /* buffer is already pinned by me and redeemable */
 
-	if (PrivateRefCount[b] == 0)
+        pin_already_banked_by_me = (    (PrivateRefCount[b] > 0) && (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                                     && (   (    (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                                             : (-(buf->freeNext))  )  == this_backend_pid )
+                             );
+	if (PrivateRefCount[b] == 0) {
 		buf->refcount++;
+        }
+        if (pin_already_banked_by_me) {
+                        elog(LOG, "PinBuffer_Locked : on buffer %d with banked pin rel=%s, blockNum=%u, with flags %X refcount=%u"
+                       ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                       ,buf->tag.blockNum, buf->flags, buf->refcount);
+            buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;
+                    if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                        buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+                    }
+        }
 	UnlockBufHdr(buf);
+        if (!pin_already_banked_by_me) {
 	PrivateRefCount[b]++;
 	Assert(PrivateRefCount[b] > 0);
 	ResourceOwnerRememberBuffer(CurrentResourceOwner,
 								BufferDescriptorGetBuffer(buf));
 }
+}
 
 /*
  * UnpinBuffer -- make buffer available for replacement.
@@ -1160,29 +1502,68 @@ PinBuffer_Locked(volatile BufferDesc *buf)
  * Most but not all callers want CurrentResourceOwner to be adjusted.
  * Those that don't should pass fixOwner = FALSE.
  */
-static void
+void
 UnpinBuffer(volatile BufferDesc *buf, bool fixOwner)
 {
+
 	int			b = buf->buf_id;
+        bool       pin_already_banked_by_me;  /* buffer is already pinned by me and redeemable */
 
-	if (fixOwner)
+	if (fixOwner) {
 		ResourceOwnerForgetBuffer(CurrentResourceOwner,
 								  BufferDescriptorGetBuffer(buf));
+        }
 
 	Assert(PrivateRefCount[b] > 0);
 	PrivateRefCount[b]--;
 	if (PrivateRefCount[b] == 0)
 	{
+
 		/* I'd better not still hold any locks on the buffer */
 		Assert(!LWLockHeldByMe(buf->content_lock));
 		Assert(!LWLockHeldByMe(buf->io_in_progress_lock));
 
 		LockBufHdr(buf);
 
+		/* this backend has released last pin - buffer should not have pin banked by me
+                ** and if AIO in progress then there should be another backend pin
+                */
+                pin_already_banked_by_me = (       (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                                             &&    (   (    (buf->flags & BM_AIO_IN_PROGRESS)
+                                                         ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                         : (-(buf->freeNext))
+                                                       )  == this_backend_pid )
+                                           );
+                if (pin_already_banked_by_me) {
+                        /*  this is a strange situation  -  caller had a banked pin (which callers are supposed not to know about)
+                        **                                  but either discovered it had it or has over-counted how many pins it has
+                        */
+                        buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;   /*   redeem the pin although it is now of no use since about to release */
+                        if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                            buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+                        }
+
+                        /*     temporarily suppress logging error to avoid performance degradation -
+                        **     either this task really does not need the buffer in which case the error is harmless
+                        **     or a more severe error will be detected later (possibly immediately below)
+                        elog(LOG, "UnpinBuffer :  released last this-backend pin on buffer %d rel=%s, blockNum=%u, but had banked pin flags %X refcount=%u"
+                       ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                       ,buf->tag.blockNum, buf->flags, buf->refcount);
+                        */
+                }
+
 		/* Decrement the shared reference count */
 		Assert(buf->refcount > 0);
 		buf->refcount--;
 
+                if (   (buf->refcount == 0) && (buf->flags & BM_AIO_IN_PROGRESS)  ) {
+
+                        elog(ERROR, "UnpinBuffer :  released last any-backend pin on buffer %d rel=%s, blockNum=%u, but AIO in progress flags %X refcount=%u"
+                            ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                            ,buf->tag.blockNum, buf->flags, buf->refcount);
+                }
+
+
 		/* Support LockBufferForCleanup() */
 		if ((buf->flags & BM_PIN_COUNT_WAITER) &&
 			buf->refcount == 1)
@@ -1657,6 +2038,7 @@ SyncOneBuffer(int buf_id, bool skip_recently_used)
 	volatile BufferDesc *bufHdr = &BufferDescriptors[buf_id];
 	int			result = 0;
 
+
 	/*
 	 * Check whether buffer needs writing.
 	 *
@@ -1789,6 +2171,8 @@ PrintBufferLeakWarning(Buffer buffer)
 	char	   *path;
 	BackendId	backend;
 
+
+
 	Assert(BufferIsValid(buffer));
 	if (BufferIsLocal(buffer))
 	{
@@ -1799,12 +2183,28 @@ PrintBufferLeakWarning(Buffer buffer)
 	else
 	{
 		buf = &BufferDescriptors[buffer - 1];
+#ifdef USE_PREFETCH
+                /*   If reason that this buffer is pinned
+                **   is that it was prefetched with async_io
+                **   and never read or discarded, then omit the
+                **   warning,  because this is expected in some
+                **   cases when a scan is closed abnormally.
+                **   Note that the buffer will be released soon by our caller.
+                */
+                if (buf->flags & BM_AIO_PREFETCH_PIN_BANKED) {
+                    pgBufferUsage.aio_read_forgot++; /* account for it */
+                    return;
+                }
+#endif /*  USE_PREFETCH */
 		loccount = PrivateRefCount[buffer - 1];
 		backend = InvalidBackendId;
 	}
 
+/* #if defined(DO_ISCAN_DISCARD_BUFFER) && defined(DO_HEAPSCAN_DISCARD_BUFFER) not yet working safely 130530 */
 	/* theoretically we should lock the bufhdr here */
 	path = relpathbackend(buf->tag.rnode, backend, buf->tag.forkNum);
+
+
 	elog(WARNING,
 		 "buffer refcount leak: [%03d] "
 		 "(rel=%s, blockNum=%u, flags=0x%x, refcount=%u %d)",
@@ -1812,6 +2212,7 @@ PrintBufferLeakWarning(Buffer buffer)
 		 buf->tag.blockNum, buf->flags,
 		 buf->refcount, loccount);
 	pfree(path);
+/* #endif defined(DO_ISCAN_DISCARD_BUFFER) && defined(DO_HEAPSCAN_DISCARD_BUFFER) not yet working safely 130530 */
 }
 
 /*
@@ -1928,7 +2329,7 @@ FlushBuffer(volatile BufferDesc *buf, SMgrRelation reln)
 	 * false, then someone else flushed the buffer before we could, so we need
 	 * not do anything.
 	 */
-	if (!StartBufferIO(buf, false))
+	if (!StartBufferIO(buf, false, 0))
 		return;
 
 	/* Setup error traceback support for ereport() */
@@ -2512,6 +2913,70 @@ FlushDatabaseBuffers(Oid dbid)
 	}
 }
 
+#ifdef USE_PREFETCH
+/*
+ * DiscardBuffer -- discard shared buffer used for a previously
+ *                  prefetched but unread block of a relation
+ *
+ * If the buffer is found and pinned with a banked pin,  then :
+ *      .  if AIO in progress, terminate AIO without waiting
+ *      .  if AIO had already completed successfully,
+ *         then mark buffer valid (in case someone else wants it)
+ *      .  redeem the banked pin and unpin it.
+ *
+ * This function is similar in purpose to ReleaseBuffer (below)
+ * but sufficiently different that it is a separate function.
+ * Two important differences are :
+ *   .   caller identifies buffer by blocknumber,  not buffer number
+ *   .   we unpin buffer *only* if the pin is banked,
+ *                      *never* if pinned but not banked.
+ *       This is essential as caller may perform a sequence of
+ *  SCAN1   . PrefetchBuffer   (and remember block was prefetched)
+ *  SCAN2   . ReadBuffer       (but fails to connect this read to the prefetch by SCAN1)
+ *  SCAN1   . DiscardBuffer    (SCAN1 terminates early)
+ *  SCAN2   . access tuples in buffer
+ *        Clearly the Discard *must not* unpin the buffer since SCAN2 needs it!
+ *
+ *
+ * caller may pass InvalidBlockNumber as blockNum to mean do nothing
+ */
+void
+DiscardBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
+{
+        BufferTag	newTag;		   /* identity of requested block */
+        uint32		newHash;	   /* hash value for newTag */
+        LWLockId	newPartitionLock;  /* buffer partition lock for it */
+        Buffer		buf_id;
+        volatile        BufferDesc *buf_desc;
+
+    if (!SmgrIsTemp(reln->rd_smgr)) {
+	Assert(RelationIsValid(reln));
+	if (BlockNumberIsValid(blockNum)) {
+
+            /* create a tag so we can lookup the buffer */
+            INIT_BUFFERTAG(newTag, reln->rd_smgr->smgr_rnode.node,
+                                       forkNum, blockNum);
+
+            /* determine its hash code and partition lock ID */
+            newHash = BufTableHashCode(&newTag);
+            newPartitionLock = BufMappingPartitionLock(newHash);
+
+            /* see if the block is in the buffer pool already */
+            LWLockAcquire(newPartitionLock, LW_SHARED);
+            buf_id = BufTableLookup(&newTag, newHash);
+            LWLockRelease(newPartitionLock);
+
+            /* If in buffers, proceed */
+            if (buf_id >= 0) {
+                buf_desc = &BufferDescriptors[buf_id];        /* found buffer descriptor */
+                BufCheckAsync(0 , reln, buf_desc , BUF_INTENTION_REJECT_UNBANK , 0 , 0 , false , 0); /* end the IO and unpin if banked */
+                pgBufferUsage.aio_read_discrd++; /* account for it */
+            }
+        }
+    }
+}
+#endif   /* USE_PREFETCH */
+
 /*
  * ReleaseBuffer -- release the pin on a buffer
  */
@@ -2520,26 +2985,23 @@ ReleaseBuffer(Buffer buffer)
 {
 	volatile BufferDesc *bufHdr;
 
+
 	if (!BufferIsValid(buffer))
 		elog(ERROR, "bad buffer ID: %d", buffer);
 
-	ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
 
 	if (BufferIsLocal(buffer))
 	{
+                ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
 		Assert(LocalRefCount[-buffer - 1] > 0);
 		LocalRefCount[-buffer - 1]--;
 		return;
 	}
-
-	bufHdr = &BufferDescriptors[buffer - 1];
-
-	Assert(PrivateRefCount[buffer - 1] > 0);
-
-	if (PrivateRefCount[buffer - 1] > 1)
-		PrivateRefCount[buffer - 1]--;
 	else
-		UnpinBuffer(bufHdr, false);
+        {
+                bufHdr = &BufferDescriptors[buffer - 1];
+                BufCheckAsync(0 , 0 , bufHdr , BUF_INTENTION_REJECT_NOADJUST , 0 , 0 , false , 0 );
+        }
 }
 
 /*
@@ -2565,14 +3027,41 @@ UnlockReleaseBuffer(Buffer buffer)
 void
 IncrBufferRefCount(Buffer buffer)
 {
+        bool       pin_already_banked_by_me = false;  /* buffer is already pinned by me and redeemable */
+        volatile BufferDesc *buf;                     /* descriptor for a shared buffer */
+
 	Assert(BufferIsPinned(buffer));
+
+        if (!(BufferIsLocal(buffer))) {
+                buf = &BufferDescriptors[buffer - 1];
+		LockBufHdr(buf);
+                pin_already_banked_by_me =
+                      (    (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                        && (   (    (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                                      : (-(buf->freeNext))  )  == this_backend_pid )
+                      );
+        }
+
+        if (!pin_already_banked_by_me) {
 	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
 	ResourceOwnerRememberBuffer(CurrentResourceOwner, buffer);
+        }
+
 	if (BufferIsLocal(buffer))
 		LocalRefCount[-buffer - 1]++;
-	else
+	else {
+                if (pin_already_banked_by_me) {
+                    buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;
+                    if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                        buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+                    }
+                }
+		UnlockBufHdr(buf);
+                if (!pin_already_banked_by_me) {
 		PrivateRefCount[buffer - 1]++;
 }
+        }
+}
 
 /*
  * MarkBufferDirtyHint
@@ -2994,61 +3483,138 @@ WaitIO(volatile BufferDesc *buf)
  *
  * In some scenarios there are race conditions in which multiple backends
  * could attempt the same I/O operation concurrently.  If someone else
- * has already started I/O on this buffer then we will block on the
+ * has already started synchronous I/O on this buffer then we will block on the
  * io_in_progress lock until he's done.
  *
+ * if an async io is in progress and we are doing synchronous io,
+ * then readbuffer uses call to smgrcompleteaio to wait,
+ * and so we treat this request as if no io in progress
+ *
  * Input operations are only attempted on buffers that are not BM_VALID,
  * and output operations only on buffers that are BM_VALID and BM_DIRTY,
  * so we can always tell if the work is already done.
  *
+ * index_for_aio is an input parm which,  if non-zero,  identifies a BufferAiocb
+ * acquired by caller and to be attached to the buffer header for use with async io
+ *
  * Returns TRUE if we successfully marked the buffer as I/O busy,
  * FALSE if someone else already did the work.
  */
 static bool
-StartBufferIO(volatile BufferDesc *buf, bool forInput)
+StartBufferIO(volatile BufferDesc *buf, bool forInput , int index_for_aio )
 {
+#ifdef USE_PREFETCH
+        struct BufferAiocb volatile * BAiocb = (struct BufferAiocb*)0;  /*  BufferAiocb for use with aio */
+#endif   /* USE_PREFETCH */
+ 
+        if (!index_for_aio)
 	Assert(!InProgressBuf);
 
 	for (;;)
 	{
+                if (!index_for_aio) {
 		/*
 		 * Grab the io_in_progress lock so that other processes can wait for
 		 * me to finish the I/O.
 		 */
 		LWLockAcquire(buf->io_in_progress_lock, LW_EXCLUSIVE);
+                }
 
 		LockBufHdr(buf);
 
-		if (!(buf->flags & BM_IO_IN_PROGRESS))
+                /*     the following test is intended to distinguish between :
+                **      .   buffer which : 
+                **           .     has io in progress
+                **             AND is not associated with a current aio
+                **      .   not the above
+                **     Here,  "recent" means an aio marked by buf->freeNext <= FREENEXT_BAIOCB_ORIGIN but no longer in progress -
+                **          this situation arises when the aio has just been cancelled and this process now wishes to recycle the buffer.
+                **          In this case,  the first such would-be recycler (i.e. me) must :
+                **             . avoid waiting for the cancelled aio to complete
+                **             . if not myself doing async read, then assume responsibility for posting other future readbuffers.
+                */
+		if (    (buf->flags & BM_AIO_IN_PROGRESS)
+                     || (!(buf->flags & BM_IO_IN_PROGRESS))
+                   )
 			break;
 
 		/*
-		 * The only way BM_IO_IN_PROGRESS could be set when the io_in_progress
+		 * The only way BM_IO_IN_PROGRESS without AIO in progress could be set when the io_in_progress
 		 * lock isn't held is if the process doing the I/O is recovering from
 		 * an error (see AbortBufferIO).  If that's the case, we must wait for
 		 * him to get unwedged.
 		 */
 		UnlockBufHdr(buf);
+                if (!index_for_aio) {
 		LWLockRelease(buf->io_in_progress_lock);
+                }
 		WaitIO(buf);
 	}
 
-	/* Once we get here, there is definitely no I/O active on this buffer */
-
+#ifdef USE_PREFETCH
+	/* Once we get here, there is definitely no synchronous I/O active on this buffer
+        ** but if being asked to attach a BufferAiocb to the buf header,
+        ** then we must also check if there is any async io currently
+        ** in progress or pinned started by a different task.
+        */
+        if (index_for_aio) {
+            BAiocb = (BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext);
+            if (    (buf->flags & (BM_AIO_IN_PROGRESS|BM_AIO_PREFETCH_PIN_BANKED))
+                 && (buf->freeNext <= FREENEXT_BAIOCB_ORIGIN)
+                 && (BAiocb->pidOfAio != this_backend_pid)
+               ) {
+                    /* someone else already doing async I/O */
+                    UnlockBufHdr(buf);
+                    return false;
+            }
+	}
+#endif   /* USE_PREFETCH */
 	if (forInput ? (buf->flags & BM_VALID) : !(buf->flags & BM_DIRTY))
 	{
 		/* someone else already did the I/O */
 		UnlockBufHdr(buf);
+                if (!index_for_aio) {
 		LWLockRelease(buf->io_in_progress_lock);
+                }
 		return false;
 	}
 
 	buf->flags |= BM_IO_IN_PROGRESS;
 
+#ifdef USE_PREFETCH
+        if (index_for_aio) {
+            BAiocb = (BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - index_for_aio);
+            /*   insist that no other buffer is using this BufferAiocb for async IO */
+            if (BAiocb->BAiocbbufh == (struct sbufdesc *)0) {
+                BAiocb->BAiocbbufh = buf;
+            }
+            if (BAiocb->BAiocbbufh != buf) {
+                               ereport(ERROR,
+                                    (errcode(ERRCODE_INTERNAL_ERROR),
+                                     errmsg("AIO control block %p to be used by %p already in use by %p"
+                                              ,BAiocb ,buf , BAiocb->BAiocbbufh)));
+            }
+            /*   note - there is no need to register self as an dependent of BAiocb
+            **   as we shall not unlock buf_desc before we free the BAiocb
+            */
+
+            buf->flags |= BM_AIO_IN_PROGRESS;
+            buf->freeNext = index_for_aio;
+            /*  at this point,  this buffer appears to have an in-progress aio_read,
+            **  and any other task which is able to look inside the buffer might try waiting on that aio -
+            **  except we have not yet issued the aio!   So we must keep the buffer header locked
+            **  from here all the way back to the BufStartAsync caller
+            */
+        } else {
+#endif   /* USE_PREFETCH */
+
 	UnlockBufHdr(buf);
 
 	InProgressBuf = buf;
 	IsForInput = forInput;
+#ifdef USE_PREFETCH
+        }
+#endif   /* USE_PREFETCH */
 
 	return true;
 }
@@ -3058,7 +3624,7 @@ StartBufferIO(volatile BufferDesc *buf, bool forInput)
  *	(Assumptions)
  *	My process is executing IO for the buffer
  *	BM_IO_IN_PROGRESS bit is set for the buffer
- *	We hold the buffer's io_in_progress lock
+ *	if no async IO in progress,  then We hold the buffer's io_in_progress lock
  *	The buffer is Pinned
  *
  * If clear_dirty is TRUE and BM_JUST_DIRTIED is not set, we clear the
@@ -3070,26 +3636,32 @@ StartBufferIO(volatile BufferDesc *buf, bool forInput)
  * BM_IO_ERROR in a failure case.  For successful completion it could
  * be 0, or BM_VALID if we just finished reading in the page.
  */
-static void
+void
 TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
 				  int set_flag_bits)
 {
-	Assert(buf == InProgressBuf);
+        int flags_on_entry;
 
 	LockBufHdr(buf);
 
+        flags_on_entry = buf->flags;
+
+        if (! (flags_on_entry & BM_AIO_IN_PROGRESS) )
+            Assert( buf == InProgressBuf );
+
 	Assert(buf->flags & BM_IO_IN_PROGRESS);
-	buf->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR);
+	buf->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR | BM_AIO_IN_PROGRESS);
 	if (clear_dirty && !(buf->flags & BM_JUST_DIRTIED))
 		buf->flags &= ~(BM_DIRTY | BM_CHECKPOINT_NEEDED);
 	buf->flags |= set_flag_bits;
 
 	UnlockBufHdr(buf);
 
+        if (! (flags_on_entry & BM_AIO_IN_PROGRESS) ) {
 	InProgressBuf = NULL;
-
 	LWLockRelease(buf->io_in_progress_lock);
 }
+}
 
 /*
  * AbortBufferIO: Clean up any active buffer I/O after an error.
diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index 1f69c9e..4133506 100644
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -77,6 +77,9 @@
 #include "utils/guc.h"
 #include "utils/resowner_private.h"
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+#include "aio.h"
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * We must leave some file descriptors free for system(), the dynamic loader,
@@ -1239,6 +1242,10 @@ FileClose(File file)
  * We could add an implementation using libaio in the future; but note that
  * this API is inappropriate for libaio, which wants to have a buffer provided
  * to read into.
+ * Also note that a new, different implementation of asynchronous prefetch
+ * using librt,  not libaio,  is provided by the two functions following this one,
+ * FileStartaio and FileCompleteaio.   These also require to have a buffer provided
+ * to read into,  which the new async_io support provides.
  */
 int
 FilePrefetch(File file, off_t offset, int amount)
@@ -1266,6 +1273,139 @@ FilePrefetch(File file, off_t offset, int amount)
 #endif
 }
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*
+ * FileInitaio - initialize the aio subsystem max number of threads and max number of requests
+ *  input parms
+ *  max_aio_threads;    maximum number of threads
+ *  max_aio_num;        maximum number of concurrent aio read requests
+ *
+ *  on linux, man page for the librt implemenation of aio_init() says :
+ *         This function is a GNU extension.
+ *  If your posix aio does not have it,   then add the following line to 
+ *        src/include/pg_config_manual.h
+ *    #define DONT_HAVE_AIO_INIT
+ *  to render it as a no-op
+ */
+void
+FileInitaio(int max_aio_threads, int max_aio_num )
+{
+#ifndef DONT_HAVE_AIO_INIT
+    struct aioinit aioinit_struct;  /*  structure to pass to aio_init */
+
+    aioinit_struct.aio_threads = max_aio_threads; /*     maximum number of threads */
+    aioinit_struct.aio_num = max_aio_num;         /*     maximum number of concurrent aio read requests */
+    aioinit_struct.aio_idle_time = 1;             /*     we dont want to alter this but aio_init does not ignore it so set to the default */
+    aio_init(&aioinit_struct);
+#endif  /* ndef DONT_HAVE_AIO_INIT */
+    return;
+}
+
+/*
+ * FileStartaio - initiate asynchronous read of a given range of the file.
+ * The logical seek position is unaffected.
+ *
+ * use standard posix aio (librt)
+ *  ASSUME   BufferAiocb.aio_buf already set to -> buffer by caller
+ *  return 0 if successfully started,  else non-zero
+ */
+int
+FileStartaio(File file, off_t offset, int amount , char *aiocbp )
+{
+	int	returnCode;
+        struct aiocb *my_aiocbp = (struct aiocb *)aiocbp;
+
+	Assert(FileIsValid(file));
+
+	DO_DB(elog(LOG, "FileStartaio: %d (%s) " INT64_FORMAT " %d",
+			   file, VfdCache[file].fileName,
+			   (int64) offset, amount));
+
+	returnCode = FileAccess(file);
+	if (returnCode >= 0) {
+
+            my_aiocbp->aio_fildes = VfdCache[file].fd;
+            my_aiocbp->aio_lio_opcode = LIO_READ;
+            my_aiocbp->aio_nbytes = amount;
+            my_aiocbp->aio_offset = offset;
+            returnCode = aio_read(my_aiocbp);
+        }
+
+	return returnCode;
+}
+
+/*
+ * FileCompleteaio - complete asynchronous aio read
+ * normal_wait indicates whether to cancel or wait -
+ *                 0 <=> cancel
+ *                 1 <=> wait
+ *
+ * use standard posix aio (librt)
+ *  return 0 if successfull and did not have to wait,
+ *         1 if successfull and had to wait,
+ *    else x'ff'
+ */
+int
+FileCompleteaio( char *aiocbp , int normal_wait )
+{
+	int	returnCode;
+	int	aio_errno;
+        struct aiocb *my_aiocbp = (struct aiocb *)aiocbp;
+        const struct aiocb const*cblist[1];
+        int fd;
+        struct timespec my_timeout = { 0 , 10000 };
+        int max_polls;
+
+        fd = my_aiocbp->aio_fildes;
+        cblist[0] = my_aiocbp;
+        returnCode = aio_errno = aio_error(my_aiocbp);
+        /* note that aio_error returns 0 if op already completed successfully */
+
+        /*  first handle normal case of waiting for op to complete  */
+        if (normal_wait) {
+            while (aio_errno == EINPROGRESS) {
+                max_polls = 256;
+                my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                returnCode = aio_suspend(cblist , 1 , &my_timeout);
+                while ((returnCode < 0) && (EAGAIN == errno) && (max_polls-- > 0)) {
+                    my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                    returnCode = aio_suspend(cblist , 1 , &my_timeout);
+                }
+                returnCode = aio_errno = aio_error(my_aiocbp);
+                /*  now return_code is from aio_error  */
+                if (returnCode == 0) {
+                    returnCode = 1;    /*  successful but had to wait */
+                }
+            }
+            if (aio_errno) {
+                elog(LOG, "FileCompleteaio: %d %d", fd, returnCode);
+                returnCode = 0xff; /*  unsuccessful */
+            }
+        } else {
+            if (aio_errno == EINPROGRESS) {
+                do {
+                        max_polls = 256;
+                        my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                        returnCode = aio_cancel(fd, my_aiocbp);
+                        while ((returnCode == AIO_NOTCANCELED) && (max_polls-- > 0)) {
+                            my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                            returnCode = aio_cancel(fd, my_aiocbp);
+                        }
+                    returnCode = aio_errno = aio_error(my_aiocbp);
+                } while (aio_errno == EINPROGRESS);
+                returnCode = 0xff; /*  unsuccessful */
+            }
+            if (returnCode != 0)
+                returnCode = 0xff; /*  unsuccessful */
+        }
+
+	DO_DB(elog(LOG, "FileCompleteaio: %d %d",
+			   fd, returnCode));
+
+	return returnCode;
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 int
 FileRead(File file, char *buffer, int amount)
 {
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 266b0da..3f13ded 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -52,6 +52,7 @@
 #include "utils/timeout.h"
 #include "utils/timestamp.h"
 
+extern pid_t this_backend_pid;     /*   pid of this backend */
 
 /* GUC variables */
 int			DeadlockTimeout = 1000;
@@ -361,6 +362,7 @@ InitProcess(void)
 	MyPgXact->xid = InvalidTransactionId;
 	MyPgXact->xmin = InvalidTransactionId;
 	MyProc->pid = MyProcPid;
+        this_backend_pid = getpid();    /*    pid of this backend */
 	/* backendId, databaseId and roleId will be filled in later */
 	MyProc->backendId = InvalidBackendId;
 	MyProc->databaseId = InvalidOid;
diff --git a/src/backend/storage/smgr/md.c b/src/backend/storage/smgr/md.c
index 3c1c81a..e0ca942 100644
--- a/src/backend/storage/smgr/md.c
+++ b/src/backend/storage/smgr/md.c
@@ -647,6 +647,62 @@ mdprefetch(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum)
 }
 
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*
+ *	mdinitaio() --  init the aio subsystem max number of threads and max number of requests
+ */
+void
+mdinitaio(int max_aio_threads, int max_aio_num)
+{
+     FileInitaio( max_aio_threads, max_aio_num );
+}
+
+/*
+ *	mdstartaio() -- start aio read of the specified block of a relation
+ */
+void
+mdstartaio(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum , char *aiocbp , int *retcode )
+{
+#ifdef USE_PREFETCH
+	off_t		seekpos;
+	MdfdVec    *v;
+        int local_retcode;
+
+	v = _mdfd_getseg(reln, forknum, blocknum, false, EXTENSION_FAIL);
+
+	seekpos = (off_t) BLCKSZ *(blocknum % ((BlockNumber) RELSEG_SIZE));
+
+	Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+
+	local_retcode = FileStartaio(v->mdfd_vfd, seekpos, BLCKSZ , aiocbp);
+	if (retcode) {
+            *retcode = local_retcode;
+        }
+#endif   /* USE_PREFETCH */
+}
+
+
+/*
+ *	mdcompleteaio() -- complete aio read of the specified block of a relation
+ *      on entry, *inoutcode should indicate :
+ *           .  non-0  <=>   check if complete and wait if not
+ *           .  0      <=>   cancel io immediately
+ */
+void
+mdcompleteaio( char *aiocbp , int *inoutcode )
+{
+#ifdef USE_PREFETCH
+        int local_retcode;
+
+	local_retcode = FileCompleteaio(aiocbp, (inoutcode ? *inoutcode : 0));
+	if (inoutcode) {
+            *inoutcode = local_retcode;
+        }
+#endif   /* USE_PREFETCH */
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+
 /*
  *	mdread() -- Read the specified block from a relation.
  */
diff --git a/src/backend/storage/smgr/smgr.c b/src/backend/storage/smgr/smgr.c
index d16f559..7f3e6ff 100644
--- a/src/backend/storage/smgr/smgr.c
+++ b/src/backend/storage/smgr/smgr.c
@@ -49,6 +49,12 @@ typedef struct f_smgr
 						 BlockNumber blocknum, char *buffer, bool skipFsync);
 	void		(*smgr_prefetch) (SMgrRelation reln, ForkNumber forknum,
 											  BlockNumber blocknum);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+	void		(*smgr_initaio) (int max_aio_threads, int max_aio_num);
+	void		(*smgr_startaio) (SMgrRelation reln, ForkNumber forknum,
+											  BlockNumber blocknum , char *aiocbp , int *retcode );
+	void		(*smgr_completeaio) ( char *aiocbp , int *inoutcode );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 	void		(*smgr_read) (SMgrRelation reln, ForkNumber forknum,
 										  BlockNumber blocknum, char *buffer);
 	void		(*smgr_write) (SMgrRelation reln, ForkNumber forknum,
@@ -66,7 +72,11 @@ typedef struct f_smgr
 static const f_smgr smgrsw[] = {
 	/* magnetic disk */
 	{mdinit, NULL, mdclose, mdcreate, mdexists, mdunlink, mdextend,
-		mdprefetch, mdread, mdwrite, mdnblocks, mdtruncate, mdimmedsync,
+		mdprefetch
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                        ,mdinitaio, mdstartaio, mdcompleteaio
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+              , mdread, mdwrite, mdnblocks, mdtruncate, mdimmedsync,
 		mdpreckpt, mdsync, mdpostckpt
 	}
 };
@@ -612,6 +622,35 @@ smgrprefetch(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum)
 	(*(smgrsw[reln->smgr_which].smgr_prefetch)) (reln, forknum, blocknum);
 }
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*
+ *	smgrinitaio() -- initialize the aio subsystem max number of threads and max number of requests
+ */
+void
+smgrinitaio(int max_aio_threads, int max_aio_num)
+{
+	(*(smgrsw[0].smgr_initaio)) ( max_aio_threads, max_aio_num );
+}
+
+/*
+ *	smgrstartaio() -- Initiate aio read of the specified block of a relation.
+ */
+void
+smgrstartaio(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum , char *aiocbp , int *retcode )
+{
+	(*(smgrsw[reln->smgr_which].smgr_startaio)) (reln, forknum, blocknum , aiocbp , retcode );
+}
+
+/*
+ *	smgrcompleteaio() -- Complete aio read of the specified block of a relation.
+ */
+void
+smgrcompleteaio(SMgrRelation reln,  char *aiocbp , int *inoutcode )
+{
+	(*(smgrsw[reln->smgr_which].smgr_completeaio)) ( aiocbp , inoutcode );
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 /*
  *	smgrread() -- read a particular block from a relation into the supplied
  *				  buffer.
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 1d094f0..d36b77d 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -2264,6 +2264,25 @@ static struct config_int ConfigureNamesInt[] =
 	},
 
 	{
+		{"max_async_io_prefetchers",
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+			PGC_USERSET,
+#else
+			PGC_INTERNAL,
+#endif
+			RESOURCES_ASYNCHRONOUS,
+			gettext_noop("Maximum number of background processes concurrently using asynchronous librt threads to prefetch pages into shared memory buffers."),
+		},
+		&max_async_io_prefetchers,
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+	        -1, 0, 8192,      /*  boot val -1 indicates to initialize to something sensible during buf_init */
+#else
+		0, 0, 0,
+#endif
+		NULL, NULL, NULL
+	},
+
+	{
 		{"max_worker_processes",
 			PGC_POSTMASTER,
 			RESOURCES_ASYNCHRONOUS,
diff --git a/src/backend/utils/mmgr/aset.c b/src/backend/utils/mmgr/aset.c
index 743455e..e392390 100644
--- a/src/backend/utils/mmgr/aset.c
+++ b/src/backend/utils/mmgr/aset.c
@@ -733,6 +733,48 @@ AllocSetAlloc(MemoryContext context, Size size)
 	 */
 	fidx = AllocSetFreeIndex(size);
 	chunk = set->freelist[fidx];
+#ifdef MEMORY_CONTEXT_CHECKING
+        /*    an instance of segfault caused by a rogue value in set->freelist[fidx]
+        **    has been seen - check for it using crude sanity check based on neighbours :
+        **    if at least one is sufficiently close, then pass,  else fail
+        */
+        if (chunk != 0) {
+            int frx, nrx; /*  frx is index,  nrx is index of failing neighbour for errmsg */
+            for (nrx = -1, frx = 0; (frx < ALLOCSET_NUM_FREELISTS); frx++) {
+                if (   (frx != fidx)     /*  not the chosen one */
+                    && ( ( (unsigned long)(set->freelist[frx]) ) != 0 ) /* not empty */
+                   ) {
+                    if (   ( (unsigned long)chunk < ( ( (unsigned long)(set->freelist[frx]) ) / 2 ) )
+                        && (  ( (unsigned long)(set->freelist[frx]) ) < 0x4000000  )
+               /***     || ( (unsigned long)chunk > ( ( (unsigned long)(set->freelist[frx]) ) * 2 ) )  ***/
+                       ) {
+                       nrx = frx;
+                    } else {
+                       nrx = -1;
+                       break;
+                    }
+                }
+            }
+
+            if (nrx >= 0) {
+                     /*  this must be a rogue value  -  put this list in the garbage */
+                     /*  build message but be careful to avoid recursively triggering same fault */
+                     set->freelist[fidx] = NULL;   /*  mark this list empty */
+                     elog(WARNING, "detected rogue value %p in freelist index %d compared with neighbour %p whose chunksize %d"
+				 , chunk , fidx , set->freelist[nrx] , set->freelist[nrx]->size);
+                     chunk = NULL;
+            }
+        }
+#else /* if not MEMORY_CONTEXT_CHECKING make very simple-minded check*/
+        if ( (chunk != 0) && ( (unsigned long)chunk <  0x40000 ) ) {
+                     /*  this must be a rogue value  -  put this list in the garbage */
+                     /*  build message but be careful to avoid recursively triggering same fault */
+                     set->freelist[fidx] = NULL;   /*  mark this list empty */
+                     elog(WARNING, "detected rogue value %p in freelist index %d"
+				 , chunk , fidx);
+                     chunk = NULL;
+        }
+#endif
 	if (chunk != NULL)
 	{
 		Assert(chunk->size >= size);
diff --git a/src/include/access/heapam.h b/src/include/access/heapam.h
index 493839f..3ce5618 100644
--- a/src/include/access/heapam.h
+++ b/src/include/access/heapam.h
@@ -175,7 +175,7 @@ extern void heap_page_prune_execute(Buffer buffer,
 extern void heap_get_root_tuples(Page page, OffsetNumber *root_offsets);
 
 /* in heap/syncscan.c */
-extern void ss_report_location(Relation rel, BlockNumber location);
+extern void ss_report_location(Relation rel, BlockNumber location , BlockNumber *prefetchHWMp);
 extern BlockNumber ss_get_location(Relation rel, BlockNumber relnblocks);
 extern void SyncScanShmemInit(void);
 extern Size SyncScanShmemSize(void);
diff --git a/src/include/access/nbtree.h b/src/include/access/nbtree.h
index f281759..34cf496 100644
--- a/src/include/access/nbtree.h
+++ b/src/include/access/nbtree.h
@@ -19,6 +19,7 @@
 #include "access/sdir.h"
 #include "access/xlog.h"
 #include "access/xlogutils.h"
+#include "access/relscan.h"
 #include "catalog/pg_index.h"
 
 /* There's room for a 16-bit vacuum cycle ID in BTPageOpaqueData */
@@ -524,6 +525,7 @@ typedef struct BTScanPosData
 	Buffer		buf;			/* if valid, the buffer is pinned */
 
 	BlockNumber nextPage;		/* page's right link when we scanned it */
+	BlockNumber prevPage;		/* page's left link when we scanned it */
 
 	/*
 	 * moreLeft and moreRight track whether we think there may be matching
@@ -603,6 +605,15 @@ typedef struct BTScanOpaqueData
 	 */
 	int			markItemIndex;	/* itemIndex, or -1 if not valid */
 
+	/* prefetch logic state */
+	unsigned int	backSeqRun;	/* number of back-sequential pages in a run */
+	BlockNumber		backSeqPos;	/* blkid last prefetched in back-sequential 
+				          		   runs */
+	BlockNumber		lastHeapPrefetchBlkno;	/* blkid last prefetched from heap */
+	int				prefetchItemIndex; /* item index within currPos last
+					                      fetched by heap prefetch */
+	int				prefetchBlockCount; /* number of prefetched heap blocks */
+	
 	/* keep these last in struct for efficiency */
 	BTScanPosData currPos;		/* current position data */
 	BTScanPosData markPos;		/* marked position, if any */
@@ -655,7 +666,11 @@ extern Buffer _bt_getroot(Relation rel, int access);
 extern Buffer _bt_gettrueroot(Relation rel);
 extern int	_bt_getrootheight(Relation rel);
 extern void _bt_checkpage(Relation rel, Buffer buf);
-extern Buffer _bt_getbuf(Relation rel, BlockNumber blkno, int access);
+extern Buffer _bt_getbuf(Relation rel, BlockNumber blkno, int access , struct pfch_index_pagelist* pfch_index_page_list);
+extern void _bt_prefetchbuf(Relation rel, BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P);
+extern struct pfch_index_item* _bt_find_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list);
+extern int _bt_add_block(BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P , uint32 discard_status);
+extern void _bt_subtract_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list);
 extern Buffer _bt_relandgetbuf(Relation rel, Buffer obuf,
 				 BlockNumber blkno, int access);
 extern void _bt_relbuf(Relation rel, Buffer buf);
diff --git a/src/include/access/relscan.h b/src/include/access/relscan.h
index 8a57698..cbf3100 100644
--- a/src/include/access/relscan.h
+++ b/src/include/access/relscan.h
@@ -44,6 +44,24 @@ typedef struct HeapScanDescData
 	bool		rs_inited;		/* false = scan not init'd yet */
 	HeapTupleData rs_ctup;		/* current tuple in scan, if any */
 	BlockNumber rs_cblock;		/* current block # in scan, if any */
+#ifdef USE_PREFETCH
+	int	    rs_prefetch_target; /* target distance (numblocks) for prefetch to reach beyond main scan */
+	BlockNumber rs_pfchblock;	/* next block # to be prefetched in scan, if any */
+
+        /*   Unread_Pfetched is a "mostly" circular list of recently prefetched blocknos of size target_prefetch_pages
+        **   the index of the first unread block is held in Unread_Pfetched_next
+        **         and is advanced when a block is read
+        **   the count of number of unread blocks is in Unread_Pfetched_count (and this subset can wrap around)
+        **   "mostly" means that there may be gaps caused by storing entries for blocks which do not need to be discarded -
+        **   these are indicated by blockno = InvalidBlockNumber,  and these slots are reused when found.
+        */
+        BlockNumber *rs_Unread_Pfetched_base;   /*  where the list of prefetched but unread blocknos starts */
+        unsigned int rs_Unread_Pfetched_next;   /*  where the next unread blockno probably is relative to start --
+                                                **  this is only a hint which may be temporarily stale.
+                                                */
+        unsigned int rs_Unread_Pfetched_count;  /*  number of valid unread blocknos in list */
+#endif   /* USE_PREFETCH */
+
 	Buffer		rs_cbuf;		/* current buffer in scan, if any */
 	/* NB: if rs_cbuf is not InvalidBuffer, we hold a pin on that buffer */
 	ItemPointerData rs_mctid;	/* marked scan position, if any */
@@ -55,6 +73,27 @@ typedef struct HeapScanDescData
 	OffsetNumber rs_vistuples[MaxHeapTuplesPerPage];	/* their offsets */
 }	HeapScanDescData;
 
+/* pfch_index_items track prefetched and unread index pages -   chunks of blocknumbers are chained in singly-linked list from scan->pfch_index_item_list */
+struct pfch_index_item {                              /*  index-relation BlockIds which we will/have prefetched */
+       BlockNumber         pfch_blocknum;    /* Blocknum which we will/have prefetched */
+       uint32              pfch_discard;     /* whether block is to be discarded when scan is closed */
+};
+
+struct pfch_block_item {
+       struct BlockIdData  pfch_blockid;     /* BlockId which we will/have prefetched */
+       uint32              pfch_discard;     /* whether block is to be discarded when scan is closed */
+};
+
+/* pfch_index_page_items track prefetched and unread index pages -
+** chunks of blocknumbers are chained backwards (newest first,  oldest last)
+** in singly-linked list from scan->pfch_index_item_list
+*/
+struct pfch_index_pagelist {                          /*  index-relation BlockIds which we will/have prefetched */
+       struct pfch_index_pagelist* pfch_index_pagelist_next;  /*  pointer to next chunk if any */
+       unsigned int    pfch_index_item_count;         /*  number of used entries in this chunk */
+       struct pfch_index_item pfch_indexid[1];        /*  in-line list of Blocknums which we will/have prefetched and whether to be discarded */
+};
+
 /*
  * We use the same IndexScanDescData structure for both amgettuple-based
  * and amgetbitmap-based index scans.  Some fields are only relevant in
@@ -75,8 +114,15 @@ typedef struct IndexScanDescData
 	/* signaling to index AM about killing index tuples */
 	bool		kill_prior_tuple;		/* last-returned tuple is dead */
 	bool		ignore_killed_tuples;	/* do not return killed entries */
-	bool		xactStartedInRecovery;	/* prevents killing/seeing killed
-										 * tuples */
+	bool		xactStartedInRecovery;	/* prevents killing/seeing killed tuples */
+										 
+#ifdef USE_PREFETCH
+        struct pfch_index_pagelist* pfch_index_page_list;  /* array of index-relation BlockIds which we will/have prefetched */
+        struct pfch_block_item* pfch_block_item_list;   /* array of heap-relation BlockIds which we will/have prefetched */
+        unsigned short int     pfch_used;	/*  number of used elements in BlockIdData array   */
+        unsigned short int     pfch_next;	/*  next element for prefetch in BlockIdData array */
+	int             do_prefetch;    /*  should I prefetch ? */
+#endif   /* USE_PREFETCH */
 
 	/* index access method's private state */
 	void	   *opaque;			/* access-method-specific info */
@@ -91,6 +137,10 @@ typedef struct IndexScanDescData
 	/* NB: if xs_cbuf is not InvalidBuffer, we hold a pin on that buffer */
 	bool		xs_recheck;		/* T means scan keys must be rechecked */
 
+	/* heap fetch statistics for read-ahead logic */
+	unsigned int heap_tids_seen;
+	unsigned int heap_tids_fetched;
+
 	/* state data for traversing HOT chains in index_getnext */
 	bool		xs_continue_hot;	/* T if must keep walking HOT chain */
 }	IndexScanDescData;
diff --git a/src/include/catalog/pg_am.h b/src/include/catalog/pg_am.h
index 759ea70..dc461b6 100644
--- a/src/include/catalog/pg_am.h
+++ b/src/include/catalog/pg_am.h
@@ -67,6 +67,7 @@ CATALOG(pg_am,2601)
 	regproc		amcanreturn;	/* can indexscan return IndexTuples? */
 	regproc		amcostestimate; /* estimate cost of an indexscan */
 	regproc		amoptions;		/* parse AM-specific parameters */
+	regproc		ampeeknexttuple;	/* peek at the next tuple different from any blocknum in pfch_list without reading a new index page */
 } FormData_pg_am;
 
 /* ----------------
@@ -117,19 +118,19 @@ typedef FormData_pg_am *Form_pg_am;
  * ----------------
  */
 
-DATA(insert OID = 403 (  btree		5 2 t f t t t t t t f t t 0 btinsert btbeginscan btgettuple btgetbitmap btrescan btendscan btmarkpos btrestrpos btbuild btbuildempty btbulkdelete btvacuumcleanup btcanreturn btcostestimate btoptions ));
+DATA(insert OID = 403 (  btree		5 2 t f t t t t t t f t t 0 btinsert btbeginscan btgettuple btgetbitmap btrescan btendscan btmarkpos btrestrpos btbuild btbuildempty btbulkdelete btvacuumcleanup btcanreturn btcostestimate btoptions btpeeknexttuple ));
 DESCR("b-tree index access method");
 #define BTREE_AM_OID 403
-DATA(insert OID = 405 (  hash		1 1 f f t f f f f f f f f 23 hashinsert hashbeginscan hashgettuple hashgetbitmap hashrescan hashendscan hashmarkpos hashrestrpos hashbuild hashbuildempty hashbulkdelete hashvacuumcleanup - hashcostestimate hashoptions ));
+DATA(insert OID = 405 (  hash		1 1 f f t f f f f f f f f 23 hashinsert hashbeginscan hashgettuple hashgetbitmap hashrescan hashendscan hashmarkpos hashrestrpos hashbuild hashbuildempty hashbulkdelete hashvacuumcleanup - hashcostestimate hashoptions - ));
 DESCR("hash index access method");
 #define HASH_AM_OID 405
-DATA(insert OID = 783 (  gist		0 8 f t f f t t f t t t f 0 gistinsert gistbeginscan gistgettuple gistgetbitmap gistrescan gistendscan gistmarkpos gistrestrpos gistbuild gistbuildempty gistbulkdelete gistvacuumcleanup - gistcostestimate gistoptions ));
+DATA(insert OID = 783 (  gist		0 8 f t f f t t f t t t f 0 gistinsert gistbeginscan gistgettuple gistgetbitmap gistrescan gistendscan gistmarkpos gistrestrpos gistbuild gistbuildempty gistbulkdelete gistvacuumcleanup - gistcostestimate gistoptions - ));
 DESCR("GiST index access method");
 #define GIST_AM_OID 783
-DATA(insert OID = 2742 (  gin		0 6 f f f f t t f f t f f 0 gininsert ginbeginscan - gingetbitmap ginrescan ginendscan ginmarkpos ginrestrpos ginbuild ginbuildempty ginbulkdelete ginvacuumcleanup - gincostestimate ginoptions ));
+DATA(insert OID = 2742 (  gin		0 6 f f f f t t f f t f f 0 gininsert ginbeginscan - gingetbitmap ginrescan ginendscan ginmarkpos ginrestrpos ginbuild ginbuildempty ginbulkdelete ginvacuumcleanup - gincostestimate ginoptions - ));
 DESCR("GIN index access method");
 #define GIN_AM_OID 2742
-DATA(insert OID = 4000 (  spgist	0 5 f f f f f t f t f f f 0 spginsert spgbeginscan spggettuple spggetbitmap spgrescan spgendscan spgmarkpos spgrestrpos spgbuild spgbuildempty spgbulkdelete spgvacuumcleanup spgcanreturn spgcostestimate spgoptions ));
+DATA(insert OID = 4000 (  spgist	0 5 f f f f f t f t f f f 0 spginsert spgbeginscan spggettuple spggetbitmap spgrescan spgendscan spgmarkpos spgrestrpos spgbuild spgbuildempty spgbulkdelete spgvacuumcleanup spgcanreturn spgcostestimate spgoptions - ));
 DESCR("SP-GiST index access method");
 #define SPGIST_AM_OID 4000
 
diff --git a/src/include/catalog/pg_proc.h b/src/include/catalog/pg_proc.h
index 72170af..f61c05a 100644
--- a/src/include/catalog/pg_proc.h
+++ b/src/include/catalog/pg_proc.h
@@ -536,6 +536,12 @@ DESCR("convert float4 to int4");
 
 DATA(insert OID = 330 (  btgettuple		   PGNSP PGUID 12 1 0 0 0 f f f f t f v 2 0 16 "2281 2281" _null_ _null_ _null_ _null_	btgettuple _null_ _null_ _null_ ));
 DESCR("btree(internal)");
+
+#ifndef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+DATA(insert OID = 3251 (  btpeeknexttuple	   PGNSP PGUID 12 1 0 0 0 f f f f t f v 1 0 16 "2281" _null_ _null_ _null_ _null_ btpeeknexttuple _null_ _null_ _null_ ));
+DESCR("btree(internal)");
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+
 DATA(insert OID = 636 (  btgetbitmap	   PGNSP PGUID 12 1 0 0 0 f f f f t f v 2 0 20 "2281 2281" _null_ _null_ _null_ _null_	btgetbitmap _null_ _null_ _null_ ));
 DESCR("btree(internal)");
 DATA(insert OID = 331 (  btinsert		   PGNSP PGUID 12 1 0 0 0 f f f f t f v 6 0 16 "2281 2281 2281 2281 2281 2281" _null_ _null_ _null_ _null_	btinsert _null_ _null_ _null_ ));
diff --git a/src/include/executor/instrument.h b/src/include/executor/instrument.h
index 8ec1033..8c86781 100644
--- a/src/include/executor/instrument.h
+++ b/src/include/executor/instrument.h
@@ -28,8 +28,18 @@ typedef struct BufferUsage
 	long		local_blks_written;		/* # of local disk blocks written */
 	long		temp_blks_read; /* # of temp blocks read */
 	long		temp_blks_written;		/* # of temp blocks written */
+
 	instr_time	blk_read_time;	/* time spent reading */
 	instr_time	blk_write_time; /* time spent writing */
+
+	long		aio_read_noneed;		/* # of prefetches for which no need for prefetch as block already in buffer pool */
+	long		aio_read_discrd;		/* # of prefetches for which no need for prefetch as block already in buffer pool */
+	long		aio_read_forgot;		/* # of prefetches for which no need for prefetch as block already in buffer pool */
+	long		aio_read_noblok;		/* # of prefetches for which no available BufferAiocb */
+	long		aio_read_failed;		/* # of aio reads for which aio itself failed or the read failed with an errno */
+	long		aio_read_wasted;		/* # of aio reads for which disk block not used */
+	long		aio_read_waited;		/* # of aio reads for which disk block used but had to wait for it */
+	long		aio_read_ontime;		/* # of aio reads for which disk block used and ready on time when requested */
 } BufferUsage;
 
 /* Flag bits included in InstrAlloc's instrument_options bitmask */
diff --git a/src/include/nodes/tidbitmap.h b/src/include/nodes/tidbitmap.h
index 444d4d8..4998dbc 100644
--- a/src/include/nodes/tidbitmap.h
+++ b/src/include/nodes/tidbitmap.h
@@ -41,6 +41,16 @@ typedef struct
 	int			ntuples;		/* -1 indicates lossy result */
 	bool		recheck;		/* should the tuples be rechecked? */
 	/* Note: recheck is always true if ntuples < 0 */
+#ifdef USE_PREFETCH
+        /*   Unread_Pfetched is a circular list of recently prefetched blocknos of size target_prefetch_pages
+        **   the index of the first unread block is held in Unread_Pfetched_next
+        **         and is advanced when a block is read
+        **   the count of number of unread blocks is in Unread_Pfetched_count (and this subset can wrap around)
+        */
+        BlockNumber *Unread_Pfetched_base;   /*  where the list of prefetched but unread blocknos starts */
+        unsigned int Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+        unsigned int Unread_Pfetched_count;  /*  number of valid unread blocknos in list */
+#endif   /* USE_PREFETCH */
 	OffsetNumber offsets[1];	/* VARIABLE LENGTH ARRAY */
 } TBMIterateResult;				/* VARIABLE LENGTH STRUCT */
 
@@ -62,5 +72,8 @@ extern bool tbm_is_empty(const TIDBitmap *tbm);
 extern TBMIterator *tbm_begin_iterate(TIDBitmap *tbm);
 extern TBMIterateResult *tbm_iterate(TBMIterator *iterator);
 extern void tbm_end_iterate(TBMIterator *iterator);
-
+extern void tbm_zero(TBMIterator *iterator); /* zero list of prefetched and unread blocknos */
+extern void tbm_add(TBMIterator *iterator, BlockNumber blockno); /* add this blockno to list of prefetched and unread blocknos */
+extern void tbm_subtract(TBMIterator *iterator, BlockNumber blockno); /* remove this blockno from list of prefetched and unread blocknos */
+extern TBMIterateResult *tbm_locate_IterateResult(TBMIterator *iterator); /* locate the TBMIterateResult of an iterator */
 #endif   /* TIDBITMAP_H */
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 5ff9e41..01cfa81 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -1,4 +1,4 @@
-/* src/include/pg_config.h.in.  Generated from configure.in by autoheader.  */
+/* src/include/pg_config.h.in.  Generated from - by autoheader.  */
 
 /* Define to the type of arg 1 of 'accept' */
 #undef ACCEPT_TYPE_ARG1
@@ -748,6 +748,10 @@
 /* Define to the appropriate snprintf format for unsigned 64-bit ints. */
 #undef UINT64_FORMAT
 
+/* Define to select librt-style async io and the gcc atomic compare_and_swap.
+   */
+#undef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+
 /* Define to 1 to build with assertion checks. (--enable-cassert) */
 #undef USE_ASSERT_CHECKING
 
diff --git a/src/include/pg_config_manual.h b/src/include/pg_config_manual.h
index d1f99fb..6040a5d 100644
--- a/src/include/pg_config_manual.h
+++ b/src/include/pg_config_manual.h
@@ -138,9 +138,11 @@
 /*
  * USE_PREFETCH code should be compiled only if we have a way to implement
  * prefetching.  (This is decoupled from USE_POSIX_FADVISE because there
- * might in future be support for alternative low-level prefetch APIs.)
+ * might in future be support for alternative low-level prefetch APIs  --
+ * -- update October 2013  -- now there is such a new prefetch capability --
+ *   async_io into postgres buffers  -   configuration parameter max_async_io_threads)
  */
-#ifdef USE_POSIX_FADVISE
+#if defined(USE_POSIX_FADVISE) || defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP)
 #define USE_PREFETCH
 #endif
 
diff --git a/src/include/storage/buf_internals.h b/src/include/storage/buf_internals.h
index c019013..1482c5a 100644
--- a/src/include/storage/buf_internals.h
+++ b/src/include/storage/buf_internals.h
@@ -22,7 +22,9 @@
 #include "storage/smgr.h"
 #include "storage/spin.h"
 #include "utils/relcache.h"
-
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+#include "aio.h"
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * Flags for buffer descriptors
@@ -38,8 +40,23 @@
 #define BM_JUST_DIRTIED			(1 << 5)		/* dirtied since write started */
 #define BM_PIN_COUNT_WAITER		(1 << 6)		/* have waiter for sole pin */
 #define BM_CHECKPOINT_NEEDED	(1 << 7)		/* must write for checkpoint */
-#define BM_PERMANENT			(1 << 8)		/* permanent relation (not
-												 * unlogged) */
+#define BM_PERMANENT			(1 << 8)	/* permanent relation (not unlogged) */
+#define BM_AIO_IN_PROGRESS		(1 << 9)	/* aio in progress    */
+#define BM_AIO_PREFETCH_PIN_BANKED	(1 << 10)	/* pinned when prefetch issued
+                                                        ** and this pin is banked - i.e.
+                                                        ** redeemable by the next use by same task
+                                                        ** note that for any one buffer, a pin can be banked
+                                                        **      by at most one process globally,
+                                                        **      that is,   only one process may bank a pin on the buffer
+                                                        **                 and it may do so only once (may not be stacked)
+                                                        */
+
+/*********
+for asynchronous aio-read prefetching, two golden rules concerning buffer pinning and buffer-header flags must be observed:
+  R1.  a buffer marked as BM_AIO_IN_PROGRESS must be pinned by at least one backend
+  R2.  a buffer marked as BM_AIO_PREFETCH_PIN_BANKED must be pinned by the backend identified by
+               (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio : (-(buf->freeNext))
+*********/
 
 typedef bits16 BufFlags;
 
@@ -140,17 +157,83 @@ typedef struct sbufdesc
 	BufFlags	flags;			/* see bit definitions above */
 	uint16		usage_count;	/* usage counter for clock sweep code */
 	unsigned	refcount;		/* # of backends holding pins on buffer */
-	int			wait_backend_pid;		/* backend PID of pin-count waiter */
+	int		wait_backend_pid;	/*  if     flags & BM_PIN_COUNT_WAITER
+                                                **  then   backend PID of pin-count waiter
+                                                **  else   not set
+                                                */
 
 	slock_t		buf_hdr_lock;	/* protects the above fields */
 
 	int			buf_id;			/* buffer's index number (from 0) */
-	int			freeNext;		/* link in freelist chain */
+        int    	volatile	freeNext;	/* overloaded and much-abused field :
+                                                ** EITHER
+                                                **     if     >= 0
+                                                **     then   link in freelist chain
+                                                **  OR
+                                                **     if     <  0
+                                                **     then    EITHER
+                                                **             if     flags & BM_AIO_IN_PROGRESS
+                                                **             then   negative of (the index of the aiocb in the BufferAiocbs array + 3)
+                                                **             else   if flags & BM_AIO_PREFETCH_PIN_BANKED
+                                                **             then   -(pid of task that issued aio_read and pinned buffer)
+                                                **             else   one of the special values -1 or -2 listed below
+                                                */
 
 	LWLock	   *io_in_progress_lock;	/* to wait for I/O to complete */
 	LWLock	   *content_lock;	/* to lock access to buffer contents */
 } BufferDesc;
 
+/*  structures for control blocks for our implementation of async io */
+
+/*  if USE_AIO_ATOMIC_BUILTIN_COMP_SWAP is not defined,  the following struct is not put into use at runtime
+**  but it is easier to let the compiler find the definition but hide the reference to aiocb
+**  which is the only type it would not understand
+*/
+
+struct BufferAiocb {
+       struct BufferAiocb volatile * volatile BAiocbnext;  /*    next free entry or value of BAIOCB_OCCUPIED means in use  */
+       struct sbufdesc    volatile * volatile BAiocbbufh;  /*    there can be at most one BufferDesc marked BM_AIO_IN_PROGRESS
+                                                           **    and using this BufferAiocb -
+                                                           **    if there is one, BAiocbbufh points to it, else BAiocbbufh is zero
+                                                           **    NOTE  BAiocbbufh should be zero for every BufferAiocb on the free list
+                                                           */
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+       struct aiocb       volatile            BAiocbthis;  /*    the aio library's control block for one async io */
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+       int                volatile  BAiocbDependentCount;  /*    count of tasks who depend on this BufferAiocb
+                                                           **    in the sense that they are waiting for io completion.
+                                                           **    only a Dependent may move the BufferAiocb onto the freelist
+                                                           **    and only when that Dependent is the *only* Dependent (count == 1)
+                                                           **    BAiocbDependentCount is protected by bufferheader spinlock
+                                                           **    and must be updated only when that spinlock is held
+                                                           */
+       pid_t              volatile  pidOfAio;              /*    pid of backend who issued an aio_read using this BAiocb -
+                                                           **    this backend must have pinned the associated buffer.
+                                                           */
+};
+
+#define BAIOCB_OCCUPIED 0x75f1        /*  distinct indicator of a BufferAiocb.BAiocbnext that is NOT on free list */
+#define BAIOCB_FREE 0x7b9d            /*  distinct indicator of a BufferAiocb.BAiocbbufh that IS     on free list */
+
+struct BAiocbAnchor {                 /*  anchor for all control blocks pertaining to aio  */
+       volatile struct BufferAiocb* BufferAiocbs;          /*  aiocbs ...                   */
+       volatile struct BufferAiocb* volatile FreeBAiocbs; /* ... and their free list   */
+};
+
+/*   values for BufCheckAsync input and retcode */
+#define BUF_INTENTION_WANT 		 1  /* wants the buffer, wait for in-progress aio and then pin */
+#define BUF_INTENTION_REJECT_KEEP_PIN 	-1  /* pin already held, do not unpin */
+#define BUF_INTENTION_REJECT_OBTAIN_PIN	-2  /* obtain pin,  caller wants it for same buffer */
+#define BUF_INTENTION_REJECT_FORGET	-3  /* unpin and tell resource owner to forget */
+#define BUF_INTENTION_REJECT_NOADJUST	-4  /* unpin and call ResourceOwnerForgetBuffer */
+#define BUF_INTENTION_REJECT_UNBANK   	-5  /* unpin only if pin banked by caller */
+
+#define BUF_INTENT_RC_CHANGED_TAG	-5
+#define BUF_INTENT_RC_BADPAGE		-4
+#define BUF_INTENT_RC_INVALID_AIO	-3    /*  invalid and aio was in progress */
+#define BUF_INTENT_RC_INVALID_NO_AIO	-1    /*  invalid and no aio was in progress */
+#define BUF_INTENT_RC_VALID		 1
+
 #define BufferDescriptorGetBuffer(bdesc) ((bdesc)->buf_id + 1)
 
 /*
@@ -159,6 +242,7 @@ typedef struct sbufdesc
  */
 #define FREENEXT_END_OF_LIST	(-1)
 #define FREENEXT_NOT_IN_LIST	(-2)
+#define FREENEXT_BAIOCB_ORIGIN	(-3)
 
 /*
  * Macros for acquiring/releasing a shared buffer header's spinlock.
diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h
index 89447d0..c652841 100644
--- a/src/include/storage/bufmgr.h
+++ b/src/include/storage/bufmgr.h
@@ -41,6 +41,7 @@ typedef enum
 	RBM_ZERO_ON_ERROR,			/* Read, but return an all-zeros page on error */
 	RBM_NORMAL_NO_LOG			/* Don't log page as invalid during WAL
 								 * replay; otherwise same as RBM_NORMAL */
+       ,RBM_NOREAD_FOR_PREFETCH   /* Don't read from disk, don't zero buffer, find buffer only */
 } ReadBufferMode;
 
 /* in globals.c ... this duplicates miscadmin.h */
@@ -57,6 +58,9 @@ extern int	target_prefetch_pages;
 extern PGDLLIMPORT char *BufferBlocks;
 extern PGDLLIMPORT int32 *PrivateRefCount;
 
+/*  in buf_async.c  */;
+extern int      max_async_io_prefetchers; /* Maximum number of backends using asynchronous librt threads to read pages into our buffers */
+
 /* in localbuf.c */
 extern PGDLLIMPORT int NLocBuffer;
 extern PGDLLIMPORT Block *LocalBufferBlockPointers;
@@ -159,9 +163,15 @@ extern PGDLLIMPORT int32 *LocalRefCount;
 #define BufferGetPage(buffer) ((Page)BufferGetBlock(buffer))
 
 /*
- * prototypes for functions in bufmgr.c
+ * prototypes for external functions in bufmgr.c and buf_async.c
  */
-extern void PrefetchBuffer(Relation reln, ForkNumber forkNum,
+extern int PrefetchBuffer(Relation reln, ForkNumber forkNum,
+			   BlockNumber blockNum , BufferAccessStrategy strategy);
+/*   return code  is an int bitmask : */
+#define PREFTCHRC_BUF_PIN_INCREASED 0x01    /*  pin count on buffer has been increased by 1 */
+#define PREFTCHRC_BLK_ALREADY_PRESENT 0x02  /*  block was already present in a buffer       */
+
+extern void DiscardBuffer(Relation reln, ForkNumber forkNum,
 			   BlockNumber blockNum);
 extern Buffer ReadBuffer(Relation reln, BlockNumber blockNum);
 extern Buffer ReadBufferExtended(Relation reln, ForkNumber forkNum,
diff --git a/src/include/storage/fd.h b/src/include/storage/fd.h
index a6df8fb..d394c6a 100644
--- a/src/include/storage/fd.h
+++ b/src/include/storage/fd.h
@@ -69,6 +69,11 @@ extern File PathNameOpenFile(FileName fileName, int fileFlags, int fileMode);
 extern File OpenTemporaryFile(bool interXact);
 extern void FileClose(File file);
 extern int	FilePrefetch(File file, off_t offset, int amount);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern void FileInitaio(int max_aio_threads, int max_aio_num );
+extern int  FileStartaio(File file, off_t offset, int amount , char *aiocbp);
+extern int  FileCompleteaio( char *aiocbp , int normal_wait );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 extern int	FileRead(File file, char *buffer, int amount);
 extern int	FileWrite(File file, char *buffer, int amount);
 extern int	FileSync(File file);
diff --git a/src/include/storage/smgr.h b/src/include/storage/smgr.h
index ba7c909..5e8d645 100644
--- a/src/include/storage/smgr.h
+++ b/src/include/storage/smgr.h
@@ -92,6 +92,12 @@ extern void smgrextend(SMgrRelation reln, ForkNumber forknum,
 		   BlockNumber blocknum, char *buffer, bool skipFsync);
 extern void smgrprefetch(SMgrRelation reln, ForkNumber forknum,
 			 BlockNumber blocknum);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern void smgrinitaio(int max_aio_threads, int max_aio_num);
+extern void smgrstartaio(SMgrRelation reln, ForkNumber forknum,
+			 BlockNumber blocknum , char *aiocbp , int *retcode);
+extern void smgrcompleteaio( SMgrRelation reln, char *aiocbp , int *inoutcode );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 extern void smgrread(SMgrRelation reln, ForkNumber forknum,
 		 BlockNumber blocknum, char *buffer);
 extern void smgrwrite(SMgrRelation reln, ForkNumber forknum,
@@ -118,6 +124,11 @@ extern void mdextend(SMgrRelation reln, ForkNumber forknum,
 		 BlockNumber blocknum, char *buffer, bool skipFsync);
 extern void mdprefetch(SMgrRelation reln, ForkNumber forknum,
 		   BlockNumber blocknum);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern void mdinitaio(int max_aio_threads, int max_aio_num);
+extern void mdstartaio(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum , char *aiocbp , int *retcode );
+extern void mdcompleteaio( char *aiocbp , int *inoutcode );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 extern void mdread(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 	   char *buffer);
 extern void mdwrite(SMgrRelation reln, ForkNumber forknum,
diff --git a/src/include/utils/rel.h b/src/include/utils/rel.h
index af4f53f..720d3f3 100644
--- a/src/include/utils/rel.h
+++ b/src/include/utils/rel.h
@@ -61,6 +61,7 @@ typedef struct RelationAmInfo
 	FmgrInfo	ammarkpos;
 	FmgrInfo	amrestrpos;
 	FmgrInfo	amcanreturn;
+	FmgrInfo	ampeeknexttuple;	/* peek at the next tuple different from any blocknum in pfch_list without reading a new index page */
 } RelationAmInfo;
 
 

--- configure.in.orig	2014-05-31 17:19:07.689208337 -0400
+++ configure.in	2014-05-31 19:53:08.552072242 -0400
@@ -1771,6 +1771,12 @@ operating system;  use --disable-thread-
 fi
 fi
 
+#  test whether this system has both the librt-style async io and the gcc atomic compare_and_swap
+#      and test operation  of the latter.
+PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP
+if test x"$pgac_cv_aio_atomic_builtin_comp_swap" = x"yes"; then
+      AC_DEFINE(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP, 1, [Define to select librt-style async io and the gcc atomic compare_and_swap.])
+fi
 
 # Select semaphore implementation type.
 if test "$PORTNAME" != "win32"; then
--- contrib/pg_prewarm/pg_prewarm.c.orig	2014-05-31 17:19:07.705208380 -0400
+++ contrib/pg_prewarm/pg_prewarm.c	2014-05-31 19:53:08.620072414 -0400
@@ -159,7 +159,7 @@ pg_prewarm(PG_FUNCTION_ARGS)
 		 */
 		for (block = first_block; block <= last_block; ++block)
 		{
-			PrefetchBuffer(rel, forkNumber, block);
+			PrefetchBuffer(rel, forkNumber, block, 0);
 			++blocks_done;
 		}
 #else
--- contrib/pg_stat_statements/pg_stat_statements--1.3.sql.orig	2014-05-31 17:21:19.249556268 -0400
+++ contrib/pg_stat_statements/pg_stat_statements--1.3.sql	2014-05-31 19:53:08.652072495 -0400
@@ -0,0 +1,52 @@
+/* contrib/pg_stat_statements/pg_stat_statements--1.3.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION pg_stat_statements VERSION '1.3'" to load this file. \quit
+
+-- Register functions.
+CREATE FUNCTION pg_stat_statements_reset()
+RETURNS void
+AS 'MODULE_PATHNAME'
+LANGUAGE C;
+
+CREATE FUNCTION pg_stat_statements(IN showtext boolean,
+    OUT userid oid,
+    OUT dbid oid,
+    OUT queryid bigint,
+    OUT query text,
+    OUT calls int8,
+    OUT total_time float8,
+    OUT rows int8,
+    OUT shared_blks_hit int8,
+    OUT shared_blks_read int8,
+    OUT shared_blks_dirtied int8,
+    OUT shared_blks_written int8,
+    OUT local_blks_hit int8,
+    OUT local_blks_read int8,
+    OUT local_blks_dirtied int8,
+    OUT local_blks_written int8,
+    OUT temp_blks_read int8,
+    OUT temp_blks_written int8,
+    OUT blk_read_time float8,
+    OUT blk_write_time float8
+   , OUT aio_read_noneed int8
+   , OUT aio_read_discrd int8
+   , OUT aio_read_forgot int8
+   , OUT aio_read_noblok int8
+   , OUT aio_read_failed int8
+   , OUT aio_read_wasted int8
+   , OUT aio_read_waited int8
+   , OUT aio_read_ontime int8
+)
+RETURNS SETOF record
+AS 'MODULE_PATHNAME', 'pg_stat_statements_1_3'
+LANGUAGE C STRICT VOLATILE;
+
+-- Register a view on the function for ease of use.
+CREATE VIEW pg_stat_statements AS
+  SELECT * FROM pg_stat_statements(true);
+
+GRANT SELECT ON pg_stat_statements TO PUBLIC;
+
+-- Don't want this to be available to non-superusers.
+REVOKE ALL ON FUNCTION pg_stat_statements_reset() FROM PUBLIC;
--- contrib/pg_stat_statements/Makefile.orig	2014-05-31 17:19:07.705208380 -0400
+++ contrib/pg_stat_statements/Makefile	2014-05-31 19:53:08.664072525 -0400
@@ -4,7 +4,8 @@ MODULE_big = pg_stat_statements
 OBJS = pg_stat_statements.o
 
 EXTENSION = pg_stat_statements
-DATA = pg_stat_statements--1.2.sql pg_stat_statements--1.1--1.2.sql \
+DATA = pg_stat_statements--1.3.sql pg_stat_statements--1.2--1.3.sql \
+	pg_stat_statements--1.2.sql pg_stat_statements--1.1--1.2.sql \
 	pg_stat_statements--1.0--1.1.sql pg_stat_statements--unpackaged--1.0.sql
 
 ifdef USE_PGXS
--- contrib/pg_stat_statements/pg_stat_statements.c.orig	2014-05-31 17:19:07.705208380 -0400
+++ contrib/pg_stat_statements/pg_stat_statements.c	2014-05-31 19:53:08.700072616 -0400
@@ -117,6 +117,7 @@ typedef enum pgssVersion
 	PGSS_V1_0 = 0,
 	PGSS_V1_1,
 	PGSS_V1_2
+	,PGSS_V1_3
 } pgssVersion;
 
 /*
@@ -148,6 +149,16 @@ typedef struct Counters
 	int64		local_blks_written;		/* # of local disk blocks written */
 	int64		temp_blks_read; /* # of temp blocks read */
 	int64		temp_blks_written;		/* # of temp blocks written */
+
+	int64		aio_read_noneed;		/* # of prefetches for which no need for prefetch as block already in buffer pool  */
+	int64		aio_read_discrd;		/* # of prefetches for which buffer not subsequently read and therefore discarded  */
+	int64		aio_read_forgot;		/* # of prefetches for which buffer not subsequently read and then forgotten about */
+	int64		aio_read_noblok;		/* # of prefetches for which no available BufferAiocb  control block               */
+	int64		aio_read_failed;		/* # of aio reads for which aio itself failed or the read failed with an errno     */
+	int64		aio_read_wasted;		/* # of aio reads for which in-progress aio cancelled and disk block not used      */
+	int64		aio_read_waited;		/* # of aio reads for which disk block used but had to wait for it                 */
+	int64		aio_read_ontime;		/* # of aio reads for which disk block used and ready on time when requested       */
+
 	double		blk_read_time;	/* time spent reading, in msec */
 	double		blk_write_time; /* time spent writing, in msec */
 	double		usage;			/* usage factor */
@@ -274,7 +285,7 @@ void		_PG_init(void);
 void		_PG_fini(void);
 
 PG_FUNCTION_INFO_V1(pg_stat_statements_reset);
-PG_FUNCTION_INFO_V1(pg_stat_statements_1_2);
+PG_FUNCTION_INFO_V1(pg_stat_statements_1_3);
 PG_FUNCTION_INFO_V1(pg_stat_statements);
 
 static void pgss_shmem_startup(void);
@@ -1026,7 +1037,25 @@ pgss_ProcessUtility(Node *parsetree, con
 		bufusage.temp_blks_read =
 			pgBufferUsage.temp_blks_read - bufusage_start.temp_blks_read;
 		bufusage.temp_blks_written =
-			pgBufferUsage.temp_blks_written - bufusage_start.temp_blks_written;
+			pgBufferUsage.temp_blks_written - bufusage.temp_blks_written;
+
+		bufusage.aio_read_noneed =
+			pgBufferUsage.aio_read_noneed - bufusage.aio_read_noneed;
+		bufusage.aio_read_discrd =
+			pgBufferUsage.aio_read_discrd - bufusage.aio_read_discrd;
+		bufusage.aio_read_forgot =
+			pgBufferUsage.aio_read_forgot - bufusage.aio_read_forgot;
+		bufusage.aio_read_noblok =
+			pgBufferUsage.aio_read_noblok - bufusage.aio_read_noblok;
+		bufusage.aio_read_failed =
+			pgBufferUsage.aio_read_failed - bufusage.aio_read_failed;
+		bufusage.aio_read_wasted =
+			pgBufferUsage.aio_read_wasted - bufusage.aio_read_wasted;
+		bufusage.aio_read_waited =
+			pgBufferUsage.aio_read_waited - bufusage.aio_read_waited;
+		bufusage.aio_read_ontime =
+			pgBufferUsage.aio_read_ontime - bufusage.aio_read_ontime;
+
 		bufusage.blk_read_time = pgBufferUsage.blk_read_time;
 		INSTR_TIME_SUBTRACT(bufusage.blk_read_time, bufusage_start.blk_read_time);
 		bufusage.blk_write_time = pgBufferUsage.blk_write_time;
@@ -1041,6 +1070,7 @@ pgss_ProcessUtility(Node *parsetree, con
 				   rows,
 				   &bufusage,
 				   NULL);
+
 	}
 	else
 	{
@@ -1224,6 +1254,16 @@ pgss_store(const char *query, uint32 que
 		e->counters.local_blks_written += bufusage->local_blks_written;
 		e->counters.temp_blks_read += bufusage->temp_blks_read;
 		e->counters.temp_blks_written += bufusage->temp_blks_written;
+
+		e->counters.aio_read_noneed     += bufusage->aio_read_noneed;
+		e->counters.aio_read_discrd     += bufusage->aio_read_discrd;
+		e->counters.aio_read_forgot     += bufusage->aio_read_forgot;
+		e->counters.aio_read_noblok     += bufusage->aio_read_noblok;
+		e->counters.aio_read_failed     += bufusage->aio_read_failed;
+		e->counters.aio_read_wasted     += bufusage->aio_read_wasted;
+		e->counters.aio_read_waited     += bufusage->aio_read_waited;
+		e->counters.aio_read_ontime     += bufusage->aio_read_ontime;
+
 		e->counters.blk_read_time += INSTR_TIME_GET_MILLISEC(bufusage->blk_read_time);
 		e->counters.blk_write_time += INSTR_TIME_GET_MILLISEC(bufusage->blk_write_time);
 		e->counters.usage += USAGE_EXEC(total_time);
@@ -1257,7 +1297,8 @@ pg_stat_statements_reset(PG_FUNCTION_ARG
 #define PG_STAT_STATEMENTS_COLS_V1_0	14
 #define PG_STAT_STATEMENTS_COLS_V1_1	18
 #define PG_STAT_STATEMENTS_COLS_V1_2	19
-#define PG_STAT_STATEMENTS_COLS			19		/* maximum of above */
+#define PG_STAT_STATEMENTS_COLS_V1_3	27
+#define PG_STAT_STATEMENTS_COLS			27		/* maximum of above */
 
 /*
  * Retrieve statement statistics.
@@ -1270,6 +1311,16 @@ pg_stat_statements_reset(PG_FUNCTION_ARG
  * function.  Unfortunately we weren't bright enough to do that for 1.1.
  */
 Datum
+pg_stat_statements_1_3(PG_FUNCTION_ARGS)
+{
+	bool		showtext = PG_GETARG_BOOL(0);
+
+	pg_stat_statements_internal(fcinfo, PGSS_V1_3, showtext);
+
+	return (Datum) 0;
+}
+
+Datum
 pg_stat_statements_1_2(PG_FUNCTION_ARGS)
 {
 	bool		showtext = PG_GETARG_BOOL(0);
@@ -1358,6 +1409,10 @@ pg_stat_statements_internal(FunctionCall
 			if (api_version != PGSS_V1_2)
 				elog(ERROR, "incorrect number of output arguments");
 			break;
+		case PG_STAT_STATEMENTS_COLS_V1_3:
+			if (api_version != PGSS_V1_3)
+				elog(ERROR, "incorrect number of output arguments");
+			break;
 		default:
 			elog(ERROR, "incorrect number of output arguments");
 	}
@@ -1534,11 +1589,24 @@ pg_stat_statements_internal(FunctionCall
 		{
 			values[i++] = Float8GetDatumFast(tmp.blk_read_time);
 			values[i++] = Float8GetDatumFast(tmp.blk_write_time);
+
+			if (api_version >= PGSS_V1_3)
+			{
+				values[i++] = Int64GetDatumFast(tmp.aio_read_noneed);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_discrd);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_forgot);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_noblok);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_failed);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_wasted);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_waited);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_ontime);
+			}
 		}
 
 		Assert(i == (api_version == PGSS_V1_0 ? PG_STAT_STATEMENTS_COLS_V1_0 :
 					 api_version == PGSS_V1_1 ? PG_STAT_STATEMENTS_COLS_V1_1 :
 					 api_version == PGSS_V1_2 ? PG_STAT_STATEMENTS_COLS_V1_2 :
+					 api_version == PGSS_V1_3 ? PG_STAT_STATEMENTS_COLS_V1_3 :
 					 -1 /* fail if you forget to update this assert */ ));
 
 		tuplestore_putvalues(tupstore, tupdesc, values, nulls);
--- contrib/pg_stat_statements/pg_stat_statements--1.2--1.3.sql.orig	2014-05-31 17:21:19.249556268 -0400
+++ contrib/pg_stat_statements/pg_stat_statements--1.2--1.3.sql	2014-05-31 19:53:08.724072677 -0400
@@ -0,0 +1,51 @@
+/* contrib/pg_stat_statements/pg_stat_statements--1.2--1.3.sql */
+
+-- complain if script is sourced in psql, rather than via ALTER EXTENSION
+\echo Use "ALTER EXTENSION pg_stat_statements UPDATE TO '1.3'" to load this file. \quit
+
+/* First we have to remove them from the extension */
+ALTER EXTENSION pg_stat_statements DROP VIEW pg_stat_statements;
+ALTER EXTENSION pg_stat_statements DROP FUNCTION pg_stat_statements();
+
+/* Then we can drop them */
+DROP VIEW pg_stat_statements;
+DROP FUNCTION pg_stat_statements();
+
+/* Now redefine */
+CREATE FUNCTION pg_stat_statements(IN showtext boolean,
+    OUT userid oid,
+    OUT dbid oid,
+    OUT queryid bigint,
+    OUT query text,
+    OUT calls int8,
+    OUT total_time float8,
+    OUT rows int8,
+    OUT shared_blks_hit int8,
+    OUT shared_blks_read int8,
+    OUT shared_blks_dirtied int8,
+    OUT shared_blks_written int8,
+    OUT local_blks_hit int8,
+    OUT local_blks_read int8,
+    OUT local_blks_dirtied int8,
+    OUT local_blks_written int8,
+    OUT temp_blks_read int8,
+    OUT temp_blks_written int8,
+    OUT blk_read_time float8,
+    OUT blk_write_time float8
+   , OUT aio_read_noneed int8
+   , OUT aio_read_discrd int8
+   , OUT aio_read_forgot int8
+   , OUT aio_read_noblok int8
+   , OUT aio_read_failed int8
+   , OUT aio_read_wasted int8
+   , OUT aio_read_waited int8
+   , OUT aio_read_ontime int8
+)
+RETURNS SETOF record
+AS 'MODULE_PATHNAME', 'pg_stat_statements_1_3'
+LANGUAGE C STRICT VOLATILE;
+
+CREATE VIEW pg_stat_statements AS
+  SELECT * FROM pg_stat_statements(true);
+
+GRANT SELECT ON pg_stat_statements TO PUBLIC;
--- postgresql-prefetching-asyncio.README.orig	2014-05-31 17:21:19.249556268 -0400
+++ postgresql-prefetching-asyncio.README	2014-05-31 19:53:08.768072788 -0400
@@ -0,0 +1,544 @@
+Postgresql  --   Extended Prefetching using Asynchronous IO
+============================================================
+
+Postgresql currently (9.3.4) provides a limited prefetching capability
+using posix_fadvise to give hints to the Operating System kernel
+about which pages it expects to read in the near future.
+This capability is used only during the heap-scan phase of bitmap-index scans.
+It is controlled via the effective_io_concurrency configuration parameter.
+
+This capability is now extended in two ways :
+   .   use asynchronous IO into Postgresql shared buffers as an
+       alternative to posix_fadvise
+   .   Implement prefetching in other types of scan :
+            .  non-bitmap (i.e. simple) index scans - index pages
+                     currently only for B-tree indexes.
+                    (developed by Claudio Freire <klaussfreire(at)gmail(dot)com>)
+            .  non-bitmap (i.e. simple) index scans - heap pages
+                          currently only for B-tree indexes.
+            .  simple heap scans
+
+Posix asynchronous IO is chosen as the function library for asynchronous IO,
+since this is well supported and also fits very well with the model of
+the prefetching process,  particularly as regards checking for completion
+of an asynchronous read.    On linux,   Posix asynchronous IO is provided
+in the librt library.    librt uses independently-schedulable threads to
+achieve the asynchronicity,   rather than kernel functionality.
+
+In this implementation,  use of asynchronous IO is limited to prefetching
+while performing one of the three types of scan
+            .  B-tree bitmap index scan - heap pages    (as already exists)
+            .  B-tree non-bitmap (i.e. simple) index scans - index and heap pages
+            .  simple heap scans
+on permanent relations.   It is not used on temporary tables nor for writes.
+
+The advantages of Posix asynchronous IO into shared buffers
+compared to posix_fadvise are :
+   .   Beneficial for non-sequential access patterns as well as sequential
+   .   No restriction on the kinds of IO which can be used
+       (other kinds of asynchronous IO impose restrictions such as
+        buffer alignment,  use of non-buffered IO).
+   .   Does not interfere with standard linux kernel read-ahead functionality.
+       (It has been stated in 
+ www.postgresql.org/message-id/CAGTBQpbu2M=-M7NUr6DWr0K8gUVmXVhwKohB-Cnj7kYS1AhH4A@mail.gmail.com
+       that :
+          "the kernel stops doing read-ahead when a call to posix_fadvise comes.
+           I noticed the performance hit, and checked the kernel's code.
+           It effectively changes the prediction mode from sequential to fadvise,
+           negating the (assumed) kernel's prefetch logic")
+   .   When the read request is issued after a prefetch has completed,
+       no delay associated with a kernel call to copy the page from
+       kernel page buffers into the Postgresql shared buffer,
+       since it is already there.
+       Also,   in a memory-constrained environment,   there is a greater
+       probability that the prefetched page will "stick" in memory
+       since the linux kernel victimizes the filesystem page cache in preference
+       to swapping out user process pages.
+   .   Statistics on prefetch success can be gathered (see "Statistics" below)
+       which helps the administrator to tune the prefetching settings.
+
+These benefits are most likely to be obtained in a system whose usage profile
+(e.g. from iostat)  shows:
+     .   high IO wait from mostly-read activity
+     .   disk access pattern is not entirely sequential
+         (so kernel readahead can't predict it but postgresql can)
+     .   sufficient spare idle CPU to run the librt pthreads
+         or,  stated another way,    the CPU subsystem is relatively powerful
+         compared to the disk subsystem.
+In such ideal conditions,  and with a workload with plenty of index scans,
+around 10% - 20% improvement in throughput has been achieved.
+In an admittedly extreme environment measured by this author,    with a workload
+consisting of 8 client applications each running similar complex queries
+(same query structure but different predicates and constants),
+including 2 Bitmap Index Scans and 17 non-bitmap index scans,
+on a dual-core Intel laptop (4 hyperthreads) with the database on a single
+USB3-attached 500GB disk drive, and no part of the database in filesystem buffers
+initially,  (filesystem freshly mounted),  comparing unpatched build
+using posix_fadvise with effective_io_concurrency 4 against same build patched
+with async IO and effective_io_concurrency 4 and max_async_io_prefetchers 32,
+elapse time repeatably improved from around 640-670 seconds to around 530-550 seconds,
+a 17% - 18% improvement. 
+
+The disadvantages of Posix asynchronous IO compared to posix_fadvise are:
+     .   probably higher CPU utilization:
+         Firstly, the extra work performed by the librt threads adds CPU
+         overhead, and secondly, if the asynchronous prefetching is effective,
+         then it will deliver better (greater) overlap of CPU with IO, which
+         will reduce elapsed times and hence increase CPU utilization percentage
+         still more (during that shorter elapsed time).
+     .   more context switching,  because of the additional threads.
+
+
+Statistics:
+___________
+
+A number of additional statistics relating to effectiveness of asynchronous IO
+are provided as an extension of the existing pg_stat_statements loadable module.
+Refer to the appendix "Additional Supplied Modules" in the current
+PostgreSQL Documentation for details of this module.
+
+The following additional statistics are provided for asynchronous IO prefetching:
+
+    . aio_read_noneed  :   number of prefetches for which no need for prefetch as block already in buffer pool
+    . aio_read_discrd  :   number of prefetches for which buffer not subsequently read and therefore discarded
+    . aio_read_forgot  :   number of prefetches for which buffer not subsequently read and then forgotten about
+    . aio_read_noblok  :   number of prefetches for which no available BufferAiocb  control block
+    . aio_read_failed  :   number of aio reads for which aio itself failed or the read failed with an errno
+    . aio_read_wasted  :   number of aio reads for which in-progress aio cancelled and disk block not used
+    . aio_read_waited  :   number of aio reads for which disk block used but had to wait for it
+    . aio_read_ontime  :   number of aio reads for which disk block used and ready on time when requested
+
+Some of these are (hopefully) self-explanatory.    Some additional notes:
+
+    . aio_read_discrd and aio_read_forgot  :
+                    prefetch was wasted work since the buffer was not subsequently read
+                    The discrd case indicates that the scanner realized this and discarded the buffer,
+                    whereas the forgot case indicates that the scanner did not realize it,
+                    which should not normally occur.
+                    A high number in either suggests lowering effective_io_concurrency.
+
+    . aio_read_noblok  :   
+                    Any significant number in relation to all the other numbers indicates that
+                    max_async_io_prefetchers should be increased.
+
+    . aio_read_waited  :
+                    The page was prefetched but the asynchronous read had not completed by the time the
+                    scanner requested to read it.     causes extra overhead in waiting and indicates
+                    prefetching is not providing much if any benefit.
+                    The disk subsystem may be underpowered/overloaded in relation to the available CPU power.
+
+    . aio_read_ontime  :
+                    The page was prefetched and the asynchronous read had completed by the time the
+                    scanner requested to read it.     Optimal behaviour.      If this number if large
+                    in relation to all the other numbers except (possibly) aio_read_noneed,
+                    then prefetching is working well.
+
+To create the extension with support for these additional statistics, use the following syntax:
+     CREATE EXTENSION pg_stat_statements VERSION '1.3'
+or,  if you run the new code against an existing database which already has the extension
+( see installation and migration below ),  you can 
+     ALTER EXTENSION pg_stat_statements UPDATE TO '1.3'
+
+A suggested set of commands for displaying these statistics might be :
+
+ /*  OPTIONALLY */ DROP extension pg_stat_statements;
+                   CREATE extension pg_stat_statements VERSION '1.3';
+ /*  run your workload   */
+                  select userid , dbid , substring(query from 1 for 24) , calls , total_time , rows , shared_blks_read , blk_read_time , blk_write_time \
+                    , aio_read_noneed , aio_read_noblok , aio_read_failed , aio_read_wasted , aio_read_waited , aio_read_ontime , aio_read_forgot       \
+                      from pg_stat_statements where shared_blks_read > 0;
+
+
+Installation and Build Configuration:
+_____________________________________
+
+1. First -  a prerequsite:
+#  as well as requiring all the usual package build tools such as gcc , make etc,
+#  as described in the instructions for building postgresql,
+#  the following is required :
+    gnu autoconf at version 2.69 :
+# run the following command
+autoconf -V
+# it *must* return
+autoconf (GNU Autoconf) 2.69
+
+2. If you don't have it or it is a different version,
+then you must obtain version 2.69 (which is the current version)
+from your distribution provider or from the gnu software download site.
+
+3. Also you must have the source tree for postgresql version 9.4 (development version).
+#   all the following commands assume your current working directory is the top of the source tree.
+
+4. cd to top of source tree :
+#   check it appears to be a postgresql source tree
+ls -ld configure.in src
+#   should show both the file and the directory
+grep PostgreSQL COPYRIGHT
+#   should show PostgreSQL Database Management System
+
+5. Apply the patch :
+patch -b -p0 -i <patch_file_path>
+#   should report no errors, 45 files patched (see list at bottom of this README)
+#   and all hunks applied
+#  check the patch was appplied to configure.in
+ls -ld configure.in.orig configure.in
+#   should show both files
+
+6. Rebuild the configure script with the patched configure.in :
+mv configure configure.orig;
+autoconf configure.in >configure;echo "rc= $? from autoconf"; chmod +x configure;
+ls -lrt configure.orig configure;
+
+7. run the new configure script :
+#   if you have run configure before,
+#   then you may first want to save existing config.status and config.log if they exist,
+#   and then specify same configure flags and options as you specified before.
+#   the patch does not alter or extend the set of configure options
+#   if unsure,   run ./configure --help
+#   if still unsure,   run ./configure
+./configure <other configure options as desired>
+
+
+
+8. now check that configure decided that this environment supports asynchronous IO :
+grep USE_AIO_ATOMIC_BUILTIN_COMP_SWAP src/include/pg_config.h
+#  it should show
+#define USE_AIO_ATOMIC_BUILTIN_COMP_SWAP 1
+#  if not,  apparently your environment does not support asynch IO  -
+#  the config.log will show how it came to that conclusion,
+#  also check for :
+#    . a librt.so somewhere in the loader's library path (probably under /lib , /lib64 , or /usr)
+#    . your gcc must support the atomic compare_and_swap __sync_bool_compare_and_swap built-in function
+#  do not proceed without this define being set.
+
+9. do you want to use the new code on an existing cluster
+   that was created using the same code base but without the patch?
+   If so then run this nasty-looking command :
+   (cut-and-paste it into a terminal window or a shell-script file)
+   Otherwise continue to step 10.
+   see Migration note below for explanation.
+###############################################################################################
+   fl=src/Makefile.global; typeset -i bkx=0; while [[ $bkx < 200 ]]; do {
+       bkfl="${fl}.bak${bkx}"; if [[ -a ${bkfl} ]]; then ((bkx=bkx+1)); else break; fi;
+   }; done;
+   if [[ -a ${bkfl} ]]; then echo "sorry cannot find a backup name for $fl";
+   elif [[ -a $fl ]]; then {
+       mv $fl $bkfl && {
+          sed -e "/^CFLAGS =/ s/\$/ -DAVOID_CATALOG_MIGRATION_FOR_ASYNCIO/" $bkfl > $fl;
+          str="diff -w $bkfl $fl";echo "$str"; eval "$str";
+       };
+   };
+   else echo "ooopppss $fl is missing";
+   fi;
+###############################################################################################
+# it should report something like
+diff -w Makefile.global.bak0 Makefile.global
+222c222
+< CFLAGS = XXXX
+---
+> CFLAGS = XXXX -DAVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+#   where XXXX is some set of flags
+
+
+10. now run the rest of the build process as usual  -
+    follow instructions in file INSTALL if that file exists,
+    else e.g. run
+make && make install
+
+If the build fails with the following error:
+undefined reference to `aio_init'
+Then edit the following file
+src/include/pg_config_manual.h
+and add the following line at the bottom:
+
+#define DONT_HAVE_AIO_INIT
+
+and then run
+make clean && make && make install
+See notes to section Runtime Configuration below for more information on this.
+
+
+
+Migration , Runtime Configuration, and Use:
+___________________________________________
+
+
+Database Migration:
+___________________
+
+The new prefetching code for non-bitmap index scans introduces a new btree-index
+function named btpeeknexttuple.    The correct way to add such a function involves
+also adding it to the catalog as an internal function in pg_proc.
+However,  this results in the new built code considering an existing database to be
+incompatible,  i.e requiring backup on the old code and restore on the new.
+This is normal behaviour for migration to a new version of postgresql,  and is
+also a valid way of migrating a database for use with this asynchronous IO feature,
+but in this case it may be inconvenient.
+
+As an alternative,    the new code may be compiled with the macro define
+AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+which does what it says by not altering the catalog.   The patched build can then
+be run against an existing database cluster initdb'd using the unpatched build.
+
+There are no known ill-effects of so doing,  but :
+     .  in any case,  it is strongly suggested to make a backup of any precious database
+        before accessing it with a patched build
+     .  be aware that if this asynchronous IO feature is eventually released as part of postgresql,
+        migration will probably be required anyway.
+
+This option to avoid catalog migration is intended as a convenience for a quick test,
+and also makes it easier to obtain performance comparisons on the same database.
+
+
+
+Runtime Configuration:
+______________________
+
+One new configuration parameter settable in postgresql.conf and
+in any other way as described in the postgresql documentation :
+
+max_async_io_prefetchers
+  Maximum number of background processes concurrently using asynchronous
+  librt threads to prefetch pages into shared memory buffers
+
+This number can be thought of as the maximum number
+of librt threads concurrently active,   each working on a list of
+from 1 to target_prefetch_pages pages ( see notes 1 and 2 ).
+
+In practice,    this number simply controls how many prefetch requests in total
+may be active concurrently :
+        max_async_io_prefetchers * target_prefetch_pages ( see note 1)
+
+default is max_connections/6
+and recall that the default for max_connections is 100
+
+
+note 1  a number based on effective_io_concurrency and approximately n * ln(n)
+        where n is effective_io_concurrency
+
+note 2  Provided that the gnu extension to Posix AIO which provides the
+aio_init() function is present,   then aio_init() is called
+to set the librt maximum number of threads to max_async_io_prefetchers,
+and to set the maximum number of concurrent aio read requests to the product of
+        max_async_io_prefetchers * target_prefetch_pages
+
+
+As well as this regular configuration parameter,
+there are several other parameters that can be set via environment variable.
+The reason why they are environment vars rather than regular configuration parameters
+is that it is not expected that they should need to be set,   but they may be useful :
+                variable name         values                  default        meaning
+   PG_TRY_PREFETCHING_FOR_BITMAP      [Y|N]                    Y         whether to prefetch bitmap heap scans
+   PG_TRY_PREFETCHING_FOR_ISCAN       [Y|N|integer[,[N|Y]]]   256,N      whether to prefetch  non-bitmap index scans
+                                                                    also numeric size of list of prefetched blocks
+                                                                    also whether to prefetch forward-sequential-pattern index pages
+   PG_TRY_PREFETCHING_FOR_BTREE       [Y|N]                    Y         whether to prefetch heap pages in non-bitmap index scans
+   PG_TRY_PREFETCHING_FOR_HEAP        [Y|N]                    N         whether to prefetch relation (un-indexed) heap scans
+
+
+The setting for PG_TRY_PREFETCHING_FOR_ISCAN is a litle complicated.
+It can be set to Y or N to control prefetching of  non-bitmap index scans;
+But in addition it can be set to an integer,   which both implies Y
+and also sets the size of a list used to remember prefetched but unread heap pages.
+This list is an optimization used to avoid re-prefetching and maximise the potential
+set of prefetchable blocks indexed by one index page.
+And if set to an integer,  this integer may be followed by either ,Y or ,N
+to specify to prefetch index pages which are being accessed forward-sequentially.
+It has been found that prefetching is not of great benefit for this access pattern,
+and so it is not the default,  but also does no harm (provided sufficient CPU capacity).
+
+
+
+Usage :
+______
+
+
+There are no changes in usage other than as noted under Configuration and Statistics.
+However,   in order to assess benefit from this feature,   it will be useful to
+understand the query access plans of your workload using EXPLAIN.    Before doing that,
+make sure that statistics are up to date using ANALYZE.
+
+
+
+Internals:
+__________
+
+
+Internal changes span two areas and the interface between them :
+
+ .  buffer manager layer
+ .  programming interface for scanner to call buffer manager
+ .  scanner layer
+
+ .  buffer manager layer
+    ____________________
+
+    changes comprise :
+       .   allocating,  pinning , unpinning buffers
+            this is complex and discussed briefly below in "Buffer Management"
+       .   acquiring and releasing a BufferAiocb, the control block
+            associated with a single aio_read,  and checking for its completion
+            a new file,  backend/storage/buffer/buf_async.c, provides three new functions,
+                  BufStartAsync        BufReleaseAsync            BufCheckAsync
+            which handle this.
+       .   calling librt asynch io functions
+            this follows the example of all other filesystem interfaces
+            and is straightforward.    
+            two new functions are provided in fd.c:
+                   FileStartaio        FileCompleteaio
+            and corresponding interfaces in smgr.c
+
+ .  programming interface for scanner to call buffer manager
+    ________________________________________________________
+       . calling interface for existing function PrefetchBuffer is modified :
+           .  one new argument,   BufferAccessStrategy strategy
+           .  now returns an int return code which indicates :
+                     whether pin count on buffer has been increased by 1
+                     whether block was already present in a buffer
+       .  new function DiscardBuffer
+           .  discard buffer used for a previously prefetched page
+                 which scanner decides it does not want to read.
+           .  same arguments as for PrefetchBuffer except for omission of BufferAccessStrategy
+           .  note - this is different from the existing function ReleaseBuffer
+                     in that ReleaseBuffer takes a buffer_descriptor as argument
+                     for a buffer which has been read, but has similar purpose.
+
+ .  scanner layer
+    _____________
+        common to all scanners is that the scanner which wishes to prefetch must do two things:
+          .  decide which pages to prefetch and call PrefetchBuffer to prefetch them
+                 nodeBitmapHeapscan already does this (but note one extra argument on PrefetchBuffer)
+          .  remember which pages it has prefetched in some list (actual or conceptual,  e.g. a page range),
+                 removing each page from this list if and when it subsequently reads the page.
+          .  at end of scan,  call DiscardBuffer for every remembered (i.e. prefetched not unread) page
+       how this list of prefetched pages is implemented varies for each of the three scanners and four scan types:
+            .  bitmap index scan - heap pages
+            .  non-bitmap (i.e. simple) index scans - index pages
+            .  non-bitmap (i.e. simple) index scans - heap pages
+            .  simple heap scans
+       The consequences of forgetting to call DiscardBuffer on a prefetched but unread page are:
+            .   counted in aio_read_forgot  (see "Statistics" above)
+            .   may incur an annoying but harmless warning in the pg_log "Buffer Leak ... "
+                  (the buffer is released at commit)
+       This does sometimes happen ...
+     
+
+
+Buffer Management
+_________________
+
+With async io,   PrefetchBuffer must allocate and pin a buffer,  which is relatively straightforward,
+but also every other part of buffer manager must know about the possibility that a buffer may be in
+a state of async_io_in_progress state and be prepared to determine the possible completion.
+That is,  one backend BK1 may start the io but another BK2 may try to read it before BK1 does.
+Posix Asynchronous IO provides a means for waiting on this or another task's read if in progress,
+namely aio_suspend(),  which this extension uses.    Therefore,  although StartBufferIO and TerminateBufferIO
+are called as part of asynchronous prefetching,   their role is limited to maintaining the buffer descriptor flags,
+and they do not track the asynchronous IO itself.    Instead,   asynchronous IOs are tracked in
+a separate set of shared control blocks,  the BufferAiocb list -
+refer to     include/storage/buf_internals.h
+Checking asynchronous io status is handled in  backend/storage/buffer/buf_async.c BufCheckAsync function.
+Read the commentary for this function for more details.
+
+Pinning and unpinning of buffers is the most complex aspect of asynch io prefetching,
+and the logic is spread throughout BufStartAsync , BufCheckAsync , and many functions in bufmgr.c.
+When a backend BK2 requests ReadBuffer of a page for which asynch read is in progress,
+buffer manager has to determine which backend BK1 pinned this buffer during previous PrefetchBuffer,
+and for example must not be re-pinned a second time if BK2 is BK1.
+Information concerning which backend initiated the prefetch is held in the BufferAiocb.
+
+The trickiest case concerns the scenario in which :
+   .  BK1 initiates prefetch and acquires a pin
+   .  BK2 possibly waits for completion and then reads the buffer,  and perhaps later on
+         releases it by ReleaseBuffer.
+   .  Since the asynchronous IO is no longer in progress,     there is no longer any
+         BufferAiocb associated with it.    Yet buffer manager must remember that BK1 holds a
+         "prefetch" pin, i.e. a pin which must not be repeated if and when BK1 finally issues ReadBuffer.
+   .  The solution to this problem is to invent the concept of a "banked" pin,
+      which is a pin obtained when prefetch was issued,   identied as in "banked" status only if and when
+      the associated asynchronous IO terminates,  and redeemable by the next use by same task,
+      either by ReadBuffer or DiscardBuffer.
+      The pid of the backend which holds a banked pin on a buffer (there can be at most one such backend)
+      is stored in the buffer descriptor.
+      This is done without increasing size of the buffer descriptor,  which is important since
+      there may be a very large number of these.     This does overload the relevant field in the descriptor.
+      Refer to include/storage/buf_internals.h for more details
+      and search for BM_AIO_PREFETCH_PIN_BANKED in storage/buffer/bufmgr.c and  backend/storage/buffer/buf_async.c
+
+______________________________________________________________________________
+The following 45 files are changed in this feature (output of the patch command) :
+
+patching file configure.in
+patching file contrib/pg_prewarm/pg_prewarm.c
+patching file contrib/pg_stat_statements/pg_stat_statements--1.3.sql
+patching file contrib/pg_stat_statements/Makefile
+patching file contrib/pg_stat_statements/pg_stat_statements.c
+patching file contrib/pg_stat_statements/pg_stat_statements--1.2--1.3.sql
+patching file postgresql-prefetching-asyncio.README
+patching file config/c-library.m4
+patching file src/backend/postmaster/postmaster.c
+patching file src/backend/executor/nodeBitmapHeapscan.c
+patching file src/backend/executor/nodeIndexscan.c
+patching file src/backend/executor/instrument.c
+patching file src/backend/storage/buffer/Makefile
+patching file src/backend/storage/buffer/bufmgr.c
+patching file src/backend/storage/buffer/buf_async.c
+patching file src/backend/storage/buffer/buf_init.c
+patching file src/backend/storage/smgr/md.c
+patching file src/backend/storage/smgr/smgr.c
+patching file src/backend/storage/file/fd.c
+patching file src/backend/storage/lmgr/proc.c
+patching file src/backend/access/heap/heapam.c
+patching file src/backend/access/heap/syncscan.c
+patching file src/backend/access/index/indexam.c
+patching file src/backend/access/index/genam.c
+patching file src/backend/access/nbtree/nbtsearch.c
+patching file src/backend/access/nbtree/nbtinsert.c
+patching file src/backend/access/nbtree/nbtpage.c
+patching file src/backend/access/nbtree/nbtree.c
+patching file src/backend/nodes/tidbitmap.c
+patching file src/backend/utils/misc/guc.c
+patching file src/backend/utils/mmgr/aset.c
+patching file src/include/executor/instrument.h
+patching file src/include/storage/bufmgr.h
+patching file src/include/storage/smgr.h
+patching file src/include/storage/fd.h
+patching file src/include/storage/buf_internals.h
+patching file src/include/catalog/pg_am.h
+patching file src/include/catalog/pg_proc.h
+patching file src/include/pg_config_manual.h
+patching file src/include/access/nbtree.h
+patching file src/include/access/heapam.h
+patching file src/include/access/relscan.h
+patching file src/include/nodes/tidbitmap.h
+patching file src/include/utils/rel.h
+patching file src/include/pg_config.h.in
+
+
+Future Possibilities:
+____________________
+
+There are several possible extensions of this feature :
+   .   Extend prefetching of index scans to types of index
+       other than B-tree.
+       This should be fairly straightforward,  but requires some
+       good base of benchmarkable workloads to prove the value.
+   .   Investigate why asynchronous IO prefetching does not greatly
+       improve sequential relation heap scans and possibly find how to
+       achieve a benefit.
+   .   Build knowledge of asycnhronous IO prefetching into the
+       Query Planner costing.
+       This is far from straightforward.    The Postgresql Query Planner's
+       costing model is based on resource consumption rather than elapsed time.
+       Use of asynchronous IO prefetching is intended to improve elapsed time
+       as the expense of (probably) higher resource consumption.
+       Although Costing understands about the reduced cost of reading buffered
+       blocks, it does not take asynchronicity or overlap of CPU with disk
+       into account.  A naive approach might be to try to tweak the Query
+       Planner's Cost Constant configuration parameters
+       such as seq_page_cost , random_page_cost
+       but this is hazardous as explained in the Documentation.
+
+
+
+John Lumby,  johnlumby(at)hotmail(dot)com
--- config/c-library.m4.orig	2014-05-31 17:19:07.685208326 -0400
+++ config/c-library.m4	2014-05-31 19:53:08.788072839 -0400
@@ -367,3 +367,152 @@ if test "$pgac_cv_type_locale_t" = 'yes
   AC_DEFINE(LOCALE_T_IN_XLOCALE, 1,
             [Define to 1 if `locale_t' requires <xlocale.h>.])
 fi])])# PGAC_HEADER_XLOCALE
+
+
+# PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP
+# ---------------------------------------
+# test whether this system has both the librt-style async io and the gcc atomic compare_and_swap
+#      and test operation of both,
+#      including verifying that aio_error can retrieve completion status
+#      of aio_read issued by a different process
+#
+AC_DEFUN([PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP],
+[AC_MSG_CHECKING([whether have both librt-style async io and the gcc atomic compare_and_swap])
+AC_CACHE_VAL(pgac_cv_aio_atomic_builtin_comp_swap,
+pgac_save_LIBS=$LIBS
+LIBS=" -lrt $pgac_save_LIBS"
+[AC_TRY_RUN([#include <stdio.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+#include <fcntl.h>
+#include "aio.h"
+#include <errno.h>
+
+char *shmem;
+
+/*  returns rc of aio_read or -1 if some error */
+int
+processA(void)
+{
+	int fd , rc;
+	struct aiocb *aiocbp = (struct aiocb *) shmem;
+	char *buf = shmem + sizeof(struct aiocb);
+
+	rc = fd = open("configure", O_RDONLY );
+	if (fd != -1) {
+
+            memset(aiocbp, 0, sizeof(struct aiocb));
+            aiocbp->aio_fildes = fd;
+            aiocbp->aio_offset = 0;
+            aiocbp->aio_buf = buf;
+            aiocbp->aio_nbytes = 8;
+            aiocbp->aio_reqprio = 0;
+            aiocbp->aio_sigevent.sigev_notify = SIGEV_NONE;
+
+            rc = aio_read(aiocbp);
+	}
+        return rc;
+}
+
+/*  returns result of aio_error  -  0 if io completed successfully */
+int
+processB(void)
+{
+	struct aiocb *aiocbp = (struct aiocb *) shmem;
+	const struct aiocb * const pl[1] = { aiocbp };
+	int rv;
+	int	returnCode;
+        struct timespec my_timeout = { 0 , 10000 };
+        int max_iters , max_polls;
+
+	rv = aio_error(aiocbp);
+        max_iters = 100;
+        while ( (max_iters-- > 0) && (rv == EINPROGRESS) ) {
+                max_polls = 256;
+                my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                returnCode = aio_suspend(pl , 1 , &my_timeout);
+                while ((returnCode < 0) && (EAGAIN == errno) && (max_polls-- > 0)) {
+                    my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                    returnCode = aio_suspend(pl , 1 , &my_timeout);
+                }
+                rv = aio_error(aiocbp);
+        }
+
+	return rv;
+}
+
+int main (int argc, char *argv[])
+ {
+   int rc;
+   int pidB;
+   int child_status;
+   struct aiocb volatile * first_aiocb;
+   struct aiocb volatile * second_aiocb;
+   struct aiocb volatile * my_aiocbp = (struct aiocb *)20000008;
+
+   first_aiocb = (struct aiocb *)20000008;
+   second_aiocb = (struct aiocb *)40000008;
+
+   /*  first test  --  __sync_bool_compare_and_swap
+   **  set zero as success if two comp-swaps both worked as expected -
+   **  first compares equal and swaps,  second compares unequal
+   */
+   rc = (__sync_bool_compare_and_swap (&my_aiocbp, first_aiocb, second_aiocb));
+   if (rc) {
+      rc = (__sync_bool_compare_and_swap (&my_aiocbp, first_aiocb, second_aiocb));
+   } else {
+      rc = -1;
+   }
+
+   if (rc == 0) {
+       /*  second test  --  process A start aio_read
+       **  and process B checks completion by polling
+       */
+        rc = -1; /* pessimistic */
+
+	shmem = mmap(NULL, sizeof(struct aiocb) + 2048,
+				 PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS,
+				 -1, 0);
+	if (shmem != MAP_FAILED) {
+            
+            /*
+             * Start the I/O request in parent process, then fork and try to wait
+             * for it to finish from the child process.
+             */
+            rc = processA();
+            if (rc >= 0) {
+
+                rc = pidB = fork();
+                if (pidB != -1) {
+                    if (pidB != 0) {
+                        /* parent */
+                        wait (&child_status);
+                        if (WIFEXITED(child_status)) {
+                            rc = WEXITSTATUS(child_status);
+                        }
+                    } else {
+                        /* child */
+                        rc = processB();
+                        exit(rc);
+                    }
+                }
+            }
+        }
+   }
+
+   return rc;
+}],
+[pgac_cv_aio_atomic_builtin_comp_swap=yes],
+[pgac_cv_aio_atomic_builtin_comp_swap=no],
+[pgac_cv_aio_atomic_builtin_comp_swap=cross])
+])dnl AC_CACHE_VAL
+AC_MSG_RESULT([$pgac_cv_aio_atomic_builtin_comp_swap])
+if test x"$pgac_cv_aio_atomic_builtin_comp_swap" != x"yes"; then
+LIBS=$pgac_save_LIBS
+fi
+])# PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP
--- src/backend/postmaster/postmaster.c.orig	2014-05-31 17:19:07.865208803 -0400
+++ src/backend/postmaster/postmaster.c	2014-05-31 19:53:08.828072940 -0400
@@ -123,6 +123,11 @@
 #include "storage/spin.h"
 #endif
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+void ReportFreeBAiocbs(void);
+int CountInuseBAiocbs(void);
+extern int hwmBufferAiocbs;         /*  high water mark of in-use  BufferAiocbs in pool           */
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * Possible types of a backend. Beyond being the possible bkend_type values in
@@ -1493,9 +1498,15 @@ ServerLoop(void)
 	fd_set		readmask;
 	int			nSockets;
 	time_t		now,
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                           count_baiocb_time,
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 				last_touch_time;
 
 	last_touch_time = time(NULL);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        count_baiocb_time = time(NULL);
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 	nSockets = initMasks(&readmask);
 
@@ -1654,6 +1665,19 @@ ServerLoop(void)
 			last_touch_time = now;
 		}
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                /*   maintain the hwm of used baiocbs every 10 seconds  */
+		if ((now - count_baiocb_time) >= 10)
+		{
+                        int inuseBufferAiocbs;         /*  current in-use  BufferAiocbs in pool */
+                        inuseBufferAiocbs = CountInuseBAiocbs();
+                        if (inuseBufferAiocbs > hwmBufferAiocbs) {
+			    hwmBufferAiocbs = inuseBufferAiocbs;
+			}
+			count_baiocb_time = now;
+		}
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 		/*
 		 * If we already sent SIGQUIT to children and they are slow to shut
 		 * down, it's time to send them SIGKILL.  This doesn't happen
@@ -3444,6 +3468,9 @@ PostmasterStateMachine(void)
 						signal_child(PgStatPID, SIGQUIT);
 				}
 			}
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                        ReportFreeBAiocbs();
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 		}
 	}
 
--- src/backend/executor/nodeBitmapHeapscan.c.orig	2014-05-31 17:19:07.809208656 -0400
+++ src/backend/executor/nodeBitmapHeapscan.c	2014-05-31 19:53:08.876073061 -0400
@@ -34,6 +34,8 @@
  *		ExecEndBitmapHeapScan		releases all storage.
  */
 #include "postgres.h"
+#include <sys/types.h> 
+#include <unistd.h>
 
 #include "access/relscan.h"
 #include "access/transam.h"
@@ -47,6 +49,10 @@
 #include "utils/snapmgr.h"
 #include "utils/tqual.h"
 
+#ifdef USE_PREFETCH
+extern unsigned int prefetch_dbOid; /*  database oid of relations on which prefetching to be done - 0 means all */
+extern unsigned int prefetch_bitmap_scans; /*  boolean whether to prefetch bitmap heap scans         */
+#endif   /* USE_PREFETCH */
 
 static TupleTableSlot *BitmapHeapNext(BitmapHeapScanState *node);
 static void bitgetpage(HeapScanDesc scan, TBMIterateResult *tbmres);
@@ -111,10 +117,21 @@ BitmapHeapNext(BitmapHeapScanState *node
 		node->tbmres = tbmres = NULL;
 
 #ifdef USE_PREFETCH
-		if (target_prefetch_pages > 0)
-		{
+		if (    prefetch_bitmap_scans
+                     && (target_prefetch_pages > 0)
+                     && (    (   (prefetch_dbOid > 0)
+                               && (prefetch_dbOid == scan->rs_rd->rd_node.dbNode)
+                             )
+                         ||  (prefetch_dbOid == 0)
+                        )
+                        /* sufficient number of blocks - at least twice the target_prefetch_pages */
+                     && (scan->rs_nblocks > (2*target_prefetch_pages))
+                   ) {
 			node->prefetch_iterator = prefetch_iterator = tbm_begin_iterate(tbm);
 			node->prefetch_pages = 0;
+                        if (prefetch_iterator) {
+                          tbm_zero(prefetch_iterator);  /* zero list of prefetched and unread blocknos */
+                        }
 			node->prefetch_target = -1;
 		}
 #endif   /* USE_PREFETCH */
@@ -138,12 +155,14 @@ BitmapHeapNext(BitmapHeapScanState *node
 			}
 
 #ifdef USE_PREFETCH
+                        if (prefetch_iterator) {
 			if (node->prefetch_pages > 0)
 			{
 				/* The main iterator has closed the distance by one page */
 				node->prefetch_pages--;
+                                tbm_subtract(prefetch_iterator, tbmres->blockno); /* remove this blockno from list of prefetched and unread blocknos */
 			}
-			else if (prefetch_iterator)
+                            else
 			{
 				/* Do not let the prefetch iterator get behind the main one */
 				TBMIterateResult *tbmpre = tbm_iterate(prefetch_iterator);
@@ -151,6 +170,7 @@ BitmapHeapNext(BitmapHeapScanState *node
 				if (tbmpre == NULL || tbmpre->blockno != tbmres->blockno)
 					elog(ERROR, "prefetch and main iterators are out of sync");
 			}
+                        }
 #endif   /* USE_PREFETCH */
 
 			/*
@@ -239,16 +259,26 @@ BitmapHeapNext(BitmapHeapScanState *node
 			while (node->prefetch_pages < node->prefetch_target)
 			{
 				TBMIterateResult *tbmpre = tbm_iterate(prefetch_iterator);
+                                int             PrefetchBufferRc; /*  return value from PrefetchBuffer  - refer to bufmgr.h */
+
 
 				if (tbmpre == NULL)
 				{
 					/* No more pages to prefetch */
-					tbm_end_iterate(prefetch_iterator);
-					node->prefetch_iterator = prefetch_iterator = NULL;
+                                        /* let ExecEndBitmapHeapScan terminate the prefetch_iterator
+				        **	tbm_end_iterate(prefetch_iterator);
+					**      node->prefetch_iterator = NULL;
+                                        */
+                                        prefetch_iterator = NULL;
 					break;
 				}
 				node->prefetch_pages++;
-				PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, tbmpre->blockno);
+				PrefetchBufferRc = PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, tbmpre->blockno , 0);
+                                /*  add this blockno to list of prefetched and unread blocknos
+                                **  if pin count did not increase then indicate so in the Unread_Pfetched list
+                                */
+                                tbm_add(prefetch_iterator
+                                   ,( (PrefetchBufferRc & PREFTCHRC_BUF_PIN_INCREASED) ? tbmpre->blockno : InvalidBlockNumber ) );
 			}
 		}
 #endif   /* USE_PREFETCH */
@@ -482,12 +512,31 @@ ExecEndBitmapHeapScan(BitmapHeapScanStat
 {
 	Relation	relation;
 	HeapScanDesc scanDesc;
+	TBMIterator *prefetch_iterator;
 
 	/*
 	 * extract information from the node
 	 */
 	relation = node->ss.ss_currentRelation;
 	scanDesc = node->ss.ss_currentScanDesc;
+	prefetch_iterator = node->prefetch_iterator;
+
+#ifdef USE_PREFETCH
+        /*  before any other cleanup,  discard any prefetched but unread buffers  */
+        if (prefetch_iterator != NULL) {
+            TBMIterateResult *tbmpre = tbm_locate_IterateResult(prefetch_iterator);
+            BlockNumber *Unread_Pfetched_base = tbmpre->Unread_Pfetched_base;
+            unsigned int Unread_Pfetched_next = tbmpre->Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+            unsigned int Unread_Pfetched_count = tbmpre->Unread_Pfetched_count;
+
+            while ((Unread_Pfetched_count--) > 0) {
+                DiscardBuffer( scanDesc->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                Unread_Pfetched_next++;
+                if (Unread_Pfetched_next >= target_prefetch_pages)
+                    Unread_Pfetched_next = 0;
+            }
+        }
+#endif   /* USE_PREFETCH */
 
 	/*
 	 * Free the exprcontext
--- src/backend/executor/nodeIndexscan.c.orig	2014-05-31 17:19:07.813208666 -0400
+++ src/backend/executor/nodeIndexscan.c	2014-05-31 19:53:08.900073122 -0400
@@ -35,8 +35,13 @@
 #include "utils/rel.h"
 
 
+
 static TupleTableSlot *IndexNext(IndexScanState *node);
 
+#ifdef USE_PREFETCH
+extern unsigned int prefetch_dbOid; /*  database oid of relations on which prefetching to be done - 0 means all */
+extern unsigned int prefetch_index_scans; /*  boolean whether to prefetch bitmap heap scans         */
+#endif   /* USE_PREFETCH */
 
 /* ----------------------------------------------------------------
  *		IndexNext
@@ -418,7 +423,12 @@ ExecEndIndexScan(IndexScanState *node)
 	 * close the index relation (no-op if we didn't open it)
 	 */
 	if (indexScanDesc)
+        {
 		index_endscan(indexScanDesc);
+
+        /*  note  -  at this point all scan controlblock resources have been freed by IndexScanEnd called by index_endscan */
+
+        }
 	if (indexRelationDesc)
 		index_close(indexRelationDesc, NoLock);
 
@@ -609,6 +619,33 @@ ExecInitIndexScan(IndexScan *node, EStat
 											   indexstate->iss_NumScanKeys,
 											 indexstate->iss_NumOrderByKeys);
 
+#ifdef USE_PREFETCH
+        /*  initialize prefetching   */
+                indexstate->iss_ScanDesc->pfch_index_page_list =  (struct pfch_index_pagelist*)0;
+                indexstate->iss_ScanDesc->pfch_block_item_list = (struct pfch_block_item*)0;
+		if (    prefetch_index_scans
+			 && (target_prefetch_pages > 0)
+			 &&	(!RelationUsesLocalBuffers(indexstate->iss_ScanDesc->heapRelation)) /* I think this must always be true for an indexed heap ? */
+			 && (    (   (prefetch_dbOid > 0)
+					   && (prefetch_dbOid == indexstate->iss_ScanDesc->heapRelation->rd_node.dbNode)
+					 )
+				 ||  (prefetch_dbOid == 0)
+				)
+		   ) {
+			indexstate->iss_ScanDesc->pfch_index_page_list = palloc( sizeof(struct pfch_index_pagelist) + ( (target_prefetch_pages-1) * sizeof(struct pfch_index_item) ) );
+			indexstate->iss_ScanDesc->pfch_block_item_list = palloc( prefetch_index_scans * sizeof(struct pfch_block_item) );
+			if (     ( (struct pfch_index_pagelist*)0 != indexstate->iss_ScanDesc->pfch_index_page_list )
+                  && ( (struct pfch_block_item*)0 != indexstate->iss_ScanDesc->pfch_block_item_list )
+               ) {
+                          indexstate->iss_ScanDesc->pfch_used = 0;
+                          indexstate->iss_ScanDesc->pfch_next = prefetch_index_scans; /* ensure first entry is at index 0 */
+                          indexstate->iss_ScanDesc->pfch_index_page_list->pfch_index_pagelist_next = (struct pfch_index_pagelist*)0;
+                          indexstate->iss_ScanDesc->pfch_index_page_list->pfch_index_item_count = 0;
+                          indexstate->iss_ScanDesc->do_prefetch = 1;
+            }
+		}
+#endif   /* USE_PREFETCH */
+
 	/*
 	 * If no run-time keys to calculate, go ahead and pass the scankeys to the
 	 * index AM.
--- src/backend/executor/instrument.c.orig	2014-05-31 17:19:07.809208656 -0400
+++ src/backend/executor/instrument.c	2014-05-31 19:53:08.932073203 -0400
@@ -41,6 +41,14 @@ InstrAlloc(int n, int instrument_options
 		{
 			instr[i].need_bufusage = need_buffers;
 			instr[i].need_timer = need_timer;
+			instr[i].bufusage_start.aio_read_noneed = 0;
+			instr[i].bufusage_start.aio_read_discrd = 0;
+			instr[i].bufusage_start.aio_read_forgot = 0;
+			instr[i].bufusage_start.aio_read_noblok = 0;
+			instr[i].bufusage_start.aio_read_failed = 0;
+			instr[i].bufusage_start.aio_read_wasted = 0;
+			instr[i].bufusage_start.aio_read_waited = 0;
+			instr[i].bufusage_start.aio_read_ontime = 0;
 		}
 	}
 
@@ -143,6 +151,16 @@ BufferUsageAccumDiff(BufferUsage *dst,
 	dst->local_blks_written += add->local_blks_written - sub->local_blks_written;
 	dst->temp_blks_read += add->temp_blks_read - sub->temp_blks_read;
 	dst->temp_blks_written += add->temp_blks_written - sub->temp_blks_written;
+
+	dst->aio_read_noneed       += add->aio_read_noneed - sub->aio_read_noneed;
+	dst->aio_read_discrd       += add->aio_read_discrd - sub->aio_read_discrd;
+	dst->aio_read_forgot       += add->aio_read_forgot - sub->aio_read_forgot;
+	dst->aio_read_noblok       += add->aio_read_noblok - sub->aio_read_noblok;
+	dst->aio_read_failed       += add->aio_read_failed - sub->aio_read_failed;
+	dst->aio_read_wasted       += add->aio_read_wasted - sub->aio_read_wasted;
+	dst->aio_read_waited       += add->aio_read_waited - sub->aio_read_waited;
+	dst->aio_read_ontime       += add->aio_read_ontime - sub->aio_read_ontime;
+
 	INSTR_TIME_ACCUM_DIFF(dst->blk_read_time,
 						  add->blk_read_time, sub->blk_read_time);
 	INSTR_TIME_ACCUM_DIFF(dst->blk_write_time,
--- src/backend/storage/buffer/Makefile.orig	2014-05-31 17:19:07.873208825 -0400
+++ src/backend/storage/buffer/Makefile	2014-05-31 19:53:08.952073254 -0400
@@ -12,6 +12,6 @@ subdir = src/backend/storage/buffer
 top_builddir = ../../../..
 include $(top_builddir)/src/Makefile.global
 
-OBJS = buf_table.o buf_init.o bufmgr.o freelist.o localbuf.o
+OBJS = buf_table.o buf_init.o bufmgr.o freelist.o localbuf.o buf_async.o
 
 include $(top_srcdir)/src/backend/common.mk
--- src/backend/storage/buffer/bufmgr.c.orig	2014-05-31 17:19:07.873208825 -0400
+++ src/backend/storage/buffer/bufmgr.c	2014-05-31 19:53:08.996073365 -0400
@@ -29,7 +29,7 @@
  *		buf_table.c -- manages the buffer lookup table
  */
 #include "postgres.h"
-
+#include <sys/types.h> 
 #include <sys/file.h>
 #include <unistd.h>
 
@@ -50,7 +50,6 @@
 #include "utils/resowner_private.h"
 #include "utils/timestamp.h"
 
-
 /* Note: these two macros only work on shared buffers, not local ones! */
 #define BufHdrGetBlock(bufHdr)	((Block) (BufferBlocks + ((Size) (bufHdr)->buf_id) * BLCKSZ))
 #define BufferGetLSN(bufHdr)	(PageGetLSN(BufHdrGetBlock(bufHdr)))
@@ -63,6 +62,8 @@
 #define BUF_WRITTEN				0x01
 #define BUF_REUSABLE			0x02
 
+extern volatile struct BAiocbAnchor *BAiocbAnchr;  /*  anchor for all control blocks pertaining to aio  */
+
 #define DROP_RELS_BSEARCH_THRESHOLD		20
 
 /* GUC variables */
@@ -78,26 +79,33 @@ bool		track_io_timing = false;
  */
 int			target_prefetch_pages = 0;
 
-/* local state for StartBufferIO and related functions */
+/* local state for StartBufferIO and related functions
+**  but ONLY for synchronous IO  -  not altered for aio
+*/
 static volatile BufferDesc *InProgressBuf = NULL;
 static bool IsForInput;
+pid_t this_backend_pid = 0;    /*    pid of this backend */
 
 /* local state for LockBufferForCleanup */
 static volatile BufferDesc *PinCountWaitBuf = NULL;
 
-
-static Buffer ReadBuffer_common(SMgrRelation reln, char relpersistence,
+extern int
+BufStartAsync(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy);
+extern int
+BufCheckAsync(SMgrRelation caller_smgr, Relation caller_reln, volatile BufferDesc *buf_desc, int intention
+        ,BufferAccessStrategy strategy , int index_for_aio , bool spinLockHeld , LWLockId PartitionLock );
+Buffer ReadBuffer_common(SMgrRelation reln, char relpersistence,
 				  ForkNumber forkNum, BlockNumber blockNum,
 				  ReadBufferMode mode, BufferAccessStrategy strategy,
-				  bool *hit);
-static bool PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy);
-static void PinBuffer_Locked(volatile BufferDesc *buf);
-static void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner);
+				  bool *hit , int index_for_aio);
+bool PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy);
+void PinBuffer_Locked(volatile BufferDesc *buf);
+void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner);
 static void BufferSync(int flags);
 static int	SyncOneBuffer(int buf_id, bool skip_recently_used);
 static void WaitIO(volatile BufferDesc *buf);
-static bool StartBufferIO(volatile BufferDesc *buf, bool forInput);
-static void TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
+static bool StartBufferIO(volatile BufferDesc *buf, bool forInput , int index_for_aio );
+void TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
 				  int set_flag_bits);
 static void shared_buffer_write_error_callback(void *arg);
 static void local_buffer_write_error_callback(void *arg);
@@ -106,24 +114,66 @@ static volatile BufferDesc *BufferAlloc(
 			ForkNumber forkNum,
 			BlockNumber blockNum,
 			BufferAccessStrategy strategy,
-			bool *foundPtr);
+			int *foundPtr , int index_for_aio );
 static void FlushBuffer(volatile BufferDesc *buf, SMgrRelation reln);
 static void AtProcExit_Buffers(int code, Datum arg);
 static int	rnode_comparator(const void *p1, const void *p2);
 
+BlockNumber BlocknumOfBuffer(Buffer buffer);
+bool BlocknotinBuffer(Buffer buffer, Relation relation, BlockNumber blockNum);
 
 /*
  * PrefetchBuffer -- initiate asynchronous read of a block of a relation
  *
- * This is named by analogy to ReadBuffer but doesn't actually allocate a
- * buffer.  Instead it tries to ensure that a future ReadBuffer for the given
- * block will not be delayed by the I/O.  Prefetching is optional.
+ * This is named by analogy to ReadBuffer but allocates a buffer only if using asynchronous I/O.
+ * Its purpose  is to try to ensure that a future ReadBuffer for the given block
+ * will not be delayed by the I/O.  Prefetching is optional.
  * No-op if prefetching isn't compiled in.
- */
-void
-PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
-{
+ *
+ * Originally the prefetch simply called posix_fadvise() to recommend read-ahead into kernel page cache.
+ * Extended to provide an alternative of issuing an asynchronous aio_read() to read into a buffer.
+ * This extension has an implication on how this bufmgr component manages concurrent requests
+ * for the same disk block.
+ *
+ * Synchronous IO (read()) does not provide a means for waiting on another task's read if in progress,
+ * and bufmgr implements its own scheme in StartBufferIO, WaitIO, and TerminateBufferIO.
+ *
+ * Asynchronous IO (aio_read()) provides a means for waiting on this or another task's read if in progress,
+ * namely aio_suspend(),  which this extension uses.    Therefore,  although StartBufferIO and TerminateBufferIO
+ * are called as part of asynchronous prefetching,   their role is limited to maintaining the buffer desc flags,
+ * and they do not track the asynchronous IO itself.    Instead,   asynchronous IOs are tracked in 
+ * a separate set of shared control blocks,  the BufferAiocb list -
+ *   refer to     include/storage/buf_internals.h and storage/buffer/buf_init.c
+ *
+ * Another implication of asynchronous IO concerns buffer pinning.
+ * The buffer used for the prefetch is pinned before aio_read is issued.
+ * It is expected that the same task (and possibly others) will later ask to read the page
+ * and eventually release and unpin the buffer.
+ * However,  if the task which issued the aio_read later decides not to read the page,
+ * and return code indicates delta_pin_count > 0 (see below)
+ * it *must* instead issue a DiscardBuffer() (see function later in this file)
+ * so that its pin is released.
+ * Therefore,  each client which uses the PrefetchBuffer service must either always read all
+ * prefetched pages,  or keep track of prefetched pages and discard unread ones at end of scan.
+ *
+ * return code:   is an int bitmask defined in bufmgr.h
+        PREFTCHRC_BUF_PIN_INCREASED 0x01      pin count on buffer has been increased by 1
+        PREFTCHRC_BLK_ALREADY_PRESENT 0x02    block was already present in a buffer
+ *
+ * PREFTCHRC_BLK_ALREADY_PRESENT is a hint to caller that the prefetch may be unnecessary
+ */
+int
+PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy)
+{
+	Buffer		buf_id; /* indicates buffer containing the requested block  */
+        int             PrefetchBufferRc = 0; /*  return value as described above  */
+        int             PinCountOnEntry = 0;  /*  pin count on entry           */
+        int             PinCountdelta = 0;    /*  pin count delta increase     */
+
+
 #ifdef USE_PREFETCH
+
+	buf_id = -1;
 	Assert(RelationIsValid(reln));
 	Assert(BlockNumberIsValid(blockNum));
 
@@ -146,7 +196,12 @@ PrefetchBuffer(Relation reln, ForkNumber
 		BufferTag	newTag;		/* identity of requested block */
 		uint32		newHash;	/* hash value for newTag */
 		LWLock	   *newPartitionLock;	/* buffer partition lock for it */
-		int			buf_id;
+        int         BufStartAsyncrc = -1;  /*  retcode from BufStartAsync :
+                                                       **        0 if started successfully (which implies buffer was newly pinned )
+                                                       **       -1 if failed for some reason
+                                                       **        1+PrivateRefCount if we found desired buffer in buffer pool
+                                                       **  and we set it likewise if we find buffer in buffer pool
+                                                       */
 
 		/* create a tag so we can lookup the buffer */
 		INIT_BUFFERTAG(newTag, reln->rd_smgr->smgr_rnode.node,
@@ -158,28 +213,119 @@ PrefetchBuffer(Relation reln, ForkNumber
 
 		/* see if the block is in the buffer pool already */
 		LWLockAcquire(newPartitionLock, LW_SHARED);
-		buf_id = BufTableLookup(&newTag, newHash);
+		buf_id = (Buffer)BufTableLookup(&newTag, newHash);
+                if (buf_id >= 0) {
+                    PinCountOnEntry = PrivateRefCount[buf_id];  /*  pin count on entry               */
+                    BufStartAsyncrc = 1 + PinCountOnEntry; /* indicate this backends pin count - see above comment */
+                    PrefetchBufferRc = PREFTCHRC_BLK_ALREADY_PRESENT;       /* indicate buffer present */
+                } else {
+                    PrefetchBufferRc = 0;                                   /* indicate buffer not present */
+                }
 		LWLockRelease(newPartitionLock);
 
+     not_in_buffers:
 		/* If not in buffers, initiate prefetch */
-		if (buf_id < 0)
+		if (buf_id < 0) {
+                    /*    try using async aio_read with a buffer */
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                    BufStartAsyncrc = BufStartAsync( reln, forkNum, blockNum , strategy );
+                    if (BufStartAsyncrc < 0) {
+                            pgBufferUsage.aio_read_noblok++;
+                    }
+#else /* not USE_AIO_ATOMIC_BUILTIN_COMP_SWAP so try the alternative that does not read the block into a postgresql buffer */
 			smgrprefetch(reln->rd_smgr, forkNum, blockNum);
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+		}
 
-		/*
-		 * If the block *is* in buffers, we do nothing.  This is not really
-		 * ideal: the block might be just about to be evicted, which would be
-		 * stupid since we know we are going to need it soon.  But the only
-		 * easy answer is to bump the usage_count, which does not seem like a
-		 * great solution: when the caller does ultimately touch the block,
-		 * usage_count would get bumped again, resulting in too much
-		 * favoritism for blocks that are involved in a prefetch sequence. A
-		 * real fix would involve some additional per-buffer state, and it's
-		 * not clear that there's enough of a problem to justify that.
+        if (  (buf_id >= 0) || (BufStartAsyncrc >= 1)  ) {
+                        /* The block *is* in buffers.    */
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                        pgBufferUsage.aio_read_noneed++;
+#ifndef USE_PREFETCH_BUT_DONT_PIN_ALREADY_PRESENT /* jury is out on whether the following wins but it ought to ...  */
+                        /*
+                        ** If this backend already had pinned it,
+                        ** or another backend had banked a pin on it,
+                        ** or there is an IO in progress,
+                        ** or it is not marked valid,
+                        ** then do nothing.
+                        ** Otherwise pin it and mark the buffer's pin as banked by this backend.
+                        ** Note  -  it may or not be pinned by another backend -
+                        **          it is ok for us to bank a pin on it
+                        **          *provided* the other backend did not bank its pin.
+                        **          The reason for this is that the banked-pin indicator is global -
+                        **          it can identify at most one process.
+                        */
+                        /* pgBufferUsage.aio_read_wasted++;  overload counter - only for debugging */
+                        if (BufStartAsyncrc == 1) {            /*   not pinned by me  */
+                              /*  pgBufferUsage.aio_read_wasted++;  overload counter - only for debugging */
+                              /*  note   -   all we can say with certainty is that the buffer is not pinned by me
+                              **             we cannot be sure that it is still in buffer pool
+                              **             so must go through the entire locking and searching all over again ...
 		 */
+                            LWLockAcquire(newPartitionLock, LW_SHARED);
+                            buf_id = (Buffer)BufTableLookup(&newTag, newHash);
+                            /* If in buffers, proceed */
+                            if (buf_id >= 0) {
+                                /*  since the block is now present,
+                                **  save the current pin count to ensure final delta is calculated correctly
+                                */
+                                PinCountOnEntry = PrivateRefCount[buf_id];  /*  pin count on entry               */
+                                if ( PinCountOnEntry == 0) { /*  paranoid check it's still not pinned by me */
+                                    volatile        BufferDesc *buf_desc;
+
+                                    buf_desc = &BufferDescriptors[buf_id];        /* found buffer descriptor */
+                                    LockBufHdr(buf_desc);
+                                    if (    (buf_desc->flags & BM_VALID)           /* buffer is valid        */
+                                         && (!(buf_desc->flags & (BM_IO_IN_PROGRESS|BM_AIO_IN_PROGRESS|BM_AIO_PREFETCH_PIN_BANKED))) /* buffer is not any of ... */
+                                       ) {
+                                        buf_desc->flags |= BM_AIO_PREFETCH_PIN_BANKED; /* bank the pin for next use by this task */
+                                        /*  note  - we can call PinBuffer_Locked with the BM_AIO_PREFETCH_PIN_BANKED flag set because it is not yet pinned by me */
+                                        buf_desc->freeNext = -(this_backend_pid);       /* remember which pid banked it */
+                                        /*  pgBufferUsage.aio_read_wasted--;      overload counter - not wasted after all - only for debugging */
+
+                                        /* Make sure we will have room to remember the buffer pin */
+                                        ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
+
+                                        PinBuffer_Locked(buf_desc);
 	}
-#endif   /* USE_PREFETCH */
+                                    else {
+                                        UnlockBufHdr(buf_desc);
+                                    }
+                                }
+                            }
+                            LWLockRelease(newPartitionLock);
+                            /*  although unlikely,  maybe it was evicted while we were puttering about  */
+                            if (buf_id < 0) {
+                                pgBufferUsage.aio_read_noneed--;   /*   back out the accounting */
+                                goto not_in_buffers;               /*   and try again           */
+                            }
+                        }
+#endif /*  USE_PREFETCH_BUT_DONT_PIN_ALREADY_PRESENT */
+
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+		}
+
+		if (buf_id >= 0) {
+			PinCountdelta = PrivateRefCount[buf_id] - PinCountOnEntry;  /*  pin count delta increase    */
+			if (  (PinCountdelta < 0) || (PinCountdelta > 1) ) {
+				  elog(ERROR,
+						 "PrefetchBuffer #%d : incremented pin count by %d on bufdesc %p refcount %u localpins %d\n"
+								  ,(buf_id+1) , PinCountdelta , &BufferDescriptors[buf_id] ,BufferDescriptors[buf_id].refcount , PrivateRefCount[buf_id]);
 }
+		} else
+		if (BufStartAsyncrc == 0) {  /* aio started successfully (which implies buffer was newly pinned ) */
+			PinCountdelta = 1;
+		}
+
+		/*  set final PrefetchBufferRc according to previous value */
+		PrefetchBufferRc |= PinCountdelta;  /* set the PREFTCHRC_BUF_PIN_INCREASED bit */
+	}
+
+#endif   /* USE_PREFETCH */
 
+	return PrefetchBufferRc; /*  return value as described above */
+}
 
 /*
  * ReadBuffer -- a shorthand for ReadBufferExtended, for reading from main
@@ -252,7 +398,7 @@ ReadBufferExtended(Relation reln, ForkNu
 	 */
 	pgstat_count_buffer_read(reln);
 	buf = ReadBuffer_common(reln->rd_smgr, reln->rd_rel->relpersistence,
-							forkNum, blockNum, mode, strategy, &hit);
+							forkNum, blockNum, mode, strategy, &hit , 0);
 	if (hit)
 		pgstat_count_buffer_hit(reln);
 	return buf;
@@ -280,7 +426,7 @@ ReadBufferWithoutRelcache(RelFileNode rn
 	Assert(InRecovery);
 
 	return ReadBuffer_common(smgr, RELPERSISTENCE_PERMANENT, forkNum, blockNum,
-							 mode, strategy, &hit);
+							 mode, strategy, &hit , 0);
 }
 
 
@@ -288,15 +434,18 @@ ReadBufferWithoutRelcache(RelFileNode rn
  * ReadBuffer_common -- common logic for all ReadBuffer variants
  *
  * *hit is set to true if the request was satisfied from shared buffer cache.
+ * index_for_aio ,  if -ve , is negative of ( index of the aiocb in the BufferAiocbs array + 3 )
+ *     which is passed through to StartBufferIO
  */
-static Buffer
+Buffer
 ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 				  BlockNumber blockNum, ReadBufferMode mode,
-				  BufferAccessStrategy strategy, bool *hit)
+				  BufferAccessStrategy strategy, bool *hit , int index_for_aio )
 {
 	volatile BufferDesc *bufHdr;
 	Block		bufBlock;
 	bool		found;
+        int             allocrc;             /*  retcode from BufferAlloc */
 	bool		isExtend;
 	bool		isLocalBuf = SmgrIsTemp(smgr);
 
@@ -328,16 +477,40 @@ ReadBuffer_common(SMgrRelation smgr, cha
 	}
 	else
 	{
+                allocrc = mode; /* pass mode to BufferAlloc since it must not wait for async io if RBM_NOREAD_FOR_PREFETCH */
 		/*
 		 * lookup the buffer.  IO_IN_PROGRESS is set if the requested block is
 		 * not currently in memory.
 		 */
 		bufHdr = BufferAlloc(smgr, relpersistence, forkNum, blockNum,
-							 strategy, &found);
-		if (found)
-			pgBufferUsage.shared_blks_hit++;
+							 strategy, &allocrc , index_for_aio );
+		if (allocrc < 0) {
+                    if (mode == RBM_ZERO_ON_ERROR || zero_damaged_pages)
+                    {
+                        ereport(WARNING,
+                                (errcode(ERRCODE_DATA_CORRUPTED),
+                                 errmsg("invalid page header in block %u of relation %s; zeroing out page",
+                                        blockNum,
+                                        relpath(smgr->smgr_rnode, forkNum))));
+                        bufBlock = BufHdrGetBlock(bufHdr);
+                        MemSet((char *) bufBlock, 0, BLCKSZ);
+                    }
 		else
+                      ereport(ERROR,
+                              (errcode(ERRCODE_DATA_CORRUPTED),
+                               errmsg("invalid page header in block %u of relation %s",
+                                      blockNum,
+                                      relpath(smgr->smgr_rnode, forkNum))));
+                        found = true;
+                }
+		else if (allocrc > 0) {
+			pgBufferUsage.shared_blks_hit++;
+                        found = true;
+                }
+		else {
 			pgBufferUsage.shared_blks_read++;
+                        found = false;
+                }
 	}
 
 	/* At this point we do NOT hold any locks. */
@@ -410,7 +583,7 @@ ReadBuffer_common(SMgrRelation smgr, cha
 				Assert(bufHdr->flags & BM_VALID);
 				bufHdr->flags &= ~BM_VALID;
 				UnlockBufHdr(bufHdr);
-			} while (!StartBufferIO(bufHdr, true));
+			} while (!StartBufferIO(bufHdr, true, 0));
 		}
 	}
 
@@ -430,6 +603,7 @@ ReadBuffer_common(SMgrRelation smgr, cha
 
 	bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);
 
+        if (mode != RBM_NOREAD_FOR_PREFETCH) {
 	if (isExtend)
 	{
 		/* new buffers are zero-filled */
@@ -499,6 +673,7 @@ ReadBuffer_common(SMgrRelation smgr, cha
 	VacuumPageMiss++;
 	if (VacuumCostActive)
 		VacuumCostBalance += VacuumCostPageMiss;
+	}
 
 	TRACE_POSTGRESQL_BUFFER_READ_DONE(forkNum, blockNum,
 									  smgr->smgr_rnode.node.spcNode,
@@ -520,21 +695,39 @@ ReadBuffer_common(SMgrRelation smgr, cha
  * the default strategy.  The selected buffer's usage_count is advanced when
  * using the default strategy, but otherwise possibly not (see PinBuffer).
  *
- * The returned buffer is pinned and is already marked as holding the
- * desired page.  If it already did have the desired page, *foundPtr is
- * set TRUE.  Otherwise, *foundPtr is set FALSE and the buffer is marked
+ * index_for_aio is an input parm which,  if non-zero,  identifies a BufferAiocb
+ * acquired by caller and to be used for any StartBufferIO performed by this routine.
+ * In this case,  if block not found in buffer pool and we allocate a new buffer,
+ * then we must maintain the spinlock on the buffer and pass it back to caller.
+ *
+ * foundPtr is input and output :
+ *  . input   -  indicates the read-buffer mode  ( see bufmgr.h )
+ *  . output  -  indicates the status of the buffer - see below
+ *
+ * Except for the case of RBM_NOREAD_FOR_PREFETCH and buffer is found,
+ * the returned buffer is pinned and is already marked as holding the
+ * desired page.
+ *  If it already did have the desired page and page content is valid,
+ *  *foundPtr is set to 1
+ *  If it already did have the desired page and mode is RBM_NOREAD_FOR_PREFETCH
+ *    and StartBufferIO returned false
+ *    (meaning it could not initialise the buffer for aio)
+ *  *foundPtr is set to 2
+ *  If it already did have the desired page but page content is invalid,
+ *  *foundPtr is set to -1
+ *   this can happen only if the buffer was read by an async read
+ *   and the aio is still in progress or pinned by the issuer of the startaio.
+ *  Otherwise, *foundPtr is set to 0 and the buffer is marked
  * as IO_IN_PROGRESS; ReadBuffer will now need to do I/O to fill it.
  *
- * *foundPtr is actually redundant with the buffer's BM_VALID flag, but
- * we keep it for simplicity in ReadBuffer.
- *
- * No locks are held either at entry or exit.
+ * No locks are held either at entry or exit EXCEPT for case noted above
+ * of passing an empty buffer back to async io caller ( index_for_aio set ).
  */
 static volatile BufferDesc *
 BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 			BlockNumber blockNum,
 			BufferAccessStrategy strategy,
-			bool *foundPtr)
+			int *foundPtr , int index_for_aio )
 {
 	BufferTag	newTag;			/* identity of requested block */
 	uint32		newHash;		/* hash value for newTag */
@@ -546,6 +739,13 @@ BufferAlloc(SMgrRelation smgr, char relp
 	int			buf_id;
 	volatile BufferDesc *buf;
 	bool		valid;
+        int             IntentionBufferrc;      /* retcode from BufCheckAsync */
+        bool            StartBufferIOrc;        /* retcode from StartBufferIO */
+        ReadBufferMode mode;
+
+
+        mode = *foundPtr;
+        *foundPtr = 0;
 
 	/* create a tag so we can lookup the buffer */
 	INIT_BUFFERTAG(newTag, smgr->smgr_rnode.node, forkNum, blockNum);
@@ -560,21 +760,53 @@ BufferAlloc(SMgrRelation smgr, char relp
 	if (buf_id >= 0)
 	{
 		/*
-		 * Found it.  Now, pin the buffer so no one can steal it from the
-		 * buffer pool, and check to see if the correct data has been loaded
-		 * into the buffer.
+		 * Found it.
 		 */
+                *foundPtr = 1;
 		buf = &BufferDescriptors[buf_id];
 
-		valid = PinBuffer(buf, strategy);
-
-		/* Can release the mapping lock as soon as we've pinned it */
+                /*   If prefetch mode,  then return immediately indicating found,
+                **   and NOTE in this case only,  we did not pin buffer.
+                **   In theory we might try to check whether the buffer is valid,  io in progress,  etc
+                **   but in practice it is simpler to abandon the prefetch if the buffer exists
+                */
+                if (mode == RBM_NOREAD_FOR_PREFETCH) {
+                    /* release the mapping lock and return    */
 		LWLockRelease(newPartitionLock);
+                } else {
+                    /*   note that the current request is for same tag as the one associated with the aio -
+                    **   so simply complete the aio and we have our buffer.
+                    **         If an aio was started on this buffer,
+                    **         check complete and wait for it if not.
+                    **         And,  if aio had been started,  then the task
+                    **         which issued the start aio already pinned it for this read,
+                    **         so if that task was me and the aio was successful,
+                    **         pass the current pin to this read without dropping and re-acquiring.
+                    **         this is all done by BufCheckAsync
+                    */
+                    IntentionBufferrc = BufCheckAsync(smgr , 0 , buf, BUF_INTENTION_WANT , strategy , index_for_aio , false , newPartitionLock );
 
-		*foundPtr = TRUE;
+                    /*       check to see if the correct data has been loaded into the buffer.  */
+                    valid = (IntentionBufferrc == BUF_INTENT_RC_VALID);
 
-		if (!valid)
-		{
+                    /*  check for serious IO errors   */
+                    if (!valid) {
+                        if (    (IntentionBufferrc != BUF_INTENT_RC_INVALID_NO_AIO)
+                             && (IntentionBufferrc != BUF_INTENT_RC_INVALID_AIO)
+                           ) {
+                            *foundPtr = -1;  /*  inform caller of serious error */
+                        }
+                        else
+                        if (IntentionBufferrc == BUF_INTENT_RC_INVALID_AIO) {
+                            goto proceed_with_not_found;  /*  yes,  I know,  a goto ... think of it as a break out of the if */
+                        }
+                     }
+
+                    /* BufCheckAsync pinned the buffer            */
+                    /* so can now release the mapping lock               */
+                    LWLockRelease(newPartitionLock);
+
+                    if (!valid) {
 			/*
 			 * We can only get here if (a) someone else is still reading in
 			 * the page, or (b) a previous read attempt failed.  We have to
@@ -582,19 +814,21 @@ BufferAlloc(SMgrRelation smgr, char relp
 			 * own read attempt if the page is still not BM_VALID.
 			 * StartBufferIO does it all.
 			 */
-			if (StartBufferIO(buf, true))
+                                if (StartBufferIO(buf, true, index_for_aio))
 			{
 				/*
 				 * If we get here, previous attempts to read the buffer must
 				 * have failed ... but we shall bravely try again.
 				 */
-				*foundPtr = FALSE;
+                                        *foundPtr = 0;
+                                }
 			}
 		}
 
 		return buf;
 	}
 
+  proceed_with_not_found:
 	/*
 	 * Didn't find it in the buffer pool.  We'll have to initialize a new
 	 * buffer.  Remember to unlock the mapping lock while doing the work.
@@ -619,8 +853,10 @@ BufferAlloc(SMgrRelation smgr, char relp
 		/* Must copy buffer flags while we still hold the spinlock */
 		oldFlags = buf->flags;
 
-		/* Pin the buffer and then release the buffer spinlock */
-		PinBuffer_Locked(buf);
+                /*         If an aio was started on this buffer,
+                **         check complete and cancel it if not.
+                */
+                BufCheckAsync(smgr , 0 , buf, BUF_INTENTION_REJECT_OBTAIN_PIN , 0 , index_for_aio, true , 0 );
 
 		/* Now it's safe to release the freelist lock */
 		if (lock_held)
@@ -791,13 +1027,18 @@ BufferAlloc(SMgrRelation smgr, char relp
 				 * then set up our own read attempt if the page is still not
 				 * BM_VALID.  StartBufferIO does it all.
 				 */
-				if (StartBufferIO(buf, true))
+                                StartBufferIOrc = StartBufferIO(buf, true , index_for_aio); /* retcode from StartBufferIO */
+				if (StartBufferIOrc)
 				{
 					/*
 					 * If we get here, previous attempts to read the buffer
 					 * must have failed ... but we shall bravely try again.
 					 */
-					*foundPtr = FALSE;
+					*foundPtr = 0;
+                                } else
+                                if (mode == RBM_NOREAD_FOR_PREFETCH) {
+					UnpinBuffer(buf, true); /* must unpin if RBM_NOREAD_FOR_PREFETCH and found */
+                                        *foundPtr = 2;  /*  inform BufStartAsync that buffer must not be used */
 				}
 			}
 
@@ -860,10 +1101,17 @@ BufferAlloc(SMgrRelation smgr, char relp
 	 * lock.  If StartBufferIO returns false, then someone else managed to
 	 * read it before we did, so there's nothing left for BufferAlloc() to do.
 	 */
-	if (StartBufferIO(buf, true))
-		*foundPtr = FALSE;
-	else
-		*foundPtr = TRUE;
+        StartBufferIOrc = StartBufferIO(buf, true , index_for_aio); /* retcode from StartBufferIO */
+        if (StartBufferIOrc) {
+		*foundPtr = 0;
+        } else {
+                if (mode == RBM_NOREAD_FOR_PREFETCH) {
+                    UnpinBuffer(buf, true); /* must unpin if RBM_NOREAD_FOR_PREFETCH and found */
+                    *foundPtr = 2;  /*  inform BufStartAsync that buffer must not be used */
+                } else {
+                    *foundPtr = 1;
+                }
+        }
 
 	return buf;
 }
@@ -970,6 +1218,10 @@ retry:
 	/*
 	 * Insert the buffer at the head of the list of free buffers.
 	 */
+        /*   avoid confusing freelist with strange-looking freeNext */
+        if (buf->freeNext <= FREENEXT_BAIOCB_ORIGIN) { /* means was used for aiocb index */
+            buf->freeNext = FREENEXT_NOT_IN_LIST;
+        }
 	StrategyFreeBuffer(buf);
 }
 
@@ -1022,6 +1274,56 @@ MarkBufferDirty(Buffer buffer)
 	UnlockBufHdr(bufHdr);
 }
 
+/*  return the blocknum of the block in a buffer if it is valid
+**  if a shared buffer,  it must be pinned
+*/
+BlockNumber
+BlocknumOfBuffer(Buffer buffer)
+{
+	volatile BufferDesc *bufHdr;
+        BlockNumber rc = 0;
+
+	if (BufferIsValid(buffer)) {
+            if  (BufferIsLocal(buffer)) {
+                bufHdr = &LocalBufferDescriptors[-buffer - 1];
+            } else {
+                bufHdr = &BufferDescriptors[buffer - 1];
+            }
+
+            rc = bufHdr->tag.blockNum;
+        }
+
+        return rc;
+}
+
+/*  report whether specified buffer contains same or different block
+**  if a shared buffer,  it must be pinned
+*/
+bool
+BlocknotinBuffer(Buffer buffer,
+					 Relation relation,
+					 BlockNumber blockNum)
+{
+	volatile BufferDesc *bufHdr;
+        bool rc = false;
+
+	if (BufferIsValid(buffer)) {
+            if  (BufferIsLocal(buffer)) {
+                bufHdr = &LocalBufferDescriptors[-buffer - 1];
+            } else {
+                bufHdr = &BufferDescriptors[buffer - 1];
+            }
+
+            rc =
+                  (   (bufHdr->tag.blockNum != blockNum)
+                   || (!(RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) ))
+                   || (bufHdr->tag.forkNum != MAIN_FORKNUM)
+                  );
+        }
+
+        return rc;
+}
+
 /*
  * ReleaseAndReadBuffer -- combine ReleaseBuffer() and ReadBuffer()
  *
@@ -1040,18 +1342,18 @@ ReleaseAndReadBuffer(Buffer buffer,
 					 Relation relation,
 					 BlockNumber blockNum)
 {
-	ForkNumber	forkNum = MAIN_FORKNUM;
 	volatile BufferDesc *bufHdr;
+        bool isDifferentBlock;   /*       requesting different block from that already in buffer ? */
 
 	if (BufferIsValid(buffer))
 	{
+	    /* if a shared buff, we have pin, so it's ok to examine tag without spinlock */
+            isDifferentBlock = BlocknotinBuffer(buffer,relation,blockNum); /*  requesting different block from that already in buffer ? */
 		if (BufferIsLocal(buffer))
 		{
 			Assert(LocalRefCount[-buffer - 1] > 0);
 			bufHdr = &LocalBufferDescriptors[-buffer - 1];
-			if (bufHdr->tag.blockNum == blockNum &&
-				RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
-				bufHdr->tag.forkNum == forkNum)
+			if (!isDifferentBlock)
 				return buffer;
 			ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
 			LocalRefCount[-buffer - 1]--;
@@ -1060,12 +1362,12 @@ ReleaseAndReadBuffer(Buffer buffer,
 		{
 			Assert(PrivateRefCount[buffer - 1] > 0);
 			bufHdr = &BufferDescriptors[buffer - 1];
-			/* we have pin, so it's ok to examine tag without spinlock */
-			if (bufHdr->tag.blockNum == blockNum &&
-				RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
-				bufHdr->tag.forkNum == forkNum)
+                        BufCheckAsync(0 , relation , bufHdr , ( isDifferentBlock ? BUF_INTENTION_REJECT_FORGET
+                                                                                 : BUF_INTENTION_REJECT_KEEP_PIN )
+                                                            , 0 , 0 , false , 0 ); /* end any IO and maybe unpin */
+			if (!isDifferentBlock) {
 				return buffer;
-			UnpinBuffer(bufHdr, true);
+                        }
 		}
 	}
 
@@ -1090,11 +1392,12 @@ ReleaseAndReadBuffer(Buffer buffer,
  * Returns TRUE if buffer is BM_VALID, else FALSE.  This provision allows
  * some callers to avoid an extra spinlock cycle.
  */
-static bool
+bool
 PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy)
 {
 	int			b = buf->buf_id;
 	bool		result;
+        bool       pin_already_banked_by_me = 0;  /* buffer is already pinned by me and redeemable */
 
 	if (PrivateRefCount[b] == 0)
 	{
@@ -1116,12 +1419,34 @@ PinBuffer(volatile BufferDesc *buf, Buff
 	else
 	{
 		/* If we previously pinned the buffer, it must surely be valid */
+                /* Errr  -   is that really true  ???    I don't think so  :
+                ** what if I pin,  try an IO,  in progress,  then mistakenly pin again
 		result = true;
+                */
+		LockBufHdr(buf);
+                pin_already_banked_by_me = (    (PrivateRefCount[b] > 0) && (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                                             && (   (    (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                                                     : (-(buf->freeNext))  )  == this_backend_pid )
+                                           );
+                if (pin_already_banked_by_me) {
+                        elog(LOG, "PinBuffer : on buffer %d with banked pin rel=%s, blockNum=%u, with flags %X refcount=%u"
+                       ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                       ,buf->tag.blockNum, buf->flags, buf->refcount);
+                    buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;
+                    if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                        buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
 	}
+                }
+		result = (buf->flags & BM_VALID) != 0;
+		UnlockBufHdr(buf);
+	}
+
+        if (!pin_already_banked_by_me) {
 	PrivateRefCount[b]++;
 	Assert(PrivateRefCount[b] > 0);
 	ResourceOwnerRememberBuffer(CurrentResourceOwner,
 								BufferDescriptorGetBuffer(buf));
+        }
 	return result;
 }
 
@@ -1138,19 +1463,36 @@ PinBuffer(volatile BufferDesc *buf, Buff
  * to save a spin lock/unlock cycle, because we need to pin a buffer before
  * its state can change under us.
  */
-static void
+void
 PinBuffer_Locked(volatile BufferDesc *buf)
 {
 	int			b = buf->buf_id;
+        bool       pin_already_banked_by_me;  /* buffer is already pinned by me and redeemable */
 
-	if (PrivateRefCount[b] == 0)
+        pin_already_banked_by_me = (    (PrivateRefCount[b] > 0) && (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                                     && (   (    (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                                             : (-(buf->freeNext))  )  == this_backend_pid )
+                             );
+	if (PrivateRefCount[b] == 0) {
 		buf->refcount++;
+        }
+        if (pin_already_banked_by_me) {
+                        elog(LOG, "PinBuffer_Locked : on buffer %d with banked pin rel=%s, blockNum=%u, with flags %X refcount=%u"
+                       ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                       ,buf->tag.blockNum, buf->flags, buf->refcount);
+            buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;
+                    if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                        buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+                    }
+        }
 	UnlockBufHdr(buf);
+        if (!pin_already_banked_by_me) {
 	PrivateRefCount[b]++;
 	Assert(PrivateRefCount[b] > 0);
 	ResourceOwnerRememberBuffer(CurrentResourceOwner,
 								BufferDescriptorGetBuffer(buf));
 }
+}
 
 /*
  * UnpinBuffer -- make buffer available for replacement.
@@ -1160,29 +1502,68 @@ PinBuffer_Locked(volatile BufferDesc *bu
  * Most but not all callers want CurrentResourceOwner to be adjusted.
  * Those that don't should pass fixOwner = FALSE.
  */
-static void
+void
 UnpinBuffer(volatile BufferDesc *buf, bool fixOwner)
 {
+
 	int			b = buf->buf_id;
+        bool       pin_already_banked_by_me;  /* buffer is already pinned by me and redeemable */
 
-	if (fixOwner)
+	if (fixOwner) {
 		ResourceOwnerForgetBuffer(CurrentResourceOwner,
 								  BufferDescriptorGetBuffer(buf));
+        }
 
 	Assert(PrivateRefCount[b] > 0);
 	PrivateRefCount[b]--;
 	if (PrivateRefCount[b] == 0)
 	{
+
 		/* I'd better not still hold any locks on the buffer */
 		Assert(!LWLockHeldByMe(buf->content_lock));
 		Assert(!LWLockHeldByMe(buf->io_in_progress_lock));
 
 		LockBufHdr(buf);
 
+		/* this backend has released last pin - buffer should not have pin banked by me
+                ** and if AIO in progress then there should be another backend pin
+                */
+                pin_already_banked_by_me = (       (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                                             &&    (   (    (buf->flags & BM_AIO_IN_PROGRESS)
+                                                         ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                         : (-(buf->freeNext))
+                                                       )  == this_backend_pid )
+                                           );
+                if (pin_already_banked_by_me) {
+                        /*  this is a strange situation  -  caller had a banked pin (which callers are supposed not to know about)
+                        **                                  but either discovered it had it or has over-counted how many pins it has
+                        */
+                        buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;   /*   redeem the pin although it is now of no use since about to release */
+                        if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                            buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+                        }
+
+                        /*     temporarily suppress logging error to avoid performance degradation -
+                        **     either this task really does not need the buffer in which case the error is harmless
+                        **     or a more severe error will be detected later (possibly immediately below)
+                        elog(LOG, "UnpinBuffer :  released last this-backend pin on buffer %d rel=%s, blockNum=%u, but had banked pin flags %X refcount=%u"
+                       ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                       ,buf->tag.blockNum, buf->flags, buf->refcount);
+                        */
+                }
+
 		/* Decrement the shared reference count */
 		Assert(buf->refcount > 0);
 		buf->refcount--;
 
+                if (   (buf->refcount == 0) && (buf->flags & BM_AIO_IN_PROGRESS)  ) {
+
+                        elog(ERROR, "UnpinBuffer :  released last any-backend pin on buffer %d rel=%s, blockNum=%u, but AIO in progress flags %X refcount=%u"
+                            ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                            ,buf->tag.blockNum, buf->flags, buf->refcount);
+                }
+
+
 		/* Support LockBufferForCleanup() */
 		if ((buf->flags & BM_PIN_COUNT_WAITER) &&
 			buf->refcount == 1)
@@ -1657,6 +2038,7 @@ SyncOneBuffer(int buf_id, bool skip_rece
 	volatile BufferDesc *bufHdr = &BufferDescriptors[buf_id];
 	int			result = 0;
 
+
 	/*
 	 * Check whether buffer needs writing.
 	 *
@@ -1789,6 +2171,8 @@ PrintBufferLeakWarning(Buffer buffer)
 	char	   *path;
 	BackendId	backend;
 
+
+
 	Assert(BufferIsValid(buffer));
 	if (BufferIsLocal(buffer))
 	{
@@ -1799,12 +2183,28 @@ PrintBufferLeakWarning(Buffer buffer)
 	else
 	{
 		buf = &BufferDescriptors[buffer - 1];
+#ifdef USE_PREFETCH
+                /*   If reason that this buffer is pinned
+                **   is that it was prefetched with async_io
+                **   and never read or discarded, then omit the
+                **   warning,  because this is expected in some
+                **   cases when a scan is closed abnormally.
+                **   Note that the buffer will be released soon by our caller.
+                */
+                if (buf->flags & BM_AIO_PREFETCH_PIN_BANKED) {
+                    pgBufferUsage.aio_read_forgot++; /* account for it */
+                    return;
+                }
+#endif /*  USE_PREFETCH */
 		loccount = PrivateRefCount[buffer - 1];
 		backend = InvalidBackendId;
 	}
 
+/* #if defined(DO_ISCAN_DISCARD_BUFFER) && defined(DO_HEAPSCAN_DISCARD_BUFFER) not yet working safely 130530 */
 	/* theoretically we should lock the bufhdr here */
 	path = relpathbackend(buf->tag.rnode, backend, buf->tag.forkNum);
+
+
 	elog(WARNING,
 		 "buffer refcount leak: [%03d] "
 		 "(rel=%s, blockNum=%u, flags=0x%x, refcount=%u %d)",
@@ -1812,6 +2212,7 @@ PrintBufferLeakWarning(Buffer buffer)
 		 buf->tag.blockNum, buf->flags,
 		 buf->refcount, loccount);
 	pfree(path);
+/* #endif defined(DO_ISCAN_DISCARD_BUFFER) && defined(DO_HEAPSCAN_DISCARD_BUFFER) not yet working safely 130530 */
 }
 
 /*
@@ -1928,7 +2329,7 @@ FlushBuffer(volatile BufferDesc *buf, SM
 	 * false, then someone else flushed the buffer before we could, so we need
 	 * not do anything.
 	 */
-	if (!StartBufferIO(buf, false))
+	if (!StartBufferIO(buf, false, 0))
 		return;
 
 	/* Setup error traceback support for ereport() */
@@ -2512,6 +2913,70 @@ FlushDatabaseBuffers(Oid dbid)
 	}
 }
 
+#ifdef USE_PREFETCH
+/*
+ * DiscardBuffer -- discard shared buffer used for a previously
+ *                  prefetched but unread block of a relation
+ *
+ * If the buffer is found and pinned with a banked pin,  then :
+ *      .  if AIO in progress, terminate AIO without waiting
+ *      .  if AIO had already completed successfully,
+ *         then mark buffer valid (in case someone else wants it)
+ *      .  redeem the banked pin and unpin it.
+ *
+ * This function is similar in purpose to ReleaseBuffer (below)
+ * but sufficiently different that it is a separate function.
+ * Two important differences are :
+ *   .   caller identifies buffer by blocknumber,  not buffer number
+ *   .   we unpin buffer *only* if the pin is banked,
+ *                      *never* if pinned but not banked.
+ *       This is essential as caller may perform a sequence of
+ *  SCAN1   . PrefetchBuffer   (and remember block was prefetched)
+ *  SCAN2   . ReadBuffer       (but fails to connect this read to the prefetch by SCAN1)
+ *  SCAN1   . DiscardBuffer    (SCAN1 terminates early)
+ *  SCAN2   . access tuples in buffer
+ *        Clearly the Discard *must not* unpin the buffer since SCAN2 needs it!
+ *
+ *
+ * caller may pass InvalidBlockNumber as blockNum to mean do nothing
+ */
+void
+DiscardBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
+{
+        BufferTag	newTag;		   /* identity of requested block */
+        uint32		newHash;	   /* hash value for newTag */
+        LWLockId	newPartitionLock;  /* buffer partition lock for it */
+        Buffer		buf_id;
+        volatile        BufferDesc *buf_desc;
+
+    if (!SmgrIsTemp(reln->rd_smgr)) {
+	Assert(RelationIsValid(reln));
+	if (BlockNumberIsValid(blockNum)) {
+
+            /* create a tag so we can lookup the buffer */
+            INIT_BUFFERTAG(newTag, reln->rd_smgr->smgr_rnode.node,
+                                       forkNum, blockNum);
+
+            /* determine its hash code and partition lock ID */
+            newHash = BufTableHashCode(&newTag);
+            newPartitionLock = BufMappingPartitionLock(newHash);
+
+            /* see if the block is in the buffer pool already */
+            LWLockAcquire(newPartitionLock, LW_SHARED);
+            buf_id = BufTableLookup(&newTag, newHash);
+            LWLockRelease(newPartitionLock);
+
+            /* If in buffers, proceed */
+            if (buf_id >= 0) {
+                buf_desc = &BufferDescriptors[buf_id];        /* found buffer descriptor */
+                BufCheckAsync(0 , reln, buf_desc , BUF_INTENTION_REJECT_UNBANK , 0 , 0 , false , 0); /* end the IO and unpin if banked */
+                pgBufferUsage.aio_read_discrd++; /* account for it */
+            }
+        }
+    }
+}
+#endif   /* USE_PREFETCH */
+
 /*
  * ReleaseBuffer -- release the pin on a buffer
  */
@@ -2520,26 +2985,23 @@ ReleaseBuffer(Buffer buffer)
 {
 	volatile BufferDesc *bufHdr;
 
+
 	if (!BufferIsValid(buffer))
 		elog(ERROR, "bad buffer ID: %d", buffer);
 
-	ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
 
 	if (BufferIsLocal(buffer))
 	{
+                ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
 		Assert(LocalRefCount[-buffer - 1] > 0);
 		LocalRefCount[-buffer - 1]--;
 		return;
 	}
-
-	bufHdr = &BufferDescriptors[buffer - 1];
-
-	Assert(PrivateRefCount[buffer - 1] > 0);
-
-	if (PrivateRefCount[buffer - 1] > 1)
-		PrivateRefCount[buffer - 1]--;
 	else
-		UnpinBuffer(bufHdr, false);
+        {
+                bufHdr = &BufferDescriptors[buffer - 1];
+                BufCheckAsync(0 , 0 , bufHdr , BUF_INTENTION_REJECT_NOADJUST , 0 , 0 , false , 0 );
+        }
 }
 
 /*
@@ -2565,14 +3027,41 @@ UnlockReleaseBuffer(Buffer buffer)
 void
 IncrBufferRefCount(Buffer buffer)
 {
+        bool       pin_already_banked_by_me = false;  /* buffer is already pinned by me and redeemable */
+        volatile BufferDesc *buf;                     /* descriptor for a shared buffer */
+
 	Assert(BufferIsPinned(buffer));
+
+        if (!(BufferIsLocal(buffer))) {
+                buf = &BufferDescriptors[buffer - 1];
+		LockBufHdr(buf);
+                pin_already_banked_by_me =
+                      (    (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                        && (   (    (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                                      : (-(buf->freeNext))  )  == this_backend_pid )
+                      );
+        }
+
+        if (!pin_already_banked_by_me) {
 	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
 	ResourceOwnerRememberBuffer(CurrentResourceOwner, buffer);
+        }
+
 	if (BufferIsLocal(buffer))
 		LocalRefCount[-buffer - 1]++;
-	else
+	else {
+                if (pin_already_banked_by_me) {
+                    buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;
+                    if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                        buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+                    }
+                }
+		UnlockBufHdr(buf);
+                if (!pin_already_banked_by_me) {
 		PrivateRefCount[buffer - 1]++;
 }
+        }
+}
 
 /*
  * MarkBufferDirtyHint
@@ -2994,61 +3483,138 @@ WaitIO(volatile BufferDesc *buf)
  *
  * In some scenarios there are race conditions in which multiple backends
  * could attempt the same I/O operation concurrently.  If someone else
- * has already started I/O on this buffer then we will block on the
+ * has already started synchronous I/O on this buffer then we will block on the
  * io_in_progress lock until he's done.
  *
+ * if an async io is in progress and we are doing synchronous io,
+ * then readbuffer uses call to smgrcompleteaio to wait,
+ * and so we treat this request as if no io in progress
+ *
  * Input operations are only attempted on buffers that are not BM_VALID,
  * and output operations only on buffers that are BM_VALID and BM_DIRTY,
  * so we can always tell if the work is already done.
  *
+ * index_for_aio is an input parm which,  if non-zero,  identifies a BufferAiocb
+ * acquired by caller and to be attached to the buffer header for use with async io
+ *
  * Returns TRUE if we successfully marked the buffer as I/O busy,
  * FALSE if someone else already did the work.
  */
 static bool
-StartBufferIO(volatile BufferDesc *buf, bool forInput)
+StartBufferIO(volatile BufferDesc *buf, bool forInput , int index_for_aio )
 {
+#ifdef USE_PREFETCH
+        struct BufferAiocb volatile * BAiocb = (struct BufferAiocb*)0;  /*  BufferAiocb for use with aio */
+#endif   /* USE_PREFETCH */
+ 
+        if (!index_for_aio)
 	Assert(!InProgressBuf);
 
 	for (;;)
 	{
+                if (!index_for_aio) {
 		/*
 		 * Grab the io_in_progress lock so that other processes can wait for
 		 * me to finish the I/O.
 		 */
 		LWLockAcquire(buf->io_in_progress_lock, LW_EXCLUSIVE);
+                }
 
 		LockBufHdr(buf);
 
-		if (!(buf->flags & BM_IO_IN_PROGRESS))
+                /*     the following test is intended to distinguish between :
+                **      .   buffer which : 
+                **           .     has io in progress
+                **             AND is not associated with a current aio
+                **      .   not the above
+                **     Here,  "recent" means an aio marked by buf->freeNext <= FREENEXT_BAIOCB_ORIGIN but no longer in progress -
+                **          this situation arises when the aio has just been cancelled and this process now wishes to recycle the buffer.
+                **          In this case,  the first such would-be recycler (i.e. me) must :
+                **             . avoid waiting for the cancelled aio to complete
+                **             . if not myself doing async read, then assume responsibility for posting other future readbuffers.
+                */
+		if (    (buf->flags & BM_AIO_IN_PROGRESS)
+                     || (!(buf->flags & BM_IO_IN_PROGRESS))
+                   )
 			break;
 
 		/*
-		 * The only way BM_IO_IN_PROGRESS could be set when the io_in_progress
+		 * The only way BM_IO_IN_PROGRESS without AIO in progress could be set when the io_in_progress
 		 * lock isn't held is if the process doing the I/O is recovering from
 		 * an error (see AbortBufferIO).  If that's the case, we must wait for
 		 * him to get unwedged.
 		 */
 		UnlockBufHdr(buf);
+                if (!index_for_aio) {
 		LWLockRelease(buf->io_in_progress_lock);
+                }
 		WaitIO(buf);
 	}
 
-	/* Once we get here, there is definitely no I/O active on this buffer */
-
+#ifdef USE_PREFETCH
+	/* Once we get here, there is definitely no synchronous I/O active on this buffer
+        ** but if being asked to attach a BufferAiocb to the buf header,
+        ** then we must also check if there is any async io currently
+        ** in progress or pinned started by a different task.
+        */
+        if (index_for_aio) {
+            BAiocb = (BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext);
+            if (    (buf->flags & (BM_AIO_IN_PROGRESS|BM_AIO_PREFETCH_PIN_BANKED))
+                 && (buf->freeNext <= FREENEXT_BAIOCB_ORIGIN)
+                 && (BAiocb->pidOfAio != this_backend_pid)
+               ) {
+                    /* someone else already doing async I/O */
+                    UnlockBufHdr(buf);
+                    return false;
+            }
+	}
+#endif   /* USE_PREFETCH */
 	if (forInput ? (buf->flags & BM_VALID) : !(buf->flags & BM_DIRTY))
 	{
 		/* someone else already did the I/O */
 		UnlockBufHdr(buf);
+                if (!index_for_aio) {
 		LWLockRelease(buf->io_in_progress_lock);
+                }
 		return false;
 	}
 
 	buf->flags |= BM_IO_IN_PROGRESS;
 
+#ifdef USE_PREFETCH
+        if (index_for_aio) {
+            BAiocb = (BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - index_for_aio);
+            /*   insist that no other buffer is using this BufferAiocb for async IO */
+            if (BAiocb->BAiocbbufh == (struct sbufdesc *)0) {
+                BAiocb->BAiocbbufh = buf;
+            }
+            if (BAiocb->BAiocbbufh != buf) {
+                               ereport(ERROR,
+                                    (errcode(ERRCODE_INTERNAL_ERROR),
+                                     errmsg("AIO control block %p to be used by %p already in use by %p"
+                                              ,BAiocb ,buf , BAiocb->BAiocbbufh)));
+            }
+            /*   note - there is no need to register self as an dependent of BAiocb
+            **   as we shall not unlock buf_desc before we free the BAiocb
+            */
+
+            buf->flags |= BM_AIO_IN_PROGRESS;
+            buf->freeNext = index_for_aio;
+            /*  at this point,  this buffer appears to have an in-progress aio_read,
+            **  and any other task which is able to look inside the buffer might try waiting on that aio -
+            **  except we have not yet issued the aio!   So we must keep the buffer header locked
+            **  from here all the way back to the BufStartAsync caller
+            */
+        } else {
+#endif   /* USE_PREFETCH */
+
 	UnlockBufHdr(buf);
 
 	InProgressBuf = buf;
 	IsForInput = forInput;
+#ifdef USE_PREFETCH
+        }
+#endif   /* USE_PREFETCH */
 
 	return true;
 }
@@ -3058,7 +3624,7 @@ StartBufferIO(volatile BufferDesc *buf,
  *	(Assumptions)
  *	My process is executing IO for the buffer
  *	BM_IO_IN_PROGRESS bit is set for the buffer
- *	We hold the buffer's io_in_progress lock
+ *	if no async IO in progress,  then We hold the buffer's io_in_progress lock
  *	The buffer is Pinned
  *
  * If clear_dirty is TRUE and BM_JUST_DIRTIED is not set, we clear the
@@ -3070,26 +3636,32 @@ StartBufferIO(volatile BufferDesc *buf,
  * BM_IO_ERROR in a failure case.  For successful completion it could
  * be 0, or BM_VALID if we just finished reading in the page.
  */
-static void
+void
 TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
 				  int set_flag_bits)
 {
-	Assert(buf == InProgressBuf);
+        int flags_on_entry;
 
 	LockBufHdr(buf);
 
+        flags_on_entry = buf->flags;
+
+        if (! (flags_on_entry & BM_AIO_IN_PROGRESS) )
+            Assert( buf == InProgressBuf );
+
 	Assert(buf->flags & BM_IO_IN_PROGRESS);
-	buf->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR);
+	buf->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR | BM_AIO_IN_PROGRESS);
 	if (clear_dirty && !(buf->flags & BM_JUST_DIRTIED))
 		buf->flags &= ~(BM_DIRTY | BM_CHECKPOINT_NEEDED);
 	buf->flags |= set_flag_bits;
 
 	UnlockBufHdr(buf);
 
+        if (! (flags_on_entry & BM_AIO_IN_PROGRESS) ) {
 	InProgressBuf = NULL;
-
 	LWLockRelease(buf->io_in_progress_lock);
 }
+}
 
 /*
  * AbortBufferIO: Clean up any active buffer I/O after an error.
--- src/backend/storage/buffer/buf_async.c.orig	2014-05-31 17:21:19.261556301 -0400
+++ src/backend/storage/buffer/buf_async.c	2014-05-31 19:53:09.036073466 -0400
@@ -0,0 +1,921 @@
+/*-------------------------------------------------------------------------
+ *
+ * buf_async.c
+ *	  buffer manager asynchronous disk read routines
+ *
+ * Portions Copyright (c) 1996-2012, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/storage/buffer/buf_async.c
+ *
+ *-------------------------------------------------------------------------
+ */
+/*
+ * Principal entry points:
+ *
+ * BufStartAsync() -- start an asynchronous read of a block into a buffer and
+ *	 pin it so that no one can destroy it while this process is using it.
+ *
+ * BufCheckAsync() -- check completion of an asynchronous read
+ *       and either claim buffer or discard it
+ *
+ * Private helper
+ *
+ * BufReleaseAsync() -- release the BAiocb resources used for an asynchronous read
+ *
+ * See also these files:
+ *		bufmgr.c -- main buffer manager functions
+ *		buf_init.c -- initialisation of resources
+ */
+#include "postgres.h"
+#include <sys/types.h> 
+#include <sys/file.h>
+#include <unistd.h>
+#include <sched.h>
+
+#include "catalog/catalog.h"
+#include "common/relpath.h"
+#include "executor/instrument.h"
+#include "miscadmin.h"
+#include "pg_trace.h"
+#include "pgstat.h"
+#include "postmaster/bgwriter.h"
+#include "storage/buf_internals.h"
+#include "storage/bufmgr.h"
+#include "storage/ipc.h"
+#include "storage/proc.h"
+#include "storage/smgr.h"
+#include "storage/standby.h"
+#include "utils/rel.h"
+#include "utils/resowner_private.h"
+
+/*
+ * GUC parameters
+ */
+int      max_async_io_prefetchers; /* Maximum number of backends using asynchronous librt threads to read pages into our buffers */
+
+/* Note: these two macros only work on shared buffers, not local ones! */
+#define BufHdrGetBlock(bufHdr)	((Block) (BufferBlocks + ((Size) (bufHdr)->buf_id) * BLCKSZ))
+#define BufferGetLSN(bufHdr)	(PageGetLSN(BufHdrGetBlock(bufHdr)))
+
+extern volatile struct BAiocbAnchor *BAiocbAnchr;  /*  anchor for all control blocks pertaining to aio  */
+extern int numBufferAiocbs;        /*  total number of  BufferAiocbs in pool  */
+extern int maxGetBAiocbTries;      /*   max times we will try to get a free BufferAiocb */
+extern int maxRelBAiocbTries;      /*   max times we will try to release a BufferAiocb back to freelist */
+extern pid_t this_backend_pid;     /*   pid of this backend */
+
+extern bool PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy);
+extern void PinBuffer_Locked(volatile BufferDesc *buf);
+extern Buffer ReadBuffer_common(SMgrRelation reln, char relpersistence,
+				  ForkNumber forkNum, BlockNumber blockNum,
+				  ReadBufferMode mode, BufferAccessStrategy strategy,
+				  bool *hit , int index_for_aio);
+extern void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner);
+extern void TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
+				  int set_flag_bits);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+int BufStartAsync(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy);
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+int BufCheckAsync(SMgrRelation caller_smgr, Relation caller_reln, volatile BufferDesc *buf_desc
+  ,int intention , BufferAccessStrategy strategy , int index_for_aio , bool spinLockHeld , LWLockId PartitionLock );
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+
+static struct BufferAiocb volatile * cachedBAiocb = (struct BufferAiocb*)0;  /*  one cached BufferAiocb for use with aio */
+
+#ifdef USE_PREFETCH
+/*  BufReleaseAsync releases a BufferAiocb and returns 0 if successful else non-zero
+**  it *must* be called :
+**     EITHER with a valid  BAiocb->BAiocbbufh -> buf_desc
+**            and that buf_desc must be spin-locked
+**     OR     with BAiocb->BAiocbbufh == 0
+*/
+static int
+BufReleaseAsync(struct BufferAiocb volatile * BAiocb)
+{
+        int    LockTries;         /*  max times we will try to release the BufferAiocb */
+        volatile struct BufferAiocb *BufferAiocbs;
+        struct BufferAiocb volatile * oldFreeBAiocb;  /*  old value of FreeBAiocbs */
+
+        int failed = 1; /* by end of this function, non-zero  will indicate if we failed to return the BAiocb */
+
+
+        if (    ( BAiocb == (struct BufferAiocb*)0 )
+             || ( BAiocb == (struct BufferAiocb*)BAIOCB_OCCUPIED )
+             || ( ((unsigned long)BAiocb) & 0x1 )
+           ) {
+                          elog(ERROR,
+                                 "AIO control block corruption on release of aiocb %p - invalid BAiocb"
+                                          ,BAiocb);
+        }
+        else 
+        if (   (0 == BAiocb->BAiocbDependentCount)     /*  no dependents  */
+            && ((struct BufferAiocb*)BAIOCB_OCCUPIED == BAiocb->BAiocbnext)  /*  not already on freelist */
+           ) {
+
+            if ((struct sbufdesc*)0 != BAiocb->BAiocbbufh) { /*  if a buffer was attached */
+                volatile        BufferDesc *buf_desc = BAiocb->BAiocbbufh;
+
+                /*  spinlock held so instead of TerminateBufferIO(buf, false , 0); ... */
+                if (buf_desc->flags & BM_AIO_PREFETCH_PIN_BANKED) { /* if a pid banked the pin */
+                    buf_desc->freeNext = -(BAiocb->pidOfAio);       /* then remember which pid */
+                }
+                else if (buf_desc->freeNext <= FREENEXT_BAIOCB_ORIGIN) {
+                    buf_desc->freeNext = FREENEXT_NOT_IN_LIST;   /* disconnect BufferAiocb from buf_desc */
+                }
+                buf_desc->flags &= ~BM_AIO_IN_PROGRESS;
+            }
+            
+            BAiocb->BAiocbbufh = (struct sbufdesc *)0; /* disconnect buf_desc from BufferAiocb */
+            BAiocb->pidOfAio = 0;                      /*  clean */
+            LockTries = maxRelBAiocbTries;         /*  max times we will try to release the BufferAiocb */
+            do {
+                register long long int dividend , remainder;
+
+                /*      retrieve old value of FreeBAiocbs  */
+                BAiocb->BAiocbnext = oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs;  /*  old value of FreeBAiocbs */
+
+                /*  this is a volatile value unprotected by any lock,  so must validate it;
+                **  safest is to verify that it is identical to one of the BufferAiocbs
+                **  to do so,  verify by direct division that its address offset from first control block 
+                **  is an integral multiple of the control block size
+                **  that lies within the range [ 0 , (numBufferAiocbs-1) ]
+                */
+                BufferAiocbs = BAiocbAnchr->BufferAiocbs;
+                remainder = ((long long int)oldFreeBAiocb - (long long int)BufferAiocbs)
+                            % (long long int)(sizeof(struct BufferAiocb));
+                failed = (int)remainder;
+                if (!failed) {
+                    dividend = ((long long int)oldFreeBAiocb - (long long int)BufferAiocbs)
+                               / (long long int)(sizeof(struct BufferAiocb));
+                     failed = ( (dividend < 0 ) || ( dividend >= numBufferAiocbs) );
+                     if (!failed) {
+                         if (__sync_bool_compare_and_swap (&(BAiocbAnchr->FreeBAiocbs), oldFreeBAiocb, BAiocb)) {
+                            LockTries = 0;   /*  end the do loop  */
+
+                            goto cheering;   /*  cant simply break because then failed would be set incorrectly */
+                         }
+                    }
+                }
+                /*  if we reach here, we have failed and failed is set to -1 */
+
+       cheering: ;
+
+                if ( LockTries > 1 ) {
+                    sched_yield();    /*   yield to another process,  (hopefully a backend) */
+                }
+            } while  (LockTries-- > 0);
+
+            if (failed) {
+#ifdef LOG_RELBAIOCB_DEPLETION
+                          elog(LOG,
+                                "BufReleaseAsync:AIO control block %p unreleased after tries= %d\n"
+                                       ,BAiocb,maxRelBAiocbTries);
+#endif  /* LOG_RELBAIOCB_DEPLETION */
+            }
+
+        }
+              else
+                          elog(LOG,
+                                "BufReleaseAsync:AIO control block %p either has dependents= %d or is already on freelist %p or has no buf_header %p\n"
+                                       ,BAiocb , BAiocb->BAiocbDependentCount , BAiocb->BAiocbnext , BAiocb->BAiocbbufh);
+        return failed;
+}
+
+/*  try using asynchronous aio_read to prefetch into a buffer
+**  return code :
+**        0 if started successfully
+**       -1 if failed for some reason
+**        1+PrivateRefCount if we found desired buffer in buffer pool
+**
+**  There is a harmless race condition here :
+**  two different backends may both arrive here simultaneously
+**  to prefetch the same buffer.    This is not unlikely when a syncscan is in progress.
+**  .  One will acquire the buffer and issue the smgrstartaio
+**  .  Other will find the buffer on return from  ReadBuffer_common with hit = true
+**  Only the first task has a pin on the buffer since ReadBuffer_common knows not to get a pin
+**  on a found buffer in prefetch mode.
+**  Therefore  -   the second task must simply abandon the prefetch if it finds the buffer in the buffer pool.
+**
+**  if we fail to acquire a BAiocb because of concurrent theft from freelist by other backend,
+**  retry up to maxGetBAiocbTries times provided that there actually was at least one BAiocb on the freelist.
+*/
+int
+BufStartAsync(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy) {
+
+        int retcode = -1;
+        struct BufferAiocb volatile * BAiocb = (struct BufferAiocb*)0;  /*  BufferAiocb for use with aio */
+        int  smgrstartaio_rc = -1;           /*  retcode from smgrstartaio */
+        bool do_unpin_buffer = false;        /* unpin must be deferred until after buffer descriptor is unlocked */
+        Buffer		buf_id;
+        bool		hit = false;
+        volatile        BufferDesc *buf_desc = (BufferDesc *)0;
+
+        int    LockTries;         /*  max times we will try to get a free BufferAiocb */
+
+        struct BufferAiocb volatile * oldFreeBAiocb;  /*  old value of FreeBAiocbs */
+        struct BufferAiocb volatile * newFreeBAiocb;  /*  new value of FreeBAiocbs */
+
+
+    /*  return immediately if no async io resources */
+    if (numBufferAiocbs > 0) {
+        buf_id = (Buffer)0;
+
+        if ( (struct BAiocbAnchor *)0 != BAiocbAnchr ) {
+
+            volatile struct BufferAiocb *BufferAiocbs;
+
+            if ((struct BufferAiocb*)0 != cachedBAiocb) {  /* any cached BufferAiocb ? */
+                BAiocb = cachedBAiocb;                     /* yes  use it  */
+                cachedBAiocb = BAiocb->BAiocbnext;
+                BAiocb->BAiocbnext = (struct BufferAiocb*)BAIOCB_OCCUPIED; /* mark as not on freelist */
+                BAiocb->BAiocbDependentCount = 0;  /* no dependent yet */
+                BAiocb->BAiocbbufh = (struct sbufdesc*)0;
+                BAiocb->pidOfAio = 0;
+            } else {
+
+                LockTries = maxGetBAiocbTries;         /*  max times we will try to get a free BufferAiocb */
+                do {
+                    register long long int dividend = -1 , remainder;
+                    /*  check if we have a free BufferAiocb */
+
+                    oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs;  /*  old value of FreeBAiocbs */
+
+                    /*  check if we have a free BufferAiocb */
+
+                    /*  BAiocbAnchr->FreeBAiocbs is a volatile value unprotected by any lock,
+                    **  and use of compare-and-swap to add and remove items from the list has
+                    **  two potential pitfalls,    both relating to the fact that we must
+                    **  access data de-referenced from this pointer before the compare-and-swap.
+                    **  1)  The value we load may be corrupt,  e.g. mixture of bytes from
+                    **      two different values,   so must validate it;
+                    **      safest is to verify that it is identical to one of the BufferAiocbs.
+                    **      to do so,  verify by direct division that its address offset from
+                    **      first control block is an integral multiple of the control block size
+                    **      that lies within the range [ 0 , (numBufferAiocbs-1) ]
+                    **      Thus we completely prevent this pitfall.
+                    **  2)  The content of the item's next pointer may have changed between the
+                    **      time we de-reference it and the time of the compare-and-swap.
+                    **      Thus even though the compare-and-swap succeeds,   we might set the
+                    **      new head of the freelist to an invalid value  (either a free item
+                    **      that is not the first in the free chain  -  resulting only in
+                    **      loss of the orphaned free items, or,  much worse, an in-use item).
+                    **      In practice this is extremely unlikely because of the implied huge delay
+                    **      in this window interval in this (current) process.    Here are two scenarios:
+                    **      legend:
+                    **         P0  -  this (current) process,  P1,  P2 , ... other processes
+                    **         content of freelist shown as BAiocbAnchr->FreeBAiocbs -> first item -> 2nd item ...
+                    **         @[X] means address of X
+                    **         |      timeline of window of exposure to problems
+                    **      successive lines in chronological order                                       content of freelist
+                    **        2.1    P0           access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I0]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **               P0          access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I1]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1        access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I0]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1       access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I1]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1        swap-remove I0,  place I1 at head of list                F -> I1 -> I2 -> I3 ...
+                    **         |           P2     access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I1]  F -> I1 -> I2 -> I3 ...
+                    **         |           P2    access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I2]  F -> I1 -> I2 -> I3 ...
+                    **         |           P2     swap-remove I1,  place I2 at head of list                F -> I2 -> I3 ...
+                    **         |        P1       complete aio,  replace I0 at head of list                 F -> I0 -> I2 -> I3 ...
+                    **               P0        swap-remove I0,  place I1 at head of list                   F -> I1 IS IN USE !! CORRUPT !!
+                    **               compare-and-swap succeeded but the value of newFreeBAiocb was stale
+                    **               and had become in-use during the window.
+                    **        2.2    P0           access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I0]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **               P0          access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I1]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1        access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I0]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1       access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I1]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1        swap-remove I0,  place I1 at head of list                F -> I1 -> I2 -> I3 ...
+                    **         |           P2     access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I1]  F -> I1 -> I2 -> I3 ...
+                    **         |           P2    access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I2]  F -> I1 -> I2 -> I3 ...
+                    **         |           P2     swap-remove I1,  place I2 at head of list                F -> I2 -> I3 ...
+                    **         |              P3  access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I2]  F -> I2 -> I3 ...
+                    **         |              P3 access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I3]  F -> I2 -> I3 ...
+                    **         |              P3  swap-remove I2,  place I3 at head of list                F -> I3 ...
+                    **         |           P2    complete aio,  replace I1 at head of list                 F -> I1 -> I3 ...
+                    **         |              P3 complete aio,  replace I2 at head of list                 F -> I2 -> I1 -> I3 ...
+                    **         |        P1       complete aio,  replace I0 at head of list                 F -> I0 -> I2 -> I1 -> I3 ...
+                    **               P0        swap-remove I0,  place I1 at head of list                   F -> I1 -> I3 ... ! I2 is orphaned !
+                    **               compare-and-swap succeeded but the value of newFreeBAiocb was stale
+                    **               and had moved further down the free list during the window.
+                    **      Unfortunately, we cannot prevent this pitfall but we can detect it (after the fact),
+                    **      by checking that the next pointer of the item we have just removed for our use still points to the same item.
+                    **      This test is not subject to any timing or uncertainty since :
+                    **       .  The fact that the compare-and-swap succeeded implies that the item we removed
+                    **          was defintely on the freelist (at the head) when it was removed,
+                    **          and therefore cannot be in use,  and therefore its next pointer is no longer volatile.
+                    **       .  Although pointers of the anchor and items on the freelist are volatile,
+                    **          the addresses of items never change -  they are in an allocated array and never move.
+                    **      E.g. in the above two scenarios,   the test is that I0.next still -> I1,
+                    **      and this is true if and only if the second item on the freelist is
+                    **      still the same at the end of the window as it was at the start of the window.
+                    **      Note that we do not insist that it did not change during the window,
+                    **           only that it is still the correct new head of freelist.
+                    **      If this test fails,  we abort immediately as the subsystem is damaged and cannot be repaired.
+                    **      Note that at least one aio must have been issued *and* completed during the window
+                    **           for this to occur,  and since the window is just one single machine instruction,
+                    **           it is very unlikely in practice.
+                    */
+                    BufferAiocbs = BAiocbAnchr->BufferAiocbs;
+                    remainder = ((long long int)oldFreeBAiocb - (long long int)BufferAiocbs)
+                                % (long long int)(sizeof(struct BufferAiocb));
+                    if (remainder == 0) {
+                        dividend = ((long long int)oldFreeBAiocb - (long long int)BufferAiocbs)
+                                    / (long long int)(sizeof(struct BufferAiocb));
+                    }
+                    if (    (remainder == 0)
+                         && ( (dividend >= 0 ) && ( dividend < numBufferAiocbs) )
+                       )
+                    {
+                        newFreeBAiocb = oldFreeBAiocb->BAiocbnext; /*  tentative new value is second on free list */
+                        /*   Here we are in the exposure window referred to in the above comments,
+                        **   so moving along rapidly ...
+                        */
+                        if (__sync_bool_compare_and_swap (&(BAiocbAnchr->FreeBAiocbs), oldFreeBAiocb, newFreeBAiocb)) {   /*  did we get it ? */
+                                /*   We have successfully swapped head of freelist pointed to by oldFreeBAiocb off the list;
+                                **   Here we check that the item we just placed at head of freelist, pointed to by newFreeBAiocb,
+                                **   is the right one
+                                **
+                                **   also check that the BAiocb we have acquired was not in use
+                                **   i.e. that scenario 2.1 above did not occur just before our compare-and-swap
+                                **   The test is that the BAiocb is not in use.
+                                **
+                                **  in one hypothetical case,
+                                **  we can be certain that there is no corruption -
+                                **  the case where newFreeBAiocb == 0 and oldFreeBAiocb->BAiocbnext != BAIOCB_OCCUPIED -
+                                **  i.e. we have set the freelist to empty but we have a baiocb chained from ours.
+                                **  in this case our comp_swap removed all BAiocbs from the list (including ours)
+                                **  so the others chained from ours are either orphaned (no harm done)
+                                **  or in use by another backend and will eventually be returned (fine).
+                                */
+                                if ((struct BufferAiocb *)0 == newFreeBAiocb) {
+                                    if ((struct BufferAiocb *)BAIOCB_OCCUPIED == oldFreeBAiocb->BAiocbnext) {
+                                        goto baiocb_corruption;
+                                    } else if ((struct BufferAiocb *)0 != oldFreeBAiocb->BAiocbnext) {
+                                      elog(LOG,
+                                         "AIO control block inconsistency on acquiring aiocb %p - its next free %p may be orphaned (no corruption has occurred)"
+                                         	,oldFreeBAiocb , oldFreeBAiocb->BAiocbnext);
+                                    }
+                                } else {
+                                    /*  case of newFreeBAiocb not null  -  so must check more carefully ... */
+                                    remainder = ((long long int)newFreeBAiocb - (long long int)BufferAiocbs)
+                                                % (long long int)(sizeof(struct BufferAiocb));
+                                    dividend = ((long long int)newFreeBAiocb - (long long int)BufferAiocbs)
+                                                / (long long int)(sizeof(struct BufferAiocb));
+
+                                    if (    (newFreeBAiocb != oldFreeBAiocb->BAiocbnext)
+                                         || (remainder != 0)
+                                         || ( (dividend < 0 ) || ( dividend >= numBufferAiocbs) )
+                                       ) {
+                                        goto baiocb_corruption;
+                                    }
+                                }
+                                BAiocb = oldFreeBAiocb;
+                                BAiocb->BAiocbnext = (struct BufferAiocb*)BAIOCB_OCCUPIED; /* mark as not on freelist */
+                                BAiocb->BAiocbDependentCount = 0;  /* no dependent yet */
+                                BAiocb->BAiocbbufh = (struct sbufdesc*)0;
+                                BAiocb->pidOfAio = 0;
+
+                                LockTries = 0;   /*  end the do loop  */
+
+                        }
+                    }
+
+                    if ( LockTries > 1 ) {
+                        sched_yield();    /*   yield to another process,  (hopefully a backend) */
+                    }
+                } while (     ((struct BufferAiocb*)0 == BAiocb)            /*  did not get a BAiocb    */
+                           && ((struct BufferAiocb*)0 != oldFreeBAiocb)     /*  there was a free BAiocb */
+                           && (LockTries-- > 0)                             /*  told to retry           */
+                        );
+            }
+        }
+
+        if ( BAiocb != (struct BufferAiocb*)0 ) {
+            /*  try an async io    */
+            BAiocb->BAiocbthis.aio_fildes = -1; /* necessary to ensure any thief realizes aio not yet started */
+            BAiocb->pidOfAio = this_backend_pid;
+
+            /*  now try to acquire a buffer :
+            **  note -   ReadBuffer_common returns hit=true if the block is found in the buffer pool,
+            **            in which case there is no need to prefetch.
+            **  otherwise ReadBuffer_common pins returned buffer and calls StartBufferIO
+            **           and StartBufferIO :
+            **      . sets buf_desc->freeNext to negative of ( index of the aiocb in the BufferAiocbs array + 3 )
+            **      . sets  BAiocb->BAiocbbufh -> buf_desc
+            **           and in this case the buffer spinlock is held.
+            **           This is essential as no other task must issue any intention with respect
+            **           to the buffer until we have started the aio_read.
+            **  Also note that ReadBuffer_common handles enlarging the ResourceOwner buffer list as needed
+            **       so I dont need to do that
+            */
+            buf_id = ReadBuffer_common(reln->rd_smgr, reln->rd_rel->relpersistence,
+                                        forkNum, blockNum
+                                       ,RBM_NOREAD_FOR_PREFETCH  /*  tells ReadBuffer not to do any read,  just alloc buf */
+                                       ,strategy , &hit , (FREENEXT_BAIOCB_ORIGIN - (BAiocb - (BAiocbAnchr->BufferAiocbs))));
+            buf_desc = &BufferDescriptors[buf_id-1];    /* find buffer descriptor */
+
+            /*  normally hit will be false as presumably it was not in the pool
+            **  when our caller looked - but it could be there now ...
+            */
+            if (hit) {
+                   /*   see earlier comments  -  we must abandon the prefetch */
+                   retcode = 1 + PrivateRefCount[buf_id];
+                   BAiocb->BAiocbbufh = (struct sbufdesc *)0; /* indicator that we must release the BAiocb before exiting */
+            } else
+            if (  (buf_id > 0) && ((BufferDesc *)0 != buf_desc) && (buf_desc == BAiocb->BAiocbbufh)  ) {
+                   /*   buff descriptor header lock should be held.
+                   **   However,  just to be safe ,   now validate that
+                   **   we are still the owner and no other task already stole it.
+                   */
+
+                   buf_desc->flags &= ~BM_AIO_PREFETCH_PIN_BANKED; /* ensure no banked pin */
+                   /*  there should not be any other pid waiting on this buffer
+                   **  so check both of BM_VALID and BM_PIN_COUNT_WAITER are not set
+                   */
+                   if (    ( !(buf_desc->flags & (BM_VALID|BM_PIN_COUNT_WAITER) ) )
+                        && (buf_desc->flags & BM_AIO_IN_PROGRESS)
+                        && ((FREENEXT_BAIOCB_ORIGIN - (BAiocb - (BAiocbAnchr->BufferAiocbs))) == buf_desc->freeNext) /* it is still mine */
+                        && (-1 == BAiocb->BAiocbthis.aio_fildes)   /* no thief stole it */
+                        && (0 == BAiocb->BAiocbDependentCount)     /* no dependent */
+                     ) {
+                        /*   we have an empty buffer for our use */
+
+                        BAiocb->BAiocbthis.aio_buf = (void *)(BufHdrGetBlock(buf_desc)); /* Location of actual buffer.  */
+
+                        /*   note - there is no need to register self as an dependent of BAiocb
+                        **   as we shall not unlock buf_desc before we free the BAiocb
+                        */
+
+                        /*   smgrstartaio retcode is returned in smgrstartaio_rc -
+                        **   it indicates whether started or not
+                        */
+                        smgrstartaio(reln->rd_smgr, forkNum, blockNum , (char *)&(BAiocb->BAiocbthis) , &smgrstartaio_rc );
+
+                        if (smgrstartaio_rc == 0) {
+                            retcode = 0;
+                            buf_desc->flags |= BM_AIO_PREFETCH_PIN_BANKED; /* bank the pin for next use by this task */
+                            /* we did not register register self as an dependent of BAiocb so no need to unregister  */
+                        } else {
+                            /*  failed - return the BufferAiocb to the free list  */
+                            do_unpin_buffer = true;    /* unpin must be deferred until after buffer descriptor is unlocked */
+                            /*  spinlock held so instead of TerminateBufferIO(buf_desc, false , 0); ... */
+                            buf_desc->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR | BM_AIO_IN_PROGRESS | BM_AIO_PREFETCH_PIN_BANKED | BM_VALID);
+                            /* we did not register register self as an dependent of BAiocb so no need to unregister  */
+
+                            /*  return the BufferAiocb to the free list  */
+                            if (
+                                   BufReleaseAsync(BAiocb)
+                               ) {         /*  failed ? */
+                                BAiocb->BAiocbnext = cachedBAiocb;    /* then ...       */
+                                cachedBAiocb = BAiocb;                /*  ... cache it  */
+                            }
+
+                            pgBufferUsage.aio_read_failed++;
+                            smgrstartaio_rc = 1;  /*   to distinguish from aio not even attempted */
+                        }
+                   }
+                   else {
+                            /*  buffer was stolen or in use by other task - return the BufferAiocb to the free list  */
+                            do_unpin_buffer = true;    /* unpin must be deferred until after buffer descriptor is unlocked */
+                   }
+
+                   UnlockBufHdr(buf_desc);
+                   if (do_unpin_buffer) {
+                        if (smgrstartaio_rc >= 0) { /*  if  aio was attempted */
+                            TerminateBufferIO(buf_desc, false , 0);
+                        }
+                        UnpinBuffer(buf_desc, true);
+                   }
+            }
+            else {
+                BAiocb->BAiocbbufh = (struct sbufdesc *)0; /* indicator that we must release the BAiocb before exiting */
+            }
+
+            if ((struct sbufdesc*)0 == BAiocb->BAiocbbufh) { /*  we did not associate a buffer */
+                                                             /*  so return the BufferAiocb to the free list  */
+                            if (
+                                   BufReleaseAsync(BAiocb)
+                               ) {         /*  failed ? */
+                                BAiocb->BAiocbnext = cachedBAiocb;    /* then ...       */
+                                cachedBAiocb = BAiocb;                /*  ... cache it  */
+                            }
+            }
+        }
+    }
+
+    return retcode;
+
+    baiocb_corruption:;
+         elog(PANIC,
+              "AIO control block corruption on acquiring aiocb %p - its next free %p conflicts with new freelist pointer %p which may be invalid (corruption may have occurred)"
+                                                    ,oldFreeBAiocb , oldFreeBAiocb->BAiocbnext , newFreeBAiocb);
+}
+#endif   /* USE_PREFETCH */
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+/*
+ * BufCheckAsync --      act upon caller's intention regarding a shared buffer,
+ *                       primarily in connection with any async io in progress on the buffer.
+ *     class  subvalue   intention has two main classes and some subvalues within those :
+ *      +ve      1            .   want    <=>  caller wants the buffer,
+ *                                             wait for in-progress aio and then always pin
+ *      -ve                   .   reject  <=>  caller does not want the buffer,
+ *                                             if there are no dependents,  then cancel the aio
+ *              -1, -2 , -3 , ... (see below)        and then optionally unpin
+ *                             Used when there may have been a previous fetch or prefetch.
+ *
+ * buffer is assumed to be an existing member of the shared buffer pool
+ *    as returned by BufTableLookup.
+ * if AIO in progress,  then :
+ *      .  terminate AIO, waiting for completion if +ve intention, else without waiting
+ *      .  if the AIO had already completed successfully,   then mark buffer valid
+ *      .  pin/unpin as requested
+ *
+ * +ve intention indicates that buffer must be pinned :
+ *   if the strategy parameter is null,  then use the PinBuffer_Locked optimization
+ *   to pin and unlock in one operation.   But always update buffer usage count.
+ *
+ * -ve intention indicates whether and how to unpin :
+ *   BUF_INTENTION_REJECT_KEEP_PIN 	-1   pin already held, do not unpin, (caller wants to keep it)
+ *   BUF_INTENTION_REJECT_OBTAIN_PIN	-2   obtain pin,  caller wants it for same buffer
+ *   BUF_INTENTION_REJECT_FORGET	-3   unpin and tell resource owner to forget
+ *   BUF_INTENTION_REJECT_NOADJUST	-4   unpin and call ResourceOwnerForgetBuffer myself
+ *                                           instead of telling UnpinBuffer to adjust CurrentResource owner
+ *                                           (quirky simulation of ReleaseBuffer logic)
+ *   BUF_INTENTION_REJECT_UNBANK   	-5   unpin only if pin banked by caller
+ *   The behaviour for the -ve case is based on that of ReleaseBuffer, adding handling of async io.
+ *
+ * pin/unpin action must take account of whether this backend hold a "disposable" pin on the particular buffer.
+ * A "disposable" pin is a pin acquired by buffer manager without caller knowing, such as :
+ *      when required to safeguard an async AIO  -  pin can be held across multiple bufmgr calls
+ *      when required to safeguard waiting for an async AIO  -  pin acquired and released within this function
+ * if a disposable pin is held,   then :
+ *      if a new pin is requested,  the disposable pin must be retained (redeemed) and any flags relating to it unset
+ *      if an unpin is requested,   then :
+ *              if    either no AIO in progress or this backend did not initiate the AIO
+ *              then  the disposable pin must be dropped (redeemed) and any flags relating to it unset
+ *              else  log warning and do nothing
+ *  i.e. in either case,   there is no longer a disposable pin after this function has completed.
+ *       Note that if    intention is BUF_INTENTION_REJECT_UNBANK,
+ *                 then caller expects there to be a disposable banked pin
+ *                      and if there isn't one,  we do nothing
+ *                 for all other intentions,  if there is no disposable pin,   we pin/unpin normally.
+ *
+ * index_for_aio indicates the BAiocb to be used for next aio (see PrefetchBuffer)
+ * BufFreelistLockHeld indicates whether freelistlock is held
+ * spinLockHeld indicates whether buffer header spinlock is held
+ * PartitionLock is the  buffer partition lock to be used
+ *
+ * return code (meaningful ONLY if intention is +ve) indicates validity of buffer :
+ *  -1    buffer is invalid and failed PageHeaderIsValid check
+ *   0    buffer is not valid
+ *   1    buffer is valid
+ *   2    buffer is valid but tag changed  -  (so content does not match the relation block that caller expects)
+ */
+int
+BufCheckAsync(SMgrRelation caller_smgr, Relation caller_reln, BufferDesc volatile * buf_desc, int intention , BufferAccessStrategy strategy , int index_for_aio , bool spinLockHeld , LWLockId PartitionLock )
+{
+
+        int retcode = BUF_INTENT_RC_INVALID_NO_AIO;
+        bool valid = false;
+
+	BufferTag origTag = buf_desc->tag;	/* original identity of selected buffer */
+
+#ifdef USE_PREFETCH
+        int smgrcompleteaio_rc;          /*  retcode from smgrcompleteaio */
+        SMgrRelation smgr = caller_smgr;
+        int aio_successful = -1;         /*  did the aio_read succeed ?  -1 = no aio,  0 unsuccessful , 1 successful */
+	BufFlags	flags_on_entry;  /*  for debugging  -  can be printed in gdb */
+        int    		freeNext_on_entry;  /*  for debugging  -  can be printed in gdb */
+        int    		BAiocbDependentCount_after_aio_finished = -1;  /*  for debugging  -  can be printed in gdb */
+        bool       disposable_pin = false;            /* this backend had a disposable pin on entry or pins the buffer while waiting for aio_read to complete */
+        bool       pin_already_banked_by_me;          /* buffer is already pinned by me and redeemable */
+        int local_intention;
+#endif   /* USE_PREFETCH */
+
+
+
+#ifdef USE_PREFETCH
+            if (!spinLockHeld) {
+                /*  lock buffer header    */
+                LockBufHdr(buf_desc);
+            }
+
+	    flags_on_entry = buf_desc->flags;
+	    freeNext_on_entry = buf_desc->freeNext;
+            pin_already_banked_by_me =
+                      (    (flags_on_entry & BM_AIO_PREFETCH_PIN_BANKED)
+                        && (   (    (flags_on_entry & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - freeNext_on_entry))->pidOfAio )
+                                                                      : (-(freeNext_on_entry))  )  == this_backend_pid )
+                      );
+
+            if (pin_already_banked_by_me) {
+                if (PrivateRefCount[buf_desc->buf_id] == 0) {  /*  but do we actually have a pin ?? */
+                    /*   this is an anomalous situation   -  somehow our disposable pin was lost without us noticing
+                    **   if AIO is in progress and we started it,
+                    **   then this is disastrous  -   two backends might both issue IO on same buffer
+                    **   otherwise,  it is harmless,  and simply means we have no disposable pin,
+                    **               but we must update flags to "notice" the fact now
+                    */
+                    if (flags_on_entry & BM_AIO_IN_PROGRESS) {
+                            elog(ERROR, "BufCheckAsync : AIO control block issuer of aio_read lost pin with BM_AIO_IN_PROGRESS on buffer %d rel=%s, blockNum=%u, flags 0x%X refcount=%u intention= %d"
+                                ,buf_desc->buf_id,relpathbackend(buf_desc->tag.rnode, InvalidBackendId, buf_desc->tag.forkNum)
+                           ,buf_desc->tag.blockNum, flags_on_entry, buf_desc->refcount ,intention);
+                    } else {
+                            elog(LOG, "BufCheckAsync : AIO control block issuer of aio_read lost pin on buffer %d rel=%s, blockNum=%u, with flags 0x%X refcount=%u intention= %d"
+                               ,buf_desc->buf_id,relpathbackend(buf_desc->tag.rnode, InvalidBackendId, buf_desc->tag.forkNum)
+                               ,buf_desc->tag.blockNum, flags_on_entry, buf_desc->refcount ,intention);
+							buf_desc->flags &= ~BM_AIO_PREFETCH_PIN_BANKED; /*   redeem the banked pin  */
+							/*   since AIO not in progress,  disconnect the buffer from banked pin */
+							buf_desc->freeNext = FREENEXT_NOT_IN_LIST;      /*   and forget the bank client */
+							pin_already_banked_by_me = false;
+                    }
+                } else {
+                    disposable_pin = true;
+                }
+            }
+
+            /*  the case of BUF_INTENTION_REJECT_UNBANK is handled specially :
+            **    if this backend has a banked pin,  then proceed just as for BUF_INTENTION_REJECT_FORGET
+            **    else the call is a no-op  --  unlock buf header and return immediately
+            */
+            local_intention = intention;
+            if (intention == BUF_INTENTION_REJECT_UNBANK) {
+                if (pin_already_banked_by_me) {
+                    local_intention = BUF_INTENTION_REJECT_FORGET;
+                } else {
+                    goto unlock_buf_header;  /*  code following the unlock will do nothing since local_intention still set to BUF_INTENTION_REJECT_UNBANK */
+                }
+            }
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+            /*       we do not expect that BM_AIO_IN_PROGRESS is set without freeNext identifying the BAiocb */
+            if ( (buf_desc->flags & BM_AIO_IN_PROGRESS) && (buf_desc->freeNext == FREENEXT_NOT_IN_LIST) ) {
+
+					elog(ERROR, "BufCheckAsync : found BM_AIO_IN_PROGRESS without a BAiocb on buffer %d rel=%s, blockNum=%u, flags %X refcount=%u"
+						,buf_desc->buf_id,relpathbackend(buf_desc->tag.rnode, InvalidBackendId, buf_desc->tag.forkNum)
+						,buf_desc->tag.blockNum, buf_desc->flags, buf_desc->refcount);
+                }
+            /*     check whether aio in progress  */
+            if  (    ( (struct BAiocbAnchor *)0 != BAiocbAnchr )
+                  && (buf_desc->flags & BM_AIO_IN_PROGRESS)
+                  && (buf_desc->freeNext <= FREENEXT_BAIOCB_ORIGIN)                       /*  has a valid BAiocb */
+                  && ((FREENEXT_BAIOCB_ORIGIN - buf_desc->freeNext) < numBufferAiocbs)    /*  double-check */
+                ) {        /* this is aio   */
+                    struct BufferAiocb volatile * BAiocb = (BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf_desc->freeNext); /*  BufferAiocb associated with this aio */
+                    if ((struct BufferAiocb*)BAIOCB_OCCUPIED == BAiocb->BAiocbnext) { /*  ensure BAiocb is occupied */
+                        aio_successful = 0;       /*  tentatively the aio_read did not succeed   */
+                        retcode = BUF_INTENT_RC_INVALID_AIO;
+
+                        if (smgr == NULL) {
+                            if (caller_reln == NULL) {
+                                smgr = smgropen(buf_desc->tag.rnode, InvalidBackendId);
+                            } else {
+                                smgr = caller_reln->rd_smgr;
+                            }
+                        }
+
+                        /*  assert that this AIO is not using the same BufferAiocb as the one caller asked us to use */
+                        if ((index_for_aio < 0) && (index_for_aio == buf_desc->freeNext)) {
+                                   ereport(ERROR,
+                                        (errcode(ERRCODE_INTERNAL_ERROR),
+                                         errmsg("AIO control block index %d to be used by %p already in use by %p"
+                                                  ,index_for_aio, buf_desc, BAiocb->BAiocbbufh)));
+                        }
+
+                        /*    Call smgrcompleteaio only if either we want buffer or there are no dependents.
+                        **    In the other case of reject and there are dependents,
+                        **    then one of them will do it.
+                        */
+                        if (   (local_intention > 0) || (0 == BAiocb->BAiocbDependentCount)  ) {
+                            if (local_intention > 0) {
+                                /*  wait for in-progress aio and then pin
+                                **  OR  if I did not issue the aio and do not have a pin
+                                **  then pin now before waiting to ensure the buffer does not become unpinned while I wait
+                                **  we may potentially wait for io to complete
+                                **  so release buf header lock so that others may also wait here
+                                */
+                                BAiocb->BAiocbDependentCount++; /* register self as dependent  */
+                                if (PrivateRefCount[buf_desc->buf_id] == 0) {   /* if this buffer not pinned by me */
+                                    disposable_pin = true; /* this backend has pinned the buffer while waiting for aio_read to complete */
+                                    PinBuffer_Locked(buf_desc);
+                                } else {
+                                    UnlockBufHdr(buf_desc);
+                                }
+                                LWLockRelease(PartitionLock);
+
+                                smgrcompleteaio_rc = 1   /*  tell smgrcompleteaio to wait  */
+                                                   + ( BAiocb->pidOfAio == this_backend_pid ); /*  and whether I initiated the aio */
+                            } else {
+                                smgrcompleteaio_rc = 0;   /*  tell smgrcompleteaio to cancel */
+                            }
+
+                            smgrcompleteaio( smgr , (char *)&(BAiocb->BAiocbthis) , &smgrcompleteaio_rc );
+                            if ( (smgrcompleteaio_rc == 0) || (smgrcompleteaio_rc == 1) ) {
+                                  aio_successful = 1;
+                            }
+
+                            /*   statistics  */
+                            if (local_intention > 0) {
+                                if (smgrcompleteaio_rc == 0) {
+                                    /*    completed successfully and did not have to wait  */
+                                    pgBufferUsage.aio_read_ontime++;
+                                } else if (smgrcompleteaio_rc == 1) {
+                                    /*    completed successfully and did have to wait  */
+                                    pgBufferUsage.aio_read_waited++;
+                                } else {
+                                    /*  bad news   -   read failed and so buffer not usable
+                                    **  the buffer is still pinned so unpin and proceed with "not found" case
+                                    */
+                                    pgBufferUsage.aio_read_failed++;
+                                }
+
+                                /*  regain locks and handle the validity of the buffer and intention regarding it    */
+                                LWLockAcquire(PartitionLock, LW_SHARED);
+                                LockBufHdr(buf_desc);
+                                BAiocb->BAiocbDependentCount--; /* unregister self as dependent  */
+                            } else {
+                                    pgBufferUsage.aio_read_wasted++;  /*   regardless of whether aio_successful */
+                            }
+
+
+                            if (local_intention > 0) {
+                                /*  verify the buffer is still ours and has same identity
+                                **  There is one slightly tricky point here -
+                                **  if there are other dependents,   then each of them will perform this same check
+                                **  this is unavoidable as the correct setting of retcode and the BM_VALID flag
+                                **  is required by each dependent,  so we may not leave it to the last one to do it.
+                                **  It should not do any harm and easier to let them all do it than try to avoid.
+                                */
+                                if ((FREENEXT_BAIOCB_ORIGIN - (BAiocb - (BAiocbAnchr->BufferAiocbs))) == buf_desc->freeNext) { /* it is still mine */
+
+                                    if (aio_successful) {
+                                        /* validate page header.   If valid,  then mark the buffer as valid */
+                                        if (PageIsVerified((Page)(BufHdrGetBlock(buf_desc)) , ((BAiocb->BAiocbthis).aio_offset/BLCKSZ))) {
+                                            buf_desc->flags |= BM_VALID;
+                                            if (BUFFERTAGS_EQUAL(origTag , buf_desc->tag)) {
+                                                retcode = BUF_INTENT_RC_VALID;
+                                            } else {
+                                                retcode = BUF_INTENT_RC_CHANGED_TAG;
+                                            }
+                                        } else {
+                                            retcode = BUF_INTENT_RC_BADPAGE;
+                                        }
+                                    }
+                                }
+                            }
+
+                            BAiocbDependentCount_after_aio_finished = BAiocb->BAiocbDependentCount;
+
+                            /*  if no dependents,   then disconnect the BAiocb and update buffer header */
+                            if (BAiocbDependentCount_after_aio_finished == 0 ) {
+
+
+                                /*  return the BufferAiocb to the free list  */
+                                buf_desc->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR | BM_AIO_IN_PROGRESS);
+                                if (
+                                           BufReleaseAsync(BAiocb)
+                                   ) {        /*  failed ? */
+                                    BAiocb->BAiocbnext = cachedBAiocb;   /* then ...       */
+                                    cachedBAiocb = BAiocb;               /*  ... cache it  */
+                                }
+                            }
+
+                        }
+                    }
+            }
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+            /*  note whether buffer is valid before unlocking spinlock */
+            valid = ((buf_desc->flags & BM_VALID) != 0);
+
+            /*  if there was a disposable pin on entry to this function (i.e. marked in buffer flags)
+            **  then unmark it  -  refer to prologue comments talking about :
+            **    if a disposable pin is held,   then :
+            **     ...
+            **    i.e. in either case,   there is no longer a disposable pin after this function has completed.
+            */
+            if (pin_already_banked_by_me) {
+                        buf_desc->flags &= ~BM_AIO_PREFETCH_PIN_BANKED; /*   redeem the banked pin  */
+                        /*   if AIO not in progress,  then disconnect the buffer from BAiocb and/or banked pin */
+                        if (!(buf_desc->flags & BM_AIO_IN_PROGRESS)) {
+                            buf_desc->freeNext = FREENEXT_NOT_IN_LIST;      /*   and forget the bank client */
+                        }
+                        /********** for debugging   *****************
+                        else elog(LOG, "BufCheckAsync : found BM_AIO_IN_PROGRESS when redeeming banked pin on buffer %d rel=%s, blockNum=%u, flags %X refcount=%u"
+                       ,buf_desc->buf_id,relpathbackend(buf_desc->tag.rnode, InvalidBackendId, buf_desc->tag.forkNum)
+                       ,buf_desc->tag.blockNum, buf_desc->flags, buf_desc->refcount);
+                        ********** for debugging     *****************/
+            }
+
+            /*  If we are to obtain new pin, then use pin optimization  -  pin and unlock.
+            **  However,   if the caller is the same backend who issued the aio_read,
+            **  then he ought to have obtained the pin at that time and must not acquire
+            **  a "second" one since this is logically the same read -  he would have obtained
+            **  a single pin if using synchronous read and we emulate that behaviour.
+            **  Its important to understand that the caller is not aware that he already obtained a pin -
+            **  because calling PrefetchBuffer did not imply a pin -
+            **  so we must track that via the pidOfAio field in the BAiocb.
+            **  And to add one further complication :
+            **      we assume that although PrefetchBuffer pinned the buffer,
+            **      it did not increment the usage count.
+            **      (because it called PinBuffer_Locked which does not do that)
+            **      so in this case,   we must increment the usage count without double-pinning.
+            **      yes its ugly  -  and theres a goto!
+            */
+            if (   (local_intention > 0)
+                || (local_intention == BUF_INTENTION_REJECT_OBTAIN_PIN)
+               ) {
+
+                /* Make sure we will have room to remember the buffer pin */
+                ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
+
+                /*    here we really want a version of PinBuffer_Locked which updates usage count ... */
+                if (   (PrivateRefCount[buf_desc->buf_id] == 0) /*   if this buffer not previously pinned by me */
+                    || pin_already_banked_by_me                 /*   or I had a disposable pin on entry */
+                   ) {
+                    if (strategy == NULL)
+                    {
+                            if (buf_desc->usage_count < BM_MAX_USAGE_COUNT)
+                                    buf_desc->usage_count++;
+                    }
+                    else
+                    {
+                            if (buf_desc->usage_count == 0)
+                                    buf_desc->usage_count = 1;
+                    }
+		}
+
+                /*  now pin buffer unless we have a disposable */
+                if (!disposable_pin) { /* this backend neither banked pin for aio nor pinned the buffer while waiting for aio_read to complete */
+                    PinBuffer_Locked(buf_desc);
+                    goto unlocked_it;
+                }
+                else
+                /*    if this task previously issued the aio or pinned the buffer while waiting for aio_read to complete
+                **       and aio was unsuccessful,    then release the pin
+                */
+                if (     disposable_pin
+                      && (aio_successful == 0)       /*  aio_read failed ? */
+                   ) {
+                           UnpinBuffer(buf_desc, true);
+                }
+            }
+
+    unlock_buf_header:
+            UnlockBufHdr(buf_desc);
+    unlocked_it:
+#endif   /* USE_PREFETCH */
+
+            /*   now do any requested pin (if not done immediately above) or unpin/forget  */
+            if (local_intention == BUF_INTENTION_REJECT_KEEP_PIN) {
+            /*   the caller is supposed to hold a pin already so there should be nothing to do ... */
+                if (PrivateRefCount[buf_desc->buf_id] == 0) {
+                    elog(LOG, "request to keep pin on unpinned buffer %d",buf_desc->buf_id);
+
+                    valid = PinBuffer(buf_desc, strategy);
+                }
+            }
+            else
+            if (   (   (local_intention == BUF_INTENTION_REJECT_FORGET)
+                    || (local_intention == BUF_INTENTION_REJECT_NOADJUST)
+                   )
+                && (PrivateRefCount[buf_desc->buf_id] > 0) /*   if this buffer was previously pinned by me ... */
+               )  {
+
+                    if (local_intention == BUF_INTENTION_REJECT_FORGET) {
+                        UnpinBuffer(buf_desc, true); /*  ... then release the pin                   */
+                    } else
+                    if (local_intention == BUF_INTENTION_REJECT_NOADJUST) {
+                        /*   following code moved from ReleaseBuffer :
+                        **   not sure why we can't simply UnpinBuffer(buf_desc, true)
+                        **   but better leave it the way it was
+                        */
+                        ResourceOwnerForgetBuffer(CurrentResourceOwner, BufferDescriptorGetBuffer(buf_desc));
+                        if (PrivateRefCount[buf_desc->buf_id] > 1) {
+                            PrivateRefCount[buf_desc->buf_id]--;
+                        } else {
+                            UnpinBuffer(buf_desc, false);
+                        }
+                    }
+            }
+
+            /*    if retcode has not been set to one of the unusual conditions
+            **        namely failed header validity or tag changed
+            **    then the setting of valid takes precedence
+            **    over whatever retcode may be currently set to.
+            */
+            if ( ( (retcode == BUF_INTENT_RC_INVALID_NO_AIO) || (retcode == BUF_INTENT_RC_INVALID_AIO) ) && valid) {
+                   retcode = BUF_INTENT_RC_VALID;
+            } else
+            if ((retcode == BUF_INTENT_RC_VALID) && (!valid)) {
+                   if (aio_successful == -1) { /* aio not attempted */
+                       retcode = BUF_INTENT_RC_INVALID_NO_AIO;
+                   } else {
+                       retcode = BUF_INTENT_RC_INVALID_AIO;
+                   }
+            }
+
+            return retcode;
+}
--- src/backend/storage/buffer/buf_init.c.orig	2014-05-31 17:19:07.873208825 -0400
+++ src/backend/storage/buffer/buf_init.c	2014-05-31 19:53:09.060073526 -0400
@@ -13,15 +13,89 @@
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
+#include <sys/types.h> 
+#include <unistd.h>
 
 #include "storage/bufmgr.h"
 #include "storage/buf_internals.h"
-
+#include <stdlib.h> /* for getenv() */
+#include <errno.h> /* for strtoul() */
 
 BufferDesc *BufferDescriptors;
 char	   *BufferBlocks;
-int32	   *PrivateRefCount;
+int32	   *PrivateRefCount;       /*  array of counts per buffer of how many times this task has pinned this buffer */
+
+volatile struct BAiocbAnchor *BAiocbAnchr = (struct BAiocbAnchor *)0;  /*  anchor for all control blocks pertaining to aio  */
 
+int CountInuseBAiocbs(void);     /*  keep compiler happy */
+void ReportFreeBAiocbs(void);    /*  keep compiler happy */
+
+extern int	MaxConnections;  /*  max number of client connections which postmaster will allow  */
+int numBufferAiocbs = 0;         /*  total number of  BufferAiocbs in pool (0 <=> no async io) */
+int hwmBufferAiocbs = 0;         /*  high water mark of in-use  BufferAiocbs in pool
+                                 **  (not required to be accurate, kindly maintained for us somehow by postmaster)
+                                 */
+
+#ifdef USE_PREFETCH
+unsigned int prefetch_dbOid = 0; /*  database oid of relations on which prefetching to be done - 0 means all */
+unsigned int prefetch_bitmap_scans = 1; /*  boolean whether to prefetch bitmap heap scans        */
+unsigned int prefetch_heap_scans = 0;   /*  boolean whether to prefetch non-bitmap heap scans    */
+unsigned int prefetch_sequential_index_scans = 0;  /*  boolean whether to prefetch sequential-access non-bitmap index scans  */
+unsigned int prefetch_index_scans = 256;  /*  boolean whether to prefetch non-bitmap index scans also numeric size of pfch_list  */
+unsigned int prefetch_btree_heaps = 1;  /*  boolean whether to prefetch heap pages in _bt_next for non-bitmap index scans   */
+#endif /* USE_PREFETCH */
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+int maxGetBAiocbTries = 1;       /*  max times we will try to get a free BufferAiocb */
+int maxRelBAiocbTries = 1;       /*  max times we will try to release a BufferAiocb back to freelist */
+
+/*  locking protocol for manipulating the BufferAiocbs and FreeBAiocbs list :
+**    1.    ownership of a BufferAiocb :
+**          to gain ownership of a BufferAiocb, a task must
+**          EITHER    remove it from FreeBAiocbs (it is now temporary owner and no other task can find it)
+**                    if decision is to attach it to a buffer descriptor header, then
+**                       .   lock the buffer descriptor header
+**                       .   check  NOT flags & BM_AIO_IN_PROGRESS
+**                       .   attach to buffer descriptor header
+**                       .   increment the BufferAiocb.dependent_count
+**                       .   unlock the buffer descriptor header
+**                     and ownership scope is from lock to unlock
+***          OR        locate it by dereferencing the pointer in a buffer descriptor,
+**                    in which case :
+**                       .   lock the buffer descriptor header
+**                       .   check  flags & BM_AIO_IN_PROGRESS
+**                       .   increment the BufferAiocb.dependent_count
+**                       .   if decision is to return to FreeBAiocbs,
+**                           then   (with buffer descriptor header still locked)
+**                                  .  turn off BM_AIO_IN_PROGRESS
+**                       .   IF        the BufferAiocb.dependent_count == 1 (I am sole dependent)
+**                       .   THEN
+**                       .       .  decrement the BufferAiocb.dependent_count
+**                               .  return to FreeBAiocbs (see below)
+**                       .   unlock the buffer descriptor header
+**                     and ownership scope is from lock to either return to FreeBAiocbs or unlock
+**    2.    adding and removing from FreeBAiocbs :
+**      two alternative methods - controlled by conditional macro definition LOCK_BAIOCB_FOR_GET_REL
+**       2.1 LOCK_BAIOCB_FOR_GET_REL is defined - use a lock
+**          .   lock BufFreelistLock exclusive
+**          .   add / remove from FreeBAiocbs
+**          .   unlock BufFreelistLock exclusive
+**          advantage of this method  -  never fails to add or remove
+**       2.2  LOCK_BAIOCB_FOR_GET_REL is not defined - use compare_and_swap
+**          .   retrieve the current Freelist pointer and validate
+**          .   compare_and_swap on/off the FreeBAiocb list
+**          .   unlock BufFreelistLock exclusive
+**          advantage of this method  -  never waits
+**          to avoid losing a FreeBAiocbs,   save it in a process-local cache and reuse
+*/
+#else   /*  not USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+        /*   this dummy structure is to ensure that references to these fields in other bufmgr runtime code
+        **   that is not conditionally ifdefd on USE_AIO_ATOMIC_BUILTIN_COMP_SWAP compiles and runs correctly
+        */
+        struct BufferAiocb dummy_BAiocbAnchr = { (struct BufferAiocb*)0 , (struct BufferAiocb*)0 };
+int maxGetBAiocbTries = -1;       /*  max times we will try to get a free BufferAiocb */
+int maxRelBAiocbTries = -1;       /*  max times we will try to release a BufferAiocb back to freelist */
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * Data Structures:
@@ -73,7 +147,14 @@ void
 InitBufferPool(void)
 {
 	bool		foundBufs,
-				foundDescs;
+				foundDescs
+                        , foundAiocbs
+          ;
+#if defined(USE_PREFETCH) || defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP)
+        char *envvarpointer = (char *)0;  /*  might point to an environment variable string */
+        char *charptr;
+#endif /* USE_PREFETCH */
+
 
 	BufferDescriptors = (BufferDesc *)
 		ShmemInitStruct("Buffer Descriptors",
@@ -83,6 +164,142 @@ InitBufferPool(void)
 		ShmemInitStruct("Buffer Blocks",
 						NBuffers * (Size) BLCKSZ, &foundBufs);
 
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        BAiocbAnchr = (struct BAiocbAnchor *)0; /*  anchor for all control blocks pertaining to aio  */
+        if (max_async_io_prefetchers < 0) {  /*  negative value indicates to initialize to something sensible during buf_init */
+            max_async_io_prefetchers = MaxConnections/6;  /*  default allows for average of MaxConnections/6 concurrent prefetchers  - reasonable ??? */
+        }
+
+        if ((target_prefetch_pages > 0) && (max_async_io_prefetchers > 0)) {
+            int ix;
+            volatile struct BufferAiocb *BufferAiocbs;
+            volatile struct BufferAiocb * volatile FreeBAiocbs;
+
+            numBufferAiocbs = (target_prefetch_pages*max_async_io_prefetchers);  /*  target_prefetch_pages per prefetcher */
+            BAiocbAnchr = (struct BAiocbAnchor *)
+		ShmemInitStruct("Buffer Aiocbs",
+                          sizeof(struct BAiocbAnchor) + (numBufferAiocbs * sizeof(struct BufferAiocb)), &foundAiocbs);
+            if (BAiocbAnchr) {
+                BufferAiocbs = BAiocbAnchr->BufferAiocbs = (struct BufferAiocb*)(((char *)BAiocbAnchr) + sizeof(struct BAiocbAnchor));
+                FreeBAiocbs = (struct BufferAiocb*)0;
+                for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                    (BufferAiocbs+ix)->BAiocbnext = FreeBAiocbs;   /*  init the free list,  last one -> 0  */
+                    (BufferAiocbs+ix)->BAiocbbufh = (struct sbufdesc*)0;
+                    (BufferAiocbs+ix)->BAiocbDependentCount = 0;
+                    (BufferAiocbs+ix)->pidOfAio = 0;
+                    FreeBAiocbs = (BufferAiocbs+ix);
+
+                }
+                BAiocbAnchr->FreeBAiocbs = FreeBAiocbs;
+                envvarpointer = getenv("PG_MAX_GET_BAIOCB_TRIES");
+                if (    (envvarpointer != (char *)0)
+                     && (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   )
+                   ) {
+                      maxGetBAiocbTries = strtol(envvarpointer, 0, 10);
+                }
+                envvarpointer = getenv("PG_MAX_REL_BAIOCB_TRIES");
+                if (    (envvarpointer != (char *)0)
+                     && (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   )
+                   ) {
+                      maxRelBAiocbTries = strtol(envvarpointer, 0, 10);
+                }
+
+                /*   init the aio subsystem max number of threads and max number of requests
+                **   max number of threads   <-->  max_async_io_prefetchers
+                **   max number of requests  <-->  numBufferAiocbs = (target_prefetch_pages*max_async_io_prefetchers)
+                **   there is no return code so we just hope.
+                */
+                smgrinitaio(max_async_io_prefetchers , numBufferAiocbs);
+
+            }
+        }
+#else   /*  not USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+        /*   this dummy structure is to ensure that references to these fields in other bufmgr runtime code
+        **   that is not conditionally ifdefd on USE_AIO_ATOMIC_BUILTIN_COMP_SWAP compiles and runs correctly
+        */
+        BAiocbAnchr = &dummy_BAiocbAnchr;
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+
+#ifdef USE_PREFETCH
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_BITMAP");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_bitmap_scans = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+                 prefetch_bitmap_scans = 1;
+             }
+        }
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_ISCAN");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_index_scans = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+                 prefetch_index_scans = 1;
+             } else
+             if (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   ) {
+                 prefetch_index_scans = strtol(envvarpointer, &charptr, 10);
+                 if (charptr && (',' == *charptr)) {   /*  optional sequential prefetch in index scans */
+					 charptr++;        /*   following the comma ... */
+					 if ( ('Y' == *charptr) || ('y' == *charptr) || ('1' == *charptr) ) {
+                         prefetch_sequential_index_scans = 1;
+					 }
+				 }
+             }
+             /*  if prefeching for ISCAN,  then we require size of pfch_list to be at least target_prefetch_pages */
+             if (   (prefetch_index_scans > 0)
+                 && (prefetch_index_scans < target_prefetch_pages)
+                ) {
+                 prefetch_index_scans = target_prefetch_pages;
+             }
+        }
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_BTREE");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_btree_heaps = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+                 prefetch_btree_heaps = 1;
+             }
+        }
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_HEAP");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_heap_scans = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+              prefetch_heap_scans = 1;
+             }
+        }
+        envvarpointer = getenv("PG_PREFETCH_DBOID");
+        if (    (envvarpointer != (char *)0)
+             && (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   )
+           ) {
+              errno = 0;   /*  required in order to distinguish error from 0 */
+              prefetch_dbOid = (unsigned int)strtoul((const char *)envvarpointer, 0, 10);
+              if (errno) {
+                  prefetch_dbOid = 0;
+              }
+        }
+        elog(LOG, "prefetching initialised with target_prefetch_pages= %d "
+                  ", max_async_io_prefetchers= %d implying aio concurrency= %d "
+                  ", prefetching_for_bitmap= %s "
+                  ", prefetching_for_heap= %s "
+                  ", prefetching_for_iscan= %d with sequential_index_page_prefetching= %s "
+                  ", prefetching_for_btree= %s"
+                   ,target_prefetch_pages ,max_async_io_prefetchers ,numBufferAiocbs
+                   ,(prefetch_bitmap_scans ? "Y" : "N")
+                   ,(prefetch_heap_scans ? "Y" : "N")
+                   ,prefetch_index_scans
+                   ,(prefetch_sequential_index_scans ? "Y" : "N")
+                   ,(prefetch_btree_heaps ? "Y" : "N")
+            );
+#endif /* USE_PREFETCH */
+
+
 	if (foundDescs || foundBufs)
 	{
 		/* both should be present or neither */
@@ -176,3 +393,80 @@ BufferShmemSize(void)
 
 	return size;
 }
+
+/*     imprecise count of number of in-use BAiocbs at any time
+ *     we scan the array read-only without latching so are subject to unstable result
+ *     (but since the array is in well-known contiguous storage,
+ *     we are not subject to segmentation violation)
+ *     This function may be called at any time and just does its best
+ *     return the count of what we counted.
+ */
+int
+CountInuseBAiocbs(void)
+{
+        volatile struct BufferAiocb *BAiocb;
+        int count = 0;
+        int ix;
+
+
+        if (BAiocbAnchr != (struct BAiocbAnchor *)0 ) {
+            BAiocb = BAiocbAnchr->BufferAiocbs;             /*   start of list */
+
+            for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                    if ((struct BufferAiocb*)BAIOCB_OCCUPIED == (BAiocb+ix)->BAiocbnext) {   /* not on freelist ? */
+                        count++;
+                    }
+            }
+        }
+        return count;
+}
+
+/*
+ * report how many free BAiocbs at shutdown
+ * DO NOT call this while backends are actively working!!
+ * this report is useful when compare_and_swap method used (see above)
+ * as it can be used to deduce how many BAiocbs were in process-local caches -
+ * (original_number_on_freelist_at_startup - this_reported_number_at_shutdown)
+ */
+void
+ReportFreeBAiocbs(void)
+{
+        volatile struct BufferAiocb *BAiocb;
+        volatile struct BufferAiocb *BufferAiocbs;
+        int count = 0;
+        int fx , ix;
+
+
+        if (BAiocbAnchr != (struct BAiocbAnchor *)0 ) {
+            BAiocb = BAiocbAnchr->FreeBAiocbs;             /*   start of free list */
+            BufferAiocbs = BAiocbAnchr->BufferAiocbs;
+
+            for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                    (BufferAiocbs+ix)->BAiocbDependentCount = 0;  /* use this as marker for finding it on freelist */
+            }
+            for (fx = (numBufferAiocbs-1);  ( (fx>=0) &&  ( BAiocb != (struct BufferAiocb*)0 ) );  fx--) {
+                    
+                    /*  check if it is a valid BufferAiocb */
+                    for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                        if ((BufferAiocbs+ix) == BAiocb) { /*  is it this one ? */
+                             break;
+                        }
+                    }
+                    if (ix >= 0) {
+                        if (BAiocb->BAiocbDependentCount) {   /* seen it already ? */
+                            elog(LOG, "ReportFreeBAiocbs closed cycle on AIO control block freelist %p"
+                                          ,BAiocb);
+                            fx = 0; /* give up at this point */
+                        }
+                        BAiocb->BAiocbDependentCount = 1;  /* use this as marker for finding it on freelist */
+                        count++;
+                        BAiocb = BAiocb->BAiocbnext;
+                    } else {
+                        elog(LOG, "ReportFreeBAiocbs invalid item on AIO control block freelist %p"
+                                          ,BAiocb);
+                        fx = 0; /* give up at this point */
+                    }
+            }
+        }
+        elog(LOG, "ReportFreeBAiocbs AIO control block list : poolsize= %d  in-use-hwm= %d  final-free= %d" ,numBufferAiocbs , hwmBufferAiocbs , count);
+}
--- src/backend/storage/smgr/md.c.orig	2014-05-31 17:19:07.881208848 -0400
+++ src/backend/storage/smgr/md.c	2014-05-31 19:53:09.084073587 -0400
@@ -647,6 +647,62 @@ mdprefetch(SMgrRelation reln, ForkNumber
 }
 
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*
+ *	mdinitaio() --  init the aio subsystem max number of threads and max number of requests
+ */
+void
+mdinitaio(int max_aio_threads, int max_aio_num)
+{
+     FileInitaio( max_aio_threads, max_aio_num );
+}
+
+/*
+ *	mdstartaio() -- start aio read of the specified block of a relation
+ */
+void
+mdstartaio(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum , char *aiocbp , int *retcode )
+{
+#ifdef USE_PREFETCH
+	off_t		seekpos;
+	MdfdVec    *v;
+        int local_retcode;
+
+	v = _mdfd_getseg(reln, forknum, blocknum, false, EXTENSION_FAIL);
+
+	seekpos = (off_t) BLCKSZ *(blocknum % ((BlockNumber) RELSEG_SIZE));
+
+	Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+
+	local_retcode = FileStartaio(v->mdfd_vfd, seekpos, BLCKSZ , aiocbp);
+	if (retcode) {
+            *retcode = local_retcode;
+        }
+#endif   /* USE_PREFETCH */
+}
+
+
+/*
+ *	mdcompleteaio() -- complete aio read of the specified block of a relation
+ *      on entry, *inoutcode should indicate :
+ *           .  non-0  <=>   check if complete and wait if not
+ *           .  0      <=>   cancel io immediately
+ */
+void
+mdcompleteaio( char *aiocbp , int *inoutcode )
+{
+#ifdef USE_PREFETCH
+        int local_retcode;
+
+	local_retcode = FileCompleteaio(aiocbp, (inoutcode ? *inoutcode : 0));
+	if (inoutcode) {
+            *inoutcode = local_retcode;
+        }
+#endif   /* USE_PREFETCH */
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+
 /*
  *	mdread() -- Read the specified block from a relation.
  */
--- src/backend/storage/smgr/smgr.c.orig	2014-05-31 17:19:07.881208848 -0400
+++ src/backend/storage/smgr/smgr.c	2014-05-31 19:53:09.108073648 -0400
@@ -49,6 +49,12 @@ typedef struct f_smgr
 						 BlockNumber blocknum, char *buffer, bool skipFsync);
 	void		(*smgr_prefetch) (SMgrRelation reln, ForkNumber forknum,
 											  BlockNumber blocknum);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+	void		(*smgr_initaio) (int max_aio_threads, int max_aio_num);
+	void		(*smgr_startaio) (SMgrRelation reln, ForkNumber forknum,
+											  BlockNumber blocknum , char *aiocbp , int *retcode );
+	void		(*smgr_completeaio) ( char *aiocbp , int *inoutcode );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 	void		(*smgr_read) (SMgrRelation reln, ForkNumber forknum,
 										  BlockNumber blocknum, char *buffer);
 	void		(*smgr_write) (SMgrRelation reln, ForkNumber forknum,
@@ -66,7 +72,11 @@ typedef struct f_smgr
 static const f_smgr smgrsw[] = {
 	/* magnetic disk */
 	{mdinit, NULL, mdclose, mdcreate, mdexists, mdunlink, mdextend,
-		mdprefetch, mdread, mdwrite, mdnblocks, mdtruncate, mdimmedsync,
+		mdprefetch
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                        ,mdinitaio, mdstartaio, mdcompleteaio
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+              , mdread, mdwrite, mdnblocks, mdtruncate, mdimmedsync,
 		mdpreckpt, mdsync, mdpostckpt
 	}
 };
@@ -612,6 +622,35 @@ smgrprefetch(SMgrRelation reln, ForkNumb
 	(*(smgrsw[reln->smgr_which].smgr_prefetch)) (reln, forknum, blocknum);
 }
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*
+ *	smgrinitaio() -- initialize the aio subsystem max number of threads and max number of requests
+ */
+void
+smgrinitaio(int max_aio_threads, int max_aio_num)
+{
+	(*(smgrsw[0].smgr_initaio)) ( max_aio_threads, max_aio_num );
+}
+
+/*
+ *	smgrstartaio() -- Initiate aio read of the specified block of a relation.
+ */
+void
+smgrstartaio(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum , char *aiocbp , int *retcode )
+{
+	(*(smgrsw[reln->smgr_which].smgr_startaio)) (reln, forknum, blocknum , aiocbp , retcode );
+}
+
+/*
+ *	smgrcompleteaio() -- Complete aio read of the specified block of a relation.
+ */
+void
+smgrcompleteaio(SMgrRelation reln,  char *aiocbp , int *inoutcode )
+{
+	(*(smgrsw[reln->smgr_which].smgr_completeaio)) ( aiocbp , inoutcode );
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 /*
  *	smgrread() -- read a particular block from a relation into the supplied
  *				  buffer.
--- src/backend/storage/file/fd.c.orig	2014-05-31 17:19:07.877208836 -0400
+++ src/backend/storage/file/fd.c	2014-05-31 19:53:09.136073719 -0400
@@ -77,6 +77,9 @@
 #include "utils/guc.h"
 #include "utils/resowner_private.h"
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+#include "aio.h"
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * We must leave some file descriptors free for system(), the dynamic loader,
@@ -1239,6 +1242,10 @@ FileClose(File file)
  * We could add an implementation using libaio in the future; but note that
  * this API is inappropriate for libaio, which wants to have a buffer provided
  * to read into.
+ * Also note that a new, different implementation of asynchronous prefetch
+ * using librt,  not libaio,  is provided by the two functions following this one,
+ * FileStartaio and FileCompleteaio.   These also require to have a buffer provided
+ * to read into,  which the new async_io support provides.
  */
 int
 FilePrefetch(File file, off_t offset, int amount)
@@ -1266,6 +1273,145 @@ FilePrefetch(File file, off_t offset, in
 #endif
 }
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*
+ * FileInitaio - initialize the aio subsystem max number of threads and max number of requests
+ *  input parms
+ *  max_aio_threads;    maximum number of threads
+ *  max_aio_num;        maximum number of concurrent aio read requests
+ *
+ *  on linux, man page for the librt implemenation of aio_init() says :
+ *         This function is a GNU extension.
+ *  If your posix aio does not have it,   then add the following line to 
+ *        src/include/pg_config_manual.h
+ *    #define DONT_HAVE_AIO_INIT
+ *  to render it as a no-op
+ */
+void
+FileInitaio(int max_aio_threads, int max_aio_num )
+{
+#ifndef DONT_HAVE_AIO_INIT
+    struct aioinit aioinit_struct;  /*  structure to pass to aio_init */
+
+    aioinit_struct.aio_threads = max_aio_threads; /*     maximum number of threads */
+    aioinit_struct.aio_num = max_aio_num;         /*     maximum number of concurrent aio read requests */
+    aioinit_struct.aio_idle_time = 1;             /*     we dont want to alter this but aio_init does not ignore it so set to the default */
+    aio_init(&aioinit_struct);
+#endif  /* ndef DONT_HAVE_AIO_INIT */
+    return;
+}
+
+/*
+ * FileStartaio - initiate asynchronous read of a given range of the file.
+ * The logical seek position is unaffected.
+ *
+ * use standard posix aio (librt)
+ *  ASSUME   BufferAiocb.aio_buf already set to -> buffer by caller
+ *  return 0 if successfully started,  else non-zero
+ */
+int
+FileStartaio(File file, off_t offset, int amount , char *aiocbp )
+{
+	int	returnCode;
+        struct aiocb *my_aiocbp = (struct aiocb *)aiocbp;
+
+	Assert(FileIsValid(file));
+
+	DO_DB(elog(LOG, "FileStartaio: %d (%s) " INT64_FORMAT " %d",
+			   file, VfdCache[file].fileName,
+			   (int64) offset, amount));
+
+	returnCode = FileAccess(file);
+	if (returnCode >= 0) {
+
+            my_aiocbp->aio_fildes = VfdCache[file].fd;
+            my_aiocbp->aio_lio_opcode = LIO_READ;
+            my_aiocbp->aio_nbytes = amount;
+            my_aiocbp->aio_offset = offset;
+            returnCode = aio_read(my_aiocbp);
+        }
+
+	return returnCode;
+}
+
+/*
+ * FileCompleteaio - complete asynchronous aio read
+ * normal_wait indicates whether to cancel or wait -
+ *                 0 <=> cancel
+ *                 1 <=> wait by polling the aiocb
+ *                 2 <=> wait by suspending on the aiocb
+ *
+ * use standard posix aio (librt)
+ *  return 0 if successfull and did not have to wait,
+ *         1 if successfull and had to wait,
+ *    else x'ff'
+ */
+int
+FileCompleteaio( char *aiocbp , int normal_wait )
+{
+	int	returnCode;
+	int	aio_errno;
+        struct aiocb *my_aiocbp = (struct aiocb *)aiocbp;
+        const struct aiocb const*cblist[1];
+        int fd;
+        struct timespec my_timeout = { 0 , 10000 };
+        struct timespec *suspend_timeout_P; /*  the timeout actually used depending on normal_wait */
+        int max_polls;
+
+        fd = my_aiocbp->aio_fildes;
+        cblist[0] = my_aiocbp;
+        returnCode = aio_errno = aio_error(my_aiocbp);
+        /* note that aio_error returns 0 if op already completed successfully */
+
+        /*  first handle normal case of waiting for op to complete  */
+        if (normal_wait) {
+            /*   if told not to poll,   then specify no timeout  */
+            suspend_timeout_P = (normal_wait == 1 ? &my_timeout : (struct timespec *)0);
+            while (aio_errno == EINPROGRESS) {
+                max_polls = 256;
+                my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                returnCode = aio_suspend(cblist , 1 , suspend_timeout_P);
+                while (    (returnCode < 0) && (max_polls-- > 0)
+                        && ((EAGAIN == errno) || (EINTR == errno))
+                      ) {
+                    my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                    returnCode = aio_suspend(cblist , 1 , suspend_timeout_P);
+                }
+                returnCode = aio_errno = aio_error(my_aiocbp);
+                /*  now return_code is from aio_error  */
+                if (returnCode == 0) {
+                    returnCode = 1;    /*  successful but had to wait */
+                }
+            }
+            if (aio_errno) {
+                elog(LOG, "FileCompleteaio: %d %d", fd, returnCode);
+                returnCode = 0xff; /*  unsuccessful */
+            }
+        } else {
+            if (aio_errno == EINPROGRESS) {
+                do {
+                        max_polls = 256;
+                        my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                        returnCode = aio_cancel(fd, my_aiocbp);
+                        while ((returnCode == AIO_NOTCANCELED) && (max_polls-- > 0)) {
+                            my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                            returnCode = aio_cancel(fd, my_aiocbp);
+                        }
+                    returnCode = aio_errno = aio_error(my_aiocbp);
+                } while (aio_errno == EINPROGRESS);
+                returnCode = 0xff; /*  unsuccessful */
+            }
+            if (returnCode != 0)
+                returnCode = 0xff; /*  unsuccessful */
+        }
+
+	DO_DB(elog(LOG, "FileCompleteaio: %d %d",
+			   fd, returnCode));
+
+	return returnCode;
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 int
 FileRead(File file, char *buffer, int amount)
 {
--- src/backend/storage/lmgr/proc.c.orig	2014-05-31 17:19:07.881208848 -0400
+++ src/backend/storage/lmgr/proc.c	2014-05-31 19:53:09.168073800 -0400
@@ -52,6 +52,7 @@
 #include "utils/timeout.h"
 #include "utils/timestamp.h"
 
+extern pid_t this_backend_pid;     /*   pid of this backend */
 
 /* GUC variables */
 int			DeadlockTimeout = 1000;
@@ -361,6 +362,7 @@ InitProcess(void)
 	MyPgXact->xid = InvalidTransactionId;
 	MyPgXact->xmin = InvalidTransactionId;
 	MyProc->pid = MyProcPid;
+        this_backend_pid = getpid();    /*    pid of this backend */
 	/* backendId, databaseId and roleId will be filled in later */
 	MyProc->backendId = InvalidBackendId;
 	MyProc->databaseId = InvalidOid;
--- src/backend/access/heap/heapam.c.orig	2014-05-31 17:19:07.785208591 -0400
+++ src/backend/access/heap/heapam.c	2014-05-31 19:53:09.228073952 -0400
@@ -71,6 +71,28 @@
 #include "utils/syscache.h"
 #include "utils/tqual.h"
 
+#ifdef USE_PREFETCH
+#include <sys/types.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+#include "executor/instrument.h"
+
+extern unsigned int prefetch_dbOid; /*  database oid of relations on which prefetching to be done - 0 means all */
+extern unsigned int prefetch_heap_scans; /*  boolean whether to prefetch non-bitmap heap scans         */
+
+/*  special values for scan->rs_prefetch_target indicating as follows :               */
+#define PREFETCH_MAYBE 0xffffffff      /*   prefetch permitted but not yet in effect  */
+#define PREFETCH_DISABLED 0xfffffffe   /*   prefetch disabled and not permitted       */
+/*  PREFETCH_WRAP_POINT indicates a pretcher who has reached the point where the scan would wrap -
+**  at this point the prefetcher runs on the spot until scan catches up.
+**  This *must* be < maximum valid setting of target_prefetch_pages aka effective_io_concurrency.
+*/
+#define PREFETCH_WRAP_POINT 0x0fffffff
+
+#endif   /* USE_PREFETCH */
+
 
 /* GUC variable */
 bool		synchronize_seqscans = true;
@@ -115,6 +137,8 @@ static XLogRecPtr log_heap_new_cid(Relat
 static HeapTuple ExtractReplicaIdentity(Relation rel, HeapTuple tup, bool key_modified,
 					   bool *copy);
 
+static void heap_unread_add(HeapScanDesc scan, BlockNumber blockno);
+static void heap_unread_subtract(HeapScanDesc scan, BlockNumber blockno);
 
 /*
  * Each tuple lock mode has a corresponding heavyweight lock, and one or two
@@ -292,10 +316,149 @@ initscan(HeapScanDesc scan, ScanKey key,
 	 * Currently, we don't have a stats counter for bitmap heap scans (but the
 	 * underlying bitmap index scans will be counted).
 	 */
-	if (!scan->rs_bitmapscan)
+#ifdef USE_PREFETCH
+        /*    by default,  no prefetching on any scan  */
+        scan->rs_prefetch_target = PREFETCH_DISABLED;  /*  tentatively disable  */
+        scan->rs_pfchblock = 0; /*  scanner will reset this to be ahead of scan */
+        scan->rs_Unread_Pfetched_base = (BlockNumber *)0; /*  list of prefetched but unread blocknos */
+        scan->rs_Unread_Pfetched_next = 0; /*  next unread blockno */
+        scan->rs_Unread_Pfetched_count = 0; /* number of valid unread blocknos */
+#endif   /* USE_PREFETCH */
+	if (!scan->rs_bitmapscan) {
+
 		pgstat_count_heap_scan(scan->rs_rd);
+#ifdef USE_PREFETCH
+                /*    bitmap scans do their own prefetching -
+                **    for others,  set up prefetching now
+                */
+                if (    prefetch_heap_scans
+                     && (target_prefetch_pages > 0)
+                     &&	(!RelationUsesLocalBuffers(scan->rs_rd))
+                   ) {
+                      /*   prefetch_dbOid may be set to a database Oid to specify only prefetch in that db */
+                      if (    (   (prefetch_dbOid > 0)
+                               && (prefetch_dbOid == scan->rs_rd->rd_node.dbNode)
+                              )
+                          ||  (prefetch_dbOid == 0)
+                         ) {
+                          scan->rs_prefetch_target = PREFETCH_MAYBE;    /*  permitted but let the scan decide */
+                      }
+                      else {
+                      }
+                }
+#endif   /* USE_PREFETCH */
+        }
+}
+
+/* add this blockno to list of prefetched and unread blocknos
+** use the one identified by the (next+count|modulo circumference) index if it is unused,
+** else search for the first available slot if there is one,
+** else error.
+*/
+static void
+heap_unread_add(HeapScanDesc scan, BlockNumber blockno)
+{
+      BlockNumber *available_P;   /*  where to store new blockno */
+      unsigned int Unread_Pfetched_index = scan->rs_Unread_Pfetched_next
+                                         + scan->rs_Unread_Pfetched_count; /* index of next unused slot */
+
+      /*  caller is not supposed to pass InvalidBlockNumber but check anyway */
+      if (blockno != InvalidBlockNumber) {
+
+		  /*  ensure there is some room somewhere   */
+		  if (scan->rs_Unread_Pfetched_count < target_prefetch_pages) {
+
+			  /*  try the "next+count" one */
+			  if (Unread_Pfetched_index >= target_prefetch_pages) {
+				  Unread_Pfetched_index -= target_prefetch_pages;  /* modulo circumference */
+			  }
+			  available_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);   /*  where to store new blockno */
+			  if (*available_P == InvalidBlockNumber) { /* unused */
+				  goto store_blockno;
+			  } else {
+                  /*  slow-search the entire list  */
+				  for (Unread_Pfetched_index = 0; Unread_Pfetched_index < target_prefetch_pages; Unread_Pfetched_index++) {
+					  available_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);   /*  where to store new blockno */
+					  if (*available_P == InvalidBlockNumber) { /* unused */
+                          /*  before storing this blockno,
+                          **  since the next pointer did not locate an unused slot,
+                          **  set it to one which is more likely to be so for the next time
+                          */
+                          scan->rs_Unread_Pfetched_next = Unread_Pfetched_index;
+						  goto store_blockno;
+					  }
+				  }
+			  }
+		  }
+
+          /*  if we reach here,    either there was no available slot
+          **  or we thought there was one and didn't find any  --
+          */
+  		  ereport(NOTICE,
+			  (errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("heap_unread_add overflowed list cannot add blockno %d", blockno)));
+
+  		  return;
 }
 
+    store_blockno:
+      *available_P = blockno;
+      scan->rs_Unread_Pfetched_count++;  /*  update count */
+
+    return;
+}
+
+/* remove specified blockno from list of prefetched and unread blocknos.
+** Usually this will be found at the rs_Unread_Pfetched_next item -
+** else search for it.    If not found,   inore it  -  no error results.
+*/
+static void
+heap_unread_subtract(HeapScanDesc scan, BlockNumber blockno)
+{
+      unsigned int Unread_Pfetched_index = scan->rs_Unread_Pfetched_next; /* index of next unread blockno */
+      BlockNumber *candidate_P;   /*  location of callers blockno - maybe */
+      BlockNumber nextUnreadPfetched;
+
+      /*  caller is not supposed to pass InvalidBlockNumber but check anyway */
+      if (    (blockno != InvalidBlockNumber)
+		   && ( scan->rs_Unread_Pfetched_count > 0 )   /*  if the list is not empty  */
+         ) {
+
+			  /*  take modulo of the circumference.
+			  **  actually rs_Unread_Pfetched_next should never exceed the circumference but check anyway.
+			  */
+			  if (Unread_Pfetched_index >= target_prefetch_pages) {
+				  Unread_Pfetched_index  -= target_prefetch_pages;
+			  }
+			  candidate_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);
+			  nextUnreadPfetched = *candidate_P;
+
+			  if ( nextUnreadPfetched == blockno ) {
+				  goto remove_blockno;
+			  } else {
+                  /*  slow-search the entire list  */
+				  for (Unread_Pfetched_index = 0; Unread_Pfetched_index < target_prefetch_pages; Unread_Pfetched_index++) {
+					  candidate_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);   /*  where to store new blockno */
+					  if (*candidate_P == blockno) { /* unused */
+						  goto remove_blockno;
+					  }
+				  }
+			  }
+
+    remove_blockno:
+			  *candidate_P = InvalidBlockNumber;
+
+			  scan->rs_Unread_Pfetched_next = (Unread_Pfetched_index+1);  /*  update next pfchd unread */
+			  if (scan->rs_Unread_Pfetched_next >= target_prefetch_pages) {
+					  scan->rs_Unread_Pfetched_next = 0;
+			  }
+			  scan->rs_Unread_Pfetched_count--;  /*  update count */
+	  }
+
+      return;
+}
+
+
 /*
  * heapgetpage - subroutine for heapgettup()
  *
@@ -304,7 +467,7 @@ initscan(HeapScanDesc scan, ScanKey key,
  * which tuples on the page are visible.
  */
 static void
-heapgetpage(HeapScanDesc scan, BlockNumber page)
+heapgetpage(HeapScanDesc scan, BlockNumber page , BlockNumber prefetchHWM)
 {
 	Buffer		buffer;
 	Snapshot	snapshot;
@@ -314,6 +477,10 @@ heapgetpage(HeapScanDesc scan, BlockNumb
 	OffsetNumber lineoff;
 	ItemId		lpp;
 	bool		all_visible;
+#ifdef USE_PREFETCH
+	int             PrefetchBufferRc;  /*   indicates whether requested prefetch block already in a buffer and if pin count on buffer has been increased */
+#endif   /* USE_PREFETCH */
+
 
 	Assert(page < scan->rs_nblocks);
 
@@ -336,6 +503,98 @@ heapgetpage(HeapScanDesc scan, BlockNumb
 									   RBM_NORMAL, scan->rs_strategy);
 	scan->rs_cblock = page;
 
+#ifdef USE_PREFETCH
+
+        heap_unread_subtract(scan, page);
+
+        /*    maybe prefetch some pages  starting with rs_pfchblock */
+        if (scan->rs_prefetch_target >= 0) {       /*   prefetching enabled on this scan ?                         */
+            int next_block_to_be_read = (page+1);  /*   next block to be read = lowest possible prefetchable block */
+            int num_to_pfch_this_time;             /*   eventually holds the number of blocks to prefetch now      */
+            int prefetchable_range;                /*   size of the area ahead of the current prefetch position    */
+
+            /*  check if prefetcher reached wrap point and the scan has now wrapped */
+            if (  (page == 0) && (scan->rs_prefetch_target == PREFETCH_WRAP_POINT)  ) {
+                scan->rs_prefetch_target = 1;
+                scan->rs_pfchblock = next_block_to_be_read;
+            } else
+            if (scan->rs_pfchblock < next_block_to_be_read) {
+                scan->rs_pfchblock = next_block_to_be_read; /* next block to be prefetched must be ahead of one we just read */
+            }
+
+            /* now we know where we would start prefetching -
+            ** next question   -  if this is a sync scan,  ensure we do not prefetch behind the HWM
+            ** debatable whether to require strict inequality or >=  -   >= works better in practice
+            */
+            if ( (!scan->rs_syncscan) || (scan->rs_pfchblock >= prefetchHWM) ) {
+
+                /* now we know where we will start prefetching -
+                ** next question   -  how many?
+                ** apply two limits :
+                **  1.   target prefetch distance
+                **  2.   number of available blocks ahead of us
+                */
+
+                /*  1.   target prefetch distance   */
+                num_to_pfch_this_time = next_block_to_be_read + scan->rs_prefetch_target; /* page beyond prefetch target */
+                num_to_pfch_this_time -= scan->rs_pfchblock;                              /*  convert to offset        */
+
+                /*   first do prefetching up to our current limit  ...
+                **   highest page number that a scan (pre)-fetches is scan->rs_nblocks-1
+                **   note  -  prefetcher does not wrap a prefetch range -
+                **            instead just stop and then start again if and when main scan wraps
+                */
+                if (scan->rs_pfchblock <= scan->rs_startblock) {  /*  if on second leg towards startblock */
+                    prefetchable_range = ((int)(scan->rs_startblock) - (int)(scan->rs_pfchblock));
+                }
+                else {                                            /*     on first leg towards nblocks     */
+                    prefetchable_range = ((int)(scan->rs_nblocks) - (int)(scan->rs_pfchblock));
+                }
+                if (prefetchable_range > 0) {           /*  if theres a range to prefetch */
+
+                    /*  2.   number of available blocks ahead of us        */
+                    if (num_to_pfch_this_time > prefetchable_range) {
+                        num_to_pfch_this_time = prefetchable_range;
+                    }
+                    while (num_to_pfch_this_time-- > 0) {
+                        PrefetchBufferRc = PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, scan->rs_pfchblock, scan->rs_strategy);
+                        /*  if pin acquired on buffer,  then remember in case of future Discard */
+                        if (PrefetchBufferRc & PREFTCHRC_BUF_PIN_INCREASED) {
+                            heap_unread_add(scan, scan->rs_pfchblock);
+						}
+                        scan->rs_pfchblock++;
+                        /*  if syncscan and requested block was already in buffer pool,
+                        **  this suggests that another scanner is ahead of us and we should advance
+                        */
+                        if ( (scan->rs_syncscan) && (PrefetchBufferRc & PREFTCHRC_BLK_ALREADY_PRESENT) ) {
+                            scan->rs_pfchblock++;
+                            num_to_pfch_this_time--;
+                        }
+                    }
+                }
+                else {
+                    /*   we must not modify scan->rs_pfchblock here
+                    **   because it is needed for possible DiscardBuffer at end of scan  ...
+                    **   ... instead ...
+                    */
+                    scan->rs_prefetch_target = PREFETCH_WRAP_POINT;  /*   mark this prefetcher as waiting to wrap */
+                }
+
+                /*   ...  then adjust prefetching limit : by doubling on each iteration */
+                if (scan->rs_prefetch_target == 0) {
+                    scan->rs_prefetch_target = 1;
+                }
+                else {
+                    scan->rs_prefetch_target *= 2;
+                    if (scan->rs_prefetch_target > target_prefetch_pages) {
+                        scan->rs_prefetch_target = target_prefetch_pages;
+                    }
+                }
+            }
+        }
+#endif   /* USE_PREFETCH */
+
+
 	if (!scan->rs_pageatatime)
 		return;
 
@@ -452,6 +711,8 @@ heapgettup(HeapScanDesc scan,
 	OffsetNumber lineoff;
 	int			linesleft;
 	ItemId		lpp;
+    BlockNumber  prefetchHWM = 0;   /*  HWM of prefetch BlockNum for all participants in same sync scan */
+    int          ix;
 
 	/*
 	 * calculate next starting lineoff, given scan direction
@@ -470,7 +731,25 @@ heapgettup(HeapScanDesc scan,
 				return;
 			}
 			page = scan->rs_startblock; /* first page */
-			heapgetpage(scan, page);
+#ifdef USE_PREFETCH
+                        /*  decide if we shall do prefetching :  only if :
+                        **   .  prefetching enabled for this scan
+                        **   .  not a bitmap scan (which do their own)
+                        **   .  sufficient number of blocks - at least twice the target_prefetch_pages
+                        */
+                        if (    (!scan->rs_bitmapscan)
+                             && (scan->rs_prefetch_target >= PREFETCH_MAYBE)
+                             && (scan->rs_nblocks > (2*target_prefetch_pages))
+                           )  {
+                                scan->rs_prefetch_target = 1;  /*  do prefetching on forward non-bitmap scan */
+                                scan->rs_Unread_Pfetched_base = (BlockNumber *)palloc(sizeof(BlockNumber)*target_prefetch_pages);
+                                /*  Initialise the list */
+                                for (ix = 0; ix < target_prefetch_pages; ix++) {
+									*(scan->rs_Unread_Pfetched_base + ix) = InvalidBlockNumber;
+								}
+                        }
+#endif   /* USE_PREFETCH */
+			heapgetpage(scan, page, prefetchHWM);
 			lineoff = FirstOffsetNumber;		/* first offnum */
 			scan->rs_inited = true;
 		}
@@ -516,7 +795,7 @@ heapgettup(HeapScanDesc scan,
 				page = scan->rs_startblock - 1;
 			else
 				page = scan->rs_nblocks - 1;
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 		}
 		else
 		{
@@ -557,7 +836,7 @@ heapgettup(HeapScanDesc scan,
 
 		page = ItemPointerGetBlockNumber(&(tuple->t_self));
 		if (page != scan->rs_cblock)
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 
 		/* Since the tuple was previously fetched, needn't lock page here */
 		dp = (Page) BufferGetPage(scan->rs_cbuf);
@@ -660,8 +939,10 @@ heapgettup(HeapScanDesc scan,
 			 * a little bit backwards on every invocation, which is confusing.
 			 * We don't guarantee any specific ordering in general, though.
 			 */
-			if (scan->rs_syncscan)
-				ss_report_location(scan->rs_rd, page);
+			if (scan->rs_syncscan) {
+                                prefetchHWM = scan->rs_pfchblock;
+				ss_report_location(scan->rs_rd, page, &prefetchHWM);
+                        }
 		}
 
 		/*
@@ -671,6 +952,22 @@ heapgettup(HeapScanDesc scan,
 		{
 			if (BufferIsValid(scan->rs_cbuf))
 				ReleaseBuffer(scan->rs_cbuf);
+#ifdef USE_PREFETCH
+                        if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                            /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                            while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+                        }
+#endif   /* USE_PREFETCH */
 			scan->rs_cbuf = InvalidBuffer;
 			scan->rs_cblock = InvalidBlockNumber;
 			tuple->t_data = NULL;
@@ -678,7 +975,7 @@ heapgettup(HeapScanDesc scan,
 			return;
 		}
 
-		heapgetpage(scan, page);
+		heapgetpage(scan, page, prefetchHWM);
 
 		LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);
 
@@ -727,6 +1024,8 @@ heapgettup_pagemode(HeapScanDesc scan,
 	OffsetNumber lineoff;
 	int			linesleft;
 	ItemId		lpp;
+    BlockNumber  prefetchHWM = 0;   /*  HWM of prefetch BlockNum for all participants in same sync scan */
+    int          ix;
 
 	/*
 	 * calculate next starting lineindex, given scan direction
@@ -745,7 +1044,25 @@ heapgettup_pagemode(HeapScanDesc scan,
 				return;
 			}
 			page = scan->rs_startblock; /* first page */
-			heapgetpage(scan, page);
+#ifdef USE_PREFETCH
+                        /*  decide if we shall do prefetching :  only if :
+                        **   .  prefetching enabled for this scan
+                        **   .  not a bitmap scan (which do their own)
+                        **   .  sufficient number of blocks - at least twice the target_prefetch_pages
+                        */
+                        if (    (!scan->rs_bitmapscan)
+                             && (scan->rs_prefetch_target >= PREFETCH_MAYBE)
+                             && (scan->rs_nblocks > (2*target_prefetch_pages))
+                           )  {
+                                scan->rs_prefetch_target = 1;  /*  do prefetching on forward non-bitmap scan */
+                                scan->rs_Unread_Pfetched_base = (BlockNumber *)palloc(sizeof(BlockNumber)*target_prefetch_pages);
+                                /*  Initialise the list */
+                                for (ix = 0; ix < target_prefetch_pages; ix++) {
+									*(scan->rs_Unread_Pfetched_base + ix) = InvalidBlockNumber;
+								}
+                        }
+#endif   /* USE_PREFETCH */
+			heapgetpage(scan, page, prefetchHWM);
 			lineindex = 0;
 			scan->rs_inited = true;
 		}
@@ -788,7 +1105,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 				page = scan->rs_startblock - 1;
 			else
 				page = scan->rs_nblocks - 1;
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 		}
 		else
 		{
@@ -826,7 +1143,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 
 		page = ItemPointerGetBlockNumber(&(tuple->t_self));
 		if (page != scan->rs_cblock)
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 
 		/* Since the tuple was previously fetched, needn't lock page here */
 		dp = (Page) BufferGetPage(scan->rs_cbuf);
@@ -921,8 +1238,10 @@ heapgettup_pagemode(HeapScanDesc scan,
 			 * a little bit backwards on every invocation, which is confusing.
 			 * We don't guarantee any specific ordering in general, though.
 			 */
-			if (scan->rs_syncscan)
-				ss_report_location(scan->rs_rd, page);
+			if (scan->rs_syncscan) {
+                                prefetchHWM = scan->rs_pfchblock;
+				ss_report_location(scan->rs_rd, page, &prefetchHWM);
+                        }
 		}
 
 		/*
@@ -932,6 +1251,22 @@ heapgettup_pagemode(HeapScanDesc scan,
 		{
 			if (BufferIsValid(scan->rs_cbuf))
 				ReleaseBuffer(scan->rs_cbuf);
+#ifdef USE_PREFETCH
+                        if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                            /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                             while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+                        }
+#endif   /* USE_PREFETCH */
 			scan->rs_cbuf = InvalidBuffer;
 			scan->rs_cblock = InvalidBlockNumber;
 			tuple->t_data = NULL;
@@ -939,7 +1274,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 			return;
 		}
 
-		heapgetpage(scan, page);
+		heapgetpage(scan, page, prefetchHWM);
 
 		dp = (Page) BufferGetPage(scan->rs_cbuf);
 		lines = scan->rs_ntuples;
@@ -1394,6 +1729,23 @@ void
 heap_rescan(HeapScanDesc scan,
 			ScanKey key)
 {
+
+#ifdef USE_PREFETCH
+         if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                       /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                            while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+         }
+#endif   /* USE_PREFETCH */
 	/*
 	 * unpin scan buffers
 	 */
@@ -1418,6 +1770,23 @@ heap_endscan(HeapScanDesc scan)
 {
 	/* Note: no locking manipulations needed */
 
+
+#ifdef USE_PREFETCH
+         if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                       /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                            while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+         }
+#endif   /* USE_PREFETCH */
 	/*
 	 * unpin scan buffers
 	 */
@@ -1435,6 +1804,10 @@ heap_endscan(HeapScanDesc scan)
 	if (scan->rs_strategy != NULL)
 		FreeAccessStrategy(scan->rs_strategy);
 
+    if (scan->rs_Unread_Pfetched_base) {
+        pfree(scan->rs_Unread_Pfetched_base);
+    }
+
 	if (scan->rs_temp_snap)
 		UnregisterSnapshot(scan->rs_snapshot);
 
@@ -1464,7 +1837,6 @@ heap_endscan(HeapScanDesc scan)
 #define HEAPDEBUG_3
 #endif   /* !defined(HEAPDEBUGALL) */
 
-
 HeapTuple
 heap_getnext(HeapScanDesc scan, ScanDirection direction)
 {
@@ -6347,6 +6719,25 @@ heap_markpos(HeapScanDesc scan)
 void
 heap_restrpos(HeapScanDesc scan)
 {
+
+
+#ifdef USE_PREFETCH
+                        if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                            /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                             while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+                        }
+#endif   /* USE_PREFETCH */
+
 	/* XXX no amrestrpos checking that ammarkpos called */
 
 	if (!ItemPointerIsValid(&scan->rs_mctid))
--- src/backend/access/heap/syncscan.c.orig	2014-05-31 17:19:07.785208591 -0400
+++ src/backend/access/heap/syncscan.c	2014-05-31 19:53:09.252074012 -0400
@@ -90,6 +90,7 @@ typedef struct ss_scan_location_t
 {
 	RelFileNode relfilenode;	/* identity of a relation */
 	BlockNumber location;		/* last-reported location in the relation */
+	BlockNumber prefetchHWM;	/* high-water-mark of prefetched Blocknum */
 } ss_scan_location_t;
 
 typedef struct ss_lru_item_t
@@ -113,7 +114,7 @@ static ss_scan_locations_t *scan_locatio
 
 /* prototypes for internal functions */
 static BlockNumber ss_search(RelFileNode relfilenode,
-		  BlockNumber location, bool set);
+		  BlockNumber location, bool set , BlockNumber *prefetchHWMp);
 
 
 /*
@@ -160,6 +161,7 @@ SyncScanShmemInit(void)
 			item->location.relfilenode.dbNode = InvalidOid;
 			item->location.relfilenode.relNode = InvalidOid;
 			item->location.location = InvalidBlockNumber;
+			item->location.prefetchHWM = InvalidBlockNumber;
 
 			item->prev = (i > 0) ?
 				(&scan_locations->items[i - 1]) : NULL;
@@ -185,7 +187,7 @@ SyncScanShmemInit(void)
  * data structure.
  */
 static BlockNumber
-ss_search(RelFileNode relfilenode, BlockNumber location, bool set)
+ss_search(RelFileNode relfilenode, BlockNumber location, bool set , BlockNumber *prefetchHWMp)
 {
 	ss_lru_item_t *item;
 
@@ -206,6 +208,22 @@ ss_search(RelFileNode relfilenode, Block
 			{
 				item->location.relfilenode = relfilenode;
 				item->location.location = location;
+                                /*  if prefetch information requested,
+                                **  then reconcile and either update or report back the new HWM.
+                                */
+                                if (prefetchHWMp)
+                                {
+                                    if (   (item->location.prefetchHWM == InvalidBlockNumber)
+                                        || (item->location.prefetchHWM < *prefetchHWMp)
+                                       )
+                                    {
+                                      item->location.prefetchHWM = *prefetchHWMp;
+                                    }
+                                    else
+                                    {
+                                      *prefetchHWMp = item->location.prefetchHWM;
+                                    }
+                                }
 			}
 			else if (set)
 				item->location.location = location;
@@ -252,7 +270,7 @@ ss_get_location(Relation rel, BlockNumbe
 	BlockNumber startloc;
 
 	LWLockAcquire(SyncScanLock, LW_EXCLUSIVE);
-	startloc = ss_search(rel->rd_node, 0, false);
+	startloc = ss_search(rel->rd_node, 0, false , 0);
 	LWLockRelease(SyncScanLock);
 
 	/*
@@ -282,7 +300,7 @@ ss_get_location(Relation rel, BlockNumbe
  * same relfilenode.
  */
 void
-ss_report_location(Relation rel, BlockNumber location)
+ss_report_location(Relation rel, BlockNumber location , BlockNumber *prefetchHWMp)
 {
 #ifdef TRACE_SYNCSCAN
 	if (trace_syncscan)
@@ -306,7 +324,7 @@ ss_report_location(Relation rel, BlockNu
 	{
 		if (LWLockConditionalAcquire(SyncScanLock, LW_EXCLUSIVE))
 		{
-			(void) ss_search(rel->rd_node, location, true);
+			(void) ss_search(rel->rd_node, location, true , prefetchHWMp);
 			LWLockRelease(SyncScanLock);
 		}
 #ifdef TRACE_SYNCSCAN
--- src/backend/access/index/indexam.c.orig	2014-05-31 17:19:07.785208591 -0400
+++ src/backend/access/index/indexam.c	2014-05-31 19:53:09.276074073 -0400
@@ -79,6 +79,55 @@
 #include "utils/tqual.h"
 
 
+#ifdef USE_PREFETCH
+bool BlocknotinBuffer(Buffer buffer, Relation relation, BlockNumber blockNum);
+BlockNumber BlocknumOfBuffer(Buffer buffer);
+void index_mark_or_evict_block(IndexScanDesc scan , BlockNumber blocknumber , int markit);
+
+extern unsigned int prefetch_index_scans; /*  boolean whether to prefetch bitmap heap scans         */
+
+/*  if specified block number is present in the prefetch array,
+**  then either mark it as not to be discarded or evict it according to input param
+*/
+void index_mark_or_evict_block(IndexScanDesc scan , BlockNumber blocknumber , int markit)
+{
+        unsigned short int pfchx , pfchy , pfchz; /*  indexes in BlockIdData array   */
+
+        if (    scan->do_prefetch
+             && ((struct pfch_block_item*)0 != scan->pfch_block_item_list)
+             /* no need to check for scan->pfch_next < prefetch_index_scans
+             ** since we will do nothing if scan->pfch_used == 0
+             */
+           ) {
+            /*  search the prefetch list to find if the block is a member */
+            for (pfchx = 0; pfchx < scan->pfch_used; pfchx++) {
+                if (BlockIdGetBlockNumber(&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)) == blocknumber) {
+                      if (markit) {
+						      /*  mark it as not to be discarded */
+						      ((scan->pfch_block_item_list)+pfchx)->pfch_discard &= ~PREFTCHRC_BUF_PIN_INCREASED;
+					  } else {
+							  /*  shuffle all following the evictee to the left
+							  **  and update next pointer if its element moves
+							  */
+							  pfchy = (scan->pfch_used - 1); /*  current rightmost */
+							  scan->pfch_used = pfchy;
+
+							  while (pfchy > pfchx) {
+								  pfchz = pfchx + 1;
+								  BlockIdCopy((&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)), (&(((scan->pfch_block_item_list)+pfchz)->pfch_blockid)));
+								  ((scan->pfch_block_item_list)+pfchx)->pfch_discard = ((scan->pfch_block_item_list)+pfchz)->pfch_discard;
+								  if (scan->pfch_next == pfchz) {
+									  scan->pfch_next = pfchx;
+								  }
+								  pfchx = pfchz; /* advance */
+							  }
+                      }
+                }
+            }
+        }
+}
+#endif /* USE_PREFETCH */
+
 /* ----------------------------------------------------------------
  *					macros used in index_ routines
  *
@@ -253,6 +302,11 @@ index_beginscan(Relation heapRelation,
 	 */
 	scan->heapRelation = heapRelation;
 	scan->xs_snapshot = snapshot;
+#ifdef USE_PREFETCH
+	scan->do_prefetch = 0;             /*  no prefetching by default  */
+        scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+	scan->pfch_block_item_list = (struct pfch_block_item*)0;
+#endif   /* USE_PREFETCH */
 
 	return scan;
 }
@@ -277,6 +331,11 @@ index_beginscan_bitmap(Relation indexRel
 	 * up by RelationGetIndexScan.
 	 */
 	scan->xs_snapshot = snapshot;
+#ifdef USE_PREFETCH
+	scan->do_prefetch = 0;             /*  no prefetching by default  */
+        scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+	scan->pfch_block_item_list = (struct pfch_block_item*)0;
+#endif   /* USE_PREFETCH */
 
 	return scan;
 }
@@ -311,6 +370,9 @@ index_beginscan_internal(Relation indexR
 									  Int32GetDatum(nkeys),
 									  Int32GetDatum(norderbys)));
 
+	scan->heap_tids_seen = 0;
+	scan->heap_tids_fetched = 0;
+	
 	return scan;
 }
 
@@ -342,6 +404,12 @@ index_rescan(IndexScanDesc scan,
 	/* Release any held pin on a heap page */
 	if (BufferIsValid(scan->xs_cbuf))
 	{
+#ifdef USE_PREFETCH
+                /*   if specified block number is present in the prefetch array,  then evict it */
+                if (scan->do_prefetch) {
+                      index_mark_or_evict_block(scan , BlocknumOfBuffer(scan->xs_cbuf) , 0);
+                }
+#endif   /* USE_PREFETCH */
 		ReleaseBuffer(scan->xs_cbuf);
 		scan->xs_cbuf = InvalidBuffer;
 	}
@@ -373,10 +441,30 @@ index_endscan(IndexScanDesc scan)
 	/* Release any held pin on a heap page */
 	if (BufferIsValid(scan->xs_cbuf))
 	{
+#ifdef USE_PREFETCH
+                /*   if specified block number is present in the prefetch array,  then evict it */
+                if (scan->do_prefetch) {
+                      index_mark_or_evict_block(scan , BlocknumOfBuffer(scan->xs_cbuf) , 0);
+                }
+#endif   /* USE_PREFETCH */
 		ReleaseBuffer(scan->xs_cbuf);
 		scan->xs_cbuf = InvalidBuffer;
 	}
 
+#ifdef USE_PREFETCH
+        /*   discard prefetched but unread buffers */
+        if (    scan->do_prefetch
+             && ((struct pfch_block_item*)0 != scan->pfch_block_item_list)
+           ) {
+            unsigned short int pfchx; /*  index in BlockIdData array   */
+                for (pfchx = 0; pfchx < scan->pfch_used; pfchx++) {
+                  if (((scan->pfch_block_item_list)+pfchx)->pfch_discard) {
+                      DiscardBuffer(scan->heapRelation, MAIN_FORKNUM, BlockIdGetBlockNumber(&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)));
+                  }
+                }
+        }
+#endif   /* USE_PREFETCH */
+
 	/* End the AM's scan */
 	FunctionCall1(procedure, PointerGetDatum(scan));
 
@@ -472,6 +560,12 @@ index_getnext_tid(IndexScanDesc scan, Sc
 		/* ... but first, release any held pin on a heap page */
 		if (BufferIsValid(scan->xs_cbuf))
 		{
+#ifdef USE_PREFETCH
+                    /*   if specified block number is present in the prefetch array,  then evict it */
+                    if (scan->do_prefetch) {
+                          index_mark_or_evict_block(scan , BlocknumOfBuffer(scan->xs_cbuf) , 0);
+                    }
+#endif   /* USE_PREFETCH */
 			ReleaseBuffer(scan->xs_cbuf);
 			scan->xs_cbuf = InvalidBuffer;
 		}
@@ -479,6 +573,11 @@ index_getnext_tid(IndexScanDesc scan, Sc
 	}
 
 	pgstat_count_index_tuples(scan->indexRelation, 1);
+	if (scan->heap_tids_seen++ >= (~0)) {
+		/* Avoid integer overflow */
+		scan->heap_tids_seen = 1;
+		scan->heap_tids_fetched = 0;
+	}
 
 	/* Return the TID of the tuple we found. */
 	return &scan->xs_ctup.t_self;
@@ -502,6 +601,10 @@ index_getnext_tid(IndexScanDesc scan, Sc
  * enough information to do it efficiently in the general case.
  * ----------------
  */
+#if defined(USE_PREFETCH) && defined(AVOID_CATALOG_MIGRATION_FOR_ASYNCIO)
+extern Datum btpeeknexttuple(IndexScanDesc scan);
+#endif /* USE_PREFETCH */
+
 HeapTuple
 index_fetch_heap(IndexScanDesc scan)
 {
@@ -509,16 +612,109 @@ index_fetch_heap(IndexScanDesc scan)
 	bool		all_dead = false;
 	bool		got_heap_tuple;
 
+
+
 	/* We can skip the buffer-switching logic if we're in mid-HOT chain. */
 	if (!scan->xs_continue_hot)
 	{
 		/* Switch to correct buffer if we don't have it already */
 		Buffer		prev_buf = scan->xs_cbuf;
 
+#ifdef USE_PREFETCH
+
+                /*   If the old block is different from new block, then evict old
+                **   block from prefetched array.   It is arguable we should leave it
+                **   in the array because it's likely to remain in the buffer pool
+                **   for a while,  but in that case , if we encounter the block
+                **   again,  prefetching it again does no harm.
+                **   (and note that,  if it's not pinned,  prefetching it will try to
+                **   pin it since prefetch tries to bank a pin for a buffer in the buffer pool).
+                **   therefore it should usually win.
+                */
+                if (    scan->do_prefetch
+                     && ( BufferIsValid(prev_buf) )
+                     && (BlocknotinBuffer(prev_buf,scan->heapRelation,ItemPointerGetBlockNumber(tid)))
+                     && (scan->pfch_next < prefetch_index_scans)  /* ensure there is an entry */
+                        ) {
+                          index_mark_or_evict_block(scan , BlocknumOfBuffer(prev_buf) , 0);
+                }
+
+#endif   /* USE_PREFETCH  */
 		scan->xs_cbuf = ReleaseAndReadBuffer(scan->xs_cbuf,
 											 scan->heapRelation,
 											 ItemPointerGetBlockNumber(tid));
 
+                /*   If the new block had been prefetched and pinned,
+                **   then mark that it no longer requires to be discarded.
+                **   Of course,  we don't evict the entry,
+                **   because we want to remember that it was recently prefetched.
+                */
+                index_mark_or_evict_block(scan , BlocknumOfBuffer(prev_buf) , 1);
+
+				scan->heap_tids_fetched++;
+
+#ifdef USE_PREFETCH
+                /*  try prefetching next data block
+                **    (next meaning one containing TIDs from matching keys
+                **     in same index page and different from any block
+                **     we previously prefetched and listed in prefetched array)
+                */
+                {
+                    FmgrInfo   *procedure;
+                    bool	found;             /*  did we find the "next" heap tid in current index page */
+                    int         PrefetchBufferRc;  /*  indicates whether requested prefetch block already in a buffer and if pin count on buffer has been increased */
+
+                    if (scan->do_prefetch) {
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+                        procedure = &scan->indexRelation->rd_aminfo->ampeeknexttuple; /* is incorrect but avoids adding function to catalog */
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                        if (RegProcedureIsValid(scan->indexRelation->rd_am->ampeeknexttuple)) {
+                            GET_SCAN_PROCEDURE(ampeeknexttuple); /* is correct but requires adding function to catalog */
+                        } else {
+                            procedure = 0;
+                        }
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+
+                        if (    procedure          /* does the index access method support peektuple? */
+#ifndef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+                             && procedure->fn_addr /* procedure->fn_addr is non-null only if in catalog */
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                           ) {
+                            int iterations = 1;      /*  how many iterations of prefetching shall we try  -
+                                                     **  if used entries in prefetch list is < target_prefetch_pages
+                                                     **  then 2,  else 1
+                                                     **  this should result in gradually and smoothly increasing up to target_prefetch_pages
+                                                     */
+                            /*  note we trust InitIndexScan verified this scan is forwards only and so set that */
+                            if (scan->pfch_used < target_prefetch_pages) {
+                                iterations = 2;
+                            }
+                            do {
+                                found =  DatumGetBool(
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+                                                         btpeeknexttuple(scan)     /*  pass scan as direct parameter since cant use fmgr because not in catalog */
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                                                         FunctionCall1(procedure, PointerGetDatum(scan)) /* use fmgr to call it because in catalog  */
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                                                     );
+                                if (found) {
+                                    /*    btpeeknexttuple set pfch_next to point to the item in block_item_list to be prefetched */
+                                    PrefetchBufferRc = PrefetchBuffer(scan->heapRelation, MAIN_FORKNUM, BlockIdGetBlockNumber((&((scan->pfch_block_item_list + scan->pfch_next))->pfch_blockid)) , 0);
+                                    /* elog(LOG,"index_fetch_heap prefetched rel %u blockNum %u"
+                                       ,scan->heapRelation->rd_node.relNode ,BlockIdGetBlockNumber(scan->pfch_block_item_list + scan->pfch_next));
+                                    */
+
+                                    /*  if pin acquired on buffer,  then remember in case of future Discard */
+                                    (scan->pfch_block_item_list + scan->pfch_next)->pfch_discard = (PrefetchBufferRc & PREFTCHRC_BUF_PIN_INCREASED);
+
+
+                                }
+                            } while (--iterations > 0);
+                        }
+                    }
+                }
+#endif   /* USE_PREFETCH */
+
 		/*
 		 * Prune page, but only if we weren't already on this page
 		 */
--- src/backend/access/index/genam.c.orig	2014-05-31 17:19:07.785208591 -0400
+++ src/backend/access/index/genam.c	2014-05-31 19:53:09.296074123 -0400
@@ -77,6 +77,12 @@ RelationGetIndexScan(Relation indexRelat
 
 	scan = (IndexScanDesc) palloc(sizeof(IndexScanDescData));
 
+#ifdef USE_PREFETCH
+        scan->do_prefetch = 0;             /*  no prefetching by default  */
+        scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+        scan->pfch_block_item_list = (struct pfch_block_item*)0;
+#endif   /* USE_PREFETCH */
+
 	scan->heapRelation = NULL;	/* may be set later */
 	scan->indexRelation = indexRelation;
 	scan->xs_snapshot = InvalidSnapshot;		/* caller must initialize this */
@@ -139,6 +145,19 @@ RelationGetIndexScan(Relation indexRelat
 void
 IndexScanEnd(IndexScanDesc scan)
 {
+#ifdef USE_PREFETCH
+	if (scan->do_prefetch) {
+		if ( (struct pfch_block_item*)0 != scan->pfch_block_item_list ) {
+			pfree(scan->pfch_block_item_list);
+			scan->pfch_block_item_list = (struct pfch_block_item*)0;
+		}
+		if ( (struct pfch_index_pagelist*)0 != scan->pfch_index_page_list ) {
+			pfree(scan->pfch_index_page_list);
+			scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+		}
+	}
+#endif   /* USE_PREFETCH */
+
 	if (scan->keyData != NULL)
 		pfree(scan->keyData);
 	if (scan->orderByData != NULL)
--- src/backend/access/nbtree/nbtsearch.c.orig	2014-05-31 17:19:07.785208591 -0400
+++ src/backend/access/nbtree/nbtsearch.c	2014-05-31 19:53:09.324074194 -0400
@@ -23,13 +23,16 @@
 #include "utils/lsyscache.h"
 #include "utils/rel.h"
 
+extern unsigned int prefetch_btree_heaps;  /*  boolean whether to prefetch heap pages in _bt_next for non-bitmap index scans   */
+extern unsigned int prefetch_sequential_index_scans;  /*  boolean whether to prefetch sequential-access non-bitmap index scans  */
 
 static bool _bt_readpage(IndexScanDesc scan, ScanDirection dir,
 			 OffsetNumber offnum);
 static void _bt_saveitem(BTScanOpaque so, int itemIndex,
 			 OffsetNumber offnum, IndexTuple itup);
-static bool _bt_steppage(IndexScanDesc scan, ScanDirection dir);
-static Buffer _bt_walk_left(Relation rel, Buffer buf);
+static bool _bt_steppage(IndexScanDesc scan, ScanDirection dir, 
+			 bool prefetch);
+static Buffer _bt_walk_left(IndexScanDesc scan, Relation rel, Buffer buf);
 static bool _bt_endpoint(IndexScanDesc scan, ScanDirection dir);
 
 
@@ -226,7 +229,7 @@ _bt_moveright(Relation rel,
 				_bt_relbuf(rel, buf);
 
 			/* re-acquire the lock in the right mode, and re-check */
-			buf = _bt_getbuf(rel, blkno, access);
+			buf = _bt_getbuf(rel, blkno, access , (struct pfch_index_pagelist*)0);
 			continue;
 		}
 
@@ -1005,7 +1008,7 @@ _bt_first(IndexScanDesc scan, ScanDirect
 		 * There's no actually-matching data on this page.  Try to advance to
 		 * the next page.  Return false if there's no matching data at all.
 		 */
-		if (!_bt_steppage(scan, dir))
+		if (!_bt_steppage(scan, dir, false))
 			return false;
 	}
 
@@ -1040,6 +1043,8 @@ _bt_next(IndexScanDesc scan, ScanDirecti
 {
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
 	BTScanPosItem *currItem;
+	BlockNumber prevblkno = ItemPointerGetBlockNumber(
+		&scan->xs_ctup.t_self);
 
 	/*
 	 * Advance to next tuple on current page; or if there's no more, try to
@@ -1052,11 +1057,53 @@ _bt_next(IndexScanDesc scan, ScanDirecti
 			/* We must acquire lock before applying _bt_steppage */
 			Assert(BufferIsValid(so->currPos.buf));
 			LockBuffer(so->currPos.buf, BT_READ);
-			if (!_bt_steppage(scan, dir))
+			if (!_bt_steppage(scan, dir, target_prefetch_pages > 0))
 				return false;
 			/* Drop the lock, but not pin, on the new page */
 			LockBuffer(so->currPos.buf, BUFFER_LOCK_UNLOCK);
 		}
+		
+		if (prefetch_btree_heaps && (target_prefetch_pages > 0)) {
+			BlockNumber currblkno = ItemPointerGetBlockNumber(
+				&so->currPos.items[so->currPos.itemIndex].heapTid);
+		
+			if (currblkno != prevblkno) {
+				if (so->prefetchBlockCount > 0)
+					so->prefetchBlockCount--;
+			
+				/* If we have heap fetch frequency stats, and it's above ~94%,
+				 * initiate heap prefetches */
+				if (so->currPos.moreRight
+					&& scan->heap_tids_seen > 256 
+					&& ( (scan->heap_tids_seen - scan->heap_tids_seen/16)
+						 <= scan->heap_tids_fetched ) )
+				{
+					bool nonsequential = false;
+				
+					if (so->prefetchItemIndex <= so->currPos.itemIndex)
+						so->prefetchItemIndex = so->currPos.itemIndex + 1;
+					while (    (so->prefetchItemIndex <= so->currPos.lastItem)
+							&& (so->prefetchBlockCount < target_prefetch_pages) )
+					{
+						ItemPointer tid = &so->currPos.items[so->prefetchItemIndex++].heapTid;
+						BlockNumber blkno = ItemPointerGetBlockNumber(tid);
+						if (blkno != so->lastHeapPrefetchBlkno) {  /*  if not a repetition of previous block */
+							/*   start prefetch on next page, providing :
+                            **   EITHER  .  we're reading non-sequentially previously or for this block
+                            **   OR      .  user explicitly specified to prefetch for sequential pattern
+							**   as it may be counterproductive otherwise
+                            */
+							nonsequential = (nonsequential || blkno != (so->lastHeapPrefetchBlkno+1));
+							if (prefetch_sequential_index_scans || nonsequential) {
+								_bt_prefetchbuf(scan->heapRelation, blkno , &scan->pfch_index_page_list );
+							}
+							so->lastHeapPrefetchBlkno = blkno;
+							so->prefetchBlockCount++;
+						}
+					}
+				}
+			}
+		}
 	}
 	else
 	{
@@ -1065,11 +1112,53 @@ _bt_next(IndexScanDesc scan, ScanDirecti
 			/* We must acquire lock before applying _bt_steppage */
 			Assert(BufferIsValid(so->currPos.buf));
 			LockBuffer(so->currPos.buf, BT_READ);
-			if (!_bt_steppage(scan, dir))
+			if (!_bt_steppage(scan, dir, target_prefetch_pages > 0))
 				return false;
 			/* Drop the lock, but not pin, on the new page */
 			LockBuffer(so->currPos.buf, BUFFER_LOCK_UNLOCK);
 		}
+		
+		if (prefetch_btree_heaps && (target_prefetch_pages > 0)) {
+			BlockNumber currblkno = ItemPointerGetBlockNumber(
+				&so->currPos.items[so->currPos.itemIndex].heapTid);
+		
+			if (currblkno != prevblkno) {
+				if (so->prefetchBlockCount > 0)
+					so->prefetchBlockCount--;
+			
+				/* If we have heap fetch frequency stats, and it's above ~94%,
+				 * initiate heap prefetches */
+				if (so->currPos.moreLeft
+					&& scan->heap_tids_seen > 256 
+					&& ( (scan->heap_tids_seen - scan->heap_tids_seen/16)
+						 <= scan->heap_tids_fetched ) )
+				{
+					bool nonsequential = false;
+			
+					if (so->prefetchItemIndex >= so->currPos.itemIndex)
+						so->prefetchItemIndex = so->currPos.itemIndex - 1;
+					while (    (so->prefetchItemIndex >= so->currPos.firstItem)
+							&& (so->prefetchBlockCount < target_prefetch_pages) )
+					{
+						ItemPointer tid = &so->currPos.items[so->prefetchItemIndex--].heapTid;
+						BlockNumber blkno = ItemPointerGetBlockNumber(tid);
+						if (blkno != so->lastHeapPrefetchBlkno) {  /*  if not a repetition of previous block */
+							/*   start prefetch on next page, providing :
+                            **   EITHER  .  we're reading non-sequentially previously or for this block
+                            **   OR      .  user explicitly specified to prefetch for sequential pattern
+							**   as it may be counterproductive otherwise
+                            */
+							nonsequential = (nonsequential || blkno != (so->lastHeapPrefetchBlkno+1));
+							if (prefetch_sequential_index_scans || nonsequential) {
+								_bt_prefetchbuf(scan->heapRelation, blkno , &scan->pfch_index_page_list );
+							}
+							so->lastHeapPrefetchBlkno = blkno;
+							so->prefetchBlockCount++;
+						}
+					}
+				}
+			}
+		}
 	}
 
 	/* OK, itemIndex says what to return */
@@ -1119,9 +1208,11 @@ _bt_readpage(IndexScanDesc scan, ScanDir
 	/*
 	 * we must save the page's right-link while scanning it; this tells us
 	 * where to step right to after we're done with these items.  There is no
-	 * corresponding need for the left-link, since splits always go right.
+	 * corresponding need for the left-link, since splits always go right,
+	 * but we need it for back-sequential scan detection.
 	 */
 	so->currPos.nextPage = opaque->btpo_next;
+	so->currPos.prevPage = opaque->btpo_prev;
 
 	/* initialize tuple workspace to empty */
 	so->currPos.nextTupleOffset = 0;
@@ -1156,6 +1247,7 @@ _bt_readpage(IndexScanDesc scan, ScanDir
 		so->currPos.firstItem = 0;
 		so->currPos.lastItem = itemIndex - 1;
 		so->currPos.itemIndex = 0;
+		so->prefetchItemIndex = 0;
 	}
 	else
 	{
@@ -1187,6 +1279,7 @@ _bt_readpage(IndexScanDesc scan, ScanDir
 		so->currPos.firstItem = itemIndex;
 		so->currPos.lastItem = MaxIndexTuplesPerPage - 1;
 		so->currPos.itemIndex = MaxIndexTuplesPerPage - 1;
+		so->prefetchItemIndex = MaxIndexTuplesPerPage - 1;
 	}
 
 	return (so->currPos.firstItem <= so->currPos.lastItem);
@@ -1224,7 +1317,7 @@ _bt_saveitem(BTScanOpaque so, int itemIn
  * locks and pins, set so->currPos.buf to InvalidBuffer, and return FALSE.
  */
 static bool
-_bt_steppage(IndexScanDesc scan, ScanDirection dir)
+_bt_steppage(IndexScanDesc scan, ScanDirection dir, bool prefetch)
 {
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
 	Relation	rel;
@@ -1278,7 +1371,7 @@ _bt_steppage(IndexScanDesc scan, ScanDir
 			/* check for interrupts while we're not holding any buffer lock */
 			CHECK_FOR_INTERRUPTS();
 			/* step right one page */
-			so->currPos.buf = _bt_getbuf(rel, blkno, BT_READ);
+			so->currPos.buf = _bt_getbuf(rel, blkno, BT_READ , scan->pfch_index_page_list);
 			/* check for deleted page */
 			page = BufferGetPage(so->currPos.buf);
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
@@ -1287,9 +1380,20 @@ _bt_steppage(IndexScanDesc scan, ScanDir
 				PredicateLockPage(rel, blkno, scan->xs_snapshot);
 				/* see if there are any matches on this page */
 				/* note that this will clear moreRight if we can stop */
-				if (_bt_readpage(scan, dir, P_FIRSTDATAKEY(opaque)))
+				if (_bt_readpage(scan, dir, P_FIRSTDATAKEY(opaque))) {
+					if (    prefetch && so->currPos.moreRight
+						/*   start prefetch on next page, providing :
+						**   EITHER  .  we're reading non-sequentially for this block
+						**   OR      .  user explicitly specified to prefetch for sequential pattern
+						**   as it may be counterproductive otherwise
+						*/
+						&& (prefetch_sequential_index_scans || opaque->btpo_next != (blkno+1))
+                       ) {
+ 						  _bt_prefetchbuf(rel, opaque->btpo_next , &scan->pfch_index_page_list);
+					}
 					break;
 			}
+			}
 			/* nope, keep going */
 			blkno = opaque->btpo_next;
 		}
@@ -1317,7 +1421,7 @@ _bt_steppage(IndexScanDesc scan, ScanDir
 			}
 
 			/* Step to next physical page */
-			so->currPos.buf = _bt_walk_left(rel, so->currPos.buf);
+			so->currPos.buf = _bt_walk_left(scan , rel, so->currPos.buf);
 
 			/* if we're physically at end of index, return failure */
 			if (so->currPos.buf == InvalidBuffer)
@@ -1332,14 +1436,58 @@ _bt_steppage(IndexScanDesc scan, ScanDir
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 			if (!P_IGNORE(opaque))
 			{
+				/* We must rely on the previously saved prevPage link! */
+				BlockNumber blkno = so->currPos.prevPage;
+				
 				PredicateLockPage(rel, BufferGetBlockNumber(so->currPos.buf), scan->xs_snapshot);
 				/* see if there are any matches on this page */
 				/* note that this will clear moreLeft if we can stop */
-				if (_bt_readpage(scan, dir, PageGetMaxOffsetNumber(page)))
+				if (_bt_readpage(scan, dir, PageGetMaxOffsetNumber(page))) {
+					if (prefetch && so->currPos.moreLeft) {
+						/* detect back-sequential runs and increase prefetch window blindly 
+						 * downwards 2 blocks at a time. This only works in our favor
+						 * for index-only scans, by merging read requests at the kernel,
+						 * so we want to inflate target_prefetch_pages since merged 
+						 * back-sequential requests are about as expensive as a single one 
+						 */
+						if (scan->xs_want_itup && blkno > 0 && opaque->btpo_prev == (blkno-1)) {
+							BlockNumber backPos;
+							unsigned int back_prefetch_pages = target_prefetch_pages * 16;
+							if (back_prefetch_pages > 64)
+								back_prefetch_pages = 64;
+							
+							if (so->backSeqRun == 0)
+								backPos = (blkno-1);
+							else
+								backPos = so->backSeqPos;
+							so->backSeqRun++;
+							
+							if (backPos > 0 && (blkno - backPos) <= back_prefetch_pages) {
+								_bt_prefetchbuf(rel, backPos-- , &scan->pfch_index_page_list);
+								/* don't start back-seq prefetch too early */
+								if (so->backSeqRun >= back_prefetch_pages
+										&& backPos > 0 
+										&& (blkno - backPos) <= back_prefetch_pages)
+								{
+									_bt_prefetchbuf(rel, backPos-- , &scan->pfch_index_page_list);
+								}
+							}
+							
+							so->backSeqPos = backPos;
+						} else {
+							/* start prefetch on next page */
+							if (so->backSeqRun != 0) {
+								if (opaque->btpo_prev > blkno || opaque->btpo_prev < so->backSeqPos)
+									so->backSeqRun = 0;
+							}
+							_bt_prefetchbuf(rel, opaque->btpo_prev , &scan->pfch_index_page_list);
+						}
+					}
 					break;
 			}
 		}
 	}
+	}
 
 	return true;
 }
@@ -1359,7 +1507,7 @@ _bt_steppage(IndexScanDesc scan, ScanDir
  * again if it's important.
  */
 static Buffer
-_bt_walk_left(Relation rel, Buffer buf)
+_bt_walk_left(IndexScanDesc scan, Relation rel, Buffer buf)
 {
 	Page		page;
 	BTPageOpaque opaque;
@@ -1387,7 +1535,7 @@ _bt_walk_left(Relation rel, Buffer buf)
 		_bt_relbuf(rel, buf);
 		/* check for interrupts while we're not holding any buffer lock */
 		CHECK_FOR_INTERRUPTS();
-		buf = _bt_getbuf(rel, blkno, BT_READ);
+		buf = _bt_getbuf(rel, blkno, BT_READ , scan->pfch_index_page_list );
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
@@ -1631,7 +1779,7 @@ _bt_endpoint(IndexScanDesc scan, ScanDir
 		 * There's no actually-matching data on this page.  Try to advance to
 		 * the next page.  Return false if there's no matching data at all.
 		 */
-		if (!_bt_steppage(scan, dir))
+		if (!_bt_steppage(scan, dir, false))
 			return false;
 	}
 
--- src/backend/access/nbtree/nbtinsert.c.orig	2014-05-31 17:19:07.785208591 -0400
+++ src/backend/access/nbtree/nbtinsert.c	2014-05-31 19:53:09.356074275 -0400
@@ -793,7 +793,7 @@ _bt_insertonpg(Relation rel,
 		{
 			Assert(!P_ISLEAF(lpageop));
 
-			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE , (struct pfch_index_pagelist*)0);
 			metapg = BufferGetPage(metabuf);
 			metad = BTPageGetMeta(metapg);
 
@@ -972,7 +972,7 @@ _bt_split(Relation rel, Buffer buf, Buff
 	bool		isleaf;
 
 	/* Acquire a new page to split into */
-	rbuf = _bt_getbuf(rel, P_NEW, BT_WRITE);
+	rbuf = _bt_getbuf(rel, P_NEW, BT_WRITE , (struct pfch_index_pagelist*)0);
 
 	/*
 	 * origpage is the original page to be split.  leftpage is a temporary
@@ -1175,7 +1175,7 @@ _bt_split(Relation rel, Buffer buf, Buff
 
 	if (!P_RIGHTMOST(oopaque))
 	{
-		sbuf = _bt_getbuf(rel, oopaque->btpo_next, BT_WRITE);
+		sbuf = _bt_getbuf(rel, oopaque->btpo_next, BT_WRITE , (struct pfch_index_pagelist*)0);
 		spage = BufferGetPage(sbuf);
 		sopaque = (BTPageOpaque) PageGetSpecialPointer(spage);
 		if (sopaque->btpo_prev != origpagenumber)
@@ -1817,7 +1817,7 @@ _bt_finish_split(Relation rel, Buffer lb
 	Assert(P_INCOMPLETE_SPLIT(lpageop));
 
 	/* Lock right sibling, the one missing the downlink */
-	rbuf = _bt_getbuf(rel, lpageop->btpo_next, BT_WRITE);
+	rbuf = _bt_getbuf(rel, lpageop->btpo_next, BT_WRITE , (struct pfch_index_pagelist*)0);
 	rpage = BufferGetPage(rbuf);
 	rpageop = (BTPageOpaque) PageGetSpecialPointer(rpage);
 
@@ -1829,7 +1829,7 @@ _bt_finish_split(Relation rel, Buffer lb
 		BTMetaPageData *metad;
 
 		/* acquire lock on the metapage */
-		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE , (struct pfch_index_pagelist*)0);
 		metapg = BufferGetPage(metabuf);
 		metad = BTPageGetMeta(metapg);
 
@@ -1877,7 +1877,7 @@ _bt_getstackbuf(Relation rel, BTStack st
 		Page		page;
 		BTPageOpaque opaque;
 
-		buf = _bt_getbuf(rel, blkno, access);
+		buf = _bt_getbuf(rel, blkno, access , (struct pfch_index_pagelist*)0);
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
@@ -2008,12 +2008,12 @@ _bt_newroot(Relation rel, Buffer lbuf, B
 	lopaque = (BTPageOpaque) PageGetSpecialPointer(lpage);
 
 	/* get a new root page */
-	rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE);
+	rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE , (struct pfch_index_pagelist*)0);
 	rootpage = BufferGetPage(rootbuf);
 	rootblknum = BufferGetBlockNumber(rootbuf);
 
 	/* acquire lock on the metapage */
-	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE , (struct pfch_index_pagelist*)0);
 	metapg = BufferGetPage(metabuf);
 	metad = BTPageGetMeta(metapg);
 
--- src/backend/access/nbtree/nbtpage.c.orig	2014-05-31 17:19:07.785208591 -0400
+++ src/backend/access/nbtree/nbtpage.c	2014-05-31 19:53:09.384074346 -0400
@@ -127,7 +127,7 @@ _bt_getroot(Relation rel, int access)
 		Assert(rootblkno != P_NONE);
 		rootlevel = metad->btm_fastlevel;
 
-		rootbuf = _bt_getbuf(rel, rootblkno, BT_READ);
+		rootbuf = _bt_getbuf(rel, rootblkno, BT_READ , (struct pfch_index_pagelist*)0);
 		rootpage = BufferGetPage(rootbuf);
 		rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);
 
@@ -153,7 +153,7 @@ _bt_getroot(Relation rel, int access)
 		rel->rd_amcache = NULL;
 	}
 
-	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
+	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ , (struct pfch_index_pagelist*)0);
 	metapg = BufferGetPage(metabuf);
 	metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
 	metad = BTPageGetMeta(metapg);
@@ -209,7 +209,7 @@ _bt_getroot(Relation rel, int access)
 		 * the new root page.  Since this is the first page in the tree, it's
 		 * a leaf as well as the root.
 		 */
-		rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE);
+		rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE , (struct pfch_index_pagelist*)0);
 		rootblkno = BufferGetBlockNumber(rootbuf);
 		rootpage = BufferGetPage(rootbuf);
 		rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);
@@ -350,7 +350,7 @@ _bt_gettrueroot(Relation rel)
 		pfree(rel->rd_amcache);
 	rel->rd_amcache = NULL;
 
-	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
+	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ , (struct pfch_index_pagelist*)0);
 	metapg = BufferGetPage(metabuf);
 	metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
 	metad = BTPageGetMeta(metapg);
@@ -436,7 +436,7 @@ _bt_getrootheight(Relation rel)
 		Page		metapg;
 		BTPageOpaque metaopaque;
 
-		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
+		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ , (struct pfch_index_pagelist*)0);
 		metapg = BufferGetPage(metabuf);
 		metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
 		metad = BTPageGetMeta(metapg);
@@ -562,6 +562,170 @@ _bt_log_reuse_page(Relation rel, BlockNu
 }
 
 /*
+ *	_bt_prefetchbuf() -- Prefetch a buffer by block number
+ *                           and keep track of prefetched and unread blocknums in pagelist.
+ *   input parms  :
+ *       rel and blockno identify block to be prefetched as usual
+ *       pfch_index_page_list_P points to the pointer anchoring the head of the index page list
+ *             Since the pagelist is a kind of optimization,
+ *             handle palloc failure by quietly omitting the keeping track.
+ */
+void
+_bt_prefetchbuf(Relation rel, BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P)
+{
+
+    int rc = 0;
+    struct pfch_index_pagelist* pfch_index_plp;      /*  pointer to current chunk */
+    struct pfch_index_item* found_item = 0;
+
+		if ((struct pfch_index_pagelist**)0 == pfch_index_page_list_P) {
+			pfch_index_plp = (struct pfch_index_pagelist*)0;
+		} else {
+			pfch_index_plp = *pfch_index_page_list_P;
+		}
+
+    	if (blkno != P_NEW && blkno != P_NONE)
+    	{
+            /* prefetch an existing block of the relation
+            ** but first,  check it has not recently already been prefetched and not yet read
+            */
+            found_item = _bt_find_block(blkno , pfch_index_plp);
+			if ((struct pfch_index_item*)0 == found_item) {  /*  not found */
+
+		        rc = PrefetchBuffer(rel, MAIN_FORKNUM, blkno , 0);
+
+                /*  add the pagenum to the list ,  indicating its discard status
+                **  since it's only an optimization,  ignore failure such as exceeded allowed space
+				*/
+                _bt_add_block( blkno , pfch_index_page_list_P , (uint32)(rc & PREFTCHRC_BUF_PIN_INCREASED));
+
+            }
+	    }
+        return;
+}
+
+/*   _bt_find_block finds the item referencing specified Block in index page list if present
+**   and returns the pointer to the pfch_index_item if found,  or null if not
+*/
+struct pfch_index_item*
+_bt_find_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list)
+{
+
+    struct pfch_index_item* found_item = 0;
+    struct pfch_index_pagelist* pfch_index_plp;      /*  pointer to current chunk */
+    int ix, tx;
+
+		pfch_index_plp = pfch_index_page_list;
+
+		while (     (struct pfch_index_pagelist*)0 != pfch_index_plp
+                &&  ( (struct pfch_index_item*)0 == found_item)
+              ) {
+			ix = 0;
+			tx = pfch_index_plp->pfch_index_item_count;
+			while (     (ix < tx)
+                    &&  ( (struct pfch_index_item*)0 == found_item)
+                  ) {
+				if (pfch_index_plp->pfch_indexid[ix].pfch_blocknum == blkno) {
+					found_item = &pfch_index_plp->pfch_indexid[ix];
+				}
+                ix++;
+			}
+			pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+		}
+
+     return found_item;
+}
+
+/*   _bt_add_block adds the specified Block to the index page list
+**   and returns 0 if successful,  non-zero if not
+*/
+int
+_bt_add_block(BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P , uint32 discard_status)
+{
+    int rc = 1;
+    int ix;
+    struct pfch_index_pagelist* pfch_index_plp;      /*  pointer to current chunk */
+    struct pfch_index_pagelist* pfch_index_page_list_anchor; /*  pointer to first chunk if any  */
+	/*  allow expansion of pagelist to 16 chunks
+	**  which accommodates backwards-sequential index scans
+	**  where the scanner increases target_prefetch_pages by a factor of up to 16
+	**   see code in _bt_steppage
+	**  note - this creates an undesirable weak dependency on this number in _bt_steppage,
+	**         but :
+	**           there is no disaster if the numbers disagree  -  just sub-optimal use of the list
+	**           to implement a proper interface would require that chunks have a variable size
+	**           which would require an extra size variable in each chunk
+	*/
+	int num_chunks = 16;
+
+		if ((struct pfch_index_pagelist**)0 == pfch_index_page_list_P) {
+			pfch_index_page_list_anchor = (struct pfch_index_pagelist*)0;
+		} else {
+			pfch_index_page_list_anchor = *pfch_index_page_list_P;
+		}
+		pfch_index_plp = pfch_index_page_list_anchor;       /* pointer to current chunk */
+
+		while ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+			ix = pfch_index_plp->pfch_index_item_count;
+			if (ix < target_prefetch_pages) {
+				pfch_index_plp->pfch_indexid[ix].pfch_blocknum = blkno;
+				pfch_index_plp->pfch_indexid[ix].pfch_discard = discard_status;
+				pfch_index_plp->pfch_index_item_count = (ix+1);
+                rc = 0;
+				goto stored_pagenum;
+			}
+			pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+			num_chunks--;              /*  keep track of number of chunks */
+		}
+
+		/*   we did not find any free space in existing chunks -
+		**   create new chunk if within our limit and we have a pfch_index_page_list
+		*/
+		if ( (num_chunks > 0) && ((struct pfch_index_pagelist*)0 != pfch_index_page_list_anchor) ) {
+			pfch_index_plp = (struct pfch_index_pagelist*)palloc( sizeof(struct pfch_index_pagelist) + ( (target_prefetch_pages-1) * sizeof(struct pfch_index_item) ) );
+			if ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+				pfch_index_plp->pfch_index_pagelist_next = pfch_index_page_list_anchor;  /* old head of list is next after this */
+				pfch_index_plp->pfch_indexid[0].pfch_blocknum = blkno;
+				pfch_index_plp->pfch_indexid[0].pfch_discard = discard_status;
+				pfch_index_plp->pfch_index_item_count = 1;
+				pfch_index_page_list_P = &pfch_index_plp;   /*  new head of list is new chunk */
+                rc = 0;
+			}
+		}
+
+    stored_pagenum:;
+     return rc;
+}
+
+/*  _bt_subtract_block removes a block from the prefetched-but-unread pagelist if present */
+void
+_bt_subtract_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list)
+{
+    struct pfch_index_pagelist* pfch_index_plp = pfch_index_page_list;
+	if ( (blkno != P_NEW) && (blkno != P_NONE) ) {
+            int ix , jx;
+                while ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+                    ix = pfch_index_plp->pfch_index_item_count;
+                    while (ix-- > 0) {
+                        if (pfch_index_plp->pfch_indexid[ix].pfch_blocknum == blkno) {
+                            /*   move the last item to the curent (now deleted) position and decrement count */
+                            jx = (pfch_index_plp->pfch_index_item_count-1); /*  index of last item ... */
+                            if (jx > ix) {                                  /*  ... is not the current one so move is required */
+                                pfch_index_plp->pfch_indexid[ix].pfch_blocknum = pfch_index_plp->pfch_indexid[jx].pfch_blocknum;
+                                pfch_index_plp->pfch_indexid[ix].pfch_discard = pfch_index_plp->pfch_indexid[jx].pfch_discard;
+                                ix = jx;
+                            }
+                            pfch_index_plp->pfch_index_item_count = ix;
+                            goto done_subtract;
+                        }
+                    }
+                    pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+                }
+        }
+    done_subtract:  return;
+}
+
+/*
  *	_bt_getbuf() -- Get a buffer by block number for read or write.
  *
  *		blkno == P_NEW means to get an unallocated index page.  The page
@@ -573,7 +737,7 @@ _bt_log_reuse_page(Relation rel, BlockNu
  *		_bt_checkpage to sanity-check the page (except in P_NEW case).
  */
 Buffer
-_bt_getbuf(Relation rel, BlockNumber blkno, int access)
+_bt_getbuf(Relation rel, BlockNumber blkno, int access , struct pfch_index_pagelist* pfch_index_page_list)
 {
 	Buffer		buf;
 
@@ -581,6 +745,10 @@ _bt_getbuf(Relation rel, BlockNumber blk
 	{
 		/* Read an existing block of the relation */
 		buf = ReadBuffer(rel, blkno);
+
+        /*  if the block is in the prefetched-but-unread pagelist,  remove it */
+        _bt_subtract_block( blkno , pfch_index_page_list);
+
 		LockBuffer(buf, access);
 		_bt_checkpage(rel, buf);
 	}
@@ -702,6 +870,10 @@ _bt_getbuf(Relation rel, BlockNumber blk
  * bufmgr when one would do.  However, now it's mainly just a notational
  * convenience.  The only case where it saves work over _bt_relbuf/_bt_getbuf
  * is when the target page is the same one already in the buffer.
+ *
+ * if prefetching of index pages is changed to use this function,
+ * then it should be extended to take the index_page_list as parameter
+ * and call_bt_subtract_block in the same way that _bt_getbuf does.
  */
 Buffer
 _bt_relandgetbuf(Relation rel, Buffer obuf, BlockNumber blkno, int access)
@@ -712,6 +884,7 @@ _bt_relandgetbuf(Relation rel, Buffer ob
 	if (BufferIsValid(obuf))
 		LockBuffer(obuf, BUFFER_LOCK_UNLOCK);
 	buf = ReleaseAndReadBuffer(obuf, rel, blkno);
+
 	LockBuffer(buf, access);
 	_bt_checkpage(rel, buf);
 	return buf;
@@ -965,7 +1138,7 @@ _bt_is_page_halfdead(Relation rel, Block
 	BTPageOpaque opaque;
 	bool		result;
 
-	buf = _bt_getbuf(rel, blk, BT_READ);
+	buf = _bt_getbuf(rel, blk, BT_READ , (struct pfch_index_pagelist*)0);
 	page = BufferGetPage(buf);
 	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
@@ -1069,7 +1242,7 @@ _bt_lock_branch_parent(Relation rel, Blo
 				Page		lpage;
 				BTPageOpaque lopaque;
 
-				lbuf = _bt_getbuf(rel, leftsib, BT_READ);
+				lbuf = _bt_getbuf(rel, leftsib, BT_READ, (struct pfch_index_pagelist*)0);
 				lpage = BufferGetPage(lbuf);
 				lopaque = (BTPageOpaque) PageGetSpecialPointer(lpage);
 
@@ -1265,7 +1438,7 @@ _bt_pagedel(Relation rel, Buffer buf)
 					BTPageOpaque lopaque;
 					Page		lpage;
 
-					lbuf = _bt_getbuf(rel, leftsib, BT_READ);
+					lbuf = _bt_getbuf(rel, leftsib, BT_READ, (struct pfch_index_pagelist*)0);
 					lpage = BufferGetPage(lbuf);
 					lopaque = (BTPageOpaque) PageGetSpecialPointer(lpage);
 					/*
@@ -1340,7 +1513,7 @@ _bt_pagedel(Relation rel, Buffer buf)
 		if (!rightsib_empty)
 			break;
 
-		buf = _bt_getbuf(rel, rightsib, BT_WRITE);
+		buf = _bt_getbuf(rel, rightsib, BT_WRITE, (struct pfch_index_pagelist*)0);
 	}
 
 	return ndeleted;
@@ -1593,7 +1766,7 @@ _bt_unlink_halfdead_page(Relation rel, B
 		target = topblkno;
 
 		/* fetch the block number of the topmost parent's left sibling */
-		buf = _bt_getbuf(rel, topblkno, BT_READ);
+		buf = _bt_getbuf(rel, topblkno, BT_READ, (struct pfch_index_pagelist*)0);
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		leftsib = opaque->btpo_prev;
@@ -1632,7 +1805,7 @@ _bt_unlink_halfdead_page(Relation rel, B
 		LockBuffer(leafbuf, BT_WRITE);
 	if (leftsib != P_NONE)
 	{
-		lbuf = _bt_getbuf(rel, leftsib, BT_WRITE);
+		lbuf = _bt_getbuf(rel, leftsib, BT_WRITE , (struct pfch_index_pagelist*)0);
 		page = BufferGetPage(lbuf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		while (P_ISDELETED(opaque) || opaque->btpo_next != target)
@@ -1646,7 +1819,7 @@ _bt_unlink_halfdead_page(Relation rel, B
 					 RelationGetRelationName(rel));
 				return false;
 			}
-			lbuf = _bt_getbuf(rel, leftsib, BT_WRITE);
+			lbuf = _bt_getbuf(rel, leftsib, BT_WRITE , (struct pfch_index_pagelist*)0);
 			page = BufferGetPage(lbuf);
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		}
@@ -1701,7 +1874,7 @@ _bt_unlink_halfdead_page(Relation rel, B
 	 * And next write-lock the (current) right sibling.
 	 */
 	rightsib = opaque->btpo_next;
-	rbuf = _bt_getbuf(rel, rightsib, BT_WRITE);
+	rbuf = _bt_getbuf(rel, rightsib, BT_WRITE , (struct pfch_index_pagelist*)0);
 	page = BufferGetPage(rbuf);
 	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 	if (opaque->btpo_prev != target)
@@ -1731,7 +1904,7 @@ _bt_unlink_halfdead_page(Relation rel, B
 		if (P_RIGHTMOST(opaque))
 		{
 			/* rightsib will be the only one left on the level */
-			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE , (struct pfch_index_pagelist*)0);
 			metapg = BufferGetPage(metabuf);
 			metad = BTPageGetMeta(metapg);
 
--- src/backend/access/nbtree/nbtree.c.orig	2014-05-31 17:19:07.785208591 -0400
+++ src/backend/access/nbtree/nbtree.c	2014-05-31 19:53:09.416074427 -0400
@@ -30,6 +30,18 @@
 #include "tcop/tcopprot.h"
 #include "utils/memutils.h"
 
+#ifdef USE_PREFETCH
+extern unsigned int prefetch_index_scans; /* boolean whether to prefetch non-bitmap index scans also numeric size of pfch_list */
+#endif   /* USE_PREFETCH */
+
+Datum
+btpeeknexttuple(
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+        IndexScanDesc scan
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+        PG_FUNCTION_ARGS
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+);
 
 /* Working state for btbuild and its callback */
 typedef struct
@@ -332,6 +344,74 @@ btgettuple(PG_FUNCTION_ARGS)
 }
 
 /*
+ *	btpeeknexttuple() -- peek at the next tuple different from any blocknum in pfch_block_item_list
+ *                           without reading a new index page
+ *                       and without causing any side-effects such as altering values in control blocks
+ *               if found,     store blocknum in next element of pfch_block_item_list
+ */
+Datum
+btpeeknexttuple(
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+        IndexScanDesc scan
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+        PG_FUNCTION_ARGS
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+)
+{
+#ifndef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+    IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+	BTScanOpaque so = (BTScanOpaque) scan->opaque;
+	bool		res = false;
+	int		itemIndex;		/* current index in items[] */
+
+        /*
+         * If we've already initialized this scan, we can just advance it in
+         * the appropriate direction.  If we haven't done so yet, bail out
+         */
+        if ( BTScanPosIsValid(so->currPos) ) {
+
+            itemIndex = so->currPos.itemIndex+1;    /*   next item */
+
+            /* This loop handles advancing till we find different data block or end of index page */
+            while (itemIndex <= so->currPos.lastItem) {
+                unsigned short int pfchx; /*  index in BlockIdData array   */
+                for (pfchx = 0; pfchx < scan->pfch_used; pfchx++) {
+                        if (BlockIdEquals((&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)) , &(so->currPos.items[itemIndex].heapTid.ip_blkid))) {
+                             goto block_match;
+                        }
+                }
+
+                /*  if we reach here,  no block in list matched this item  */
+                res = true;
+                /*   set item in prefetch list
+                **   prefer unused entry if there is one,  else overwrite
+                */
+                if (scan->pfch_used < prefetch_index_scans) {
+                    scan->pfch_next = scan->pfch_used;
+                } else {
+                    scan->pfch_next++;
+                    if (scan->pfch_next >= prefetch_index_scans) {
+                        scan->pfch_next = 0;
+                    }
+                }
+
+                BlockIdCopy((&((scan->pfch_block_item_list + scan->pfch_next)->pfch_blockid)) , &(so->currPos.items[itemIndex].heapTid.ip_blkid));
+                if (scan->pfch_used <= scan->pfch_next) {
+                     scan->pfch_used = (scan->pfch_next + 1);
+                }
+
+                goto peek_complete;
+
+      block_match:         itemIndex++;
+            }
+	}
+
+ peek_complete:
+	PG_RETURN_BOOL(res);
+}
+
+/*
  * btgetbitmap() -- gets all matching tuples, and adds them to a bitmap
  */
 Datum
@@ -425,6 +505,12 @@ btbeginscan(PG_FUNCTION_ARGS)
 	so->killedItems = NULL;		/* until needed */
 	so->numKilled = 0;
 
+	so->backSeqRun = 0;
+	so->backSeqPos = 0;
+	so->prefetchItemIndex = 0;
+	so->lastHeapPrefetchBlkno = P_NONE;
+	so->prefetchBlockCount = 0;
+	
 	/*
 	 * We don't know yet whether the scan will be index-only, so we do not
 	 * allocate the tuple workspace arrays until btrescan.  However, we set up
@@ -516,6 +602,23 @@ btendscan(PG_FUNCTION_ARGS)
 {
 	IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
+        struct pfch_index_pagelist* pfch_index_plp;
+        int ix;
+
+#ifdef USE_PREFETCH
+
+	/* discard all prefetched but unread index pages listed in the pagelist */
+        pfch_index_plp = scan->pfch_index_page_list;
+        while ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+                    ix = pfch_index_plp->pfch_index_item_count;
+                    while (ix-- > 0) {
+                        if (pfch_index_plp->pfch_indexid[ix].pfch_discard) {
+                            DiscardBuffer( scan->indexRelation , MAIN_FORKNUM , pfch_index_plp->pfch_indexid[ix].pfch_blocknum);
+                        }
+                    }
+                    pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+        }
+#endif /* USE_PREFETCH */
 
 	/* we aren't holding any read locks, but gotta drop the pins */
 	if (BTScanPosIsValid(so->currPos))
--- src/backend/nodes/tidbitmap.c.orig	2014-05-31 17:19:07.817208676 -0400
+++ src/backend/nodes/tidbitmap.c	2014-05-31 19:53:09.440074487 -0400
@@ -44,6 +44,9 @@
 #include "nodes/bitmapset.h"
 #include "nodes/tidbitmap.h"
 #include "utils/hsearch.h"
+#ifdef USE_PREFETCH
+extern int	target_prefetch_pages;
+#endif   /* USE_PREFETCH */
 
 /*
  * The maximum number of tuples per page is not large (typically 256 with
@@ -572,7 +575,12 @@ tbm_begin_iterate(TIDBitmap *tbm)
 	 * needs of the TBMIterateResult sub-struct.
 	 */
 	iterator = (TBMIterator *) palloc(sizeof(TBMIterator) +
-								 MAX_TUPLES_PER_PAGE * sizeof(OffsetNumber));
+								 MAX_TUPLES_PER_PAGE * sizeof(OffsetNumber)
+#ifdef USE_PREFETCH
+                                          		      /*  space for remembering every prefetched but unread blockno */
+                                          		      +  (target_prefetch_pages * sizeof(BlockNumber))
+#endif   /* USE_PREFETCH */
+                                         );
 	iterator->tbm = tbm;
 
 	/*
@@ -1020,3 +1028,68 @@ tbm_comparator(const void *left, const v
 		return 1;
 	return 0;
 }
+
+void
+tbm_zero(TBMIterator *iterator) /* zero list of prefetched and unread blocknos */
+{
+      /* locate the list of prefetched but unread blocknos immediately following the array of offsets
+      ** and note that tbm_begin_iterate allocates space for (1 + MAX_TUPLES_PER_PAGE) offsets  -
+      ** 1 included in struct TBMIterator and MAX_TUPLES_PER_PAGE additional
+      */
+      iterator->output.Unread_Pfetched_base = ((BlockNumber *)(&(iterator->output.offsets[MAX_TUPLES_PER_PAGE+1])));
+      iterator->output.Unread_Pfetched_next = iterator->output.Unread_Pfetched_count = 0;
+}
+
+void
+tbm_add(TBMIterator *iterator, BlockNumber blockno) /* add this blockno to list of prefetched and unread blocknos */
+{
+      unsigned int Unread_Pfetched_index = iterator->output.Unread_Pfetched_next + iterator->output.Unread_Pfetched_count++;
+
+      if (iterator->output.Unread_Pfetched_count > target_prefetch_pages) {
+		ereport(ERROR,
+			(errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("tbm_add overflowed list cannot add blockno %d", blockno)));
+      }
+
+      if (Unread_Pfetched_index >= target_prefetch_pages)
+          Unread_Pfetched_index -= target_prefetch_pages;
+      *(iterator->output.Unread_Pfetched_base + Unread_Pfetched_index) = blockno;
+}
+
+void
+tbm_subtract(TBMIterator *iterator, BlockNumber blockno) /* remove this blockno from list of prefetched and unread blocknos */
+{
+      unsigned int Unread_Pfetched_index = iterator->output.Unread_Pfetched_next++;
+      BlockNumber nextUnreadPfetched;
+
+      /*    make a weak check that the next blockno is the one to be removed,
+      **    although actually in case of disagreement,   we ignore callers blockno and remove next anyway,
+      **    which is really what caller wants
+      */
+      if ( iterator->output.Unread_Pfetched_count == 0 ) {
+		ereport(ERROR,
+			(errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("tbm_subtract empty list cannot subtract blockno %d", blockno)));
+      }
+
+      if (Unread_Pfetched_index >= target_prefetch_pages)
+          Unread_Pfetched_index = 0;
+      nextUnreadPfetched = *(iterator->output.Unread_Pfetched_base + Unread_Pfetched_index);
+      if (   ( nextUnreadPfetched != blockno ) 
+          && ( nextUnreadPfetched != InvalidBlockNumber ) /* dont report it if the block in the list was InvalidBlockNumber */
+         ) {
+		ereport(NOTICE,
+			(errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("tbm_subtract will subtract blockno %d not %d",
+					nextUnreadPfetched, blockno)));
+      }
+      if (iterator->output.Unread_Pfetched_next >= target_prefetch_pages)
+          iterator->output.Unread_Pfetched_next = 0;
+      iterator->output.Unread_Pfetched_count--;
+}
+
+TBMIterateResult *
+tbm_locate_IterateResult(TBMIterator *iterator)
+{
+   return &(iterator->output);
+}
--- src/backend/utils/misc/guc.c.orig	2014-05-31 17:19:07.949209027 -0400
+++ src/backend/utils/misc/guc.c	2014-05-31 19:53:09.484074599 -0400
@@ -2264,6 +2264,25 @@ static struct config_int ConfigureNamesI
 	},
 
 	{
+		{"max_async_io_prefetchers",
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+			PGC_USERSET,
+#else
+			PGC_INTERNAL,
+#endif
+			RESOURCES_ASYNCHRONOUS,
+			gettext_noop("Maximum number of background processes concurrently using asynchronous librt threads to prefetch pages into shared memory buffers."),
+		},
+		&max_async_io_prefetchers,
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+	        -1, 0, 8192,      /*  boot val -1 indicates to initialize to something sensible during buf_init */
+#else
+		0, 0, 0,
+#endif
+		NULL, NULL, NULL
+	},
+
+	{
 		{"max_worker_processes",
 			PGC_POSTMASTER,
 			RESOURCES_ASYNCHRONOUS,
--- src/backend/utils/mmgr/aset.c.orig	2014-05-31 17:19:07.949209027 -0400
+++ src/backend/utils/mmgr/aset.c	2014-05-31 19:53:09.496074629 -0400
@@ -733,6 +733,48 @@ AllocSetAlloc(MemoryContext context, Siz
 	 */
 	fidx = AllocSetFreeIndex(size);
 	chunk = set->freelist[fidx];
+#ifdef MEMORY_CONTEXT_CHECKING
+        /*    an instance of segfault caused by a rogue value in set->freelist[fidx]
+        **    has been seen - check for it using crude sanity check based on neighbours :
+        **    if at least one is sufficiently close, then pass,  else fail
+        */
+        if (chunk != 0) {
+            int frx, nrx; /*  frx is index,  nrx is index of failing neighbour for errmsg */
+            for (nrx = -1, frx = 0; (frx < ALLOCSET_NUM_FREELISTS); frx++) {
+                if (   (frx != fidx)     /*  not the chosen one */
+                    && ( ( (unsigned long)(set->freelist[frx]) ) != 0 ) /* not empty */
+                   ) {
+                    if (   ( (unsigned long)chunk < ( ( (unsigned long)(set->freelist[frx]) ) / 2 ) )
+                        && (  ( (unsigned long)(set->freelist[frx]) ) < 0x4000000  )
+               /***     || ( (unsigned long)chunk > ( ( (unsigned long)(set->freelist[frx]) ) * 2 ) )  ***/
+                       ) {
+                       nrx = frx;
+                    } else {
+                       nrx = -1;
+                       break;
+                    }
+                }
+            }
+
+            if (nrx >= 0) {
+                     /*  this must be a rogue value  -  put this list in the garbage */
+                     /*  build message but be careful to avoid recursively triggering same fault */
+                     set->freelist[fidx] = NULL;   /*  mark this list empty */
+                     elog(WARNING, "detected rogue value %p in freelist index %d compared with neighbour %p whose chunksize %d"
+				 , chunk , fidx , set->freelist[nrx] , set->freelist[nrx]->size);
+                     chunk = NULL;
+            }
+        }
+#else /* if not MEMORY_CONTEXT_CHECKING make very simple-minded check*/
+        if ( (chunk != 0) && ( (unsigned long)chunk <  0x40000 ) ) {
+                     /*  this must be a rogue value  -  put this list in the garbage */
+                     /*  build message but be careful to avoid recursively triggering same fault */
+                     set->freelist[fidx] = NULL;   /*  mark this list empty */
+                     elog(WARNING, "detected rogue value %p in freelist index %d"
+				 , chunk , fidx);
+                     chunk = NULL;
+        }
+#endif
 	if (chunk != NULL)
 	{
 		Assert(chunk->size >= size);
--- src/include/executor/instrument.h.orig	2014-05-31 17:19:07.997209154 -0400
+++ src/include/executor/instrument.h	2014-05-31 19:53:09.536074730 -0400
@@ -28,8 +28,18 @@ typedef struct BufferUsage
 	long		local_blks_written;		/* # of local disk blocks written */
 	long		temp_blks_read; /* # of temp blocks read */
 	long		temp_blks_written;		/* # of temp blocks written */
+
 	instr_time	blk_read_time;	/* time spent reading */
 	instr_time	blk_write_time; /* time spent writing */
+
+	long		aio_read_noneed;		/* # of prefetches for which no need for prefetch as block already in buffer pool */
+	long		aio_read_discrd;		/* # of prefetches for which no need for prefetch as block already in buffer pool */
+	long		aio_read_forgot;		/* # of prefetches for which no need for prefetch as block already in buffer pool */
+	long		aio_read_noblok;		/* # of prefetches for which no available BufferAiocb */
+	long		aio_read_failed;		/* # of aio reads for which aio itself failed or the read failed with an errno */
+	long		aio_read_wasted;		/* # of aio reads for which disk block not used */
+	long		aio_read_waited;		/* # of aio reads for which disk block used but had to wait for it */
+	long		aio_read_ontime;		/* # of aio reads for which disk block used and ready on time when requested */
 } BufferUsage;
 
 /* Flag bits included in InstrAlloc's instrument_options bitmask */
--- src/include/storage/bufmgr.h.orig	2014-05-31 17:19:08.005209176 -0400
+++ src/include/storage/bufmgr.h	2014-05-31 19:53:09.572074821 -0400
@@ -41,6 +41,7 @@ typedef enum
 	RBM_ZERO_ON_ERROR,			/* Read, but return an all-zeros page on error */
 	RBM_NORMAL_NO_LOG			/* Don't log page as invalid during WAL
 								 * replay; otherwise same as RBM_NORMAL */
+       ,RBM_NOREAD_FOR_PREFETCH   /* Don't read from disk, don't zero buffer, find buffer only */
 } ReadBufferMode;
 
 /* in globals.c ... this duplicates miscadmin.h */
@@ -57,6 +58,9 @@ extern int	target_prefetch_pages;
 extern PGDLLIMPORT char *BufferBlocks;
 extern PGDLLIMPORT int32 *PrivateRefCount;
 
+/*  in buf_async.c  */;
+extern int      max_async_io_prefetchers; /* Maximum number of backends using asynchronous librt threads to read pages into our buffers */
+
 /* in localbuf.c */
 extern PGDLLIMPORT int NLocBuffer;
 extern PGDLLIMPORT Block *LocalBufferBlockPointers;
@@ -159,9 +163,15 @@ extern PGDLLIMPORT int32 *LocalRefCount;
 #define BufferGetPage(buffer) ((Page)BufferGetBlock(buffer))
 
 /*
- * prototypes for functions in bufmgr.c
+ * prototypes for external functions in bufmgr.c and buf_async.c
  */
-extern void PrefetchBuffer(Relation reln, ForkNumber forkNum,
+extern int PrefetchBuffer(Relation reln, ForkNumber forkNum,
+			   BlockNumber blockNum , BufferAccessStrategy strategy);
+/*   return code  is an int bitmask : */
+#define PREFTCHRC_BUF_PIN_INCREASED 0x01    /*  pin count on buffer has been increased by 1 */
+#define PREFTCHRC_BLK_ALREADY_PRESENT 0x02  /*  block was already present in a buffer       */
+
+extern void DiscardBuffer(Relation reln, ForkNumber forkNum,
 			   BlockNumber blockNum);
 extern Buffer ReadBuffer(Relation reln, BlockNumber blockNum);
 extern Buffer ReadBufferExtended(Relation reln, ForkNumber forkNum,
--- src/include/storage/smgr.h.orig	2014-05-31 17:19:08.009209186 -0400
+++ src/include/storage/smgr.h	2014-05-31 19:53:09.604074902 -0400
@@ -92,6 +92,12 @@ extern void smgrextend(SMgrRelation reln
 		   BlockNumber blocknum, char *buffer, bool skipFsync);
 extern void smgrprefetch(SMgrRelation reln, ForkNumber forknum,
 			 BlockNumber blocknum);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern void smgrinitaio(int max_aio_threads, int max_aio_num);
+extern void smgrstartaio(SMgrRelation reln, ForkNumber forknum,
+			 BlockNumber blocknum , char *aiocbp , int *retcode);
+extern void smgrcompleteaio( SMgrRelation reln, char *aiocbp , int *inoutcode );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 extern void smgrread(SMgrRelation reln, ForkNumber forknum,
 		 BlockNumber blocknum, char *buffer);
 extern void smgrwrite(SMgrRelation reln, ForkNumber forknum,
@@ -118,6 +124,11 @@ extern void mdextend(SMgrRelation reln,
 		 BlockNumber blocknum, char *buffer, bool skipFsync);
 extern void mdprefetch(SMgrRelation reln, ForkNumber forknum,
 		   BlockNumber blocknum);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern void mdinitaio(int max_aio_threads, int max_aio_num);
+extern void mdstartaio(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum , char *aiocbp , int *retcode );
+extern void mdcompleteaio( char *aiocbp , int *inoutcode );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 extern void mdread(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 	   char *buffer);
 extern void mdwrite(SMgrRelation reln, ForkNumber forknum,
--- src/include/storage/fd.h.orig	2014-05-31 17:19:08.009209186 -0400
+++ src/include/storage/fd.h	2014-05-31 19:53:09.632074973 -0400
@@ -69,6 +69,11 @@ extern File PathNameOpenFile(FileName fi
 extern File OpenTemporaryFile(bool interXact);
 extern void FileClose(File file);
 extern int	FilePrefetch(File file, off_t offset, int amount);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern void FileInitaio(int max_aio_threads, int max_aio_num );
+extern int  FileStartaio(File file, off_t offset, int amount , char *aiocbp);
+extern int  FileCompleteaio( char *aiocbp , int normal_wait );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 extern int	FileRead(File file, char *buffer, int amount);
 extern int	FileWrite(File file, char *buffer, int amount);
 extern int	FileSync(File file);
--- src/include/storage/buf_internals.h.orig	2014-05-31 17:19:08.005209176 -0400
+++ src/include/storage/buf_internals.h	2014-05-31 19:53:09.656075033 -0400
@@ -22,7 +22,9 @@
 #include "storage/smgr.h"
 #include "storage/spin.h"
 #include "utils/relcache.h"
-
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+#include "aio.h"
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * Flags for buffer descriptors
@@ -38,8 +40,23 @@
 #define BM_JUST_DIRTIED			(1 << 5)		/* dirtied since write started */
 #define BM_PIN_COUNT_WAITER		(1 << 6)		/* have waiter for sole pin */
 #define BM_CHECKPOINT_NEEDED	(1 << 7)		/* must write for checkpoint */
-#define BM_PERMANENT			(1 << 8)		/* permanent relation (not
-												 * unlogged) */
+#define BM_PERMANENT			(1 << 8)	/* permanent relation (not unlogged) */
+#define BM_AIO_IN_PROGRESS		(1 << 9)	/* aio in progress    */
+#define BM_AIO_PREFETCH_PIN_BANKED	(1 << 10)	/* pinned when prefetch issued
+                                                        ** and this pin is banked - i.e.
+                                                        ** redeemable by the next use by same task
+                                                        ** note that for any one buffer, a pin can be banked
+                                                        **      by at most one process globally,
+                                                        **      that is,   only one process may bank a pin on the buffer
+                                                        **                 and it may do so only once (may not be stacked)
+                                                        */
+
+/*********
+for asynchronous aio-read prefetching, two golden rules concerning buffer pinning and buffer-header flags must be observed:
+  R1.  a buffer marked as BM_AIO_IN_PROGRESS must be pinned by at least one backend
+  R2.  a buffer marked as BM_AIO_PREFETCH_PIN_BANKED must be pinned by the backend identified by
+               (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio : (-(buf->freeNext))
+*********/
 
 typedef bits16 BufFlags;
 
@@ -140,17 +157,83 @@ typedef struct sbufdesc
 	BufFlags	flags;			/* see bit definitions above */
 	uint16		usage_count;	/* usage counter for clock sweep code */
 	unsigned	refcount;		/* # of backends holding pins on buffer */
-	int			wait_backend_pid;		/* backend PID of pin-count waiter */
+	int		wait_backend_pid;	/*  if     flags & BM_PIN_COUNT_WAITER
+                                                **  then   backend PID of pin-count waiter
+                                                **  else   not set
+                                                */
 
 	slock_t		buf_hdr_lock;	/* protects the above fields */
 
 	int			buf_id;			/* buffer's index number (from 0) */
-	int			freeNext;		/* link in freelist chain */
+        int    	volatile	freeNext;	/* overloaded and much-abused field :
+                                                ** EITHER
+                                                **     if     >= 0
+                                                **     then   link in freelist chain
+                                                **  OR
+                                                **     if     <  0
+                                                **     then    EITHER
+                                                **             if     flags & BM_AIO_IN_PROGRESS
+                                                **             then   negative of (the index of the aiocb in the BufferAiocbs array + 3)
+                                                **             else   if flags & BM_AIO_PREFETCH_PIN_BANKED
+                                                **             then   -(pid of task that issued aio_read and pinned buffer)
+                                                **             else   one of the special values -1 or -2 listed below
+                                                */
 
 	LWLock	   *io_in_progress_lock;	/* to wait for I/O to complete */
 	LWLock	   *content_lock;	/* to lock access to buffer contents */
 } BufferDesc;
 
+/*  structures for control blocks for our implementation of async io */
+
+/*  if USE_AIO_ATOMIC_BUILTIN_COMP_SWAP is not defined,  the following struct is not put into use at runtime
+**  but it is easier to let the compiler find the definition but hide the reference to aiocb
+**  which is the only type it would not understand
+*/
+
+struct BufferAiocb {
+       struct BufferAiocb volatile * volatile BAiocbnext;  /*    next free entry or value of BAIOCB_OCCUPIED means in use  */
+       struct sbufdesc    volatile * volatile BAiocbbufh;  /*    there can be at most one BufferDesc marked BM_AIO_IN_PROGRESS
+                                                           **    and using this BufferAiocb -
+                                                           **    if there is one, BAiocbbufh points to it, else BAiocbbufh is zero
+                                                           **    NOTE  BAiocbbufh should be zero for every BufferAiocb on the free list
+                                                           */
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+       struct aiocb       volatile            BAiocbthis;  /*    the aio library's control block for one async io */
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+       int                volatile  BAiocbDependentCount;  /*    count of tasks who depend on this BufferAiocb
+                                                           **    in the sense that they are waiting for io completion.
+                                                           **    only a Dependent may move the BufferAiocb onto the freelist
+                                                           **    and only when that Dependent is the *only* Dependent (count == 1)
+                                                           **    BAiocbDependentCount is protected by bufferheader spinlock
+                                                           **    and must be updated only when that spinlock is held
+                                                           */
+       pid_t              volatile  pidOfAio;              /*    pid of backend who issued an aio_read using this BAiocb -
+                                                           **    this backend must have pinned the associated buffer.
+                                                           */
+};
+
+#define BAIOCB_OCCUPIED 0x75f1        /*  distinct indicator of a BufferAiocb.BAiocbnext that is NOT on free list */
+#define BAIOCB_FREE 0x7b9d            /*  distinct indicator of a BufferAiocb.BAiocbbufh that IS     on free list */
+
+struct BAiocbAnchor {                 /*  anchor for all control blocks pertaining to aio  */
+       volatile struct BufferAiocb* BufferAiocbs;          /*  aiocbs ...                   */
+       volatile struct BufferAiocb* volatile FreeBAiocbs; /* ... and their free list   */
+};
+
+/*   values for BufCheckAsync input and retcode */
+#define BUF_INTENTION_WANT 		 1  /* wants the buffer, wait for in-progress aio and then pin */
+#define BUF_INTENTION_REJECT_KEEP_PIN 	-1  /* pin already held, do not unpin */
+#define BUF_INTENTION_REJECT_OBTAIN_PIN	-2  /* obtain pin,  caller wants it for same buffer */
+#define BUF_INTENTION_REJECT_FORGET	-3  /* unpin and tell resource owner to forget */
+#define BUF_INTENTION_REJECT_NOADJUST	-4  /* unpin and call ResourceOwnerForgetBuffer */
+#define BUF_INTENTION_REJECT_UNBANK   	-5  /* unpin only if pin banked by caller */
+
+#define BUF_INTENT_RC_CHANGED_TAG	-5
+#define BUF_INTENT_RC_BADPAGE		-4
+#define BUF_INTENT_RC_INVALID_AIO	-3    /*  invalid and aio was in progress */
+#define BUF_INTENT_RC_INVALID_NO_AIO	-1    /*  invalid and no aio was in progress */
+#define BUF_INTENT_RC_VALID		 1
+
 #define BufferDescriptorGetBuffer(bdesc) ((bdesc)->buf_id + 1)
 
 /*
@@ -159,6 +242,7 @@ typedef struct sbufdesc
  */
 #define FREENEXT_END_OF_LIST	(-1)
 #define FREENEXT_NOT_IN_LIST	(-2)
+#define FREENEXT_BAIOCB_ORIGIN	(-3)
 
 /*
  * Macros for acquiring/releasing a shared buffer header's spinlock.
--- src/include/catalog/pg_am.h.orig	2014-05-31 17:19:07.989209134 -0400
+++ src/include/catalog/pg_am.h	2014-05-31 19:53:09.676075084 -0400
@@ -67,6 +67,7 @@ CATALOG(pg_am,2601)
 	regproc		amcanreturn;	/* can indexscan return IndexTuples? */
 	regproc		amcostestimate; /* estimate cost of an indexscan */
 	regproc		amoptions;		/* parse AM-specific parameters */
+	regproc		ampeeknexttuple;	/* peek at the next tuple different from any blocknum in pfch_list without reading a new index page */
 } FormData_pg_am;
 
 /* ----------------
@@ -117,19 +118,19 @@ typedef FormData_pg_am *Form_pg_am;
  * ----------------
  */
 
-DATA(insert OID = 403 (  btree		5 2 t f t t t t t t f t t 0 btinsert btbeginscan btgettuple btgetbitmap btrescan btendscan btmarkpos btrestrpos btbuild btbuildempty btbulkdelete btvacuumcleanup btcanreturn btcostestimate btoptions ));
+DATA(insert OID = 403 (  btree		5 2 t f t t t t t t f t t 0 btinsert btbeginscan btgettuple btgetbitmap btrescan btendscan btmarkpos btrestrpos btbuild btbuildempty btbulkdelete btvacuumcleanup btcanreturn btcostestimate btoptions btpeeknexttuple ));
 DESCR("b-tree index access method");
 #define BTREE_AM_OID 403
-DATA(insert OID = 405 (  hash		1 1 f f t f f f f f f f f 23 hashinsert hashbeginscan hashgettuple hashgetbitmap hashrescan hashendscan hashmarkpos hashrestrpos hashbuild hashbuildempty hashbulkdelete hashvacuumcleanup - hashcostestimate hashoptions ));
+DATA(insert OID = 405 (  hash		1 1 f f t f f f f f f f f 23 hashinsert hashbeginscan hashgettuple hashgetbitmap hashrescan hashendscan hashmarkpos hashrestrpos hashbuild hashbuildempty hashbulkdelete hashvacuumcleanup - hashcostestimate hashoptions - ));
 DESCR("hash index access method");
 #define HASH_AM_OID 405
-DATA(insert OID = 783 (  gist		0 8 f t f f t t f t t t f 0 gistinsert gistbeginscan gistgettuple gistgetbitmap gistrescan gistendscan gistmarkpos gistrestrpos gistbuild gistbuildempty gistbulkdelete gistvacuumcleanup - gistcostestimate gistoptions ));
+DATA(insert OID = 783 (  gist		0 8 f t f f t t f t t t f 0 gistinsert gistbeginscan gistgettuple gistgetbitmap gistrescan gistendscan gistmarkpos gistrestrpos gistbuild gistbuildempty gistbulkdelete gistvacuumcleanup - gistcostestimate gistoptions - ));
 DESCR("GiST index access method");
 #define GIST_AM_OID 783
-DATA(insert OID = 2742 (  gin		0 6 f f f f t t f f t f f 0 gininsert ginbeginscan - gingetbitmap ginrescan ginendscan ginmarkpos ginrestrpos ginbuild ginbuildempty ginbulkdelete ginvacuumcleanup - gincostestimate ginoptions ));
+DATA(insert OID = 2742 (  gin		0 6 f f f f t t f f t f f 0 gininsert ginbeginscan - gingetbitmap ginrescan ginendscan ginmarkpos ginrestrpos ginbuild ginbuildempty ginbulkdelete ginvacuumcleanup - gincostestimate ginoptions - ));
 DESCR("GIN index access method");
 #define GIN_AM_OID 2742
-DATA(insert OID = 4000 (  spgist	0 5 f f f f f t f t f f f 0 spginsert spgbeginscan spggettuple spggetbitmap spgrescan spgendscan spgmarkpos spgrestrpos spgbuild spgbuildempty spgbulkdelete spgvacuumcleanup spgcanreturn spgcostestimate spgoptions ));
+DATA(insert OID = 4000 (  spgist	0 5 f f f f f t f t f f f 0 spginsert spgbeginscan spggettuple spggetbitmap spgrescan spgendscan spgmarkpos spgrestrpos spgbuild spgbuildempty spgbulkdelete spgvacuumcleanup spgcanreturn spgcostestimate spgoptions - ));
 DESCR("SP-GiST index access method");
 #define SPGIST_AM_OID 4000
 
--- src/include/catalog/pg_proc.h.orig	2014-05-31 17:19:07.993209143 -0400
+++ src/include/catalog/pg_proc.h	2014-05-31 19:53:09.724075206 -0400
@@ -536,6 +536,12 @@ DESCR("convert float4 to int4");
 
 DATA(insert OID = 330 (  btgettuple		   PGNSP PGUID 12 1 0 0 0 f f f f t f v 2 0 16 "2281 2281" _null_ _null_ _null_ _null_	btgettuple _null_ _null_ _null_ ));
 DESCR("btree(internal)");
+
+#ifndef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+DATA(insert OID = 3251 (  btpeeknexttuple	   PGNSP PGUID 12 1 0 0 0 f f f f t f v 1 0 16 "2281" _null_ _null_ _null_ _null_ btpeeknexttuple _null_ _null_ _null_ ));
+DESCR("btree(internal)");
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+
 DATA(insert OID = 636 (  btgetbitmap	   PGNSP PGUID 12 1 0 0 0 f f f f t f v 2 0 20 "2281 2281" _null_ _null_ _null_ _null_	btgetbitmap _null_ _null_ _null_ ));
 DESCR("btree(internal)");
 DATA(insert OID = 331 (  btinsert		   PGNSP PGUID 12 1 0 0 0 f f f f t f v 6 0 16 "2281 2281 2281 2281 2281 2281" _null_ _null_ _null_ _null_	btinsert _null_ _null_ _null_ ));
--- src/include/pg_config_manual.h.orig	2014-05-31 17:19:08.005209176 -0400
+++ src/include/pg_config_manual.h	2014-05-31 19:53:09.740075246 -0400
@@ -138,9 +138,11 @@
 /*
  * USE_PREFETCH code should be compiled only if we have a way to implement
  * prefetching.  (This is decoupled from USE_POSIX_FADVISE because there
- * might in future be support for alternative low-level prefetch APIs.)
+ * might in future be support for alternative low-level prefetch APIs  --
+ * -- update October 2013  -- now there is such a new prefetch capability --
+ *   async_io into postgres buffers  -   configuration parameter max_async_io_threads)
  */
-#ifdef USE_POSIX_FADVISE
+#if defined(USE_POSIX_FADVISE) || defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP)
 #define USE_PREFETCH
 #endif
 
--- src/include/access/nbtree.h.orig	2014-05-31 17:19:07.989209134 -0400
+++ src/include/access/nbtree.h	2014-05-31 19:53:09.768075317 -0400
@@ -19,6 +19,7 @@
 #include "access/sdir.h"
 #include "access/xlog.h"
 #include "access/xlogutils.h"
+#include "access/relscan.h"
 #include "catalog/pg_index.h"
 
 /* There's room for a 16-bit vacuum cycle ID in BTPageOpaqueData */
@@ -524,6 +525,7 @@ typedef struct BTScanPosData
 	Buffer		buf;			/* if valid, the buffer is pinned */
 
 	BlockNumber nextPage;		/* page's right link when we scanned it */
+	BlockNumber prevPage;		/* page's left link when we scanned it */
 
 	/*
 	 * moreLeft and moreRight track whether we think there may be matching
@@ -603,6 +605,15 @@ typedef struct BTScanOpaqueData
 	 */
 	int			markItemIndex;	/* itemIndex, or -1 if not valid */
 
+	/* prefetch logic state */
+	unsigned int	backSeqRun;	/* number of back-sequential pages in a run */
+	BlockNumber		backSeqPos;	/* blkid last prefetched in back-sequential 
+				          		   runs */
+	BlockNumber		lastHeapPrefetchBlkno;	/* blkid last prefetched from heap */
+	int				prefetchItemIndex; /* item index within currPos last
+					                      fetched by heap prefetch */
+	int				prefetchBlockCount; /* number of prefetched heap blocks */
+	
 	/* keep these last in struct for efficiency */
 	BTScanPosData currPos;		/* current position data */
 	BTScanPosData markPos;		/* marked position, if any */
@@ -655,7 +666,11 @@ extern Buffer _bt_getroot(Relation rel,
 extern Buffer _bt_gettrueroot(Relation rel);
 extern int	_bt_getrootheight(Relation rel);
 extern void _bt_checkpage(Relation rel, Buffer buf);
-extern Buffer _bt_getbuf(Relation rel, BlockNumber blkno, int access);
+extern Buffer _bt_getbuf(Relation rel, BlockNumber blkno, int access , struct pfch_index_pagelist* pfch_index_page_list);
+extern void _bt_prefetchbuf(Relation rel, BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P);
+extern struct pfch_index_item* _bt_find_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list);
+extern int _bt_add_block(BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P , uint32 discard_status);
+extern void _bt_subtract_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list);
 extern Buffer _bt_relandgetbuf(Relation rel, Buffer obuf,
 				 BlockNumber blkno, int access);
 extern void _bt_relbuf(Relation rel, Buffer buf);
--- src/include/access/heapam.h.orig	2014-05-31 17:19:07.989209134 -0400
+++ src/include/access/heapam.h	2014-05-31 19:53:09.792075377 -0400
@@ -175,7 +175,7 @@ extern void heap_page_prune_execute(Buff
 extern void heap_get_root_tuples(Page page, OffsetNumber *root_offsets);
 
 /* in heap/syncscan.c */
-extern void ss_report_location(Relation rel, BlockNumber location);
+extern void ss_report_location(Relation rel, BlockNumber location , BlockNumber *prefetchHWMp);
 extern BlockNumber ss_get_location(Relation rel, BlockNumber relnblocks);
 extern void SyncScanShmemInit(void);
 extern Size SyncScanShmemSize(void);
--- src/include/access/relscan.h.orig	2014-05-31 17:19:07.989209134 -0400
+++ src/include/access/relscan.h	2014-05-31 19:53:09.808075418 -0400
@@ -44,6 +44,24 @@ typedef struct HeapScanDescData
 	bool		rs_inited;		/* false = scan not init'd yet */
 	HeapTupleData rs_ctup;		/* current tuple in scan, if any */
 	BlockNumber rs_cblock;		/* current block # in scan, if any */
+#ifdef USE_PREFETCH
+	int	    rs_prefetch_target; /* target distance (numblocks) for prefetch to reach beyond main scan */
+	BlockNumber rs_pfchblock;	/* next block # to be prefetched in scan, if any */
+
+        /*   Unread_Pfetched is a "mostly" circular list of recently prefetched blocknos of size target_prefetch_pages
+        **   the index of the first unread block is held in Unread_Pfetched_next
+        **         and is advanced when a block is read
+        **   the count of number of unread blocks is in Unread_Pfetched_count (and this subset can wrap around)
+        **   "mostly" means that there may be gaps caused by storing entries for blocks which do not need to be discarded -
+        **   these are indicated by blockno = InvalidBlockNumber,  and these slots are reused when found.
+        */
+        BlockNumber *rs_Unread_Pfetched_base;   /*  where the list of prefetched but unread blocknos starts */
+        unsigned int rs_Unread_Pfetched_next;   /*  where the next unread blockno probably is relative to start --
+                                                **  this is only a hint which may be temporarily stale.
+                                                */
+        unsigned int rs_Unread_Pfetched_count;  /*  number of valid unread blocknos in list */
+#endif   /* USE_PREFETCH */
+
 	Buffer		rs_cbuf;		/* current buffer in scan, if any */
 	/* NB: if rs_cbuf is not InvalidBuffer, we hold a pin on that buffer */
 	ItemPointerData rs_mctid;	/* marked scan position, if any */
@@ -55,6 +73,27 @@ typedef struct HeapScanDescData
 	OffsetNumber rs_vistuples[MaxHeapTuplesPerPage];	/* their offsets */
 }	HeapScanDescData;
 
+/* pfch_index_items track prefetched and unread index pages -   chunks of blocknumbers are chained in singly-linked list from scan->pfch_index_item_list */
+struct pfch_index_item {                              /*  index-relation BlockIds which we will/have prefetched */
+       BlockNumber         pfch_blocknum;    /* Blocknum which we will/have prefetched */
+       uint32              pfch_discard;     /* whether block is to be discarded when scan is closed */
+};
+
+struct pfch_block_item {
+       struct BlockIdData  pfch_blockid;     /* BlockId which we will/have prefetched */
+       uint32              pfch_discard;     /* whether block is to be discarded when scan is closed */
+};
+
+/* pfch_index_page_items track prefetched and unread index pages -
+** chunks of blocknumbers are chained backwards (newest first,  oldest last)
+** in singly-linked list from scan->pfch_index_item_list
+*/
+struct pfch_index_pagelist {                          /*  index-relation BlockIds which we will/have prefetched */
+       struct pfch_index_pagelist* pfch_index_pagelist_next;  /*  pointer to next chunk if any */
+       unsigned int    pfch_index_item_count;         /*  number of used entries in this chunk */
+       struct pfch_index_item pfch_indexid[1];        /*  in-line list of Blocknums which we will/have prefetched and whether to be discarded */
+};
+
 /*
  * We use the same IndexScanDescData structure for both amgettuple-based
  * and amgetbitmap-based index scans.  Some fields are only relevant in
@@ -75,8 +114,15 @@ typedef struct IndexScanDescData
 	/* signaling to index AM about killing index tuples */
 	bool		kill_prior_tuple;		/* last-returned tuple is dead */
 	bool		ignore_killed_tuples;	/* do not return killed entries */
-	bool		xactStartedInRecovery;	/* prevents killing/seeing killed
-										 * tuples */
+	bool		xactStartedInRecovery;	/* prevents killing/seeing killed tuples */
+										 
+#ifdef USE_PREFETCH
+        struct pfch_index_pagelist* pfch_index_page_list;  /* array of index-relation BlockIds which we will/have prefetched */
+        struct pfch_block_item* pfch_block_item_list;   /* array of heap-relation BlockIds which we will/have prefetched */
+        unsigned short int     pfch_used;	/*  number of used elements in BlockIdData array   */
+        unsigned short int     pfch_next;	/*  next element for prefetch in BlockIdData array */
+	int             do_prefetch;    /*  should I prefetch ? */
+#endif   /* USE_PREFETCH */
 
 	/* index access method's private state */
 	void	   *opaque;			/* access-method-specific info */
@@ -91,6 +137,10 @@ typedef struct IndexScanDescData
 	/* NB: if xs_cbuf is not InvalidBuffer, we hold a pin on that buffer */
 	bool		xs_recheck;		/* T means scan keys must be rechecked */
 
+	/* heap fetch statistics for read-ahead logic */
+	unsigned int heap_tids_seen;
+	unsigned int heap_tids_fetched;
+
 	/* state data for traversing HOT chains in index_getnext */
 	bool		xs_continue_hot;	/* T if must keep walking HOT chain */
 }	IndexScanDescData;
--- src/include/nodes/tidbitmap.h.orig	2014-05-31 17:19:08.001209165 -0400
+++ src/include/nodes/tidbitmap.h	2014-05-31 19:53:09.836075488 -0400
@@ -41,6 +41,16 @@ typedef struct
 	int			ntuples;		/* -1 indicates lossy result */
 	bool		recheck;		/* should the tuples be rechecked? */
 	/* Note: recheck is always true if ntuples < 0 */
+#ifdef USE_PREFETCH
+        /*   Unread_Pfetched is a circular list of recently prefetched blocknos of size target_prefetch_pages
+        **   the index of the first unread block is held in Unread_Pfetched_next
+        **         and is advanced when a block is read
+        **   the count of number of unread blocks is in Unread_Pfetched_count (and this subset can wrap around)
+        */
+        BlockNumber *Unread_Pfetched_base;   /*  where the list of prefetched but unread blocknos starts */
+        unsigned int Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+        unsigned int Unread_Pfetched_count;  /*  number of valid unread blocknos in list */
+#endif   /* USE_PREFETCH */
 	OffsetNumber offsets[1];	/* VARIABLE LENGTH ARRAY */
 } TBMIterateResult;				/* VARIABLE LENGTH STRUCT */
 
@@ -62,5 +72,8 @@ extern bool tbm_is_empty(const TIDBitmap
 extern TBMIterator *tbm_begin_iterate(TIDBitmap *tbm);
 extern TBMIterateResult *tbm_iterate(TBMIterator *iterator);
 extern void tbm_end_iterate(TBMIterator *iterator);
-
+extern void tbm_zero(TBMIterator *iterator); /* zero list of prefetched and unread blocknos */
+extern void tbm_add(TBMIterator *iterator, BlockNumber blockno); /* add this blockno to list of prefetched and unread blocknos */
+extern void tbm_subtract(TBMIterator *iterator, BlockNumber blockno); /* remove this blockno from list of prefetched and unread blocknos */
+extern TBMIterateResult *tbm_locate_IterateResult(TBMIterator *iterator); /* locate the TBMIterateResult of an iterator */
 #endif   /* TIDBITMAP_H */
--- src/include/utils/rel.h.orig	2014-05-31 17:19:08.013209197 -0400
+++ src/include/utils/rel.h	2014-05-31 19:53:09.864075560 -0400
@@ -61,6 +61,7 @@ typedef struct RelationAmInfo
 	FmgrInfo	ammarkpos;
 	FmgrInfo	amrestrpos;
 	FmgrInfo	amcanreturn;
+	FmgrInfo	ampeeknexttuple;	/* peek at the next tuple different from any blocknum in pfch_list without reading a new index page */
 } RelationAmInfo;
 
 
--- src/include/pg_config.h.in.orig	2014-05-31 17:19:08.001209165 -0400
+++ src/include/pg_config.h.in	2014-05-31 19:53:09.896075641 -0400
@@ -1,4 +1,4 @@
-/* src/include/pg_config.h.in.  Generated from configure.in by autoheader.  */
+/* src/include/pg_config.h.in.  Generated from - by autoheader.  */
 
 /* Define to the type of arg 1 of 'accept' */
 #undef ACCEPT_TYPE_ARG1
@@ -748,6 +748,10 @@
 /* Define to the appropriate snprintf format for unsigned 64-bit ints. */
 #undef UINT64_FORMAT
 
+/* Define to select librt-style async io and the gcc atomic compare_and_swap.
+   */
+#undef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+
 /* Define to 1 to build with assertion checks. (--enable-cassert) */
 #undef USE_ASSERT_CHECKING
 

--- configure.in.orig	2014-06-08 11:26:27.000000000 -0400
+++ configure.in	2014-06-08 21:59:36.140095486 -0400
@@ -1771,6 +1771,12 @@ operating system;  use --disable-thread-
 fi
 fi
 
+#  test whether this system has both the librt-style async io and the gcc atomic compare_and_swap
+#      and test operation  of the latter.
+PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP
+if test x"$pgac_cv_aio_atomic_builtin_comp_swap" = x"yes"; then
+      AC_DEFINE(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP, 1, [Define to select librt-style async io and the gcc atomic compare_and_swap.])
+fi
 
 # Select semaphore implementation type.
 if test "$PORTNAME" != "win32"; then
--- contrib/pg_prewarm/pg_prewarm.c.orig	2014-06-08 11:26:27.000000000 -0400
+++ contrib/pg_prewarm/pg_prewarm.c	2014-06-08 21:59:36.248095674 -0400
@@ -159,7 +159,7 @@ pg_prewarm(PG_FUNCTION_ARGS)
 		 */
 		for (block = first_block; block <= last_block; ++block)
 		{
-			PrefetchBuffer(rel, forkNumber, block);
+			PrefetchBuffer(rel, forkNumber, block, 0);
 			++blocks_done;
 		}
 #else
--- contrib/pg_stat_statements/pg_stat_statements--1.3.sql.orig	2014-06-08 17:57:22.088976836 -0400
+++ contrib/pg_stat_statements/pg_stat_statements--1.3.sql	2014-06-08 21:59:36.272095715 -0400
@@ -0,0 +1,52 @@
+/* contrib/pg_stat_statements/pg_stat_statements--1.3.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION pg_stat_statements VERSION '1.3'" to load this file. \quit
+
+-- Register functions.
+CREATE FUNCTION pg_stat_statements_reset()
+RETURNS void
+AS 'MODULE_PATHNAME'
+LANGUAGE C;
+
+CREATE FUNCTION pg_stat_statements(IN showtext boolean,
+    OUT userid oid,
+    OUT dbid oid,
+    OUT queryid bigint,
+    OUT query text,
+    OUT calls int8,
+    OUT total_time float8,
+    OUT rows int8,
+    OUT shared_blks_hit int8,
+    OUT shared_blks_read int8,
+    OUT shared_blks_dirtied int8,
+    OUT shared_blks_written int8,
+    OUT local_blks_hit int8,
+    OUT local_blks_read int8,
+    OUT local_blks_dirtied int8,
+    OUT local_blks_written int8,
+    OUT temp_blks_read int8,
+    OUT temp_blks_written int8,
+    OUT blk_read_time float8,
+    OUT blk_write_time float8
+   , OUT aio_read_noneed int8
+   , OUT aio_read_discrd int8
+   , OUT aio_read_forgot int8
+   , OUT aio_read_noblok int8
+   , OUT aio_read_failed int8
+   , OUT aio_read_wasted int8
+   , OUT aio_read_waited int8
+   , OUT aio_read_ontime int8
+)
+RETURNS SETOF record
+AS 'MODULE_PATHNAME', 'pg_stat_statements_1_3'
+LANGUAGE C STRICT VOLATILE;
+
+-- Register a view on the function for ease of use.
+CREATE VIEW pg_stat_statements AS
+  SELECT * FROM pg_stat_statements(true);
+
+GRANT SELECT ON pg_stat_statements TO PUBLIC;
+
+-- Don't want this to be available to non-superusers.
+REVOKE ALL ON FUNCTION pg_stat_statements_reset() FROM PUBLIC;
--- contrib/pg_stat_statements/Makefile.orig	2014-06-08 11:26:27.000000000 -0400
+++ contrib/pg_stat_statements/Makefile	2014-06-08 21:59:36.292095750 -0400
@@ -4,7 +4,8 @@ MODULE_big = pg_stat_statements
 OBJS = pg_stat_statements.o
 
 EXTENSION = pg_stat_statements
-DATA = pg_stat_statements--1.2.sql pg_stat_statements--1.1--1.2.sql \
+DATA = pg_stat_statements--1.3.sql pg_stat_statements--1.2--1.3.sql \
+	pg_stat_statements--1.2.sql pg_stat_statements--1.1--1.2.sql \
 	pg_stat_statements--1.0--1.1.sql pg_stat_statements--unpackaged--1.0.sql
 
 ifdef USE_PGXS
--- contrib/pg_stat_statements/pg_stat_statements.c.orig	2014-06-08 11:26:27.000000000 -0400
+++ contrib/pg_stat_statements/pg_stat_statements.c	2014-06-08 21:59:36.324095805 -0400
@@ -117,6 +117,7 @@ typedef enum pgssVersion
 	PGSS_V1_0 = 0,
 	PGSS_V1_1,
 	PGSS_V1_2
+	,PGSS_V1_3
 } pgssVersion;
 
 /*
@@ -148,6 +149,16 @@ typedef struct Counters
 	int64		local_blks_written;		/* # of local disk blocks written */
 	int64		temp_blks_read; /* # of temp blocks read */
 	int64		temp_blks_written;		/* # of temp blocks written */
+
+	int64		aio_read_noneed;		/* # of prefetches for which no need for prefetch as block already in buffer pool  */
+	int64		aio_read_discrd;		/* # of prefetches for which buffer not subsequently read and therefore discarded  */
+	int64		aio_read_forgot;		/* # of prefetches for which buffer not subsequently read and then forgotten about */
+	int64		aio_read_noblok;		/* # of prefetches for which no available BufferAiocb  control block               */
+	int64		aio_read_failed;		/* # of aio reads for which aio itself failed or the read failed with an errno     */
+	int64		aio_read_wasted;		/* # of aio reads for which in-progress aio cancelled and disk block not used      */
+	int64		aio_read_waited;		/* # of aio reads for which disk block used but had to wait for it                 */
+	int64		aio_read_ontime;		/* # of aio reads for which disk block used and ready on time when requested       */
+
 	double		blk_read_time;	/* time spent reading, in msec */
 	double		blk_write_time; /* time spent writing, in msec */
 	double		usage;			/* usage factor */
@@ -275,6 +286,7 @@ void		_PG_fini(void);
 
 PG_FUNCTION_INFO_V1(pg_stat_statements_reset);
 PG_FUNCTION_INFO_V1(pg_stat_statements_1_2);
+PG_FUNCTION_INFO_V1(pg_stat_statements_1_3);
 PG_FUNCTION_INFO_V1(pg_stat_statements);
 
 static void pgss_shmem_startup(void);
@@ -1026,7 +1038,25 @@ pgss_ProcessUtility(Node *parsetree, con
 		bufusage.temp_blks_read =
 			pgBufferUsage.temp_blks_read - bufusage_start.temp_blks_read;
 		bufusage.temp_blks_written =
-			pgBufferUsage.temp_blks_written - bufusage_start.temp_blks_written;
+			pgBufferUsage.temp_blks_written - bufusage.temp_blks_written;
+
+		bufusage.aio_read_noneed =
+			pgBufferUsage.aio_read_noneed - bufusage.aio_read_noneed;
+		bufusage.aio_read_discrd =
+			pgBufferUsage.aio_read_discrd - bufusage.aio_read_discrd;
+		bufusage.aio_read_forgot =
+			pgBufferUsage.aio_read_forgot - bufusage.aio_read_forgot;
+		bufusage.aio_read_noblok =
+			pgBufferUsage.aio_read_noblok - bufusage.aio_read_noblok;
+		bufusage.aio_read_failed =
+			pgBufferUsage.aio_read_failed - bufusage.aio_read_failed;
+		bufusage.aio_read_wasted =
+			pgBufferUsage.aio_read_wasted - bufusage.aio_read_wasted;
+		bufusage.aio_read_waited =
+			pgBufferUsage.aio_read_waited - bufusage.aio_read_waited;
+		bufusage.aio_read_ontime =
+			pgBufferUsage.aio_read_ontime - bufusage.aio_read_ontime;
+
 		bufusage.blk_read_time = pgBufferUsage.blk_read_time;
 		INSTR_TIME_SUBTRACT(bufusage.blk_read_time, bufusage_start.blk_read_time);
 		bufusage.blk_write_time = pgBufferUsage.blk_write_time;
@@ -1041,6 +1071,7 @@ pgss_ProcessUtility(Node *parsetree, con
 				   rows,
 				   &bufusage,
 				   NULL);
+
 	}
 	else
 	{
@@ -1224,6 +1255,16 @@ pgss_store(const char *query, uint32 que
 		e->counters.local_blks_written += bufusage->local_blks_written;
 		e->counters.temp_blks_read += bufusage->temp_blks_read;
 		e->counters.temp_blks_written += bufusage->temp_blks_written;
+
+		e->counters.aio_read_noneed     += bufusage->aio_read_noneed;
+		e->counters.aio_read_discrd     += bufusage->aio_read_discrd;
+		e->counters.aio_read_forgot     += bufusage->aio_read_forgot;
+		e->counters.aio_read_noblok     += bufusage->aio_read_noblok;
+		e->counters.aio_read_failed     += bufusage->aio_read_failed;
+		e->counters.aio_read_wasted     += bufusage->aio_read_wasted;
+		e->counters.aio_read_waited     += bufusage->aio_read_waited;
+		e->counters.aio_read_ontime     += bufusage->aio_read_ontime;
+
 		e->counters.blk_read_time += INSTR_TIME_GET_MILLISEC(bufusage->blk_read_time);
 		e->counters.blk_write_time += INSTR_TIME_GET_MILLISEC(bufusage->blk_write_time);
 		e->counters.usage += USAGE_EXEC(total_time);
@@ -1257,7 +1298,8 @@ pg_stat_statements_reset(PG_FUNCTION_ARG
 #define PG_STAT_STATEMENTS_COLS_V1_0	14
 #define PG_STAT_STATEMENTS_COLS_V1_1	18
 #define PG_STAT_STATEMENTS_COLS_V1_2	19
-#define PG_STAT_STATEMENTS_COLS			19		/* maximum of above */
+#define PG_STAT_STATEMENTS_COLS_V1_3	27
+#define PG_STAT_STATEMENTS_COLS			27		/* maximum of above */
 
 /*
  * Retrieve statement statistics.
@@ -1270,6 +1312,16 @@ pg_stat_statements_reset(PG_FUNCTION_ARG
  * function.  Unfortunately we weren't bright enough to do that for 1.1.
  */
 Datum
+pg_stat_statements_1_3(PG_FUNCTION_ARGS)
+{
+	bool		showtext = PG_GETARG_BOOL(0);
+
+	pg_stat_statements_internal(fcinfo, PGSS_V1_3, showtext);
+
+	return (Datum) 0;
+}
+
+Datum
 pg_stat_statements_1_2(PG_FUNCTION_ARGS)
 {
 	bool		showtext = PG_GETARG_BOOL(0);
@@ -1358,6 +1410,10 @@ pg_stat_statements_internal(FunctionCall
 			if (api_version != PGSS_V1_2)
 				elog(ERROR, "incorrect number of output arguments");
 			break;
+		case PG_STAT_STATEMENTS_COLS_V1_3:
+			if (api_version != PGSS_V1_3)
+				elog(ERROR, "incorrect number of output arguments");
+			break;
 		default:
 			elog(ERROR, "incorrect number of output arguments");
 	}
@@ -1534,11 +1590,24 @@ pg_stat_statements_internal(FunctionCall
 		{
 			values[i++] = Float8GetDatumFast(tmp.blk_read_time);
 			values[i++] = Float8GetDatumFast(tmp.blk_write_time);
+
+			if (api_version >= PGSS_V1_3)
+			{
+				values[i++] = Int64GetDatumFast(tmp.aio_read_noneed);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_discrd);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_forgot);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_noblok);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_failed);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_wasted);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_waited);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_ontime);
+			}
 		}
 
 		Assert(i == (api_version == PGSS_V1_0 ? PG_STAT_STATEMENTS_COLS_V1_0 :
 					 api_version == PGSS_V1_1 ? PG_STAT_STATEMENTS_COLS_V1_1 :
 					 api_version == PGSS_V1_2 ? PG_STAT_STATEMENTS_COLS_V1_2 :
+					 api_version == PGSS_V1_3 ? PG_STAT_STATEMENTS_COLS_V1_3 :
 					 -1 /* fail if you forget to update this assert */ ));
 
 		tuplestore_putvalues(tupstore, tupdesc, values, nulls);
--- contrib/pg_stat_statements/pg_stat_statements--1.2--1.3.sql.orig	2014-06-08 17:57:22.088976836 -0400
+++ contrib/pg_stat_statements/pg_stat_statements--1.2--1.3.sql	2014-06-08 21:59:36.348095847 -0400
@@ -0,0 +1,51 @@
+/* contrib/pg_stat_statements/pg_stat_statements--1.2--1.3.sql */
+
+-- complain if script is sourced in psql, rather than via ALTER EXTENSION
+\echo Use "ALTER EXTENSION pg_stat_statements UPDATE TO '1.3'" to load this file. \quit
+
+/* First we have to remove them from the extension */
+ALTER EXTENSION pg_stat_statements DROP VIEW pg_stat_statements;
+ALTER EXTENSION pg_stat_statements DROP FUNCTION pg_stat_statements();
+
+/* Then we can drop them */
+DROP VIEW pg_stat_statements;
+DROP FUNCTION pg_stat_statements();
+
+/* Now redefine */
+CREATE FUNCTION pg_stat_statements(IN showtext boolean,
+    OUT userid oid,
+    OUT dbid oid,
+    OUT queryid bigint,
+    OUT query text,
+    OUT calls int8,
+    OUT total_time float8,
+    OUT rows int8,
+    OUT shared_blks_hit int8,
+    OUT shared_blks_read int8,
+    OUT shared_blks_dirtied int8,
+    OUT shared_blks_written int8,
+    OUT local_blks_hit int8,
+    OUT local_blks_read int8,
+    OUT local_blks_dirtied int8,
+    OUT local_blks_written int8,
+    OUT temp_blks_read int8,
+    OUT temp_blks_written int8,
+    OUT blk_read_time float8,
+    OUT blk_write_time float8
+   , OUT aio_read_noneed int8
+   , OUT aio_read_discrd int8
+   , OUT aio_read_forgot int8
+   , OUT aio_read_noblok int8
+   , OUT aio_read_failed int8
+   , OUT aio_read_wasted int8
+   , OUT aio_read_waited int8
+   , OUT aio_read_ontime int8
+)
+RETURNS SETOF record
+AS 'MODULE_PATHNAME', 'pg_stat_statements_1_3'
+LANGUAGE C STRICT VOLATILE;
+
+CREATE VIEW pg_stat_statements AS
+  SELECT * FROM pg_stat_statements(true);
+
+GRANT SELECT ON pg_stat_statements TO PUBLIC;
--- postgresql-prefetching-asyncio.README.orig	2014-06-08 17:57:22.088976836 -0400
+++ postgresql-prefetching-asyncio.README	2014-06-08 21:59:36.372095888 -0400
@@ -0,0 +1,544 @@
+Postgresql  --   Extended Prefetching using Asynchronous IO
+============================================================
+
+Postgresql currently (9.3.4) provides a limited prefetching capability
+using posix_fadvise to give hints to the Operating System kernel
+about which pages it expects to read in the near future.
+This capability is used only during the heap-scan phase of bitmap-index scans.
+It is controlled via the effective_io_concurrency configuration parameter.
+
+This capability is now extended in two ways :
+   .   use asynchronous IO into Postgresql shared buffers as an
+       alternative to posix_fadvise
+   .   Implement prefetching in other types of scan :
+            .  non-bitmap (i.e. simple) index scans - index pages
+                     currently only for B-tree indexes.
+                    (developed by Claudio Freire <klaussfreire(at)gmail(dot)com>)
+            .  non-bitmap (i.e. simple) index scans - heap pages
+                          currently only for B-tree indexes.
+            .  simple heap scans
+
+Posix asynchronous IO is chosen as the function library for asynchronous IO,
+since this is well supported and also fits very well with the model of
+the prefetching process,  particularly as regards checking for completion
+of an asynchronous read.    On linux,   Posix asynchronous IO is provided
+in the librt library.    librt uses independently-schedulable threads to
+achieve the asynchronicity,   rather than kernel functionality.
+
+In this implementation,  use of asynchronous IO is limited to prefetching
+while performing one of the three types of scan
+            .  B-tree bitmap index scan - heap pages    (as already exists)
+            .  B-tree non-bitmap (i.e. simple) index scans - index and heap pages
+            .  simple heap scans
+on permanent relations.   It is not used on temporary tables nor for writes.
+
+The advantages of Posix asynchronous IO into shared buffers
+compared to posix_fadvise are :
+   .   Beneficial for non-sequential access patterns as well as sequential
+   .   No restriction on the kinds of IO which can be used
+       (other kinds of asynchronous IO impose restrictions such as
+        buffer alignment,  use of non-buffered IO).
+   .   Does not interfere with standard linux kernel read-ahead functionality.
+       (It has been stated in 
+ www.postgresql.org/message-id/CAGTBQpbu2M=-M7NUr6DWr0K8gUVmXVhwKohB-Cnj7kYS1AhH4A@mail.gmail.com
+       that :
+          "the kernel stops doing read-ahead when a call to posix_fadvise comes.
+           I noticed the performance hit, and checked the kernel's code.
+           It effectively changes the prediction mode from sequential to fadvise,
+           negating the (assumed) kernel's prefetch logic")
+   .   When the read request is issued after a prefetch has completed,
+       no delay associated with a kernel call to copy the page from
+       kernel page buffers into the Postgresql shared buffer,
+       since it is already there.
+       Also,   in a memory-constrained environment,   there is a greater
+       probability that the prefetched page will "stick" in memory
+       since the linux kernel victimizes the filesystem page cache in preference
+       to swapping out user process pages.
+   .   Statistics on prefetch success can be gathered (see "Statistics" below)
+       which helps the administrator to tune the prefetching settings.
+
+These benefits are most likely to be obtained in a system whose usage profile
+(e.g. from iostat)  shows:
+     .   high IO wait from mostly-read activity
+     .   disk access pattern is not entirely sequential
+         (so kernel readahead can't predict it but postgresql can)
+     .   sufficient spare idle CPU to run the librt pthreads
+         or,  stated another way,    the CPU subsystem is relatively powerful
+         compared to the disk subsystem.
+In such ideal conditions,  and with a workload with plenty of index scans,
+around 10% - 20% improvement in throughput has been achieved.
+In an admittedly extreme environment measured by this author,    with a workload
+consisting of 8 client applications each running similar complex queries
+(same query structure but different predicates and constants),
+including 2 Bitmap Index Scans and 17 non-bitmap index scans,
+on a dual-core Intel laptop (4 hyperthreads) with the database on a single
+USB3-attached 500GB disk drive, and no part of the database in filesystem buffers
+initially,  (filesystem freshly mounted),  comparing unpatched build
+using posix_fadvise with effective_io_concurrency 4 against same build patched
+with async IO and effective_io_concurrency 4 and max_async_io_prefetchers 32,
+elapse time repeatably improved from around 640-670 seconds to around 530-550 seconds,
+a 17% - 18% improvement. 
+
+The disadvantages of Posix asynchronous IO compared to posix_fadvise are:
+     .   probably higher CPU utilization:
+         Firstly, the extra work performed by the librt threads adds CPU
+         overhead, and secondly, if the asynchronous prefetching is effective,
+         then it will deliver better (greater) overlap of CPU with IO, which
+         will reduce elapsed times and hence increase CPU utilization percentage
+         still more (during that shorter elapsed time).
+     .   more context switching,  because of the additional threads.
+
+
+Statistics:
+___________
+
+A number of additional statistics relating to effectiveness of asynchronous IO
+are provided as an extension of the existing pg_stat_statements loadable module.
+Refer to the appendix "Additional Supplied Modules" in the current
+PostgreSQL Documentation for details of this module.
+
+The following additional statistics are provided for asynchronous IO prefetching:
+
+    . aio_read_noneed  :   number of prefetches for which no need for prefetch as block already in buffer pool
+    . aio_read_discrd  :   number of prefetches for which buffer not subsequently read and therefore discarded
+    . aio_read_forgot  :   number of prefetches for which buffer not subsequently read and then forgotten about
+    . aio_read_noblok  :   number of prefetches for which no available BufferAiocb  control block
+    . aio_read_failed  :   number of aio reads for which aio itself failed or the read failed with an errno
+    . aio_read_wasted  :   number of aio reads for which in-progress aio cancelled and disk block not used
+    . aio_read_waited  :   number of aio reads for which disk block used but had to wait for it
+    . aio_read_ontime  :   number of aio reads for which disk block used and ready on time when requested
+
+Some of these are (hopefully) self-explanatory.    Some additional notes:
+
+    . aio_read_discrd and aio_read_forgot  :
+                    prefetch was wasted work since the buffer was not subsequently read
+                    The discrd case indicates that the scanner realized this and discarded the buffer,
+                    whereas the forgot case indicates that the scanner did not realize it,
+                    which should not normally occur.
+                    A high number in either suggests lowering effective_io_concurrency.
+
+    . aio_read_noblok  :   
+                    Any significant number in relation to all the other numbers indicates that
+                    max_async_io_prefetchers should be increased.
+
+    . aio_read_waited  :
+                    The page was prefetched but the asynchronous read had not completed by the time the
+                    scanner requested to read it.     causes extra overhead in waiting and indicates
+                    prefetching is not providing much if any benefit.
+                    The disk subsystem may be underpowered/overloaded in relation to the available CPU power.
+
+    . aio_read_ontime  :
+                    The page was prefetched and the asynchronous read had completed by the time the
+                    scanner requested to read it.     Optimal behaviour.      If this number if large
+                    in relation to all the other numbers except (possibly) aio_read_noneed,
+                    then prefetching is working well.
+
+To create the extension with support for these additional statistics, use the following syntax:
+     CREATE EXTENSION pg_stat_statements VERSION '1.3'
+or,  if you run the new code against an existing database which already has the extension
+( see installation and migration below ),  you can 
+     ALTER EXTENSION pg_stat_statements UPDATE TO '1.3'
+
+A suggested set of commands for displaying these statistics might be :
+
+ /*  OPTIONALLY */ DROP extension pg_stat_statements;
+                   CREATE extension pg_stat_statements VERSION '1.3';
+ /*  run your workload   */
+                  select userid , dbid , substring(query from 1 for 24) , calls , total_time , rows , shared_blks_read , blk_read_time , blk_write_time \
+                    , aio_read_noneed , aio_read_noblok , aio_read_failed , aio_read_wasted , aio_read_waited , aio_read_ontime , aio_read_forgot       \
+                      from pg_stat_statements where shared_blks_read > 0;
+
+
+Installation and Build Configuration:
+_____________________________________
+
+1. First -  a prerequsite:
+#  as well as requiring all the usual package build tools such as gcc , make etc,
+#  as described in the instructions for building postgresql,
+#  the following is required :
+    gnu autoconf at version 2.69 :
+# run the following command
+autoconf -V
+# it *must* return
+autoconf (GNU Autoconf) 2.69
+
+2. If you don't have it or it is a different version,
+then you must obtain version 2.69 (which is the current version)
+from your distribution provider or from the gnu software download site.
+
+3. Also you must have the source tree for postgresql version 9.4 (development version).
+#   all the following commands assume your current working directory is the top of the source tree.
+
+4. cd to top of source tree :
+#   check it appears to be a postgresql source tree
+ls -ld configure.in src
+#   should show both the file and the directory
+grep PostgreSQL COPYRIGHT
+#   should show PostgreSQL Database Management System
+
+5. Apply the patch :
+patch -b -p0 -i <patch_file_path>
+#   should report no errors, 45 files patched (see list at bottom of this README)
+#   and all hunks applied
+#  check the patch was appplied to configure.in
+ls -ld configure.in.orig configure.in
+#   should show both files
+
+6. Rebuild the configure script with the patched configure.in :
+mv configure configure.orig;
+autoconf configure.in >configure;echo "rc= $? from autoconf"; chmod +x configure;
+ls -lrt configure.orig configure;
+
+7. run the new configure script :
+#   if you have run configure before,
+#   then you may first want to save existing config.status and config.log if they exist,
+#   and then specify same configure flags and options as you specified before.
+#   the patch does not alter or extend the set of configure options
+#   if unsure,   run ./configure --help
+#   if still unsure,   run ./configure
+./configure <other configure options as desired>
+
+
+
+8. now check that configure decided that this environment supports asynchronous IO :
+grep USE_AIO_ATOMIC_BUILTIN_COMP_SWAP src/include/pg_config.h
+#  it should show
+#define USE_AIO_ATOMIC_BUILTIN_COMP_SWAP 1
+#  if not,  apparently your environment does not support asynch IO  -
+#  the config.log will show how it came to that conclusion,
+#  also check for :
+#    . a librt.so somewhere in the loader's library path (probably under /lib , /lib64 , or /usr)
+#    . your gcc must support the atomic compare_and_swap __sync_bool_compare_and_swap built-in function
+#  do not proceed without this define being set.
+
+9. do you want to use the new code on an existing cluster
+   that was created using the same code base but without the patch?
+   If so then run this nasty-looking command :
+   (cut-and-paste it into a terminal window or a shell-script file)
+   Otherwise continue to step 10.
+   see Migration note below for explanation.
+###############################################################################################
+   fl=src/Makefile.global; typeset -i bkx=0; while [[ $bkx < 200 ]]; do {
+       bkfl="${fl}.bak${bkx}"; if [[ -a ${bkfl} ]]; then ((bkx=bkx+1)); else break; fi;
+   }; done;
+   if [[ -a ${bkfl} ]]; then echo "sorry cannot find a backup name for $fl";
+   elif [[ -a $fl ]]; then {
+       mv $fl $bkfl && {
+          sed -e "/^CFLAGS =/ s/\$/ -DAVOID_CATALOG_MIGRATION_FOR_ASYNCIO/" $bkfl > $fl;
+          str="diff -w $bkfl $fl";echo "$str"; eval "$str";
+       };
+   };
+   else echo "ooopppss $fl is missing";
+   fi;
+###############################################################################################
+# it should report something like
+diff -w Makefile.global.bak0 Makefile.global
+222c222
+< CFLAGS = XXXX
+---
+> CFLAGS = XXXX -DAVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+#   where XXXX is some set of flags
+
+
+10. now run the rest of the build process as usual  -
+    follow instructions in file INSTALL if that file exists,
+    else e.g. run
+make && make install
+
+If the build fails with the following error:
+undefined reference to `aio_init'
+Then edit the following file
+src/include/pg_config_manual.h
+and add the following line at the bottom:
+
+#define DONT_HAVE_AIO_INIT
+
+and then run
+make clean && make && make install
+See notes to section Runtime Configuration below for more information on this.
+
+
+
+Migration , Runtime Configuration, and Use:
+___________________________________________
+
+
+Database Migration:
+___________________
+
+The new prefetching code for non-bitmap index scans introduces a new btree-index
+function named btpeeknexttuple.    The correct way to add such a function involves
+also adding it to the catalog as an internal function in pg_proc.
+However,  this results in the new built code considering an existing database to be
+incompatible,  i.e requiring backup on the old code and restore on the new.
+This is normal behaviour for migration to a new version of postgresql,  and is
+also a valid way of migrating a database for use with this asynchronous IO feature,
+but in this case it may be inconvenient.
+
+As an alternative,    the new code may be compiled with the macro define
+AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+which does what it says by not altering the catalog.   The patched build can then
+be run against an existing database cluster initdb'd using the unpatched build.
+
+There are no known ill-effects of so doing,  but :
+     .  in any case,  it is strongly suggested to make a backup of any precious database
+        before accessing it with a patched build
+     .  be aware that if this asynchronous IO feature is eventually released as part of postgresql,
+        migration will probably be required anyway.
+
+This option to avoid catalog migration is intended as a convenience for a quick test,
+and also makes it easier to obtain performance comparisons on the same database.
+
+
+
+Runtime Configuration:
+______________________
+
+One new configuration parameter settable in postgresql.conf and
+in any other way as described in the postgresql documentation :
+
+max_async_io_prefetchers
+  Maximum number of background processes concurrently using asynchronous
+  librt threads to prefetch pages into shared memory buffers
+
+This number can be thought of as the maximum number
+of librt threads concurrently active,   each working on a list of
+from 1 to target_prefetch_pages pages ( see notes 1 and 2 ).
+
+In practice,    this number simply controls how many prefetch requests in total
+may be active concurrently :
+        max_async_io_prefetchers * target_prefetch_pages ( see note 1)
+
+default is max_connections/6
+and recall that the default for max_connections is 100
+
+
+note 1  a number based on effective_io_concurrency and approximately n * ln(n)
+        where n is effective_io_concurrency
+
+note 2  Provided that the gnu extension to Posix AIO which provides the
+aio_init() function is present,   then aio_init() is called
+to set the librt maximum number of threads to max_async_io_prefetchers,
+and to set the maximum number of concurrent aio read requests to the product of
+        max_async_io_prefetchers * target_prefetch_pages
+
+
+As well as this regular configuration parameter,
+there are several other parameters that can be set via environment variable.
+The reason why they are environment vars rather than regular configuration parameters
+is that it is not expected that they should need to be set,   but they may be useful :
+                variable name         values                  default        meaning
+   PG_TRY_PREFETCHING_FOR_BITMAP      [Y|N]                    Y         whether to prefetch bitmap heap scans
+   PG_TRY_PREFETCHING_FOR_ISCAN       [Y|N|integer[,[N|Y]]]   256,N      whether to prefetch  non-bitmap index scans
+                                                                    also numeric size of list of prefetched blocks
+                                                                    also whether to prefetch forward-sequential-pattern index pages
+   PG_TRY_PREFETCHING_FOR_BTREE       [Y|N]                    Y         whether to prefetch heap pages in non-bitmap index scans
+   PG_TRY_PREFETCHING_FOR_HEAP        [Y|N]                    N         whether to prefetch relation (un-indexed) heap scans
+
+
+The setting for PG_TRY_PREFETCHING_FOR_ISCAN is a litle complicated.
+It can be set to Y or N to control prefetching of  non-bitmap index scans;
+But in addition it can be set to an integer,   which both implies Y
+and also sets the size of a list used to remember prefetched but unread heap pages.
+This list is an optimization used to avoid re-prefetching and maximise the potential
+set of prefetchable blocks indexed by one index page.
+And if set to an integer,  this integer may be followed by either ,Y or ,N
+to specify to prefetch index pages which are being accessed forward-sequentially.
+It has been found that prefetching is not of great benefit for this access pattern,
+and so it is not the default,  but also does no harm (provided sufficient CPU capacity).
+
+
+
+Usage :
+______
+
+
+There are no changes in usage other than as noted under Configuration and Statistics.
+However,   in order to assess benefit from this feature,   it will be useful to
+understand the query access plans of your workload using EXPLAIN.    Before doing that,
+make sure that statistics are up to date using ANALYZE.
+
+
+
+Internals:
+__________
+
+
+Internal changes span two areas and the interface between them :
+
+ .  buffer manager layer
+ .  programming interface for scanner to call buffer manager
+ .  scanner layer
+
+ .  buffer manager layer
+    ____________________
+
+    changes comprise :
+       .   allocating,  pinning , unpinning buffers
+            this is complex and discussed briefly below in "Buffer Management"
+       .   acquiring and releasing a BufferAiocb, the control block
+            associated with a single aio_read,  and checking for its completion
+            a new file,  backend/storage/buffer/buf_async.c, provides three new functions,
+                  BufStartAsync        BufReleaseAsync            BufCheckAsync
+            which handle this.
+       .   calling librt asynch io functions
+            this follows the example of all other filesystem interfaces
+            and is straightforward.    
+            two new functions are provided in fd.c:
+                   FileStartaio        FileCompleteaio
+            and corresponding interfaces in smgr.c
+
+ .  programming interface for scanner to call buffer manager
+    ________________________________________________________
+       . calling interface for existing function PrefetchBuffer is modified :
+           .  one new argument,   BufferAccessStrategy strategy
+           .  now returns an int return code which indicates :
+                     whether pin count on buffer has been increased by 1
+                     whether block was already present in a buffer
+       .  new function DiscardBuffer
+           .  discard buffer used for a previously prefetched page
+                 which scanner decides it does not want to read.
+           .  same arguments as for PrefetchBuffer except for omission of BufferAccessStrategy
+           .  note - this is different from the existing function ReleaseBuffer
+                     in that ReleaseBuffer takes a buffer_descriptor as argument
+                     for a buffer which has been read, but has similar purpose.
+
+ .  scanner layer
+    _____________
+        common to all scanners is that the scanner which wishes to prefetch must do two things:
+          .  decide which pages to prefetch and call PrefetchBuffer to prefetch them
+                 nodeBitmapHeapscan already does this (but note one extra argument on PrefetchBuffer)
+          .  remember which pages it has prefetched in some list (actual or conceptual,  e.g. a page range),
+                 removing each page from this list if and when it subsequently reads the page.
+          .  at end of scan,  call DiscardBuffer for every remembered (i.e. prefetched not unread) page
+       how this list of prefetched pages is implemented varies for each of the three scanners and four scan types:
+            .  bitmap index scan - heap pages
+            .  non-bitmap (i.e. simple) index scans - index pages
+            .  non-bitmap (i.e. simple) index scans - heap pages
+            .  simple heap scans
+       The consequences of forgetting to call DiscardBuffer on a prefetched but unread page are:
+            .   counted in aio_read_forgot  (see "Statistics" above)
+            .   may incur an annoying but harmless warning in the pg_log "Buffer Leak ... "
+                  (the buffer is released at commit)
+       This does sometimes happen ...
+     
+
+
+Buffer Management
+_________________
+
+With async io,   PrefetchBuffer must allocate and pin a buffer,  which is relatively straightforward,
+but also every other part of buffer manager must know about the possibility that a buffer may be in
+a state of async_io_in_progress state and be prepared to determine the possible completion.
+That is,  one backend BK1 may start the io but another BK2 may try to read it before BK1 does.
+Posix Asynchronous IO provides a means for waiting on this or another task's read if in progress,
+namely aio_suspend(),  which this extension uses.    Therefore,  although StartBufferIO and TerminateBufferIO
+are called as part of asynchronous prefetching,   their role is limited to maintaining the buffer descriptor flags,
+and they do not track the asynchronous IO itself.    Instead,   asynchronous IOs are tracked in
+a separate set of shared control blocks,  the BufferAiocb list -
+refer to     include/storage/buf_internals.h
+Checking asynchronous io status is handled in  backend/storage/buffer/buf_async.c BufCheckAsync function.
+Read the commentary for this function for more details.
+
+Pinning and unpinning of buffers is the most complex aspect of asynch io prefetching,
+and the logic is spread throughout BufStartAsync , BufCheckAsync , and many functions in bufmgr.c.
+When a backend BK2 requests ReadBuffer of a page for which asynch read is in progress,
+buffer manager has to determine which backend BK1 pinned this buffer during previous PrefetchBuffer,
+and for example must not be re-pinned a second time if BK2 is BK1.
+Information concerning which backend initiated the prefetch is held in the BufferAiocb.
+
+The trickiest case concerns the scenario in which :
+   .  BK1 initiates prefetch and acquires a pin
+   .  BK2 possibly waits for completion and then reads the buffer,  and perhaps later on
+         releases it by ReleaseBuffer.
+   .  Since the asynchronous IO is no longer in progress,     there is no longer any
+         BufferAiocb associated with it.    Yet buffer manager must remember that BK1 holds a
+         "prefetch" pin, i.e. a pin which must not be repeated if and when BK1 finally issues ReadBuffer.
+   .  The solution to this problem is to invent the concept of a "banked" pin,
+      which is a pin obtained when prefetch was issued,   identied as in "banked" status only if and when
+      the associated asynchronous IO terminates,  and redeemable by the next use by same task,
+      either by ReadBuffer or DiscardBuffer.
+      The pid of the backend which holds a banked pin on a buffer (there can be at most one such backend)
+      is stored in the buffer descriptor.
+      This is done without increasing size of the buffer descriptor,  which is important since
+      there may be a very large number of these.     This does overload the relevant field in the descriptor.
+      Refer to include/storage/buf_internals.h for more details
+      and search for BM_AIO_PREFETCH_PIN_BANKED in storage/buffer/bufmgr.c and  backend/storage/buffer/buf_async.c
+
+______________________________________________________________________________
+The following 45 files are changed in this feature (output of the patch command) :
+
+patching file configure.in
+patching file contrib/pg_prewarm/pg_prewarm.c
+patching file contrib/pg_stat_statements/pg_stat_statements--1.3.sql
+patching file contrib/pg_stat_statements/Makefile
+patching file contrib/pg_stat_statements/pg_stat_statements.c
+patching file contrib/pg_stat_statements/pg_stat_statements--1.2--1.3.sql
+patching file postgresql-prefetching-asyncio.README
+patching file config/c-library.m4
+patching file src/backend/postmaster/postmaster.c
+patching file src/backend/executor/nodeBitmapHeapscan.c
+patching file src/backend/executor/nodeIndexscan.c
+patching file src/backend/executor/instrument.c
+patching file src/backend/storage/buffer/Makefile
+patching file src/backend/storage/buffer/bufmgr.c
+patching file src/backend/storage/buffer/buf_async.c
+patching file src/backend/storage/buffer/buf_init.c
+patching file src/backend/storage/smgr/md.c
+patching file src/backend/storage/smgr/smgr.c
+patching file src/backend/storage/file/fd.c
+patching file src/backend/storage/lmgr/proc.c
+patching file src/backend/access/heap/heapam.c
+patching file src/backend/access/heap/syncscan.c
+patching file src/backend/access/index/indexam.c
+patching file src/backend/access/index/genam.c
+patching file src/backend/access/nbtree/nbtsearch.c
+patching file src/backend/access/nbtree/nbtinsert.c
+patching file src/backend/access/nbtree/nbtpage.c
+patching file src/backend/access/nbtree/nbtree.c
+patching file src/backend/nodes/tidbitmap.c
+patching file src/backend/utils/misc/guc.c
+patching file src/backend/utils/mmgr/aset.c
+patching file src/include/executor/instrument.h
+patching file src/include/storage/bufmgr.h
+patching file src/include/storage/smgr.h
+patching file src/include/storage/fd.h
+patching file src/include/storage/buf_internals.h
+patching file src/include/catalog/pg_am.h
+patching file src/include/catalog/pg_proc.h
+patching file src/include/pg_config_manual.h
+patching file src/include/access/nbtree.h
+patching file src/include/access/heapam.h
+patching file src/include/access/relscan.h
+patching file src/include/nodes/tidbitmap.h
+patching file src/include/utils/rel.h
+patching file src/include/pg_config.h.in
+
+
+Future Possibilities:
+____________________
+
+There are several possible extensions of this feature :
+   .   Extend prefetching of index scans to types of index
+       other than B-tree.
+       This should be fairly straightforward,  but requires some
+       good base of benchmarkable workloads to prove the value.
+   .   Investigate why asynchronous IO prefetching does not greatly
+       improve sequential relation heap scans and possibly find how to
+       achieve a benefit.
+   .   Build knowledge of asycnhronous IO prefetching into the
+       Query Planner costing.
+       This is far from straightforward.    The Postgresql Query Planner's
+       costing model is based on resource consumption rather than elapsed time.
+       Use of asynchronous IO prefetching is intended to improve elapsed time
+       as the expense of (probably) higher resource consumption.
+       Although Costing understands about the reduced cost of reading buffered
+       blocks, it does not take asynchronicity or overlap of CPU with disk
+       into account.  A naive approach might be to try to tweak the Query
+       Planner's Cost Constant configuration parameters
+       such as seq_page_cost , random_page_cost
+       but this is hazardous as explained in the Documentation.
+
+
+
+John Lumby,  johnlumby(at)hotmail(dot)com
--- config/c-library.m4.orig	2014-06-08 11:26:27.000000000 -0400
+++ config/c-library.m4	2014-06-08 21:59:36.448096020 -0400
@@ -367,3 +367,152 @@ if test "$pgac_cv_type_locale_t" = 'yes
   AC_DEFINE(LOCALE_T_IN_XLOCALE, 1,
             [Define to 1 if `locale_t' requires <xlocale.h>.])
 fi])])# PGAC_HEADER_XLOCALE
+
+
+# PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP
+# ---------------------------------------
+# test whether this system has both the librt-style async io and the gcc atomic compare_and_swap
+#      and test operation of both,
+#      including verifying that aio_error can retrieve completion status
+#      of aio_read issued by a different process
+#
+AC_DEFUN([PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP],
+[AC_MSG_CHECKING([whether have both librt-style async io and the gcc atomic compare_and_swap])
+AC_CACHE_VAL(pgac_cv_aio_atomic_builtin_comp_swap,
+pgac_save_LIBS=$LIBS
+LIBS=" -lrt $pgac_save_LIBS"
+[AC_TRY_RUN([#include <stdio.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+#include <fcntl.h>
+#include "aio.h"
+#include <errno.h>
+
+char *shmem;
+
+/*  returns rc of aio_read or -1 if some error */
+int
+processA(void)
+{
+	int fd , rc;
+	struct aiocb *aiocbp = (struct aiocb *) shmem;
+	char *buf = shmem + sizeof(struct aiocb);
+
+	rc = fd = open("configure", O_RDONLY );
+	if (fd != -1) {
+
+            memset(aiocbp, 0, sizeof(struct aiocb));
+            aiocbp->aio_fildes = fd;
+            aiocbp->aio_offset = 0;
+            aiocbp->aio_buf = buf;
+            aiocbp->aio_nbytes = 8;
+            aiocbp->aio_reqprio = 0;
+            aiocbp->aio_sigevent.sigev_notify = SIGEV_NONE;
+
+            rc = aio_read(aiocbp);
+	}
+        return rc;
+}
+
+/*  returns result of aio_error  -  0 if io completed successfully */
+int
+processB(void)
+{
+	struct aiocb *aiocbp = (struct aiocb *) shmem;
+	const struct aiocb * const pl[1] = { aiocbp };
+	int rv;
+	int	returnCode;
+        struct timespec my_timeout = { 0 , 10000 };
+        int max_iters , max_polls;
+
+	rv = aio_error(aiocbp);
+        max_iters = 100;
+        while ( (max_iters-- > 0) && (rv == EINPROGRESS) ) {
+                max_polls = 256;
+                my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                returnCode = aio_suspend(pl , 1 , &my_timeout);
+                while ((returnCode < 0) && (EAGAIN == errno) && (max_polls-- > 0)) {
+                    my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                    returnCode = aio_suspend(pl , 1 , &my_timeout);
+                }
+                rv = aio_error(aiocbp);
+        }
+
+	return rv;
+}
+
+int main (int argc, char *argv[])
+ {
+   int rc;
+   int pidB;
+   int child_status;
+   struct aiocb volatile * first_aiocb;
+   struct aiocb volatile * second_aiocb;
+   struct aiocb volatile * my_aiocbp = (struct aiocb *)20000008;
+
+   first_aiocb = (struct aiocb *)20000008;
+   second_aiocb = (struct aiocb *)40000008;
+
+   /*  first test  --  __sync_bool_compare_and_swap
+   **  set zero as success if two comp-swaps both worked as expected -
+   **  first compares equal and swaps,  second compares unequal
+   */
+   rc = (__sync_bool_compare_and_swap (&my_aiocbp, first_aiocb, second_aiocb));
+   if (rc) {
+      rc = (__sync_bool_compare_and_swap (&my_aiocbp, first_aiocb, second_aiocb));
+   } else {
+      rc = -1;
+   }
+
+   if (rc == 0) {
+       /*  second test  --  process A start aio_read
+       **  and process B checks completion by polling
+       */
+        rc = -1; /* pessimistic */
+
+	shmem = mmap(NULL, sizeof(struct aiocb) + 2048,
+				 PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS,
+				 -1, 0);
+	if (shmem != MAP_FAILED) {
+            
+            /*
+             * Start the I/O request in parent process, then fork and try to wait
+             * for it to finish from the child process.
+             */
+            rc = processA();
+            if (rc >= 0) {
+
+                rc = pidB = fork();
+                if (pidB != -1) {
+                    if (pidB != 0) {
+                        /* parent */
+                        wait (&child_status);
+                        if (WIFEXITED(child_status)) {
+                            rc = WEXITSTATUS(child_status);
+                        }
+                    } else {
+                        /* child */
+                        rc = processB();
+                        exit(rc);
+                    }
+                }
+            }
+        }
+   }
+
+   return rc;
+}],
+[pgac_cv_aio_atomic_builtin_comp_swap=yes],
+[pgac_cv_aio_atomic_builtin_comp_swap=no],
+[pgac_cv_aio_atomic_builtin_comp_swap=cross])
+])dnl AC_CACHE_VAL
+AC_MSG_RESULT([$pgac_cv_aio_atomic_builtin_comp_swap])
+if test x"$pgac_cv_aio_atomic_builtin_comp_swap" != x"yes"; then
+LIBS=$pgac_save_LIBS
+fi
+])# PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP
--- src/backend/postmaster/postmaster.c.orig	2014-06-08 11:26:30.000000000 -0400
+++ src/backend/postmaster/postmaster.c	2014-06-08 21:59:36.532096166 -0400
@@ -123,6 +123,11 @@
 #include "storage/spin.h"
 #endif
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+void ReportFreeBAiocbs(void);
+int CountInuseBAiocbs(void);
+extern int hwmBufferAiocbs;         /*  high water mark of in-use  BufferAiocbs in pool           */
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * Possible types of a backend. Beyond being the possible bkend_type values in
@@ -1493,9 +1498,15 @@ ServerLoop(void)
 	fd_set		readmask;
 	int			nSockets;
 	time_t		now,
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                           count_baiocb_time,
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 				last_touch_time;
 
 	last_touch_time = time(NULL);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        count_baiocb_time = time(NULL);
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 	nSockets = initMasks(&readmask);
 
@@ -1654,6 +1665,19 @@ ServerLoop(void)
 			last_touch_time = now;
 		}
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                /*   maintain the hwm of used baiocbs every 10 seconds  */
+		if ((now - count_baiocb_time) >= 10)
+		{
+                        int inuseBufferAiocbs;         /*  current in-use  BufferAiocbs in pool */
+                        inuseBufferAiocbs = CountInuseBAiocbs();
+                        if (inuseBufferAiocbs > hwmBufferAiocbs) {
+			    hwmBufferAiocbs = inuseBufferAiocbs;
+			}
+			count_baiocb_time = now;
+		}
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 		/*
 		 * If we already sent SIGQUIT to children and they are slow to shut
 		 * down, it's time to send them SIGKILL.  This doesn't happen
@@ -3444,6 +3468,9 @@ PostmasterStateMachine(void)
 						signal_child(PgStatPID, SIGQUIT);
 				}
 			}
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                        ReportFreeBAiocbs();
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 		}
 	}
 
--- src/backend/executor/nodeBitmapHeapscan.c.orig	2014-06-08 11:26:29.000000000 -0400
+++ src/backend/executor/nodeBitmapHeapscan.c	2014-06-08 21:59:36.552096200 -0400
@@ -34,6 +34,8 @@
  *		ExecEndBitmapHeapScan		releases all storage.
  */
 #include "postgres.h"
+#include <sys/types.h> 
+#include <unistd.h>
 
 #include "access/relscan.h"
 #include "access/transam.h"
@@ -47,6 +49,10 @@
 #include "utils/snapmgr.h"
 #include "utils/tqual.h"
 
+#ifdef USE_PREFETCH
+extern unsigned int prefetch_dbOid; /*  database oid of relations on which prefetching to be done - 0 means all */
+extern unsigned int prefetch_bitmap_scans; /*  boolean whether to prefetch bitmap heap scans         */
+#endif   /* USE_PREFETCH */
 
 static TupleTableSlot *BitmapHeapNext(BitmapHeapScanState *node);
 static void bitgetpage(HeapScanDesc scan, TBMIterateResult *tbmres);
@@ -111,10 +117,21 @@ BitmapHeapNext(BitmapHeapScanState *node
 		node->tbmres = tbmres = NULL;
 
 #ifdef USE_PREFETCH
-		if (target_prefetch_pages > 0)
-		{
+		if (    prefetch_bitmap_scans
+                     && (target_prefetch_pages > 0)
+                     && (    (   (prefetch_dbOid > 0)
+                               && (prefetch_dbOid == scan->rs_rd->rd_node.dbNode)
+                             )
+                         ||  (prefetch_dbOid == 0)
+                        )
+                        /* sufficient number of blocks - at least twice the target_prefetch_pages */
+                     && (scan->rs_nblocks > (2*target_prefetch_pages))
+                   ) {
 			node->prefetch_iterator = prefetch_iterator = tbm_begin_iterate(tbm);
 			node->prefetch_pages = 0;
+                        if (prefetch_iterator) {
+                          tbm_zero(prefetch_iterator);  /* zero list of prefetched and unread blocknos */
+                        }
 			node->prefetch_target = -1;
 		}
 #endif   /* USE_PREFETCH */
@@ -138,12 +155,14 @@ BitmapHeapNext(BitmapHeapScanState *node
 			}
 
 #ifdef USE_PREFETCH
+                        if (prefetch_iterator) {
 			if (node->prefetch_pages > 0)
 			{
 				/* The main iterator has closed the distance by one page */
 				node->prefetch_pages--;
+                                tbm_subtract(prefetch_iterator, tbmres->blockno); /* remove this blockno from list of prefetched and unread blocknos */
 			}
-			else if (prefetch_iterator)
+                            else
 			{
 				/* Do not let the prefetch iterator get behind the main one */
 				TBMIterateResult *tbmpre = tbm_iterate(prefetch_iterator);
@@ -151,6 +170,7 @@ BitmapHeapNext(BitmapHeapScanState *node
 				if (tbmpre == NULL || tbmpre->blockno != tbmres->blockno)
 					elog(ERROR, "prefetch and main iterators are out of sync");
 			}
+                        }
 #endif   /* USE_PREFETCH */
 
 			/*
@@ -239,16 +259,26 @@ BitmapHeapNext(BitmapHeapScanState *node
 			while (node->prefetch_pages < node->prefetch_target)
 			{
 				TBMIterateResult *tbmpre = tbm_iterate(prefetch_iterator);
+                                int             PrefetchBufferRc; /*  return value from PrefetchBuffer  - refer to bufmgr.h */
+
 
 				if (tbmpre == NULL)
 				{
 					/* No more pages to prefetch */
-					tbm_end_iterate(prefetch_iterator);
-					node->prefetch_iterator = prefetch_iterator = NULL;
+                                        /* let ExecEndBitmapHeapScan terminate the prefetch_iterator
+				        **	tbm_end_iterate(prefetch_iterator);
+					**      node->prefetch_iterator = NULL;
+                                        */
+                                        prefetch_iterator = NULL;
 					break;
 				}
 				node->prefetch_pages++;
-				PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, tbmpre->blockno);
+				PrefetchBufferRc = PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, tbmpre->blockno , 0);
+                                /*  add this blockno to list of prefetched and unread blocknos
+                                **  if pin count did not increase then indicate so in the Unread_Pfetched list
+                                */
+                                tbm_add(prefetch_iterator
+                                   ,( (PrefetchBufferRc & PREFTCHRC_BUF_PIN_INCREASED) ? tbmpre->blockno : InvalidBlockNumber ) );
 			}
 		}
 #endif   /* USE_PREFETCH */
@@ -482,12 +512,31 @@ ExecEndBitmapHeapScan(BitmapHeapScanStat
 {
 	Relation	relation;
 	HeapScanDesc scanDesc;
+	TBMIterator *prefetch_iterator;
 
 	/*
 	 * extract information from the node
 	 */
 	relation = node->ss.ss_currentRelation;
 	scanDesc = node->ss.ss_currentScanDesc;
+	prefetch_iterator = node->prefetch_iterator;
+
+#ifdef USE_PREFETCH
+        /*  before any other cleanup,  discard any prefetched but unread buffers  */
+        if (prefetch_iterator != NULL) {
+            TBMIterateResult *tbmpre = tbm_locate_IterateResult(prefetch_iterator);
+            BlockNumber *Unread_Pfetched_base = tbmpre->Unread_Pfetched_base;
+            unsigned int Unread_Pfetched_next = tbmpre->Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+            unsigned int Unread_Pfetched_count = tbmpre->Unread_Pfetched_count;
+
+            while ((Unread_Pfetched_count--) > 0) {
+                DiscardBuffer( scanDesc->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                Unread_Pfetched_next++;
+                if (Unread_Pfetched_next >= target_prefetch_pages)
+                    Unread_Pfetched_next = 0;
+            }
+        }
+#endif   /* USE_PREFETCH */
 
 	/*
 	 * Free the exprcontext
--- src/backend/executor/nodeIndexscan.c.orig	2014-06-08 11:26:29.000000000 -0400
+++ src/backend/executor/nodeIndexscan.c	2014-06-08 21:59:36.584096256 -0400
@@ -35,8 +35,13 @@
 #include "utils/rel.h"
 
 
+
 static TupleTableSlot *IndexNext(IndexScanState *node);
 
+#ifdef USE_PREFETCH
+extern unsigned int prefetch_dbOid; /*  database oid of relations on which prefetching to be done - 0 means all */
+extern unsigned int prefetch_index_scans; /*  boolean whether to prefetch bitmap heap scans         */
+#endif   /* USE_PREFETCH */
 
 /* ----------------------------------------------------------------
  *		IndexNext
@@ -418,7 +423,12 @@ ExecEndIndexScan(IndexScanState *node)
 	 * close the index relation (no-op if we didn't open it)
 	 */
 	if (indexScanDesc)
+        {
 		index_endscan(indexScanDesc);
+
+        /*  note  -  at this point all scan controlblock resources have been freed by IndexScanEnd called by index_endscan */
+
+        }
 	if (indexRelationDesc)
 		index_close(indexRelationDesc, NoLock);
 
@@ -609,6 +619,33 @@ ExecInitIndexScan(IndexScan *node, EStat
 											   indexstate->iss_NumScanKeys,
 											 indexstate->iss_NumOrderByKeys);
 
+#ifdef USE_PREFETCH
+        /*  initialize prefetching   */
+                indexstate->iss_ScanDesc->pfch_index_page_list =  (struct pfch_index_pagelist*)0;
+                indexstate->iss_ScanDesc->pfch_block_item_list = (struct pfch_block_item*)0;
+		if (    prefetch_index_scans
+			 && (target_prefetch_pages > 0)
+			 &&	(!RelationUsesLocalBuffers(indexstate->iss_ScanDesc->heapRelation)) /* I think this must always be true for an indexed heap ? */
+			 && (    (   (prefetch_dbOid > 0)
+					   && (prefetch_dbOid == indexstate->iss_ScanDesc->heapRelation->rd_node.dbNode)
+					 )
+				 ||  (prefetch_dbOid == 0)
+				)
+		   ) {
+			indexstate->iss_ScanDesc->pfch_index_page_list = palloc( sizeof(struct pfch_index_pagelist) + ( (target_prefetch_pages-1) * sizeof(struct pfch_index_item) ) );
+			indexstate->iss_ScanDesc->pfch_block_item_list = palloc( prefetch_index_scans * sizeof(struct pfch_block_item) );
+			if (     ( (struct pfch_index_pagelist*)0 != indexstate->iss_ScanDesc->pfch_index_page_list )
+                  && ( (struct pfch_block_item*)0 != indexstate->iss_ScanDesc->pfch_block_item_list )
+               ) {
+                          indexstate->iss_ScanDesc->pfch_used = 0;
+                          indexstate->iss_ScanDesc->pfch_next = prefetch_index_scans; /* ensure first entry is at index 0 */
+                          indexstate->iss_ScanDesc->pfch_index_page_list->pfch_index_pagelist_next = (struct pfch_index_pagelist*)0;
+                          indexstate->iss_ScanDesc->pfch_index_page_list->pfch_index_item_count = 0;
+                          indexstate->iss_ScanDesc->do_prefetch = 1;
+            }
+		}
+#endif   /* USE_PREFETCH */
+
 	/*
 	 * If no run-time keys to calculate, go ahead and pass the scankeys to the
 	 * index AM.
--- src/backend/executor/instrument.c.orig	2014-06-08 11:26:29.000000000 -0400
+++ src/backend/executor/instrument.c	2014-06-08 21:59:36.620096318 -0400
@@ -41,6 +41,14 @@ InstrAlloc(int n, int instrument_options
 		{
 			instr[i].need_bufusage = need_buffers;
 			instr[i].need_timer = need_timer;
+			instr[i].bufusage_start.aio_read_noneed = 0;
+			instr[i].bufusage_start.aio_read_discrd = 0;
+			instr[i].bufusage_start.aio_read_forgot = 0;
+			instr[i].bufusage_start.aio_read_noblok = 0;
+			instr[i].bufusage_start.aio_read_failed = 0;
+			instr[i].bufusage_start.aio_read_wasted = 0;
+			instr[i].bufusage_start.aio_read_waited = 0;
+			instr[i].bufusage_start.aio_read_ontime = 0;
 		}
 	}
 
@@ -143,6 +151,16 @@ BufferUsageAccumDiff(BufferUsage *dst,
 	dst->local_blks_written += add->local_blks_written - sub->local_blks_written;
 	dst->temp_blks_read += add->temp_blks_read - sub->temp_blks_read;
 	dst->temp_blks_written += add->temp_blks_written - sub->temp_blks_written;
+
+	dst->aio_read_noneed       += add->aio_read_noneed - sub->aio_read_noneed;
+	dst->aio_read_discrd       += add->aio_read_discrd - sub->aio_read_discrd;
+	dst->aio_read_forgot       += add->aio_read_forgot - sub->aio_read_forgot;
+	dst->aio_read_noblok       += add->aio_read_noblok - sub->aio_read_noblok;
+	dst->aio_read_failed       += add->aio_read_failed - sub->aio_read_failed;
+	dst->aio_read_wasted       += add->aio_read_wasted - sub->aio_read_wasted;
+	dst->aio_read_waited       += add->aio_read_waited - sub->aio_read_waited;
+	dst->aio_read_ontime       += add->aio_read_ontime - sub->aio_read_ontime;
+
 	INSTR_TIME_ACCUM_DIFF(dst->blk_read_time,
 						  add->blk_read_time, sub->blk_read_time);
 	INSTR_TIME_ACCUM_DIFF(dst->blk_write_time,
--- src/backend/storage/buffer/Makefile.orig	2014-06-08 11:26:30.000000000 -0400
+++ src/backend/storage/buffer/Makefile	2014-06-08 21:59:36.652096374 -0400
@@ -12,6 +12,6 @@ subdir = src/backend/storage/buffer
 top_builddir = ../../../..
 include $(top_builddir)/src/Makefile.global
 
-OBJS = buf_table.o buf_init.o bufmgr.o freelist.o localbuf.o
+OBJS = buf_table.o buf_init.o bufmgr.o freelist.o localbuf.o buf_async.o
 
 include $(top_srcdir)/src/backend/common.mk
--- src/backend/storage/buffer/bufmgr.c.orig	2014-06-08 11:26:30.000000000 -0400
+++ src/backend/storage/buffer/bufmgr.c	2014-06-08 21:59:36.696096450 -0400
@@ -29,7 +29,7 @@
  *		buf_table.c -- manages the buffer lookup table
  */
 #include "postgres.h"
-
+#include <sys/types.h> 
 #include <sys/file.h>
 #include <unistd.h>
 
@@ -50,7 +50,6 @@
 #include "utils/resowner_private.h"
 #include "utils/timestamp.h"
 
-
 /* Note: these two macros only work on shared buffers, not local ones! */
 #define BufHdrGetBlock(bufHdr)	((Block) (BufferBlocks + ((Size) (bufHdr)->buf_id) * BLCKSZ))
 #define BufferGetLSN(bufHdr)	(PageGetLSN(BufHdrGetBlock(bufHdr)))
@@ -63,6 +62,11 @@
 #define BUF_WRITTEN				0x01
 #define BUF_REUSABLE			0x02
 
+extern volatile struct BAiocbAnchor *BAiocbAnchr;  /*  anchor for all control blocks pertaining to aio  */
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern int numBufferAiocbs;        /*  total number of  BufferAiocbs in pool  */
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 #define DROP_RELS_BSEARCH_THRESHOLD		20
 
 /* GUC variables */
@@ -78,26 +82,33 @@ bool		track_io_timing = false;
  */
 int			target_prefetch_pages = 0;
 
-/* local state for StartBufferIO and related functions */
+/* local state for StartBufferIO and related functions
+**  but ONLY for synchronous IO  -  not altered for aio
+*/
 static volatile BufferDesc *InProgressBuf = NULL;
 static bool IsForInput;
+pid_t this_backend_pid = 0;    /*    pid of this backend */
 
 /* local state for LockBufferForCleanup */
 static volatile BufferDesc *PinCountWaitBuf = NULL;
 
-
-static Buffer ReadBuffer_common(SMgrRelation reln, char relpersistence,
+extern int
+BufStartAsync(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy);
+extern int
+BufCheckAsync(SMgrRelation caller_smgr, Relation caller_reln, volatile BufferDesc *buf_desc, int intention
+        ,BufferAccessStrategy strategy , int index_for_aio , bool spinLockHeld , LWLockId PartitionLock );
+Buffer ReadBuffer_common(SMgrRelation reln, char relpersistence,
 				  ForkNumber forkNum, BlockNumber blockNum,
 				  ReadBufferMode mode, BufferAccessStrategy strategy,
-				  bool *hit);
-static bool PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy);
-static void PinBuffer_Locked(volatile BufferDesc *buf);
-static void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner);
+				  bool *hit , int index_for_aio);
+bool PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy);
+void PinBuffer_Locked(volatile BufferDesc *buf);
+void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner);
 static void BufferSync(int flags);
 static int	SyncOneBuffer(int buf_id, bool skip_recently_used);
 static void WaitIO(volatile BufferDesc *buf);
-static bool StartBufferIO(volatile BufferDesc *buf, bool forInput);
-static void TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
+static bool StartBufferIO(volatile BufferDesc *buf, bool forInput , int index_for_aio );
+void TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
 				  int set_flag_bits);
 static void shared_buffer_write_error_callback(void *arg);
 static void local_buffer_write_error_callback(void *arg);
@@ -106,24 +117,66 @@ static volatile BufferDesc *BufferAlloc(
 			ForkNumber forkNum,
 			BlockNumber blockNum,
 			BufferAccessStrategy strategy,
-			bool *foundPtr);
+			int *foundPtr , int index_for_aio );
 static void FlushBuffer(volatile BufferDesc *buf, SMgrRelation reln);
 static void AtProcExit_Buffers(int code, Datum arg);
 static int	rnode_comparator(const void *p1, const void *p2);
 
+BlockNumber BlocknumOfBuffer(Buffer buffer);
+bool BlocknotinBuffer(Buffer buffer, Relation relation, BlockNumber blockNum);
 
 /*
  * PrefetchBuffer -- initiate asynchronous read of a block of a relation
  *
- * This is named by analogy to ReadBuffer but doesn't actually allocate a
- * buffer.  Instead it tries to ensure that a future ReadBuffer for the given
- * block will not be delayed by the I/O.  Prefetching is optional.
+ * This is named by analogy to ReadBuffer but allocates a buffer only if using asynchronous I/O.
+ * Its purpose  is to try to ensure that a future ReadBuffer for the given block
+ * will not be delayed by the I/O.  Prefetching is optional.
  * No-op if prefetching isn't compiled in.
- */
-void
-PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
-{
+ *
+ * Originally the prefetch simply called posix_fadvise() to recommend read-ahead into kernel page cache.
+ * Extended to provide an alternative of issuing an asynchronous aio_read() to read into a buffer.
+ * This extension has an implication on how this bufmgr component manages concurrent requests
+ * for the same disk block.
+ *
+ * Synchronous IO (read()) does not provide a means for waiting on another task's read if in progress,
+ * and bufmgr implements its own scheme in StartBufferIO, WaitIO, and TerminateBufferIO.
+ *
+ * Asynchronous IO (aio_read()) provides a means for waiting on this or another task's read if in progress,
+ * namely aio_suspend(),  which this extension uses.    Therefore,  although StartBufferIO and TerminateBufferIO
+ * are called as part of asynchronous prefetching,   their role is limited to maintaining the buffer desc flags,
+ * and they do not track the asynchronous IO itself.    Instead,   asynchronous IOs are tracked in 
+ * a separate set of shared control blocks,  the BufferAiocb list -
+ *   refer to     include/storage/buf_internals.h and storage/buffer/buf_init.c
+ *
+ * Another implication of asynchronous IO concerns buffer pinning.
+ * The buffer used for the prefetch is pinned before aio_read is issued.
+ * It is expected that the same task (and possibly others) will later ask to read the page
+ * and eventually release and unpin the buffer.
+ * However,  if the task which issued the aio_read later decides not to read the page,
+ * and return code indicates delta_pin_count > 0 (see below)
+ * it *must* instead issue a DiscardBuffer() (see function later in this file)
+ * so that its pin is released.
+ * Therefore,  each client which uses the PrefetchBuffer service must either always read all
+ * prefetched pages,  or keep track of prefetched pages and discard unread ones at end of scan.
+ *
+ * return code:   is an int bitmask defined in bufmgr.h
+        PREFTCHRC_BUF_PIN_INCREASED 0x01      pin count on buffer has been increased by 1
+        PREFTCHRC_BLK_ALREADY_PRESENT 0x02    block was already present in a buffer
+ *
+ * PREFTCHRC_BLK_ALREADY_PRESENT is a hint to caller that the prefetch may be unnecessary
+ */
+int
+PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy)
+{
+	Buffer		buf_id; /* indicates buffer containing the requested block  */
+        int             PrefetchBufferRc = 0; /*  return value as described above  */
+        int             PinCountOnEntry = 0;  /*  pin count on entry           */
+        int             PinCountdelta = 0;    /*  pin count delta increase     */
+
+
 #ifdef USE_PREFETCH
+
+	buf_id = -1;
 	Assert(RelationIsValid(reln));
 	Assert(BlockNumberIsValid(blockNum));
 
@@ -146,7 +199,12 @@ PrefetchBuffer(Relation reln, ForkNumber
 		BufferTag	newTag;		/* identity of requested block */
 		uint32		newHash;	/* hash value for newTag */
 		LWLock	   *newPartitionLock;	/* buffer partition lock for it */
-		int			buf_id;
+        int         BufStartAsyncrc = -1;  /*  retcode from BufStartAsync :
+                                                       **        0 if started successfully (which implies buffer was newly pinned )
+                                                       **       -1 if failed for some reason
+                                                       **        1+PrivateRefCount if we found desired buffer in buffer pool
+                                                       **  and we set it likewise if we find buffer in buffer pool
+                                                       */
 
 		/* create a tag so we can lookup the buffer */
 		INIT_BUFFERTAG(newTag, reln->rd_smgr->smgr_rnode.node,
@@ -158,28 +216,121 @@ PrefetchBuffer(Relation reln, ForkNumber
 
 		/* see if the block is in the buffer pool already */
 		LWLockAcquire(newPartitionLock, LW_SHARED);
-		buf_id = BufTableLookup(&newTag, newHash);
+		buf_id = (Buffer)BufTableLookup(&newTag, newHash);
+                if (buf_id >= 0) {
+                    PinCountOnEntry = PrivateRefCount[buf_id];  /*  pin count on entry               */
+                    BufStartAsyncrc = 1 + PinCountOnEntry; /* indicate this backends pin count - see above comment */
+                    PrefetchBufferRc = PREFTCHRC_BLK_ALREADY_PRESENT;       /* indicate buffer present */
+                } else {
+                    PrefetchBufferRc = 0;                                   /* indicate buffer not present */
+                }
 		LWLockRelease(newPartitionLock);
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+     not_in_buffers:
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 		/* If not in buffers, initiate prefetch */
-		if (buf_id < 0)
+		if (buf_id < 0) {
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                    /*    try using async aio_read with a buffer */
+                    BufStartAsyncrc = BufStartAsync( reln, forkNum, blockNum , strategy );
+                    if (BufStartAsyncrc < 0) {
+                            pgBufferUsage.aio_read_noblok++;
+                    }
+#else /* not USE_AIO_ATOMIC_BUILTIN_COMP_SWAP so try the alternative that does not read the block into a postgresql buffer */
 			smgrprefetch(reln->rd_smgr, forkNum, blockNum);
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+		}
 
-		/*
-		 * If the block *is* in buffers, we do nothing.  This is not really
-		 * ideal: the block might be just about to be evicted, which would be
-		 * stupid since we know we are going to need it soon.  But the only
-		 * easy answer is to bump the usage_count, which does not seem like a
-		 * great solution: when the caller does ultimately touch the block,
-		 * usage_count would get bumped again, resulting in too much
-		 * favoritism for blocks that are involved in a prefetch sequence. A
-		 * real fix would involve some additional per-buffer state, and it's
-		 * not clear that there's enough of a problem to justify that.
+        if (  (buf_id >= 0) || (BufStartAsyncrc >= 1)  ) {
+                        /* The block *is* in buffers.    */
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                        pgBufferUsage.aio_read_noneed++;
+#ifndef USE_PREFETCH_BUT_DONT_PIN_ALREADY_PRESENT /* jury is out on whether the following wins but it ought to ...  */
+                        /*
+                        ** If this backend already had pinned it,
+                        ** or another backend had banked a pin on it,
+                        ** or there is an IO in progress,
+                        ** or it is not marked valid,
+                        ** then do nothing.
+                        ** Otherwise pin it and mark the buffer's pin as banked by this backend.
+                        ** Note  -  it may or not be pinned by another backend -
+                        **          it is ok for us to bank a pin on it
+                        **          *provided* the other backend did not bank its pin.
+                        **          The reason for this is that the banked-pin indicator is global -
+                        **          it can identify at most one process.
+                        */
+                        /* pgBufferUsage.aio_read_wasted++;  overload counter - only for debugging */
+                        if (BufStartAsyncrc == 1) {            /*   not pinned by me  */
+                              /*  pgBufferUsage.aio_read_wasted++;  overload counter - only for debugging */
+                              /*  note   -   all we can say with certainty is that the buffer is not pinned by me
+                              **             we cannot be sure that it is still in buffer pool
+                              **             so must go through the entire locking and searching all over again ...
 		 */
+                            LWLockAcquire(newPartitionLock, LW_SHARED);
+                            buf_id = (Buffer)BufTableLookup(&newTag, newHash);
+                            /* If in buffers, proceed */
+                            if (buf_id >= 0) {
+                                /*  since the block is now present,
+                                **  save the current pin count to ensure final delta is calculated correctly
+                                */
+                                PinCountOnEntry = PrivateRefCount[buf_id];  /*  pin count on entry               */
+                                if ( PinCountOnEntry == 0) { /*  paranoid check it's still not pinned by me */
+                                    volatile        BufferDesc *buf_desc;
+
+                                    buf_desc = &BufferDescriptors[buf_id];        /* found buffer descriptor */
+                                    LockBufHdr(buf_desc);
+                                    if (    (buf_desc->flags & BM_VALID)           /* buffer is valid        */
+                                         && (!(buf_desc->flags & (BM_IO_IN_PROGRESS|BM_AIO_IN_PROGRESS|BM_AIO_PREFETCH_PIN_BANKED))) /* buffer is not any of ... */
+                                       ) {
+                                        buf_desc->flags |= BM_AIO_PREFETCH_PIN_BANKED; /* bank the pin for next use by this task */
+                                        /*  note  - we can call PinBuffer_Locked with the BM_AIO_PREFETCH_PIN_BANKED flag set because it is not yet pinned by me */
+                                        buf_desc->freeNext = -(this_backend_pid);       /* remember which pid banked it */
+                                        /*  pgBufferUsage.aio_read_wasted--;      overload counter - not wasted after all - only for debugging */
+
+                                        /* Make sure we will have room to remember the buffer pin */
+                                        ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
+
+                                        PinBuffer_Locked(buf_desc);
+	}
+                                    else {
+                                        UnlockBufHdr(buf_desc);
+                                    }
+                                }
+                            }
+                            LWLockRelease(newPartitionLock);
+                            /*  although unlikely,  maybe it was evicted while we were puttering about  */
+                            if (buf_id < 0) {
+                                pgBufferUsage.aio_read_noneed--;   /*   back out the accounting */
+                                goto not_in_buffers;               /*   and try again           */
 	}
-#endif   /* USE_PREFETCH */
 }
+#endif /*  USE_PREFETCH_BUT_DONT_PIN_ALREADY_PRESENT */
 
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+		}
+
+		if (buf_id >= 0) {
+			PinCountdelta = PrivateRefCount[buf_id] - PinCountOnEntry;  /*  pin count delta increase    */
+			if (  (PinCountdelta < 0) || (PinCountdelta > 1) ) {
+				  elog(ERROR,
+						 "PrefetchBuffer #%d : incremented pin count by %d on bufdesc %p refcount %u localpins %d\n"
+								  ,(buf_id+1) , PinCountdelta , &BufferDescriptors[buf_id] ,BufferDescriptors[buf_id].refcount , PrivateRefCount[buf_id]);
+}
+		} else
+		if (BufStartAsyncrc == 0) {  /* aio started successfully (which implies buffer was newly pinned ) */
+			PinCountdelta = 1;
+		}
+
+		/*  set final PrefetchBufferRc according to previous value */
+		PrefetchBufferRc |= PinCountdelta;  /* set the PREFTCHRC_BUF_PIN_INCREASED bit */
+	}
+
+#endif   /* USE_PREFETCH */
+
+	return PrefetchBufferRc; /*  return value as described above */
+}
 
 /*
  * ReadBuffer -- a shorthand for ReadBufferExtended, for reading from main
@@ -252,7 +403,7 @@ ReadBufferExtended(Relation reln, ForkNu
 	 */
 	pgstat_count_buffer_read(reln);
 	buf = ReadBuffer_common(reln->rd_smgr, reln->rd_rel->relpersistence,
-							forkNum, blockNum, mode, strategy, &hit);
+							forkNum, blockNum, mode, strategy, &hit , 0);
 	if (hit)
 		pgstat_count_buffer_hit(reln);
 	return buf;
@@ -280,7 +431,7 @@ ReadBufferWithoutRelcache(RelFileNode rn
 	Assert(InRecovery);
 
 	return ReadBuffer_common(smgr, RELPERSISTENCE_PERMANENT, forkNum, blockNum,
-							 mode, strategy, &hit);
+							 mode, strategy, &hit , 0);
 }
 
 
@@ -288,15 +439,18 @@ ReadBufferWithoutRelcache(RelFileNode rn
  * ReadBuffer_common -- common logic for all ReadBuffer variants
  *
  * *hit is set to true if the request was satisfied from shared buffer cache.
+ * index_for_aio ,  if -ve , is negative of ( index of the aiocb in the BufferAiocbs array + 3 )
+ *     which is passed through to StartBufferIO
  */
-static Buffer
+Buffer
 ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 				  BlockNumber blockNum, ReadBufferMode mode,
-				  BufferAccessStrategy strategy, bool *hit)
+				  BufferAccessStrategy strategy, bool *hit , int index_for_aio )
 {
 	volatile BufferDesc *bufHdr;
 	Block		bufBlock;
 	bool		found;
+        int             allocrc;             /*  retcode from BufferAlloc */
 	bool		isExtend;
 	bool		isLocalBuf = SmgrIsTemp(smgr);
 
@@ -328,16 +482,40 @@ ReadBuffer_common(SMgrRelation smgr, cha
 	}
 	else
 	{
+                allocrc = mode; /* pass mode to BufferAlloc since it must not wait for async io if RBM_NOREAD_FOR_PREFETCH */
 		/*
 		 * lookup the buffer.  IO_IN_PROGRESS is set if the requested block is
 		 * not currently in memory.
 		 */
 		bufHdr = BufferAlloc(smgr, relpersistence, forkNum, blockNum,
-							 strategy, &found);
-		if (found)
-			pgBufferUsage.shared_blks_hit++;
+							 strategy, &allocrc , index_for_aio );
+		if (allocrc < 0) {
+                    if (mode == RBM_ZERO_ON_ERROR || zero_damaged_pages)
+                    {
+                        ereport(WARNING,
+                                (errcode(ERRCODE_DATA_CORRUPTED),
+                                 errmsg("invalid page header in block %u of relation %s; zeroing out page",
+                                        blockNum,
+                                        relpath(smgr->smgr_rnode, forkNum))));
+                        bufBlock = BufHdrGetBlock(bufHdr);
+                        MemSet((char *) bufBlock, 0, BLCKSZ);
+                    }
 		else
+                      ereport(ERROR,
+                              (errcode(ERRCODE_DATA_CORRUPTED),
+                               errmsg("invalid page header in block %u of relation %s",
+                                      blockNum,
+                                      relpath(smgr->smgr_rnode, forkNum))));
+                        found = true;
+                }
+		else if (allocrc > 0) {
+			pgBufferUsage.shared_blks_hit++;
+                        found = true;
+                }
+		else {
 			pgBufferUsage.shared_blks_read++;
+                        found = false;
+                }
 	}
 
 	/* At this point we do NOT hold any locks. */
@@ -410,7 +588,7 @@ ReadBuffer_common(SMgrRelation smgr, cha
 				Assert(bufHdr->flags & BM_VALID);
 				bufHdr->flags &= ~BM_VALID;
 				UnlockBufHdr(bufHdr);
-			} while (!StartBufferIO(bufHdr, true));
+			} while (!StartBufferIO(bufHdr, true, 0));
 		}
 	}
 
@@ -430,6 +608,7 @@ ReadBuffer_common(SMgrRelation smgr, cha
 
 	bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);
 
+        if (mode != RBM_NOREAD_FOR_PREFETCH) {
 	if (isExtend)
 	{
 		/* new buffers are zero-filled */
@@ -499,6 +678,7 @@ ReadBuffer_common(SMgrRelation smgr, cha
 	VacuumPageMiss++;
 	if (VacuumCostActive)
 		VacuumCostBalance += VacuumCostPageMiss;
+	}
 
 	TRACE_POSTGRESQL_BUFFER_READ_DONE(forkNum, blockNum,
 									  smgr->smgr_rnode.node.spcNode,
@@ -520,21 +700,39 @@ ReadBuffer_common(SMgrRelation smgr, cha
  * the default strategy.  The selected buffer's usage_count is advanced when
  * using the default strategy, but otherwise possibly not (see PinBuffer).
  *
- * The returned buffer is pinned and is already marked as holding the
- * desired page.  If it already did have the desired page, *foundPtr is
- * set TRUE.  Otherwise, *foundPtr is set FALSE and the buffer is marked
+ * index_for_aio is an input parm which,  if non-zero,  identifies a BufferAiocb
+ * acquired by caller and to be used for any StartBufferIO performed by this routine.
+ * In this case,  if block not found in buffer pool and we allocate a new buffer,
+ * then we must maintain the spinlock on the buffer and pass it back to caller.
+ *
+ * foundPtr is input and output :
+ *  . input   -  indicates the read-buffer mode  ( see bufmgr.h )
+ *  . output  -  indicates the status of the buffer - see below
+ *
+ * Except for the case of RBM_NOREAD_FOR_PREFETCH and buffer is found,
+ * the returned buffer is pinned and is already marked as holding the
+ * desired page.
+ *  If it already did have the desired page and page content is valid,
+ *  *foundPtr is set to 1
+ *  If it already did have the desired page and mode is RBM_NOREAD_FOR_PREFETCH
+ *    and StartBufferIO returned false
+ *    (meaning it could not initialise the buffer for aio)
+ *  *foundPtr is set to 2
+ *  If it already did have the desired page but page content is invalid,
+ *  *foundPtr is set to -1
+ *   this can happen only if the buffer was read by an async read
+ *   and the aio is still in progress or pinned by the issuer of the startaio.
+ *  Otherwise, *foundPtr is set to 0 and the buffer is marked
  * as IO_IN_PROGRESS; ReadBuffer will now need to do I/O to fill it.
  *
- * *foundPtr is actually redundant with the buffer's BM_VALID flag, but
- * we keep it for simplicity in ReadBuffer.
- *
- * No locks are held either at entry or exit.
+ * No locks are held either at entry or exit EXCEPT for case noted above
+ * of passing an empty buffer back to async io caller ( index_for_aio set ).
  */
 static volatile BufferDesc *
 BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 			BlockNumber blockNum,
 			BufferAccessStrategy strategy,
-			bool *foundPtr)
+			int *foundPtr , int index_for_aio )
 {
 	BufferTag	newTag;			/* identity of requested block */
 	uint32		newHash;		/* hash value for newTag */
@@ -546,6 +744,13 @@ BufferAlloc(SMgrRelation smgr, char relp
 	int			buf_id;
 	volatile BufferDesc *buf;
 	bool		valid;
+        int             IntentionBufferrc;      /* retcode from BufCheckAsync */
+        bool            StartBufferIOrc;        /* retcode from StartBufferIO */
+        ReadBufferMode mode;
+
+
+        mode = *foundPtr;
+        *foundPtr = 0;
 
 	/* create a tag so we can lookup the buffer */
 	INIT_BUFFERTAG(newTag, smgr->smgr_rnode.node, forkNum, blockNum);
@@ -560,21 +765,53 @@ BufferAlloc(SMgrRelation smgr, char relp
 	if (buf_id >= 0)
 	{
 		/*
-		 * Found it.  Now, pin the buffer so no one can steal it from the
-		 * buffer pool, and check to see if the correct data has been loaded
-		 * into the buffer.
+		 * Found it.
 		 */
+                *foundPtr = 1;
 		buf = &BufferDescriptors[buf_id];
 
-		valid = PinBuffer(buf, strategy);
-
-		/* Can release the mapping lock as soon as we've pinned it */
+                /*   If prefetch mode,  then return immediately indicating found,
+                **   and NOTE in this case only,  we did not pin buffer.
+                **   In theory we might try to check whether the buffer is valid,  io in progress,  etc
+                **   but in practice it is simpler to abandon the prefetch if the buffer exists
+                */
+                if (mode == RBM_NOREAD_FOR_PREFETCH) {
+                    /* release the mapping lock and return    */
 		LWLockRelease(newPartitionLock);
+                } else {
+                    /*   note that the current request is for same tag as the one associated with the aio -
+                    **   so simply complete the aio and we have our buffer.
+                    **         If an aio was started on this buffer,
+                    **         check complete and wait for it if not.
+                    **         And,  if aio had been started,  then the task
+                    **         which issued the start aio already pinned it for this read,
+                    **         so if that task was me and the aio was successful,
+                    **         pass the current pin to this read without dropping and re-acquiring.
+                    **         this is all done by BufCheckAsync
+                    */
+                    IntentionBufferrc = BufCheckAsync(smgr , 0 , buf, BUF_INTENTION_WANT , strategy , index_for_aio , false , newPartitionLock );
 
-		*foundPtr = TRUE;
+                    /*       check to see if the correct data has been loaded into the buffer.  */
+                    valid = (IntentionBufferrc == BUF_INTENT_RC_VALID);
 
-		if (!valid)
-		{
+                    /*  check for serious IO errors   */
+                    if (!valid) {
+                        if (    (IntentionBufferrc != BUF_INTENT_RC_INVALID_NO_AIO)
+                             && (IntentionBufferrc != BUF_INTENT_RC_INVALID_AIO)
+                           ) {
+                            *foundPtr = -1;  /*  inform caller of serious error */
+                        }
+                        else
+                        if (IntentionBufferrc == BUF_INTENT_RC_INVALID_AIO) {
+                            goto proceed_with_not_found;  /*  yes,  I know,  a goto ... think of it as a break out of the if */
+                        }
+                     }
+
+                    /* BufCheckAsync pinned the buffer            */
+                    /* so can now release the mapping lock               */
+                    LWLockRelease(newPartitionLock);
+
+                    if (!valid) {
 			/*
 			 * We can only get here if (a) someone else is still reading in
 			 * the page, or (b) a previous read attempt failed.  We have to
@@ -582,19 +819,21 @@ BufferAlloc(SMgrRelation smgr, char relp
 			 * own read attempt if the page is still not BM_VALID.
 			 * StartBufferIO does it all.
 			 */
-			if (StartBufferIO(buf, true))
+                                if (StartBufferIO(buf, true, index_for_aio))
 			{
 				/*
 				 * If we get here, previous attempts to read the buffer must
 				 * have failed ... but we shall bravely try again.
 				 */
-				*foundPtr = FALSE;
+                                        *foundPtr = 0;
+                                }
 			}
 		}
 
 		return buf;
 	}
 
+  proceed_with_not_found:
 	/*
 	 * Didn't find it in the buffer pool.  We'll have to initialize a new
 	 * buffer.  Remember to unlock the mapping lock while doing the work.
@@ -619,8 +858,10 @@ BufferAlloc(SMgrRelation smgr, char relp
 		/* Must copy buffer flags while we still hold the spinlock */
 		oldFlags = buf->flags;
 
-		/* Pin the buffer and then release the buffer spinlock */
-		PinBuffer_Locked(buf);
+                /*         If an aio was started on this buffer,
+                **         check complete and cancel it if not.
+                */
+                BufCheckAsync(smgr , 0 , buf, BUF_INTENTION_REJECT_OBTAIN_PIN , 0 , index_for_aio, true , 0 );
 
 		/* Now it's safe to release the freelist lock */
 		if (lock_held)
@@ -791,13 +1032,18 @@ BufferAlloc(SMgrRelation smgr, char relp
 				 * then set up our own read attempt if the page is still not
 				 * BM_VALID.  StartBufferIO does it all.
 				 */
-				if (StartBufferIO(buf, true))
+                                StartBufferIOrc = StartBufferIO(buf, true , index_for_aio); /* retcode from StartBufferIO */
+				if (StartBufferIOrc)
 				{
 					/*
 					 * If we get here, previous attempts to read the buffer
 					 * must have failed ... but we shall bravely try again.
 					 */
-					*foundPtr = FALSE;
+					*foundPtr = 0;
+                                } else
+                                if (mode == RBM_NOREAD_FOR_PREFETCH) {
+					UnpinBuffer(buf, true); /* must unpin if RBM_NOREAD_FOR_PREFETCH and found */
+                                        *foundPtr = 2;  /*  inform BufStartAsync that buffer must not be used */
 				}
 			}
 
@@ -860,10 +1106,17 @@ BufferAlloc(SMgrRelation smgr, char relp
 	 * lock.  If StartBufferIO returns false, then someone else managed to
 	 * read it before we did, so there's nothing left for BufferAlloc() to do.
 	 */
-	if (StartBufferIO(buf, true))
-		*foundPtr = FALSE;
-	else
-		*foundPtr = TRUE;
+        StartBufferIOrc = StartBufferIO(buf, true , index_for_aio); /* retcode from StartBufferIO */
+        if (StartBufferIOrc) {
+		*foundPtr = 0;
+        } else {
+                if (mode == RBM_NOREAD_FOR_PREFETCH) {
+                    UnpinBuffer(buf, true); /* must unpin if RBM_NOREAD_FOR_PREFETCH and found */
+                    *foundPtr = 2;  /*  inform BufStartAsync that buffer must not be used */
+                } else {
+                    *foundPtr = 1;
+                }
+        }
 
 	return buf;
 }
@@ -970,6 +1223,10 @@ retry:
 	/*
 	 * Insert the buffer at the head of the list of free buffers.
 	 */
+        /*   avoid confusing freelist with strange-looking freeNext */
+        if (buf->freeNext <= FREENEXT_BAIOCB_ORIGIN) { /* means was used for aiocb index */
+            buf->freeNext = FREENEXT_NOT_IN_LIST;
+        }
 	StrategyFreeBuffer(buf);
 }
 
@@ -1022,6 +1279,56 @@ MarkBufferDirty(Buffer buffer)
 	UnlockBufHdr(bufHdr);
 }
 
+/*  return the blocknum of the block in a buffer if it is valid
+**  if a shared buffer,  it must be pinned
+*/
+BlockNumber
+BlocknumOfBuffer(Buffer buffer)
+{
+	volatile BufferDesc *bufHdr;
+        BlockNumber rc = 0;
+
+	if (BufferIsValid(buffer)) {
+            if  (BufferIsLocal(buffer)) {
+                bufHdr = &LocalBufferDescriptors[-buffer - 1];
+            } else {
+                bufHdr = &BufferDescriptors[buffer - 1];
+            }
+
+            rc = bufHdr->tag.blockNum;
+        }
+
+        return rc;
+}
+
+/*  report whether specified buffer contains same or different block
+**  if a shared buffer,  it must be pinned
+*/
+bool
+BlocknotinBuffer(Buffer buffer,
+					 Relation relation,
+					 BlockNumber blockNum)
+{
+	volatile BufferDesc *bufHdr;
+        bool rc = false;
+
+	if (BufferIsValid(buffer)) {
+            if  (BufferIsLocal(buffer)) {
+                bufHdr = &LocalBufferDescriptors[-buffer - 1];
+            } else {
+                bufHdr = &BufferDescriptors[buffer - 1];
+            }
+
+            rc =
+                  (   (bufHdr->tag.blockNum != blockNum)
+                   || (!(RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) ))
+                   || (bufHdr->tag.forkNum != MAIN_FORKNUM)
+                  );
+        }
+
+        return rc;
+}
+
 /*
  * ReleaseAndReadBuffer -- combine ReleaseBuffer() and ReadBuffer()
  *
@@ -1040,18 +1347,18 @@ ReleaseAndReadBuffer(Buffer buffer,
 					 Relation relation,
 					 BlockNumber blockNum)
 {
-	ForkNumber	forkNum = MAIN_FORKNUM;
 	volatile BufferDesc *bufHdr;
+        bool isDifferentBlock;   /*       requesting different block from that already in buffer ? */
 
 	if (BufferIsValid(buffer))
 	{
+	    /* if a shared buff, we have pin, so it's ok to examine tag without spinlock */
+            isDifferentBlock = BlocknotinBuffer(buffer,relation,blockNum); /*  requesting different block from that already in buffer ? */
 		if (BufferIsLocal(buffer))
 		{
 			Assert(LocalRefCount[-buffer - 1] > 0);
 			bufHdr = &LocalBufferDescriptors[-buffer - 1];
-			if (bufHdr->tag.blockNum == blockNum &&
-				RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
-				bufHdr->tag.forkNum == forkNum)
+			if (!isDifferentBlock)
 				return buffer;
 			ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
 			LocalRefCount[-buffer - 1]--;
@@ -1060,12 +1367,12 @@ ReleaseAndReadBuffer(Buffer buffer,
 		{
 			Assert(PrivateRefCount[buffer - 1] > 0);
 			bufHdr = &BufferDescriptors[buffer - 1];
-			/* we have pin, so it's ok to examine tag without spinlock */
-			if (bufHdr->tag.blockNum == blockNum &&
-				RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
-				bufHdr->tag.forkNum == forkNum)
+                        BufCheckAsync(0 , relation , bufHdr , ( isDifferentBlock ? BUF_INTENTION_REJECT_FORGET
+                                                                                 : BUF_INTENTION_REJECT_KEEP_PIN )
+                                                            , 0 , 0 , false , 0 ); /* end any IO and maybe unpin */
+			if (!isDifferentBlock) {
 				return buffer;
-			UnpinBuffer(bufHdr, true);
+                        }
 		}
 	}
 
@@ -1090,11 +1397,12 @@ ReleaseAndReadBuffer(Buffer buffer,
  * Returns TRUE if buffer is BM_VALID, else FALSE.  This provision allows
  * some callers to avoid an extra spinlock cycle.
  */
-static bool
+bool
 PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy)
 {
 	int			b = buf->buf_id;
 	bool		result;
+        bool       pin_already_banked_by_me = 0;  /* buffer is already pinned by me and redeemable */
 
 	if (PrivateRefCount[b] == 0)
 	{
@@ -1116,12 +1424,34 @@ PinBuffer(volatile BufferDesc *buf, Buff
 	else
 	{
 		/* If we previously pinned the buffer, it must surely be valid */
+                /* Errr  -   is that really true  ???    I don't think so  :
+                ** what if I pin,  try an IO,  in progress,  then mistakenly pin again
 		result = true;
+                */
+		LockBufHdr(buf);
+                pin_already_banked_by_me = (    (PrivateRefCount[b] > 0) && (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                                             && (   (    (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                                                     : (-(buf->freeNext))  )  == this_backend_pid )
+                                           );
+                if (pin_already_banked_by_me) {
+                        elog(LOG, "PinBuffer : on buffer %d with banked pin rel=%s, blockNum=%u, with flags %X refcount=%u"
+                       ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                       ,buf->tag.blockNum, buf->flags, buf->refcount);
+                    buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;
+                    if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                        buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+	}
+                }
+		result = (buf->flags & BM_VALID) != 0;
+		UnlockBufHdr(buf);
 	}
+
+        if (!pin_already_banked_by_me) {
 	PrivateRefCount[b]++;
 	Assert(PrivateRefCount[b] > 0);
 	ResourceOwnerRememberBuffer(CurrentResourceOwner,
 								BufferDescriptorGetBuffer(buf));
+        }
 	return result;
 }
 
@@ -1138,19 +1468,36 @@ PinBuffer(volatile BufferDesc *buf, Buff
  * to save a spin lock/unlock cycle, because we need to pin a buffer before
  * its state can change under us.
  */
-static void
+void
 PinBuffer_Locked(volatile BufferDesc *buf)
 {
 	int			b = buf->buf_id;
+        bool       pin_already_banked_by_me;  /* buffer is already pinned by me and redeemable */
 
-	if (PrivateRefCount[b] == 0)
+        pin_already_banked_by_me = (    (PrivateRefCount[b] > 0) && (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                                     && (   (    (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                                             : (-(buf->freeNext))  )  == this_backend_pid )
+                             );
+	if (PrivateRefCount[b] == 0) {
 		buf->refcount++;
+        }
+        if (pin_already_banked_by_me) {
+                        elog(LOG, "PinBuffer_Locked : on buffer %d with banked pin rel=%s, blockNum=%u, with flags %X refcount=%u"
+                       ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                       ,buf->tag.blockNum, buf->flags, buf->refcount);
+            buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;
+                    if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                        buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+                    }
+        }
 	UnlockBufHdr(buf);
+        if (!pin_already_banked_by_me) {
 	PrivateRefCount[b]++;
 	Assert(PrivateRefCount[b] > 0);
 	ResourceOwnerRememberBuffer(CurrentResourceOwner,
 								BufferDescriptorGetBuffer(buf));
 }
+}
 
 /*
  * UnpinBuffer -- make buffer available for replacement.
@@ -1160,29 +1507,68 @@ PinBuffer_Locked(volatile BufferDesc *bu
  * Most but not all callers want CurrentResourceOwner to be adjusted.
  * Those that don't should pass fixOwner = FALSE.
  */
-static void
+void
 UnpinBuffer(volatile BufferDesc *buf, bool fixOwner)
 {
+
 	int			b = buf->buf_id;
+        bool       pin_already_banked_by_me;  /* buffer is already pinned by me and redeemable */
 
-	if (fixOwner)
+	if (fixOwner) {
 		ResourceOwnerForgetBuffer(CurrentResourceOwner,
 								  BufferDescriptorGetBuffer(buf));
+        }
 
 	Assert(PrivateRefCount[b] > 0);
 	PrivateRefCount[b]--;
 	if (PrivateRefCount[b] == 0)
 	{
+
 		/* I'd better not still hold any locks on the buffer */
 		Assert(!LWLockHeldByMe(buf->content_lock));
 		Assert(!LWLockHeldByMe(buf->io_in_progress_lock));
 
 		LockBufHdr(buf);
 
+		/* this backend has released last pin - buffer should not have pin banked by me
+                ** and if AIO in progress then there should be another backend pin
+                */
+                pin_already_banked_by_me = (       (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                                             &&    (   (    (buf->flags & BM_AIO_IN_PROGRESS)
+                                                         ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                         : (-(buf->freeNext))
+                                                       )  == this_backend_pid )
+                                           );
+                if (pin_already_banked_by_me) {
+                        /*  this is a strange situation  -  caller had a banked pin (which callers are supposed not to know about)
+                        **                                  but either discovered it had it or has over-counted how many pins it has
+                        */
+                        buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;   /*   redeem the pin although it is now of no use since about to release */
+                        if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                            buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+                        }
+
+                        /*     temporarily suppress logging error to avoid performance degradation -
+                        **     either this task really does not need the buffer in which case the error is harmless
+                        **     or a more severe error will be detected later (possibly immediately below)
+                        elog(LOG, "UnpinBuffer :  released last this-backend pin on buffer %d rel=%s, blockNum=%u, but had banked pin flags %X refcount=%u"
+                       ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                       ,buf->tag.blockNum, buf->flags, buf->refcount);
+                        */
+                }
+
 		/* Decrement the shared reference count */
 		Assert(buf->refcount > 0);
 		buf->refcount--;
 
+                if (   (buf->refcount == 0) && (buf->flags & BM_AIO_IN_PROGRESS)  ) {
+
+                        elog(ERROR, "UnpinBuffer :  released last any-backend pin on buffer %d rel=%s, blockNum=%u, but AIO in progress flags %X refcount=%u"
+                            ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                            ,buf->tag.blockNum, buf->flags, buf->refcount);
+                }
+
+
 		/* Support LockBufferForCleanup() */
 		if ((buf->flags & BM_PIN_COUNT_WAITER) &&
 			buf->refcount == 1)
@@ -1657,6 +2043,7 @@ SyncOneBuffer(int buf_id, bool skip_rece
 	volatile BufferDesc *bufHdr = &BufferDescriptors[buf_id];
 	int			result = 0;
 
+
 	/*
 	 * Check whether buffer needs writing.
 	 *
@@ -1744,6 +2131,16 @@ void
 InitBufferPoolBackend(void)
 {
 	on_shmem_exit(AtProcExit_Buffers, 0);
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        /*   init the aio subsystem max number of threads and max number of requests
+        **   max number of threads   <-->  max_async_io_prefetchers
+        **   max number of requests  <-->  numBufferAiocbs = (target_prefetch_pages*max_async_io_prefetchers)
+        **   there is no return code so we just hope.
+        */
+        smgrinitaio(max_async_io_prefetchers , numBufferAiocbs);
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 }
 
 /*
@@ -1789,6 +2186,8 @@ PrintBufferLeakWarning(Buffer buffer)
 	char	   *path;
 	BackendId	backend;
 
+
+
 	Assert(BufferIsValid(buffer));
 	if (BufferIsLocal(buffer))
 	{
@@ -1799,12 +2198,28 @@ PrintBufferLeakWarning(Buffer buffer)
 	else
 	{
 		buf = &BufferDescriptors[buffer - 1];
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                /*   If reason that this buffer is pinned
+                **   is that it was prefetched with async_io
+                **   and never read or discarded, then omit the
+                **   warning,  because this is expected in some
+                **   cases when a scan is closed abnormally.
+                **   Note that the buffer will be released soon by our caller.
+                */
+                if (buf->flags & BM_AIO_PREFETCH_PIN_BANKED) {
+                    pgBufferUsage.aio_read_forgot++; /* account for it */
+                    return;
+                }
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 		loccount = PrivateRefCount[buffer - 1];
 		backend = InvalidBackendId;
 	}
 
+/* #if defined(DO_ISCAN_DISCARD_BUFFER) && defined(DO_HEAPSCAN_DISCARD_BUFFER) not yet working safely 130530 */
 	/* theoretically we should lock the bufhdr here */
 	path = relpathbackend(buf->tag.rnode, backend, buf->tag.forkNum);
+
+
 	elog(WARNING,
 		 "buffer refcount leak: [%03d] "
 		 "(rel=%s, blockNum=%u, flags=0x%x, refcount=%u %d)",
@@ -1812,6 +2227,7 @@ PrintBufferLeakWarning(Buffer buffer)
 		 buf->tag.blockNum, buf->flags,
 		 buf->refcount, loccount);
 	pfree(path);
+/* #endif defined(DO_ISCAN_DISCARD_BUFFER) && defined(DO_HEAPSCAN_DISCARD_BUFFER) not yet working safely 130530 */
 }
 
 /*
@@ -1928,7 +2344,7 @@ FlushBuffer(volatile BufferDesc *buf, SM
 	 * false, then someone else flushed the buffer before we could, so we need
 	 * not do anything.
 	 */
-	if (!StartBufferIO(buf, false))
+	if (!StartBufferIO(buf, false, 0))
 		return;
 
 	/* Setup error traceback support for ereport() */
@@ -2512,6 +2928,70 @@ FlushDatabaseBuffers(Oid dbid)
 	}
 }
 
+#ifdef USE_PREFETCH
+/*
+ * DiscardBuffer -- discard shared buffer used for a previously
+ *                  prefetched but unread block of a relation
+ *
+ * If the buffer is found and pinned with a banked pin,  then :
+ *      .  if AIO in progress, terminate AIO without waiting
+ *      .  if AIO had already completed successfully,
+ *         then mark buffer valid (in case someone else wants it)
+ *      .  redeem the banked pin and unpin it.
+ *
+ * This function is similar in purpose to ReleaseBuffer (below)
+ * but sufficiently different that it is a separate function.
+ * Two important differences are :
+ *   .   caller identifies buffer by blocknumber,  not buffer number
+ *   .   we unpin buffer *only* if the pin is banked,
+ *                      *never* if pinned but not banked.
+ *       This is essential as caller may perform a sequence of
+ *  SCAN1   . PrefetchBuffer   (and remember block was prefetched)
+ *  SCAN2   . ReadBuffer       (but fails to connect this read to the prefetch by SCAN1)
+ *  SCAN1   . DiscardBuffer    (SCAN1 terminates early)
+ *  SCAN2   . access tuples in buffer
+ *        Clearly the Discard *must not* unpin the buffer since SCAN2 needs it!
+ *
+ *
+ * caller may pass InvalidBlockNumber as blockNum to mean do nothing
+ */
+void
+DiscardBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
+{
+        BufferTag	newTag;		   /* identity of requested block */
+        uint32		newHash;	   /* hash value for newTag */
+        LWLockId	newPartitionLock;  /* buffer partition lock for it */
+        Buffer		buf_id;
+        volatile        BufferDesc *buf_desc;
+
+    if (!SmgrIsTemp(reln->rd_smgr)) {
+	Assert(RelationIsValid(reln));
+	if (BlockNumberIsValid(blockNum)) {
+
+            /* create a tag so we can lookup the buffer */
+            INIT_BUFFERTAG(newTag, reln->rd_smgr->smgr_rnode.node,
+                                       forkNum, blockNum);
+
+            /* determine its hash code and partition lock ID */
+            newHash = BufTableHashCode(&newTag);
+            newPartitionLock = BufMappingPartitionLock(newHash);
+
+            /* see if the block is in the buffer pool already */
+            LWLockAcquire(newPartitionLock, LW_SHARED);
+            buf_id = BufTableLookup(&newTag, newHash);
+            LWLockRelease(newPartitionLock);
+
+            /* If in buffers, proceed */
+            if (buf_id >= 0) {
+                buf_desc = &BufferDescriptors[buf_id];        /* found buffer descriptor */
+                BufCheckAsync(0 , reln, buf_desc , BUF_INTENTION_REJECT_UNBANK , 0 , 0 , false , 0); /* end the IO and unpin if banked */
+                pgBufferUsage.aio_read_discrd++; /* account for it */
+            }
+        }
+    }
+}
+#endif   /* USE_PREFETCH */
+
 /*
  * ReleaseBuffer -- release the pin on a buffer
  */
@@ -2520,26 +3000,23 @@ ReleaseBuffer(Buffer buffer)
 {
 	volatile BufferDesc *bufHdr;
 
+
 	if (!BufferIsValid(buffer))
 		elog(ERROR, "bad buffer ID: %d", buffer);
 
-	ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
 
 	if (BufferIsLocal(buffer))
 	{
+                ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
 		Assert(LocalRefCount[-buffer - 1] > 0);
 		LocalRefCount[-buffer - 1]--;
 		return;
 	}
-
-	bufHdr = &BufferDescriptors[buffer - 1];
-
-	Assert(PrivateRefCount[buffer - 1] > 0);
-
-	if (PrivateRefCount[buffer - 1] > 1)
-		PrivateRefCount[buffer - 1]--;
 	else
-		UnpinBuffer(bufHdr, false);
+        {
+                bufHdr = &BufferDescriptors[buffer - 1];
+                BufCheckAsync(0 , 0 , bufHdr , BUF_INTENTION_REJECT_NOADJUST , 0 , 0 , false , 0 );
+        }
 }
 
 /*
@@ -2565,14 +3042,41 @@ UnlockReleaseBuffer(Buffer buffer)
 void
 IncrBufferRefCount(Buffer buffer)
 {
+        bool       pin_already_banked_by_me = false;  /* buffer is already pinned by me and redeemable */
+        volatile BufferDesc *buf;                     /* descriptor for a shared buffer */
+
 	Assert(BufferIsPinned(buffer));
+
+        if (!(BufferIsLocal(buffer))) {
+                buf = &BufferDescriptors[buffer - 1];
+		LockBufHdr(buf);
+                pin_already_banked_by_me =
+                      (    (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                        && (   (    (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                                      : (-(buf->freeNext))  )  == this_backend_pid )
+                      );
+        }
+
+        if (!pin_already_banked_by_me) {
 	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
 	ResourceOwnerRememberBuffer(CurrentResourceOwner, buffer);
+        }
+
 	if (BufferIsLocal(buffer))
 		LocalRefCount[-buffer - 1]++;
-	else
+	else {
+                if (pin_already_banked_by_me) {
+                    buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;
+                    if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                        buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+                    }
+                }
+		UnlockBufHdr(buf);
+                if (!pin_already_banked_by_me) {
 		PrivateRefCount[buffer - 1]++;
 }
+        }
+}
 
 /*
  * MarkBufferDirtyHint
@@ -2994,61 +3498,138 @@ WaitIO(volatile BufferDesc *buf)
  *
  * In some scenarios there are race conditions in which multiple backends
  * could attempt the same I/O operation concurrently.  If someone else
- * has already started I/O on this buffer then we will block on the
+ * has already started synchronous I/O on this buffer then we will block on the
  * io_in_progress lock until he's done.
  *
+ * if an async io is in progress and we are doing synchronous io,
+ * then readbuffer uses call to smgrcompleteaio to wait,
+ * and so we treat this request as if no io in progress
+ *
  * Input operations are only attempted on buffers that are not BM_VALID,
  * and output operations only on buffers that are BM_VALID and BM_DIRTY,
  * so we can always tell if the work is already done.
  *
+ * index_for_aio is an input parm which,  if non-zero,  identifies a BufferAiocb
+ * acquired by caller and to be attached to the buffer header for use with async io
+ *
  * Returns TRUE if we successfully marked the buffer as I/O busy,
  * FALSE if someone else already did the work.
  */
 static bool
-StartBufferIO(volatile BufferDesc *buf, bool forInput)
+StartBufferIO(volatile BufferDesc *buf, bool forInput , int index_for_aio )
 {
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        struct BufferAiocb volatile * BAiocb = (struct BufferAiocb*)0;  /*  BufferAiocb for use with aio */
+#endif   /* USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+ 
+        if (!index_for_aio)
 	Assert(!InProgressBuf);
 
 	for (;;)
 	{
+                if (!index_for_aio) {
 		/*
 		 * Grab the io_in_progress lock so that other processes can wait for
 		 * me to finish the I/O.
 		 */
 		LWLockAcquire(buf->io_in_progress_lock, LW_EXCLUSIVE);
+                }
 
 		LockBufHdr(buf);
 
-		if (!(buf->flags & BM_IO_IN_PROGRESS))
+                /*     the following test is intended to distinguish between :
+                **      .   buffer which : 
+                **           .     has io in progress
+                **             AND is not associated with a current aio
+                **      .   not the above
+                **     Here,  "recent" means an aio marked by buf->freeNext <= FREENEXT_BAIOCB_ORIGIN but no longer in progress -
+                **          this situation arises when the aio has just been cancelled and this process now wishes to recycle the buffer.
+                **          In this case,  the first such would-be recycler (i.e. me) must :
+                **             . avoid waiting for the cancelled aio to complete
+                **             . if not myself doing async read, then assume responsibility for posting other future readbuffers.
+                */
+		if (    (buf->flags & BM_AIO_IN_PROGRESS)
+                     || (!(buf->flags & BM_IO_IN_PROGRESS))
+                   )
 			break;
 
 		/*
-		 * The only way BM_IO_IN_PROGRESS could be set when the io_in_progress
+		 * The only way BM_IO_IN_PROGRESS without AIO in progress could be set when the io_in_progress
 		 * lock isn't held is if the process doing the I/O is recovering from
 		 * an error (see AbortBufferIO).  If that's the case, we must wait for
 		 * him to get unwedged.
 		 */
 		UnlockBufHdr(buf);
+                if (!index_for_aio) {
 		LWLockRelease(buf->io_in_progress_lock);
+                }
 		WaitIO(buf);
 	}
 
-	/* Once we get here, there is definitely no I/O active on this buffer */
-
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+	/* Once we get here, there is definitely no synchronous I/O active on this buffer
+        ** but if being asked to attach a BufferAiocb to the buf header,
+        ** then we must also check if there is any async io currently
+        ** in progress or pinned started by a different task.
+        */
+        if (index_for_aio) {
+            BAiocb = (BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext);
+            if (    (buf->flags & (BM_AIO_IN_PROGRESS|BM_AIO_PREFETCH_PIN_BANKED))
+                 && (buf->freeNext <= FREENEXT_BAIOCB_ORIGIN)
+                 && (BAiocb->pidOfAio != this_backend_pid)
+               ) {
+                    /* someone else already doing async I/O */
+                    UnlockBufHdr(buf);
+                    return false;
+            }
+	}
+#endif   /* USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 	if (forInput ? (buf->flags & BM_VALID) : !(buf->flags & BM_DIRTY))
 	{
 		/* someone else already did the I/O */
 		UnlockBufHdr(buf);
+                if (!index_for_aio) {
 		LWLockRelease(buf->io_in_progress_lock);
+                }
 		return false;
 	}
 
 	buf->flags |= BM_IO_IN_PROGRESS;
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        if (index_for_aio) {
+            BAiocb = (BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - index_for_aio);
+            /*   insist that no other buffer is using this BufferAiocb for async IO */
+            if (BAiocb->BAiocbbufh == (struct sbufdesc *)0) {
+                BAiocb->BAiocbbufh = buf;
+            }
+            if (BAiocb->BAiocbbufh != buf) {
+                               ereport(ERROR,
+                                    (errcode(ERRCODE_INTERNAL_ERROR),
+                                     errmsg("AIO control block %p to be used by %p already in use by %p"
+                                              ,BAiocb ,buf , BAiocb->BAiocbbufh)));
+            }
+            /*   note - there is no need to register self as an dependent of BAiocb
+            **   as we shall not unlock buf_desc before we free the BAiocb
+            */
+
+            buf->flags |= BM_AIO_IN_PROGRESS;
+            buf->freeNext = index_for_aio;
+            /*  at this point,  this buffer appears to have an in-progress aio_read,
+            **  and any other task which is able to look inside the buffer might try waiting on that aio -
+            **  except we have not yet issued the aio!   So we must keep the buffer header locked
+            **  from here all the way back to the BufStartAsync caller
+            */
+        } else {
+#endif   /* USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 	UnlockBufHdr(buf);
 
 	InProgressBuf = buf;
 	IsForInput = forInput;
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        }
+#endif   /* USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 	return true;
 }
@@ -3058,7 +3639,7 @@ StartBufferIO(volatile BufferDesc *buf,
  *	(Assumptions)
  *	My process is executing IO for the buffer
  *	BM_IO_IN_PROGRESS bit is set for the buffer
- *	We hold the buffer's io_in_progress lock
+ *	if no async IO in progress,  then We hold the buffer's io_in_progress lock
  *	The buffer is Pinned
  *
  * If clear_dirty is TRUE and BM_JUST_DIRTIED is not set, we clear the
@@ -3070,26 +3651,32 @@ StartBufferIO(volatile BufferDesc *buf,
  * BM_IO_ERROR in a failure case.  For successful completion it could
  * be 0, or BM_VALID if we just finished reading in the page.
  */
-static void
+void
 TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
 				  int set_flag_bits)
 {
-	Assert(buf == InProgressBuf);
+        int flags_on_entry;
 
 	LockBufHdr(buf);
 
+        flags_on_entry = buf->flags;
+
+        if (! (flags_on_entry & BM_AIO_IN_PROGRESS) )
+            Assert( buf == InProgressBuf );
+
 	Assert(buf->flags & BM_IO_IN_PROGRESS);
-	buf->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR);
+	buf->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR | BM_AIO_IN_PROGRESS);
 	if (clear_dirty && !(buf->flags & BM_JUST_DIRTIED))
 		buf->flags &= ~(BM_DIRTY | BM_CHECKPOINT_NEEDED);
 	buf->flags |= set_flag_bits;
 
 	UnlockBufHdr(buf);
 
+        if (! (flags_on_entry & BM_AIO_IN_PROGRESS) ) {
 	InProgressBuf = NULL;
-
 	LWLockRelease(buf->io_in_progress_lock);
 }
+}
 
 /*
  * AbortBufferIO: Clean up any active buffer I/O after an error.
--- src/backend/storage/buffer/buf_async.c.orig	2014-06-08 17:57:22.096976856 -0400
+++ src/backend/storage/buffer/buf_async.c	2014-06-08 21:59:36.748096540 -0400
@@ -0,0 +1,923 @@
+/*-------------------------------------------------------------------------
+ *
+ * buf_async.c
+ *	  buffer manager asynchronous disk read routines
+ *
+ * Portions Copyright (c) 1996-2012, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/storage/buffer/buf_async.c
+ *
+ *-------------------------------------------------------------------------
+ */
+/*
+ * Principal entry points:
+ *
+ * BufStartAsync() -- start an asynchronous read of a block into a buffer and
+ *	 pin it so that no one can destroy it while this process is using it.
+ *
+ * BufCheckAsync() -- check completion of an asynchronous read
+ *       and either claim buffer or discard it
+ *
+ * Private helper
+ *
+ * BufReleaseAsync() -- release the BAiocb resources used for an asynchronous read
+ *
+ * See also these files:
+ *		bufmgr.c -- main buffer manager functions
+ *		buf_init.c -- initialisation of resources
+ */
+#include "postgres.h"
+#include <sys/types.h> 
+#include <sys/file.h>
+#include <unistd.h>
+#include <sched.h>
+
+#include "catalog/catalog.h"
+#include "common/relpath.h"
+#include "executor/instrument.h"
+#include "miscadmin.h"
+#include "pg_trace.h"
+#include "pgstat.h"
+#include "postmaster/bgwriter.h"
+#include "storage/buf_internals.h"
+#include "storage/bufmgr.h"
+#include "storage/ipc.h"
+#include "storage/proc.h"
+#include "storage/smgr.h"
+#include "storage/standby.h"
+#include "utils/rel.h"
+#include "utils/resowner_private.h"
+
+/*
+ * GUC parameters
+ */
+int      max_async_io_prefetchers; /* Maximum number of backends using asynchronous librt threads to read pages into our buffers */
+
+/* Note: these two macros only work on shared buffers, not local ones! */
+#define BufHdrGetBlock(bufHdr)	((Block) (BufferBlocks + ((Size) (bufHdr)->buf_id) * BLCKSZ))
+#define BufferGetLSN(bufHdr)	(PageGetLSN(BufHdrGetBlock(bufHdr)))
+
+extern volatile struct BAiocbAnchor *BAiocbAnchr;  /*  anchor for all control blocks pertaining to aio  */
+extern int numBufferAiocbs;        /*  total number of  BufferAiocbs in pool  */
+extern int maxGetBAiocbTries;      /*   max times we will try to get a free BufferAiocb */
+extern int maxRelBAiocbTries;      /*   max times we will try to release a BufferAiocb back to freelist */
+extern pid_t this_backend_pid;     /*   pid of this backend */
+
+extern bool PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy);
+extern void PinBuffer_Locked(volatile BufferDesc *buf);
+extern Buffer ReadBuffer_common(SMgrRelation reln, char relpersistence,
+				  ForkNumber forkNum, BlockNumber blockNum,
+				  ReadBufferMode mode, BufferAccessStrategy strategy,
+				  bool *hit , int index_for_aio);
+extern void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner);
+extern void TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
+				  int set_flag_bits);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+int BufStartAsync(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy);
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+int BufCheckAsync(SMgrRelation caller_smgr, Relation caller_reln, volatile BufferDesc *buf_desc
+  ,int intention , BufferAccessStrategy strategy , int index_for_aio , bool spinLockHeld , LWLockId PartitionLock );
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+
+static struct BufferAiocb volatile * cachedBAiocb = (struct BufferAiocb*)0;  /*  one cached BufferAiocb for use with aio */
+
+#ifdef USE_PREFETCH
+/*  BufReleaseAsync releases a BufferAiocb and returns 0 if successful else non-zero
+**  it *must* be called :
+**     EITHER with a valid  BAiocb->BAiocbbufh -> buf_desc
+**            and that buf_desc must be spin-locked
+**     OR     with BAiocb->BAiocbbufh == 0
+*/
+static int
+BufReleaseAsync(struct BufferAiocb volatile * BAiocb)
+{
+        int    LockTries;         /*  max times we will try to release the BufferAiocb */
+        volatile struct BufferAiocb *BufferAiocbs;
+        struct BufferAiocb volatile * oldFreeBAiocb;  /*  old value of FreeBAiocbs */
+
+        int failed = 1; /* by end of this function, non-zero  will indicate if we failed to return the BAiocb */
+
+
+        if (    ( BAiocb == (struct BufferAiocb*)0 )
+             || ( BAiocb == (struct BufferAiocb*)BAIOCB_OCCUPIED )
+             || ( ((unsigned long)BAiocb) & 0x1 )
+           ) {
+                          elog(ERROR,
+                                 "AIO control block corruption on release of aiocb %p - invalid BAiocb"
+                                          ,BAiocb);
+        }
+        else 
+        if (   (0 == BAiocb->BAiocbDependentCount)     /*  no dependents  */
+            && ((struct BufferAiocb*)BAIOCB_OCCUPIED == BAiocb->BAiocbnext)  /*  not already on freelist */
+           ) {
+
+            if ((struct sbufdesc*)0 != BAiocb->BAiocbbufh) { /*  if a buffer was attached */
+                volatile        BufferDesc *buf_desc = BAiocb->BAiocbbufh;
+
+                /*  spinlock held so instead of TerminateBufferIO(buf, false , 0); ... */
+                if (buf_desc->flags & BM_AIO_PREFETCH_PIN_BANKED) { /* if a pid banked the pin */
+                    buf_desc->freeNext = -(BAiocb->pidOfAio);       /* then remember which pid */
+                }
+                else if (buf_desc->freeNext <= FREENEXT_BAIOCB_ORIGIN) {
+                    buf_desc->freeNext = FREENEXT_NOT_IN_LIST;   /* disconnect BufferAiocb from buf_desc */
+                }
+                buf_desc->flags &= ~BM_AIO_IN_PROGRESS;
+            }
+            
+            BAiocb->BAiocbbufh = (struct sbufdesc *)0; /* disconnect buf_desc from BufferAiocb */
+            BAiocb->pidOfAio = 0;                      /*  clean */
+            LockTries = maxRelBAiocbTries;         /*  max times we will try to release the BufferAiocb */
+            do {
+                register long long int dividend , remainder;
+
+                /*      retrieve old value of FreeBAiocbs  */
+                BAiocb->BAiocbnext = oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs;  /*  old value of FreeBAiocbs */
+
+                /*  this is a volatile value unprotected by any lock,  so must validate it;
+                **  safest is to verify that it is identical to one of the BufferAiocbs
+                **  to do so,  verify by direct division that its address offset from first control block 
+                **  is an integral multiple of the control block size
+                **  that lies within the range [ 0 , (numBufferAiocbs-1) ]
+                */
+                BufferAiocbs = BAiocbAnchr->BufferAiocbs;
+                remainder = ((long long int)oldFreeBAiocb - (long long int)BufferAiocbs)
+                            % (long long int)(sizeof(struct BufferAiocb));
+                failed = (int)remainder;
+                if (!failed) {
+                    dividend = ((long long int)oldFreeBAiocb - (long long int)BufferAiocbs)
+                               / (long long int)(sizeof(struct BufferAiocb));
+                     failed = ( (dividend < 0 ) || ( dividend >= numBufferAiocbs) );
+                     if (!failed) {
+                         if (__sync_bool_compare_and_swap (&(BAiocbAnchr->FreeBAiocbs), oldFreeBAiocb, BAiocb)) {
+                            LockTries = 0;   /*  end the do loop  */
+
+                            goto cheering;   /*  cant simply break because then failed would be set incorrectly */
+                         }
+                    }
+                }
+                /*  if we reach here, we have failed and failed is set to -1 */
+
+       cheering: ;
+
+                if ( LockTries > 1 ) {
+                    sched_yield();    /*   yield to another process,  (hopefully a backend) */
+                }
+            } while  (LockTries-- > 0);
+
+            if (failed) {
+#ifdef LOG_RELBAIOCB_DEPLETION
+                          elog(LOG,
+                                "BufReleaseAsync:AIO control block %p unreleased after tries= %d\n"
+                                       ,BAiocb,maxRelBAiocbTries);
+#endif  /* LOG_RELBAIOCB_DEPLETION */
+            }
+
+        }
+              else
+                          elog(LOG,
+                                "BufReleaseAsync:AIO control block %p either has dependents= %d or is already on freelist %p or has no buf_header %p\n"
+                                       ,BAiocb , BAiocb->BAiocbDependentCount , BAiocb->BAiocbnext , BAiocb->BAiocbbufh);
+        return failed;
+}
+
+/*  try using asynchronous aio_read to prefetch into a buffer
+**  return code :
+**        0 if started successfully
+**       -1 if failed for some reason
+**        1+PrivateRefCount if we found desired buffer in buffer pool
+**
+**  There is a harmless race condition here :
+**  two different backends may both arrive here simultaneously
+**  to prefetch the same buffer.    This is not unlikely when a syncscan is in progress.
+**  .  One will acquire the buffer and issue the smgrstartaio
+**  .  Other will find the buffer on return from  ReadBuffer_common with hit = true
+**  Only the first task has a pin on the buffer since ReadBuffer_common knows not to get a pin
+**  on a found buffer in prefetch mode.
+**  Therefore  -   the second task must simply abandon the prefetch if it finds the buffer in the buffer pool.
+**
+**  if we fail to acquire a BAiocb because of concurrent theft from freelist by other backend,
+**  retry up to maxGetBAiocbTries times provided that there actually was at least one BAiocb on the freelist.
+*/
+int
+BufStartAsync(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy) {
+
+        int retcode = -1;
+        struct BufferAiocb volatile * BAiocb = (struct BufferAiocb*)0;  /*  BufferAiocb for use with aio */
+        int  smgrstartaio_rc = -1;           /*  retcode from smgrstartaio */
+        bool do_unpin_buffer = false;        /* unpin must be deferred until after buffer descriptor is unlocked */
+        Buffer		buf_id;
+        bool		hit = false;
+        volatile        BufferDesc *buf_desc = (BufferDesc *)0;
+
+        int    LockTries;         /*  max times we will try to get a free BufferAiocb */
+
+        struct BufferAiocb volatile * oldFreeBAiocb;  /*  old value of FreeBAiocbs */
+        struct BufferAiocb volatile * newFreeBAiocb;  /*  new value of FreeBAiocbs */
+
+
+    /*  return immediately if no async io resources */
+    if (numBufferAiocbs > 0) {
+        buf_id = (Buffer)0;
+
+        if ( (struct BAiocbAnchor *)0 != BAiocbAnchr ) {
+
+            volatile struct BufferAiocb *BufferAiocbs;
+
+            if ((struct BufferAiocb*)0 != cachedBAiocb) {  /* any cached BufferAiocb ? */
+                BAiocb = cachedBAiocb;                     /* yes  use it  */
+                cachedBAiocb = BAiocb->BAiocbnext;
+                BAiocb->BAiocbnext = (struct BufferAiocb*)BAIOCB_OCCUPIED; /* mark as not on freelist */
+                BAiocb->BAiocbDependentCount = 0;  /* no dependent yet */
+                BAiocb->BAiocbbufh = (struct sbufdesc*)0;
+                BAiocb->pidOfAio = 0;
+            } else {
+
+                LockTries = maxGetBAiocbTries;         /*  max times we will try to get a free BufferAiocb */
+                do {
+                    register long long int dividend = -1 , remainder;
+                    /*  check if we have a free BufferAiocb */
+
+                    oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs;  /*  old value of FreeBAiocbs */
+
+                    /*  check if we have a free BufferAiocb */
+
+                    /*  BAiocbAnchr->FreeBAiocbs is a volatile value unprotected by any lock,
+                    **  and use of compare-and-swap to add and remove items from the list has
+                    **  two potential pitfalls,    both relating to the fact that we must
+                    **  access data de-referenced from this pointer before the compare-and-swap.
+                    **  1)  The value we load may be corrupt,  e.g. mixture of bytes from
+                    **      two different values,   so must validate it;
+                    **      safest is to verify that it is identical to one of the BufferAiocbs.
+                    **      to do so,  verify by direct division that its address offset from
+                    **      first control block is an integral multiple of the control block size
+                    **      that lies within the range [ 0 , (numBufferAiocbs-1) ]
+                    **      Thus we completely prevent this pitfall.
+                    **  2)  The content of the item's next pointer may have changed between the
+                    **      time we de-reference it and the time of the compare-and-swap.
+                    **      Thus even though the compare-and-swap succeeds,   we might set the
+                    **      new head of the freelist to an invalid value  (either a free item
+                    **      that is not the first in the free chain  -  resulting only in
+                    **      loss of the orphaned free items, or,  much worse, an in-use item).
+                    **      In practice this is extremely unlikely because of the implied huge delay
+                    **      in this window interval in this (current) process.    Here are two scenarios:
+                    **      legend:
+                    **         P0  -  this (current) process,  P1,  P2 , ... other processes
+                    **         content of freelist shown as BAiocbAnchr->FreeBAiocbs -> first item -> 2nd item ...
+                    **         @[X] means address of X
+                    **         |      timeline of window of exposure to problems
+                    **      successive lines in chronological order                                       content of freelist
+                    **        2.1    P0           access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I0]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **               P0          access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I1]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1        access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I0]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1       access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I1]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1        swap-remove I0,  place I1 at head of list                F -> I1 -> I2 -> I3 ...
+                    **         |           P2     access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I1]  F -> I1 -> I2 -> I3 ...
+                    **         |           P2    access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I2]  F -> I1 -> I2 -> I3 ...
+                    **         |           P2     swap-remove I1,  place I2 at head of list                F -> I2 -> I3 ...
+                    **         |        P1       complete aio,  replace I0 at head of list                 F -> I0 -> I2 -> I3 ...
+                    **               P0        swap-remove I0,  place I1 at head of list                   F -> I1 IS IN USE !! CORRUPT !!
+                    **               compare-and-swap succeeded but the value of newFreeBAiocb was stale
+                    **               and had become in-use during the window.
+                    **        2.2    P0           access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I0]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **               P0          access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I1]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1        access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I0]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1       access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I1]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1        swap-remove I0,  place I1 at head of list                F -> I1 -> I2 -> I3 ...
+                    **         |           P2     access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I1]  F -> I1 -> I2 -> I3 ...
+                    **         |           P2    access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I2]  F -> I1 -> I2 -> I3 ...
+                    **         |           P2     swap-remove I1,  place I2 at head of list                F -> I2 -> I3 ...
+                    **         |              P3  access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I2]  F -> I2 -> I3 ...
+                    **         |              P3 access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I3]  F -> I2 -> I3 ...
+                    **         |              P3  swap-remove I2,  place I3 at head of list                F -> I3 ...
+                    **         |           P2    complete aio,  replace I1 at head of list                 F -> I1 -> I3 ...
+                    **         |              P3 complete aio,  replace I2 at head of list                 F -> I2 -> I1 -> I3 ...
+                    **         |        P1       complete aio,  replace I0 at head of list                 F -> I0 -> I2 -> I1 -> I3 ...
+                    **               P0        swap-remove I0,  place I1 at head of list                   F -> I1 -> I3 ... ! I2 is orphaned !
+                    **               compare-and-swap succeeded but the value of newFreeBAiocb was stale
+                    **               and had moved further down the free list during the window.
+                    **      Unfortunately, we cannot prevent this pitfall but we can detect it (after the fact),
+                    **      by checking that the next pointer of the item we have just removed for our use still points to the same item.
+                    **      This test is not subject to any timing or uncertainty since :
+                    **       .  The fact that the compare-and-swap succeeded implies that the item we removed
+                    **          was defintely on the freelist (at the head) when it was removed,
+                    **          and therefore cannot be in use,  and therefore its next pointer is no longer volatile.
+                    **       .  Although pointers of the anchor and items on the freelist are volatile,
+                    **          the addresses of items never change -  they are in an allocated array and never move.
+                    **      E.g. in the above two scenarios,   the test is that I0.next still -> I1,
+                    **      and this is true if and only if the second item on the freelist is
+                    **      still the same at the end of the window as it was at the start of the window.
+                    **      Note that we do not insist that it did not change during the window,
+                    **           only that it is still the correct new head of freelist.
+                    **      If this test fails,  we abort immediately as the subsystem is damaged and cannot be repaired.
+                    **      Note that at least one aio must have been issued *and* completed during the window
+                    **           for this to occur,  and since the window is just one single machine instruction,
+                    **           it is very unlikely in practice.
+                    */
+                    BufferAiocbs = BAiocbAnchr->BufferAiocbs;
+                    remainder = ((long long int)oldFreeBAiocb - (long long int)BufferAiocbs)
+                                % (long long int)(sizeof(struct BufferAiocb));
+                    if (remainder == 0) {
+                        dividend = ((long long int)oldFreeBAiocb - (long long int)BufferAiocbs)
+                                    / (long long int)(sizeof(struct BufferAiocb));
+                    }
+                    if (    (remainder == 0)
+                         && ( (dividend >= 0 ) && ( dividend < numBufferAiocbs) )
+                       )
+                    {
+                        newFreeBAiocb = oldFreeBAiocb->BAiocbnext; /*  tentative new value is second on free list */
+                        /*   Here we are in the exposure window referred to in the above comments,
+                        **   so moving along rapidly ...
+                        */
+                        if (__sync_bool_compare_and_swap (&(BAiocbAnchr->FreeBAiocbs), oldFreeBAiocb, newFreeBAiocb)) {   /*  did we get it ? */
+                                /*   We have successfully swapped head of freelist pointed to by oldFreeBAiocb off the list;
+                                **   Here we check that the item we just placed at head of freelist, pointed to by newFreeBAiocb,
+                                **   is the right one
+                                **
+                                **   also check that the BAiocb we have acquired was not in use
+                                **   i.e. that scenario 2.1 above did not occur just before our compare-and-swap
+                                **   The test is that the BAiocb is not in use.
+                                **
+                                **  in one hypothetical case,
+                                **  we can be certain that there is no corruption -
+                                **  the case where newFreeBAiocb == 0 and oldFreeBAiocb->BAiocbnext != BAIOCB_OCCUPIED -
+                                **  i.e. we have set the freelist to empty but we have a baiocb chained from ours.
+                                **  in this case our comp_swap removed all BAiocbs from the list (including ours)
+                                **  so the others chained from ours are either orphaned (no harm done)
+                                **  or in use by another backend and will eventually be returned (fine).
+                                */
+                                if ((struct BufferAiocb *)0 == newFreeBAiocb) {
+                                    if ((struct BufferAiocb *)BAIOCB_OCCUPIED == oldFreeBAiocb->BAiocbnext) {
+                                        goto baiocb_corruption;
+                                    } else if ((struct BufferAiocb *)0 != oldFreeBAiocb->BAiocbnext) {
+                                      elog(LOG,
+                                         "AIO control block inconsistency on acquiring aiocb %p - its next free %p may be orphaned (no corruption has occurred)"
+                                         	,oldFreeBAiocb , oldFreeBAiocb->BAiocbnext);
+                                    }
+                                } else {
+                                    /*  case of newFreeBAiocb not null  -  so must check more carefully ... */
+                                    remainder = ((long long int)newFreeBAiocb - (long long int)BufferAiocbs)
+                                                % (long long int)(sizeof(struct BufferAiocb));
+                                    dividend = ((long long int)newFreeBAiocb - (long long int)BufferAiocbs)
+                                                / (long long int)(sizeof(struct BufferAiocb));
+
+                                    if (    (newFreeBAiocb != oldFreeBAiocb->BAiocbnext)
+                                         || (remainder != 0)
+                                         || ( (dividend < 0 ) || ( dividend >= numBufferAiocbs) )
+                                       ) {
+                                        goto baiocb_corruption;
+                                    }
+                                }
+                                BAiocb = oldFreeBAiocb;
+                                BAiocb->BAiocbnext = (struct BufferAiocb*)BAIOCB_OCCUPIED; /* mark as not on freelist */
+                                BAiocb->BAiocbDependentCount = 0;  /* no dependent yet */
+                                BAiocb->BAiocbbufh = (struct sbufdesc*)0;
+                                BAiocb->pidOfAio = 0;
+
+                                LockTries = 0;   /*  end the do loop  */
+
+                        }
+                    }
+
+                    if ( LockTries > 1 ) {
+                        sched_yield();    /*   yield to another process,  (hopefully a backend) */
+                    }
+                } while (     ((struct BufferAiocb*)0 == BAiocb)            /*  did not get a BAiocb    */
+                           && ((struct BufferAiocb*)0 != oldFreeBAiocb)     /*  there was a free BAiocb */
+                           && (LockTries-- > 0)                             /*  told to retry           */
+                        );
+            }
+        }
+
+        if ( BAiocb != (struct BufferAiocb*)0 ) {
+            /*  try an async io    */
+            BAiocb->BAiocbthis.aio_fildes = -1; /* necessary to ensure any thief realizes aio not yet started */
+            BAiocb->pidOfAio = this_backend_pid;
+
+            /*  now try to acquire a buffer :
+            **  note -   ReadBuffer_common returns hit=true if the block is found in the buffer pool,
+            **            in which case there is no need to prefetch.
+            **  otherwise ReadBuffer_common pins returned buffer and calls StartBufferIO
+            **           and StartBufferIO :
+            **      . sets buf_desc->freeNext to negative of ( index of the aiocb in the BufferAiocbs array + 3 )
+            **      . sets  BAiocb->BAiocbbufh -> buf_desc
+            **           and in this case the buffer spinlock is held.
+            **           This is essential as no other task must issue any intention with respect
+            **           to the buffer until we have started the aio_read.
+            **  Also note that ReadBuffer_common handles enlarging the ResourceOwner buffer list as needed
+            **       so I dont need to do that
+            */
+            buf_id = ReadBuffer_common(reln->rd_smgr, reln->rd_rel->relpersistence,
+                                        forkNum, blockNum
+                                       ,RBM_NOREAD_FOR_PREFETCH  /*  tells ReadBuffer not to do any read,  just alloc buf */
+                                       ,strategy , &hit , (FREENEXT_BAIOCB_ORIGIN - (BAiocb - (BAiocbAnchr->BufferAiocbs))));
+            buf_desc = &BufferDescriptors[buf_id-1];    /* find buffer descriptor */
+
+            /*  normally hit will be false as presumably it was not in the pool
+            **  when our caller looked - but it could be there now ...
+            */
+            if (hit) {
+                   /*   see earlier comments  -  we must abandon the prefetch */
+                   retcode = 1 + PrivateRefCount[buf_id];
+                   BAiocb->BAiocbbufh = (struct sbufdesc *)0; /* indicator that we must release the BAiocb before exiting */
+            } else
+            if (  (buf_id > 0) && ((BufferDesc *)0 != buf_desc) && (buf_desc == BAiocb->BAiocbbufh)  ) {
+                   /*   buff descriptor header lock should be held.
+                   **   However,  just to be safe ,   now validate that
+                   **   we are still the owner and no other task already stole it.
+                   */
+
+                   buf_desc->flags &= ~BM_AIO_PREFETCH_PIN_BANKED; /* ensure no banked pin */
+                   /*  there should not be any other pid waiting on this buffer
+                   **  so check both of BM_VALID and BM_PIN_COUNT_WAITER are not set
+                   */
+                   if (    ( !(buf_desc->flags & (BM_VALID|BM_PIN_COUNT_WAITER) ) )
+                        && (buf_desc->flags & BM_AIO_IN_PROGRESS)
+                        && ((FREENEXT_BAIOCB_ORIGIN - (BAiocb - (BAiocbAnchr->BufferAiocbs))) == buf_desc->freeNext) /* it is still mine */
+                        && (-1 == BAiocb->BAiocbthis.aio_fildes)   /* no thief stole it */
+                        && (0 == BAiocb->BAiocbDependentCount)     /* no dependent */
+                     ) {
+                        /*   we have an empty buffer for our use */
+
+                        BAiocb->BAiocbthis.aio_buf = (void *)(BufHdrGetBlock(buf_desc)); /* Location of actual buffer.  */
+
+                        /*   note - there is no need to register self as an dependent of BAiocb
+                        **   as we shall not unlock buf_desc before we free the BAiocb
+                        */
+
+                        /*   smgrstartaio retcode is returned in smgrstartaio_rc -
+                        **   it indicates whether started or not
+                        */
+                        smgrstartaio(reln->rd_smgr, forkNum, blockNum , (char *)&(BAiocb->BAiocbthis) , &smgrstartaio_rc );
+
+                        if (smgrstartaio_rc == 0) {
+                            retcode = 0;
+                            buf_desc->flags |= BM_AIO_PREFETCH_PIN_BANKED; /* bank the pin for next use by this task */
+                            /* we did not register register self as an dependent of BAiocb so no need to unregister  */
+                        } else {
+                            /*  failed - return the BufferAiocb to the free list  */
+                            do_unpin_buffer = true;    /* unpin must be deferred until after buffer descriptor is unlocked */
+                            /*  spinlock held so instead of TerminateBufferIO(buf_desc, false , 0); ... */
+                            buf_desc->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR | BM_AIO_IN_PROGRESS | BM_AIO_PREFETCH_PIN_BANKED | BM_VALID);
+                            /* we did not register register self as an dependent of BAiocb so no need to unregister  */
+
+                            /*  return the BufferAiocb to the free list  */
+                            if (
+                                   BufReleaseAsync(BAiocb)
+                               ) {         /*  failed ? */
+                                BAiocb->BAiocbnext = cachedBAiocb;    /* then ...       */
+                                cachedBAiocb = BAiocb;                /*  ... cache it  */
+                            }
+
+                            pgBufferUsage.aio_read_failed++;
+                            smgrstartaio_rc = 1;  /*   to distinguish from aio not even attempted */
+                        }
+                   }
+                   else {
+                            /*  buffer was stolen or in use by other task - return the BufferAiocb to the free list  */
+                            do_unpin_buffer = true;    /* unpin must be deferred until after buffer descriptor is unlocked */
+                   }
+
+                   UnlockBufHdr(buf_desc);
+                   if (do_unpin_buffer) {
+                        if (smgrstartaio_rc >= 0) { /*  if  aio was attempted */
+                            TerminateBufferIO(buf_desc, false , 0);
+                        }
+                        UnpinBuffer(buf_desc, true);
+                   }
+            }
+            else {
+                BAiocb->BAiocbbufh = (struct sbufdesc *)0; /* indicator that we must release the BAiocb before exiting */
+            }
+
+            if ((struct sbufdesc*)0 == BAiocb->BAiocbbufh) { /*  we did not associate a buffer */
+                                                             /*  so return the BufferAiocb to the free list  */
+                            if (
+                                   BufReleaseAsync(BAiocb)
+                               ) {         /*  failed ? */
+                                BAiocb->BAiocbnext = cachedBAiocb;    /* then ...       */
+                                cachedBAiocb = BAiocb;                /*  ... cache it  */
+                            }
+            }
+        }
+    }
+
+    return retcode;
+
+    baiocb_corruption:;
+         elog(PANIC,
+              "AIO control block corruption on acquiring aiocb %p - its next free %p conflicts with new freelist pointer %p which may be invalid (corruption may have occurred)"
+                                                    ,oldFreeBAiocb , oldFreeBAiocb->BAiocbnext , newFreeBAiocb);
+}
+#endif   /* USE_PREFETCH */
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+/*
+ * BufCheckAsync --      act upon caller's intention regarding a shared buffer,
+ *                       primarily in connection with any async io in progress on the buffer.
+ *     class  subvalue   intention has two main classes and some subvalues within those :
+ *      +ve      1            .   want    <=>  caller wants the buffer,
+ *                                             wait for in-progress aio and then always pin
+ *      -ve                   .   reject  <=>  caller does not want the buffer,
+ *                                             if there are no dependents,  then cancel the aio
+ *              -1, -2 , -3 , ... (see below)        and then optionally unpin
+ *                             Used when there may have been a previous fetch or prefetch.
+ *
+ * buffer is assumed to be an existing member of the shared buffer pool
+ *    as returned by BufTableLookup.
+ * if AIO in progress,  then :
+ *      .  terminate AIO, waiting for completion if +ve intention, else without waiting
+ *      .  if the AIO had already completed successfully,   then mark buffer valid
+ *      .  pin/unpin as requested
+ *
+ * +ve intention indicates that buffer must be pinned :
+ *   if the strategy parameter is null,  then use the PinBuffer_Locked optimization
+ *   to pin and unlock in one operation.   But always update buffer usage count.
+ *
+ * -ve intention indicates whether and how to unpin :
+ *   BUF_INTENTION_REJECT_KEEP_PIN 	-1   pin already held, do not unpin, (caller wants to keep it)
+ *   BUF_INTENTION_REJECT_OBTAIN_PIN	-2   obtain pin,  caller wants it for same buffer
+ *   BUF_INTENTION_REJECT_FORGET	-3   unpin and tell resource owner to forget
+ *   BUF_INTENTION_REJECT_NOADJUST	-4   unpin and call ResourceOwnerForgetBuffer myself
+ *                                           instead of telling UnpinBuffer to adjust CurrentResource owner
+ *                                           (quirky simulation of ReleaseBuffer logic)
+ *   BUF_INTENTION_REJECT_UNBANK   	-5   unpin only if pin banked by caller
+ *   The behaviour for the -ve case is based on that of ReleaseBuffer, adding handling of async io.
+ *
+ * pin/unpin action must take account of whether this backend hold a "disposable" pin on the particular buffer.
+ * A "disposable" pin is a pin acquired by buffer manager without caller knowing, such as :
+ *      when required to safeguard an async AIO  -  pin can be held across multiple bufmgr calls
+ *      when required to safeguard waiting for an async AIO  -  pin acquired and released within this function
+ * if a disposable pin is held,   then :
+ *      if a new pin is requested,  the disposable pin must be retained (redeemed) and any flags relating to it unset
+ *      if an unpin is requested,   then :
+ *              if    either no AIO in progress or this backend did not initiate the AIO
+ *              then  the disposable pin must be dropped (redeemed) and any flags relating to it unset
+ *              else  log warning and do nothing
+ *  i.e. in either case,   there is no longer a disposable pin after this function has completed.
+ *       Note that if    intention is BUF_INTENTION_REJECT_UNBANK,
+ *                 then caller expects there to be a disposable banked pin
+ *                      and if there isn't one,  we do nothing
+ *                 for all other intentions,  if there is no disposable pin,   we pin/unpin normally.
+ *
+ * index_for_aio indicates the BAiocb to be used for next aio (see PrefetchBuffer)
+ * BufFreelistLockHeld indicates whether freelistlock is held
+ * spinLockHeld indicates whether buffer header spinlock is held
+ * PartitionLock is the  buffer partition lock to be used
+ *
+ * return code (meaningful ONLY if intention is +ve) indicates validity of buffer :
+ *  -1    buffer is invalid and failed PageHeaderIsValid check
+ *   0    buffer is not valid
+ *   1    buffer is valid
+ *   2    buffer is valid but tag changed  -  (so content does not match the relation block that caller expects)
+ */
+int
+BufCheckAsync(SMgrRelation caller_smgr, Relation caller_reln, BufferDesc volatile * buf_desc, int intention , BufferAccessStrategy strategy , int index_for_aio , bool spinLockHeld , LWLockId PartitionLock )
+{
+
+        int retcode = BUF_INTENT_RC_INVALID_NO_AIO;
+        bool valid = false;
+
+
+#ifdef USE_PREFETCH
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        int smgrcompleteaio_rc;          /*  retcode from smgrcompleteaio */
+        SMgrRelation smgr = caller_smgr;
+        int    		BAiocbDependentCount_after_aio_finished = -1;  /*  for debugging  -  can be printed in gdb */
+	    BufferTag origTag = buf_desc->tag;	/* original identity of selected buffer */
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+        int aio_successful = -1;         /*  did the aio_read succeed ?  -1 = no aio,  0 unsuccessful , 1 successful */
+        BufFlags	flags_on_entry;  /*  for debugging  -  can be printed in gdb */
+        int    		freeNext_on_entry;  /*  for debugging  -  can be printed in gdb */
+        bool       disposable_pin = false;            /* this backend had a disposable pin on entry or pins the buffer while waiting for aio_read to complete */
+        bool       pin_already_banked_by_me;          /* buffer is already pinned by me and redeemable */
+        int local_intention;
+#endif   /* USE_PREFETCH */
+
+
+
+#ifdef USE_PREFETCH
+            if (!spinLockHeld) {
+                /*  lock buffer header    */
+                LockBufHdr(buf_desc);
+            }
+
+	    flags_on_entry = buf_desc->flags;
+	    freeNext_on_entry = buf_desc->freeNext;
+            pin_already_banked_by_me =
+                      (    (flags_on_entry & BM_AIO_PREFETCH_PIN_BANKED)
+                        && (   (    (flags_on_entry & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - freeNext_on_entry))->pidOfAio )
+                                                                      : (-(freeNext_on_entry))  )  == this_backend_pid )
+                      );
+
+            if (pin_already_banked_by_me) {
+                if (PrivateRefCount[buf_desc->buf_id] == 0) {  /*  but do we actually have a pin ?? */
+                    /*   this is an anomalous situation   -  somehow our disposable pin was lost without us noticing
+                    **   if AIO is in progress and we started it,
+                    **   then this is disastrous  -   two backends might both issue IO on same buffer
+                    **   otherwise,  it is harmless,  and simply means we have no disposable pin,
+                    **               but we must update flags to "notice" the fact now
+                    */
+                    if (flags_on_entry & BM_AIO_IN_PROGRESS) {
+                            elog(ERROR, "BufCheckAsync : AIO control block issuer of aio_read lost pin with BM_AIO_IN_PROGRESS on buffer %d rel=%s, blockNum=%u, flags 0x%X refcount=%u intention= %d"
+                                ,buf_desc->buf_id,relpathbackend(buf_desc->tag.rnode, InvalidBackendId, buf_desc->tag.forkNum)
+                           ,buf_desc->tag.blockNum, flags_on_entry, buf_desc->refcount ,intention);
+                    } else {
+                            elog(LOG, "BufCheckAsync : AIO control block issuer of aio_read lost pin on buffer %d rel=%s, blockNum=%u, with flags 0x%X refcount=%u intention= %d"
+                               ,buf_desc->buf_id,relpathbackend(buf_desc->tag.rnode, InvalidBackendId, buf_desc->tag.forkNum)
+                               ,buf_desc->tag.blockNum, flags_on_entry, buf_desc->refcount ,intention);
+							buf_desc->flags &= ~BM_AIO_PREFETCH_PIN_BANKED; /*   redeem the banked pin  */
+							/*   since AIO not in progress,  disconnect the buffer from banked pin */
+							buf_desc->freeNext = FREENEXT_NOT_IN_LIST;      /*   and forget the bank client */
+							pin_already_banked_by_me = false;
+                    }
+                } else {
+                    disposable_pin = true;
+                }
+            }
+
+            /*  the case of BUF_INTENTION_REJECT_UNBANK is handled specially :
+            **    if this backend has a banked pin,  then proceed just as for BUF_INTENTION_REJECT_FORGET
+            **    else the call is a no-op  --  unlock buf header and return immediately
+            */
+            local_intention = intention;
+            if (intention == BUF_INTENTION_REJECT_UNBANK) {
+                if (pin_already_banked_by_me) {
+                    local_intention = BUF_INTENTION_REJECT_FORGET;
+                } else {
+                    goto unlock_buf_header;  /*  code following the unlock will do nothing since local_intention still set to BUF_INTENTION_REJECT_UNBANK */
+                }
+            }
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+            /*       we do not expect that BM_AIO_IN_PROGRESS is set without freeNext identifying the BAiocb */
+            if ( (buf_desc->flags & BM_AIO_IN_PROGRESS) && (buf_desc->freeNext == FREENEXT_NOT_IN_LIST) ) {
+
+					elog(ERROR, "BufCheckAsync : found BM_AIO_IN_PROGRESS without a BAiocb on buffer %d rel=%s, blockNum=%u, flags %X refcount=%u"
+						,buf_desc->buf_id,relpathbackend(buf_desc->tag.rnode, InvalidBackendId, buf_desc->tag.forkNum)
+						,buf_desc->tag.blockNum, buf_desc->flags, buf_desc->refcount);
+                }
+            /*     check whether aio in progress  */
+            if  (    ( (struct BAiocbAnchor *)0 != BAiocbAnchr )
+                  && (buf_desc->flags & BM_AIO_IN_PROGRESS)
+                  && (buf_desc->freeNext <= FREENEXT_BAIOCB_ORIGIN)                       /*  has a valid BAiocb */
+                  && ((FREENEXT_BAIOCB_ORIGIN - buf_desc->freeNext) < numBufferAiocbs)    /*  double-check */
+                ) {        /* this is aio   */
+                    struct BufferAiocb volatile * BAiocb = (BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf_desc->freeNext); /*  BufferAiocb associated with this aio */
+                    if ((struct BufferAiocb*)BAIOCB_OCCUPIED == BAiocb->BAiocbnext) { /*  ensure BAiocb is occupied */
+                        aio_successful = 0;       /*  tentatively the aio_read did not succeed   */
+                        retcode = BUF_INTENT_RC_INVALID_AIO;
+
+                        if (smgr == NULL) {
+                            if (caller_reln == NULL) {
+                                smgr = smgropen(buf_desc->tag.rnode, InvalidBackendId);
+                            } else {
+                                smgr = caller_reln->rd_smgr;
+                            }
+                        }
+
+                        /*  assert that this AIO is not using the same BufferAiocb as the one caller asked us to use */
+                        if ((index_for_aio < 0) && (index_for_aio == buf_desc->freeNext)) {
+                                   ereport(ERROR,
+                                        (errcode(ERRCODE_INTERNAL_ERROR),
+                                         errmsg("AIO control block index %d to be used by %p already in use by %p"
+                                                  ,index_for_aio, buf_desc, BAiocb->BAiocbbufh)));
+                        }
+
+                        /*    Call smgrcompleteaio only if either we want buffer or there are no dependents.
+                        **    In the other case of reject and there are dependents,
+                        **    then one of them will do it.
+                        */
+                        if (   (local_intention > 0) || (0 == BAiocb->BAiocbDependentCount)  ) {
+                            if (local_intention > 0) {
+                                /*  wait for in-progress aio and then pin
+                                **  OR  if I did not issue the aio and do not have a pin
+                                **  then pin now before waiting to ensure the buffer does not become unpinned while I wait
+                                **  we may potentially wait for io to complete
+                                **  so release buf header lock so that others may also wait here
+                                */
+                                BAiocb->BAiocbDependentCount++; /* register self as dependent  */
+                                if (PrivateRefCount[buf_desc->buf_id] == 0) {   /* if this buffer not pinned by me */
+                                    disposable_pin = true; /* this backend has pinned the buffer while waiting for aio_read to complete */
+                                    PinBuffer_Locked(buf_desc);
+                                } else {
+                                    UnlockBufHdr(buf_desc);
+                                }
+                                LWLockRelease(PartitionLock);
+
+                                smgrcompleteaio_rc = 1   /*  tell smgrcompleteaio to wait  */
+                                                   + ( BAiocb->pidOfAio == this_backend_pid ); /*  and whether I initiated the aio */
+                            } else {
+                                smgrcompleteaio_rc = 0;   /*  tell smgrcompleteaio to cancel */
+                            }
+
+                            smgrcompleteaio( smgr , (char *)&(BAiocb->BAiocbthis) , &smgrcompleteaio_rc );
+                            if ( (smgrcompleteaio_rc == 0) || (smgrcompleteaio_rc == 1) ) {
+                                  aio_successful = 1;
+                            }
+
+                            /*   statistics  */
+                            if (local_intention > 0) {
+                                if (smgrcompleteaio_rc == 0) {
+                                    /*    completed successfully and did not have to wait  */
+                                    pgBufferUsage.aio_read_ontime++;
+                                } else if (smgrcompleteaio_rc == 1) {
+                                    /*    completed successfully and did have to wait  */
+                                    pgBufferUsage.aio_read_waited++;
+                                } else {
+                                    /*  bad news   -   read failed and so buffer not usable
+                                    **  the buffer is still pinned so unpin and proceed with "not found" case
+                                    */
+                                    pgBufferUsage.aio_read_failed++;
+                                }
+
+                                /*  regain locks and handle the validity of the buffer and intention regarding it    */
+                                LWLockAcquire(PartitionLock, LW_SHARED);
+                                LockBufHdr(buf_desc);
+                                BAiocb->BAiocbDependentCount--; /* unregister self as dependent  */
+                            } else {
+                                    pgBufferUsage.aio_read_wasted++;  /*   regardless of whether aio_successful */
+                            }
+
+
+                            if (local_intention > 0) {
+                                /*  verify the buffer is still ours and has same identity
+                                **  There is one slightly tricky point here -
+                                **  if there are other dependents,   then each of them will perform this same check
+                                **  this is unavoidable as the correct setting of retcode and the BM_VALID flag
+                                **  is required by each dependent,  so we may not leave it to the last one to do it.
+                                **  It should not do any harm and easier to let them all do it than try to avoid.
+                                */
+                                if ((FREENEXT_BAIOCB_ORIGIN - (BAiocb - (BAiocbAnchr->BufferAiocbs))) == buf_desc->freeNext) { /* it is still mine */
+
+                                    if (aio_successful) {
+                                        /* validate page header.   If valid,  then mark the buffer as valid */
+                                        if (PageIsVerified((Page)(BufHdrGetBlock(buf_desc)) , ((BAiocb->BAiocbthis).aio_offset/BLCKSZ))) {
+                                            buf_desc->flags |= BM_VALID;
+                                            if (BUFFERTAGS_EQUAL(origTag , buf_desc->tag)) {
+                                                retcode = BUF_INTENT_RC_VALID;
+                                            } else {
+                                                retcode = BUF_INTENT_RC_CHANGED_TAG;
+                                            }
+                                        } else {
+                                            retcode = BUF_INTENT_RC_BADPAGE;
+                                        }
+                                    }
+                                }
+                            }
+
+                            BAiocbDependentCount_after_aio_finished = BAiocb->BAiocbDependentCount;
+
+                            /*  if no dependents,   then disconnect the BAiocb and update buffer header */
+                            if (BAiocbDependentCount_after_aio_finished == 0 ) {
+
+
+                                /*  return the BufferAiocb to the free list  */
+                                buf_desc->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR | BM_AIO_IN_PROGRESS);
+                                if (
+                                           BufReleaseAsync(BAiocb)
+                                   ) {        /*  failed ? */
+                                    BAiocb->BAiocbnext = cachedBAiocb;   /* then ...       */
+                                    cachedBAiocb = BAiocb;               /*  ... cache it  */
+                                }
+                            }
+
+                        }
+                    }
+            }
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+            /*  note whether buffer is valid before unlocking spinlock */
+            valid = ((buf_desc->flags & BM_VALID) != 0);
+
+            /*  if there was a disposable pin on entry to this function (i.e. marked in buffer flags)
+            **  then unmark it  -  refer to prologue comments talking about :
+            **    if a disposable pin is held,   then :
+            **     ...
+            **    i.e. in either case,   there is no longer a disposable pin after this function has completed.
+            */
+            if (pin_already_banked_by_me) {
+                        buf_desc->flags &= ~BM_AIO_PREFETCH_PIN_BANKED; /*   redeem the banked pin  */
+                        /*   if AIO not in progress,  then disconnect the buffer from BAiocb and/or banked pin */
+                        if (!(buf_desc->flags & BM_AIO_IN_PROGRESS)) {
+                            buf_desc->freeNext = FREENEXT_NOT_IN_LIST;      /*   and forget the bank client */
+                        }
+                        /********** for debugging   *****************
+                        else elog(LOG, "BufCheckAsync : found BM_AIO_IN_PROGRESS when redeeming banked pin on buffer %d rel=%s, blockNum=%u, flags %X refcount=%u"
+                       ,buf_desc->buf_id,relpathbackend(buf_desc->tag.rnode, InvalidBackendId, buf_desc->tag.forkNum)
+                       ,buf_desc->tag.blockNum, buf_desc->flags, buf_desc->refcount);
+                        ********** for debugging     *****************/
+            }
+
+            /*  If we are to obtain new pin, then use pin optimization  -  pin and unlock.
+            **  However,   if the caller is the same backend who issued the aio_read,
+            **  then he ought to have obtained the pin at that time and must not acquire
+            **  a "second" one since this is logically the same read -  he would have obtained
+            **  a single pin if using synchronous read and we emulate that behaviour.
+            **  Its important to understand that the caller is not aware that he already obtained a pin -
+            **  because calling PrefetchBuffer did not imply a pin -
+            **  so we must track that via the pidOfAio field in the BAiocb.
+            **  And to add one further complication :
+            **      we assume that although PrefetchBuffer pinned the buffer,
+            **      it did not increment the usage count.
+            **      (because it called PinBuffer_Locked which does not do that)
+            **      so in this case,   we must increment the usage count without double-pinning.
+            **      yes its ugly  -  and theres a goto!
+            */
+            if (   (local_intention > 0)
+                || (local_intention == BUF_INTENTION_REJECT_OBTAIN_PIN)
+               ) {
+
+                /* Make sure we will have room to remember the buffer pin */
+                ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
+
+                /*    here we really want a version of PinBuffer_Locked which updates usage count ... */
+                if (   (PrivateRefCount[buf_desc->buf_id] == 0) /*   if this buffer not previously pinned by me */
+                    || pin_already_banked_by_me                 /*   or I had a disposable pin on entry */
+                   ) {
+                    if (strategy == NULL)
+                    {
+                            if (buf_desc->usage_count < BM_MAX_USAGE_COUNT)
+                                    buf_desc->usage_count++;
+                    }
+                    else
+                    {
+                            if (buf_desc->usage_count == 0)
+                                    buf_desc->usage_count = 1;
+                    }
+		}
+
+                /*  now pin buffer unless we have a disposable */
+                if (!disposable_pin) { /* this backend neither banked pin for aio nor pinned the buffer while waiting for aio_read to complete */
+                    PinBuffer_Locked(buf_desc);
+                    goto unlocked_it;
+                }
+                else
+                /*    if this task previously issued the aio or pinned the buffer while waiting for aio_read to complete
+                **       and aio was unsuccessful,    then release the pin
+                */
+                if (     disposable_pin
+                      && (aio_successful == 0)       /*  aio_read failed ? */
+                   ) {
+                           UnpinBuffer(buf_desc, true);
+                }
+            }
+
+    unlock_buf_header:
+            UnlockBufHdr(buf_desc);
+    unlocked_it:
+#endif   /* USE_PREFETCH */
+
+            /*   now do any requested pin (if not done immediately above) or unpin/forget  */
+            if (local_intention == BUF_INTENTION_REJECT_KEEP_PIN) {
+            /*   the caller is supposed to hold a pin already so there should be nothing to do ... */
+                if (PrivateRefCount[buf_desc->buf_id] == 0) {
+                    elog(LOG, "request to keep pin on unpinned buffer %d",buf_desc->buf_id);
+
+                    valid = PinBuffer(buf_desc, strategy);
+                }
+            }
+            else
+            if (   (   (local_intention == BUF_INTENTION_REJECT_FORGET)
+                    || (local_intention == BUF_INTENTION_REJECT_NOADJUST)
+                   )
+                && (PrivateRefCount[buf_desc->buf_id] > 0) /*   if this buffer was previously pinned by me ... */
+               )  {
+
+                    if (local_intention == BUF_INTENTION_REJECT_FORGET) {
+                        UnpinBuffer(buf_desc, true); /*  ... then release the pin                   */
+                    } else
+                    if (local_intention == BUF_INTENTION_REJECT_NOADJUST) {
+                        /*   following code moved from ReleaseBuffer :
+                        **   not sure why we can't simply UnpinBuffer(buf_desc, true)
+                        **   but better leave it the way it was
+                        */
+                        ResourceOwnerForgetBuffer(CurrentResourceOwner, BufferDescriptorGetBuffer(buf_desc));
+                        if (PrivateRefCount[buf_desc->buf_id] > 1) {
+                            PrivateRefCount[buf_desc->buf_id]--;
+                        } else {
+                            UnpinBuffer(buf_desc, false);
+                        }
+                    }
+            }
+
+            /*    if retcode has not been set to one of the unusual conditions
+            **        namely failed header validity or tag changed
+            **    then the setting of valid takes precedence
+            **    over whatever retcode may be currently set to.
+            */
+            if ( ( (retcode == BUF_INTENT_RC_INVALID_NO_AIO) || (retcode == BUF_INTENT_RC_INVALID_AIO) ) && valid) {
+                   retcode = BUF_INTENT_RC_VALID;
+            } else
+            if ((retcode == BUF_INTENT_RC_VALID) && (!valid)) {
+                   if (aio_successful == -1) { /* aio not attempted */
+                       retcode = BUF_INTENT_RC_INVALID_NO_AIO;
+                   } else {
+                       retcode = BUF_INTENT_RC_INVALID_AIO;
+                   }
+            }
+
+            return retcode;
+}
--- src/backend/storage/buffer/buf_init.c.orig	2014-06-08 11:26:30.000000000 -0400
+++ src/backend/storage/buffer/buf_init.c	2014-06-08 21:59:36.776096588 -0400
@@ -13,15 +13,89 @@
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
+#include <sys/types.h> 
+#include <unistd.h>
 
 #include "storage/bufmgr.h"
 #include "storage/buf_internals.h"
-
+#include <stdlib.h> /* for getenv() */
+#include <errno.h> /* for strtoul() */
 
 BufferDesc *BufferDescriptors;
 char	   *BufferBlocks;
-int32	   *PrivateRefCount;
+int32	   *PrivateRefCount;       /*  array of counts per buffer of how many times this task has pinned this buffer */
+
+volatile struct BAiocbAnchor *BAiocbAnchr = (struct BAiocbAnchor *)0;  /*  anchor for all control blocks pertaining to aio  */
+
+int CountInuseBAiocbs(void);     /*  keep compiler happy */
+void ReportFreeBAiocbs(void);    /*  keep compiler happy */
+
+extern int	MaxConnections;  /*  max number of client connections which postmaster will allow  */
+int numBufferAiocbs = 0;         /*  total number of  BufferAiocbs in pool (0 <=> no async io) */
+int hwmBufferAiocbs = 0;         /*  high water mark of in-use  BufferAiocbs in pool
+                                 **  (not required to be accurate, kindly maintained for us somehow by postmaster)
+                                 */
 
+#ifdef USE_PREFETCH
+unsigned int prefetch_dbOid = 0; /*  database oid of relations on which prefetching to be done - 0 means all */
+unsigned int prefetch_bitmap_scans = 1; /*  boolean whether to prefetch bitmap heap scans        */
+unsigned int prefetch_heap_scans = 0;   /*  boolean whether to prefetch non-bitmap heap scans    */
+unsigned int prefetch_sequential_index_scans = 0;  /*  boolean whether to prefetch sequential-access non-bitmap index scans  */
+unsigned int prefetch_index_scans = 256;  /*  boolean whether to prefetch non-bitmap index scans also numeric size of pfch_list  */
+unsigned int prefetch_btree_heaps = 1;  /*  boolean whether to prefetch heap pages in _bt_next for non-bitmap index scans   */
+#endif /* USE_PREFETCH */
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+int maxGetBAiocbTries = 1;       /*  max times we will try to get a free BufferAiocb */
+int maxRelBAiocbTries = 1;       /*  max times we will try to release a BufferAiocb back to freelist */
+
+/*  locking protocol for manipulating the BufferAiocbs and FreeBAiocbs list :
+**    1.    ownership of a BufferAiocb :
+**          to gain ownership of a BufferAiocb, a task must
+**          EITHER    remove it from FreeBAiocbs (it is now temporary owner and no other task can find it)
+**                    if decision is to attach it to a buffer descriptor header, then
+**                       .   lock the buffer descriptor header
+**                       .   check  NOT flags & BM_AIO_IN_PROGRESS
+**                       .   attach to buffer descriptor header
+**                       .   increment the BufferAiocb.dependent_count
+**                       .   unlock the buffer descriptor header
+**                     and ownership scope is from lock to unlock
+***          OR        locate it by dereferencing the pointer in a buffer descriptor,
+**                    in which case :
+**                       .   lock the buffer descriptor header
+**                       .   check  flags & BM_AIO_IN_PROGRESS
+**                       .   increment the BufferAiocb.dependent_count
+**                       .   if decision is to return to FreeBAiocbs,
+**                           then   (with buffer descriptor header still locked)
+**                                  .  turn off BM_AIO_IN_PROGRESS
+**                       .   IF        the BufferAiocb.dependent_count == 1 (I am sole dependent)
+**                       .   THEN
+**                       .       .  decrement the BufferAiocb.dependent_count
+**                               .  return to FreeBAiocbs (see below)
+**                       .   unlock the buffer descriptor header
+**                     and ownership scope is from lock to either return to FreeBAiocbs or unlock
+**    2.    adding and removing from FreeBAiocbs :
+**      two alternative methods - controlled by conditional macro definition LOCK_BAIOCB_FOR_GET_REL
+**       2.1 LOCK_BAIOCB_FOR_GET_REL is defined - use a lock
+**          .   lock BufFreelistLock exclusive
+**          .   add / remove from FreeBAiocbs
+**          .   unlock BufFreelistLock exclusive
+**          advantage of this method  -  never fails to add or remove
+**       2.2  LOCK_BAIOCB_FOR_GET_REL is not defined - use compare_and_swap
+**          .   retrieve the current Freelist pointer and validate
+**          .   compare_and_swap on/off the FreeBAiocb list
+**          .   unlock BufFreelistLock exclusive
+**          advantage of this method  -  never waits
+**          to avoid losing a FreeBAiocbs,   save it in a process-local cache and reuse
+*/
+#else   /*  not USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+        /*   this dummy structure is to ensure that references to these fields in other bufmgr runtime code
+        **   that is not conditionally ifdefd on USE_AIO_ATOMIC_BUILTIN_COMP_SWAP compiles and runs correctly
+        */
+        struct BAiocbAnchor dummy_BAiocbAnchr = { (struct BufferAiocb*)0 , (struct BufferAiocb*)0 };
+int maxGetBAiocbTries = -1;       /*  max times we will try to get a free BufferAiocb */
+int maxRelBAiocbTries = -1;       /*  max times we will try to release a BufferAiocb back to freelist */
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * Data Structures:
@@ -73,7 +147,16 @@ void
 InitBufferPool(void)
 {
 	bool		foundBufs,
-				foundDescs;
+				foundDescs
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+              , foundAiocbs
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+          ;
+#if defined(USE_PREFETCH) || defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP)
+        char *envvarpointer = (char *)0;  /*  might point to an environment variable string */
+        char *charptr;
+#endif /* USE_PREFETCH */
+
 
 	BufferDescriptors = (BufferDesc *)
 		ShmemInitStruct("Buffer Descriptors",
@@ -83,6 +166,134 @@ InitBufferPool(void)
 		ShmemInitStruct("Buffer Blocks",
 						NBuffers * (Size) BLCKSZ, &foundBufs);
 
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        BAiocbAnchr = (struct BAiocbAnchor *)0; /*  anchor for all control blocks pertaining to aio  */
+        if (max_async_io_prefetchers < 0) {  /*  negative value indicates to initialize to something sensible during buf_init */
+            max_async_io_prefetchers = MaxConnections/6;  /*  default allows for average of MaxConnections/6 concurrent prefetchers  - reasonable ??? */
+        }
+
+        if ((target_prefetch_pages > 0) && (max_async_io_prefetchers > 0)) {
+            int ix;
+            volatile struct BufferAiocb *BufferAiocbs;
+            volatile struct BufferAiocb * volatile FreeBAiocbs;
+
+            numBufferAiocbs = (target_prefetch_pages*max_async_io_prefetchers);  /*  target_prefetch_pages per prefetcher */
+            BAiocbAnchr = (struct BAiocbAnchor *)
+		ShmemInitStruct("Buffer Aiocbs",
+                          sizeof(struct BAiocbAnchor) + (numBufferAiocbs * sizeof(struct BufferAiocb)), &foundAiocbs);
+            if (BAiocbAnchr) {
+                BufferAiocbs = BAiocbAnchr->BufferAiocbs = (struct BufferAiocb*)(((char *)BAiocbAnchr) + sizeof(struct BAiocbAnchor));
+                FreeBAiocbs = (struct BufferAiocb*)0;
+                for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                    (BufferAiocbs+ix)->BAiocbnext = FreeBAiocbs;   /*  init the free list,  last one -> 0  */
+                    (BufferAiocbs+ix)->BAiocbbufh = (struct sbufdesc*)0;
+                    (BufferAiocbs+ix)->BAiocbDependentCount = 0;
+                    (BufferAiocbs+ix)->pidOfAio = 0;
+                    FreeBAiocbs = (BufferAiocbs+ix);
+
+                }
+                BAiocbAnchr->FreeBAiocbs = FreeBAiocbs;
+                envvarpointer = getenv("PG_MAX_GET_BAIOCB_TRIES");
+                if (    (envvarpointer != (char *)0)
+                     && (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   )
+                   ) {
+                      maxGetBAiocbTries = strtol(envvarpointer, 0, 10);
+                }
+                envvarpointer = getenv("PG_MAX_REL_BAIOCB_TRIES");
+                if (    (envvarpointer != (char *)0)
+                     && (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   )
+                   ) {
+                      maxRelBAiocbTries = strtol(envvarpointer, 0, 10);
+                }
+            }
+        }
+#else   /*  not USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+        /*   this dummy structure is to ensure that references to these fields in other bufmgr runtime code
+        **   that is not conditionally ifdefd on USE_AIO_ATOMIC_BUILTIN_COMP_SWAP compiles and runs correctly
+        */
+        BAiocbAnchr = &dummy_BAiocbAnchr;
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+
+#ifdef USE_PREFETCH
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_BITMAP");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_bitmap_scans = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+                 prefetch_bitmap_scans = 1;
+             }
+        }
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_ISCAN");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_index_scans = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+                 prefetch_index_scans = 1;
+             } else
+             if (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   ) {
+                 prefetch_index_scans = strtol(envvarpointer, &charptr, 10);
+                 if (charptr && (',' == *charptr)) {   /*  optional sequential prefetch in index scans */
+					 charptr++;        /*   following the comma ... */
+					 if ( ('Y' == *charptr) || ('y' == *charptr) || ('1' == *charptr) ) {
+                         prefetch_sequential_index_scans = 1;
+					 }
+				 }
+             }
+             /*  if prefeching for ISCAN,  then we require size of pfch_list to be at least target_prefetch_pages */
+             if (   (prefetch_index_scans > 0)
+                 && (prefetch_index_scans < target_prefetch_pages)
+                ) {
+                 prefetch_index_scans = target_prefetch_pages;
+             }
+        }
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_BTREE");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_btree_heaps = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+                 prefetch_btree_heaps = 1;
+             }
+        }
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_HEAP");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_heap_scans = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+              prefetch_heap_scans = 1;
+             }
+        }
+        envvarpointer = getenv("PG_PREFETCH_DBOID");
+        if (    (envvarpointer != (char *)0)
+             && (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   )
+           ) {
+              errno = 0;   /*  required in order to distinguish error from 0 */
+              prefetch_dbOid = (unsigned int)strtoul((const char *)envvarpointer, 0, 10);
+              if (errno) {
+                  prefetch_dbOid = 0;
+              }
+        }
+        elog(LOG, "prefetching initialised with target_prefetch_pages= %d "
+                  ", max_async_io_prefetchers= %d implying aio concurrency= %d "
+                  ", prefetching_for_bitmap= %s "
+                  ", prefetching_for_heap= %s "
+                  ", prefetching_for_iscan= %d with sequential_index_page_prefetching= %s "
+                  ", prefetching_for_btree= %s"
+                   ,target_prefetch_pages ,max_async_io_prefetchers ,numBufferAiocbs
+                   ,(prefetch_bitmap_scans ? "Y" : "N")
+                   ,(prefetch_heap_scans ? "Y" : "N")
+                   ,prefetch_index_scans
+                   ,(prefetch_sequential_index_scans ? "Y" : "N")
+                   ,(prefetch_btree_heaps ? "Y" : "N")
+            );
+#endif /* USE_PREFETCH */
+
+
 	if (foundDescs || foundBufs)
 	{
 		/* both should be present or neither */
@@ -176,3 +387,82 @@ BufferShmemSize(void)
 
 	return size;
 }
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*     imprecise count of number of in-use BAiocbs at any time
+ *     we scan the array read-only without latching so are subject to unstable result
+ *     (but since the array is in well-known contiguous storage,
+ *     we are not subject to segmentation violation)
+ *     This function may be called at any time and just does its best
+ *     return the count of what we counted.
+ */
+int
+CountInuseBAiocbs(void)
+{
+        volatile struct BufferAiocb *BAiocb;
+        int count = 0;
+        int ix;
+
+
+        if (BAiocbAnchr != (struct BAiocbAnchor *)0 ) {
+            BAiocb = BAiocbAnchr->BufferAiocbs;             /*   start of list */
+
+            for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                    if ((struct BufferAiocb*)BAIOCB_OCCUPIED == (BAiocb+ix)->BAiocbnext) {   /* not on freelist ? */
+                        count++;
+                    }
+            }
+        }
+        return count;
+}
+
+/*
+ * report how many free BAiocbs at shutdown
+ * DO NOT call this while backends are actively working!!
+ * this report is useful when compare_and_swap method used (see above)
+ * as it can be used to deduce how many BAiocbs were in process-local caches -
+ * (original_number_on_freelist_at_startup - this_reported_number_at_shutdown)
+ */
+void
+ReportFreeBAiocbs(void)
+{
+        volatile struct BufferAiocb *BAiocb;
+        volatile struct BufferAiocb *BufferAiocbs;
+        int count = 0;
+        int fx , ix;
+
+
+        if (BAiocbAnchr != (struct BAiocbAnchor *)0 ) {
+            BAiocb = BAiocbAnchr->FreeBAiocbs;             /*   start of free list */
+            BufferAiocbs = BAiocbAnchr->BufferAiocbs;
+
+            for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                    (BufferAiocbs+ix)->BAiocbDependentCount = 0;  /* use this as marker for finding it on freelist */
+            }
+            for (fx = (numBufferAiocbs-1);  ( (fx>=0) &&  ( BAiocb != (struct BufferAiocb*)0 ) );  fx--) {
+                    
+                    /*  check if it is a valid BufferAiocb */
+                    for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                        if ((BufferAiocbs+ix) == BAiocb) { /*  is it this one ? */
+                             break;
+                        }
+                    }
+                    if (ix >= 0) {
+                        if (BAiocb->BAiocbDependentCount) {   /* seen it already ? */
+                            elog(LOG, "ReportFreeBAiocbs closed cycle on AIO control block freelist %p"
+                                          ,BAiocb);
+                            fx = 0; /* give up at this point */
+                        }
+                        BAiocb->BAiocbDependentCount = 1;  /* use this as marker for finding it on freelist */
+                        count++;
+                        BAiocb = BAiocb->BAiocbnext;
+                    } else {
+                        elog(LOG, "ReportFreeBAiocbs invalid item on AIO control block freelist %p"
+                                          ,BAiocb);
+                        fx = 0; /* give up at this point */
+                    }
+            }
+        }
+        elog(LOG, "ReportFreeBAiocbs AIO control block list : poolsize= %d  in-use-hwm= %d  final-free= %d" ,numBufferAiocbs , hwmBufferAiocbs , count);
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
--- src/backend/storage/smgr/md.c.orig	2014-06-08 11:26:30.000000000 -0400
+++ src/backend/storage/smgr/md.c	2014-06-08 21:59:36.804096637 -0400
@@ -647,6 +647,62 @@ mdprefetch(SMgrRelation reln, ForkNumber
 }
 
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*
+ *	mdinitaio() --  init the aio subsystem max number of threads and max number of requests
+ */
+void
+mdinitaio(int max_aio_threads, int max_aio_num)
+{
+     FileInitaio( max_aio_threads, max_aio_num );
+}
+
+/*
+ *	mdstartaio() -- start aio read of the specified block of a relation
+ */
+void
+mdstartaio(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum , char *aiocbp , int *retcode )
+{
+#ifdef USE_PREFETCH
+	off_t		seekpos;
+	MdfdVec    *v;
+        int local_retcode;
+
+	v = _mdfd_getseg(reln, forknum, blocknum, false, EXTENSION_FAIL);
+
+	seekpos = (off_t) BLCKSZ *(blocknum % ((BlockNumber) RELSEG_SIZE));
+
+	Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+
+	local_retcode = FileStartaio(v->mdfd_vfd, seekpos, BLCKSZ , aiocbp);
+	if (retcode) {
+            *retcode = local_retcode;
+        }
+#endif   /* USE_PREFETCH */
+}
+
+
+/*
+ *	mdcompleteaio() -- complete aio read of the specified block of a relation
+ *      on entry, *inoutcode should indicate :
+ *           .  non-0  <=>   check if complete and wait if not
+ *           .  0      <=>   cancel io immediately
+ */
+void
+mdcompleteaio( char *aiocbp , int *inoutcode )
+{
+#ifdef USE_PREFETCH
+        int local_retcode;
+
+	local_retcode = FileCompleteaio(aiocbp, (inoutcode ? *inoutcode : 0));
+	if (inoutcode) {
+            *inoutcode = local_retcode;
+        }
+#endif   /* USE_PREFETCH */
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+
 /*
  *	mdread() -- Read the specified block from a relation.
  */
--- src/backend/storage/smgr/smgr.c.orig	2014-06-08 11:26:30.000000000 -0400
+++ src/backend/storage/smgr/smgr.c	2014-06-08 21:59:36.828096679 -0400
@@ -49,6 +49,12 @@ typedef struct f_smgr
 						 BlockNumber blocknum, char *buffer, bool skipFsync);
 	void		(*smgr_prefetch) (SMgrRelation reln, ForkNumber forknum,
 											  BlockNumber blocknum);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+	void		(*smgr_initaio) (int max_aio_threads, int max_aio_num);
+	void		(*smgr_startaio) (SMgrRelation reln, ForkNumber forknum,
+											  BlockNumber blocknum , char *aiocbp , int *retcode );
+	void		(*smgr_completeaio) ( char *aiocbp , int *inoutcode );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 	void		(*smgr_read) (SMgrRelation reln, ForkNumber forknum,
 										  BlockNumber blocknum, char *buffer);
 	void		(*smgr_write) (SMgrRelation reln, ForkNumber forknum,
@@ -66,7 +72,11 @@ typedef struct f_smgr
 static const f_smgr smgrsw[] = {
 	/* magnetic disk */
 	{mdinit, NULL, mdclose, mdcreate, mdexists, mdunlink, mdextend,
-		mdprefetch, mdread, mdwrite, mdnblocks, mdtruncate, mdimmedsync,
+		mdprefetch
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                        ,mdinitaio, mdstartaio, mdcompleteaio
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+              , mdread, mdwrite, mdnblocks, mdtruncate, mdimmedsync,
 		mdpreckpt, mdsync, mdpostckpt
 	}
 };
@@ -612,6 +622,35 @@ smgrprefetch(SMgrRelation reln, ForkNumb
 	(*(smgrsw[reln->smgr_which].smgr_prefetch)) (reln, forknum, blocknum);
 }
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*
+ *	smgrinitaio() -- initialize the aio subsystem max number of threads and max number of requests
+ */
+void
+smgrinitaio(int max_aio_threads, int max_aio_num)
+{
+	(*(smgrsw[0].smgr_initaio)) ( max_aio_threads, max_aio_num );
+}
+
+/*
+ *	smgrstartaio() -- Initiate aio read of the specified block of a relation.
+ */
+void
+smgrstartaio(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum , char *aiocbp , int *retcode )
+{
+	(*(smgrsw[reln->smgr_which].smgr_startaio)) (reln, forknum, blocknum , aiocbp , retcode );
+}
+
+/*
+ *	smgrcompleteaio() -- Complete aio read of the specified block of a relation.
+ */
+void
+smgrcompleteaio(SMgrRelation reln,  char *aiocbp , int *inoutcode )
+{
+	(*(smgrsw[reln->smgr_which].smgr_completeaio)) ( aiocbp , inoutcode );
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 /*
  *	smgrread() -- read a particular block from a relation into the supplied
  *				  buffer.
--- src/backend/storage/file/fd.c.orig	2014-06-08 11:26:30.000000000 -0400
+++ src/backend/storage/file/fd.c	2014-06-08 21:59:36.856096727 -0400
@@ -77,6 +77,9 @@
 #include "utils/guc.h"
 #include "utils/resowner_private.h"
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+#include "aio.h"
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * We must leave some file descriptors free for system(), the dynamic loader,
@@ -1239,6 +1242,10 @@ FileClose(File file)
  * We could add an implementation using libaio in the future; but note that
  * this API is inappropriate for libaio, which wants to have a buffer provided
  * to read into.
+ * Also note that a new, different implementation of asynchronous prefetch
+ * using librt,  not libaio,  is provided by the two functions following this one,
+ * FileStartaio and FileCompleteaio.   These also require to have a buffer provided
+ * to read into,  which the new async_io support provides.
  */
 int
 FilePrefetch(File file, off_t offset, int amount)
@@ -1266,6 +1273,145 @@ FilePrefetch(File file, off_t offset, in
 #endif
 }
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*
+ * FileInitaio - initialize the aio subsystem max number of threads and max number of requests
+ *  input parms
+ *  max_aio_threads;    maximum number of threads
+ *  max_aio_num;        maximum number of concurrent aio read requests
+ *
+ *  on linux, man page for the librt implemenation of aio_init() says :
+ *         This function is a GNU extension.
+ *  If your posix aio does not have it,   then add the following line to 
+ *        src/include/pg_config_manual.h
+ *    #define DONT_HAVE_AIO_INIT
+ *  to render it as a no-op
+ */
+void
+FileInitaio(int max_aio_threads, int max_aio_num )
+{
+#ifndef DONT_HAVE_AIO_INIT
+    struct aioinit aioinit_struct;  /*  structure to pass to aio_init */
+
+    aioinit_struct.aio_threads = max_aio_threads; /*     maximum number of threads */
+    aioinit_struct.aio_num = max_aio_num;         /*     maximum number of concurrent aio read requests */
+    aioinit_struct.aio_idle_time = 1;             /*     we dont want to alter this but aio_init does not ignore it so set to the default */
+    aio_init(&aioinit_struct);
+#endif  /* ndef DONT_HAVE_AIO_INIT */
+    return;
+}
+
+/*
+ * FileStartaio - initiate asynchronous read of a given range of the file.
+ * The logical seek position is unaffected.
+ *
+ * use standard posix aio (librt)
+ *  ASSUME   BufferAiocb.aio_buf already set to -> buffer by caller
+ *  return 0 if successfully started,  else non-zero
+ */
+int
+FileStartaio(File file, off_t offset, int amount , char *aiocbp )
+{
+	int	returnCode;
+        struct aiocb *my_aiocbp = (struct aiocb *)aiocbp;
+
+	Assert(FileIsValid(file));
+
+	DO_DB(elog(LOG, "FileStartaio: %d (%s) " INT64_FORMAT " %d",
+			   file, VfdCache[file].fileName,
+			   (int64) offset, amount));
+
+	returnCode = FileAccess(file);
+	if (returnCode >= 0) {
+
+            my_aiocbp->aio_fildes = VfdCache[file].fd;
+            my_aiocbp->aio_lio_opcode = LIO_READ;
+            my_aiocbp->aio_nbytes = amount;
+            my_aiocbp->aio_offset = offset;
+            returnCode = aio_read(my_aiocbp);
+        }
+
+	return returnCode;
+}
+
+/*
+ * FileCompleteaio - complete asynchronous aio read
+ * normal_wait indicates whether to cancel or wait -
+ *                 0 <=> cancel
+ *                 1 <=> wait by polling the aiocb
+ *                 2 <=> wait by suspending on the aiocb
+ *
+ * use standard posix aio (librt)
+ *  return 0 if successfull and did not have to wait,
+ *         1 if successfull and had to wait,
+ *    else x'ff'
+ */
+int
+FileCompleteaio( char *aiocbp , int normal_wait )
+{
+	int	returnCode;
+	int	aio_errno;
+        struct aiocb *my_aiocbp = (struct aiocb *)aiocbp;
+        const struct aiocb const*cblist[1];
+        int fd;
+        struct timespec my_timeout = { 0 , 10000 };
+        struct timespec *suspend_timeout_P; /*  the timeout actually used depending on normal_wait */
+        int max_polls;
+
+        fd = my_aiocbp->aio_fildes;
+        cblist[0] = my_aiocbp;
+        returnCode = aio_errno = aio_error(my_aiocbp);
+        /* note that aio_error returns 0 if op already completed successfully */
+
+        /*  first handle normal case of waiting for op to complete  */
+        if (normal_wait) {
+            /*   if told not to poll,   then specify no timeout  */
+            suspend_timeout_P = (normal_wait == 1 ? &my_timeout : (struct timespec *)0);
+            while (aio_errno == EINPROGRESS) {
+                max_polls = 256;
+                my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                returnCode = aio_suspend(cblist , 1 , suspend_timeout_P);
+                while (    (returnCode < 0) && (max_polls-- > 0)
+                        && ((EAGAIN == errno) || (EINTR == errno))
+                      ) {
+                    my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                    returnCode = aio_suspend(cblist , 1 , suspend_timeout_P);
+                }
+                returnCode = aio_errno = aio_error(my_aiocbp);
+                /*  now return_code is from aio_error  */
+                if (returnCode == 0) {
+                    returnCode = 1;    /*  successful but had to wait */
+                }
+            }
+            if (aio_errno) {
+                elog(LOG, "FileCompleteaio: %d %d", fd, returnCode);
+                returnCode = 0xff; /*  unsuccessful */
+            }
+        } else {
+            if (aio_errno == EINPROGRESS) {
+                do {
+                        max_polls = 256;
+                        my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                        returnCode = aio_cancel(fd, my_aiocbp);
+                        while ((returnCode == AIO_NOTCANCELED) && (max_polls-- > 0)) {
+                            my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                            returnCode = aio_cancel(fd, my_aiocbp);
+                        }
+                    returnCode = aio_errno = aio_error(my_aiocbp);
+                } while (aio_errno == EINPROGRESS);
+                returnCode = 0xff; /*  unsuccessful */
+            }
+            if (returnCode != 0)
+                returnCode = 0xff; /*  unsuccessful */
+        }
+
+	DO_DB(elog(LOG, "FileCompleteaio: %d %d",
+			   fd, returnCode));
+
+	return returnCode;
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 int
 FileRead(File file, char *buffer, int amount)
 {
--- src/backend/storage/lmgr/proc.c.orig	2014-06-08 11:26:30.000000000 -0400
+++ src/backend/storage/lmgr/proc.c	2014-06-08 21:59:36.888096782 -0400
@@ -52,6 +52,7 @@
 #include "utils/timeout.h"
 #include "utils/timestamp.h"
 
+extern pid_t this_backend_pid;     /*   pid of this backend */
 
 /* GUC variables */
 int			DeadlockTimeout = 1000;
@@ -361,6 +362,7 @@ InitProcess(void)
 	MyPgXact->xid = InvalidTransactionId;
 	MyPgXact->xmin = InvalidTransactionId;
 	MyProc->pid = MyProcPid;
+        this_backend_pid = getpid();    /*    pid of this backend */
 	/* backendId, databaseId and roleId will be filled in later */
 	MyProc->backendId = InvalidBackendId;
 	MyProc->databaseId = InvalidOid;
--- src/backend/access/heap/heapam.c.orig	2014-06-08 11:26:29.000000000 -0400
+++ src/backend/access/heap/heapam.c	2014-06-08 21:59:36.932096859 -0400
@@ -71,6 +71,28 @@
 #include "utils/syscache.h"
 #include "utils/tqual.h"
 
+#ifdef USE_PREFETCH
+#include <sys/types.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+#include "executor/instrument.h"
+
+extern unsigned int prefetch_dbOid; /*  database oid of relations on which prefetching to be done - 0 means all */
+extern unsigned int prefetch_heap_scans; /*  boolean whether to prefetch non-bitmap heap scans         */
+
+/*  special values for scan->rs_prefetch_target indicating as follows :               */
+#define PREFETCH_MAYBE 0xffffffff      /*   prefetch permitted but not yet in effect  */
+#define PREFETCH_DISABLED 0xfffffffe   /*   prefetch disabled and not permitted       */
+/*  PREFETCH_WRAP_POINT indicates a pretcher who has reached the point where the scan would wrap -
+**  at this point the prefetcher runs on the spot until scan catches up.
+**  This *must* be < maximum valid setting of target_prefetch_pages aka effective_io_concurrency.
+*/
+#define PREFETCH_WRAP_POINT 0x0fffffff
+
+#endif   /* USE_PREFETCH */
+
 
 /* GUC variable */
 bool		synchronize_seqscans = true;
@@ -115,6 +137,8 @@ static XLogRecPtr log_heap_new_cid(Relat
 static HeapTuple ExtractReplicaIdentity(Relation rel, HeapTuple tup, bool key_modified,
 					   bool *copy);
 
+static void heap_unread_add(HeapScanDesc scan, BlockNumber blockno);
+static void heap_unread_subtract(HeapScanDesc scan, BlockNumber blockno);
 
 /*
  * Each tuple lock mode has a corresponding heavyweight lock, and one or two
@@ -292,9 +316,149 @@ initscan(HeapScanDesc scan, ScanKey key,
 	 * Currently, we don't have a stats counter for bitmap heap scans (but the
 	 * underlying bitmap index scans will be counted).
 	 */
-	if (!scan->rs_bitmapscan)
+#ifdef USE_PREFETCH
+        /*    by default,  no prefetching on any scan  */
+        scan->rs_prefetch_target = PREFETCH_DISABLED;  /*  tentatively disable  */
+        scan->rs_pfchblock = 0; /*  scanner will reset this to be ahead of scan */
+        scan->rs_Unread_Pfetched_base = (BlockNumber *)0; /*  list of prefetched but unread blocknos */
+        scan->rs_Unread_Pfetched_next = 0; /*  next unread blockno */
+        scan->rs_Unread_Pfetched_count = 0; /* number of valid unread blocknos */
+#endif   /* USE_PREFETCH */
+	if (!scan->rs_bitmapscan) {
+
 		pgstat_count_heap_scan(scan->rs_rd);
+#ifdef USE_PREFETCH
+                /*    bitmap scans do their own prefetching -
+                **    for others,  set up prefetching now
+                */
+                if (    prefetch_heap_scans
+                     && (target_prefetch_pages > 0)
+                     &&	(!RelationUsesLocalBuffers(scan->rs_rd))
+                   ) {
+                      /*   prefetch_dbOid may be set to a database Oid to specify only prefetch in that db */
+                      if (    (   (prefetch_dbOid > 0)
+                               && (prefetch_dbOid == scan->rs_rd->rd_node.dbNode)
+                              )
+                          ||  (prefetch_dbOid == 0)
+                         ) {
+                          scan->rs_prefetch_target = PREFETCH_MAYBE;    /*  permitted but let the scan decide */
+                      }
+                      else {
+                      }
+                }
+#endif   /* USE_PREFETCH */
+        }
+}
+
+/* add this blockno to list of prefetched and unread blocknos
+** use the one identified by the (next+count|modulo circumference) index if it is unused,
+** else search for the first available slot if there is one,
+** else error.
+*/
+static void
+heap_unread_add(HeapScanDesc scan, BlockNumber blockno)
+{
+      BlockNumber *available_P;   /*  where to store new blockno */
+      unsigned int Unread_Pfetched_index = scan->rs_Unread_Pfetched_next
+                                         + scan->rs_Unread_Pfetched_count; /* index of next unused slot */
+
+      /*  caller is not supposed to pass InvalidBlockNumber but check anyway */
+      if (blockno != InvalidBlockNumber) {
+
+		  /*  ensure there is some room somewhere   */
+		  if (scan->rs_Unread_Pfetched_count < target_prefetch_pages) {
+
+			  /*  try the "next+count" one */
+			  if (Unread_Pfetched_index >= target_prefetch_pages) {
+				  Unread_Pfetched_index -= target_prefetch_pages;  /* modulo circumference */
+			  }
+			  available_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);   /*  where to store new blockno */
+			  if (*available_P == InvalidBlockNumber) { /* unused */
+				  goto store_blockno;
+			  } else {
+                  /*  slow-search the entire list  */
+				  for (Unread_Pfetched_index = 0; Unread_Pfetched_index < target_prefetch_pages; Unread_Pfetched_index++) {
+					  available_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);   /*  where to store new blockno */
+					  if (*available_P == InvalidBlockNumber) { /* unused */
+                          /*  before storing this blockno,
+                          **  since the next pointer did not locate an unused slot,
+                          **  set it to one which is more likely to be so for the next time
+                          */
+                          scan->rs_Unread_Pfetched_next = Unread_Pfetched_index;
+						  goto store_blockno;
+					  }
+				  }
+			  }
+		  }
+
+          /*  if we reach here,    either there was no available slot
+          **  or we thought there was one and didn't find any  --
+          */
+  		  ereport(NOTICE,
+			  (errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("heap_unread_add overflowed list cannot add blockno %d", blockno)));
+
+  		  return;
+
+    store_blockno:
+		  *available_P = blockno;
+		  scan->rs_Unread_Pfetched_count++;  /*  update count */
+
+	  }
+
+    return;
+}
+
+/* remove specified blockno from list of prefetched and unread blocknos.
+** Usually this will be found at the rs_Unread_Pfetched_next item -
+** else search for it.    If not found,   inore it  -  no error results.
+*/
+static void
+heap_unread_subtract(HeapScanDesc scan, BlockNumber blockno)
+{
+      unsigned int Unread_Pfetched_index = scan->rs_Unread_Pfetched_next; /* index of next unread blockno */
+      BlockNumber *candidate_P;   /*  location of callers blockno - maybe */
+      BlockNumber nextUnreadPfetched;
+
+      /*  caller is not supposed to pass InvalidBlockNumber but check anyway */
+      if (    (blockno != InvalidBlockNumber)
+		   && ( scan->rs_Unread_Pfetched_count > 0 )   /*  if the list is not empty  */
+         ) {
+
+			  /*  take modulo of the circumference.
+			  **  actually rs_Unread_Pfetched_next should never exceed the circumference but check anyway.
+			  */
+			  if (Unread_Pfetched_index >= target_prefetch_pages) {
+				  Unread_Pfetched_index  -= target_prefetch_pages;
 }
+			  candidate_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);
+			  nextUnreadPfetched = *candidate_P;
+
+			  if ( nextUnreadPfetched == blockno ) {
+				  goto remove_blockno;
+			  } else {
+                  /*  slow-search the entire list  */
+				  for (Unread_Pfetched_index = 0; Unread_Pfetched_index < target_prefetch_pages; Unread_Pfetched_index++) {
+					  candidate_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);   /*  where to store new blockno */
+					  if (*candidate_P == blockno) { /* unused */
+						  goto remove_blockno;
+					  }
+				  }
+			  }
+
+    remove_blockno:
+			  *candidate_P = InvalidBlockNumber;
+
+			  scan->rs_Unread_Pfetched_next = (Unread_Pfetched_index+1);  /*  update next pfchd unread */
+			  if (scan->rs_Unread_Pfetched_next >= target_prefetch_pages) {
+					  scan->rs_Unread_Pfetched_next = 0;
+			  }
+			  scan->rs_Unread_Pfetched_count--;  /*  update count */
+	  }
+
+      return;
+}
+
 
 /*
  * heapgetpage - subroutine for heapgettup()
@@ -304,7 +468,7 @@ initscan(HeapScanDesc scan, ScanKey key,
  * which tuples on the page are visible.
  */
 static void
-heapgetpage(HeapScanDesc scan, BlockNumber page)
+heapgetpage(HeapScanDesc scan, BlockNumber page , BlockNumber prefetchHWM)
 {
 	Buffer		buffer;
 	Snapshot	snapshot;
@@ -314,6 +478,10 @@ heapgetpage(HeapScanDesc scan, BlockNumb
 	OffsetNumber lineoff;
 	ItemId		lpp;
 	bool		all_visible;
+#ifdef USE_PREFETCH
+	int             PrefetchBufferRc;  /*   indicates whether requested prefetch block already in a buffer and if pin count on buffer has been increased */
+#endif   /* USE_PREFETCH */
+
 
 	Assert(page < scan->rs_nblocks);
 
@@ -336,6 +504,98 @@ heapgetpage(HeapScanDesc scan, BlockNumb
 									   RBM_NORMAL, scan->rs_strategy);
 	scan->rs_cblock = page;
 
+#ifdef USE_PREFETCH
+
+        heap_unread_subtract(scan, page);
+
+        /*    maybe prefetch some pages  starting with rs_pfchblock */
+        if (scan->rs_prefetch_target >= 0) {       /*   prefetching enabled on this scan ?                         */
+            int next_block_to_be_read = (page+1);  /*   next block to be read = lowest possible prefetchable block */
+            int num_to_pfch_this_time;             /*   eventually holds the number of blocks to prefetch now      */
+            int prefetchable_range;                /*   size of the area ahead of the current prefetch position    */
+
+            /*  check if prefetcher reached wrap point and the scan has now wrapped */
+            if (  (page == 0) && (scan->rs_prefetch_target == PREFETCH_WRAP_POINT)  ) {
+                scan->rs_prefetch_target = 1;
+                scan->rs_pfchblock = next_block_to_be_read;
+            } else
+            if (scan->rs_pfchblock < next_block_to_be_read) {
+                scan->rs_pfchblock = next_block_to_be_read; /* next block to be prefetched must be ahead of one we just read */
+            }
+
+            /* now we know where we would start prefetching -
+            ** next question   -  if this is a sync scan,  ensure we do not prefetch behind the HWM
+            ** debatable whether to require strict inequality or >=  -   >= works better in practice
+            */
+            if ( (!scan->rs_syncscan) || (scan->rs_pfchblock >= prefetchHWM) ) {
+
+                /* now we know where we will start prefetching -
+                ** next question   -  how many?
+                ** apply two limits :
+                **  1.   target prefetch distance
+                **  2.   number of available blocks ahead of us
+                */
+
+                /*  1.   target prefetch distance   */
+                num_to_pfch_this_time = next_block_to_be_read + scan->rs_prefetch_target; /* page beyond prefetch target */
+                num_to_pfch_this_time -= scan->rs_pfchblock;                              /*  convert to offset        */
+
+                /*   first do prefetching up to our current limit  ...
+                **   highest page number that a scan (pre)-fetches is scan->rs_nblocks-1
+                **   note  -  prefetcher does not wrap a prefetch range -
+                **            instead just stop and then start again if and when main scan wraps
+                */
+                if (scan->rs_pfchblock <= scan->rs_startblock) {  /*  if on second leg towards startblock */
+                    prefetchable_range = ((int)(scan->rs_startblock) - (int)(scan->rs_pfchblock));
+                }
+                else {                                            /*     on first leg towards nblocks     */
+                    prefetchable_range = ((int)(scan->rs_nblocks) - (int)(scan->rs_pfchblock));
+                }
+                if (prefetchable_range > 0) {           /*  if theres a range to prefetch */
+
+                    /*  2.   number of available blocks ahead of us        */
+                    if (num_to_pfch_this_time > prefetchable_range) {
+                        num_to_pfch_this_time = prefetchable_range;
+                    }
+                    while (num_to_pfch_this_time-- > 0) {
+                        PrefetchBufferRc = PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, scan->rs_pfchblock, scan->rs_strategy);
+                        /*  if pin acquired on buffer,  then remember in case of future Discard */
+                        if (PrefetchBufferRc & PREFTCHRC_BUF_PIN_INCREASED) {
+                            heap_unread_add(scan, scan->rs_pfchblock);
+						}
+                        scan->rs_pfchblock++;
+                        /*  if syncscan and requested block was already in buffer pool,
+                        **  this suggests that another scanner is ahead of us and we should advance
+                        */
+                        if ( (scan->rs_syncscan) && (PrefetchBufferRc & PREFTCHRC_BLK_ALREADY_PRESENT) ) {
+                            scan->rs_pfchblock++;
+                            num_to_pfch_this_time--;
+                        }
+                    }
+                }
+                else {
+                    /*   we must not modify scan->rs_pfchblock here
+                    **   because it is needed for possible DiscardBuffer at end of scan  ...
+                    **   ... instead ...
+                    */
+                    scan->rs_prefetch_target = PREFETCH_WRAP_POINT;  /*   mark this prefetcher as waiting to wrap */
+                }
+
+                /*   ...  then adjust prefetching limit : by doubling on each iteration */
+                if (scan->rs_prefetch_target == 0) {
+                    scan->rs_prefetch_target = 1;
+                }
+                else {
+                    scan->rs_prefetch_target *= 2;
+                    if (scan->rs_prefetch_target > target_prefetch_pages) {
+                        scan->rs_prefetch_target = target_prefetch_pages;
+                    }
+                }
+            }
+        }
+#endif   /* USE_PREFETCH */
+
+
 	if (!scan->rs_pageatatime)
 		return;
 
@@ -452,6 +712,8 @@ heapgettup(HeapScanDesc scan,
 	OffsetNumber lineoff;
 	int			linesleft;
 	ItemId		lpp;
+    BlockNumber  prefetchHWM = 0;   /*  HWM of prefetch BlockNum for all participants in same sync scan */
+    int          ix;
 
 	/*
 	 * calculate next starting lineoff, given scan direction
@@ -470,7 +732,25 @@ heapgettup(HeapScanDesc scan,
 				return;
 			}
 			page = scan->rs_startblock; /* first page */
-			heapgetpage(scan, page);
+#ifdef USE_PREFETCH
+                        /*  decide if we shall do prefetching :  only if :
+                        **   .  prefetching enabled for this scan
+                        **   .  not a bitmap scan (which do their own)
+                        **   .  sufficient number of blocks - at least twice the target_prefetch_pages
+                        */
+                        if (    (!scan->rs_bitmapscan)
+                             && (scan->rs_prefetch_target >= PREFETCH_MAYBE)
+                             && (scan->rs_nblocks > (2*target_prefetch_pages))
+                           )  {
+                                scan->rs_prefetch_target = 1;  /*  do prefetching on forward non-bitmap scan */
+                                scan->rs_Unread_Pfetched_base = (BlockNumber *)palloc(sizeof(BlockNumber)*target_prefetch_pages);
+                                /*  Initialise the list */
+                                for (ix = 0; ix < target_prefetch_pages; ix++) {
+									*(scan->rs_Unread_Pfetched_base + ix) = InvalidBlockNumber;
+								}
+                        }
+#endif   /* USE_PREFETCH */
+			heapgetpage(scan, page, prefetchHWM);
 			lineoff = FirstOffsetNumber;		/* first offnum */
 			scan->rs_inited = true;
 		}
@@ -516,7 +796,7 @@ heapgettup(HeapScanDesc scan,
 				page = scan->rs_startblock - 1;
 			else
 				page = scan->rs_nblocks - 1;
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 		}
 		else
 		{
@@ -557,7 +837,7 @@ heapgettup(HeapScanDesc scan,
 
 		page = ItemPointerGetBlockNumber(&(tuple->t_self));
 		if (page != scan->rs_cblock)
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 
 		/* Since the tuple was previously fetched, needn't lock page here */
 		dp = (Page) BufferGetPage(scan->rs_cbuf);
@@ -660,8 +940,10 @@ heapgettup(HeapScanDesc scan,
 			 * a little bit backwards on every invocation, which is confusing.
 			 * We don't guarantee any specific ordering in general, though.
 			 */
-			if (scan->rs_syncscan)
-				ss_report_location(scan->rs_rd, page);
+			if (scan->rs_syncscan) {
+                                prefetchHWM = scan->rs_pfchblock;
+				ss_report_location(scan->rs_rd, page, &prefetchHWM);
+                        }
 		}
 
 		/*
@@ -671,6 +953,22 @@ heapgettup(HeapScanDesc scan,
 		{
 			if (BufferIsValid(scan->rs_cbuf))
 				ReleaseBuffer(scan->rs_cbuf);
+#ifdef USE_PREFETCH
+                        if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                            /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                            while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+                        }
+#endif   /* USE_PREFETCH */
 			scan->rs_cbuf = InvalidBuffer;
 			scan->rs_cblock = InvalidBlockNumber;
 			tuple->t_data = NULL;
@@ -678,7 +976,7 @@ heapgettup(HeapScanDesc scan,
 			return;
 		}
 
-		heapgetpage(scan, page);
+		heapgetpage(scan, page, prefetchHWM);
 
 		LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);
 
@@ -727,6 +1025,8 @@ heapgettup_pagemode(HeapScanDesc scan,
 	OffsetNumber lineoff;
 	int			linesleft;
 	ItemId		lpp;
+    BlockNumber  prefetchHWM = 0;   /*  HWM of prefetch BlockNum for all participants in same sync scan */
+    int          ix;
 
 	/*
 	 * calculate next starting lineindex, given scan direction
@@ -745,7 +1045,25 @@ heapgettup_pagemode(HeapScanDesc scan,
 				return;
 			}
 			page = scan->rs_startblock; /* first page */
-			heapgetpage(scan, page);
+#ifdef USE_PREFETCH
+                        /*  decide if we shall do prefetching :  only if :
+                        **   .  prefetching enabled for this scan
+                        **   .  not a bitmap scan (which do their own)
+                        **   .  sufficient number of blocks - at least twice the target_prefetch_pages
+                        */
+                        if (    (!scan->rs_bitmapscan)
+                             && (scan->rs_prefetch_target >= PREFETCH_MAYBE)
+                             && (scan->rs_nblocks > (2*target_prefetch_pages))
+                           )  {
+                                scan->rs_prefetch_target = 1;  /*  do prefetching on forward non-bitmap scan */
+                                scan->rs_Unread_Pfetched_base = (BlockNumber *)palloc(sizeof(BlockNumber)*target_prefetch_pages);
+                                /*  Initialise the list */
+                                for (ix = 0; ix < target_prefetch_pages; ix++) {
+									*(scan->rs_Unread_Pfetched_base + ix) = InvalidBlockNumber;
+								}
+                        }
+#endif   /* USE_PREFETCH */
+			heapgetpage(scan, page, prefetchHWM);
 			lineindex = 0;
 			scan->rs_inited = true;
 		}
@@ -788,7 +1106,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 				page = scan->rs_startblock - 1;
 			else
 				page = scan->rs_nblocks - 1;
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 		}
 		else
 		{
@@ -826,7 +1144,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 
 		page = ItemPointerGetBlockNumber(&(tuple->t_self));
 		if (page != scan->rs_cblock)
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 
 		/* Since the tuple was previously fetched, needn't lock page here */
 		dp = (Page) BufferGetPage(scan->rs_cbuf);
@@ -921,8 +1239,10 @@ heapgettup_pagemode(HeapScanDesc scan,
 			 * a little bit backwards on every invocation, which is confusing.
 			 * We don't guarantee any specific ordering in general, though.
 			 */
-			if (scan->rs_syncscan)
-				ss_report_location(scan->rs_rd, page);
+			if (scan->rs_syncscan) {
+                                prefetchHWM = scan->rs_pfchblock;
+				ss_report_location(scan->rs_rd, page, &prefetchHWM);
+                        }
 		}
 
 		/*
@@ -932,6 +1252,22 @@ heapgettup_pagemode(HeapScanDesc scan,
 		{
 			if (BufferIsValid(scan->rs_cbuf))
 				ReleaseBuffer(scan->rs_cbuf);
+#ifdef USE_PREFETCH
+                        if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                            /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                             while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+                        }
+#endif   /* USE_PREFETCH */
 			scan->rs_cbuf = InvalidBuffer;
 			scan->rs_cblock = InvalidBlockNumber;
 			tuple->t_data = NULL;
@@ -939,7 +1275,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 			return;
 		}
 
-		heapgetpage(scan, page);
+		heapgetpage(scan, page, prefetchHWM);
 
 		dp = (Page) BufferGetPage(scan->rs_cbuf);
 		lines = scan->rs_ntuples;
@@ -1394,6 +1730,23 @@ void
 heap_rescan(HeapScanDesc scan,
 			ScanKey key)
 {
+
+#ifdef USE_PREFETCH
+         if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                       /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                            while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+         }
+#endif   /* USE_PREFETCH */
 	/*
 	 * unpin scan buffers
 	 */
@@ -1418,6 +1771,23 @@ heap_endscan(HeapScanDesc scan)
 {
 	/* Note: no locking manipulations needed */
 
+
+#ifdef USE_PREFETCH
+         if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                       /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                            while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+         }
+#endif   /* USE_PREFETCH */
 	/*
 	 * unpin scan buffers
 	 */
@@ -1435,6 +1805,10 @@ heap_endscan(HeapScanDesc scan)
 	if (scan->rs_strategy != NULL)
 		FreeAccessStrategy(scan->rs_strategy);
 
+    if (scan->rs_Unread_Pfetched_base) {
+        pfree(scan->rs_Unread_Pfetched_base);
+    }
+
 	if (scan->rs_temp_snap)
 		UnregisterSnapshot(scan->rs_snapshot);
 
@@ -1464,7 +1838,6 @@ heap_endscan(HeapScanDesc scan)
 #define HEAPDEBUG_3
 #endif   /* !defined(HEAPDEBUGALL) */
 
-
 HeapTuple
 heap_getnext(HeapScanDesc scan, ScanDirection direction)
 {
@@ -6347,6 +6720,25 @@ heap_markpos(HeapScanDesc scan)
 void
 heap_restrpos(HeapScanDesc scan)
 {
+
+
+#ifdef USE_PREFETCH
+                        if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                            /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                             while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+                        }
+#endif   /* USE_PREFETCH */
+
 	/* XXX no amrestrpos checking that ammarkpos called */
 
 	if (!ItemPointerIsValid(&scan->rs_mctid))
--- src/backend/access/heap/syncscan.c.orig	2014-06-08 11:26:29.000000000 -0400
+++ src/backend/access/heap/syncscan.c	2014-06-08 21:59:36.968096921 -0400
@@ -90,6 +90,7 @@ typedef struct ss_scan_location_t
 {
 	RelFileNode relfilenode;	/* identity of a relation */
 	BlockNumber location;		/* last-reported location in the relation */
+	BlockNumber prefetchHWM;	/* high-water-mark of prefetched Blocknum */
 } ss_scan_location_t;
 
 typedef struct ss_lru_item_t
@@ -113,7 +114,7 @@ static ss_scan_locations_t *scan_locatio
 
 /* prototypes for internal functions */
 static BlockNumber ss_search(RelFileNode relfilenode,
-		  BlockNumber location, bool set);
+		  BlockNumber location, bool set , BlockNumber *prefetchHWMp);
 
 
 /*
@@ -160,6 +161,7 @@ SyncScanShmemInit(void)
 			item->location.relfilenode.dbNode = InvalidOid;
 			item->location.relfilenode.relNode = InvalidOid;
 			item->location.location = InvalidBlockNumber;
+			item->location.prefetchHWM = InvalidBlockNumber;
 
 			item->prev = (i > 0) ?
 				(&scan_locations->items[i - 1]) : NULL;
@@ -185,7 +187,7 @@ SyncScanShmemInit(void)
  * data structure.
  */
 static BlockNumber
-ss_search(RelFileNode relfilenode, BlockNumber location, bool set)
+ss_search(RelFileNode relfilenode, BlockNumber location, bool set , BlockNumber *prefetchHWMp)
 {
 	ss_lru_item_t *item;
 
@@ -206,6 +208,22 @@ ss_search(RelFileNode relfilenode, Block
 			{
 				item->location.relfilenode = relfilenode;
 				item->location.location = location;
+                                /*  if prefetch information requested,
+                                **  then reconcile and either update or report back the new HWM.
+                                */
+                                if (prefetchHWMp)
+                                {
+                                    if (   (item->location.prefetchHWM == InvalidBlockNumber)
+                                        || (item->location.prefetchHWM < *prefetchHWMp)
+                                       )
+                                    {
+                                      item->location.prefetchHWM = *prefetchHWMp;
+                                    }
+                                    else
+                                    {
+                                      *prefetchHWMp = item->location.prefetchHWM;
+                                    }
+                                }
 			}
 			else if (set)
 				item->location.location = location;
@@ -252,7 +270,7 @@ ss_get_location(Relation rel, BlockNumbe
 	BlockNumber startloc;
 
 	LWLockAcquire(SyncScanLock, LW_EXCLUSIVE);
-	startloc = ss_search(rel->rd_node, 0, false);
+	startloc = ss_search(rel->rd_node, 0, false , 0);
 	LWLockRelease(SyncScanLock);
 
 	/*
@@ -282,7 +300,7 @@ ss_get_location(Relation rel, BlockNumbe
  * same relfilenode.
  */
 void
-ss_report_location(Relation rel, BlockNumber location)
+ss_report_location(Relation rel, BlockNumber location , BlockNumber *prefetchHWMp)
 {
 #ifdef TRACE_SYNCSCAN
 	if (trace_syncscan)
@@ -306,7 +324,7 @@ ss_report_location(Relation rel, BlockNu
 	{
 		if (LWLockConditionalAcquire(SyncScanLock, LW_EXCLUSIVE))
 		{
-			(void) ss_search(rel->rd_node, location, true);
+			(void) ss_search(rel->rd_node, location, true , prefetchHWMp);
 			LWLockRelease(SyncScanLock);
 		}
 #ifdef TRACE_SYNCSCAN
--- src/backend/access/index/indexam.c.orig	2014-06-08 11:26:29.000000000 -0400
+++ src/backend/access/index/indexam.c	2014-06-08 21:59:37.012096997 -0400
@@ -79,6 +79,55 @@
 #include "utils/tqual.h"
 
 
+#ifdef USE_PREFETCH
+bool BlocknotinBuffer(Buffer buffer, Relation relation, BlockNumber blockNum);
+BlockNumber BlocknumOfBuffer(Buffer buffer);
+void index_mark_or_evict_block(IndexScanDesc scan , BlockNumber blocknumber , int markit);
+
+extern unsigned int prefetch_index_scans; /*  boolean whether to prefetch bitmap heap scans         */
+
+/*  if specified block number is present in the prefetch array,
+**  then either mark it as not to be discarded or evict it according to input param
+*/
+void index_mark_or_evict_block(IndexScanDesc scan , BlockNumber blocknumber , int markit)
+{
+        unsigned short int pfchx , pfchy , pfchz; /*  indexes in BlockIdData array   */
+
+        if (    scan->do_prefetch
+             && ((struct pfch_block_item*)0 != scan->pfch_block_item_list)
+             /* no need to check for scan->pfch_next < prefetch_index_scans
+             ** since we will do nothing if scan->pfch_used == 0
+             */
+           ) {
+            /*  search the prefetch list to find if the block is a member */
+            for (pfchx = 0; pfchx < scan->pfch_used; pfchx++) {
+                if (BlockIdGetBlockNumber(&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)) == blocknumber) {
+                      if (markit) {
+						      /*  mark it as not to be discarded */
+						      ((scan->pfch_block_item_list)+pfchx)->pfch_discard &= ~PREFTCHRC_BUF_PIN_INCREASED;
+					  } else {
+							  /*  shuffle all following the evictee to the left
+							  **  and update next pointer if its element moves
+							  */
+							  pfchy = (scan->pfch_used - 1); /*  current rightmost */
+							  scan->pfch_used = pfchy;
+
+							  while (pfchy > pfchx) {
+								  pfchz = pfchx + 1;
+								  BlockIdCopy((&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)), (&(((scan->pfch_block_item_list)+pfchz)->pfch_blockid)));
+								  ((scan->pfch_block_item_list)+pfchx)->pfch_discard = ((scan->pfch_block_item_list)+pfchz)->pfch_discard;
+								  if (scan->pfch_next == pfchz) {
+									  scan->pfch_next = pfchx;
+								  }
+								  pfchx = pfchz; /* advance */
+							  }
+                      }
+                }
+            }
+        }
+}
+#endif /* USE_PREFETCH */
+
 /* ----------------------------------------------------------------
  *					macros used in index_ routines
  *
@@ -253,6 +302,11 @@ index_beginscan(Relation heapRelation,
 	 */
 	scan->heapRelation = heapRelation;
 	scan->xs_snapshot = snapshot;
+#ifdef USE_PREFETCH
+	scan->do_prefetch = 0;             /*  no prefetching by default  */
+        scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+	scan->pfch_block_item_list = (struct pfch_block_item*)0;
+#endif   /* USE_PREFETCH */
 
 	return scan;
 }
@@ -277,6 +331,11 @@ index_beginscan_bitmap(Relation indexRel
 	 * up by RelationGetIndexScan.
 	 */
 	scan->xs_snapshot = snapshot;
+#ifdef USE_PREFETCH
+	scan->do_prefetch = 0;             /*  no prefetching by default  */
+        scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+	scan->pfch_block_item_list = (struct pfch_block_item*)0;
+#endif   /* USE_PREFETCH */
 
 	return scan;
 }
@@ -311,6 +370,9 @@ index_beginscan_internal(Relation indexR
 									  Int32GetDatum(nkeys),
 									  Int32GetDatum(norderbys)));
 
+	scan->heap_tids_seen = 0;
+	scan->heap_tids_fetched = 0;
+	
 	return scan;
 }
 
@@ -342,6 +404,12 @@ index_rescan(IndexScanDesc scan,
 	/* Release any held pin on a heap page */
 	if (BufferIsValid(scan->xs_cbuf))
 	{
+#ifdef USE_PREFETCH
+                /*   if specified block number is present in the prefetch array,  then evict it */
+                if (scan->do_prefetch) {
+                      index_mark_or_evict_block(scan , BlocknumOfBuffer(scan->xs_cbuf) , 0);
+                }
+#endif   /* USE_PREFETCH */
 		ReleaseBuffer(scan->xs_cbuf);
 		scan->xs_cbuf = InvalidBuffer;
 	}
@@ -373,10 +441,30 @@ index_endscan(IndexScanDesc scan)
 	/* Release any held pin on a heap page */
 	if (BufferIsValid(scan->xs_cbuf))
 	{
+#ifdef USE_PREFETCH
+                /*   if specified block number is present in the prefetch array,  then evict it */
+                if (scan->do_prefetch) {
+                      index_mark_or_evict_block(scan , BlocknumOfBuffer(scan->xs_cbuf) , 0);
+                }
+#endif   /* USE_PREFETCH */
 		ReleaseBuffer(scan->xs_cbuf);
 		scan->xs_cbuf = InvalidBuffer;
 	}
 
+#ifdef USE_PREFETCH
+        /*   discard prefetched but unread buffers */
+        if (    scan->do_prefetch
+             && ((struct pfch_block_item*)0 != scan->pfch_block_item_list)
+           ) {
+            unsigned short int pfchx; /*  index in BlockIdData array   */
+                for (pfchx = 0; pfchx < scan->pfch_used; pfchx++) {
+                  if (((scan->pfch_block_item_list)+pfchx)->pfch_discard) {
+                      DiscardBuffer(scan->heapRelation, MAIN_FORKNUM, BlockIdGetBlockNumber(&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)));
+                  }
+                }
+        }
+#endif   /* USE_PREFETCH */
+
 	/* End the AM's scan */
 	FunctionCall1(procedure, PointerGetDatum(scan));
 
@@ -472,6 +560,12 @@ index_getnext_tid(IndexScanDesc scan, Sc
 		/* ... but first, release any held pin on a heap page */
 		if (BufferIsValid(scan->xs_cbuf))
 		{
+#ifdef USE_PREFETCH
+                    /*   if specified block number is present in the prefetch array,  then evict it */
+                    if (scan->do_prefetch) {
+                          index_mark_or_evict_block(scan , BlocknumOfBuffer(scan->xs_cbuf) , 0);
+                    }
+#endif   /* USE_PREFETCH */
 			ReleaseBuffer(scan->xs_cbuf);
 			scan->xs_cbuf = InvalidBuffer;
 		}
@@ -479,6 +573,11 @@ index_getnext_tid(IndexScanDesc scan, Sc
 	}
 
 	pgstat_count_index_tuples(scan->indexRelation, 1);
+	if (scan->heap_tids_seen++ >= (~0)) {
+		/* Avoid integer overflow */
+		scan->heap_tids_seen = 1;
+		scan->heap_tids_fetched = 0;
+	}
 
 	/* Return the TID of the tuple we found. */
 	return &scan->xs_ctup.t_self;
@@ -502,6 +601,10 @@ index_getnext_tid(IndexScanDesc scan, Sc
  * enough information to do it efficiently in the general case.
  * ----------------
  */
+#if defined(USE_PREFETCH) && defined(AVOID_CATALOG_MIGRATION_FOR_ASYNCIO)
+extern Datum btpeeknexttuple(IndexScanDesc scan);
+#endif /* USE_PREFETCH */
+
 HeapTuple
 index_fetch_heap(IndexScanDesc scan)
 {
@@ -509,16 +612,109 @@ index_fetch_heap(IndexScanDesc scan)
 	bool		all_dead = false;
 	bool		got_heap_tuple;
 
+
+
 	/* We can skip the buffer-switching logic if we're in mid-HOT chain. */
 	if (!scan->xs_continue_hot)
 	{
 		/* Switch to correct buffer if we don't have it already */
 		Buffer		prev_buf = scan->xs_cbuf;
 
+#ifdef USE_PREFETCH
+
+                /*   If the old block is different from new block, then evict old
+                **   block from prefetched array.   It is arguable we should leave it
+                **   in the array because it's likely to remain in the buffer pool
+                **   for a while,  but in that case , if we encounter the block
+                **   again,  prefetching it again does no harm.
+                **   (and note that,  if it's not pinned,  prefetching it will try to
+                **   pin it since prefetch tries to bank a pin for a buffer in the buffer pool).
+                **   therefore it should usually win.
+                */
+                if (    scan->do_prefetch
+                     && ( BufferIsValid(prev_buf) )
+                     && (BlocknotinBuffer(prev_buf,scan->heapRelation,ItemPointerGetBlockNumber(tid)))
+                     && (scan->pfch_next < prefetch_index_scans)  /* ensure there is an entry */
+                        ) {
+                          index_mark_or_evict_block(scan , BlocknumOfBuffer(prev_buf) , 0);
+                }
+
+#endif   /* USE_PREFETCH  */
 		scan->xs_cbuf = ReleaseAndReadBuffer(scan->xs_cbuf,
 											 scan->heapRelation,
 											 ItemPointerGetBlockNumber(tid));
 
+                /*   If the new block had been prefetched and pinned,
+                **   then mark that it no longer requires to be discarded.
+                **   Of course,  we don't evict the entry,
+                **   because we want to remember that it was recently prefetched.
+                */
+                index_mark_or_evict_block(scan , BlocknumOfBuffer(prev_buf) , 1);
+
+				scan->heap_tids_fetched++;
+
+#ifdef USE_PREFETCH
+                /*  try prefetching next data block
+                **    (next meaning one containing TIDs from matching keys
+                **     in same index page and different from any block
+                **     we previously prefetched and listed in prefetched array)
+                */
+                {
+                    FmgrInfo   *procedure;
+                    bool	found;             /*  did we find the "next" heap tid in current index page */
+                    int         PrefetchBufferRc;  /*  indicates whether requested prefetch block already in a buffer and if pin count on buffer has been increased */
+
+                    if (scan->do_prefetch) {
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+                        procedure = &scan->indexRelation->rd_aminfo->ampeeknexttuple; /* is incorrect but avoids adding function to catalog */
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                        if (RegProcedureIsValid(scan->indexRelation->rd_am->ampeeknexttuple)) {
+                            GET_SCAN_PROCEDURE(ampeeknexttuple); /* is correct but requires adding function to catalog */
+                        } else {
+                            procedure = 0;
+                        }
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+
+                        if (    procedure          /* does the index access method support peektuple? */
+#ifndef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+                             && procedure->fn_addr /* procedure->fn_addr is non-null only if in catalog */
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                           ) {
+                            int iterations = 1;      /*  how many iterations of prefetching shall we try  -
+                                                     **  if used entries in prefetch list is < target_prefetch_pages
+                                                     **  then 2,  else 1
+                                                     **  this should result in gradually and smoothly increasing up to target_prefetch_pages
+                                                     */
+                            /*  note we trust InitIndexScan verified this scan is forwards only and so set that */
+                            if (scan->pfch_used < target_prefetch_pages) {
+                                iterations = 2;
+                            }
+                            do {
+                                found =  DatumGetBool(
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+                                                         btpeeknexttuple(scan)     /*  pass scan as direct parameter since cant use fmgr because not in catalog */
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                                                         FunctionCall1(procedure, PointerGetDatum(scan)) /* use fmgr to call it because in catalog  */
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                                                     );
+                                if (found) {
+                                    /*    btpeeknexttuple set pfch_next to point to the item in block_item_list to be prefetched */
+                                    PrefetchBufferRc = PrefetchBuffer(scan->heapRelation, MAIN_FORKNUM, BlockIdGetBlockNumber((&((scan->pfch_block_item_list + scan->pfch_next))->pfch_blockid)) , 0);
+                                    /* elog(LOG,"index_fetch_heap prefetched rel %u blockNum %u"
+                                       ,scan->heapRelation->rd_node.relNode ,BlockIdGetBlockNumber(scan->pfch_block_item_list + scan->pfch_next));
+                                    */
+
+                                    /*  if pin acquired on buffer,  then remember in case of future Discard */
+                                    (scan->pfch_block_item_list + scan->pfch_next)->pfch_discard = (PrefetchBufferRc & PREFTCHRC_BUF_PIN_INCREASED);
+
+
+                                }
+                            } while (--iterations > 0);
+                        }
+                    }
+                }
+#endif   /* USE_PREFETCH */
+
 		/*
 		 * Prune page, but only if we weren't already on this page
 		 */
--- src/backend/access/index/genam.c.orig	2014-06-08 11:26:29.000000000 -0400
+++ src/backend/access/index/genam.c	2014-06-08 21:59:37.036097039 -0400
@@ -77,6 +77,12 @@ RelationGetIndexScan(Relation indexRelat
 
 	scan = (IndexScanDesc) palloc(sizeof(IndexScanDescData));
 
+#ifdef USE_PREFETCH
+        scan->do_prefetch = 0;             /*  no prefetching by default  */
+        scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+        scan->pfch_block_item_list = (struct pfch_block_item*)0;
+#endif   /* USE_PREFETCH */
+
 	scan->heapRelation = NULL;	/* may be set later */
 	scan->indexRelation = indexRelation;
 	scan->xs_snapshot = InvalidSnapshot;		/* caller must initialize this */
@@ -139,6 +145,19 @@ RelationGetIndexScan(Relation indexRelat
 void
 IndexScanEnd(IndexScanDesc scan)
 {
+#ifdef USE_PREFETCH
+	if (scan->do_prefetch) {
+		if ( (struct pfch_block_item*)0 != scan->pfch_block_item_list ) {
+			pfree(scan->pfch_block_item_list);
+			scan->pfch_block_item_list = (struct pfch_block_item*)0;
+		}
+		if ( (struct pfch_index_pagelist*)0 != scan->pfch_index_page_list ) {
+			pfree(scan->pfch_index_page_list);
+			scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+		}
+	}
+#endif   /* USE_PREFETCH */
+
 	if (scan->keyData != NULL)
 		pfree(scan->keyData);
 	if (scan->orderByData != NULL)
--- src/backend/access/nbtree/nbtsearch.c.orig	2014-06-08 11:26:29.000000000 -0400
+++ src/backend/access/nbtree/nbtsearch.c	2014-06-08 21:59:37.064097087 -0400
@@ -23,13 +23,16 @@
 #include "utils/lsyscache.h"
 #include "utils/rel.h"
 
+extern unsigned int prefetch_btree_heaps;  /*  boolean whether to prefetch heap pages in _bt_next for non-bitmap index scans   */
+extern unsigned int prefetch_sequential_index_scans;  /*  boolean whether to prefetch sequential-access non-bitmap index scans  */
 
 static bool _bt_readpage(IndexScanDesc scan, ScanDirection dir,
 			 OffsetNumber offnum);
 static void _bt_saveitem(BTScanOpaque so, int itemIndex,
 			 OffsetNumber offnum, IndexTuple itup);
-static bool _bt_steppage(IndexScanDesc scan, ScanDirection dir);
-static Buffer _bt_walk_left(Relation rel, Buffer buf);
+static bool _bt_steppage(IndexScanDesc scan, ScanDirection dir, 
+			 bool prefetch);
+static Buffer _bt_walk_left(IndexScanDesc scan, Relation rel, Buffer buf);
 static bool _bt_endpoint(IndexScanDesc scan, ScanDirection dir);
 
 
@@ -226,7 +229,7 @@ _bt_moveright(Relation rel,
 				_bt_relbuf(rel, buf);
 
 			/* re-acquire the lock in the right mode, and re-check */
-			buf = _bt_getbuf(rel, blkno, access);
+			buf = _bt_getbuf(rel, blkno, access , (struct pfch_index_pagelist*)0);
 			continue;
 		}
 
@@ -1005,7 +1008,7 @@ _bt_first(IndexScanDesc scan, ScanDirect
 		 * There's no actually-matching data on this page.  Try to advance to
 		 * the next page.  Return false if there's no matching data at all.
 		 */
-		if (!_bt_steppage(scan, dir))
+		if (!_bt_steppage(scan, dir, false))
 			return false;
 	}
 
@@ -1040,6 +1043,8 @@ _bt_next(IndexScanDesc scan, ScanDirecti
 {
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
 	BTScanPosItem *currItem;
+	BlockNumber prevblkno = ItemPointerGetBlockNumber(
+		&scan->xs_ctup.t_self);
 
 	/*
 	 * Advance to next tuple on current page; or if there's no more, try to
@@ -1052,11 +1057,53 @@ _bt_next(IndexScanDesc scan, ScanDirecti
 			/* We must acquire lock before applying _bt_steppage */
 			Assert(BufferIsValid(so->currPos.buf));
 			LockBuffer(so->currPos.buf, BT_READ);
-			if (!_bt_steppage(scan, dir))
+			if (!_bt_steppage(scan, dir, target_prefetch_pages > 0))
 				return false;
 			/* Drop the lock, but not pin, on the new page */
 			LockBuffer(so->currPos.buf, BUFFER_LOCK_UNLOCK);
 		}
+		
+		if (prefetch_btree_heaps && (target_prefetch_pages > 0)) {
+			BlockNumber currblkno = ItemPointerGetBlockNumber(
+				&so->currPos.items[so->currPos.itemIndex].heapTid);
+		
+			if (currblkno != prevblkno) {
+				if (so->prefetchBlockCount > 0)
+					so->prefetchBlockCount--;
+			
+				/* If we have heap fetch frequency stats, and it's above ~94%,
+				 * initiate heap prefetches */
+				if (so->currPos.moreRight
+					&& scan->heap_tids_seen > 256 
+					&& ( (scan->heap_tids_seen - scan->heap_tids_seen/16)
+						 <= scan->heap_tids_fetched ) )
+				{
+					bool nonsequential = false;
+				
+					if (so->prefetchItemIndex <= so->currPos.itemIndex)
+						so->prefetchItemIndex = so->currPos.itemIndex + 1;
+					while (    (so->prefetchItemIndex <= so->currPos.lastItem)
+							&& (so->prefetchBlockCount < target_prefetch_pages) )
+					{
+						ItemPointer tid = &so->currPos.items[so->prefetchItemIndex++].heapTid;
+						BlockNumber blkno = ItemPointerGetBlockNumber(tid);
+						if (blkno != so->lastHeapPrefetchBlkno) {  /*  if not a repetition of previous block */
+							/*   start prefetch on next page, providing :
+                            **   EITHER  .  we're reading non-sequentially previously or for this block
+                            **   OR      .  user explicitly specified to prefetch for sequential pattern
+							**   as it may be counterproductive otherwise
+                            */
+							nonsequential = (nonsequential || blkno != (so->lastHeapPrefetchBlkno+1));
+							if (prefetch_sequential_index_scans || nonsequential) {
+								_bt_prefetchbuf(scan->heapRelation, blkno , &scan->pfch_index_page_list );
+							}
+							so->lastHeapPrefetchBlkno = blkno;
+							so->prefetchBlockCount++;
+						}
+					}
+				}
+			}
+		}
 	}
 	else
 	{
@@ -1065,11 +1112,53 @@ _bt_next(IndexScanDesc scan, ScanDirecti
 			/* We must acquire lock before applying _bt_steppage */
 			Assert(BufferIsValid(so->currPos.buf));
 			LockBuffer(so->currPos.buf, BT_READ);
-			if (!_bt_steppage(scan, dir))
+			if (!_bt_steppage(scan, dir, target_prefetch_pages > 0))
 				return false;
 			/* Drop the lock, but not pin, on the new page */
 			LockBuffer(so->currPos.buf, BUFFER_LOCK_UNLOCK);
 		}
+		
+		if (prefetch_btree_heaps && (target_prefetch_pages > 0)) {
+			BlockNumber currblkno = ItemPointerGetBlockNumber(
+				&so->currPos.items[so->currPos.itemIndex].heapTid);
+		
+			if (currblkno != prevblkno) {
+				if (so->prefetchBlockCount > 0)
+					so->prefetchBlockCount--;
+			
+				/* If we have heap fetch frequency stats, and it's above ~94%,
+				 * initiate heap prefetches */
+				if (so->currPos.moreLeft
+					&& scan->heap_tids_seen > 256 
+					&& ( (scan->heap_tids_seen - scan->heap_tids_seen/16)
+						 <= scan->heap_tids_fetched ) )
+				{
+					bool nonsequential = false;
+			
+					if (so->prefetchItemIndex >= so->currPos.itemIndex)
+						so->prefetchItemIndex = so->currPos.itemIndex - 1;
+					while (    (so->prefetchItemIndex >= so->currPos.firstItem)
+							&& (so->prefetchBlockCount < target_prefetch_pages) )
+					{
+						ItemPointer tid = &so->currPos.items[so->prefetchItemIndex--].heapTid;
+						BlockNumber blkno = ItemPointerGetBlockNumber(tid);
+						if (blkno != so->lastHeapPrefetchBlkno) {  /*  if not a repetition of previous block */
+							/*   start prefetch on next page, providing :
+                            **   EITHER  .  we're reading non-sequentially previously or for this block
+                            **   OR      .  user explicitly specified to prefetch for sequential pattern
+							**   as it may be counterproductive otherwise
+                            */
+							nonsequential = (nonsequential || blkno != (so->lastHeapPrefetchBlkno+1));
+							if (prefetch_sequential_index_scans || nonsequential) {
+								_bt_prefetchbuf(scan->heapRelation, blkno , &scan->pfch_index_page_list );
+							}
+							so->lastHeapPrefetchBlkno = blkno;
+							so->prefetchBlockCount++;
+						}
+					}
+				}
+			}
+		}
 	}
 
 	/* OK, itemIndex says what to return */
@@ -1119,9 +1208,11 @@ _bt_readpage(IndexScanDesc scan, ScanDir
 	/*
 	 * we must save the page's right-link while scanning it; this tells us
 	 * where to step right to after we're done with these items.  There is no
-	 * corresponding need for the left-link, since splits always go right.
+	 * corresponding need for the left-link, since splits always go right,
+	 * but we need it for back-sequential scan detection.
 	 */
 	so->currPos.nextPage = opaque->btpo_next;
+	so->currPos.prevPage = opaque->btpo_prev;
 
 	/* initialize tuple workspace to empty */
 	so->currPos.nextTupleOffset = 0;
@@ -1156,6 +1247,7 @@ _bt_readpage(IndexScanDesc scan, ScanDir
 		so->currPos.firstItem = 0;
 		so->currPos.lastItem = itemIndex - 1;
 		so->currPos.itemIndex = 0;
+		so->prefetchItemIndex = 0;
 	}
 	else
 	{
@@ -1187,6 +1279,7 @@ _bt_readpage(IndexScanDesc scan, ScanDir
 		so->currPos.firstItem = itemIndex;
 		so->currPos.lastItem = MaxIndexTuplesPerPage - 1;
 		so->currPos.itemIndex = MaxIndexTuplesPerPage - 1;
+		so->prefetchItemIndex = MaxIndexTuplesPerPage - 1;
 	}
 
 	return (so->currPos.firstItem <= so->currPos.lastItem);
@@ -1224,7 +1317,7 @@ _bt_saveitem(BTScanOpaque so, int itemIn
  * locks and pins, set so->currPos.buf to InvalidBuffer, and return FALSE.
  */
 static bool
-_bt_steppage(IndexScanDesc scan, ScanDirection dir)
+_bt_steppage(IndexScanDesc scan, ScanDirection dir, bool prefetch)
 {
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
 	Relation	rel;
@@ -1278,7 +1371,7 @@ _bt_steppage(IndexScanDesc scan, ScanDir
 			/* check for interrupts while we're not holding any buffer lock */
 			CHECK_FOR_INTERRUPTS();
 			/* step right one page */
-			so->currPos.buf = _bt_getbuf(rel, blkno, BT_READ);
+			so->currPos.buf = _bt_getbuf(rel, blkno, BT_READ , scan->pfch_index_page_list);
 			/* check for deleted page */
 			page = BufferGetPage(so->currPos.buf);
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
@@ -1287,9 +1380,20 @@ _bt_steppage(IndexScanDesc scan, ScanDir
 				PredicateLockPage(rel, blkno, scan->xs_snapshot);
 				/* see if there are any matches on this page */
 				/* note that this will clear moreRight if we can stop */
-				if (_bt_readpage(scan, dir, P_FIRSTDATAKEY(opaque)))
+				if (_bt_readpage(scan, dir, P_FIRSTDATAKEY(opaque))) {
+					if (    prefetch && so->currPos.moreRight
+						/*   start prefetch on next page, providing :
+						**   EITHER  .  we're reading non-sequentially for this block
+						**   OR      .  user explicitly specified to prefetch for sequential pattern
+						**   as it may be counterproductive otherwise
+						*/
+						&& (prefetch_sequential_index_scans || opaque->btpo_next != (blkno+1))
+                       ) {
+ 						  _bt_prefetchbuf(rel, opaque->btpo_next , &scan->pfch_index_page_list);
+					}
 					break;
 			}
+			}
 			/* nope, keep going */
 			blkno = opaque->btpo_next;
 		}
@@ -1317,7 +1421,7 @@ _bt_steppage(IndexScanDesc scan, ScanDir
 			}
 
 			/* Step to next physical page */
-			so->currPos.buf = _bt_walk_left(rel, so->currPos.buf);
+			so->currPos.buf = _bt_walk_left(scan , rel, so->currPos.buf);
 
 			/* if we're physically at end of index, return failure */
 			if (so->currPos.buf == InvalidBuffer)
@@ -1332,14 +1436,58 @@ _bt_steppage(IndexScanDesc scan, ScanDir
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 			if (!P_IGNORE(opaque))
 			{
+				/* We must rely on the previously saved prevPage link! */
+				BlockNumber blkno = so->currPos.prevPage;
+				
 				PredicateLockPage(rel, BufferGetBlockNumber(so->currPos.buf), scan->xs_snapshot);
 				/* see if there are any matches on this page */
 				/* note that this will clear moreLeft if we can stop */
-				if (_bt_readpage(scan, dir, PageGetMaxOffsetNumber(page)))
+				if (_bt_readpage(scan, dir, PageGetMaxOffsetNumber(page))) {
+					if (prefetch && so->currPos.moreLeft) {
+						/* detect back-sequential runs and increase prefetch window blindly 
+						 * downwards 2 blocks at a time. This only works in our favor
+						 * for index-only scans, by merging read requests at the kernel,
+						 * so we want to inflate target_prefetch_pages since merged 
+						 * back-sequential requests are about as expensive as a single one 
+						 */
+						if (scan->xs_want_itup && blkno > 0 && opaque->btpo_prev == (blkno-1)) {
+							BlockNumber backPos;
+							unsigned int back_prefetch_pages = target_prefetch_pages * 16;
+							if (back_prefetch_pages > 64)
+								back_prefetch_pages = 64;
+							
+							if (so->backSeqRun == 0)
+								backPos = (blkno-1);
+							else
+								backPos = so->backSeqPos;
+							so->backSeqRun++;
+							
+							if (backPos > 0 && (blkno - backPos) <= back_prefetch_pages) {
+								_bt_prefetchbuf(rel, backPos-- , &scan->pfch_index_page_list);
+								/* don't start back-seq prefetch too early */
+								if (so->backSeqRun >= back_prefetch_pages
+										&& backPos > 0 
+										&& (blkno - backPos) <= back_prefetch_pages)
+								{
+									_bt_prefetchbuf(rel, backPos-- , &scan->pfch_index_page_list);
+								}
+							}
+							
+							so->backSeqPos = backPos;
+						} else {
+							/* start prefetch on next page */
+							if (so->backSeqRun != 0) {
+								if (opaque->btpo_prev > blkno || opaque->btpo_prev < so->backSeqPos)
+									so->backSeqRun = 0;
+							}
+							_bt_prefetchbuf(rel, opaque->btpo_prev , &scan->pfch_index_page_list);
+						}
+					}
 					break;
 			}
 		}
 	}
+	}
 
 	return true;
 }
@@ -1359,7 +1507,7 @@ _bt_steppage(IndexScanDesc scan, ScanDir
  * again if it's important.
  */
 static Buffer
-_bt_walk_left(Relation rel, Buffer buf)
+_bt_walk_left(IndexScanDesc scan, Relation rel, Buffer buf)
 {
 	Page		page;
 	BTPageOpaque opaque;
@@ -1387,7 +1535,7 @@ _bt_walk_left(Relation rel, Buffer buf)
 		_bt_relbuf(rel, buf);
 		/* check for interrupts while we're not holding any buffer lock */
 		CHECK_FOR_INTERRUPTS();
-		buf = _bt_getbuf(rel, blkno, BT_READ);
+		buf = _bt_getbuf(rel, blkno, BT_READ , scan->pfch_index_page_list );
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
@@ -1631,7 +1779,7 @@ _bt_endpoint(IndexScanDesc scan, ScanDir
 		 * There's no actually-matching data on this page.  Try to advance to
 		 * the next page.  Return false if there's no matching data at all.
 		 */
-		if (!_bt_steppage(scan, dir))
+		if (!_bt_steppage(scan, dir, false))
 			return false;
 	}
 
--- src/backend/access/nbtree/nbtinsert.c.orig	2014-06-08 11:26:29.000000000 -0400
+++ src/backend/access/nbtree/nbtinsert.c	2014-06-08 21:59:37.104097157 -0400
@@ -793,7 +793,7 @@ _bt_insertonpg(Relation rel,
 		{
 			Assert(!P_ISLEAF(lpageop));
 
-			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE , (struct pfch_index_pagelist*)0);
 			metapg = BufferGetPage(metabuf);
 			metad = BTPageGetMeta(metapg);
 
@@ -972,7 +972,7 @@ _bt_split(Relation rel, Buffer buf, Buff
 	bool		isleaf;
 
 	/* Acquire a new page to split into */
-	rbuf = _bt_getbuf(rel, P_NEW, BT_WRITE);
+	rbuf = _bt_getbuf(rel, P_NEW, BT_WRITE , (struct pfch_index_pagelist*)0);
 
 	/*
 	 * origpage is the original page to be split.  leftpage is a temporary
@@ -1175,7 +1175,7 @@ _bt_split(Relation rel, Buffer buf, Buff
 
 	if (!P_RIGHTMOST(oopaque))
 	{
-		sbuf = _bt_getbuf(rel, oopaque->btpo_next, BT_WRITE);
+		sbuf = _bt_getbuf(rel, oopaque->btpo_next, BT_WRITE , (struct pfch_index_pagelist*)0);
 		spage = BufferGetPage(sbuf);
 		sopaque = (BTPageOpaque) PageGetSpecialPointer(spage);
 		if (sopaque->btpo_prev != origpagenumber)
@@ -1817,7 +1817,7 @@ _bt_finish_split(Relation rel, Buffer lb
 	Assert(P_INCOMPLETE_SPLIT(lpageop));
 
 	/* Lock right sibling, the one missing the downlink */
-	rbuf = _bt_getbuf(rel, lpageop->btpo_next, BT_WRITE);
+	rbuf = _bt_getbuf(rel, lpageop->btpo_next, BT_WRITE , (struct pfch_index_pagelist*)0);
 	rpage = BufferGetPage(rbuf);
 	rpageop = (BTPageOpaque) PageGetSpecialPointer(rpage);
 
@@ -1829,7 +1829,7 @@ _bt_finish_split(Relation rel, Buffer lb
 		BTMetaPageData *metad;
 
 		/* acquire lock on the metapage */
-		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE , (struct pfch_index_pagelist*)0);
 		metapg = BufferGetPage(metabuf);
 		metad = BTPageGetMeta(metapg);
 
@@ -1877,7 +1877,7 @@ _bt_getstackbuf(Relation rel, BTStack st
 		Page		page;
 		BTPageOpaque opaque;
 
-		buf = _bt_getbuf(rel, blkno, access);
+		buf = _bt_getbuf(rel, blkno, access , (struct pfch_index_pagelist*)0);
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
@@ -2008,12 +2008,12 @@ _bt_newroot(Relation rel, Buffer lbuf, B
 	lopaque = (BTPageOpaque) PageGetSpecialPointer(lpage);
 
 	/* get a new root page */
-	rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE);
+	rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE , (struct pfch_index_pagelist*)0);
 	rootpage = BufferGetPage(rootbuf);
 	rootblknum = BufferGetBlockNumber(rootbuf);
 
 	/* acquire lock on the metapage */
-	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE , (struct pfch_index_pagelist*)0);
 	metapg = BufferGetPage(metabuf);
 	metad = BTPageGetMeta(metapg);
 
--- src/backend/access/nbtree/nbtpage.c.orig	2014-06-08 11:26:29.000000000 -0400
+++ src/backend/access/nbtree/nbtpage.c	2014-06-08 21:59:37.136097213 -0400
@@ -127,7 +127,7 @@ _bt_getroot(Relation rel, int access)
 		Assert(rootblkno != P_NONE);
 		rootlevel = metad->btm_fastlevel;
 
-		rootbuf = _bt_getbuf(rel, rootblkno, BT_READ);
+		rootbuf = _bt_getbuf(rel, rootblkno, BT_READ , (struct pfch_index_pagelist*)0);
 		rootpage = BufferGetPage(rootbuf);
 		rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);
 
@@ -153,7 +153,7 @@ _bt_getroot(Relation rel, int access)
 		rel->rd_amcache = NULL;
 	}
 
-	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
+	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ , (struct pfch_index_pagelist*)0);
 	metapg = BufferGetPage(metabuf);
 	metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
 	metad = BTPageGetMeta(metapg);
@@ -209,7 +209,7 @@ _bt_getroot(Relation rel, int access)
 		 * the new root page.  Since this is the first page in the tree, it's
 		 * a leaf as well as the root.
 		 */
-		rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE);
+		rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE , (struct pfch_index_pagelist*)0);
 		rootblkno = BufferGetBlockNumber(rootbuf);
 		rootpage = BufferGetPage(rootbuf);
 		rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);
@@ -350,7 +350,7 @@ _bt_gettrueroot(Relation rel)
 		pfree(rel->rd_amcache);
 	rel->rd_amcache = NULL;
 
-	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
+	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ , (struct pfch_index_pagelist*)0);
 	metapg = BufferGetPage(metabuf);
 	metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
 	metad = BTPageGetMeta(metapg);
@@ -436,7 +436,7 @@ _bt_getrootheight(Relation rel)
 		Page		metapg;
 		BTPageOpaque metaopaque;
 
-		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
+		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ , (struct pfch_index_pagelist*)0);
 		metapg = BufferGetPage(metabuf);
 		metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
 		metad = BTPageGetMeta(metapg);
@@ -562,6 +562,170 @@ _bt_log_reuse_page(Relation rel, BlockNu
 }
 
 /*
+ *	_bt_prefetchbuf() -- Prefetch a buffer by block number
+ *                           and keep track of prefetched and unread blocknums in pagelist.
+ *   input parms  :
+ *       rel and blockno identify block to be prefetched as usual
+ *       pfch_index_page_list_P points to the pointer anchoring the head of the index page list
+ *             Since the pagelist is a kind of optimization,
+ *             handle palloc failure by quietly omitting the keeping track.
+ */
+void
+_bt_prefetchbuf(Relation rel, BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P)
+{
+
+    int rc = 0;
+    struct pfch_index_pagelist* pfch_index_plp;      /*  pointer to current chunk */
+    struct pfch_index_item* found_item = 0;
+
+		if ((struct pfch_index_pagelist**)0 == pfch_index_page_list_P) {
+			pfch_index_plp = (struct pfch_index_pagelist*)0;
+		} else {
+			pfch_index_plp = *pfch_index_page_list_P;
+		}
+
+    	if (blkno != P_NEW && blkno != P_NONE)
+    	{
+            /* prefetch an existing block of the relation
+            ** but first,  check it has not recently already been prefetched and not yet read
+            */
+            found_item = _bt_find_block(blkno , pfch_index_plp);
+			if ((struct pfch_index_item*)0 == found_item) {  /*  not found */
+
+		        rc = PrefetchBuffer(rel, MAIN_FORKNUM, blkno , 0);
+
+                /*  add the pagenum to the list ,  indicating its discard status
+                **  since it's only an optimization,  ignore failure such as exceeded allowed space
+				*/
+                _bt_add_block( blkno , pfch_index_page_list_P , (uint32)(rc & PREFTCHRC_BUF_PIN_INCREASED));
+
+            }
+	    }
+        return;
+}
+
+/*   _bt_find_block finds the item referencing specified Block in index page list if present
+**   and returns the pointer to the pfch_index_item if found,  or null if not
+*/
+struct pfch_index_item*
+_bt_find_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list)
+{
+
+    struct pfch_index_item* found_item = 0;
+    struct pfch_index_pagelist* pfch_index_plp;      /*  pointer to current chunk */
+    int ix, tx;
+
+		pfch_index_plp = pfch_index_page_list;
+
+		while (     (struct pfch_index_pagelist*)0 != pfch_index_plp
+                &&  ( (struct pfch_index_item*)0 == found_item)
+              ) {
+			ix = 0;
+			tx = pfch_index_plp->pfch_index_item_count;
+			while (     (ix < tx)
+                    &&  ( (struct pfch_index_item*)0 == found_item)
+                  ) {
+				if (pfch_index_plp->pfch_indexid[ix].pfch_blocknum == blkno) {
+					found_item = &pfch_index_plp->pfch_indexid[ix];
+				}
+                ix++;
+			}
+			pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+		}
+
+     return found_item;
+}
+
+/*   _bt_add_block adds the specified Block to the index page list
+**   and returns 0 if successful,  non-zero if not
+*/
+int
+_bt_add_block(BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P , uint32 discard_status)
+{
+    int rc = 1;
+    int ix;
+    struct pfch_index_pagelist* pfch_index_plp;      /*  pointer to current chunk */
+    struct pfch_index_pagelist* pfch_index_page_list_anchor; /*  pointer to first chunk if any  */
+	/*  allow expansion of pagelist to 16 chunks
+	**  which accommodates backwards-sequential index scans
+	**  where the scanner increases target_prefetch_pages by a factor of up to 16
+	**   see code in _bt_steppage
+	**  note - this creates an undesirable weak dependency on this number in _bt_steppage,
+	**         but :
+	**           there is no disaster if the numbers disagree  -  just sub-optimal use of the list
+	**           to implement a proper interface would require that chunks have a variable size
+	**           which would require an extra size variable in each chunk
+	*/
+	int num_chunks = 16;
+
+		if ((struct pfch_index_pagelist**)0 == pfch_index_page_list_P) {
+			pfch_index_page_list_anchor = (struct pfch_index_pagelist*)0;
+		} else {
+			pfch_index_page_list_anchor = *pfch_index_page_list_P;
+		}
+		pfch_index_plp = pfch_index_page_list_anchor;       /* pointer to current chunk */
+
+		while ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+			ix = pfch_index_plp->pfch_index_item_count;
+			if (ix < target_prefetch_pages) {
+				pfch_index_plp->pfch_indexid[ix].pfch_blocknum = blkno;
+				pfch_index_plp->pfch_indexid[ix].pfch_discard = discard_status;
+				pfch_index_plp->pfch_index_item_count = (ix+1);
+                rc = 0;
+				goto stored_pagenum;
+			}
+			pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+			num_chunks--;              /*  keep track of number of chunks */
+		}
+
+		/*   we did not find any free space in existing chunks -
+		**   create new chunk if within our limit and we have a pfch_index_page_list
+		*/
+		if ( (num_chunks > 0) && ((struct pfch_index_pagelist*)0 != pfch_index_page_list_anchor) ) {
+			pfch_index_plp = (struct pfch_index_pagelist*)palloc( sizeof(struct pfch_index_pagelist) + ( (target_prefetch_pages-1) * sizeof(struct pfch_index_item) ) );
+			if ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+				pfch_index_plp->pfch_index_pagelist_next = pfch_index_page_list_anchor;  /* old head of list is next after this */
+				pfch_index_plp->pfch_indexid[0].pfch_blocknum = blkno;
+				pfch_index_plp->pfch_indexid[0].pfch_discard = discard_status;
+				pfch_index_plp->pfch_index_item_count = 1;
+				pfch_index_page_list_P = &pfch_index_plp;   /*  new head of list is new chunk */
+                rc = 0;
+			}
+		}
+
+    stored_pagenum:;
+     return rc;
+}
+
+/*  _bt_subtract_block removes a block from the prefetched-but-unread pagelist if present */
+void
+_bt_subtract_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list)
+{
+    struct pfch_index_pagelist* pfch_index_plp = pfch_index_page_list;
+	if ( (blkno != P_NEW) && (blkno != P_NONE) ) {
+            int ix , jx;
+                while ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+                    ix = pfch_index_plp->pfch_index_item_count;
+                    while (ix-- > 0) {
+                        if (pfch_index_plp->pfch_indexid[ix].pfch_blocknum == blkno) {
+                            /*   move the last item to the curent (now deleted) position and decrement count */
+                            jx = (pfch_index_plp->pfch_index_item_count-1); /*  index of last item ... */
+                            if (jx > ix) {                                  /*  ... is not the current one so move is required */
+                                pfch_index_plp->pfch_indexid[ix].pfch_blocknum = pfch_index_plp->pfch_indexid[jx].pfch_blocknum;
+                                pfch_index_plp->pfch_indexid[ix].pfch_discard = pfch_index_plp->pfch_indexid[jx].pfch_discard;
+                                ix = jx;
+                            }
+                            pfch_index_plp->pfch_index_item_count = ix;
+                            goto done_subtract;
+                        }
+                    }
+                    pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+                }
+        }
+    done_subtract:  return;
+}
+
+/*
  *	_bt_getbuf() -- Get a buffer by block number for read or write.
  *
  *		blkno == P_NEW means to get an unallocated index page.  The page
@@ -573,7 +737,7 @@ _bt_log_reuse_page(Relation rel, BlockNu
  *		_bt_checkpage to sanity-check the page (except in P_NEW case).
  */
 Buffer
-_bt_getbuf(Relation rel, BlockNumber blkno, int access)
+_bt_getbuf(Relation rel, BlockNumber blkno, int access , struct pfch_index_pagelist* pfch_index_page_list)
 {
 	Buffer		buf;
 
@@ -581,6 +745,10 @@ _bt_getbuf(Relation rel, BlockNumber blk
 	{
 		/* Read an existing block of the relation */
 		buf = ReadBuffer(rel, blkno);
+
+        /*  if the block is in the prefetched-but-unread pagelist,  remove it */
+        _bt_subtract_block( blkno , pfch_index_page_list);
+
 		LockBuffer(buf, access);
 		_bt_checkpage(rel, buf);
 	}
@@ -702,6 +870,10 @@ _bt_getbuf(Relation rel, BlockNumber blk
  * bufmgr when one would do.  However, now it's mainly just a notational
  * convenience.  The only case where it saves work over _bt_relbuf/_bt_getbuf
  * is when the target page is the same one already in the buffer.
+ *
+ * if prefetching of index pages is changed to use this function,
+ * then it should be extended to take the index_page_list as parameter
+ * and call_bt_subtract_block in the same way that _bt_getbuf does.
  */
 Buffer
 _bt_relandgetbuf(Relation rel, Buffer obuf, BlockNumber blkno, int access)
@@ -712,6 +884,7 @@ _bt_relandgetbuf(Relation rel, Buffer ob
 	if (BufferIsValid(obuf))
 		LockBuffer(obuf, BUFFER_LOCK_UNLOCK);
 	buf = ReleaseAndReadBuffer(obuf, rel, blkno);
+
 	LockBuffer(buf, access);
 	_bt_checkpage(rel, buf);
 	return buf;
@@ -965,7 +1138,7 @@ _bt_is_page_halfdead(Relation rel, Block
 	BTPageOpaque opaque;
 	bool		result;
 
-	buf = _bt_getbuf(rel, blk, BT_READ);
+	buf = _bt_getbuf(rel, blk, BT_READ , (struct pfch_index_pagelist*)0);
 	page = BufferGetPage(buf);
 	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
@@ -1069,7 +1242,7 @@ _bt_lock_branch_parent(Relation rel, Blo
 				Page		lpage;
 				BTPageOpaque lopaque;
 
-				lbuf = _bt_getbuf(rel, leftsib, BT_READ);
+				lbuf = _bt_getbuf(rel, leftsib, BT_READ, (struct pfch_index_pagelist*)0);
 				lpage = BufferGetPage(lbuf);
 				lopaque = (BTPageOpaque) PageGetSpecialPointer(lpage);
 
@@ -1265,7 +1438,7 @@ _bt_pagedel(Relation rel, Buffer buf)
 					BTPageOpaque lopaque;
 					Page		lpage;
 
-					lbuf = _bt_getbuf(rel, leftsib, BT_READ);
+					lbuf = _bt_getbuf(rel, leftsib, BT_READ, (struct pfch_index_pagelist*)0);
 					lpage = BufferGetPage(lbuf);
 					lopaque = (BTPageOpaque) PageGetSpecialPointer(lpage);
 					/*
@@ -1340,7 +1513,7 @@ _bt_pagedel(Relation rel, Buffer buf)
 		if (!rightsib_empty)
 			break;
 
-		buf = _bt_getbuf(rel, rightsib, BT_WRITE);
+		buf = _bt_getbuf(rel, rightsib, BT_WRITE, (struct pfch_index_pagelist*)0);
 	}
 
 	return ndeleted;
@@ -1593,7 +1766,7 @@ _bt_unlink_halfdead_page(Relation rel, B
 		target = topblkno;
 
 		/* fetch the block number of the topmost parent's left sibling */
-		buf = _bt_getbuf(rel, topblkno, BT_READ);
+		buf = _bt_getbuf(rel, topblkno, BT_READ, (struct pfch_index_pagelist*)0);
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		leftsib = opaque->btpo_prev;
@@ -1632,7 +1805,7 @@ _bt_unlink_halfdead_page(Relation rel, B
 		LockBuffer(leafbuf, BT_WRITE);
 	if (leftsib != P_NONE)
 	{
-		lbuf = _bt_getbuf(rel, leftsib, BT_WRITE);
+		lbuf = _bt_getbuf(rel, leftsib, BT_WRITE , (struct pfch_index_pagelist*)0);
 		page = BufferGetPage(lbuf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		while (P_ISDELETED(opaque) || opaque->btpo_next != target)
@@ -1646,7 +1819,7 @@ _bt_unlink_halfdead_page(Relation rel, B
 					 RelationGetRelationName(rel));
 				return false;
 			}
-			lbuf = _bt_getbuf(rel, leftsib, BT_WRITE);
+			lbuf = _bt_getbuf(rel, leftsib, BT_WRITE , (struct pfch_index_pagelist*)0);
 			page = BufferGetPage(lbuf);
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		}
@@ -1701,7 +1874,7 @@ _bt_unlink_halfdead_page(Relation rel, B
 	 * And next write-lock the (current) right sibling.
 	 */
 	rightsib = opaque->btpo_next;
-	rbuf = _bt_getbuf(rel, rightsib, BT_WRITE);
+	rbuf = _bt_getbuf(rel, rightsib, BT_WRITE , (struct pfch_index_pagelist*)0);
 	page = BufferGetPage(rbuf);
 	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 	if (opaque->btpo_prev != target)
@@ -1731,7 +1904,7 @@ _bt_unlink_halfdead_page(Relation rel, B
 		if (P_RIGHTMOST(opaque))
 		{
 			/* rightsib will be the only one left on the level */
-			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE , (struct pfch_index_pagelist*)0);
 			metapg = BufferGetPage(metabuf);
 			metad = BTPageGetMeta(metapg);
 
--- src/backend/access/nbtree/nbtree.c.orig	2014-06-08 11:26:29.000000000 -0400
+++ src/backend/access/nbtree/nbtree.c	2014-06-08 21:59:37.172097275 -0400
@@ -30,6 +30,18 @@
 #include "tcop/tcopprot.h"
 #include "utils/memutils.h"
 
+#ifdef USE_PREFETCH
+extern unsigned int prefetch_index_scans; /* boolean whether to prefetch non-bitmap index scans also numeric size of pfch_list */
+#endif   /* USE_PREFETCH */
+
+Datum
+btpeeknexttuple(
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+        IndexScanDesc scan
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+        PG_FUNCTION_ARGS
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+);
 
 /* Working state for btbuild and its callback */
 typedef struct
@@ -332,6 +344,74 @@ btgettuple(PG_FUNCTION_ARGS)
 }
 
 /*
+ *	btpeeknexttuple() -- peek at the next tuple different from any blocknum in pfch_block_item_list
+ *                           without reading a new index page
+ *                       and without causing any side-effects such as altering values in control blocks
+ *               if found,     store blocknum in next element of pfch_block_item_list
+ */
+Datum
+btpeeknexttuple(
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+        IndexScanDesc scan
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+        PG_FUNCTION_ARGS
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+)
+{
+#ifndef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+    IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+	BTScanOpaque so = (BTScanOpaque) scan->opaque;
+	bool		res = false;
+	int		itemIndex;		/* current index in items[] */
+
+        /*
+         * If we've already initialized this scan, we can just advance it in
+         * the appropriate direction.  If we haven't done so yet, bail out
+         */
+        if ( BTScanPosIsValid(so->currPos) ) {
+
+            itemIndex = so->currPos.itemIndex+1;    /*   next item */
+
+            /* This loop handles advancing till we find different data block or end of index page */
+            while (itemIndex <= so->currPos.lastItem) {
+                unsigned short int pfchx; /*  index in BlockIdData array   */
+                for (pfchx = 0; pfchx < scan->pfch_used; pfchx++) {
+                        if (BlockIdEquals((&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)) , &(so->currPos.items[itemIndex].heapTid.ip_blkid))) {
+                             goto block_match;
+                        }
+                }
+
+                /*  if we reach here,  no block in list matched this item  */
+                res = true;
+                /*   set item in prefetch list
+                **   prefer unused entry if there is one,  else overwrite
+                */
+                if (scan->pfch_used < prefetch_index_scans) {
+                    scan->pfch_next = scan->pfch_used;
+                } else {
+                    scan->pfch_next++;
+                    if (scan->pfch_next >= prefetch_index_scans) {
+                        scan->pfch_next = 0;
+                    }
+                }
+
+                BlockIdCopy((&((scan->pfch_block_item_list + scan->pfch_next)->pfch_blockid)) , &(so->currPos.items[itemIndex].heapTid.ip_blkid));
+                if (scan->pfch_used <= scan->pfch_next) {
+                     scan->pfch_used = (scan->pfch_next + 1);
+                }
+
+                goto peek_complete;
+
+      block_match:         itemIndex++;
+            }
+	}
+
+ peek_complete:
+	PG_RETURN_BOOL(res);
+}
+
+/*
  * btgetbitmap() -- gets all matching tuples, and adds them to a bitmap
  */
 Datum
@@ -425,6 +505,12 @@ btbeginscan(PG_FUNCTION_ARGS)
 	so->killedItems = NULL;		/* until needed */
 	so->numKilled = 0;
 
+	so->backSeqRun = 0;
+	so->backSeqPos = 0;
+	so->prefetchItemIndex = 0;
+	so->lastHeapPrefetchBlkno = P_NONE;
+	so->prefetchBlockCount = 0;
+	
 	/*
 	 * We don't know yet whether the scan will be index-only, so we do not
 	 * allocate the tuple workspace arrays until btrescan.  However, we set up
@@ -516,6 +602,23 @@ btendscan(PG_FUNCTION_ARGS)
 {
 	IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
+        struct pfch_index_pagelist* pfch_index_plp;
+        int ix;
+
+#ifdef USE_PREFETCH
+
+	/* discard all prefetched but unread index pages listed in the pagelist */
+        pfch_index_plp = scan->pfch_index_page_list;
+        while ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+                    ix = pfch_index_plp->pfch_index_item_count;
+                    while (ix-- > 0) {
+                        if (pfch_index_plp->pfch_indexid[ix].pfch_discard) {
+                            DiscardBuffer( scan->indexRelation , MAIN_FORKNUM , pfch_index_plp->pfch_indexid[ix].pfch_blocknum);
+                        }
+                    }
+                    pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+        }
+#endif /* USE_PREFETCH */
 
 	/* we aren't holding any read locks, but gotta drop the pins */
 	if (BTScanPosIsValid(so->currPos))
--- src/backend/nodes/tidbitmap.c.orig	2014-06-08 11:26:29.000000000 -0400
+++ src/backend/nodes/tidbitmap.c	2014-06-08 21:59:37.216097351 -0400
@@ -44,6 +44,9 @@
 #include "nodes/bitmapset.h"
 #include "nodes/tidbitmap.h"
 #include "utils/hsearch.h"
+#ifdef USE_PREFETCH
+extern int	target_prefetch_pages;
+#endif   /* USE_PREFETCH */
 
 /*
  * The maximum number of tuples per page is not large (typically 256 with
@@ -572,7 +575,12 @@ tbm_begin_iterate(TIDBitmap *tbm)
 	 * needs of the TBMIterateResult sub-struct.
 	 */
 	iterator = (TBMIterator *) palloc(sizeof(TBMIterator) +
-								 MAX_TUPLES_PER_PAGE * sizeof(OffsetNumber));
+								 MAX_TUPLES_PER_PAGE * sizeof(OffsetNumber)
+#ifdef USE_PREFETCH
+                                          		      /*  space for remembering every prefetched but unread blockno */
+                                          		      +  (target_prefetch_pages * sizeof(BlockNumber))
+#endif   /* USE_PREFETCH */
+                                         );
 	iterator->tbm = tbm;
 
 	/*
@@ -1020,3 +1028,68 @@ tbm_comparator(const void *left, const v
 		return 1;
 	return 0;
 }
+
+void
+tbm_zero(TBMIterator *iterator) /* zero list of prefetched and unread blocknos */
+{
+      /* locate the list of prefetched but unread blocknos immediately following the array of offsets
+      ** and note that tbm_begin_iterate allocates space for (1 + MAX_TUPLES_PER_PAGE) offsets  -
+      ** 1 included in struct TBMIterator and MAX_TUPLES_PER_PAGE additional
+      */
+      iterator->output.Unread_Pfetched_base = ((BlockNumber *)(&(iterator->output.offsets[MAX_TUPLES_PER_PAGE+1])));
+      iterator->output.Unread_Pfetched_next = iterator->output.Unread_Pfetched_count = 0;
+}
+
+void
+tbm_add(TBMIterator *iterator, BlockNumber blockno) /* add this blockno to list of prefetched and unread blocknos */
+{
+      unsigned int Unread_Pfetched_index = iterator->output.Unread_Pfetched_next + iterator->output.Unread_Pfetched_count++;
+
+      if (iterator->output.Unread_Pfetched_count > target_prefetch_pages) {
+		ereport(ERROR,
+			(errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("tbm_add overflowed list cannot add blockno %d", blockno)));
+      }
+
+      if (Unread_Pfetched_index >= target_prefetch_pages)
+          Unread_Pfetched_index -= target_prefetch_pages;
+      *(iterator->output.Unread_Pfetched_base + Unread_Pfetched_index) = blockno;
+}
+
+void
+tbm_subtract(TBMIterator *iterator, BlockNumber blockno) /* remove this blockno from list of prefetched and unread blocknos */
+{
+      unsigned int Unread_Pfetched_index = iterator->output.Unread_Pfetched_next++;
+      BlockNumber nextUnreadPfetched;
+
+      /*    make a weak check that the next blockno is the one to be removed,
+      **    although actually in case of disagreement,   we ignore callers blockno and remove next anyway,
+      **    which is really what caller wants
+      */
+      if ( iterator->output.Unread_Pfetched_count == 0 ) {
+		ereport(ERROR,
+			(errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("tbm_subtract empty list cannot subtract blockno %d", blockno)));
+      }
+
+      if (Unread_Pfetched_index >= target_prefetch_pages)
+          Unread_Pfetched_index = 0;
+      nextUnreadPfetched = *(iterator->output.Unread_Pfetched_base + Unread_Pfetched_index);
+      if (   ( nextUnreadPfetched != blockno ) 
+          && ( nextUnreadPfetched != InvalidBlockNumber ) /* dont report it if the block in the list was InvalidBlockNumber */
+         ) {
+		ereport(NOTICE,
+			(errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("tbm_subtract will subtract blockno %d not %d",
+					nextUnreadPfetched, blockno)));
+      }
+      if (iterator->output.Unread_Pfetched_next >= target_prefetch_pages)
+          iterator->output.Unread_Pfetched_next = 0;
+      iterator->output.Unread_Pfetched_count--;
+}
+
+TBMIterateResult *
+tbm_locate_IterateResult(TBMIterator *iterator)
+{
+   return &(iterator->output);
+}
--- src/backend/utils/misc/guc.c.orig	2014-06-08 11:26:31.000000000 -0400
+++ src/backend/utils/misc/guc.c	2014-06-08 21:59:37.284097469 -0400
@@ -2264,6 +2264,25 @@ static struct config_int ConfigureNamesI
 	},
 
 	{
+		{"max_async_io_prefetchers",
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+			PGC_USERSET,
+#else
+			PGC_INTERNAL,
+#endif
+			RESOURCES_ASYNCHRONOUS,
+			gettext_noop("Maximum number of background processes concurrently using asynchronous librt threads to prefetch pages into shared memory buffers."),
+		},
+		&max_async_io_prefetchers,
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+	        -1, 0, 8192,      /*  boot val -1 indicates to initialize to something sensible during buf_init */
+#else
+		0, 0, 0,
+#endif
+		NULL, NULL, NULL
+	},
+
+	{
 		{"max_worker_processes",
 			PGC_POSTMASTER,
 			RESOURCES_ASYNCHRONOUS,
--- src/backend/utils/mmgr/aset.c.orig	2014-06-08 11:26:31.000000000 -0400
+++ src/backend/utils/mmgr/aset.c	2014-06-08 21:59:37.324097538 -0400
@@ -733,6 +733,48 @@ AllocSetAlloc(MemoryContext context, Siz
 	 */
 	fidx = AllocSetFreeIndex(size);
 	chunk = set->freelist[fidx];
+#ifdef MEMORY_CONTEXT_CHECKING
+        /*    an instance of segfault caused by a rogue value in set->freelist[fidx]
+        **    has been seen - check for it using crude sanity check based on neighbours :
+        **    if at least one is sufficiently close, then pass,  else fail
+        */
+        if (chunk != 0) {
+            int frx, nrx; /*  frx is index,  nrx is index of failing neighbour for errmsg */
+            for (nrx = -1, frx = 0; (frx < ALLOCSET_NUM_FREELISTS); frx++) {
+                if (   (frx != fidx)     /*  not the chosen one */
+                    && ( ( (unsigned long)(set->freelist[frx]) ) != 0 ) /* not empty */
+                   ) {
+                    if (   ( (unsigned long)chunk < ( ( (unsigned long)(set->freelist[frx]) ) / 2 ) )
+                        && (  ( (unsigned long)(set->freelist[frx]) ) < 0x4000000  )
+               /***     || ( (unsigned long)chunk > ( ( (unsigned long)(set->freelist[frx]) ) * 2 ) )  ***/
+                       ) {
+                       nrx = frx;
+                    } else {
+                       nrx = -1;
+                       break;
+                    }
+                }
+            }
+
+            if (nrx >= 0) {
+                     /*  this must be a rogue value  -  put this list in the garbage */
+                     /*  build message but be careful to avoid recursively triggering same fault */
+                     set->freelist[fidx] = NULL;   /*  mark this list empty */
+                     elog(WARNING, "detected rogue value %p in freelist index %d compared with neighbour %p whose chunksize %d"
+				 , chunk , fidx , set->freelist[nrx] , set->freelist[nrx]->size);
+                     chunk = NULL;
+            }
+        }
+#else /* if not MEMORY_CONTEXT_CHECKING make very simple-minded check*/
+        if ( (chunk != 0) && ( (unsigned long)chunk <  0x40000 ) ) {
+                     /*  this must be a rogue value  -  put this list in the garbage */
+                     /*  build message but be careful to avoid recursively triggering same fault */
+                     set->freelist[fidx] = NULL;   /*  mark this list empty */
+                     elog(WARNING, "detected rogue value %p in freelist index %d"
+				 , chunk , fidx);
+                     chunk = NULL;
+        }
+#endif
 	if (chunk != NULL)
 	{
 		Assert(chunk->size >= size);
--- src/include/executor/instrument.h.orig	2014-06-08 11:26:31.000000000 -0400
+++ src/include/executor/instrument.h	2014-06-08 21:59:37.608098030 -0400
@@ -28,8 +28,18 @@ typedef struct BufferUsage
 	long		local_blks_written;		/* # of local disk blocks written */
 	long		temp_blks_read; /* # of temp blocks read */
 	long		temp_blks_written;		/* # of temp blocks written */
+
 	instr_time	blk_read_time;	/* time spent reading */
 	instr_time	blk_write_time; /* time spent writing */
+
+	long		aio_read_noneed;		/* # of prefetches for which no need for prefetch as block already in buffer pool */
+	long		aio_read_discrd;		/* # of prefetches for which no need for prefetch as block already in buffer pool */
+	long		aio_read_forgot;		/* # of prefetches for which no need for prefetch as block already in buffer pool */
+	long		aio_read_noblok;		/* # of prefetches for which no available BufferAiocb */
+	long		aio_read_failed;		/* # of aio reads for which aio itself failed or the read failed with an errno */
+	long		aio_read_wasted;		/* # of aio reads for which disk block not used */
+	long		aio_read_waited;		/* # of aio reads for which disk block used but had to wait for it */
+	long		aio_read_ontime;		/* # of aio reads for which disk block used and ready on time when requested */
 } BufferUsage;
 
 /* Flag bits included in InstrAlloc's instrument_options bitmask */
--- src/include/storage/bufmgr.h.orig	2014-06-08 11:26:31.000000000 -0400
+++ src/include/storage/bufmgr.h	2014-06-08 21:59:37.644098093 -0400
@@ -41,6 +41,7 @@ typedef enum
 	RBM_ZERO_ON_ERROR,			/* Read, but return an all-zeros page on error */
 	RBM_NORMAL_NO_LOG			/* Don't log page as invalid during WAL
 								 * replay; otherwise same as RBM_NORMAL */
+       ,RBM_NOREAD_FOR_PREFETCH   /* Don't read from disk, don't zero buffer, find buffer only */
 } ReadBufferMode;
 
 /* in globals.c ... this duplicates miscadmin.h */
@@ -57,6 +58,9 @@ extern int	target_prefetch_pages;
 extern PGDLLIMPORT char *BufferBlocks;
 extern PGDLLIMPORT int32 *PrivateRefCount;
 
+/*  in buf_async.c  */;
+extern int      max_async_io_prefetchers; /* Maximum number of backends using asynchronous librt threads to read pages into our buffers */
+
 /* in localbuf.c */
 extern PGDLLIMPORT int NLocBuffer;
 extern PGDLLIMPORT Block *LocalBufferBlockPointers;
@@ -159,9 +163,15 @@ extern PGDLLIMPORT int32 *LocalRefCount;
 #define BufferGetPage(buffer) ((Page)BufferGetBlock(buffer))
 
 /*
- * prototypes for functions in bufmgr.c
+ * prototypes for external functions in bufmgr.c and buf_async.c
  */
-extern void PrefetchBuffer(Relation reln, ForkNumber forkNum,
+extern int PrefetchBuffer(Relation reln, ForkNumber forkNum,
+			   BlockNumber blockNum , BufferAccessStrategy strategy);
+/*   return code  is an int bitmask : */
+#define PREFTCHRC_BUF_PIN_INCREASED 0x01    /*  pin count on buffer has been increased by 1 */
+#define PREFTCHRC_BLK_ALREADY_PRESENT 0x02  /*  block was already present in a buffer       */
+
+extern void DiscardBuffer(Relation reln, ForkNumber forkNum,
 			   BlockNumber blockNum);
 extern Buffer ReadBuffer(Relation reln, BlockNumber blockNum);
 extern Buffer ReadBufferExtended(Relation reln, ForkNumber forkNum,
--- src/include/storage/smgr.h.orig	2014-06-08 11:26:31.000000000 -0400
+++ src/include/storage/smgr.h	2014-06-08 21:59:37.664098127 -0400
@@ -92,6 +92,12 @@ extern void smgrextend(SMgrRelation reln
 		   BlockNumber blocknum, char *buffer, bool skipFsync);
 extern void smgrprefetch(SMgrRelation reln, ForkNumber forknum,
 			 BlockNumber blocknum);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern void smgrinitaio(int max_aio_threads, int max_aio_num);
+extern void smgrstartaio(SMgrRelation reln, ForkNumber forknum,
+			 BlockNumber blocknum , char *aiocbp , int *retcode);
+extern void smgrcompleteaio( SMgrRelation reln, char *aiocbp , int *inoutcode );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 extern void smgrread(SMgrRelation reln, ForkNumber forknum,
 		 BlockNumber blocknum, char *buffer);
 extern void smgrwrite(SMgrRelation reln, ForkNumber forknum,
@@ -118,6 +124,11 @@ extern void mdextend(SMgrRelation reln,
 		 BlockNumber blocknum, char *buffer, bool skipFsync);
 extern void mdprefetch(SMgrRelation reln, ForkNumber forknum,
 		   BlockNumber blocknum);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern void mdinitaio(int max_aio_threads, int max_aio_num);
+extern void mdstartaio(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum , char *aiocbp , int *retcode );
+extern void mdcompleteaio( char *aiocbp , int *inoutcode );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 extern void mdread(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 	   char *buffer);
 extern void mdwrite(SMgrRelation reln, ForkNumber forknum,
--- src/include/storage/fd.h.orig	2014-06-08 11:26:31.000000000 -0400
+++ src/include/storage/fd.h	2014-06-08 21:59:37.684098162 -0400
@@ -69,6 +69,11 @@ extern File PathNameOpenFile(FileName fi
 extern File OpenTemporaryFile(bool interXact);
 extern void FileClose(File file);
 extern int	FilePrefetch(File file, off_t offset, int amount);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern void FileInitaio(int max_aio_threads, int max_aio_num );
+extern int  FileStartaio(File file, off_t offset, int amount , char *aiocbp);
+extern int  FileCompleteaio( char *aiocbp , int normal_wait );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 extern int	FileRead(File file, char *buffer, int amount);
 extern int	FileWrite(File file, char *buffer, int amount);
 extern int	FileSync(File file);
--- src/include/storage/buf_internals.h.orig	2014-06-08 11:26:31.000000000 -0400
+++ src/include/storage/buf_internals.h	2014-06-08 21:59:37.708098204 -0400
@@ -22,7 +22,9 @@
 #include "storage/smgr.h"
 #include "storage/spin.h"
 #include "utils/relcache.h"
-
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+#include "aio.h"
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * Flags for buffer descriptors
@@ -38,8 +40,23 @@
 #define BM_JUST_DIRTIED			(1 << 5)		/* dirtied since write started */
 #define BM_PIN_COUNT_WAITER		(1 << 6)		/* have waiter for sole pin */
 #define BM_CHECKPOINT_NEEDED	(1 << 7)		/* must write for checkpoint */
-#define BM_PERMANENT			(1 << 8)		/* permanent relation (not
-												 * unlogged) */
+#define BM_PERMANENT			(1 << 8)	/* permanent relation (not unlogged) */
+#define BM_AIO_IN_PROGRESS		(1 << 9)	/* aio in progress    */
+#define BM_AIO_PREFETCH_PIN_BANKED	(1 << 10)	/* pinned when prefetch issued
+                                                        ** and this pin is banked - i.e.
+                                                        ** redeemable by the next use by same task
+                                                        ** note that for any one buffer, a pin can be banked
+                                                        **      by at most one process globally,
+                                                        **      that is,   only one process may bank a pin on the buffer
+                                                        **                 and it may do so only once (may not be stacked)
+                                                        */
+
+/*********
+for asynchronous aio-read prefetching, two golden rules concerning buffer pinning and buffer-header flags must be observed:
+  R1.  a buffer marked as BM_AIO_IN_PROGRESS must be pinned by at least one backend
+  R2.  a buffer marked as BM_AIO_PREFETCH_PIN_BANKED must be pinned by the backend identified by
+               (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio : (-(buf->freeNext))
+*********/
 
 typedef bits16 BufFlags;
 
@@ -140,17 +157,83 @@ typedef struct sbufdesc
 	BufFlags	flags;			/* see bit definitions above */
 	uint16		usage_count;	/* usage counter for clock sweep code */
 	unsigned	refcount;		/* # of backends holding pins on buffer */
-	int			wait_backend_pid;		/* backend PID of pin-count waiter */
+	int		wait_backend_pid;	/*  if     flags & BM_PIN_COUNT_WAITER
+                                                **  then   backend PID of pin-count waiter
+                                                **  else   not set
+                                                */
 
 	slock_t		buf_hdr_lock;	/* protects the above fields */
 
 	int			buf_id;			/* buffer's index number (from 0) */
-	int			freeNext;		/* link in freelist chain */
+        int    	volatile	freeNext;	/* overloaded and much-abused field :
+                                                ** EITHER
+                                                **     if     >= 0
+                                                **     then   link in freelist chain
+                                                **  OR
+                                                **     if     <  0
+                                                **     then    EITHER
+                                                **             if     flags & BM_AIO_IN_PROGRESS
+                                                **             then   negative of (the index of the aiocb in the BufferAiocbs array + 3)
+                                                **             else   if flags & BM_AIO_PREFETCH_PIN_BANKED
+                                                **             then   -(pid of task that issued aio_read and pinned buffer)
+                                                **             else   one of the special values -1 or -2 listed below
+                                                */
 
 	LWLock	   *io_in_progress_lock;	/* to wait for I/O to complete */
 	LWLock	   *content_lock;	/* to lock access to buffer contents */
 } BufferDesc;
 
+/*  structures for control blocks for our implementation of async io */
+
+/*  if USE_AIO_ATOMIC_BUILTIN_COMP_SWAP is not defined,  the following struct is not put into use at runtime
+**  but it is easier to let the compiler find the definition but hide the reference to aiocb
+**  which is the only type it would not understand
+*/
+
+struct BufferAiocb {
+       struct BufferAiocb volatile * volatile BAiocbnext;  /*    next free entry or value of BAIOCB_OCCUPIED means in use  */
+       struct sbufdesc    volatile * volatile BAiocbbufh;  /*    there can be at most one BufferDesc marked BM_AIO_IN_PROGRESS
+                                                           **    and using this BufferAiocb -
+                                                           **    if there is one, BAiocbbufh points to it, else BAiocbbufh is zero
+                                                           **    NOTE  BAiocbbufh should be zero for every BufferAiocb on the free list
+                                                           */
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+       struct aiocb       volatile            BAiocbthis;  /*    the aio library's control block for one async io */
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+       int                volatile  BAiocbDependentCount;  /*    count of tasks who depend on this BufferAiocb
+                                                           **    in the sense that they are waiting for io completion.
+                                                           **    only a Dependent may move the BufferAiocb onto the freelist
+                                                           **    and only when that Dependent is the *only* Dependent (count == 1)
+                                                           **    BAiocbDependentCount is protected by bufferheader spinlock
+                                                           **    and must be updated only when that spinlock is held
+                                                           */
+       pid_t              volatile  pidOfAio;              /*    pid of backend who issued an aio_read using this BAiocb -
+                                                           **    this backend must have pinned the associated buffer.
+                                                           */
+};
+
+#define BAIOCB_OCCUPIED 0x75f1        /*  distinct indicator of a BufferAiocb.BAiocbnext that is NOT on free list */
+#define BAIOCB_FREE 0x7b9d            /*  distinct indicator of a BufferAiocb.BAiocbbufh that IS     on free list */
+
+struct BAiocbAnchor {                 /*  anchor for all control blocks pertaining to aio  */
+       volatile struct BufferAiocb* BufferAiocbs;          /*  aiocbs ...                   */
+       volatile struct BufferAiocb* volatile FreeBAiocbs; /* ... and their free list   */
+};
+
+/*   values for BufCheckAsync input and retcode */
+#define BUF_INTENTION_WANT 		 1  /* wants the buffer, wait for in-progress aio and then pin */
+#define BUF_INTENTION_REJECT_KEEP_PIN 	-1  /* pin already held, do not unpin */
+#define BUF_INTENTION_REJECT_OBTAIN_PIN	-2  /* obtain pin,  caller wants it for same buffer */
+#define BUF_INTENTION_REJECT_FORGET	-3  /* unpin and tell resource owner to forget */
+#define BUF_INTENTION_REJECT_NOADJUST	-4  /* unpin and call ResourceOwnerForgetBuffer */
+#define BUF_INTENTION_REJECT_UNBANK   	-5  /* unpin only if pin banked by caller */
+
+#define BUF_INTENT_RC_CHANGED_TAG	-5
+#define BUF_INTENT_RC_BADPAGE		-4
+#define BUF_INTENT_RC_INVALID_AIO	-3    /*  invalid and aio was in progress */
+#define BUF_INTENT_RC_INVALID_NO_AIO	-1    /*  invalid and no aio was in progress */
+#define BUF_INTENT_RC_VALID		 1
+
 #define BufferDescriptorGetBuffer(bdesc) ((bdesc)->buf_id + 1)
 
 /*
@@ -159,6 +242,7 @@ typedef struct sbufdesc
  */
 #define FREENEXT_END_OF_LIST	(-1)
 #define FREENEXT_NOT_IN_LIST	(-2)
+#define FREENEXT_BAIOCB_ORIGIN	(-3)
 
 /*
  * Macros for acquiring/releasing a shared buffer header's spinlock.
--- src/include/catalog/pg_am.h.orig	2014-06-08 11:26:31.000000000 -0400
+++ src/include/catalog/pg_am.h	2014-06-08 21:59:37.744098266 -0400
@@ -67,6 +67,7 @@ CATALOG(pg_am,2601)
 	regproc		amcanreturn;	/* can indexscan return IndexTuples? */
 	regproc		amcostestimate; /* estimate cost of an indexscan */
 	regproc		amoptions;		/* parse AM-specific parameters */
+	regproc		ampeeknexttuple;	/* peek at the next tuple different from any blocknum in pfch_list without reading a new index page */
 } FormData_pg_am;
 
 /* ----------------
@@ -117,19 +118,19 @@ typedef FormData_pg_am *Form_pg_am;
  * ----------------
  */
 
-DATA(insert OID = 403 (  btree		5 2 t f t t t t t t f t t 0 btinsert btbeginscan btgettuple btgetbitmap btrescan btendscan btmarkpos btrestrpos btbuild btbuildempty btbulkdelete btvacuumcleanup btcanreturn btcostestimate btoptions ));
+DATA(insert OID = 403 (  btree		5 2 t f t t t t t t f t t 0 btinsert btbeginscan btgettuple btgetbitmap btrescan btendscan btmarkpos btrestrpos btbuild btbuildempty btbulkdelete btvacuumcleanup btcanreturn btcostestimate btoptions btpeeknexttuple ));
 DESCR("b-tree index access method");
 #define BTREE_AM_OID 403
-DATA(insert OID = 405 (  hash		1 1 f f t f f f f f f f f 23 hashinsert hashbeginscan hashgettuple hashgetbitmap hashrescan hashendscan hashmarkpos hashrestrpos hashbuild hashbuildempty hashbulkdelete hashvacuumcleanup - hashcostestimate hashoptions ));
+DATA(insert OID = 405 (  hash		1 1 f f t f f f f f f f f 23 hashinsert hashbeginscan hashgettuple hashgetbitmap hashrescan hashendscan hashmarkpos hashrestrpos hashbuild hashbuildempty hashbulkdelete hashvacuumcleanup - hashcostestimate hashoptions - ));
 DESCR("hash index access method");
 #define HASH_AM_OID 405
-DATA(insert OID = 783 (  gist		0 8 f t f f t t f t t t f 0 gistinsert gistbeginscan gistgettuple gistgetbitmap gistrescan gistendscan gistmarkpos gistrestrpos gistbuild gistbuildempty gistbulkdelete gistvacuumcleanup - gistcostestimate gistoptions ));
+DATA(insert OID = 783 (  gist		0 8 f t f f t t f t t t f 0 gistinsert gistbeginscan gistgettuple gistgetbitmap gistrescan gistendscan gistmarkpos gistrestrpos gistbuild gistbuildempty gistbulkdelete gistvacuumcleanup - gistcostestimate gistoptions - ));
 DESCR("GiST index access method");
 #define GIST_AM_OID 783
-DATA(insert OID = 2742 (  gin		0 6 f f f f t t f f t f f 0 gininsert ginbeginscan - gingetbitmap ginrescan ginendscan ginmarkpos ginrestrpos ginbuild ginbuildempty ginbulkdelete ginvacuumcleanup - gincostestimate ginoptions ));
+DATA(insert OID = 2742 (  gin		0 6 f f f f t t f f t f f 0 gininsert ginbeginscan - gingetbitmap ginrescan ginendscan ginmarkpos ginrestrpos ginbuild ginbuildempty ginbulkdelete ginvacuumcleanup - gincostestimate ginoptions - ));
 DESCR("GIN index access method");
 #define GIN_AM_OID 2742
-DATA(insert OID = 4000 (  spgist	0 5 f f f f f t f t f f f 0 spginsert spgbeginscan spggettuple spggetbitmap spgrescan spgendscan spgmarkpos spgrestrpos spgbuild spgbuildempty spgbulkdelete spgvacuumcleanup spgcanreturn spgcostestimate spgoptions ));
+DATA(insert OID = 4000 (  spgist	0 5 f f f f f t f t f f f 0 spginsert spgbeginscan spggettuple spggetbitmap spgrescan spgendscan spgmarkpos spgrestrpos spgbuild spgbuildempty spgbulkdelete spgvacuumcleanup spgcanreturn spgcostestimate spgoptions - ));
 DESCR("SP-GiST index access method");
 #define SPGIST_AM_OID 4000
 
--- src/include/catalog/pg_proc.h.orig	2014-06-08 11:26:31.000000000 -0400
+++ src/include/catalog/pg_proc.h	2014-06-08 21:59:37.796098356 -0400
@@ -536,6 +536,12 @@ DESCR("convert float4 to int4");
 
 DATA(insert OID = 330 (  btgettuple		   PGNSP PGUID 12 1 0 0 0 f f f f t f v 2 0 16 "2281 2281" _null_ _null_ _null_ _null_	btgettuple _null_ _null_ _null_ ));
 DESCR("btree(internal)");
+
+#ifndef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+DATA(insert OID = 3255 (  btpeeknexttuple	   PGNSP PGUID 12 1 0 0 0 f f f f t f v 1 0 16 "2281" _null_ _null_ _null_ _null_ btpeeknexttuple _null_ _null_ _null_ ));
+DESCR("btree(internal)");
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+
 DATA(insert OID = 636 (  btgetbitmap	   PGNSP PGUID 12 1 0 0 0 f f f f t f v 2 0 20 "2281 2281" _null_ _null_ _null_ _null_	btgetbitmap _null_ _null_ _null_ ));
 DESCR("btree(internal)");
 DATA(insert OID = 331 (  btinsert		   PGNSP PGUID 12 1 0 0 0 f f f f t f v 6 0 16 "2281 2281 2281 2281 2281 2281" _null_ _null_ _null_ _null_	btinsert _null_ _null_ _null_ ));
--- src/include/pg_config_manual.h.orig	2014-06-08 11:26:31.000000000 -0400
+++ src/include/pg_config_manual.h	2014-06-08 21:59:37.836098426 -0400
@@ -138,9 +138,11 @@
 /*
  * USE_PREFETCH code should be compiled only if we have a way to implement
  * prefetching.  (This is decoupled from USE_POSIX_FADVISE because there
- * might in future be support for alternative low-level prefetch APIs.)
+ * might in future be support for alternative low-level prefetch APIs  --
+ * -- update October 2013  -- now there is such a new prefetch capability --
+ *   async_io into postgres buffers  -   configuration parameter max_async_io_threads)
  */
-#ifdef USE_POSIX_FADVISE
+#if defined(USE_POSIX_FADVISE) || defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP)
 #define USE_PREFETCH
 #endif
 
--- src/include/access/nbtree.h.orig	2014-06-08 11:26:31.000000000 -0400
+++ src/include/access/nbtree.h	2014-06-08 21:59:37.864098474 -0400
@@ -19,6 +19,7 @@
 #include "access/sdir.h"
 #include "access/xlog.h"
 #include "access/xlogutils.h"
+#include "access/relscan.h"
 #include "catalog/pg_index.h"
 
 /* There's room for a 16-bit vacuum cycle ID in BTPageOpaqueData */
@@ -524,6 +525,7 @@ typedef struct BTScanPosData
 	Buffer		buf;			/* if valid, the buffer is pinned */
 
 	BlockNumber nextPage;		/* page's right link when we scanned it */
+	BlockNumber prevPage;		/* page's left link when we scanned it */
 
 	/*
 	 * moreLeft and moreRight track whether we think there may be matching
@@ -603,6 +605,15 @@ typedef struct BTScanOpaqueData
 	 */
 	int			markItemIndex;	/* itemIndex, or -1 if not valid */
 
+	/* prefetch logic state */
+	unsigned int	backSeqRun;	/* number of back-sequential pages in a run */
+	BlockNumber		backSeqPos;	/* blkid last prefetched in back-sequential 
+				          		   runs */
+	BlockNumber		lastHeapPrefetchBlkno;	/* blkid last prefetched from heap */
+	int				prefetchItemIndex; /* item index within currPos last
+					                      fetched by heap prefetch */
+	int				prefetchBlockCount; /* number of prefetched heap blocks */
+	
 	/* keep these last in struct for efficiency */
 	BTScanPosData currPos;		/* current position data */
 	BTScanPosData markPos;		/* marked position, if any */
@@ -655,7 +666,11 @@ extern Buffer _bt_getroot(Relation rel,
 extern Buffer _bt_gettrueroot(Relation rel);
 extern int	_bt_getrootheight(Relation rel);
 extern void _bt_checkpage(Relation rel, Buffer buf);
-extern Buffer _bt_getbuf(Relation rel, BlockNumber blkno, int access);
+extern Buffer _bt_getbuf(Relation rel, BlockNumber blkno, int access , struct pfch_index_pagelist* pfch_index_page_list);
+extern void _bt_prefetchbuf(Relation rel, BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P);
+extern struct pfch_index_item* _bt_find_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list);
+extern int _bt_add_block(BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P , uint32 discard_status);
+extern void _bt_subtract_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list);
 extern Buffer _bt_relandgetbuf(Relation rel, Buffer obuf,
 				 BlockNumber blkno, int access);
 extern void _bt_relbuf(Relation rel, Buffer buf);
--- src/include/access/heapam.h.orig	2014-06-08 11:26:31.000000000 -0400
+++ src/include/access/heapam.h	2014-06-08 21:59:37.892098523 -0400
@@ -175,7 +175,7 @@ extern void heap_page_prune_execute(Buff
 extern void heap_get_root_tuples(Page page, OffsetNumber *root_offsets);
 
 /* in heap/syncscan.c */
-extern void ss_report_location(Relation rel, BlockNumber location);
+extern void ss_report_location(Relation rel, BlockNumber location , BlockNumber *prefetchHWMp);
 extern BlockNumber ss_get_location(Relation rel, BlockNumber relnblocks);
 extern void SyncScanShmemInit(void);
 extern Size SyncScanShmemSize(void);
--- src/include/access/relscan.h.orig	2014-06-08 11:26:31.000000000 -0400
+++ src/include/access/relscan.h	2014-06-08 21:59:37.920098571 -0400
@@ -44,6 +44,24 @@ typedef struct HeapScanDescData
 	bool		rs_inited;		/* false = scan not init'd yet */
 	HeapTupleData rs_ctup;		/* current tuple in scan, if any */
 	BlockNumber rs_cblock;		/* current block # in scan, if any */
+#ifdef USE_PREFETCH
+	int	    rs_prefetch_target; /* target distance (numblocks) for prefetch to reach beyond main scan */
+	BlockNumber rs_pfchblock;	/* next block # to be prefetched in scan, if any */
+
+        /*   Unread_Pfetched is a "mostly" circular list of recently prefetched blocknos of size target_prefetch_pages
+        **   the index of the first unread block is held in Unread_Pfetched_next
+        **         and is advanced when a block is read
+        **   the count of number of unread blocks is in Unread_Pfetched_count (and this subset can wrap around)
+        **   "mostly" means that there may be gaps caused by storing entries for blocks which do not need to be discarded -
+        **   these are indicated by blockno = InvalidBlockNumber,  and these slots are reused when found.
+        */
+        BlockNumber *rs_Unread_Pfetched_base;   /*  where the list of prefetched but unread blocknos starts */
+        unsigned int rs_Unread_Pfetched_next;   /*  where the next unread blockno probably is relative to start --
+                                                **  this is only a hint which may be temporarily stale.
+                                                */
+        unsigned int rs_Unread_Pfetched_count;  /*  number of valid unread blocknos in list */
+#endif   /* USE_PREFETCH */
+
 	Buffer		rs_cbuf;		/* current buffer in scan, if any */
 	/* NB: if rs_cbuf is not InvalidBuffer, we hold a pin on that buffer */
 	ItemPointerData rs_mctid;	/* marked scan position, if any */
@@ -55,6 +73,27 @@ typedef struct HeapScanDescData
 	OffsetNumber rs_vistuples[MaxHeapTuplesPerPage];	/* their offsets */
 }	HeapScanDescData;
 
+/* pfch_index_items track prefetched and unread index pages -   chunks of blocknumbers are chained in singly-linked list from scan->pfch_index_item_list */
+struct pfch_index_item {                              /*  index-relation BlockIds which we will/have prefetched */
+       BlockNumber         pfch_blocknum;    /* Blocknum which we will/have prefetched */
+       uint32              pfch_discard;     /* whether block is to be discarded when scan is closed */
+};
+
+struct pfch_block_item {
+       struct BlockIdData  pfch_blockid;     /* BlockId which we will/have prefetched */
+       uint32              pfch_discard;     /* whether block is to be discarded when scan is closed */
+};
+
+/* pfch_index_page_items track prefetched and unread index pages -
+** chunks of blocknumbers are chained backwards (newest first,  oldest last)
+** in singly-linked list from scan->pfch_index_item_list
+*/
+struct pfch_index_pagelist {                          /*  index-relation BlockIds which we will/have prefetched */
+       struct pfch_index_pagelist* pfch_index_pagelist_next;  /*  pointer to next chunk if any */
+       unsigned int    pfch_index_item_count;         /*  number of used entries in this chunk */
+       struct pfch_index_item pfch_indexid[1];        /*  in-line list of Blocknums which we will/have prefetched and whether to be discarded */
+};
+
 /*
  * We use the same IndexScanDescData structure for both amgettuple-based
  * and amgetbitmap-based index scans.  Some fields are only relevant in
@@ -75,8 +114,15 @@ typedef struct IndexScanDescData
 	/* signaling to index AM about killing index tuples */
 	bool		kill_prior_tuple;		/* last-returned tuple is dead */
 	bool		ignore_killed_tuples;	/* do not return killed entries */
-	bool		xactStartedInRecovery;	/* prevents killing/seeing killed
-										 * tuples */
+	bool		xactStartedInRecovery;	/* prevents killing/seeing killed tuples */
+										 
+#ifdef USE_PREFETCH
+        struct pfch_index_pagelist* pfch_index_page_list;  /* array of index-relation BlockIds which we will/have prefetched */
+        struct pfch_block_item* pfch_block_item_list;   /* array of heap-relation BlockIds which we will/have prefetched */
+        unsigned short int     pfch_used;	/*  number of used elements in BlockIdData array   */
+        unsigned short int     pfch_next;	/*  next element for prefetch in BlockIdData array */
+	int             do_prefetch;    /*  should I prefetch ? */
+#endif   /* USE_PREFETCH */
 
 	/* index access method's private state */
 	void	   *opaque;			/* access-method-specific info */
@@ -91,6 +137,10 @@ typedef struct IndexScanDescData
 	/* NB: if xs_cbuf is not InvalidBuffer, we hold a pin on that buffer */
 	bool		xs_recheck;		/* T means scan keys must be rechecked */
 
+	/* heap fetch statistics for read-ahead logic */
+	unsigned int heap_tids_seen;
+	unsigned int heap_tids_fetched;
+
 	/* state data for traversing HOT chains in index_getnext */
 	bool		xs_continue_hot;	/* T if must keep walking HOT chain */
 }	IndexScanDescData;
--- src/include/nodes/tidbitmap.h.orig	2014-06-08 11:26:31.000000000 -0400
+++ src/include/nodes/tidbitmap.h	2014-06-08 21:59:37.952098627 -0400
@@ -41,6 +41,16 @@ typedef struct
 	int			ntuples;		/* -1 indicates lossy result */
 	bool		recheck;		/* should the tuples be rechecked? */
 	/* Note: recheck is always true if ntuples < 0 */
+#ifdef USE_PREFETCH
+        /*   Unread_Pfetched is a circular list of recently prefetched blocknos of size target_prefetch_pages
+        **   the index of the first unread block is held in Unread_Pfetched_next
+        **         and is advanced when a block is read
+        **   the count of number of unread blocks is in Unread_Pfetched_count (and this subset can wrap around)
+        */
+        BlockNumber *Unread_Pfetched_base;   /*  where the list of prefetched but unread blocknos starts */
+        unsigned int Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+        unsigned int Unread_Pfetched_count;  /*  number of valid unread blocknos in list */
+#endif   /* USE_PREFETCH */
 	OffsetNumber offsets[1];	/* VARIABLE LENGTH ARRAY */
 } TBMIterateResult;				/* VARIABLE LENGTH STRUCT */
 
@@ -62,5 +72,8 @@ extern bool tbm_is_empty(const TIDBitmap
 extern TBMIterator *tbm_begin_iterate(TIDBitmap *tbm);
 extern TBMIterateResult *tbm_iterate(TBMIterator *iterator);
 extern void tbm_end_iterate(TBMIterator *iterator);
-
+extern void tbm_zero(TBMIterator *iterator); /* zero list of prefetched and unread blocknos */
+extern void tbm_add(TBMIterator *iterator, BlockNumber blockno); /* add this blockno to list of prefetched and unread blocknos */
+extern void tbm_subtract(TBMIterator *iterator, BlockNumber blockno); /* remove this blockno from list of prefetched and unread blocknos */
+extern TBMIterateResult *tbm_locate_IterateResult(TBMIterator *iterator); /* locate the TBMIterateResult of an iterator */
 #endif   /* TIDBITMAP_H */
--- src/include/utils/rel.h.orig	2014-06-08 11:26:32.000000000 -0400
+++ src/include/utils/rel.h	2014-06-08 21:59:37.996098703 -0400
@@ -61,6 +61,7 @@ typedef struct RelationAmInfo
 	FmgrInfo	ammarkpos;
 	FmgrInfo	amrestrpos;
 	FmgrInfo	amcanreturn;
+	FmgrInfo	ampeeknexttuple;	/* peek at the next tuple different from any blocknum in pfch_list without reading a new index page */
 } RelationAmInfo;
 
 
--- src/include/pg_config.h.in.orig	2014-06-08 11:26:31.000000000 -0400
+++ src/include/pg_config.h.in	2014-06-08 21:59:38.024098751 -0400
@@ -1,4 +1,4 @@
-/* src/include/pg_config.h.in.  Generated from configure.in by autoheader.  */
+/* src/include/pg_config.h.in.  Generated from - by autoheader.  */
 
 /* Define to the type of arg 1 of 'accept' */
 #undef ACCEPT_TYPE_ARG1
@@ -748,6 +748,10 @@
 /* Define to the appropriate snprintf format for unsigned 64-bit ints. */
 #undef UINT64_FORMAT
 
+/* Define to select librt-style async io and the gcc atomic compare_and_swap.
+   */
+#undef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+
 /* Define to 1 to build with assertion checks. (--enable-cassert) */
 #undef USE_ASSERT_CHECKING
 

--- configure.in.orig	2014-06-25 16:37:59.233618849 -0400
+++ configure.in	2014-06-25 18:10:50.760519904 -0400
@@ -1771,6 +1771,12 @@ operating system;  use --disable-thread-
 fi
 fi
 
+#  test whether this system has both the librt-style async io and the gcc atomic compare_and_swap
+#      and test operation  of the latter.
+PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP
+if test x"$pgac_cv_aio_atomic_builtin_comp_swap" = x"yes"; then
+      AC_DEFINE(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP, 1, [Define to select librt-style async io and the gcc atomic compare_and_swap.])
+fi
 
 # Select semaphore implementation type.
 if test "$PORTNAME" != "win32"; then
--- contrib/pg_prewarm/pg_prewarm.c.orig	2014-06-25 16:37:59.281618859 -0400
+++ contrib/pg_prewarm/pg_prewarm.c	2014-06-25 18:10:50.772519950 -0400
@@ -159,7 +159,7 @@ pg_prewarm(PG_FUNCTION_ARGS)
 		 */
 		for (block = first_block; block <= last_block; ++block)
 		{
-			PrefetchBuffer(rel, forkNumber, block);
+			PrefetchBuffer(rel, forkNumber, block, 0);
 			++blocks_done;
 		}
 #else
--- contrib/pg_stat_statements/pg_stat_statements--1.3.sql.orig	2014-06-25 17:33:03.164961952 -0400
+++ contrib/pg_stat_statements/pg_stat_statements--1.3.sql	2014-06-25 18:10:50.792520029 -0400
@@ -0,0 +1,52 @@
+/* contrib/pg_stat_statements/pg_stat_statements--1.3.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION pg_stat_statements VERSION '1.3'" to load this file. \quit
+
+-- Register functions.
+CREATE FUNCTION pg_stat_statements_reset()
+RETURNS void
+AS 'MODULE_PATHNAME'
+LANGUAGE C;
+
+CREATE FUNCTION pg_stat_statements(IN showtext boolean,
+    OUT userid oid,
+    OUT dbid oid,
+    OUT queryid bigint,
+    OUT query text,
+    OUT calls int8,
+    OUT total_time float8,
+    OUT rows int8,
+    OUT shared_blks_hit int8,
+    OUT shared_blks_read int8,
+    OUT shared_blks_dirtied int8,
+    OUT shared_blks_written int8,
+    OUT local_blks_hit int8,
+    OUT local_blks_read int8,
+    OUT local_blks_dirtied int8,
+    OUT local_blks_written int8,
+    OUT temp_blks_read int8,
+    OUT temp_blks_written int8,
+    OUT blk_read_time float8,
+    OUT blk_write_time float8
+   , OUT aio_read_noneed int8
+   , OUT aio_read_discrd int8
+   , OUT aio_read_forgot int8
+   , OUT aio_read_noblok int8
+   , OUT aio_read_failed int8
+   , OUT aio_read_wasted int8
+   , OUT aio_read_waited int8
+   , OUT aio_read_ontime int8
+)
+RETURNS SETOF record
+AS 'MODULE_PATHNAME', 'pg_stat_statements_1_3'
+LANGUAGE C STRICT VOLATILE;
+
+-- Register a view on the function for ease of use.
+CREATE VIEW pg_stat_statements AS
+  SELECT * FROM pg_stat_statements(true);
+
+GRANT SELECT ON pg_stat_statements TO PUBLIC;
+
+-- Don't want this to be available to non-superusers.
+REVOKE ALL ON FUNCTION pg_stat_statements_reset() FROM PUBLIC;
--- contrib/pg_stat_statements/Makefile.orig	2014-06-25 16:37:59.281618859 -0400
+++ contrib/pg_stat_statements/Makefile	2014-06-25 18:10:50.812520107 -0400
@@ -4,7 +4,8 @@ MODULE_big = pg_stat_statements
 OBJS = pg_stat_statements.o
 
 EXTENSION = pg_stat_statements
-DATA = pg_stat_statements--1.2.sql pg_stat_statements--1.1--1.2.sql \
+DATA = pg_stat_statements--1.3.sql pg_stat_statements--1.2--1.3.sql \
+	pg_stat_statements--1.2.sql pg_stat_statements--1.1--1.2.sql \
 	pg_stat_statements--1.0--1.1.sql pg_stat_statements--unpackaged--1.0.sql
 
 ifdef USE_PGXS
--- contrib/pg_stat_statements/pg_stat_statements.c.orig	2014-06-25 16:37:59.285618860 -0400
+++ contrib/pg_stat_statements/pg_stat_statements.c	2014-06-25 18:10:50.844520233 -0400
@@ -117,6 +117,7 @@ typedef enum pgssVersion
 	PGSS_V1_0 = 0,
 	PGSS_V1_1,
 	PGSS_V1_2
+	,PGSS_V1_3
 } pgssVersion;
 
 /*
@@ -148,6 +149,16 @@ typedef struct Counters
 	int64		local_blks_written;		/* # of local disk blocks written */
 	int64		temp_blks_read; /* # of temp blocks read */
 	int64		temp_blks_written;		/* # of temp blocks written */
+
+	int64		aio_read_noneed;		/* # of prefetches for which no need for prefetch as block already in buffer pool  */
+	int64		aio_read_discrd;		/* # of prefetches for which buffer not subsequently read and therefore discarded  */
+	int64		aio_read_forgot;		/* # of prefetches for which buffer not subsequently read and then forgotten about */
+	int64		aio_read_noblok;		/* # of prefetches for which no available BufferAiocb  control block               */
+	int64		aio_read_failed;		/* # of aio reads for which aio itself failed or the read failed with an errno     */
+	int64		aio_read_wasted;		/* # of aio reads for which in-progress aio cancelled and disk block not used      */
+	int64		aio_read_waited;		/* # of aio reads for which disk block used but had to wait for it                 */
+	int64		aio_read_ontime;		/* # of aio reads for which disk block used and ready on time when requested       */
+
 	double		blk_read_time;	/* time spent reading, in msec */
 	double		blk_write_time; /* time spent writing, in msec */
 	double		usage;			/* usage factor */
@@ -275,6 +286,7 @@ void		_PG_fini(void);
 
 PG_FUNCTION_INFO_V1(pg_stat_statements_reset);
 PG_FUNCTION_INFO_V1(pg_stat_statements_1_2);
+PG_FUNCTION_INFO_V1(pg_stat_statements_1_3);
 PG_FUNCTION_INFO_V1(pg_stat_statements);
 
 static void pgss_shmem_startup(void);
@@ -1026,7 +1038,25 @@ pgss_ProcessUtility(Node *parsetree, con
 		bufusage.temp_blks_read =
 			pgBufferUsage.temp_blks_read - bufusage_start.temp_blks_read;
 		bufusage.temp_blks_written =
-			pgBufferUsage.temp_blks_written - bufusage_start.temp_blks_written;
+			pgBufferUsage.temp_blks_written - bufusage.temp_blks_written;
+
+		bufusage.aio_read_noneed =
+			pgBufferUsage.aio_read_noneed - bufusage.aio_read_noneed;
+		bufusage.aio_read_discrd =
+			pgBufferUsage.aio_read_discrd - bufusage.aio_read_discrd;
+		bufusage.aio_read_forgot =
+			pgBufferUsage.aio_read_forgot - bufusage.aio_read_forgot;
+		bufusage.aio_read_noblok =
+			pgBufferUsage.aio_read_noblok - bufusage.aio_read_noblok;
+		bufusage.aio_read_failed =
+			pgBufferUsage.aio_read_failed - bufusage.aio_read_failed;
+		bufusage.aio_read_wasted =
+			pgBufferUsage.aio_read_wasted - bufusage.aio_read_wasted;
+		bufusage.aio_read_waited =
+			pgBufferUsage.aio_read_waited - bufusage.aio_read_waited;
+		bufusage.aio_read_ontime =
+			pgBufferUsage.aio_read_ontime - bufusage.aio_read_ontime;
+
 		bufusage.blk_read_time = pgBufferUsage.blk_read_time;
 		INSTR_TIME_SUBTRACT(bufusage.blk_read_time, bufusage_start.blk_read_time);
 		bufusage.blk_write_time = pgBufferUsage.blk_write_time;
@@ -1041,6 +1071,7 @@ pgss_ProcessUtility(Node *parsetree, con
 				   rows,
 				   &bufusage,
 				   NULL);
+
 	}
 	else
 	{
@@ -1224,6 +1255,16 @@ pgss_store(const char *query, uint32 que
 		e->counters.local_blks_written += bufusage->local_blks_written;
 		e->counters.temp_blks_read += bufusage->temp_blks_read;
 		e->counters.temp_blks_written += bufusage->temp_blks_written;
+
+		e->counters.aio_read_noneed     += bufusage->aio_read_noneed;
+		e->counters.aio_read_discrd     += bufusage->aio_read_discrd;
+		e->counters.aio_read_forgot     += bufusage->aio_read_forgot;
+		e->counters.aio_read_noblok     += bufusage->aio_read_noblok;
+		e->counters.aio_read_failed     += bufusage->aio_read_failed;
+		e->counters.aio_read_wasted     += bufusage->aio_read_wasted;
+		e->counters.aio_read_waited     += bufusage->aio_read_waited;
+		e->counters.aio_read_ontime     += bufusage->aio_read_ontime;
+
 		e->counters.blk_read_time += INSTR_TIME_GET_MILLISEC(bufusage->blk_read_time);
 		e->counters.blk_write_time += INSTR_TIME_GET_MILLISEC(bufusage->blk_write_time);
 		e->counters.usage += USAGE_EXEC(total_time);
@@ -1257,7 +1298,8 @@ pg_stat_statements_reset(PG_FUNCTION_ARG
 #define PG_STAT_STATEMENTS_COLS_V1_0	14
 #define PG_STAT_STATEMENTS_COLS_V1_1	18
 #define PG_STAT_STATEMENTS_COLS_V1_2	19
-#define PG_STAT_STATEMENTS_COLS			19		/* maximum of above */
+#define PG_STAT_STATEMENTS_COLS_V1_3	27
+#define PG_STAT_STATEMENTS_COLS			27		/* maximum of above */
 
 /*
  * Retrieve statement statistics.
@@ -1270,6 +1312,16 @@ pg_stat_statements_reset(PG_FUNCTION_ARG
  * function.  Unfortunately we weren't bright enough to do that for 1.1.
  */
 Datum
+pg_stat_statements_1_3(PG_FUNCTION_ARGS)
+{
+	bool		showtext = PG_GETARG_BOOL(0);
+
+	pg_stat_statements_internal(fcinfo, PGSS_V1_3, showtext);
+
+	return (Datum) 0;
+}
+
+Datum
 pg_stat_statements_1_2(PG_FUNCTION_ARGS)
 {
 	bool		showtext = PG_GETARG_BOOL(0);
@@ -1358,6 +1410,10 @@ pg_stat_statements_internal(FunctionCall
 			if (api_version != PGSS_V1_2)
 				elog(ERROR, "incorrect number of output arguments");
 			break;
+		case PG_STAT_STATEMENTS_COLS_V1_3:
+			if (api_version != PGSS_V1_3)
+				elog(ERROR, "incorrect number of output arguments");
+			break;
 		default:
 			elog(ERROR, "incorrect number of output arguments");
 	}
@@ -1534,11 +1590,24 @@ pg_stat_statements_internal(FunctionCall
 		{
 			values[i++] = Float8GetDatumFast(tmp.blk_read_time);
 			values[i++] = Float8GetDatumFast(tmp.blk_write_time);
+
+			if (api_version >= PGSS_V1_3)
+			{
+				values[i++] = Int64GetDatumFast(tmp.aio_read_noneed);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_discrd);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_forgot);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_noblok);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_failed);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_wasted);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_waited);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_ontime);
+			}
 		}
 
 		Assert(i == (api_version == PGSS_V1_0 ? PG_STAT_STATEMENTS_COLS_V1_0 :
 					 api_version == PGSS_V1_1 ? PG_STAT_STATEMENTS_COLS_V1_1 :
 					 api_version == PGSS_V1_2 ? PG_STAT_STATEMENTS_COLS_V1_2 :
+					 api_version == PGSS_V1_3 ? PG_STAT_STATEMENTS_COLS_V1_3 :
 					 -1 /* fail if you forget to update this assert */ ));
 
 		tuplestore_putvalues(tupstore, tupdesc, values, nulls);
--- contrib/pg_stat_statements/pg_stat_statements--1.2--1.3.sql.orig	2014-06-25 17:33:03.168961964 -0400
+++ contrib/pg_stat_statements/pg_stat_statements--1.2--1.3.sql	2014-06-25 18:10:50.872520343 -0400
@@ -0,0 +1,51 @@
+/* contrib/pg_stat_statements/pg_stat_statements--1.2--1.3.sql */
+
+-- complain if script is sourced in psql, rather than via ALTER EXTENSION
+\echo Use "ALTER EXTENSION pg_stat_statements UPDATE TO '1.3'" to load this file. \quit
+
+/* First we have to remove them from the extension */
+ALTER EXTENSION pg_stat_statements DROP VIEW pg_stat_statements;
+ALTER EXTENSION pg_stat_statements DROP FUNCTION pg_stat_statements();
+
+/* Then we can drop them */
+DROP VIEW pg_stat_statements;
+DROP FUNCTION pg_stat_statements();
+
+/* Now redefine */
+CREATE FUNCTION pg_stat_statements(IN showtext boolean,
+    OUT userid oid,
+    OUT dbid oid,
+    OUT queryid bigint,
+    OUT query text,
+    OUT calls int8,
+    OUT total_time float8,
+    OUT rows int8,
+    OUT shared_blks_hit int8,
+    OUT shared_blks_read int8,
+    OUT shared_blks_dirtied int8,
+    OUT shared_blks_written int8,
+    OUT local_blks_hit int8,
+    OUT local_blks_read int8,
+    OUT local_blks_dirtied int8,
+    OUT local_blks_written int8,
+    OUT temp_blks_read int8,
+    OUT temp_blks_written int8,
+    OUT blk_read_time float8,
+    OUT blk_write_time float8
+   , OUT aio_read_noneed int8
+   , OUT aio_read_discrd int8
+   , OUT aio_read_forgot int8
+   , OUT aio_read_noblok int8
+   , OUT aio_read_failed int8
+   , OUT aio_read_wasted int8
+   , OUT aio_read_waited int8
+   , OUT aio_read_ontime int8
+)
+RETURNS SETOF record
+AS 'MODULE_PATHNAME', 'pg_stat_statements_1_3'
+LANGUAGE C STRICT VOLATILE;
+
+CREATE VIEW pg_stat_statements AS
+  SELECT * FROM pg_stat_statements(true);
+
+GRANT SELECT ON pg_stat_statements TO PUBLIC;
--- postgresql-prefetching-asyncio.README.orig	2014-06-25 17:33:03.168961964 -0400
+++ postgresql-prefetching-asyncio.README	2014-06-25 18:10:50.892520421 -0400
@@ -0,0 +1,588 @@
+Postgresql  --   Extended Prefetching using Asynchronous IO
+============================================================
+
+Postgresql currently (9.3.4) provides a limited prefetching capability
+using posix_fadvise to give hints to the Operating System kernel
+about which pages it expects to read in the near future.
+This capability is used only during the heap-scan phase of bitmap-index scans.
+It is controlled via the effective_io_concurrency configuration parameter.
+
+This capability is now extended in two ways :
+   .   use asynchronous IO into Postgresql shared buffers as an
+       alternative to posix_fadvise
+   .   Implement prefetching in other types of scan :
+            .  non-bitmap (i.e. simple) index scans - index pages
+                     currently only for B-tree indexes.
+                    (developed by Claudio Freire <klaussfreire(at)gmail(dot)com>)
+            .  non-bitmap (i.e. simple) index scans - heap pages
+                          currently only for B-tree indexes.
+            .  simple heap scans
+
+Posix asynchronous IO is chosen as the function library for asynchronous IO,
+since this is well supported and also fits very well with the model of
+the prefetching process,  particularly as regards checking for completion
+of an asynchronous read.    On linux,   Posix asynchronous IO is provided
+in the librt library.    librt uses independently-schedulable threads to
+achieve the asynchronicity,   rather than kernel functionality.
+
+In this implementation,  use of asynchronous IO is limited to prefetching
+while performing one of the three types of scan
+            .  B-tree bitmap index scan - heap pages    (as already exists)
+            .  B-tree non-bitmap (i.e. simple) index scans - index and heap pages
+            .  simple heap scans
+on permanent relations.   It is not used on temporary tables nor for writes.
+
+The advantages of Posix asynchronous IO into shared buffers
+compared to posix_fadvise are :
+   .   Beneficial for non-sequential access patterns as well as sequential
+   .   No restriction on the kinds of IO which can be used
+       (other kinds of asynchronous IO impose restrictions such as
+        buffer alignment,  use of non-buffered IO).
+   .   Does not interfere with standard linux kernel read-ahead functionality.
+       (It has been stated in 
+ www.postgresql.org/message-id/CAGTBQpbu2M=-M7NUr6DWr0K8gUVmXVhwKohB-Cnj7kYS1AhH4A@mail.gmail.com
+       that :
+          "the kernel stops doing read-ahead when a call to posix_fadvise comes.
+           I noticed the performance hit, and checked the kernel's code.
+           It effectively changes the prediction mode from sequential to fadvise,
+           negating the (assumed) kernel's prefetch logic")
+   .   When the read request is issued after a prefetch has completed,
+       no delay associated with a kernel call to copy the page from
+       kernel page buffers into the Postgresql shared buffer,
+       since it is already there.
+       Also,   in a memory-constrained environment,   there is a greater
+       probability that the prefetched page will "stick" in memory
+       since the linux kernel victimizes the filesystem page cache in preference
+       to swapping out user process pages.
+   .   Statistics on prefetch success can be gathered (see "Statistics" below)
+       which helps the administrator to tune the prefetching settings.
+
+These benefits are most likely to be obtained in a system whose usage profile
+(e.g. from iostat)  shows:
+     .   high IO wait from mostly-read activity
+     .   disk access pattern is not entirely sequential
+         (so kernel readahead can't predict it but postgresql can)
+     .   sufficient spare idle CPU to run the librt pthreads
+         or,  stated another way,    the CPU subsystem is relatively powerful
+         compared to the disk subsystem.
+In such ideal conditions,  and with a workload with plenty of index scans,
+around 10% - 20% improvement in throughput has been achieved.
+In an admittedly extreme environment measured by this author,    with a workload
+consisting of 8 client applications each running similar complex queries
+(same query structure but different predicates and constants),
+including 2 Bitmap Index Scans and 17 non-bitmap index scans,
+on a dual-core Intel laptop (4 hyperthreads) with the database on a single
+USB3-attached 500GB disk drive, and no part of the database in filesystem buffers
+initially,  (filesystem freshly mounted),  comparing unpatched build
+using posix_fadvise with effective_io_concurrency 4 against same build patched
+with async IO and effective_io_concurrency 4 and max_async_io_prefetchers 32,
+elapse time repeatably improved from around 640-670 seconds to around 530-550 seconds,
+a 17% - 18% improvement. 
+
+The disadvantages of Posix asynchronous IO compared to posix_fadvise are:
+     .   probably higher CPU utilization:
+         Firstly, the extra work performed by the librt threads adds CPU
+         overhead, and secondly, if the asynchronous prefetching is effective,
+         then it will deliver better (greater) overlap of CPU with IO, which
+         will reduce elapsed times and hence increase CPU utilization percentage
+         still more (during that shorter elapsed time).
+     .   more context switching,  because of the additional threads.
+
+
+Statistics:
+___________
+
+A number of additional statistics relating to effectiveness of asynchronous IO
+are provided as an extension of the existing pg_stat_statements loadable module.
+Refer to the appendix "Additional Supplied Modules" in the current
+PostgreSQL Documentation for details of this module.
+
+The following additional statistics are provided for asynchronous IO prefetching:
+
+    . aio_read_noneed  :   number of prefetches for which no need for prefetch as block already in buffer pool
+    . aio_read_discrd  :   number of prefetches for which buffer not subsequently read and therefore discarded
+    . aio_read_forgot  :   number of prefetches for which buffer not subsequently read and then forgotten about
+    . aio_read_noblok  :   number of prefetches for which no available BufferAiocb  control block
+    . aio_read_failed  :   number of aio reads for which aio itself failed or the read failed with an errno
+    . aio_read_wasted  :   number of aio reads for which in-progress aio cancelled and disk block not used
+    . aio_read_waited  :   number of aio reads for which disk block used but had to wait for it
+    . aio_read_ontime  :   number of aio reads for which disk block used and ready on time when requested
+
+Some of these are (hopefully) self-explanatory.    Some additional notes:
+
+    . aio_read_discrd and aio_read_forgot  :
+                    prefetch was wasted work since the buffer was not subsequently read
+                    The discrd case indicates that the scanner realized this and discarded the buffer,
+                    whereas the forgot case indicates that the scanner did not realize it,
+                    which should not normally occur.
+                    A high number in either suggests lowering effective_io_concurrency.
+
+    . aio_read_noblok  :   
+                    Any significant number in relation to all the other numbers indicates that
+                    max_async_io_prefetchers should be increased.
+
+    . aio_read_waited  :
+                    The page was prefetched but the asynchronous read had not completed by the time the
+                    scanner requested to read it.     causes extra overhead in waiting and indicates
+                    prefetching is not providing much if any benefit.
+                    The disk subsystem may be underpowered/overloaded in relation to the available CPU power.
+
+    . aio_read_ontime  :
+                    The page was prefetched and the asynchronous read had completed by the time the
+                    scanner requested to read it.     Optimal behaviour.      If this number if large
+                    in relation to all the other numbers except (possibly) aio_read_noneed,
+                    then prefetching is working well.
+
+To create the extension with support for these additional statistics, use the following syntax:
+     CREATE EXTENSION pg_stat_statements VERSION '1.3'
+or,  if you run the new code against an existing database which already has the extension
+( see installation and migration below ),  you can 
+     ALTER EXTENSION pg_stat_statements UPDATE TO '1.3'
+
+A suggested set of commands for displaying these statistics might be :
+
+ /*  OPTIONALLY */ DROP extension pg_stat_statements;
+                   CREATE extension pg_stat_statements VERSION '1.3';
+ /*  run your workload   */
+                  select userid , dbid , substring(query from 1 for 24) , calls , total_time , rows , shared_blks_read , blk_read_time , blk_write_time \
+                    , aio_read_noneed , aio_read_noblok , aio_read_failed , aio_read_wasted , aio_read_waited , aio_read_ontime , aio_read_forgot       \
+                      from pg_stat_statements where shared_blks_read > 0;
+
+
+Installation and Build Configuration:
+_____________________________________
+
+1. First -  a prerequsite:
+#  as well as requiring all the usual package build tools such as gcc , make etc,
+#  as described in the instructions for building postgresql,
+#  the following is required :
+    gnu autoconf at version 2.69 :
+# run the following command
+autoconf -V
+# it *must* return
+autoconf (GNU Autoconf) 2.69
+
+2. If you don't have it or it is a different version,
+then you must obtain version 2.69 (which is the current version)
+from your distribution provider or from the gnu software download site.
+
+3. Also you must have the source tree for postgresql version 9.4 (development version).
+#   all the following commands assume your current working directory is the top of the source tree.
+
+4. cd to top of source tree :
+#   check it appears to be a postgresql source tree
+ls -ld configure.in src
+#   should show both the file and the directory
+grep PostgreSQL COPYRIGHT
+#   should show PostgreSQL Database Management System
+
+5. Apply the patch :
+patch -b -p0 -i <patch_file_path>
+#   should report no errors, 48 files patched (see list at bottom of this README)
+#   and all hunks applied
+#  check the patch was appplied to configure.in
+ls -ld configure.in.orig configure.in
+#   should show both files
+
+6. Rebuild the configure script with the patched configure.in :
+mv configure configure.orig;
+autoconf configure.in >configure;echo "rc= $? from autoconf"; chmod +x configure;
+ls -lrt configure.orig configure;
+
+7. run the new configure script :
+#   if you have run configure before,
+#   then you may first want to save existing config.status and config.log if they exist,
+#   and then specify same configure flags and options as you specified before.
+#   the patch does not alter or extend the set of configure options
+#   if unsure,   run ./configure --help
+#   if still unsure,   run ./configure
+./configure <other configure options as desired>
+
+
+
+8. now check that configure decided that this environment supports asynchronous IO :
+grep USE_AIO_ATOMIC_BUILTIN_COMP_SWAP src/include/pg_config.h
+#  it should show
+#define USE_AIO_ATOMIC_BUILTIN_COMP_SWAP 1
+#  if not,  apparently your environment does not support asynch IO  -
+#  the config.log will show how it came to that conclusion,
+#  also check for :
+#    . a librt.so somewhere in the loader's library path (probably under /lib , /lib64 , or /usr)
+#    . your gcc must support the atomic compare_and_swap __sync_bool_compare_and_swap built-in function
+#  do not proceed without this define being set.
+
+9. do you want to use the new code on an existing cluster
+   that was created using the same code base but without the patch?
+   If so then run this nasty-looking command :
+   (cut-and-paste it into a terminal window or a shell-script file)
+   Otherwise continue to step 10.
+   see Migration note below for explanation.
+###############################################################################################
+   fl=src/Makefile.global; typeset -i bkx=0; while [[ $bkx < 200 ]]; do {
+       bkfl="${fl}.bak${bkx}"; if [[ -a ${bkfl} ]]; then ((bkx=bkx+1)); else break; fi;
+   }; done;
+   if [[ -a ${bkfl} ]]; then echo "sorry cannot find a backup name for $fl";
+   elif [[ -a $fl ]]; then {
+       mv $fl $bkfl && {
+          sed -e "/^CFLAGS =/ s/\$/ -DAVOID_CATALOG_MIGRATION_FOR_ASYNCIO/" $bkfl > $fl;
+          str="diff -w $bkfl $fl";echo "$str"; eval "$str";
+       };
+   };
+   else echo "ooopppss $fl is missing";
+   fi;
+###############################################################################################
+# it should report something like
+diff -w Makefile.global.bak0 Makefile.global
+222c222
+< CFLAGS = XXXX
+---
+> CFLAGS = XXXX -DAVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+#   where XXXX is some set of flags
+
+
+10. now run the rest of the build process as usual  -
+    follow instructions in file INSTALL if that file exists,
+    else e.g. run
+make && make install
+
+If the build fails with the following error:
+undefined reference to `aio_init'
+Then edit the following file
+src/include/pg_config_manual.h
+and add the following line at the bottom:
+
+#define DONT_HAVE_AIO_INIT
+
+and then run
+make clean && make && make install
+See notes to section Runtime Configuration below for more information on this.
+
+If you would like to use the sigevent mechanism for signalling completion
+of asynchronous io to non-originating backends,  instead of the polling method,
+(see section Checking AIO Completion below)
+then add these lines to src/include/pg_config_manual.h
+
+#define USE_AIO_SIGEVENT 1
+#define AIO_SIGEVENT_SIGNALNUM SIGIO  /* or signal num of your choice */
+
+Here's the context
+
+/*
+ * USE_PREFETCH code should be compiled only if we have a way to implement
+ * prefetching.  (This is decoupled from USE_POSIX_FADVISE because there
+ * might in future be support for alternative low-level prefetch APIs  --
+ * -- update October 2013  -- now there is such a new prefetch capability --
+ *   async_io into postgres buffers  -   configuration parameter max_async_io_threads)
+ */
+#if defined(USE_POSIX_FADVISE) || defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP)
+#define USE_AIO_SIGEVENT 1
+/* AIO_SIGEVENT_SIGNALNUM is the signal used to indicate completion
+ * of an aio operation.  Choose a signal that is not used elsewhere
+ * in postgresql and which can be caught by signal handler.
+*/
+#define AIO_SIGEVENT_SIGNALNUM SIGIO
+#define USE_PREFETCH
+#endif
+
+
+
+
+Migration , Runtime Configuration, and Use:
+___________________________________________
+
+
+Database Migration:
+___________________
+
+The new prefetching code for non-bitmap index scans introduces a new btree-index
+function named btpeeknexttuple.    The correct way to add such a function involves
+also adding it to the catalog as an internal function in pg_proc.
+However,  this results in the new built code considering an existing database to be
+incompatible,  i.e requiring backup on the old code and restore on the new.
+This is normal behaviour for migration to a new version of postgresql,  and is
+also a valid way of migrating a database for use with this asynchronous IO feature,
+but in this case it may be inconvenient.
+
+As an alternative,    the new code may be compiled with the macro define
+AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+which does what it says by not altering the catalog.   The patched build can then
+be run against an existing database cluster initdb'd using the unpatched build.
+
+There are no known ill-effects of so doing,  but :
+     .  in any case,  it is strongly suggested to make a backup of any precious database
+        before accessing it with a patched build
+     .  be aware that if this asynchronous IO feature is eventually released as part of postgresql,
+        migration will probably be required anyway.
+
+This option to avoid catalog migration is intended as a convenience for a quick test,
+and also makes it easier to obtain performance comparisons on the same database.
+
+
+
+Runtime Configuration:
+______________________
+
+One new configuration parameter settable in postgresql.conf and
+in any other way as described in the postgresql documentation :
+
+max_async_io_prefetchers
+  Maximum number of background processes concurrently using asynchronous
+  librt threads to prefetch pages into shared memory buffers
+
+This number can be thought of as the maximum number
+of librt threads concurrently active,   each working on a list of
+from 1 to target_prefetch_pages pages ( see notes 1 and 2 ).
+
+In practice,    this number simply controls how many prefetch requests in total
+may be active concurrently :
+        max_async_io_prefetchers * target_prefetch_pages ( see note 1)
+
+default is max_connections/6
+and recall that the default for max_connections is 100
+
+
+note 1  a number based on effective_io_concurrency and approximately n * ln(n)
+        where n is effective_io_concurrency
+
+note 2  Provided that the gnu extension to Posix AIO which provides the
+aio_init() function is present,   then aio_init() is called
+to set the librt maximum number of threads to max_async_io_prefetchers,
+and to set the maximum number of concurrent aio read requests to the product of
+        max_async_io_prefetchers * target_prefetch_pages
+
+
+As well as this regular configuration parameter,
+there are several other parameters that can be set via environment variable.
+The reason why they are environment vars rather than regular configuration parameters
+is that it is not expected that they should need to be set,   but they may be useful :
+                variable name         values                  default        meaning
+   PG_TRY_PREFETCHING_FOR_BITMAP      [Y|N]                    Y         whether to prefetch bitmap heap scans
+   PG_TRY_PREFETCHING_FOR_ISCAN       [Y|N|integer[,[N|Y]]]   256,N      whether to prefetch  non-bitmap index scans
+                                                                    also numeric size of list of prefetched blocks
+                                                                    also whether to prefetch forward-sequential-pattern index pages
+   PG_TRY_PREFETCHING_FOR_BTREE       [Y|N]                    Y         whether to prefetch heap pages in non-bitmap index scans
+   PG_TRY_PREFETCHING_FOR_HEAP        [Y|N]                    N         whether to prefetch relation (un-indexed) heap scans
+
+
+The setting for PG_TRY_PREFETCHING_FOR_ISCAN is a litle complicated.
+It can be set to Y or N to control prefetching of  non-bitmap index scans;
+But in addition it can be set to an integer,   which both implies Y
+and also sets the size of a list used to remember prefetched but unread heap pages.
+This list is an optimization used to avoid re-prefetching and maximise the potential
+set of prefetchable blocks indexed by one index page.
+And if set to an integer,  this integer may be followed by either ,Y or ,N
+to specify to prefetch index pages which are being accessed forward-sequentially.
+It has been found that prefetching is not of great benefit for this access pattern,
+and so it is not the default,  but also does no harm (provided sufficient CPU capacity).
+
+
+
+Usage :
+______
+
+
+There are no changes in usage other than as noted under Configuration and Statistics.
+However,   in order to assess benefit from this feature,   it will be useful to
+understand the query access plans of your workload using EXPLAIN.    Before doing that,
+make sure that statistics are up to date using ANALYZE.
+
+
+
+Internals:
+__________
+
+
+Internal changes span two areas and the interface between them :
+
+ .  buffer manager layer
+ .  programming interface for scanner to call buffer manager
+ .  scanner layer
+
+ .  buffer manager layer
+    ____________________
+
+    changes comprise :
+       .   allocating,  pinning , unpinning buffers
+            this is complex and discussed briefly below in "Buffer Management"
+       .   acquiring and releasing a BufferAiocb, the control block
+            associated with a single aio_read,  and checking for its completion
+            a new file,  backend/storage/buffer/buf_async.c, provides three new functions,
+                  BufStartAsync        BufReleaseAsync            BufCheckAsync
+            which handle this.
+       .   calling librt asynch io functions
+            this follows the example of all other filesystem interfaces
+            and is straightforward.    
+            two new functions are provided in fd.c:
+                   FileStartaio        FileCompleteaio
+            and corresponding interfaces in smgr.c
+
+ .  programming interface for scanner to call buffer manager
+    ________________________________________________________
+       . calling interface for existing function PrefetchBuffer is modified :
+           .  one new argument,   BufferAccessStrategy strategy
+           .  now returns an int return code which indicates :
+                     whether pin count on buffer has been increased by 1
+                     whether block was already present in a buffer
+       .  new function DiscardBuffer
+           .  discard buffer used for a previously prefetched page
+                 which scanner decides it does not want to read.
+           .  same arguments as for PrefetchBuffer except for omission of BufferAccessStrategy
+           .  note - this is different from the existing function ReleaseBuffer
+                     in that ReleaseBuffer takes a buffer_descriptor as argument
+                     for a buffer which has been read, but has similar purpose.
+
+ .  scanner layer
+    _____________
+        common to all scanners is that the scanner which wishes to prefetch must do two things:
+          .  decide which pages to prefetch and call PrefetchBuffer to prefetch them
+                 nodeBitmapHeapscan already does this (but note one extra argument on PrefetchBuffer)
+          .  remember which pages it has prefetched in some list (actual or conceptual,  e.g. a page range),
+                 removing each page from this list if and when it subsequently reads the page.
+          .  at end of scan,  call DiscardBuffer for every remembered (i.e. prefetched not unread) page
+       how this list of prefetched pages is implemented varies for each of the three scanners and four scan types:
+            .  bitmap index scan - heap pages
+            .  non-bitmap (i.e. simple) index scans - index pages
+            .  non-bitmap (i.e. simple) index scans - heap pages
+            .  simple heap scans
+       The consequences of forgetting to call DiscardBuffer on a prefetched but unread page are:
+            .   counted in aio_read_forgot  (see "Statistics" above)
+            .   may incur an annoying but harmless warning in the pg_log "Buffer Leak ... "
+                  (the buffer is released at commit)
+       This does sometimes happen ...
+     
+
+
+Buffer Management
+_________________
+
+With async io,   PrefetchBuffer must allocate and pin a buffer,  which is relatively straightforward,
+but also every other part of buffer manager must know about the possibility that a buffer may be in
+a state of async_io_in_progress state and be prepared to determine the possible completion.
+That is,  one backend BK1 may start the io but another BK2 may try to read it before BK1 does.
+Posix Asynchronous IO provides a means for waiting on this or another task's read if in progress,
+namely aio_suspend(),  which this extension uses.    Therefore,  although StartBufferIO and TerminateBufferIO
+are called as part of asynchronous prefetching,   their role is limited to maintaining the buffer descriptor flags,
+and they do not track the asynchronous IO itself.    Instead,   asynchronous IOs are tracked in
+a separate set of shared control blocks,  the BufferAiocb list -
+refer to     include/storage/buf_internals.h
+Checking asynchronous io status is handled in  backend/storage/buffer/buf_async.c BufCheckAsync function.
+Read the commentary for this function for more details.
+
+Checking AIO Completion
+_______________________
+There is a difference in how completion is checked depending on whether the backend doing the checking is :
+    .   the same as the backend which stared the asynch io (the "originator")
+    .   a different backend ("non-originator")
+The "originator" case is most common and also simplest  -
+      FileCompleteaio simply issues the appriopriate aio_xxxx calls and suspends if not complete.
+The "non-originator" case is more complex and two methods are currently designed and implemented :
+    .  polling the aiocb for completion
+    .  use of LWlocks and sigevent to cause a signal to be delivered to the originator.
+       The originator locks eXclusive at aio start and releases after delivery of the signal.
+       The non-originator locks Shared and releases.
+
+Pinning and unpinning of buffers is the most complex aspect of asynch io prefetching,
+and the logic is spread throughout BufStartAsync , BufCheckAsync , and many functions in bufmgr.c.
+When a backend BK2 requests ReadBuffer of a page for which asynch read is in progress,
+buffer manager has to determine which backend BK1 pinned this buffer during previous PrefetchBuffer,
+and for example must not be re-pinned a second time if BK2 is BK1.
+Information concerning which backend initiated the prefetch is held in the BufferAiocb.
+
+The trickiest case concerns the scenario in which :
+   .  BK1 initiates prefetch and acquires a pin
+   .  BK2 possibly waits for completion and then reads the buffer,  and perhaps later on
+         releases it by ReleaseBuffer.
+   .  Since the asynchronous IO is no longer in progress,     there is no longer any
+         BufferAiocb associated with it.    Yet buffer manager must remember that BK1 holds a
+         "prefetch" pin, i.e. a pin which must not be repeated if and when BK1 finally issues ReadBuffer.
+   .  The solution to this problem is to invent the concept of a "banked" pin,
+      which is a pin obtained when prefetch was issued,   identied as in "banked" status only if and when
+      the associated asynchronous IO terminates,  and redeemable by the next use by same task,
+      either by ReadBuffer or DiscardBuffer.
+      The pid of the backend which holds a banked pin on a buffer (there can be at most one such backend)
+      is stored in the buffer descriptor.
+      This is done without increasing size of the buffer descriptor,  which is important since
+      there may be a very large number of these.     This does overload the relevant field in the descriptor.
+      Refer to include/storage/buf_internals.h for more details
+      and search for BM_AIO_PREFETCH_PIN_BANKED in storage/buffer/bufmgr.c and  backend/storage/buffer/buf_async.c
+
+______________________________________________________________________________
+The following 48 files are changed in this feature (output of the patch command) :
+
+patching file configure.in
+patching file contrib/pg_prewarm/pg_prewarm.c
+patching file contrib/pg_stat_statements/pg_stat_statements--1.3.sql
+patching file contrib/pg_stat_statements/Makefile
+patching file contrib/pg_stat_statements/pg_stat_statements.c
+patching file contrib/pg_stat_statements/pg_stat_statements--1.2--1.3.sql
+patching file postgresql-prefetching-asyncio.README
+patching file config/c-library.m4
+patching file src/backend/postmaster/postmaster.c
+patching file src/backend/executor/nodeBitmapHeapscan.c
+patching file src/backend/executor/nodeIndexscan.c
+patching file src/backend/executor/instrument.c
+patching file src/backend/storage/buffer/Makefile
+patching file src/backend/storage/buffer/bufmgr.c
+patching file src/backend/storage/buffer/buf_async.c
+patching file src/backend/storage/buffer/buf_init.c
+patching file src/backend/storage/smgr/md.c
+patching file src/backend/storage/smgr/smgr.c
+patching file src/backend/storage/file/fd.c
+patching file src/backend/storage/lmgr/lwlock.c
+patching file src/backend/storage/lmgr/proc.c
+patching file src/backend/access/heap/heapam.c
+patching file src/backend/access/heap/syncscan.c
+patching file src/backend/access/index/indexam.c
+patching file src/backend/access/index/genam.c
+patching file src/backend/access/nbtree/nbtsearch.c
+patching file src/backend/access/nbtree/nbtinsert.c
+patching file src/backend/access/nbtree/nbtpage.c
+patching file src/backend/access/nbtree/nbtree.c
+patching file src/backend/nodes/tidbitmap.c
+patching file src/backend/utils/misc/guc.c
+patching file src/backend/utils/mmgr/aset.c
+patching file src/include/executor/instrument.h
+patching file src/include/storage/bufmgr.h
+patching file src/include/storage/lwlock.h
+patching file src/include/storage/smgr.h
+patching file src/include/storage/fd.h
+patching file src/include/storage/buf_internals.h
+patching file src/include/catalog/pg_am.h
+patching file src/include/catalog/pg_proc.h
+patching file src/include/pg_config_manual.h
+patching file src/include/miscadmin.h
+patching file src/include/access/nbtree.h
+patching file src/include/access/heapam.h
+patching file src/include/access/relscan.h
+patching file src/include/nodes/tidbitmap.h
+patching file src/include/utils/rel.h
+patching file src/include/pg_config.h.in
+
+
+Future Possibilities:
+____________________
+
+There are several possible extensions of this feature :
+   .   Extend prefetching of index scans to types of index
+       other than B-tree.
+       This should be fairly straightforward,  but requires some
+       good base of benchmarkable workloads to prove the value.
+   .   Investigate why asynchronous IO prefetching does not greatly
+       improve sequential relation heap scans and possibly find how to
+       achieve a benefit.
+   .   Build knowledge of asycnhronous IO prefetching into the
+       Query Planner costing.
+       This is far from straightforward.    The Postgresql Query Planner's
+       costing model is based on resource consumption rather than elapsed time.
+       Use of asynchronous IO prefetching is intended to improve elapsed time
+       as the expense of (probably) higher resource consumption.
+       Although Costing understands about the reduced cost of reading buffered
+       blocks, it does not take asynchronicity or overlap of CPU with disk
+       into account.  A naive approach might be to try to tweak the Query
+       Planner's Cost Constant configuration parameters
+       such as seq_page_cost , random_page_cost
+       but this is hazardous as explained in the Documentation.
+
+
+
+John Lumby,  johnlumby(at)hotmail(dot)com
--- config/c-library.m4.orig	2014-06-25 16:37:59.229618848 -0400
+++ config/c-library.m4	2014-06-25 18:10:50.920520531 -0400
@@ -367,3 +367,152 @@ if test "$pgac_cv_type_locale_t" = 'yes
   AC_DEFINE(LOCALE_T_IN_XLOCALE, 1,
             [Define to 1 if `locale_t' requires <xlocale.h>.])
 fi])])# PGAC_HEADER_XLOCALE
+
+
+# PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP
+# ---------------------------------------
+# test whether this system has both the librt-style async io and the gcc atomic compare_and_swap
+#      and test operation of both,
+#      including verifying that aio_error can retrieve completion status
+#      of aio_read issued by a different process
+#
+AC_DEFUN([PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP],
+[AC_MSG_CHECKING([whether have both librt-style async io and the gcc atomic compare_and_swap])
+AC_CACHE_VAL(pgac_cv_aio_atomic_builtin_comp_swap,
+pgac_save_LIBS=$LIBS
+LIBS=" -lrt $pgac_save_LIBS"
+[AC_TRY_RUN([#include <stdio.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+#include <fcntl.h>
+#include "aio.h"
+#include <errno.h>
+
+char *shmem;
+
+/*  returns rc of aio_read or -1 if some error */
+int
+processA(void)
+{
+	int fd , rc;
+	struct aiocb *aiocbp = (struct aiocb *) shmem;
+	char *buf = shmem + sizeof(struct aiocb);
+
+	rc = fd = open("configure", O_RDONLY );
+	if (fd != -1) {
+
+            memset(aiocbp, 0, sizeof(struct aiocb));
+            aiocbp->aio_fildes = fd;
+            aiocbp->aio_offset = 0;
+            aiocbp->aio_buf = buf;
+            aiocbp->aio_nbytes = 8;
+            aiocbp->aio_reqprio = 0;
+            aiocbp->aio_sigevent.sigev_notify = SIGEV_NONE;
+
+            rc = aio_read(aiocbp);
+	}
+        return rc;
+}
+
+/*  returns result of aio_error  -  0 if io completed successfully */
+int
+processB(void)
+{
+	struct aiocb *aiocbp = (struct aiocb *) shmem;
+	const struct aiocb * const pl[1] = { aiocbp };
+	int rv;
+	int	returnCode;
+        struct timespec my_timeout = { 0 , 10000 };
+        int max_iters , max_polls;
+
+	rv = aio_error(aiocbp);
+        max_iters = 100;
+        while ( (max_iters-- > 0) && (rv == EINPROGRESS) ) {
+                max_polls = 256;
+                my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                returnCode = aio_suspend(pl , 1 , &my_timeout);
+                while ((returnCode < 0) && (EAGAIN == errno) && (max_polls-- > 0)) {
+                    my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                    returnCode = aio_suspend(pl , 1 , &my_timeout);
+                }
+                rv = aio_error(aiocbp);
+        }
+
+	return rv;
+}
+
+int main (int argc, char *argv[])
+ {
+   int rc;
+   int pidB;
+   int child_status;
+   struct aiocb volatile * first_aiocb;
+   struct aiocb volatile * second_aiocb;
+   struct aiocb volatile * my_aiocbp = (struct aiocb *)20000008;
+
+   first_aiocb = (struct aiocb *)20000008;
+   second_aiocb = (struct aiocb *)40000008;
+
+   /*  first test  --  __sync_bool_compare_and_swap
+   **  set zero as success if two comp-swaps both worked as expected -
+   **  first compares equal and swaps,  second compares unequal
+   */
+   rc = (__sync_bool_compare_and_swap (&my_aiocbp, first_aiocb, second_aiocb));
+   if (rc) {
+      rc = (__sync_bool_compare_and_swap (&my_aiocbp, first_aiocb, second_aiocb));
+   } else {
+      rc = -1;
+   }
+
+   if (rc == 0) {
+       /*  second test  --  process A start aio_read
+       **  and process B checks completion by polling
+       */
+        rc = -1; /* pessimistic */
+
+	shmem = mmap(NULL, sizeof(struct aiocb) + 2048,
+				 PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS,
+				 -1, 0);
+	if (shmem != MAP_FAILED) {
+            
+            /*
+             * Start the I/O request in parent process, then fork and try to wait
+             * for it to finish from the child process.
+             */
+            rc = processA();
+            if (rc >= 0) {
+
+                rc = pidB = fork();
+                if (pidB != -1) {
+                    if (pidB != 0) {
+                        /* parent */
+                        wait (&child_status);
+                        if (WIFEXITED(child_status)) {
+                            rc = WEXITSTATUS(child_status);
+                        }
+                    } else {
+                        /* child */
+                        rc = processB();
+                        exit(rc);
+                    }
+                }
+            }
+        }
+   }
+
+   return rc;
+}],
+[pgac_cv_aio_atomic_builtin_comp_swap=yes],
+[pgac_cv_aio_atomic_builtin_comp_swap=no],
+[pgac_cv_aio_atomic_builtin_comp_swap=cross])
+])dnl AC_CACHE_VAL
+AC_MSG_RESULT([$pgac_cv_aio_atomic_builtin_comp_swap])
+if test x"$pgac_cv_aio_atomic_builtin_comp_swap" != x"yes"; then
+LIBS=$pgac_save_LIBS
+fi
+])# PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP
--- src/backend/postmaster/postmaster.c.orig	2014-06-25 16:37:59.445618891 -0400
+++ src/backend/postmaster/postmaster.c	2014-06-25 18:10:50.960520688 -0400
@@ -123,6 +123,11 @@
 #include "storage/spin.h"
 #endif
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+void ReportFreeBAiocbs(void);
+int CountInuseBAiocbs(void);
+extern int hwmBufferAiocbs;         /*  high water mark of in-use  BufferAiocbs in pool           */
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * Possible types of a backend. Beyond being the possible bkend_type values in
@@ -1489,9 +1494,15 @@ ServerLoop(void)
 	fd_set		readmask;
 	int			nSockets;
 	time_t		now,
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                           count_baiocb_time,
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 				last_touch_time;
 
 	last_touch_time = time(NULL);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        count_baiocb_time = time(NULL);
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 	nSockets = initMasks(&readmask);
 
@@ -1650,6 +1661,19 @@ ServerLoop(void)
 			last_touch_time = now;
 		}
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                /*   maintain the hwm of used baiocbs every 10 seconds  */
+		if ((now - count_baiocb_time) >= 10)
+		{
+                        int inuseBufferAiocbs;         /*  current in-use  BufferAiocbs in pool */
+                        inuseBufferAiocbs = CountInuseBAiocbs();
+                        if (inuseBufferAiocbs > hwmBufferAiocbs) {
+			    hwmBufferAiocbs = inuseBufferAiocbs;
+			}
+			count_baiocb_time = now;
+		}
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 		/*
 		 * If we already sent SIGQUIT to children and they are slow to shut
 		 * down, it's time to send them SIGKILL.  This doesn't happen
@@ -3440,6 +3464,9 @@ PostmasterStateMachine(void)
 						signal_child(PgStatPID, SIGQUIT);
 				}
 			}
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                        ReportFreeBAiocbs();
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 		}
 	}
 
--- src/backend/executor/nodeBitmapHeapscan.c.orig	2014-06-25 16:37:59.389618879 -0400
+++ src/backend/executor/nodeBitmapHeapscan.c	2014-06-25 18:10:50.988520798 -0400
@@ -34,6 +34,8 @@
  *		ExecEndBitmapHeapScan		releases all storage.
  */
 #include "postgres.h"
+#include <sys/types.h> 
+#include <unistd.h>
 
 #include "access/relscan.h"
 #include "access/transam.h"
@@ -47,6 +49,10 @@
 #include "utils/snapmgr.h"
 #include "utils/tqual.h"
 
+#ifdef USE_PREFETCH
+extern unsigned int prefetch_dbOid; /*  database oid of relations on which prefetching to be done - 0 means all */
+extern unsigned int prefetch_bitmap_scans; /*  boolean whether to prefetch bitmap heap scans         */
+#endif   /* USE_PREFETCH */
 
 static TupleTableSlot *BitmapHeapNext(BitmapHeapScanState *node);
 static void bitgetpage(HeapScanDesc scan, TBMIterateResult *tbmres);
@@ -111,10 +117,21 @@ BitmapHeapNext(BitmapHeapScanState *node
 		node->tbmres = tbmres = NULL;
 
 #ifdef USE_PREFETCH
-		if (target_prefetch_pages > 0)
-		{
+		if (    prefetch_bitmap_scans
+                     && (target_prefetch_pages > 0)
+                     && (    (   (prefetch_dbOid > 0)
+                               && (prefetch_dbOid == scan->rs_rd->rd_node.dbNode)
+                             )
+                         ||  (prefetch_dbOid == 0)
+                        )
+                        /* sufficient number of blocks - at least twice the target_prefetch_pages */
+                     && (scan->rs_nblocks > (2*target_prefetch_pages))
+                   ) {
 			node->prefetch_iterator = prefetch_iterator = tbm_begin_iterate(tbm);
 			node->prefetch_pages = 0;
+                        if (prefetch_iterator) {
+                          tbm_zero(prefetch_iterator);  /* zero list of prefetched and unread blocknos */
+                        }
 			node->prefetch_target = -1;
 		}
 #endif   /* USE_PREFETCH */
@@ -138,12 +155,14 @@ BitmapHeapNext(BitmapHeapScanState *node
 			}
 
 #ifdef USE_PREFETCH
+                        if (prefetch_iterator) {
 			if (node->prefetch_pages > 0)
 			{
 				/* The main iterator has closed the distance by one page */
 				node->prefetch_pages--;
+                                tbm_subtract(prefetch_iterator, tbmres->blockno); /* remove this blockno from list of prefetched and unread blocknos */
 			}
-			else if (prefetch_iterator)
+                            else
 			{
 				/* Do not let the prefetch iterator get behind the main one */
 				TBMIterateResult *tbmpre = tbm_iterate(prefetch_iterator);
@@ -151,6 +170,7 @@ BitmapHeapNext(BitmapHeapScanState *node
 				if (tbmpre == NULL || tbmpre->blockno != tbmres->blockno)
 					elog(ERROR, "prefetch and main iterators are out of sync");
 			}
+                        }
 #endif   /* USE_PREFETCH */
 
 			/*
@@ -239,16 +259,26 @@ BitmapHeapNext(BitmapHeapScanState *node
 			while (node->prefetch_pages < node->prefetch_target)
 			{
 				TBMIterateResult *tbmpre = tbm_iterate(prefetch_iterator);
+                                int             PrefetchBufferRc; /*  return value from PrefetchBuffer  - refer to bufmgr.h */
+
 
 				if (tbmpre == NULL)
 				{
 					/* No more pages to prefetch */
-					tbm_end_iterate(prefetch_iterator);
-					node->prefetch_iterator = prefetch_iterator = NULL;
+                                        /* let ExecEndBitmapHeapScan terminate the prefetch_iterator
+				        **	tbm_end_iterate(prefetch_iterator);
+					**      node->prefetch_iterator = NULL;
+                                        */
+                                        prefetch_iterator = NULL;
 					break;
 				}
 				node->prefetch_pages++;
-				PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, tbmpre->blockno);
+				PrefetchBufferRc = PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, tbmpre->blockno , 0);
+                                /*  add this blockno to list of prefetched and unread blocknos
+                                **  if pin count did not increase then indicate so in the Unread_Pfetched list
+                                */
+                                tbm_add(prefetch_iterator
+                                   ,( (PrefetchBufferRc & PREFTCHRC_BUF_PIN_INCREASED) ? tbmpre->blockno : InvalidBlockNumber ) );
 			}
 		}
 #endif   /* USE_PREFETCH */
@@ -482,12 +512,31 @@ ExecEndBitmapHeapScan(BitmapHeapScanStat
 {
 	Relation	relation;
 	HeapScanDesc scanDesc;
+	TBMIterator *prefetch_iterator;
 
 	/*
 	 * extract information from the node
 	 */
 	relation = node->ss.ss_currentRelation;
 	scanDesc = node->ss.ss_currentScanDesc;
+	prefetch_iterator = node->prefetch_iterator;
+
+#ifdef USE_PREFETCH
+        /*  before any other cleanup,  discard any prefetched but unread buffers  */
+        if (prefetch_iterator != NULL) {
+            TBMIterateResult *tbmpre = tbm_locate_IterateResult(prefetch_iterator);
+            BlockNumber *Unread_Pfetched_base = tbmpre->Unread_Pfetched_base;
+            unsigned int Unread_Pfetched_next = tbmpre->Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+            unsigned int Unread_Pfetched_count = tbmpre->Unread_Pfetched_count;
+
+            while ((Unread_Pfetched_count--) > 0) {
+                DiscardBuffer( scanDesc->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                Unread_Pfetched_next++;
+                if (Unread_Pfetched_next >= target_prefetch_pages)
+                    Unread_Pfetched_next = 0;
+            }
+        }
+#endif   /* USE_PREFETCH */
 
 	/*
 	 * Free the exprcontext
--- src/backend/executor/nodeIndexscan.c.orig	2014-06-25 16:37:59.393618881 -0400
+++ src/backend/executor/nodeIndexscan.c	2014-06-25 18:10:51.012520893 -0400
@@ -35,8 +35,13 @@
 #include "utils/rel.h"
 
 
+
 static TupleTableSlot *IndexNext(IndexScanState *node);
 
+#ifdef USE_PREFETCH
+extern unsigned int prefetch_dbOid; /*  database oid of relations on which prefetching to be done - 0 means all */
+extern unsigned int prefetch_index_scans; /*  boolean whether to prefetch bitmap heap scans         */
+#endif   /* USE_PREFETCH */
 
 /* ----------------------------------------------------------------
  *		IndexNext
@@ -418,7 +423,12 @@ ExecEndIndexScan(IndexScanState *node)
 	 * close the index relation (no-op if we didn't open it)
 	 */
 	if (indexScanDesc)
+        {
 		index_endscan(indexScanDesc);
+
+        /*  note  -  at this point all scan controlblock resources have been freed by IndexScanEnd called by index_endscan */
+
+        }
 	if (indexRelationDesc)
 		index_close(indexRelationDesc, NoLock);
 
@@ -609,6 +619,33 @@ ExecInitIndexScan(IndexScan *node, EStat
 											   indexstate->iss_NumScanKeys,
 											 indexstate->iss_NumOrderByKeys);
 
+#ifdef USE_PREFETCH
+        /*  initialize prefetching   */
+                indexstate->iss_ScanDesc->pfch_index_page_list =  (struct pfch_index_pagelist*)0;
+                indexstate->iss_ScanDesc->pfch_block_item_list = (struct pfch_block_item*)0;
+		if (    prefetch_index_scans
+			 && (target_prefetch_pages > 0)
+			 &&	(!RelationUsesLocalBuffers(indexstate->iss_ScanDesc->heapRelation)) /* I think this must always be true for an indexed heap ? */
+			 && (    (   (prefetch_dbOid > 0)
+					   && (prefetch_dbOid == indexstate->iss_ScanDesc->heapRelation->rd_node.dbNode)
+					 )
+				 ||  (prefetch_dbOid == 0)
+				)
+		   ) {
+			indexstate->iss_ScanDesc->pfch_index_page_list = palloc( sizeof(struct pfch_index_pagelist) + ( (target_prefetch_pages-1) * sizeof(struct pfch_index_item) ) );
+			indexstate->iss_ScanDesc->pfch_block_item_list = palloc( prefetch_index_scans * sizeof(struct pfch_block_item) );
+			if (     ( (struct pfch_index_pagelist*)0 != indexstate->iss_ScanDesc->pfch_index_page_list )
+                  && ( (struct pfch_block_item*)0 != indexstate->iss_ScanDesc->pfch_block_item_list )
+               ) {
+                          indexstate->iss_ScanDesc->pfch_used = 0;
+                          indexstate->iss_ScanDesc->pfch_next = prefetch_index_scans; /* ensure first entry is at index 0 */
+                          indexstate->iss_ScanDesc->pfch_index_page_list->pfch_index_pagelist_next = (struct pfch_index_pagelist*)0;
+                          indexstate->iss_ScanDesc->pfch_index_page_list->pfch_index_item_count = 0;
+                          indexstate->iss_ScanDesc->do_prefetch = 1;
+            }
+		}
+#endif   /* USE_PREFETCH */
+
 	/*
 	 * If no run-time keys to calculate, go ahead and pass the scankeys to the
 	 * index AM.
--- src/backend/executor/instrument.c.orig	2014-06-25 16:37:59.389618879 -0400
+++ src/backend/executor/instrument.c	2014-06-25 18:10:51.036520987 -0400
@@ -41,6 +41,14 @@ InstrAlloc(int n, int instrument_options
 		{
 			instr[i].need_bufusage = need_buffers;
 			instr[i].need_timer = need_timer;
+			instr[i].bufusage_start.aio_read_noneed = 0;
+			instr[i].bufusage_start.aio_read_discrd = 0;
+			instr[i].bufusage_start.aio_read_forgot = 0;
+			instr[i].bufusage_start.aio_read_noblok = 0;
+			instr[i].bufusage_start.aio_read_failed = 0;
+			instr[i].bufusage_start.aio_read_wasted = 0;
+			instr[i].bufusage_start.aio_read_waited = 0;
+			instr[i].bufusage_start.aio_read_ontime = 0;
 		}
 	}
 
@@ -143,6 +151,16 @@ BufferUsageAccumDiff(BufferUsage *dst,
 	dst->local_blks_written += add->local_blks_written - sub->local_blks_written;
 	dst->temp_blks_read += add->temp_blks_read - sub->temp_blks_read;
 	dst->temp_blks_written += add->temp_blks_written - sub->temp_blks_written;
+
+	dst->aio_read_noneed       += add->aio_read_noneed - sub->aio_read_noneed;
+	dst->aio_read_discrd       += add->aio_read_discrd - sub->aio_read_discrd;
+	dst->aio_read_forgot       += add->aio_read_forgot - sub->aio_read_forgot;
+	dst->aio_read_noblok       += add->aio_read_noblok - sub->aio_read_noblok;
+	dst->aio_read_failed       += add->aio_read_failed - sub->aio_read_failed;
+	dst->aio_read_wasted       += add->aio_read_wasted - sub->aio_read_wasted;
+	dst->aio_read_waited       += add->aio_read_waited - sub->aio_read_waited;
+	dst->aio_read_ontime       += add->aio_read_ontime - sub->aio_read_ontime;
+
 	INSTR_TIME_ACCUM_DIFF(dst->blk_read_time,
 						  add->blk_read_time, sub->blk_read_time);
 	INSTR_TIME_ACCUM_DIFF(dst->blk_write_time,
--- src/backend/storage/buffer/Makefile.orig	2014-06-25 16:37:59.457618893 -0400
+++ src/backend/storage/buffer/Makefile	2014-06-25 18:10:51.060521080 -0400
@@ -12,6 +12,6 @@ subdir = src/backend/storage/buffer
 top_builddir = ../../../..
 include $(top_builddir)/src/Makefile.global
 
-OBJS = buf_table.o buf_init.o bufmgr.o freelist.o localbuf.o
+OBJS = buf_table.o buf_init.o bufmgr.o freelist.o localbuf.o buf_async.o
 
 include $(top_srcdir)/src/backend/common.mk
--- src/backend/storage/buffer/bufmgr.c.orig	2014-06-25 16:37:59.457618893 -0400
+++ src/backend/storage/buffer/bufmgr.c	2014-06-25 18:10:51.092521205 -0400
@@ -29,7 +29,7 @@
  *		buf_table.c -- manages the buffer lookup table
  */
 #include "postgres.h"
-
+#include <sys/types.h> 
 #include <sys/file.h>
 #include <unistd.h>
 
@@ -50,7 +50,6 @@
 #include "utils/resowner_private.h"
 #include "utils/timestamp.h"
 
-
 /* Note: these two macros only work on shared buffers, not local ones! */
 #define BufHdrGetBlock(bufHdr)	((Block) (BufferBlocks + ((Size) (bufHdr)->buf_id) * BLCKSZ))
 #define BufferGetLSN(bufHdr)	(PageGetLSN(BufHdrGetBlock(bufHdr)))
@@ -63,6 +62,14 @@
 #define BUF_WRITTEN				0x01
 #define BUF_REUSABLE			0x02
 
+extern volatile struct BAiocbAnchor *BAiocbAnchr;  /*  anchor for all control blocks pertaining to aio  */
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern int numBufferAiocbs;        /*  total number of  BufferAiocbs in pool  */
+#if defined(USE_PREFETCH) && defined(USE_AIO_SIGEVENT)
+extern struct BAiocbIolock_chain_item volatile * volatile BAiocbIolock_anchor; /* anchor for chain of awaiting-release LWLock ptrs */
+#endif /* defined(USE_PREFETCH) && defined(USE_AIO_SIGEVENT)  */
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 #define DROP_RELS_BSEARCH_THRESHOLD		20
 
 /* GUC variables */
@@ -78,26 +85,33 @@ bool		track_io_timing = false;
  */
 int			target_prefetch_pages = 0;
 
-/* local state for StartBufferIO and related functions */
+/* local state for StartBufferIO and related functions
+**  but ONLY for synchronous IO  -  not altered for aio
+*/
 static volatile BufferDesc *InProgressBuf = NULL;
 static bool IsForInput;
+pid_t this_backend_pid = 0;    /*    pid of this backend */
 
 /* local state for LockBufferForCleanup */
 static volatile BufferDesc *PinCountWaitBuf = NULL;
 
-
-static Buffer ReadBuffer_common(SMgrRelation reln, char relpersistence,
+extern int
+BufStartAsync(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy);
+extern int
+BufCheckAsync(SMgrRelation caller_smgr, Relation caller_reln, volatile BufferDesc *buf_desc, int intention
+        ,BufferAccessStrategy strategy , int index_for_aio , bool spinLockHeld , LWLockId PartitionLock );
+Buffer ReadBuffer_common(SMgrRelation reln, char relpersistence,
 				  ForkNumber forkNum, BlockNumber blockNum,
 				  ReadBufferMode mode, BufferAccessStrategy strategy,
-				  bool *hit);
-static bool PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy);
-static void PinBuffer_Locked(volatile BufferDesc *buf);
-static void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner);
+				  bool *hit , int index_for_aio);
+bool PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy);
+void PinBuffer_Locked(volatile BufferDesc *buf);
+void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner);
 static void BufferSync(int flags);
 static int	SyncOneBuffer(int buf_id, bool skip_recently_used);
 static void WaitIO(volatile BufferDesc *buf);
-static bool StartBufferIO(volatile BufferDesc *buf, bool forInput);
-static void TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
+static bool StartBufferIO(volatile BufferDesc *buf, bool forInput , int index_for_aio );
+void TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
 				  int set_flag_bits);
 static void shared_buffer_write_error_callback(void *arg);
 static void local_buffer_write_error_callback(void *arg);
@@ -106,28 +120,76 @@ static volatile BufferDesc *BufferAlloc(
 			ForkNumber forkNum,
 			BlockNumber blockNum,
 			BufferAccessStrategy strategy,
-			bool *foundPtr);
+			int *foundPtr , int index_for_aio );
 static void FlushBuffer(volatile BufferDesc *buf, SMgrRelation reln);
 static void AtProcExit_Buffers(int code, Datum arg);
 static void CheckForBufferLeaks(void);
 static int	rnode_comparator(const void *p1, const void *p2);
 
+BlockNumber BlocknumOfBuffer(Buffer buffer);
+bool BlocknotinBuffer(Buffer buffer, Relation relation, BlockNumber blockNum);
 
 /*
  * PrefetchBuffer -- initiate asynchronous read of a block of a relation
  *
- * This is named by analogy to ReadBuffer but doesn't actually allocate a
- * buffer.  Instead it tries to ensure that a future ReadBuffer for the given
- * block will not be delayed by the I/O.  Prefetching is optional.
+ * This is named by analogy to ReadBuffer but allocates a buffer only if using asynchronous I/O.
+ * Its purpose  is to try to ensure that a future ReadBuffer for the given block
+ * will not be delayed by the I/O.  Prefetching is optional.
  * No-op if prefetching isn't compiled in.
- */
-void
-PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
-{
+ *
+ * Originally the prefetch simply called posix_fadvise() to recommend read-ahead into kernel page cache.
+ * Extended to provide an alternative of issuing an asynchronous aio_read() to read into a buffer.
+ * This extension has an implication on how this bufmgr component manages concurrent requests
+ * for the same disk block.
+ *
+ * Synchronous IO (read()) does not provide a means for waiting on another task's read if in progress,
+ * and bufmgr implements its own scheme in StartBufferIO, WaitIO, and TerminateBufferIO.
+ *
+ * Asynchronous IO (aio_read()) provides a means for waiting on this or another task's read if in progress,
+ * namely aio_suspend(),  which this extension uses.    Therefore,  although StartBufferIO and TerminateBufferIO
+ * are called as part of asynchronous prefetching,   their role is limited to maintaining the buffer desc flags,
+ * and they do not track the asynchronous IO itself.    Instead,   asynchronous IOs are tracked in 
+ * a separate set of shared control blocks,  the BufferAiocb list -
+ *   refer to     include/storage/buf_internals.h and storage/buffer/buf_init.c
+ *
+ * Another implication of asynchronous IO concerns buffer pinning.
+ * The buffer used for the prefetch is pinned before aio_read is issued.
+ * It is expected that the same task (and possibly others) will later ask to read the page
+ * and eventually release and unpin the buffer.
+ * However,  if the task which issued the aio_read later decides not to read the page,
+ * and return code indicates delta_pin_count > 0 (see below)
+ * it *must* instead issue a DiscardBuffer() (see function later in this file)
+ * so that its pin is released.
+ * Therefore,  each client which uses the PrefetchBuffer service must either always read all
+ * prefetched pages,  or keep track of prefetched pages and discard unread ones at end of scan.
+ *
+ * return code:   is an int bitmask defined in bufmgr.h
+        PREFTCHRC_BUF_PIN_INCREASED 0x01      pin count on buffer has been increased by 1
+        PREFTCHRC_BLK_ALREADY_PRESENT 0x02    block was already present in a buffer
+ *
+ * PREFTCHRC_BLK_ALREADY_PRESENT is a hint to caller that the prefetch may be unnecessary
+ */
+int
+PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy)
+{
+	Buffer		buf_id; /* indicates buffer containing the requested block  */
+        int             PrefetchBufferRc = 0; /*  return value as described above  */
+        int             PinCountOnEntry = 0;  /*  pin count on entry           */
+        int             PinCountdelta = 0;    /*  pin count delta increase     */
+
+
 #ifdef USE_PREFETCH
+
+	buf_id = -1;
 	Assert(RelationIsValid(reln));
 	Assert(BlockNumberIsValid(blockNum));
 
+#if defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP) && defined(USE_AIO_SIGEVENT)
+	/*  now is a good time to check whether any BAiocb waiter-locks are pending for release */
+	if (BAiocbIolock_anchor != (struct BAiocbIolock_chain_item*)0) {
+			ProcessPendingBAiocbIolocks();	 
+	}
+#endif /* defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP) && defined(USE_AIO_SIGEVENT)  */
 	/* Open it at the smgr level if not already done */
 	RelationOpenSmgr(reln);
 
@@ -147,7 +209,12 @@ PrefetchBuffer(Relation reln, ForkNumber
 		BufferTag	newTag;		/* identity of requested block */
 		uint32		newHash;	/* hash value for newTag */
 		LWLock	   *newPartitionLock;	/* buffer partition lock for it */
-		int			buf_id;
+        int         BufStartAsyncrc = -1;  /*  retcode from BufStartAsync :
+                                                       **        0 if started successfully (which implies buffer was newly pinned )
+                                                       **       -1 if failed for some reason
+                                                       **        1+PrivateRefCount if we found desired buffer in buffer pool
+                                                       **  and we set it likewise if we find buffer in buffer pool
+                                                       */
 
 		/* create a tag so we can lookup the buffer */
 		INIT_BUFFERTAG(newTag, reln->rd_smgr->smgr_rnode.node,
@@ -159,28 +226,121 @@ PrefetchBuffer(Relation reln, ForkNumber
 
 		/* see if the block is in the buffer pool already */
 		LWLockAcquire(newPartitionLock, LW_SHARED);
-		buf_id = BufTableLookup(&newTag, newHash);
+		buf_id = (Buffer)BufTableLookup(&newTag, newHash);
+                if (buf_id >= 0) {
+                    PinCountOnEntry = PrivateRefCount[buf_id];  /*  pin count on entry               */
+                    BufStartAsyncrc = 1 + PinCountOnEntry; /* indicate this backends pin count - see above comment */
+                    PrefetchBufferRc = PREFTCHRC_BLK_ALREADY_PRESENT;       /* indicate buffer present */
+                } else {
+                    PrefetchBufferRc = 0;                                   /* indicate buffer not present */
+                }
 		LWLockRelease(newPartitionLock);
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+     not_in_buffers:
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 		/* If not in buffers, initiate prefetch */
-		if (buf_id < 0)
+		if (buf_id < 0) {
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                    /*    try using async aio_read with a buffer */
+                    BufStartAsyncrc = BufStartAsync( reln, forkNum, blockNum , strategy );
+                    if (BufStartAsyncrc < 0) {
+                            pgBufferUsage.aio_read_noblok++;
+                    }
+#else /* not USE_AIO_ATOMIC_BUILTIN_COMP_SWAP so try the alternative that does not read the block into a postgresql buffer */
 			smgrprefetch(reln->rd_smgr, forkNum, blockNum);
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+		}
 
-		/*
-		 * If the block *is* in buffers, we do nothing.  This is not really
-		 * ideal: the block might be just about to be evicted, which would be
-		 * stupid since we know we are going to need it soon.  But the only
-		 * easy answer is to bump the usage_count, which does not seem like a
-		 * great solution: when the caller does ultimately touch the block,
-		 * usage_count would get bumped again, resulting in too much
-		 * favoritism for blocks that are involved in a prefetch sequence. A
-		 * real fix would involve some additional per-buffer state, and it's
-		 * not clear that there's enough of a problem to justify that.
+        if (  (buf_id >= 0) || (BufStartAsyncrc >= 1)  ) {
+                        /* The block *is* in buffers.    */
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                        pgBufferUsage.aio_read_noneed++;
+#ifndef USE_PREFETCH_BUT_DONT_PIN_ALREADY_PRESENT /* jury is out on whether the following wins but it ought to ...  */
+                        /*
+                        ** If this backend already had pinned it,
+                        ** or another backend had banked a pin on it,
+                        ** or there is an IO in progress,
+                        ** or it is not marked valid,
+                        ** then do nothing.
+                        ** Otherwise pin it and mark the buffer's pin as banked by this backend.
+                        ** Note  -  it may or not be pinned by another backend -
+                        **          it is ok for us to bank a pin on it
+                        **          *provided* the other backend did not bank its pin.
+                        **          The reason for this is that the banked-pin indicator is global -
+                        **          it can identify at most one process.
+                        */
+                        /* pgBufferUsage.aio_read_wasted++;  overload counter - only for debugging */
+                        if (BufStartAsyncrc == 1) {            /*   not pinned by me  */
+                              /*  pgBufferUsage.aio_read_wasted++;  overload counter - only for debugging */
+                              /*  note   -   all we can say with certainty is that the buffer is not pinned by me
+                              **             we cannot be sure that it is still in buffer pool
+                              **             so must go through the entire locking and searching all over again ...
 		 */
+                            LWLockAcquire(newPartitionLock, LW_SHARED);
+                            buf_id = (Buffer)BufTableLookup(&newTag, newHash);
+                            /* If in buffers, proceed */
+                            if (buf_id >= 0) {
+                                /*  since the block is now present,
+                                **  save the current pin count to ensure final delta is calculated correctly
+                                */
+                                PinCountOnEntry = PrivateRefCount[buf_id];  /*  pin count on entry               */
+                                if ( PinCountOnEntry == 0) { /*  paranoid check it's still not pinned by me */
+                                    volatile        BufferDesc *buf_desc;
+
+                                    buf_desc = &BufferDescriptors[buf_id];        /* found buffer descriptor */
+                                    LockBufHdr(buf_desc);
+                                    if (    (buf_desc->flags & BM_VALID)           /* buffer is valid        */
+                                         && (!(buf_desc->flags & (BM_IO_IN_PROGRESS|BM_AIO_IN_PROGRESS|BM_AIO_PREFETCH_PIN_BANKED))) /* buffer is not any of ... */
+                                       ) {
+                                        buf_desc->flags |= BM_AIO_PREFETCH_PIN_BANKED; /* bank the pin for next use by this task */
+                                        /*  note  - we can call PinBuffer_Locked with the BM_AIO_PREFETCH_PIN_BANKED flag set because it is not yet pinned by me */
+                                        buf_desc->freeNext = -(this_backend_pid);       /* remember which pid banked it */
+                                        /*  pgBufferUsage.aio_read_wasted--;      overload counter - not wasted after all - only for debugging */
+
+                                        /* Make sure we will have room to remember the buffer pin */
+                                        ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
+
+                                        PinBuffer_Locked(buf_desc);
+	}
+                                    else {
+                                        UnlockBufHdr(buf_desc);
+                                    }
+                                }
+                            }
+                            LWLockRelease(newPartitionLock);
+                            /*  although unlikely,  maybe it was evicted while we were puttering about  */
+                            if (buf_id < 0) {
+                                pgBufferUsage.aio_read_noneed--;   /*   back out the accounting */
+                                goto not_in_buffers;               /*   and try again           */
 	}
-#endif   /* USE_PREFETCH */
 }
+#endif /*  USE_PREFETCH_BUT_DONT_PIN_ALREADY_PRESENT */
 
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+		}
+
+		if (buf_id >= 0) {
+			PinCountdelta = PrivateRefCount[buf_id] - PinCountOnEntry;  /*  pin count delta increase    */
+			if (  (PinCountdelta < 0) || (PinCountdelta > 1) ) {
+				  elog(ERROR,
+						 "PrefetchBuffer #%d : incremented pin count by %d on bufdesc %p refcount %u localpins %d\n"
+								  ,(buf_id+1) , PinCountdelta , &BufferDescriptors[buf_id] ,BufferDescriptors[buf_id].refcount , PrivateRefCount[buf_id]);
+}
+		} else
+		if (BufStartAsyncrc == 0) {  /* aio started successfully (which implies buffer was newly pinned ) */
+			PinCountdelta = 1;
+		}
+
+		/*  set final PrefetchBufferRc according to previous value */
+		PrefetchBufferRc |= PinCountdelta;  /* set the PREFTCHRC_BUF_PIN_INCREASED bit */
+	}
+
+#endif   /* USE_PREFETCH */
+
+	return PrefetchBufferRc; /*  return value as described above */
+}
 
 /*
  * ReadBuffer -- a shorthand for ReadBufferExtended, for reading from main
@@ -234,6 +394,13 @@ ReadBufferExtended(Relation reln, ForkNu
 	bool		hit;
 	Buffer		buf;
 
+#if defined(USE_PREFETCH) && defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP) && defined(USE_AIO_SIGEVENT)
+	/*  now is a good time to check whether any BAiocb waiter-locks are pending for release */
+	if (BAiocbIolock_anchor != (struct BAiocbIolock_chain_item*)0) {
+			ProcessPendingBAiocbIolocks();	 
+	}
+#endif /* defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP) && defined(USE_AIO_SIGEVENT)  */
+
 	/* Open it at the smgr level if not already done */
 	RelationOpenSmgr(reln);
 
@@ -253,7 +420,7 @@ ReadBufferExtended(Relation reln, ForkNu
 	 */
 	pgstat_count_buffer_read(reln);
 	buf = ReadBuffer_common(reln->rd_smgr, reln->rd_rel->relpersistence,
-							forkNum, blockNum, mode, strategy, &hit);
+							forkNum, blockNum, mode, strategy, &hit , 0);
 	if (hit)
 		pgstat_count_buffer_hit(reln);
 	return buf;
@@ -281,7 +448,7 @@ ReadBufferWithoutRelcache(RelFileNode rn
 	Assert(InRecovery);
 
 	return ReadBuffer_common(smgr, RELPERSISTENCE_PERMANENT, forkNum, blockNum,
-							 mode, strategy, &hit);
+							 mode, strategy, &hit , 0);
 }
 
 
@@ -289,15 +456,18 @@ ReadBufferWithoutRelcache(RelFileNode rn
  * ReadBuffer_common -- common logic for all ReadBuffer variants
  *
  * *hit is set to true if the request was satisfied from shared buffer cache.
+ * index_for_aio ,  if -ve , is negative of ( index of the aiocb in the BufferAiocbs array + 3 )
+ *     which is passed through to StartBufferIO
  */
-static Buffer
+Buffer
 ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 				  BlockNumber blockNum, ReadBufferMode mode,
-				  BufferAccessStrategy strategy, bool *hit)
+				  BufferAccessStrategy strategy, bool *hit , int index_for_aio )
 {
 	volatile BufferDesc *bufHdr;
 	Block		bufBlock;
 	bool		found;
+        int             allocrc;             /*  retcode from BufferAlloc */
 	bool		isExtend;
 	bool		isLocalBuf = SmgrIsTemp(smgr);
 
@@ -329,16 +499,40 @@ ReadBuffer_common(SMgrRelation smgr, cha
 	}
 	else
 	{
+                allocrc = mode; /* pass mode to BufferAlloc since it must not wait for async io if RBM_NOREAD_FOR_PREFETCH */
 		/*
 		 * lookup the buffer.  IO_IN_PROGRESS is set if the requested block is
 		 * not currently in memory.
 		 */
 		bufHdr = BufferAlloc(smgr, relpersistence, forkNum, blockNum,
-							 strategy, &found);
-		if (found)
-			pgBufferUsage.shared_blks_hit++;
+							 strategy, &allocrc , index_for_aio );
+		if (allocrc < 0) {
+                    if (mode == RBM_ZERO_ON_ERROR || zero_damaged_pages)
+                    {
+                        ereport(WARNING,
+                                (errcode(ERRCODE_DATA_CORRUPTED),
+                                 errmsg("invalid page header in block %u of relation %s; zeroing out page",
+                                        blockNum,
+                                        relpath(smgr->smgr_rnode, forkNum))));
+                        bufBlock = BufHdrGetBlock(bufHdr);
+                        MemSet((char *) bufBlock, 0, BLCKSZ);
+                    }
 		else
+                      ereport(ERROR,
+                              (errcode(ERRCODE_DATA_CORRUPTED),
+                               errmsg("invalid page header in block %u of relation %s",
+                                      blockNum,
+                                      relpath(smgr->smgr_rnode, forkNum))));
+                        found = true;
+                }
+		else if (allocrc > 0) {
+			pgBufferUsage.shared_blks_hit++;
+                        found = true;
+                }
+		else {
 			pgBufferUsage.shared_blks_read++;
+                        found = false;
+                }
 	}
 
 	/* At this point we do NOT hold any locks. */
@@ -411,7 +605,7 @@ ReadBuffer_common(SMgrRelation smgr, cha
 				Assert(bufHdr->flags & BM_VALID);
 				bufHdr->flags &= ~BM_VALID;
 				UnlockBufHdr(bufHdr);
-			} while (!StartBufferIO(bufHdr, true));
+			} while (!StartBufferIO(bufHdr, true, 0));
 		}
 	}
 
@@ -431,6 +625,7 @@ ReadBuffer_common(SMgrRelation smgr, cha
 
 	bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);
 
+        if (mode != RBM_NOREAD_FOR_PREFETCH) {
 	if (isExtend)
 	{
 		/* new buffers are zero-filled */
@@ -500,6 +695,7 @@ ReadBuffer_common(SMgrRelation smgr, cha
 	VacuumPageMiss++;
 	if (VacuumCostActive)
 		VacuumCostBalance += VacuumCostPageMiss;
+	}
 
 	TRACE_POSTGRESQL_BUFFER_READ_DONE(forkNum, blockNum,
 									  smgr->smgr_rnode.node.spcNode,
@@ -521,21 +717,39 @@ ReadBuffer_common(SMgrRelation smgr, cha
  * the default strategy.  The selected buffer's usage_count is advanced when
  * using the default strategy, but otherwise possibly not (see PinBuffer).
  *
- * The returned buffer is pinned and is already marked as holding the
- * desired page.  If it already did have the desired page, *foundPtr is
- * set TRUE.  Otherwise, *foundPtr is set FALSE and the buffer is marked
+ * index_for_aio is an input parm which,  if non-zero,  identifies a BufferAiocb
+ * acquired by caller and to be used for any StartBufferIO performed by this routine.
+ * In this case,  if block not found in buffer pool and we allocate a new buffer,
+ * then we must maintain the spinlock on the buffer and pass it back to caller.
+ *
+ * foundPtr is input and output :
+ *  . input   -  indicates the read-buffer mode  ( see bufmgr.h )
+ *  . output  -  indicates the status of the buffer - see below
+ *
+ * Except for the case of RBM_NOREAD_FOR_PREFETCH and buffer is found,
+ * the returned buffer is pinned and is already marked as holding the
+ * desired page.
+ *  If it already did have the desired page and page content is valid,
+ *  *foundPtr is set to 1
+ *  If it already did have the desired page and mode is RBM_NOREAD_FOR_PREFETCH
+ *    and StartBufferIO returned false
+ *    (meaning it could not initialise the buffer for aio)
+ *  *foundPtr is set to 2
+ *  If it already did have the desired page but page content is invalid,
+ *  *foundPtr is set to -1
+ *   this can happen only if the buffer was read by an async read
+ *   and the aio is still in progress or pinned by the issuer of the startaio.
+ *  Otherwise, *foundPtr is set to 0 and the buffer is marked
  * as IO_IN_PROGRESS; ReadBuffer will now need to do I/O to fill it.
  *
- * *foundPtr is actually redundant with the buffer's BM_VALID flag, but
- * we keep it for simplicity in ReadBuffer.
- *
- * No locks are held either at entry or exit.
+ * No locks are held either at entry or exit EXCEPT for case noted above
+ * of passing an empty buffer back to async io caller ( index_for_aio set ).
  */
 static volatile BufferDesc *
 BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 			BlockNumber blockNum,
 			BufferAccessStrategy strategy,
-			bool *foundPtr)
+			int *foundPtr , int index_for_aio )
 {
 	BufferTag	newTag;			/* identity of requested block */
 	uint32		newHash;		/* hash value for newTag */
@@ -547,6 +761,13 @@ BufferAlloc(SMgrRelation smgr, char relp
 	int			buf_id;
 	volatile BufferDesc *buf;
 	bool		valid;
+        int             IntentionBufferrc;      /* retcode from BufCheckAsync */
+        bool            StartBufferIOrc;        /* retcode from StartBufferIO */
+        ReadBufferMode mode;
+
+
+        mode = *foundPtr;
+        *foundPtr = 0;
 
 	/* create a tag so we can lookup the buffer */
 	INIT_BUFFERTAG(newTag, smgr->smgr_rnode.node, forkNum, blockNum);
@@ -561,21 +782,53 @@ BufferAlloc(SMgrRelation smgr, char relp
 	if (buf_id >= 0)
 	{
 		/*
-		 * Found it.  Now, pin the buffer so no one can steal it from the
-		 * buffer pool, and check to see if the correct data has been loaded
-		 * into the buffer.
+		 * Found it.
 		 */
+                *foundPtr = 1;
 		buf = &BufferDescriptors[buf_id];
 
-		valid = PinBuffer(buf, strategy);
-
-		/* Can release the mapping lock as soon as we've pinned it */
+                /*   If prefetch mode,  then return immediately indicating found,
+                **   and NOTE in this case only,  we did not pin buffer.
+                **   In theory we might try to check whether the buffer is valid,  io in progress,  etc
+                **   but in practice it is simpler to abandon the prefetch if the buffer exists
+                */
+                if (mode == RBM_NOREAD_FOR_PREFETCH) {
+                    /* release the mapping lock and return    */
 		LWLockRelease(newPartitionLock);
+                } else {
+                    /*   note that the current request is for same tag as the one associated with the aio -
+                    **   so simply complete the aio and we have our buffer.
+                    **         If an aio was started on this buffer,
+                    **         check complete and wait for it if not.
+                    **         And,  if aio had been started,  then the task
+                    **         which issued the start aio already pinned it for this read,
+                    **         so if that task was me and the aio was successful,
+                    **         pass the current pin to this read without dropping and re-acquiring.
+                    **         this is all done by BufCheckAsync
+                    */
+                    IntentionBufferrc = BufCheckAsync(smgr , 0 , buf, BUF_INTENTION_WANT , strategy , index_for_aio , false , newPartitionLock );
 
-		*foundPtr = TRUE;
+                    /*       check to see if the correct data has been loaded into the buffer.  */
+                    valid = (IntentionBufferrc == BUF_INTENT_RC_VALID);
 
-		if (!valid)
-		{
+                    /*  check for serious IO errors   */
+                    if (!valid) {
+                        if (    (IntentionBufferrc != BUF_INTENT_RC_INVALID_NO_AIO)
+                             && (IntentionBufferrc != BUF_INTENT_RC_INVALID_AIO)
+                           ) {
+                            *foundPtr = -1;  /*  inform caller of serious error */
+                        }
+                        else
+                        if (IntentionBufferrc == BUF_INTENT_RC_INVALID_AIO) {
+                            goto proceed_with_not_found;  /*  yes,  I know,  a goto ... think of it as a break out of the if */
+                        }
+                     }
+
+                    /* BufCheckAsync pinned the buffer            */
+                    /* so can now release the mapping lock               */
+                    LWLockRelease(newPartitionLock);
+
+                    if (!valid) {
 			/*
 			 * We can only get here if (a) someone else is still reading in
 			 * the page, or (b) a previous read attempt failed.  We have to
@@ -583,19 +836,21 @@ BufferAlloc(SMgrRelation smgr, char relp
 			 * own read attempt if the page is still not BM_VALID.
 			 * StartBufferIO does it all.
 			 */
-			if (StartBufferIO(buf, true))
+                        if (StartBufferIO(buf, true, index_for_aio))
 			{
 				/*
 				 * If we get here, previous attempts to read the buffer must
 				 * have failed ... but we shall bravely try again.
 				 */
-				*foundPtr = FALSE;
+							*foundPtr = 0;
+						}
 			}
 		}
 
 		return buf;
 	}
 
+  proceed_with_not_found:
 	/*
 	 * Didn't find it in the buffer pool.  We'll have to initialize a new
 	 * buffer.  Remember to unlock the mapping lock while doing the work.
@@ -620,8 +875,10 @@ BufferAlloc(SMgrRelation smgr, char relp
 		/* Must copy buffer flags while we still hold the spinlock */
 		oldFlags = buf->flags;
 
-		/* Pin the buffer and then release the buffer spinlock */
-		PinBuffer_Locked(buf);
+        /*         If an aio was started on this buffer,
+        **         check complete and cancel it if not.
+        */
+        BufCheckAsync(smgr , 0 , buf, BUF_INTENTION_REJECT_OBTAIN_PIN , 0 , index_for_aio, true , 0 );
 
 		/* Now it's safe to release the freelist lock */
 		if (lock_held)
@@ -792,13 +1049,18 @@ BufferAlloc(SMgrRelation smgr, char relp
 				 * then set up our own read attempt if the page is still not
 				 * BM_VALID.  StartBufferIO does it all.
 				 */
-				if (StartBufferIO(buf, true))
+                                StartBufferIOrc = StartBufferIO(buf, true , index_for_aio); /* retcode from StartBufferIO */
+				if (StartBufferIOrc)
 				{
 					/*
 					 * If we get here, previous attempts to read the buffer
 					 * must have failed ... but we shall bravely try again.
 					 */
-					*foundPtr = FALSE;
+					*foundPtr = 0;
+                                } else
+                                if (mode == RBM_NOREAD_FOR_PREFETCH) {
+					UnpinBuffer(buf, true); /* must unpin if RBM_NOREAD_FOR_PREFETCH and found */
+                                        *foundPtr = 2;  /*  inform BufStartAsync that buffer must not be used */
 				}
 			}
 
@@ -861,10 +1123,17 @@ BufferAlloc(SMgrRelation smgr, char relp
 	 * lock.  If StartBufferIO returns false, then someone else managed to
 	 * read it before we did, so there's nothing left for BufferAlloc() to do.
 	 */
-	if (StartBufferIO(buf, true))
-		*foundPtr = FALSE;
-	else
-		*foundPtr = TRUE;
+        StartBufferIOrc = StartBufferIO(buf, true , index_for_aio); /* retcode from StartBufferIO */
+        if (StartBufferIOrc) {
+		*foundPtr = 0;
+        } else {
+                if (mode == RBM_NOREAD_FOR_PREFETCH) {
+                    UnpinBuffer(buf, true); /* must unpin if RBM_NOREAD_FOR_PREFETCH and found */
+                    *foundPtr = 2;  /*  inform BufStartAsync that buffer must not be used */
+                } else {
+                    *foundPtr = 1;
+                }
+        }
 
 	return buf;
 }
@@ -971,6 +1240,10 @@ retry:
 	/*
 	 * Insert the buffer at the head of the list of free buffers.
 	 */
+        /*   avoid confusing freelist with strange-looking freeNext */
+        if (buf->freeNext <= FREENEXT_BAIOCB_ORIGIN) { /* means was used for aiocb index */
+            buf->freeNext = FREENEXT_NOT_IN_LIST;
+        }
 	StrategyFreeBuffer(buf);
 }
 
@@ -1023,6 +1296,56 @@ MarkBufferDirty(Buffer buffer)
 	UnlockBufHdr(bufHdr);
 }
 
+/*  return the blocknum of the block in a buffer if it is valid
+**  if a shared buffer,  it must be pinned
+*/
+BlockNumber
+BlocknumOfBuffer(Buffer buffer)
+{
+	volatile BufferDesc *bufHdr;
+        BlockNumber rc = 0;
+
+	if (BufferIsValid(buffer)) {
+            if  (BufferIsLocal(buffer)) {
+                bufHdr = &LocalBufferDescriptors[-buffer - 1];
+            } else {
+                bufHdr = &BufferDescriptors[buffer - 1];
+            }
+
+            rc = bufHdr->tag.blockNum;
+        }
+
+        return rc;
+}
+
+/*  report whether specified buffer contains same or different block
+**  if a shared buffer,  it must be pinned
+*/
+bool
+BlocknotinBuffer(Buffer buffer,
+					 Relation relation,
+					 BlockNumber blockNum)
+{
+	volatile BufferDesc *bufHdr;
+        bool rc = false;
+
+	if (BufferIsValid(buffer)) {
+            if  (BufferIsLocal(buffer)) {
+                bufHdr = &LocalBufferDescriptors[-buffer - 1];
+            } else {
+                bufHdr = &BufferDescriptors[buffer - 1];
+            }
+
+            rc =
+                  (   (bufHdr->tag.blockNum != blockNum)
+                   || (!(RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) ))
+                   || (bufHdr->tag.forkNum != MAIN_FORKNUM)
+                  );
+        }
+
+        return rc;
+}
+
 /*
  * ReleaseAndReadBuffer -- combine ReleaseBuffer() and ReadBuffer()
  *
@@ -1041,18 +1364,18 @@ ReleaseAndReadBuffer(Buffer buffer,
 					 Relation relation,
 					 BlockNumber blockNum)
 {
-	ForkNumber	forkNum = MAIN_FORKNUM;
 	volatile BufferDesc *bufHdr;
+        bool isDifferentBlock;   /*       requesting different block from that already in buffer ? */
 
 	if (BufferIsValid(buffer))
 	{
+	    /* if a shared buff, we have pin, so it's ok to examine tag without spinlock */
+            isDifferentBlock = BlocknotinBuffer(buffer,relation,blockNum); /*  requesting different block from that already in buffer ? */
 		if (BufferIsLocal(buffer))
 		{
 			Assert(LocalRefCount[-buffer - 1] > 0);
 			bufHdr = &LocalBufferDescriptors[-buffer - 1];
-			if (bufHdr->tag.blockNum == blockNum &&
-				RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
-				bufHdr->tag.forkNum == forkNum)
+			if (!isDifferentBlock)
 				return buffer;
 			ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
 			LocalRefCount[-buffer - 1]--;
@@ -1061,12 +1384,12 @@ ReleaseAndReadBuffer(Buffer buffer,
 		{
 			Assert(PrivateRefCount[buffer - 1] > 0);
 			bufHdr = &BufferDescriptors[buffer - 1];
-			/* we have pin, so it's ok to examine tag without spinlock */
-			if (bufHdr->tag.blockNum == blockNum &&
-				RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
-				bufHdr->tag.forkNum == forkNum)
+                        BufCheckAsync(0 , relation , bufHdr , ( isDifferentBlock ? BUF_INTENTION_REJECT_FORGET
+                                                                                 : BUF_INTENTION_REJECT_KEEP_PIN )
+                                                            , 0 , 0 , false , 0 ); /* end any IO and maybe unpin */
+			if (!isDifferentBlock) {
 				return buffer;
-			UnpinBuffer(bufHdr, true);
+                        }
 		}
 	}
 
@@ -1091,11 +1414,12 @@ ReleaseAndReadBuffer(Buffer buffer,
  * Returns TRUE if buffer is BM_VALID, else FALSE.  This provision allows
  * some callers to avoid an extra spinlock cycle.
  */
-static bool
+bool
 PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy)
 {
 	int			b = buf->buf_id;
 	bool		result;
+        bool       pin_already_banked_by_me = 0;  /* buffer is already pinned by me and redeemable */
 
 	if (PrivateRefCount[b] == 0)
 	{
@@ -1117,12 +1441,34 @@ PinBuffer(volatile BufferDesc *buf, Buff
 	else
 	{
 		/* If we previously pinned the buffer, it must surely be valid */
+                /* Errr  -   is that really true  ???    I don't think so  :
+                ** what if I pin,  try an IO,  in progress,  then mistakenly pin again
 		result = true;
+                */
+		LockBufHdr(buf);
+                pin_already_banked_by_me = (    (PrivateRefCount[b] > 0) && (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                                             && (   (    (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                                                     : (-(buf->freeNext))  )  == this_backend_pid )
+                                           );
+                if (pin_already_banked_by_me) {
+                        elog(LOG, "PinBuffer : on buffer %d with banked pin rel=%s, blockNum=%u, with flags %X refcount=%u"
+                       ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                       ,buf->tag.blockNum, buf->flags, buf->refcount);
+                    buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;
+                    if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                        buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+	}
+                }
+		result = (buf->flags & BM_VALID) != 0;
+		UnlockBufHdr(buf);
 	}
+
+        if (!pin_already_banked_by_me) {
 	PrivateRefCount[b]++;
 	Assert(PrivateRefCount[b] > 0);
 	ResourceOwnerRememberBuffer(CurrentResourceOwner,
 								BufferDescriptorGetBuffer(buf));
+        }
 	return result;
 }
 
@@ -1139,19 +1485,36 @@ PinBuffer(volatile BufferDesc *buf, Buff
  * to save a spin lock/unlock cycle, because we need to pin a buffer before
  * its state can change under us.
  */
-static void
+void
 PinBuffer_Locked(volatile BufferDesc *buf)
 {
 	int			b = buf->buf_id;
+        bool       pin_already_banked_by_me;  /* buffer is already pinned by me and redeemable */
 
-	if (PrivateRefCount[b] == 0)
+        pin_already_banked_by_me = (    (PrivateRefCount[b] > 0) && (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                                     && (   (    (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                                             : (-(buf->freeNext))  )  == this_backend_pid )
+                             );
+	if (PrivateRefCount[b] == 0) {
 		buf->refcount++;
+        }
+        if (pin_already_banked_by_me) {
+                        elog(LOG, "PinBuffer_Locked : on buffer %d with banked pin rel=%s, blockNum=%u, with flags %X refcount=%u"
+                       ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                       ,buf->tag.blockNum, buf->flags, buf->refcount);
+            buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;
+                    if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                        buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+                    }
+        }
 	UnlockBufHdr(buf);
+        if (!pin_already_banked_by_me) {
 	PrivateRefCount[b]++;
 	Assert(PrivateRefCount[b] > 0);
 	ResourceOwnerRememberBuffer(CurrentResourceOwner,
 								BufferDescriptorGetBuffer(buf));
 }
+}
 
 /*
  * UnpinBuffer -- make buffer available for replacement.
@@ -1161,29 +1524,68 @@ PinBuffer_Locked(volatile BufferDesc *bu
  * Most but not all callers want CurrentResourceOwner to be adjusted.
  * Those that don't should pass fixOwner = FALSE.
  */
-static void
+void
 UnpinBuffer(volatile BufferDesc *buf, bool fixOwner)
 {
+
 	int			b = buf->buf_id;
+        bool       pin_already_banked_by_me;  /* buffer is already pinned by me and redeemable */
 
-	if (fixOwner)
+	if (fixOwner) {
 		ResourceOwnerForgetBuffer(CurrentResourceOwner,
 								  BufferDescriptorGetBuffer(buf));
+        }
 
 	Assert(PrivateRefCount[b] > 0);
 	PrivateRefCount[b]--;
 	if (PrivateRefCount[b] == 0)
 	{
+
 		/* I'd better not still hold any locks on the buffer */
 		Assert(!LWLockHeldByMe(buf->content_lock));
 		Assert(!LWLockHeldByMe(buf->io_in_progress_lock));
 
 		LockBufHdr(buf);
 
+		/* this backend has released last pin - buffer should not have pin banked by me
+                ** and if AIO in progress then there should be another backend pin
+                */
+                pin_already_banked_by_me = (       (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                                             &&    (   (    (buf->flags & BM_AIO_IN_PROGRESS)
+                                                         ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                         : (-(buf->freeNext))
+                                                       )  == this_backend_pid )
+                                           );
+                if (pin_already_banked_by_me) {
+                        /*  this is a strange situation  -  caller had a banked pin (which callers are supposed not to know about)
+                        **                                  but either discovered it had it or has over-counted how many pins it has
+                        */
+                        buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;   /*   redeem the pin although it is now of no use since about to release */
+                        if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                            buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+                        }
+
+                        /*     temporarily suppress logging error to avoid performance degradation -
+                        **     either this task really does not need the buffer in which case the error is harmless
+                        **     or a more severe error will be detected later (possibly immediately below)
+                        elog(LOG, "UnpinBuffer :  released last this-backend pin on buffer %d rel=%s, blockNum=%u, but had banked pin flags %X refcount=%u"
+                       ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                       ,buf->tag.blockNum, buf->flags, buf->refcount);
+                        */
+                }
+
 		/* Decrement the shared reference count */
 		Assert(buf->refcount > 0);
 		buf->refcount--;
 
+                if (   (buf->refcount == 0) && (buf->flags & BM_AIO_IN_PROGRESS)  ) {
+
+                        elog(ERROR, "UnpinBuffer :  released last any-backend pin on buffer %d rel=%s, blockNum=%u, but AIO in progress flags %X refcount=%u"
+                            ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                            ,buf->tag.blockNum, buf->flags, buf->refcount);
+                }
+
+
 		/* Support LockBufferForCleanup() */
 		if ((buf->flags & BM_PIN_COUNT_WAITER) &&
 			buf->refcount == 1)
@@ -1658,6 +2060,7 @@ SyncOneBuffer(int buf_id, bool skip_rece
 	volatile BufferDesc *bufHdr = &BufferDescriptors[buf_id];
 	int			result = 0;
 
+
 	/*
 	 * Check whether buffer needs writing.
 	 *
@@ -1724,6 +2127,16 @@ void
 InitBufferPoolBackend(void)
 {
 	on_shmem_exit(AtProcExit_Buffers, 0);
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        /*   init the aio subsystem max number of threads and max number of requests
+        **   max number of threads   <-->  max_async_io_prefetchers
+        **   max number of requests  <-->  numBufferAiocbs = (target_prefetch_pages*max_async_io_prefetchers)
+        **   there is no return code so we just hope.
+        */
+        smgrinitaio(max_async_io_prefetchers , numBufferAiocbs);
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 }
 
 /*
@@ -1779,6 +2192,8 @@ PrintBufferLeakWarning(Buffer buffer)
 	char	   *path;
 	BackendId	backend;
 
+
+
 	Assert(BufferIsValid(buffer));
 	if (BufferIsLocal(buffer))
 	{
@@ -1789,12 +2204,28 @@ PrintBufferLeakWarning(Buffer buffer)
 	else
 	{
 		buf = &BufferDescriptors[buffer - 1];
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                /*   If reason that this buffer is pinned
+                **   is that it was prefetched with async_io
+                **   and never read or discarded, then omit the
+                **   warning,  because this is expected in some
+                **   cases when a scan is closed abnormally.
+                **   Note that the buffer will be released soon by our caller.
+                */
+                if (buf->flags & BM_AIO_PREFETCH_PIN_BANKED) {
+                    pgBufferUsage.aio_read_forgot++; /* account for it */
+                    return;
+                }
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 		loccount = PrivateRefCount[buffer - 1];
 		backend = InvalidBackendId;
 	}
 
+/* #if defined(DO_ISCAN_DISCARD_BUFFER) && defined(DO_HEAPSCAN_DISCARD_BUFFER) not yet working safely 130530 */
 	/* theoretically we should lock the bufhdr here */
 	path = relpathbackend(buf->tag.rnode, backend, buf->tag.forkNum);
+
+
 	elog(WARNING,
 		 "buffer refcount leak: [%03d] "
 		 "(rel=%s, blockNum=%u, flags=0x%x, refcount=%u %d)",
@@ -1802,6 +2233,7 @@ PrintBufferLeakWarning(Buffer buffer)
 		 buf->tag.blockNum, buf->flags,
 		 buf->refcount, loccount);
 	pfree(path);
+/* #endif defined(DO_ISCAN_DISCARD_BUFFER) && defined(DO_HEAPSCAN_DISCARD_BUFFER) not yet working safely 130530 */
 }
 
 /*
@@ -1918,7 +2350,7 @@ FlushBuffer(volatile BufferDesc *buf, SM
 	 * false, then someone else flushed the buffer before we could, so we need
 	 * not do anything.
 	 */
-	if (!StartBufferIO(buf, false))
+	if (!StartBufferIO(buf, false, 0))
 		return;
 
 	/* Setup error traceback support for ereport() */
@@ -2502,6 +2934,77 @@ FlushDatabaseBuffers(Oid dbid)
 	}
 }
 
+#ifdef USE_PREFETCH
+/*
+ * DiscardBuffer -- discard shared buffer used for a previously
+ *                  prefetched but unread block of a relation
+ *
+ * If the buffer is found and pinned with a banked pin,  then :
+ *      .  if AIO in progress, terminate AIO without waiting
+ *      .  if AIO had already completed successfully,
+ *         then mark buffer valid (in case someone else wants it)
+ *      .  redeem the banked pin and unpin it.
+ *
+ * This function is similar in purpose to ReleaseBuffer (below)
+ * but sufficiently different that it is a separate function.
+ * Two important differences are :
+ *   .   caller identifies buffer by blocknumber,  not buffer number
+ *   .   we unpin buffer *only* if the pin is banked,
+ *                      *never* if pinned but not banked.
+ *       This is essential as caller may perform a sequence of
+ *  SCAN1   . PrefetchBuffer   (and remember block was prefetched)
+ *  SCAN2   . ReadBuffer       (but fails to connect this read to the prefetch by SCAN1)
+ *  SCAN1   . DiscardBuffer    (SCAN1 terminates early)
+ *  SCAN2   . access tuples in buffer
+ *        Clearly the Discard *must not* unpin the buffer since SCAN2 needs it!
+ *
+ *
+ * caller may pass InvalidBlockNumber as blockNum to mean do nothing
+ */
+void
+DiscardBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
+{
+        BufferTag	newTag;		   /* identity of requested block */
+        uint32		newHash;	   /* hash value for newTag */
+        LWLockId	newPartitionLock;  /* buffer partition lock for it */
+        Buffer		buf_id;
+        volatile        BufferDesc *buf_desc;
+
+#if defined(USE_PREFETCH) && defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP) && defined(USE_AIO_SIGEVENT)
+	/*  now is a good time to check whether any BAiocb waiter-locks are pending for release */
+	if (BAiocbIolock_anchor != (struct BAiocbIolock_chain_item*)0) {
+			ProcessPendingBAiocbIolocks();	 
+	}
+#endif /* defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP) && defined(USE_AIO_SIGEVENT)  */
+
+    if (!SmgrIsTemp(reln->rd_smgr)) {
+	Assert(RelationIsValid(reln));
+	if (BlockNumberIsValid(blockNum)) {
+
+            /* create a tag so we can lookup the buffer */
+            INIT_BUFFERTAG(newTag, reln->rd_smgr->smgr_rnode.node,
+                                       forkNum, blockNum);
+
+            /* determine its hash code and partition lock ID */
+            newHash = BufTableHashCode(&newTag);
+            newPartitionLock = BufMappingPartitionLock(newHash);
+
+            /* see if the block is in the buffer pool already */
+            LWLockAcquire(newPartitionLock, LW_SHARED);
+            buf_id = BufTableLookup(&newTag, newHash);
+            LWLockRelease(newPartitionLock);
+
+            /* If in buffers, proceed */
+            if (buf_id >= 0) {
+                buf_desc = &BufferDescriptors[buf_id];        /* found buffer descriptor */
+                BufCheckAsync(0 , reln, buf_desc , BUF_INTENTION_REJECT_UNBANK , 0 , 0 , false , 0); /* end the IO and unpin if banked */
+                pgBufferUsage.aio_read_discrd++; /* account for it */
+            }
+        }
+    }
+}
+#endif   /* USE_PREFETCH */
+
 /*
  * ReleaseBuffer -- release the pin on a buffer
  */
@@ -2510,26 +3013,23 @@ ReleaseBuffer(Buffer buffer)
 {
 	volatile BufferDesc *bufHdr;
 
+
 	if (!BufferIsValid(buffer))
 		elog(ERROR, "bad buffer ID: %d", buffer);
 
-	ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
 
 	if (BufferIsLocal(buffer))
 	{
+                ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
 		Assert(LocalRefCount[-buffer - 1] > 0);
 		LocalRefCount[-buffer - 1]--;
 		return;
 	}
-
-	bufHdr = &BufferDescriptors[buffer - 1];
-
-	Assert(PrivateRefCount[buffer - 1] > 0);
-
-	if (PrivateRefCount[buffer - 1] > 1)
-		PrivateRefCount[buffer - 1]--;
 	else
-		UnpinBuffer(bufHdr, false);
+        {
+                bufHdr = &BufferDescriptors[buffer - 1];
+                BufCheckAsync(0 , 0 , bufHdr , BUF_INTENTION_REJECT_NOADJUST , 0 , 0 , false , 0 );
+        }
 }
 
 /*
@@ -2555,14 +3055,41 @@ UnlockReleaseBuffer(Buffer buffer)
 void
 IncrBufferRefCount(Buffer buffer)
 {
+        bool       pin_already_banked_by_me = false;  /* buffer is already pinned by me and redeemable */
+        volatile BufferDesc *buf;                     /* descriptor for a shared buffer */
+
 	Assert(BufferIsPinned(buffer));
+
+        if (!(BufferIsLocal(buffer))) {
+                buf = &BufferDescriptors[buffer - 1];
+		LockBufHdr(buf);
+                pin_already_banked_by_me =
+                      (    (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                        && (   (    (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                                      : (-(buf->freeNext))  )  == this_backend_pid )
+                      );
+        }
+
+        if (!pin_already_banked_by_me) {
 	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
 	ResourceOwnerRememberBuffer(CurrentResourceOwner, buffer);
+        }
+
 	if (BufferIsLocal(buffer))
 		LocalRefCount[-buffer - 1]++;
-	else
+	else {
+                if (pin_already_banked_by_me) {
+                    buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;
+                    if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                        buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+                    }
+                }
+		UnlockBufHdr(buf);
+                if (!pin_already_banked_by_me) {
 		PrivateRefCount[buffer - 1]++;
 }
+        }
+}
 
 /*
  * MarkBufferDirtyHint
@@ -2984,61 +3511,138 @@ WaitIO(volatile BufferDesc *buf)
  *
  * In some scenarios there are race conditions in which multiple backends
  * could attempt the same I/O operation concurrently.  If someone else
- * has already started I/O on this buffer then we will block on the
+ * has already started synchronous I/O on this buffer then we will block on the
  * io_in_progress lock until he's done.
  *
+ * if an async io is in progress and we are doing synchronous io,
+ * then readbuffer uses call to smgrcompleteaio to wait,
+ * and so we treat this request as if no io in progress
+ *
  * Input operations are only attempted on buffers that are not BM_VALID,
  * and output operations only on buffers that are BM_VALID and BM_DIRTY,
  * so we can always tell if the work is already done.
  *
+ * index_for_aio is an input parm which,  if non-zero,  identifies a BufferAiocb
+ * acquired by caller and to be attached to the buffer header for use with async io
+ *
  * Returns TRUE if we successfully marked the buffer as I/O busy,
  * FALSE if someone else already did the work.
  */
 static bool
-StartBufferIO(volatile BufferDesc *buf, bool forInput)
+StartBufferIO(volatile BufferDesc *buf, bool forInput , int index_for_aio )
 {
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        struct BufferAiocb volatile * BAiocb = (struct BufferAiocb*)0;  /*  BufferAiocb for use with aio */
+#endif   /* USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+ 
+        if (!index_for_aio)
 	Assert(!InProgressBuf);
 
 	for (;;)
 	{
+                if (!index_for_aio) {
 		/*
 		 * Grab the io_in_progress lock so that other processes can wait for
 		 * me to finish the I/O.
 		 */
 		LWLockAcquire(buf->io_in_progress_lock, LW_EXCLUSIVE);
+                }
 
 		LockBufHdr(buf);
 
-		if (!(buf->flags & BM_IO_IN_PROGRESS))
+                /*     the following test is intended to distinguish between :
+                **      .   buffer which : 
+                **           .     has io in progress
+                **             AND is not associated with a current aio
+                **      .   not the above
+                **     Here,  "recent" means an aio marked by buf->freeNext <= FREENEXT_BAIOCB_ORIGIN but no longer in progress -
+                **          this situation arises when the aio has just been cancelled and this process now wishes to recycle the buffer.
+                **          In this case,  the first such would-be recycler (i.e. me) must :
+                **             . avoid waiting for the cancelled aio to complete
+                **             . if not myself doing async read, then assume responsibility for posting other future readbuffers.
+                */
+		if (    (buf->flags & BM_AIO_IN_PROGRESS)
+             || (!(buf->flags & BM_IO_IN_PROGRESS))
+           )
 			break;
 
 		/*
-		 * The only way BM_IO_IN_PROGRESS could be set when the io_in_progress
+		 * The only way BM_IO_IN_PROGRESS without AIO in progress could be set when the io_in_progress
 		 * lock isn't held is if the process doing the I/O is recovering from
 		 * an error (see AbortBufferIO).  If that's the case, we must wait for
 		 * him to get unwedged.
 		 */
 		UnlockBufHdr(buf);
+                if (!index_for_aio) {
 		LWLockRelease(buf->io_in_progress_lock);
+                }
 		WaitIO(buf);
 	}
 
-	/* Once we get here, there is definitely no I/O active on this buffer */
-
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+	/* Once we get here, there is definitely no synchronous I/O active on this buffer
+        ** but if being asked to attach a BufferAiocb to the buf header,
+        ** then we must also check if there is any async io currently
+        ** in progress or pinned started by a different task.
+        */
+        if (index_for_aio) {
+            BAiocb = (BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext);
+            if (    (buf->flags & (BM_AIO_IN_PROGRESS|BM_AIO_PREFETCH_PIN_BANKED))
+                 && (buf->freeNext <= FREENEXT_BAIOCB_ORIGIN)
+                 && (BAiocb->pidOfAio != this_backend_pid)
+               ) {
+                    /* someone else already doing async I/O */
+                    UnlockBufHdr(buf);
+                    return false;
+            }
+	}
+#endif   /* USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 	if (forInput ? (buf->flags & BM_VALID) : !(buf->flags & BM_DIRTY))
 	{
 		/* someone else already did the I/O */
 		UnlockBufHdr(buf);
+                if (!index_for_aio) {
 		LWLockRelease(buf->io_in_progress_lock);
+                }
 		return false;
 	}
 
 	buf->flags |= BM_IO_IN_PROGRESS;
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        if (index_for_aio) {
+            BAiocb = (BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - index_for_aio);
+            /*   insist that no other buffer is using this BufferAiocb for async IO */
+            if (BAiocb->BAiocbbufh == (struct sbufdesc *)0) {
+                BAiocb->BAiocbbufh = buf;
+            }
+            if (BAiocb->BAiocbbufh != buf) {
+                               ereport(ERROR,
+                                    (errcode(ERRCODE_INTERNAL_ERROR),
+                                     errmsg("AIO control block %p to be used by %p already in use by %p"
+                                              ,BAiocb ,buf , BAiocb->BAiocbbufh)));
+            }
+            /*   note - there is no need to register self as an dependent of BAiocb
+            **   as we shall not unlock buf_desc before we free the BAiocb
+            */
+
+            buf->flags |= BM_AIO_IN_PROGRESS;
+            buf->freeNext = index_for_aio;
+            /*  at this point,  this buffer appears to have an in-progress aio_read,
+            **  and any other task which is able to look inside the buffer might try waiting on that aio -
+            **  except we have not yet issued the aio!   So we must keep the buffer header locked
+            **  from here all the way back to the BufStartAsync caller
+            */
+        } else {
+#endif   /* USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 	UnlockBufHdr(buf);
 
 	InProgressBuf = buf;
 	IsForInput = forInput;
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        }
+#endif   /* USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 	return true;
 }
@@ -3048,7 +3652,7 @@ StartBufferIO(volatile BufferDesc *buf,
  *	(Assumptions)
  *	My process is executing IO for the buffer
  *	BM_IO_IN_PROGRESS bit is set for the buffer
- *	We hold the buffer's io_in_progress lock
+ *	if no async IO in progress,  then We hold the buffer's io_in_progress lock
  *	The buffer is Pinned
  *
  * If clear_dirty is TRUE and BM_JUST_DIRTIED is not set, we clear the
@@ -3060,26 +3664,32 @@ StartBufferIO(volatile BufferDesc *buf,
  * BM_IO_ERROR in a failure case.  For successful completion it could
  * be 0, or BM_VALID if we just finished reading in the page.
  */
-static void
+void
 TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
 				  int set_flag_bits)
 {
-	Assert(buf == InProgressBuf);
+        int flags_on_entry;
 
 	LockBufHdr(buf);
 
+        flags_on_entry = buf->flags;
+
+        if (! (flags_on_entry & BM_AIO_IN_PROGRESS) )
+            Assert( buf == InProgressBuf );
+
 	Assert(buf->flags & BM_IO_IN_PROGRESS);
-	buf->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR);
+	buf->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR | BM_AIO_IN_PROGRESS);
 	if (clear_dirty && !(buf->flags & BM_JUST_DIRTIED))
 		buf->flags &= ~(BM_DIRTY | BM_CHECKPOINT_NEEDED);
 	buf->flags |= set_flag_bits;
 
 	UnlockBufHdr(buf);
 
+        if (! (flags_on_entry & BM_AIO_IN_PROGRESS) ) {
 	InProgressBuf = NULL;
-
 	LWLockRelease(buf->io_in_progress_lock);
 }
+}
 
 /*
  * AbortBufferIO: Clean up any active buffer I/O after an error.
--- src/backend/storage/buffer/buf_async.c.orig	2014-06-25 17:33:03.176961989 -0400
+++ src/backend/storage/buffer/buf_async.c	2014-06-25 18:10:51.120521316 -0400
@@ -0,0 +1,931 @@
+/*-------------------------------------------------------------------------
+ *
+ * buf_async.c
+ *	  buffer manager asynchronous disk read routines
+ *
+ * Portions Copyright (c) 1996-2012, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/storage/buffer/buf_async.c
+ *
+ *-------------------------------------------------------------------------
+ */
+/*
+ * Principal entry points:
+ *
+ * BufStartAsync() -- start an asynchronous read of a block into a buffer and
+ *	 pin it so that no one can destroy it while this process is using it.
+ *
+ * BufCheckAsync() -- check completion of an asynchronous read
+ *       and either claim buffer or discard it
+ *
+ * Private helper
+ *
+ * BufReleaseAsync() -- release the BAiocb resources used for an asynchronous read
+ *
+ * See also these files:
+ *		bufmgr.c -- main buffer manager functions
+ *		buf_init.c -- initialisation of resources
+ */
+#include "postgres.h"
+#include <sys/types.h> 
+#include <sys/file.h>
+#include <unistd.h>
+#include <sched.h>
+
+#include "catalog/catalog.h"
+#include "common/relpath.h"
+#include "executor/instrument.h"
+#include "miscadmin.h"
+#include "pg_trace.h"
+#include "pgstat.h"
+#include "postmaster/bgwriter.h"
+#include "storage/buf_internals.h"
+#include "storage/bufmgr.h"
+#include "storage/ipc.h"
+#include "storage/proc.h"
+#include "storage/smgr.h"
+#include "storage/standby.h"
+#include "utils/rel.h"
+#include "utils/resowner_private.h"
+
+/*
+ * GUC parameters
+ */
+int      max_async_io_prefetchers; /* Maximum number of backends using asynchronous librt threads to read pages into our buffers */
+
+/* Note: these two macros only work on shared buffers, not local ones! */
+#define BufHdrGetBlock(bufHdr)	((Block) (BufferBlocks + ((Size) (bufHdr)->buf_id) * BLCKSZ))
+#define BufferGetLSN(bufHdr)	(PageGetLSN(BufHdrGetBlock(bufHdr)))
+
+extern volatile struct BAiocbAnchor *BAiocbAnchr;  /*  anchor for all control blocks pertaining to aio  */
+extern int numBufferAiocbs;        /*  total number of  BufferAiocbs in pool  */
+extern int maxGetBAiocbTries;      /*   max times we will try to get a free BufferAiocb */
+extern int maxRelBAiocbTries;      /*   max times we will try to release a BufferAiocb back to freelist */
+extern pid_t this_backend_pid;     /*   pid of this backend */
+
+extern bool PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy);
+extern void PinBuffer_Locked(volatile BufferDesc *buf);
+extern Buffer ReadBuffer_common(SMgrRelation reln, char relpersistence,
+				  ForkNumber forkNum, BlockNumber blockNum,
+				  ReadBufferMode mode, BufferAccessStrategy strategy,
+				  bool *hit , int index_for_aio);
+extern void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner);
+extern void TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
+				  int set_flag_bits);
+int BufCheckAsync(SMgrRelation caller_smgr, Relation caller_reln, volatile BufferDesc *buf_desc
+  ,int intention , BufferAccessStrategy strategy , int index_for_aio , bool spinLockHeld , LWLockId PartitionLock );
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+int BufStartAsync(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy);
+
+static struct BufferAiocb volatile * cachedBAiocb = (struct BufferAiocb*)0;  /*  one cached BufferAiocb for use with aio */
+
+/*  BufReleaseAsync releases a BufferAiocb and returns 0 if successful else non-zero
+**  it *must* be called :
+**     EITHER with a valid  BAiocb->BAiocbbufh -> buf_desc
+**            and that buf_desc must be spin-locked
+**     OR     with BAiocb->BAiocbbufh == 0
+*/
+static int
+BufReleaseAsync(struct BufferAiocb volatile * BAiocb)
+{
+        int    LockTries;         /*  max times we will try to release the BufferAiocb */
+        volatile struct BufferAiocb *BufferAiocbs;
+        struct BufferAiocb volatile * oldFreeBAiocb;  /*  old value of FreeBAiocbs */
+
+        int failed = 1; /* by end of this function, non-zero  will indicate if we failed to return the BAiocb */
+
+
+        if (    ( BAiocb == (struct BufferAiocb*)0 )
+             || ( BAiocb == (struct BufferAiocb*)BAIOCB_OCCUPIED )
+             || ( ((unsigned long)BAiocb) & 0x1 )
+           ) {
+                          elog(ERROR,
+                                 "AIO control block corruption on release of aiocb %p - invalid BAiocb"
+                                          ,BAiocb);
+        }
+        else 
+        if (   (0 == BAiocb->BAiocbDependentCount)     /*  no dependents  */
+            && ((struct BufferAiocb*)BAIOCB_OCCUPIED == BAiocb->BAiocbnext)  /*  not already on freelist */
+           ) {
+
+            if ((struct sbufdesc*)0 != BAiocb->BAiocbbufh) { /*  if a buffer was attached */
+                volatile        BufferDesc *buf_desc = BAiocb->BAiocbbufh;
+
+                /*  spinlock held so instead of TerminateBufferIO(buf, false , 0); ... */
+                if (buf_desc->flags & BM_AIO_PREFETCH_PIN_BANKED) { /* if a pid banked the pin */
+                    buf_desc->freeNext = -(BAiocb->pidOfAio);       /* then remember which pid */
+                }
+                else if (buf_desc->freeNext <= FREENEXT_BAIOCB_ORIGIN) {
+                    buf_desc->freeNext = FREENEXT_NOT_IN_LIST;   /* disconnect BufferAiocb from buf_desc */
+                }
+                buf_desc->flags &= ~BM_AIO_IN_PROGRESS;
+            }
+            
+            BAiocb->BAiocbbufh = (struct sbufdesc *)0; /* disconnect buf_desc from BufferAiocb */
+            BAiocb->pidOfAio = 0;                      /*  clean */
+            LockTries = maxRelBAiocbTries;         /*  max times we will try to release the BufferAiocb */
+            do {
+                register long long int dividend , remainder;
+
+                /*      retrieve old value of FreeBAiocbs  */
+                BAiocb->BAiocbnext = oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs;  /*  old value of FreeBAiocbs */
+
+                /*  this is a volatile value unprotected by any lock,  so must validate it;
+                **  safest is to verify that it is identical to one of the BufferAiocbs
+                **  to do so,  verify by direct division that its address offset from first control block 
+                **  is an integral multiple of the control block size
+                **  that lies within the range [ 0 , (numBufferAiocbs-1) ]
+                */
+                BufferAiocbs = BAiocbAnchr->BufferAiocbs;
+                remainder = ((long long int)oldFreeBAiocb - (long long int)BufferAiocbs)
+                            % (long long int)(sizeof(struct BufferAiocb));
+                failed = (int)remainder;
+                if (!failed) {
+                    dividend = ((long long int)oldFreeBAiocb - (long long int)BufferAiocbs)
+                               / (long long int)(sizeof(struct BufferAiocb));
+                     failed = ( (dividend < 0 ) || ( dividend >= numBufferAiocbs) );
+                     if (!failed) {
+                         if (__sync_bool_compare_and_swap (&(BAiocbAnchr->FreeBAiocbs), oldFreeBAiocb, BAiocb)) {
+                            LockTries = 0;   /*  end the do loop  */
+
+                            goto cheering;   /*  cant simply break because then failed would be set incorrectly */
+                         }
+                    }
+                }
+                /*  if we reach here, we have failed and failed is set to -1 */
+
+       cheering: ;
+
+                if ( LockTries > 1 ) {
+                    sched_yield();    /*   yield to another process,  (hopefully a backend) */
+                }
+            } while  (LockTries-- > 0);
+
+            if (failed) {
+#ifdef LOG_RELBAIOCB_DEPLETION
+                          elog(LOG,
+                                "BufReleaseAsync:AIO control block %p unreleased after tries= %d\n"
+                                       ,BAiocb,maxRelBAiocbTries);
+#endif  /* LOG_RELBAIOCB_DEPLETION */
+            }
+
+        }
+              else
+                          elog(LOG,
+                                "BufReleaseAsync:AIO control block %p either has dependents= %d or is already on freelist %p or has no buf_header %p\n"
+                                       ,BAiocb , BAiocb->BAiocbDependentCount , BAiocb->BAiocbnext , BAiocb->BAiocbbufh);
+        return failed;
+}
+
+/*  try using asynchronous aio_read to prefetch into a buffer
+**  return code :
+**        0 if started successfully
+**       -1 if failed for some reason
+**        1+PrivateRefCount if we found desired buffer in buffer pool
+**
+**  There is a harmless race condition here :
+**  two different backends may both arrive here simultaneously
+**  to prefetch the same buffer.    This is not unlikely when a syncscan is in progress.
+**  .  One will acquire the buffer and issue the smgrstartaio
+**  .  Other will find the buffer on return from  ReadBuffer_common with hit = true
+**  Only the first task has a pin on the buffer since ReadBuffer_common knows not to get a pin
+**  on a found buffer in prefetch mode.
+**  Therefore  -   the second task must simply abandon the prefetch if it finds the buffer in the buffer pool.
+**
+**  if we fail to acquire a BAiocb because of concurrent theft from freelist by other backend,
+**  retry up to maxGetBAiocbTries times provided that there actually was at least one BAiocb on the freelist.
+*/
+int
+BufStartAsync(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy) {
+
+        int retcode = -1;
+        struct BufferAiocb volatile * BAiocb = (struct BufferAiocb*)0;  /*  BufferAiocb for use with aio */
+        int  smgrstartaio_rc = -1;           /*  retcode from smgrstartaio */
+        bool do_unpin_buffer = false;        /* unpin must be deferred until after buffer descriptor is unlocked */
+        Buffer		buf_id;
+        bool		hit = false;
+        volatile        BufferDesc *buf_desc = (BufferDesc *)0;
+
+        int    LockTries;         /*  max times we will try to get a free BufferAiocb */
+
+        struct BufferAiocb volatile * oldFreeBAiocb;  /*  old value of FreeBAiocbs */
+        struct BufferAiocb volatile * newFreeBAiocb;  /*  new value of FreeBAiocbs */
+
+
+    /*  return immediately if no async io resources */
+    if (numBufferAiocbs > 0) {
+        buf_id = (Buffer)0;
+
+        if ( (struct BAiocbAnchor *)0 != BAiocbAnchr ) {
+
+            volatile struct BufferAiocb *BufferAiocbs;
+
+            if ((struct BufferAiocb*)0 != cachedBAiocb) {  /* any cached BufferAiocb ? */
+                BAiocb = cachedBAiocb;                     /* yes  use it  */
+                cachedBAiocb = BAiocb->BAiocbnext;
+                BAiocb->BAiocbnext = (struct BufferAiocb*)BAIOCB_OCCUPIED; /* mark as not on freelist */
+                BAiocb->BAiocbDependentCount = 0;  /* no dependent yet */
+                BAiocb->BAiocbbufh = (struct sbufdesc*)0;
+                BAiocb->pidOfAio = 0;
+            } else {
+
+                LockTries = maxGetBAiocbTries;         /*  max times we will try to get a free BufferAiocb */
+                do {
+                    register long long int dividend = -1 , remainder;
+                    /*  check if we have a free BufferAiocb */
+
+                    oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs;  /*  old value of FreeBAiocbs */
+
+                    /*  check if we have a free BufferAiocb */
+
+                    /*  BAiocbAnchr->FreeBAiocbs is a volatile value unprotected by any lock,
+                    **  and use of compare-and-swap to add and remove items from the list has
+                    **  two potential pitfalls,    both relating to the fact that we must
+                    **  access data de-referenced from this pointer before the compare-and-swap.
+                    **  1)  The value we load may be corrupt,  e.g. mixture of bytes from
+                    **      two different values,   so must validate it;
+                    **      safest is to verify that it is identical to one of the BufferAiocbs.
+                    **      to do so,  verify by direct division that its address offset from
+                    **      first control block is an integral multiple of the control block size
+                    **      that lies within the range [ 0 , (numBufferAiocbs-1) ]
+                    **      Thus we completely prevent this pitfall.
+                    **  2)  The content of the item's next pointer may have changed between the
+                    **      time we de-reference it and the time of the compare-and-swap.
+                    **      Thus even though the compare-and-swap succeeds,   we might set the
+                    **      new head of the freelist to an invalid value  (either a free item
+                    **      that is not the first in the free chain  -  resulting only in
+                    **      loss of the orphaned free items, or,  much worse, an in-use item).
+                    **      In practice this is extremely unlikely because of the implied huge delay
+                    **      in this window interval in this (current) process.    Here are two scenarios:
+                    **      legend:
+                    **         P0  -  this (current) process,  P1,  P2 , ... other processes
+                    **         content of freelist shown as BAiocbAnchr->FreeBAiocbs -> first item -> 2nd item ...
+                    **         @[X] means address of X
+                    **         |      timeline of window of exposure to problems
+                    **      successive lines in chronological order                                       content of freelist
+                    **        2.1    P0           access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I0]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **               P0          access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I1]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1        access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I0]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1       access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I1]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1        swap-remove I0,  place I1 at head of list                F -> I1 -> I2 -> I3 ...
+                    **         |           P2     access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I1]  F -> I1 -> I2 -> I3 ...
+                    **         |           P2    access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I2]  F -> I1 -> I2 -> I3 ...
+                    **         |           P2     swap-remove I1,  place I2 at head of list                F -> I2 -> I3 ...
+                    **         |        P1       complete aio,  replace I0 at head of list                 F -> I0 -> I2 -> I3 ...
+                    **               P0        swap-remove I0,  place I1 at head of list                   F -> I1 IS IN USE !! CORRUPT !!
+                    **               compare-and-swap succeeded but the value of newFreeBAiocb was stale
+                    **               and had become in-use during the window.
+                    **        2.2    P0           access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I0]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **               P0          access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I1]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1        access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I0]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1       access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I1]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1        swap-remove I0,  place I1 at head of list                F -> I1 -> I2 -> I3 ...
+                    **         |           P2     access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I1]  F -> I1 -> I2 -> I3 ...
+                    **         |           P2    access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I2]  F -> I1 -> I2 -> I3 ...
+                    **         |           P2     swap-remove I1,  place I2 at head of list                F -> I2 -> I3 ...
+                    **         |              P3  access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I2]  F -> I2 -> I3 ...
+                    **         |              P3 access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I3]  F -> I2 -> I3 ...
+                    **         |              P3  swap-remove I2,  place I3 at head of list                F -> I3 ...
+                    **         |           P2    complete aio,  replace I1 at head of list                 F -> I1 -> I3 ...
+                    **         |              P3 complete aio,  replace I2 at head of list                 F -> I2 -> I1 -> I3 ...
+                    **         |        P1       complete aio,  replace I0 at head of list                 F -> I0 -> I2 -> I1 -> I3 ...
+                    **               P0        swap-remove I0,  place I1 at head of list                   F -> I1 -> I3 ... ! I2 is orphaned !
+                    **               compare-and-swap succeeded but the value of newFreeBAiocb was stale
+                    **               and had moved further down the free list during the window.
+                    **      Unfortunately, we cannot prevent this pitfall but we can detect it (after the fact),
+                    **      by checking that the next pointer of the item we have just removed for our use still points to the same item.
+                    **      This test is not subject to any timing or uncertainty since :
+                    **       .  The fact that the compare-and-swap succeeded implies that the item we removed
+                    **          was defintely on the freelist (at the head) when it was removed,
+                    **          and therefore cannot be in use,  and therefore its next pointer is no longer volatile.
+                    **       .  Although pointers of the anchor and items on the freelist are volatile,
+                    **          the addresses of items never change -  they are in an allocated array and never move.
+                    **      E.g. in the above two scenarios,   the test is that I0.next still -> I1,
+                    **      and this is true if and only if the second item on the freelist is
+                    **      still the same at the end of the window as it was at the start of the window.
+                    **      Note that we do not insist that it did not change during the window,
+                    **           only that it is still the correct new head of freelist.
+                    **      If this test fails,  we abort immediately as the subsystem is damaged and cannot be repaired.
+                    **      Note that at least one aio must have been issued *and* completed during the window
+                    **           for this to occur,  and since the window is just one single machine instruction,
+                    **           it is very unlikely in practice.
+                    */
+                    BufferAiocbs = BAiocbAnchr->BufferAiocbs;
+                    remainder = ((long long int)oldFreeBAiocb - (long long int)BufferAiocbs)
+                                % (long long int)(sizeof(struct BufferAiocb));
+                    if (remainder == 0) {
+                        dividend = ((long long int)oldFreeBAiocb - (long long int)BufferAiocbs)
+                                    / (long long int)(sizeof(struct BufferAiocb));
+                    }
+                    if (    (remainder == 0)
+                         && ( (dividend >= 0 ) && ( dividend < numBufferAiocbs) )
+                       )
+                    {
+                        newFreeBAiocb = oldFreeBAiocb->BAiocbnext; /*  tentative new value is second on free list */
+                        /*   Here we are in the exposure window referred to in the above comments,
+                        **   so moving along rapidly ...
+                        */
+                        if (__sync_bool_compare_and_swap (&(BAiocbAnchr->FreeBAiocbs), oldFreeBAiocb, newFreeBAiocb)) {   /*  did we get it ? */
+                                /*   We have successfully swapped head of freelist pointed to by oldFreeBAiocb off the list;
+                                **   Here we check that the item we just placed at head of freelist, pointed to by newFreeBAiocb,
+                                **   is the right one
+                                **
+                                **   also check that the BAiocb we have acquired was not in use
+                                **   i.e. that scenario 2.1 above did not occur just before our compare-and-swap
+                                **   The test is that the BAiocb is not in use.
+                                **
+                                **  in one hypothetical case,
+                                **  we can be certain that there is no corruption -
+                                **  the case where newFreeBAiocb == 0 and oldFreeBAiocb->BAiocbnext != BAIOCB_OCCUPIED -
+                                **  i.e. we have set the freelist to empty but we have a baiocb chained from ours.
+                                **  in this case our comp_swap removed all BAiocbs from the list (including ours)
+                                **  so the others chained from ours are either orphaned (no harm done)
+                                **  or in use by another backend and will eventually be returned (fine).
+                                */
+                                if ((struct BufferAiocb *)0 == newFreeBAiocb) {
+                                    if ((struct BufferAiocb *)BAIOCB_OCCUPIED == oldFreeBAiocb->BAiocbnext) {
+                                        goto baiocb_corruption;
+                                    } else if ((struct BufferAiocb *)0 != oldFreeBAiocb->BAiocbnext) {
+                                      elog(LOG,
+                                         "AIO control block inconsistency on acquiring aiocb %p - its next free %p may be orphaned (no corruption has occurred)"
+                                         	,oldFreeBAiocb , oldFreeBAiocb->BAiocbnext);
+                                    }
+                                } else {
+                                    /*  case of newFreeBAiocb not null  -  so must check more carefully ... */
+                                    remainder = ((long long int)newFreeBAiocb - (long long int)BufferAiocbs)
+                                                % (long long int)(sizeof(struct BufferAiocb));
+                                    dividend = ((long long int)newFreeBAiocb - (long long int)BufferAiocbs)
+                                                / (long long int)(sizeof(struct BufferAiocb));
+
+                                    if (    (newFreeBAiocb != oldFreeBAiocb->BAiocbnext)
+                                         || (remainder != 0)
+                                         || ( (dividend < 0 ) || ( dividend >= numBufferAiocbs) )
+                                       ) {
+                                        goto baiocb_corruption;
+                                    }
+                                }
+                                BAiocb = oldFreeBAiocb;
+                                BAiocb->BAiocbnext = (struct BufferAiocb*)BAIOCB_OCCUPIED; /* mark as not on freelist */
+                                BAiocb->BAiocbDependentCount = 0;  /* no dependent yet */
+                                BAiocb->BAiocbbufh = (struct sbufdesc*)0;
+                                BAiocb->pidOfAio = 0;
+
+                                LockTries = 0;   /*  end the do loop  */
+
+                        }
+                    }
+
+                    if ( LockTries > 1 ) {
+                        sched_yield();    /*   yield to another process,  (hopefully a backend) */
+                    }
+                } while (     ((struct BufferAiocb*)0 == BAiocb)            /*  did not get a BAiocb    */
+                           && ((struct BufferAiocb*)0 != oldFreeBAiocb)     /*  there was a free BAiocb */
+                           && (LockTries-- > 0)                             /*  told to retry           */
+                        );
+            }
+        }
+
+        if ( BAiocb != (struct BufferAiocb*)0 ) {
+            /*  try an async io    */
+            BAiocb->BAiocbthis.aio_fildes = -1; /* necessary to ensure any thief realizes aio not yet started */
+            BAiocb->pidOfAio = this_backend_pid;
+
+            /*  now try to acquire a buffer :
+            **  note -   ReadBuffer_common returns hit=true if the block is found in the buffer pool,
+            **            in which case there is no need to prefetch.
+            **  otherwise ReadBuffer_common pins returned buffer and calls StartBufferIO
+            **           and StartBufferIO :
+            **      . sets buf_desc->freeNext to negative of ( index of the aiocb in the BufferAiocbs array + 3 )
+            **      . sets  BAiocb->BAiocbbufh -> buf_desc
+            **           and in this case the buffer spinlock is held.
+            **           This is essential as no other task must issue any intention with respect
+            **           to the buffer until we have started the aio_read.
+            **  Also note that ReadBuffer_common handles enlarging the ResourceOwner buffer list as needed
+            **       so I dont need to do that
+            */
+            buf_id = ReadBuffer_common(reln->rd_smgr, reln->rd_rel->relpersistence,
+                                        forkNum, blockNum
+                                       ,RBM_NOREAD_FOR_PREFETCH  /*  tells ReadBuffer not to do any read,  just alloc buf */
+                                       ,strategy , &hit , (FREENEXT_BAIOCB_ORIGIN - (BAiocb - (BAiocbAnchr->BufferAiocbs))));
+            buf_desc = &BufferDescriptors[buf_id-1];    /* find buffer descriptor */
+
+            /*  normally hit will be false as presumably it was not in the pool
+            **  when our caller looked - but it could be there now ...
+            */
+            if (hit) {
+                   /*   see earlier comments  -  we must abandon the prefetch */
+                   retcode = 1 + PrivateRefCount[buf_id];
+                   BAiocb->BAiocbbufh = (struct sbufdesc *)0; /* indicator that we must release the BAiocb before exiting */
+            } else
+            if (  (buf_id > 0) && ((BufferDesc *)0 != buf_desc) && (buf_desc == BAiocb->BAiocbbufh)  ) {
+                   /*   buff descriptor header lock should be held.
+                   **   However,  just to be safe ,   now validate that
+                   **   we are still the owner and no other task already stole it.
+                   */
+
+                   buf_desc->flags &= ~BM_AIO_PREFETCH_PIN_BANKED; /* ensure no banked pin */
+                   /*  there should not be any other pid waiting on this buffer
+                   **  so check both of BM_VALID and BM_PIN_COUNT_WAITER are not set
+                   */
+                   if (    ( !(buf_desc->flags & (BM_VALID|BM_PIN_COUNT_WAITER) ) )
+                        && (buf_desc->flags & BM_AIO_IN_PROGRESS)
+                        && ((FREENEXT_BAIOCB_ORIGIN - (BAiocb - (BAiocbAnchr->BufferAiocbs))) == buf_desc->freeNext) /* it is still mine */
+                        && (-1 == BAiocb->BAiocbthis.aio_fildes)   /* no thief stole it */
+                        && (0 == BAiocb->BAiocbDependentCount)     /* no dependent */
+                     ) {
+                        /*   we have an empty buffer for our use */
+
+                        BAiocb->BAiocbthis.aio_buf = (void *)(BufHdrGetBlock(buf_desc)); /* Location of actual buffer.  */
+
+                        /*   note - there is no need to register self as an dependent of BAiocb
+                        **   as we shall not unlock buf_desc before we free the BAiocb
+                        */
+
+                        /*   smgrstartaio retcode is returned in smgrstartaio_rc -
+                        **   it indicates whether started or not
+                        */
+                        smgrstartaio(reln->rd_smgr, forkNum, blockNum , (char *)&(BAiocb->BAiocbthis) , &smgrstartaio_rc
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on BAiocb lock instead of polling */
+                                    , (void *)&(BAiocb->BAiocbIolockItem)
+#endif /*  USE_AIO_SIGEVENT    */
+                                    );
+
+                        if (smgrstartaio_rc == 0) {
+                            retcode = 0;
+                            buf_desc->flags |= BM_AIO_PREFETCH_PIN_BANKED; /* bank the pin for next use by this task */
+                            /* we did not register register self as an dependent of BAiocb so no need to unregister  */
+                        } else {
+                            /*  failed - return the BufferAiocb to the free list  */
+                            do_unpin_buffer = true;    /* unpin must be deferred until after buffer descriptor is unlocked */
+                            /*  spinlock held so instead of TerminateBufferIO(buf_desc, false , 0); ... */
+                            buf_desc->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR | BM_AIO_IN_PROGRESS | BM_AIO_PREFETCH_PIN_BANKED | BM_VALID);
+                            /* we did not register register self as an dependent of BAiocb so no need to unregister  */
+
+                            /*  return the BufferAiocb to the free list  */
+                            if (
+                                   BufReleaseAsync(BAiocb)
+                               ) {         /*  failed ? */
+                                BAiocb->BAiocbnext = cachedBAiocb;    /* then ...       */
+                                cachedBAiocb = BAiocb;                /*  ... cache it  */
+                            }
+
+                            pgBufferUsage.aio_read_failed++;
+                            smgrstartaio_rc = 1;  /*   to distinguish from aio not even attempted */
+                        }
+                   }
+                   else {
+                            /*  buffer was stolen or in use by other task - return the BufferAiocb to the free list  */
+                            do_unpin_buffer = true;    /* unpin must be deferred until after buffer descriptor is unlocked */
+                   }
+
+                   UnlockBufHdr(buf_desc);
+                   if (do_unpin_buffer) {
+                        if (smgrstartaio_rc >= 0) { /*  if  aio was attempted */
+                            TerminateBufferIO(buf_desc, false , 0);
+                        }
+                        UnpinBuffer(buf_desc, true);
+                   }
+            }
+            else {
+                BAiocb->BAiocbbufh = (struct sbufdesc *)0; /* indicator that we must release the BAiocb before exiting */
+            }
+
+            if ((struct sbufdesc*)0 == BAiocb->BAiocbbufh) { /*  we did not associate a buffer */
+                                                             /*  so return the BufferAiocb to the free list  */
+                            if (
+                                   BufReleaseAsync(BAiocb)
+                               ) {         /*  failed ? */
+                                BAiocb->BAiocbnext = cachedBAiocb;    /* then ...       */
+                                cachedBAiocb = BAiocb;                /*  ... cache it  */
+                            }
+            }
+        }
+    }
+
+    return retcode;
+
+    baiocb_corruption:;
+         elog(PANIC,
+              "AIO control block corruption on acquiring aiocb %p - its next free %p conflicts with new freelist pointer %p which may be invalid (corruption may have occurred)"
+                                                    ,oldFreeBAiocb , oldFreeBAiocb->BAiocbnext , newFreeBAiocb);
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+/*
+ * BufCheckAsync --      act upon caller's intention regarding a shared buffer,
+ *                       primarily in connection with any async io in progress on the buffer.
+ *     class  subvalue   intention has two main classes and some subvalues within those :
+ *      +ve      1            .   want    <=>  caller wants the buffer,
+ *                                             wait for in-progress aio and then always pin
+ *      -ve                   .   reject  <=>  caller does not want the buffer,
+ *                                             if there are no dependents,  then cancel the aio
+ *              -1, -2 , -3 , ... (see below)        and then optionally unpin
+ *                             Used when there may have been a previous fetch or prefetch.
+ *
+ * buffer is assumed to be an existing member of the shared buffer pool
+ *    as returned by BufTableLookup.
+ * if AIO in progress,  then :
+ *      .  terminate AIO, waiting for completion if +ve intention, else without waiting
+ *      .  if the AIO had already completed successfully,   then mark buffer valid
+ *      .  pin/unpin as requested
+ *
+ * +ve intention indicates that buffer must be pinned :
+ *   if the strategy parameter is null,  then use the PinBuffer_Locked optimization
+ *   to pin and unlock in one operation.   But always update buffer usage count.
+ *
+ * -ve intention indicates whether and how to unpin :
+ *   BUF_INTENTION_REJECT_KEEP_PIN 	-1   pin already held, do not unpin, (caller wants to keep it)
+ *   BUF_INTENTION_REJECT_OBTAIN_PIN	-2   obtain pin,  caller wants it for same buffer
+ *   BUF_INTENTION_REJECT_FORGET	-3   unpin and tell resource owner to forget
+ *   BUF_INTENTION_REJECT_NOADJUST	-4   unpin and call ResourceOwnerForgetBuffer myself
+ *                                           instead of telling UnpinBuffer to adjust CurrentResource owner
+ *                                           (quirky simulation of ReleaseBuffer logic)
+ *   BUF_INTENTION_REJECT_UNBANK   	-5   unpin only if pin banked by caller
+ *   The behaviour for the -ve case is based on that of ReleaseBuffer, adding handling of async io.
+ *
+ * pin/unpin action must take account of whether this backend hold a "disposable" pin on the particular buffer.
+ * A "disposable" pin is a pin acquired by buffer manager without caller knowing, such as :
+ *      when required to safeguard an async AIO  -  pin can be held across multiple bufmgr calls
+ *      when required to safeguard waiting for an async AIO  -  pin acquired and released within this function
+ * if a disposable pin is held,   then :
+ *      if a new pin is requested,  the disposable pin must be retained (redeemed) and any flags relating to it unset
+ *      if an unpin is requested,   then :
+ *              if    either no AIO in progress or this backend did not initiate the AIO
+ *              then  the disposable pin must be dropped (redeemed) and any flags relating to it unset
+ *              else  log warning and do nothing
+ *  i.e. in either case,   there is no longer a disposable pin after this function has completed.
+ *       Note that if    intention is BUF_INTENTION_REJECT_UNBANK,
+ *                 then caller expects there to be a disposable banked pin
+ *                      and if there isn't one,  we do nothing
+ *                 for all other intentions,  if there is no disposable pin,   we pin/unpin normally.
+ *
+ * index_for_aio indicates the BAiocb to be used for next aio (see PrefetchBuffer)
+ * BufFreelistLockHeld indicates whether freelistlock is held
+ * spinLockHeld indicates whether buffer header spinlock is held
+ * PartitionLock is the  buffer partition lock to be used
+ *
+ * return code (meaningful ONLY if intention is +ve) indicates validity of buffer :
+ *  -1    buffer is invalid and failed PageHeaderIsValid check
+ *   0    buffer is not valid
+ *   1    buffer is valid
+ *   2    buffer is valid but tag changed  -  (so content does not match the relation block that caller expects)
+ */
+int
+BufCheckAsync(SMgrRelation caller_smgr, Relation caller_reln, BufferDesc volatile * buf_desc, int intention , BufferAccessStrategy strategy , int index_for_aio , bool spinLockHeld , LWLockId PartitionLock )
+{
+
+        int retcode = BUF_INTENT_RC_INVALID_NO_AIO;
+        bool valid = false;
+
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        int smgrcompleteaio_rc;          /*  retcode from smgrcompleteaio */
+        SMgrRelation smgr = caller_smgr;
+        int    		BAiocbDependentCount_after_aio_finished = -1;  /*  for debugging  -  can be printed in gdb */
+	    BufferTag origTag = buf_desc->tag;	/* original identity of selected buffer */
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+        BufFlags	flags_on_entry;  /*  for debugging  -  can be printed in gdb */
+        int    		freeNext_on_entry;  /*  for debugging  -  can be printed in gdb */
+        bool       disposable_pin = false;            /* this backend had a disposable pin on entry or pins the buffer while waiting for aio_read to complete */
+        bool       pin_already_banked_by_me;          /* buffer is already pinned by me and redeemable */
+
+        int aio_successful = -1;         /*  did the aio_read succeed ?  -1 = no aio,  0 unsuccessful , 1 successful */
+        int local_intention = intention; /*  copy of intention which in one special case below may be set differently to intention */
+
+            if (!spinLockHeld) {
+                /*  lock buffer header    */
+                LockBufHdr(buf_desc);
+            }
+
+	    flags_on_entry = buf_desc->flags;
+	    freeNext_on_entry = buf_desc->freeNext;
+            pin_already_banked_by_me =
+                      (    (flags_on_entry & BM_AIO_PREFETCH_PIN_BANKED)
+                        && (   (    (flags_on_entry & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - freeNext_on_entry))->pidOfAio )
+                                                                      : (-(freeNext_on_entry))  )  == this_backend_pid )
+                      );
+
+            if (pin_already_banked_by_me) {
+                if (PrivateRefCount[buf_desc->buf_id] == 0) {  /*  but do we actually have a pin ?? */
+                    /*   this is an anomalous situation   -  somehow our disposable pin was lost without us noticing
+                    **   if AIO is in progress and we started it,
+                    **   then this is disastrous  -   two backends might both issue IO on same buffer
+                    **   otherwise,  it is harmless,  and simply means we have no disposable pin,
+                    **               but we must update flags to "notice" the fact now
+                    */
+                    if (flags_on_entry & BM_AIO_IN_PROGRESS) {
+                            elog(ERROR, "BufCheckAsync : AIO control block issuer of aio_read lost pin with BM_AIO_IN_PROGRESS on buffer %d rel=%s, blockNum=%u, flags 0x%X refcount=%u intention= %d"
+                                ,buf_desc->buf_id,relpathbackend(buf_desc->tag.rnode, InvalidBackendId, buf_desc->tag.forkNum)
+                           ,buf_desc->tag.blockNum, flags_on_entry, buf_desc->refcount ,intention);
+                    } else {
+                            elog(LOG, "BufCheckAsync : AIO control block issuer of aio_read lost pin on buffer %d rel=%s, blockNum=%u, with flags 0x%X refcount=%u intention= %d"
+                               ,buf_desc->buf_id,relpathbackend(buf_desc->tag.rnode, InvalidBackendId, buf_desc->tag.forkNum)
+                               ,buf_desc->tag.blockNum, flags_on_entry, buf_desc->refcount ,intention);
+							buf_desc->flags &= ~BM_AIO_PREFETCH_PIN_BANKED; /*   redeem the banked pin  */
+							/*   since AIO not in progress,  disconnect the buffer from banked pin */
+							buf_desc->freeNext = FREENEXT_NOT_IN_LIST;      /*   and forget the bank client */
+							pin_already_banked_by_me = false;
+                    }
+                } else {
+                    disposable_pin = true;
+                }
+            }
+
+            /*  the case of BUF_INTENTION_REJECT_UNBANK is handled specially :
+            **    if this backend has a banked pin,  then proceed just as for BUF_INTENTION_REJECT_FORGET
+            **    else the call is a no-op  --  unlock buf header and return immediately
+            */
+
+            if (intention == BUF_INTENTION_REJECT_UNBANK) {
+                if (pin_already_banked_by_me) {
+                    local_intention = BUF_INTENTION_REJECT_FORGET;
+                } else {
+                    goto unlock_buf_header;  /*  code following the unlock will do nothing since local_intention still set to BUF_INTENTION_REJECT_UNBANK */
+                }
+            }
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+            /*       we do not expect that BM_AIO_IN_PROGRESS is set without freeNext identifying the BAiocb */
+            if ( (buf_desc->flags & BM_AIO_IN_PROGRESS) && (buf_desc->freeNext == FREENEXT_NOT_IN_LIST) ) {
+
+					elog(ERROR, "BufCheckAsync : found BM_AIO_IN_PROGRESS without a BAiocb on buffer %d rel=%s, blockNum=%u, flags %X refcount=%u"
+						,buf_desc->buf_id,relpathbackend(buf_desc->tag.rnode, InvalidBackendId, buf_desc->tag.forkNum)
+						,buf_desc->tag.blockNum, buf_desc->flags, buf_desc->refcount);
+                }
+            /*     check whether aio in progress  */
+            if  (    ( (struct BAiocbAnchor *)0 != BAiocbAnchr )
+                  && (buf_desc->flags & BM_AIO_IN_PROGRESS)
+                  && (buf_desc->freeNext <= FREENEXT_BAIOCB_ORIGIN)                       /*  has a valid BAiocb */
+                  && ((FREENEXT_BAIOCB_ORIGIN - buf_desc->freeNext) < numBufferAiocbs)    /*  double-check */
+                ) {        /* this is aio   */
+                    struct BufferAiocb volatile * BAiocb = (BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf_desc->freeNext); /*  BufferAiocb associated with this aio */
+                    if ((struct BufferAiocb*)BAIOCB_OCCUPIED == BAiocb->BAiocbnext) { /*  ensure BAiocb is occupied */
+                        aio_successful = 0;       /*  tentatively the aio_read did not succeed   */
+                        retcode = BUF_INTENT_RC_INVALID_AIO;
+
+                        if (smgr == NULL) {
+                            if (caller_reln == NULL) {
+                                smgr = smgropen(buf_desc->tag.rnode, InvalidBackendId);
+                            } else {
+                                smgr = caller_reln->rd_smgr;
+                            }
+                        }
+
+                        /*  assert that this AIO is not using the same BufferAiocb as the one caller asked us to use */
+                        if ((index_for_aio < 0) && (index_for_aio == buf_desc->freeNext)) {
+                                   ereport(ERROR,
+                                        (errcode(ERRCODE_INTERNAL_ERROR),
+                                         errmsg("AIO control block index %d to be used by %p already in use by %p"
+                                                  ,index_for_aio, buf_desc, BAiocb->BAiocbbufh)));
+                        }
+
+                        /*    Call smgrcompleteaio only if either we want buffer or there are no dependents.
+                        **    In the other case of reject and there are dependents,
+                        **    then one of them will do it.
+                        */
+                        if (   (local_intention > 0) || (0 == BAiocb->BAiocbDependentCount)  ) {
+                            if (local_intention > 0) {
+                                /*  wait for in-progress aio and then pin
+                                **  OR  if I did not issue the aio and do not have a pin
+                                **  then pin now before waiting to ensure the buffer does not become unpinned while I wait
+                                **  we may potentially wait for io to complete
+                                **  so release buf header lock so that others may also wait here
+                                */
+                                BAiocb->BAiocbDependentCount++; /* register self as dependent  */
+                                if (PrivateRefCount[buf_desc->buf_id] == 0) {   /* if this buffer not pinned by me */
+                                    disposable_pin = true; /* this backend has pinned the buffer while waiting for aio_read to complete */
+                                    PinBuffer_Locked(buf_desc);
+                                } else {
+                                    UnlockBufHdr(buf_desc);
+                                }
+                                LWLockRelease(PartitionLock);
+
+                                smgrcompleteaio_rc = 1   /*  tell smgrcompleteaio to wait  */
+                                                   + ( BAiocb->pidOfAio == this_backend_pid ); /*  and whether I initiated the aio */
+                            } else {
+                                smgrcompleteaio_rc = 0;   /*  tell smgrcompleteaio to cancel */
+                            }
+
+                            smgrcompleteaio( smgr , (char *)&(BAiocb->BAiocbthis) , &smgrcompleteaio_rc
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on BAiocb lock instead of polling */
+                                           , (void *)&(BAiocb->BAiocbIolockItem)
+#endif /*  USE_AIO_SIGEVENT    */
+                                           );
+                            if ( (smgrcompleteaio_rc == 0) || (smgrcompleteaio_rc == 1) ) {
+                                  aio_successful = 1;
+                            }
+
+                            /*   statistics  */
+                            if (local_intention > 0) {
+                                if (smgrcompleteaio_rc == 0) {
+                                    /*    completed successfully and did not have to wait  */
+                                    pgBufferUsage.aio_read_ontime++;
+                                } else if (smgrcompleteaio_rc == 1) {
+                                    /*    completed successfully and did have to wait  */
+                                    pgBufferUsage.aio_read_waited++;
+                                } else {
+                                    /*  bad news   -   read failed and so buffer not usable
+                                    **  the buffer is still pinned so unpin and proceed with "not found" case
+                                    */
+                                    pgBufferUsage.aio_read_failed++;
+                                }
+
+                                /*  regain locks and handle the validity of the buffer and intention regarding it    */
+                                LWLockAcquire(PartitionLock, LW_SHARED);
+                                LockBufHdr(buf_desc);
+                                BAiocb->BAiocbDependentCount--; /* unregister self as dependent  */
+                            } else {
+                                    pgBufferUsage.aio_read_wasted++;  /*   regardless of whether aio_successful */
+                            }
+
+
+                            if (local_intention > 0) {
+                                /*  verify the buffer is still ours and has same identity
+                                **  There is one slightly tricky point here -
+                                **  if there are other dependents,   then each of them will perform this same check
+                                **  this is unavoidable as the correct setting of retcode and the BM_VALID flag
+                                **  is required by each dependent,  so we may not leave it to the last one to do it.
+                                **  It should not do any harm and easier to let them all do it than try to avoid.
+                                */
+                                if ((FREENEXT_BAIOCB_ORIGIN - (BAiocb - (BAiocbAnchr->BufferAiocbs))) == buf_desc->freeNext) { /* it is still mine */
+
+                                    if (aio_successful) {
+                                        /* validate page header.   If valid,  then mark the buffer as valid */
+                                        if (PageIsVerified((Page)(BufHdrGetBlock(buf_desc)) , ((BAiocb->BAiocbthis).aio_offset/BLCKSZ))) {
+                                            buf_desc->flags |= BM_VALID;
+                                            if (BUFFERTAGS_EQUAL(origTag , buf_desc->tag)) {
+                                                retcode = BUF_INTENT_RC_VALID;
+                                            } else {
+                                                retcode = BUF_INTENT_RC_CHANGED_TAG;
+                                            }
+                                        } else {
+                                            retcode = BUF_INTENT_RC_BADPAGE;
+                                        }
+                                    }
+                                }
+                            }
+
+                            BAiocbDependentCount_after_aio_finished = BAiocb->BAiocbDependentCount;
+
+                            /*  if no dependents,   then disconnect the BAiocb and update buffer header */
+                            if (BAiocbDependentCount_after_aio_finished == 0 ) {
+
+
+                                /*  return the BufferAiocb to the free list  */
+                                buf_desc->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR | BM_AIO_IN_PROGRESS);
+
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on BAiocb lock instead of polling */
+                            //  FIXME    the BAiocb must not be freed for reuse until we are certain
+                            //           that the aio completion signal has been delivered
+                            //           and the BAiocbIolock_chain_item placed on *our* 
+                            //           chain of awaiting-release LWLock ptrs.
+                            //           Reason is not because this backend needs that item,
+                            //           but because it must not be placed on some other backend's chain.
+#endif /*  USE_AIO_SIGEVENT */
+                                if (
+                                           BufReleaseAsync(BAiocb)
+                                   ) {        /*  failed ? */
+                                    BAiocb->BAiocbnext = cachedBAiocb;   /* then ...       */
+                                    cachedBAiocb = BAiocb;               /*  ... cache it  */
+                                }
+                            }
+
+                        }
+                    }
+            }
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+            /*  note whether buffer is valid before unlocking spinlock */
+            valid = ((buf_desc->flags & BM_VALID) != 0);
+
+            /*  if there was a disposable pin on entry to this function (i.e. marked in buffer flags)
+            **  then unmark it  -  refer to prologue comments talking about :
+            **    if a disposable pin is held,   then :
+            **     ...
+            **    i.e. in either case,   there is no longer a disposable pin after this function has completed.
+            */
+            if (pin_already_banked_by_me) {
+                        buf_desc->flags &= ~BM_AIO_PREFETCH_PIN_BANKED; /*   redeem the banked pin  */
+                        /*   if AIO not in progress,  then disconnect the buffer from BAiocb and/or banked pin */
+                        if (!(buf_desc->flags & BM_AIO_IN_PROGRESS)) {
+                            buf_desc->freeNext = FREENEXT_NOT_IN_LIST;      /*   and forget the bank client */
+                        }
+                        /********** for debugging   *****************
+                        else elog(LOG, "BufCheckAsync : found BM_AIO_IN_PROGRESS when redeeming banked pin on buffer %d rel=%s, blockNum=%u, flags %X refcount=%u"
+                       ,buf_desc->buf_id,relpathbackend(buf_desc->tag.rnode, InvalidBackendId, buf_desc->tag.forkNum)
+                       ,buf_desc->tag.blockNum, buf_desc->flags, buf_desc->refcount);
+                        ********** for debugging     *****************/
+            }
+
+            /*  If we are to obtain new pin, then use pin optimization  -  pin and unlock.
+            **  However,   if the caller is the same backend who issued the aio_read,
+            **  then he ought to have obtained the pin at that time and must not acquire
+            **  a "second" one since this is logically the same read -  he would have obtained
+            **  a single pin if using synchronous read and we emulate that behaviour.
+            **  Its important to understand that the caller is not aware that he already obtained a pin -
+            **  because calling PrefetchBuffer did not imply a pin -
+            **  so we must track that via the pidOfAio field in the BAiocb.
+            **  And to add one further complication :
+            **      we assume that although PrefetchBuffer pinned the buffer,
+            **      it did not increment the usage count.
+            **      (because it called PinBuffer_Locked which does not do that)
+            **      so in this case,   we must increment the usage count without double-pinning.
+            **      yes its ugly  -  and theres a goto!
+            */
+            if (   (local_intention > 0)
+                || (local_intention == BUF_INTENTION_REJECT_OBTAIN_PIN)
+               ) {
+
+                /* Make sure we will have room to remember the buffer pin */
+                ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
+
+                /*    here we really want a version of PinBuffer_Locked which updates usage count ... */
+                if (   (PrivateRefCount[buf_desc->buf_id] == 0) /*   if this buffer not previously pinned by me */
+                    || pin_already_banked_by_me                 /*   or I had a disposable pin on entry */
+                   ) {
+                    if (strategy == NULL)
+                    {
+                            if (buf_desc->usage_count < BM_MAX_USAGE_COUNT)
+                                    buf_desc->usage_count++;
+                    }
+                    else
+                    {
+                            if (buf_desc->usage_count == 0)
+                                    buf_desc->usage_count = 1;
+                    }
+		}
+
+                /*  now pin buffer unless we have a disposable */
+                if (!disposable_pin) { /* this backend neither banked pin for aio nor pinned the buffer while waiting for aio_read to complete */
+                    PinBuffer_Locked(buf_desc);
+                    goto unlocked_it;
+                }
+                else
+                /*    if this task previously issued the aio or pinned the buffer while waiting for aio_read to complete
+                **       and aio was unsuccessful,    then release the pin
+                */
+                if (     disposable_pin
+                      && (aio_successful == 0)       /*  aio_read failed ? */
+                   ) {
+                           UnpinBuffer(buf_desc, true);
+                }
+            }
+
+    unlock_buf_header:
+            UnlockBufHdr(buf_desc);
+    unlocked_it:
+
+            /*   now do any requested pin (if not done immediately above) or unpin/forget  */
+            if (local_intention == BUF_INTENTION_REJECT_KEEP_PIN) {
+            /*   the caller is supposed to hold a pin already so there should be nothing to do ... */
+                if (PrivateRefCount[buf_desc->buf_id] == 0) {
+                    elog(LOG, "request to keep pin on unpinned buffer %d",buf_desc->buf_id);
+
+                    valid = PinBuffer(buf_desc, strategy);
+                }
+            }
+            else
+            if (   (   (local_intention == BUF_INTENTION_REJECT_FORGET)
+                    || (local_intention == BUF_INTENTION_REJECT_NOADJUST)
+                   )
+                && (PrivateRefCount[buf_desc->buf_id] > 0) /*   if this buffer was previously pinned by me ... */
+               )  {
+
+                    if (local_intention == BUF_INTENTION_REJECT_FORGET) {
+                        UnpinBuffer(buf_desc, true); /*  ... then release the pin                   */
+                    } else
+                    if (local_intention == BUF_INTENTION_REJECT_NOADJUST) {
+                        /*   following code moved from ReleaseBuffer :
+                        **   not sure why we can't simply UnpinBuffer(buf_desc, true)
+                        **   but better leave it the way it was
+                        */
+                        ResourceOwnerForgetBuffer(CurrentResourceOwner, BufferDescriptorGetBuffer(buf_desc));
+                        if (PrivateRefCount[buf_desc->buf_id] > 1) {
+                            PrivateRefCount[buf_desc->buf_id]--;
+                        } else {
+                            UnpinBuffer(buf_desc, false);
+                        }
+                    }
+            }
+
+            /*    if retcode has not been set to one of the unusual conditions
+            **        namely failed header validity or tag changed
+            **    then the setting of valid takes precedence
+            **    over whatever retcode may be currently set to.
+            */
+            if ( ( (retcode == BUF_INTENT_RC_INVALID_NO_AIO) || (retcode == BUF_INTENT_RC_INVALID_AIO) ) && valid) {
+                   retcode = BUF_INTENT_RC_VALID;
+            } else
+            if ((retcode == BUF_INTENT_RC_VALID) && (!valid)) {
+                   if (aio_successful == -1) { /* aio not attempted */
+                       retcode = BUF_INTENT_RC_INVALID_NO_AIO;
+                   } else {
+                       retcode = BUF_INTENT_RC_INVALID_AIO;
+                   }
+            }
+
+            return retcode;
+}
--- src/backend/storage/buffer/buf_init.c.orig	2014-06-25 16:37:59.457618893 -0400
+++ src/backend/storage/buffer/buf_init.c	2014-06-25 18:10:51.132521363 -0400
@@ -13,15 +13,193 @@
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
+#include <sys/types.h> 
+#include <unistd.h>
 
 #include "storage/bufmgr.h"
 #include "storage/buf_internals.h"
-
+#include <stdlib.h> /* for getenv() */
+#include <errno.h> /* for strtoul() */
+#if defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP) && defined(USE_AIO_SIGEVENT)
+#include <sched.h>
+#endif /*  defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP) && defined(USE_AIO_SIGEVENT)  */
 
 BufferDesc *BufferDescriptors;
 char	   *BufferBlocks;
-int32	   *PrivateRefCount;
+int32	   *PrivateRefCount;       /*  array of counts per buffer of how many times this task has pinned this buffer */
+
+volatile struct BAiocbAnchor *BAiocbAnchr = (struct BAiocbAnchor *)0;  /*  anchor for all control blocks pertaining to aio  */
+
+int CountInuseBAiocbs(void);     /*  keep compiler happy */
+void ReportFreeBAiocbs(void);    /*  keep compiler happy */
+
+extern int	MaxConnections;  /*  max number of client connections which postmaster will allow  */
+int numBufferAiocbs = 0;         /*  total number of  BufferAiocbs in pool (0 <=> no async io) */
+int hwmBufferAiocbs = 0;         /*  high water mark of in-use  BufferAiocbs in pool
+                                 **  (not required to be accurate, kindly maintained for us somehow by postmaster)
+                                 */
+
+#ifdef USE_PREFETCH
+unsigned int prefetch_dbOid = 0; /*  database oid of relations on which prefetching to be done - 0 means all */
+unsigned int prefetch_bitmap_scans = 1; /*  boolean whether to prefetch bitmap heap scans        */
+unsigned int prefetch_heap_scans = 0;   /*  boolean whether to prefetch non-bitmap heap scans    */
+unsigned int prefetch_sequential_index_scans = 0;  /*  boolean whether to prefetch sequential-access non-bitmap index scans  */
+unsigned int prefetch_index_scans = 256;  /*  boolean whether to prefetch non-bitmap index scans also numeric size of pfch_list  */
+unsigned int prefetch_btree_heaps = 1;  /*  boolean whether to prefetch heap pages in _bt_next for non-bitmap index scans   */
+#endif /* USE_PREFETCH */
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+int maxGetBAiocbTries = 1;       /*  max times we will try to get a free BufferAiocb */
+int maxRelBAiocbTries = 1;       /*  max times we will try to release a BufferAiocb back to freelist */
+
+/*  locking protocol for manipulating the BufferAiocbs and FreeBAiocbs list :
+**    1.    ownership of a BufferAiocb :
+**          to gain ownership of a BufferAiocb, a task must
+**          EITHER    remove it from FreeBAiocbs (it is now temporary owner and no other task can find it)
+**                    if decision is to attach it to a buffer descriptor header, then
+**                       .   lock the buffer descriptor header
+**                       .   check  NOT flags & BM_AIO_IN_PROGRESS
+**                       .   attach to buffer descriptor header
+**                       .   increment the BufferAiocb.dependent_count
+**                       .   unlock the buffer descriptor header
+**                     and ownership scope is from lock to unlock
+***          OR        locate it by dereferencing the pointer in a buffer descriptor,
+**                    in which case :
+**                       .   lock the buffer descriptor header
+**                       .   check  flags & BM_AIO_IN_PROGRESS
+**                       .   increment the BufferAiocb.dependent_count
+**                       .   if decision is to return to FreeBAiocbs,
+**                           then   (with buffer descriptor header still locked)
+**                                  .  turn off BM_AIO_IN_PROGRESS
+**                       .   IF        the BufferAiocb.dependent_count == 1 (I am sole dependent)
+**                       .   THEN
+**                       .       .  decrement the BufferAiocb.dependent_count
+**                               .  return to FreeBAiocbs (see below)
+**                       .   unlock the buffer descriptor header
+**                     and ownership scope is from lock to either return to FreeBAiocbs or unlock
+**    2.    adding and removing from FreeBAiocbs :
+**      two alternative methods - controlled by conditional macro definition LOCK_BAIOCB_FOR_GET_REL
+**       2.1 LOCK_BAIOCB_FOR_GET_REL is defined - use a lock
+**          .   lock BufFreelistLock exclusive
+**          .   add / remove from FreeBAiocbs
+**          .   unlock BufFreelistLock exclusive
+**          advantage of this method  -  never fails to add or remove
+**       2.2  LOCK_BAIOCB_FOR_GET_REL is not defined - use compare_and_swap
+**          .   retrieve the current Freelist pointer and validate
+**          .   compare_and_swap on/off the FreeBAiocb list
+**          .   unlock BufFreelistLock exclusive
+**          advantage of this method  -  never waits
+**          to avoid losing a FreeBAiocbs,   save it in a process-local cache and reuse
+*/
+#ifdef USE_AIO_SIGEVENT                                    /*    signal non-originator waiters instead of letting them poll */
+				/*    signal environment to invoke handlaiosignal    */
+sigset_t emptymask, blockaiomask, tempmask;
+struct sigaction chsigaction , olsigAIOaction;
+
+/*  the following loack anchor is defined in globals.c
+ *  because referenced inside the CHECK_FOR_INTERRUPTS macro
+*/
+struct BAiocbIolock_chain_item volatile * volatile BAiocbIolock_anchor = (struct BAiocbIolock_chain_item*)0; /* anchor for chain of awaiting-release LWLock ptrs */
+
+/*  ProcessPendingBAiocbIolocks is the companion to handlaiosignal
+ *  which is invoked from all CHECK_FOR_INTERRUPTS to check if there are any
+ *  pending BAiocbIolockItems (representing just-completed aio operations)
+*/
+void ProcessPendingBAiocbIolocks(void);
+void ProcessPendingBAiocbIolocks() {
+          int cs_rc;
+		  struct BAiocbIolock_chain_item volatile * volatile BAiocbIolockItemP; /* first item on chain */
+          struct BAiocbIolock_chain_item volatile * volatile instantaneous_BAiocbIolock_next; /* next item on chain */
 
+		  BAiocbIolockItemP = BAiocbIolock_anchor;              /* current first item on chain */
+          while (BAiocbIolockItemP != (struct BAiocbIolock_chain_item*)0) /* there is an item */
+          {
+                 instantaneous_BAiocbIolock_next = BAiocbIolockItemP->next; /* next item on chain */
+                 cs_rc = (__sync_bool_compare_and_swap (&(BAiocbIolock_anchor), BAiocbIolockItemP, instantaneous_BAiocbIolock_next)); /* swap it off chain */
+                 if (cs_rc) {
+
+                     /*     there are two places where originator might release the BAiocb waiter-lock :
+                      *      .  here   (always called)
+                      *      .  in completeaio (if originator actually calls completeaio)
+                      *      so each case must first check whether lock has already been released.
+                      *      Fortunately there is no possibility that both cases execute simultaneously
+                      *      because they are both performed in main-line (non-thread, non-interrupt) code
+                      *      by same process.
+                      */
+                     if (LWLockHeldByMe((struct LWLock *)(BAiocbIolockItemP->BAiocbIolock))) { /*  still waiter-locked */
+    				     LWLockRelease((struct LWLock *)BAiocbIolockItemP->BAiocbIolock);   /*  removed item so release lock */
+						 BAiocbIolockItemP->next = (struct BAiocbIolock_chain_item*)0;
+					 }
+				 } else {
+					 sched_yield();    /*   yield to another process,  (hopefully a backend) */
+                 }
+				 BAiocbIolockItemP = BAiocbIolock_anchor;   /* current first item on chain */
+          }
+}
+
+  /*  handlaiosignal is invoked whenever io completion signal is issued.
+  *   siginfP->si_value.sival_ptr contains the address of the LWlock
+  *   on which non-originator waiters wait and which we are to release
+  */
+void handlaiosignal(int sigsent, siginfo_t * siginfP, void *ucontext);
+void handlaiosignal(int sigsent, siginfo_t * siginfP, void *ucontext)
+{
+      if (sigsent == AIO_SIGEVENT_SIGNALNUM) {   /*  aio completion */
+          /*   wake up any non-originator waiters */
+          /*   we would like to issue a   ...
+          LWLockRelease((LWLock *)siginfP->si_value.sival_ptr);
+          ... here but that fails as the lwlock code is not re-entrant.
+          So instead,   hang the LWLockptr off the chain of awaiting-release
+          LWLock ptrs using atomic compare_and_swap ad infinitum.
+          */
+          struct BAiocbIolock_chain_item volatile * BAiocbIolockItemP = ((struct BAiocbIolock_chain_item*)siginfP->si_value.sival_ptr); /* address the item */
+          struct BAiocbIolock_chain_item volatile * anyItemP; /* for running the chain */
+          int cs_rc = 0;
+
+          /*  two sanity checks :
+          *      .  next ptr of this item should be 0
+          *      .  anchor must not -> this item
+          */
+		  if (BAiocbIolockItemP->next == (struct BAiocbIolock_chain_item*)0) {
+              /*  run the chain and check that my item is not yet on it */
+              anyItemP = BAiocbIolock_anchor;              /* current first item on chain */
+              while (    (anyItemP != (struct BAiocbIolock_chain_item*)0)   /*  not reached end */
+                      && (anyItemP != BAiocbIolockItemP)                    /*  not my item */
+                    ) {
+                  anyItemP = anyItemP->next;
+			  }
+              if (anyItemP == (struct BAiocbIolock_chain_item*)0) {
+				  BAiocbIolockItemP->next = BAiocbIolock_anchor;              /* current first item on chain */
+				  cs_rc = (__sync_bool_compare_and_swap (&(BAiocbIolock_anchor), BAiocbIolockItemP->next, BAiocbIolockItemP)); /* swap it onto chain */
+				  while (!cs_rc) {
+					  sched_yield();    /*   yield to another process,  (hopefully a backend) */
+					  anyItemP = BAiocbIolock_anchor;              /* current first item on chain */
+					  while (    (anyItemP != (struct BAiocbIolock_chain_item*)0)   /*  not reached end */
+							  && (anyItemP != BAiocbIolockItemP)                    /*  not my item */
+							) {
+						  anyItemP = anyItemP->next;
+					  }
+					  if (anyItemP == (struct BAiocbIolock_chain_item*)0) {
+						  BAiocbIolockItemP->next = BAiocbIolock_anchor;                              /* current first item on chain */
+						  cs_rc = (__sync_bool_compare_and_swap (&(BAiocbIolock_anchor), BAiocbIolockItemP->next, BAiocbIolockItemP)); /* swap it onto chain */
+					  }
+					  else abort();
+				  }
+			  }
+			  else abort();
+		  }
+          else abort();
+      }
+}
+#endif /*  USE_AIO_SIGEVENT    */
+#else   /*  not USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+        /*   this dummy structure is to ensure that references to these fields in other bufmgr runtime code
+        **   that is not conditionally ifdefd on USE_AIO_ATOMIC_BUILTIN_COMP_SWAP compiles and runs correctly
+        */
+        struct BAiocbAnchor dummy_BAiocbAnchr = { (struct BufferAiocb*)0 , (struct BufferAiocb*)0 };
+int maxGetBAiocbTries = -1;       /*  max times we will try to get a free BufferAiocb */
+int maxRelBAiocbTries = -1;       /*  max times we will try to release a BufferAiocb back to freelist */
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * Data Structures:
@@ -73,7 +251,16 @@ void
 InitBufferPool(void)
 {
 	bool		foundBufs,
-				foundDescs;
+				foundDescs
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+              , foundAiocbs
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+          ;
+#if defined(USE_PREFETCH) || defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP)
+        char *envvarpointer = (char *)0;  /*  might point to an environment variable string */
+        char *charptr;
+#endif /* USE_PREFETCH */
+
 
 	BufferDescriptors = (BufferDesc *)
 		ShmemInitStruct("Buffer Descriptors",
@@ -83,6 +270,152 @@ InitBufferPool(void)
 		ShmemInitStruct("Buffer Blocks",
 						NBuffers * (Size) BLCKSZ, &foundBufs);
 
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        BAiocbAnchr = (struct BAiocbAnchor *)0; /*  anchor for all control blocks pertaining to aio  */
+        if (max_async_io_prefetchers < 0) {  /*  negative value indicates to initialize to something sensible during buf_init */
+            max_async_io_prefetchers = MaxConnections/6;  /*  default allows for average of MaxConnections/6 concurrent prefetchers  - reasonable ??? */
+        }
+
+        if ((target_prefetch_pages > 0) && (max_async_io_prefetchers > 0)) {
+            int ix;
+            volatile struct BufferAiocb *BufferAiocbs;
+            volatile struct BufferAiocb * volatile FreeBAiocbs;
+
+            numBufferAiocbs = (target_prefetch_pages*max_async_io_prefetchers);  /*  target_prefetch_pages per prefetcher */
+            BAiocbAnchr = (struct BAiocbAnchor *)
+	        	ShmemInitStruct("Buffer Aiocbs",
+                          sizeof(struct BAiocbAnchor) + (numBufferAiocbs * sizeof(struct BufferAiocb)), &foundAiocbs);
+            if (BAiocbAnchr) {
+                BufferAiocbs = BAiocbAnchr->BufferAiocbs = (struct BufferAiocb*)(((char *)BAiocbAnchr) + sizeof(struct BAiocbAnchor));
+                FreeBAiocbs = (struct BufferAiocb*)0;
+                for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                    (BufferAiocbs+ix)->BAiocbnext = FreeBAiocbs;   /*  init the free list,  last one -> 0  */
+                    (BufferAiocbs+ix)->BAiocbbufh = (struct sbufdesc*)0;
+                    (BufferAiocbs+ix)->BAiocbDependentCount = 0;
+                    (BufferAiocbs+ix)->pidOfAio = 0;
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on BAiocb lock instead of polling */
+			        (BufferAiocbs+ix)->BAiocbIolockItem.BAiocbIolock = LWLockAssign();
+			        (BufferAiocbs+ix)->BAiocbIolockItem.next = (struct BAiocbIolock_chain_item*)0; /* initially not on a chain */
+#endif /*  USE_AIO_SIGEVENT    */
+                    FreeBAiocbs = (BufferAiocbs+ix);
+
+                }
+                BAiocbAnchr->FreeBAiocbs = FreeBAiocbs;
+                envvarpointer = getenv("PG_MAX_GET_BAIOCB_TRIES");
+                if (    (envvarpointer != (char *)0)
+                     && (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   )
+                   ) {
+                      maxGetBAiocbTries = strtol(envvarpointer, 0, 10);
+                }
+                envvarpointer = getenv("PG_MAX_REL_BAIOCB_TRIES");
+                if (    (envvarpointer != (char *)0)
+                     && (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   )
+                   ) {
+                      maxRelBAiocbTries = strtol(envvarpointer, 0, 10);
+                }
+
+#ifdef USE_AIO_SIGEVENT                                    /*    signal waiters instead of letting them poll */
+				/*    set our signal environment to invoke handlaiosignal    */
+				sigemptyset(&emptymask);
+				sigemptyset(&blockaiomask);
+				sigaddset(&blockaiomask, AIO_SIGEVENT_SIGNALNUM);
+				sigprocmask(SIG_SETMASK,&emptymask,0);       /*  normal mask allows all signals */
+
+				/*  chsigaction.sa_handler = (void(*)(int, siginfo_t *, void *ucontext))&handlaiosignal;  */
+				chsigaction.sa_sigaction = (void(*)(int, siginfo_t *, void *ucontext))&handlaiosignal; /*  call for siginfo */
+				chsigaction.sa_mask = emptymask;
+				chsigaction.sa_flags = SA_SIGINFO; /*  call for siginfo */
+				sigaction(AIO_SIGEVENT_SIGNALNUM, &chsigaction, &olsigAIOaction);
+#endif /*  USE_AIO_SIGEVENT    */
+            }
+        }
+#else   /*  not USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+        /*   this dummy structure is to ensure that references to these fields in other bufmgr runtime code
+        **   that is not conditionally ifdefd on USE_AIO_ATOMIC_BUILTIN_COMP_SWAP compiles and runs correctly
+        */
+        BAiocbAnchr = &dummy_BAiocbAnchr;
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+
+#ifdef USE_PREFETCH
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_BITMAP");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_bitmap_scans = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+                 prefetch_bitmap_scans = 1;
+             }
+        }
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_ISCAN");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_index_scans = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+                 prefetch_index_scans = 1;
+             } else
+             if (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   ) {
+                 prefetch_index_scans = strtol(envvarpointer, &charptr, 10);
+                 if (charptr && (',' == *charptr)) {   /*  optional sequential prefetch in index scans */
+					 charptr++;        /*   following the comma ... */
+					 if ( ('Y' == *charptr) || ('y' == *charptr) || ('1' == *charptr) ) {
+                         prefetch_sequential_index_scans = 1;
+					 }
+				 }
+             }
+             /*  if prefeching for ISCAN,  then we require size of pfch_list to be at least target_prefetch_pages */
+             if (   (prefetch_index_scans > 0)
+                 && (prefetch_index_scans < target_prefetch_pages)
+                ) {
+                 prefetch_index_scans = target_prefetch_pages;
+             }
+        }
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_BTREE");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_btree_heaps = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+                 prefetch_btree_heaps = 1;
+             }
+        }
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_HEAP");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_heap_scans = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+              prefetch_heap_scans = 1;
+             }
+        }
+        envvarpointer = getenv("PG_PREFETCH_DBOID");
+        if (    (envvarpointer != (char *)0)
+             && (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   )
+           ) {
+              errno = 0;   /*  required in order to distinguish error from 0 */
+              prefetch_dbOid = (unsigned int)strtoul((const char *)envvarpointer, 0, 10);
+              if (errno) {
+                  prefetch_dbOid = 0;
+              }
+        }
+        elog(LOG, "prefetching initialised with target_prefetch_pages= %d "
+                  ", max_async_io_prefetchers= %d implying aio concurrency= %d "
+                  ", prefetching_for_bitmap= %s "
+                  ", prefetching_for_heap= %s "
+                  ", prefetching_for_iscan= %d with sequential_index_page_prefetching= %s "
+                  ", prefetching_for_btree= %s"
+                   ,target_prefetch_pages ,max_async_io_prefetchers ,numBufferAiocbs
+                   ,(prefetch_bitmap_scans ? "Y" : "N")
+                   ,(prefetch_heap_scans ? "Y" : "N")
+                   ,prefetch_index_scans
+                   ,(prefetch_sequential_index_scans ? "Y" : "N")
+                   ,(prefetch_btree_heaps ? "Y" : "N")
+            );
+#endif /* USE_PREFETCH */
+
+
 	if (foundDescs || foundBufs)
 	{
 		/* both should be present or neither */
@@ -176,3 +509,82 @@ BufferShmemSize(void)
 
 	return size;
 }
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*     imprecise count of number of in-use BAiocbs at any time
+ *     we scan the array read-only without latching so are subject to unstable result
+ *     (but since the array is in well-known contiguous storage,
+ *     we are not subject to segmentation violation)
+ *     This function may be called at any time and just does its best
+ *     return the count of what we counted.
+ */
+int
+CountInuseBAiocbs(void)
+{
+        volatile struct BufferAiocb *BAiocb;
+        int count = 0;
+        int ix;
+
+
+        if (BAiocbAnchr != (struct BAiocbAnchor *)0 ) {
+            BAiocb = BAiocbAnchr->BufferAiocbs;             /*   start of list */
+
+            for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                    if ((struct BufferAiocb*)BAIOCB_OCCUPIED == (BAiocb+ix)->BAiocbnext) {   /* not on freelist ? */
+                        count++;
+                    }
+            }
+        }
+        return count;
+}
+
+/*
+ * report how many free BAiocbs at shutdown
+ * DO NOT call this while backends are actively working!!
+ * this report is useful when compare_and_swap method used (see above)
+ * as it can be used to deduce how many BAiocbs were in process-local caches -
+ * (original_number_on_freelist_at_startup - this_reported_number_at_shutdown)
+ */
+void
+ReportFreeBAiocbs(void)
+{
+        volatile struct BufferAiocb *BAiocb;
+        volatile struct BufferAiocb *BufferAiocbs;
+        int count = 0;
+        int fx , ix;
+
+
+        if (BAiocbAnchr != (struct BAiocbAnchor *)0 ) {
+            BAiocb = BAiocbAnchr->FreeBAiocbs;             /*   start of free list */
+            BufferAiocbs = BAiocbAnchr->BufferAiocbs;
+
+            for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                    (BufferAiocbs+ix)->BAiocbDependentCount = 0;  /* use this as marker for finding it on freelist */
+            }
+            for (fx = (numBufferAiocbs-1);  ( (fx>=0) &&  ( BAiocb != (struct BufferAiocb*)0 ) );  fx--) {
+                    
+                    /*  check if it is a valid BufferAiocb */
+                    for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                        if ((BufferAiocbs+ix) == BAiocb) { /*  is it this one ? */
+                             break;
+                        }
+                    }
+                    if (ix >= 0) {
+                        if (BAiocb->BAiocbDependentCount) {   /* seen it already ? */
+                            elog(LOG, "ReportFreeBAiocbs closed cycle on AIO control block freelist %p"
+                                          ,BAiocb);
+                            fx = 0; /* give up at this point */
+                        }
+                        BAiocb->BAiocbDependentCount = 1;  /* use this as marker for finding it on freelist */
+                        count++;
+                        BAiocb = BAiocb->BAiocbnext;
+                    } else {
+                        elog(LOG, "ReportFreeBAiocbs invalid item on AIO control block freelist %p"
+                                          ,BAiocb);
+                        fx = 0; /* give up at this point */
+                    }
+            }
+        }
+        elog(LOG, "ReportFreeBAiocbs AIO control block list : poolsize= %d  in-use-hwm= %d  final-free= %d" ,numBufferAiocbs , hwmBufferAiocbs , count);
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
--- src/backend/storage/smgr/md.c.orig	2014-06-25 16:37:59.465618895 -0400
+++ src/backend/storage/smgr/md.c	2014-06-25 18:10:51.144521410 -0400
@@ -647,6 +647,78 @@ mdprefetch(SMgrRelation reln, ForkNumber
 }
 
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*
+ *	mdinitaio() --  init the aio subsystem max number of threads and max number of requests
+ */
+void
+mdinitaio(int max_aio_threads, int max_aio_num)
+{
+     FileInitaio( max_aio_threads, max_aio_num );
+}
+
+/*
+ *	mdstartaio() -- start aio read of the specified block of a relation
+ */
+void
+mdstartaio(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum , char *aiocbp , int *retcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , void *BAiocbIolockaiocbp
+#endif /*  USE_AIO_SIGEVENT    */
+ )
+{
+#ifdef USE_PREFETCH
+	off_t		seekpos;
+	MdfdVec    *v;
+        int local_retcode;
+
+	v = _mdfd_getseg(reln, forknum, blocknum, false, EXTENSION_FAIL);
+
+	seekpos = (off_t) BLCKSZ *(blocknum % ((BlockNumber) RELSEG_SIZE));
+
+	Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+
+	local_retcode = FileStartaio(v->mdfd_vfd, seekpos, BLCKSZ , aiocbp
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , BAiocbIolockaiocbp
+#endif /*  USE_AIO_SIGEVENT    */
+                                   );
+	if (retcode) {
+            *retcode = local_retcode;
+        }
+#endif   /* USE_PREFETCH */
+}
+
+
+/*
+ *	mdcompleteaio() -- complete aio read of the specified block of a relation
+ *      on entry, *inoutcode should indicate :
+ *           .  non-0  <=>   check if complete and wait if not
+ *           .  0      <=>   cancel io immediately
+ */
+void
+mdcompleteaio( char *aiocbp , int *inoutcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , void *BAiocbIolockaiocbp
+#endif /*  USE_AIO_SIGEVENT    */
+             )
+{
+#ifdef USE_PREFETCH
+        int local_retcode;
+
+	local_retcode = FileCompleteaio(aiocbp, (inoutcode ? *inoutcode : 0)
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , BAiocbIolockaiocbp
+#endif /*  USE_AIO_SIGEVENT    */
+                                       );
+	if (inoutcode) {
+            *inoutcode = local_retcode;
+        }
+#endif   /* USE_PREFETCH */
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+
 /*
  *	mdread() -- Read the specified block from a relation.
  */
--- src/backend/storage/smgr/smgr.c.orig	2014-06-25 16:37:59.465618895 -0400
+++ src/backend/storage/smgr/smgr.c	2014-06-25 18:10:51.164521488 -0400
@@ -49,6 +49,20 @@ typedef struct f_smgr
 						 BlockNumber blocknum, char *buffer, bool skipFsync);
 	void		(*smgr_prefetch) (SMgrRelation reln, ForkNumber forknum,
 											  BlockNumber blocknum);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+	void		(*smgr_initaio) (int max_aio_threads, int max_aio_num);
+	void		(*smgr_startaio) (SMgrRelation reln, ForkNumber forknum,
+											  BlockNumber blocknum , char *aiocbp , int *retcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , void *BAiocbIolockaiocbp
+#endif /*  USE_AIO_SIGEVENT    */
+                                 );
+	void		(*smgr_completeaio) ( char *aiocbp , int *inoutcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , void *BAiocbIolockaiocbp
+#endif /*  USE_AIO_SIGEVENT    */
+                                    );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 	void		(*smgr_read) (SMgrRelation reln, ForkNumber forknum,
 										  BlockNumber blocknum, char *buffer);
 	void		(*smgr_write) (SMgrRelation reln, ForkNumber forknum,
@@ -66,7 +80,11 @@ typedef struct f_smgr
 static const f_smgr smgrsw[] = {
 	/* magnetic disk */
 	{mdinit, NULL, mdclose, mdcreate, mdexists, mdunlink, mdextend,
-		mdprefetch, mdread, mdwrite, mdnblocks, mdtruncate, mdimmedsync,
+		mdprefetch
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                        ,mdinitaio, mdstartaio, mdcompleteaio
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+              , mdread, mdwrite, mdnblocks, mdtruncate, mdimmedsync,
 		mdpreckpt, mdsync, mdpostckpt
 	}
 };
@@ -612,6 +630,51 @@ smgrprefetch(SMgrRelation reln, ForkNumb
 	(*(smgrsw[reln->smgr_which].smgr_prefetch)) (reln, forknum, blocknum);
 }
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*
+ *	smgrinitaio() -- initialize the aio subsystem max number of threads and max number of requests
+ */
+void
+smgrinitaio(int max_aio_threads, int max_aio_num)
+{
+	(*(smgrsw[0].smgr_initaio)) ( max_aio_threads, max_aio_num );
+}
+
+/*
+ *	smgrstartaio() -- Initiate aio read of the specified block of a relation.
+ */
+void
+smgrstartaio(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum , char *aiocbp , int *retcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , void *BAiocbIolockaiocbp
+#endif /*  USE_AIO_SIGEVENT    */
+             )
+{
+	(*(smgrsw[reln->smgr_which].smgr_startaio)) (reln, forknum, blocknum , aiocbp , retcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , BAiocbIolockaiocbp
+#endif /*  USE_AIO_SIGEVENT    */
+                                                );
+}
+
+/*
+ *	smgrcompleteaio() -- Complete aio read of the specified block of a relation.
+ */
+void
+smgrcompleteaio(SMgrRelation reln,  char *aiocbp , int *inoutcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , void *BAiocbIolockaiocbp
+#endif /*  USE_AIO_SIGEVENT    */
+               )
+{
+	(*(smgrsw[reln->smgr_which].smgr_completeaio)) ( aiocbp , inoutcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , BAiocbIolockaiocbp
+#endif /*  USE_AIO_SIGEVENT    */
+                                                   );
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 /*
  *	smgrread() -- read a particular block from a relation into the supplied
  *				  buffer.
--- src/backend/storage/file/fd.c.orig	2014-06-25 16:37:59.457618893 -0400
+++ src/backend/storage/file/fd.c	2014-06-25 18:10:51.180521551 -0400
@@ -77,6 +77,9 @@
 #include "utils/guc.h"
 #include "utils/resowner_private.h"
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+#include "aio.h"
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * We must leave some file descriptors free for system(), the dynamic loader,
@@ -1239,6 +1242,10 @@ FileClose(File file)
  * We could add an implementation using libaio in the future; but note that
  * this API is inappropriate for libaio, which wants to have a buffer provided
  * to read into.
+ * Also note that a new, different implementation of asynchronous prefetch
+ * using librt,  not libaio,  is provided by the two functions following this one,
+ * FileStartaio and FileCompleteaio.   These also require to have a buffer provided
+ * to read into,  which the new async_io support provides.
  */
 int
 FilePrefetch(File file, off_t offset, int amount)
@@ -1266,6 +1273,224 @@ FilePrefetch(File file, off_t offset, in
 #endif
 }
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*
+ * FileInitaio - initialize the aio subsystem max number of threads and max number of requests
+ *  input parms
+ *  max_aio_threads;    maximum number of threads
+ *  max_aio_num;        maximum number of concurrent aio read requests
+ *
+ *  on linux, man page for the librt implemenation of aio_init() says :
+ *         This function is a GNU extension.
+ *  If your posix aio does not have it,   then add the following line to 
+ *        src/include/pg_config_manual.h
+ *    #define DONT_HAVE_AIO_INIT
+ *  to render it as a no-op
+ */
+void
+FileInitaio(int max_aio_threads, int max_aio_num )
+{
+#ifndef DONT_HAVE_AIO_INIT
+    struct aioinit aioinit_struct;  /*  structure to pass to aio_init */
+
+    aioinit_struct.aio_threads = max_aio_threads; /*     maximum number of threads */
+    aioinit_struct.aio_num = max_aio_num;         /*     maximum number of concurrent aio read requests */
+    aioinit_struct.aio_idle_time = 1;             /*     we dont want to alter this but aio_init does not ignore it so set to the default */
+    aio_init(&aioinit_struct);
+#endif  /* ndef DONT_HAVE_AIO_INIT */
+    return;
+}
+
+/*
+ * FileStartaio - initiate asynchronous read of a given range of the file.
+ * The logical seek position is unaffected.
+ *
+ * use standard posix aio (librt)
+ *  ASSUME   BufferAiocb.aio_buf already set to -> buffer by caller
+ *  return 0 if successfully started,  else non-zero
+ */
+int
+FileStartaio(File file, off_t offset, int amount , char *aiocbp
+#ifdef USE_AIO_SIGEVENT                                    /*    signal  non-originator waiters instead of letting them poll */
+            , void *BAiocbIolockaiocbp
+#endif /*  USE_AIO_SIGEVENT    */
+                      )
+{
+	int	returnCode;
+        struct aiocb *my_aiocbp = (struct aiocb *)aiocbp;
+
+	Assert(FileIsValid(file));
+
+	DO_DB(elog(LOG, "FileStartaio: %d (%s) " INT64_FORMAT " %d",
+			   file, VfdCache[file].fileName,
+			   (int64) offset, amount));
+
+	returnCode = FileAccess(file);
+	if (returnCode >= 0) {
+
+            my_aiocbp->aio_fildes = VfdCache[file].fd;
+            my_aiocbp->aio_lio_opcode = LIO_READ;
+            my_aiocbp->aio_nbytes = amount;
+            my_aiocbp->aio_offset = offset;
+#ifdef USE_AIO_SIGEVENT                                    /*    signal non-originator waiters instead of letting them poll */
+            my_aiocbp->aio_sigevent.sigev_notify = SIGEV_SIGNAL;
+            my_aiocbp->aio_sigevent.sigev_signo = AIO_SIGEVENT_SIGNALNUM;
+            my_aiocbp->aio_sigevent.sigev_value.sival_ptr = BAiocbIolockaiocbp; /* address of BAiocbIolock_chain_item
+                                                                 containing LWlock on which non-originator waiters may wait */
+            /* set the next ptr and get eXclusiv lock before the aio because the signal handler may be called before return from the aio_read */
+            ((struct BAiocbIolock_chain_item*)BAiocbIolockaiocbp)->next = (struct BAiocbIolock_chain_item*)0;
+            if (!LWLockHeldByMe((struct LWLock *)(((struct BAiocbIolock_chain_item*)BAiocbIolockaiocbp)->BAiocbIolock))) { /* not waiter-locked */
+					LWLockAcquire( (LWLock *)(((struct BAiocbIolock_chain_item*)BAiocbIolockaiocbp)->BAiocbIolock), LW_EXCLUSIVE );
+			}
+#endif /*  USE_AIO_SIGEVENT    */
+            returnCode = aio_read(my_aiocbp);
+#ifdef USE_AIO_SIGEVENT                                    /*    signal non-originator waiters instead of letting them poll */
+            if (returnCode != 0) {
+                /*   the BAiocbIolock waiter-lock in this BAiocb should be currently locked,
+                 *   but best to check.
+                */
+                if (LWLockHeldByMe((struct LWLock *)(((struct BAiocbIolock_chain_item*)BAiocbIolockaiocbp)->BAiocbIolock))) { /* waiter-locked */
+                    /*  acquire exclusive use of LWlock until aio completes  */
+					LWLockRelease( (LWLock *)(((struct BAiocbIolock_chain_item*)BAiocbIolockaiocbp)->BAiocbIolock) );
+				}
+			}
+#endif /*  USE_AIO_SIGEVENT    */
+        }
+
+	return returnCode;
+}
+
+/*
+ * FileCompleteaio - complete asynchronous aio read
+ * normal_wait indicates whether to cancel or wait -
+ *                 0 <=> cancel
+ *                 1 <=> wait by polling the aiocb or waiting on lock
+ *                 2 <=> wait by suspending on the aiocb
+ *
+ * use standard posix aio (librt)
+ *  return 0 if successfull and did not have to wait,
+ *         1 if successfull and had to wait,
+ *    else x'ff'
+ */
+int
+FileCompleteaio( char *aiocbp , int normal_wait
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+            , void *BAiocbIolockaiocbp
+#endif /*  USE_AIO_SIGEVENT    */
+ )
+{
+	int	returnCode;
+	int	aio_errno;
+        struct aiocb *my_aiocbp = (struct aiocb *)aiocbp;
+        const struct aiocb const*cblist[1];
+        int fd;
+        struct timespec my_timeout = { 0 , 10000 };
+        struct timespec *suspend_timeout_P; /*  the timeout actually used depending on normal_wait */
+        int max_polls;
+
+        fd = my_aiocbp->aio_fildes;
+        cblist[0] = my_aiocbp;
+
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+            /*  if we are non-originator,  then we must now wait on the BAiocb waiter-lock,
+             *  even if the aio has completed.
+             *  Reason:
+             *     it is possible that this backend will eventually be the one to free the BAiocb
+             *     At the time that a BAiocb is freed,  there must not be a pending LWLockRelease,
+             *        i.e. no pending queued completion events.
+             *     waiting on the waiter-lock must be done only when *not* holding the buff-header spinlock,
+             *        otherwise there is a risk of deadlock.
+             *   Adding these conditions together ==>> we must wait now to ensure the LWLockRelease has been done
+            */
+            if (normal_wait == 1) { /*   told to wait on lock   i.e. non-originator */
+					/*  wait to acquire shared use of LWlock until aio completes  */
+                    LWLockAcquire( (LWLock *)(((struct BAiocbIolock_chain_item*)BAiocbIolockaiocbp)->BAiocbIolock), LW_SHARED);
+                    LWLockRelease( (LWLock *)(((struct BAiocbIolock_chain_item*)BAiocbIolockaiocbp)->BAiocbIolock));
+			}
+#endif /*  USE_AIO_SIGEVENT    */
+
+        returnCode = aio_errno = aio_error(my_aiocbp);
+        /* note that aio_error returns 0 if op already completed successfully */
+
+        /*  first handle normal case of waiting for op to complete  */
+        if (normal_wait) {
+            /*   if told not to poll,   then specify no timeout  */
+            suspend_timeout_P = ( (normal_wait == 1) ? &my_timeout : (struct timespec *)0 );
+
+            while (aio_errno == EINPROGRESS) {
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                /*   when using the aio_sigevent and normal_wait == 1,
+                 *   it should be impossible for the aio to be still in-progress at this point
+                 *   since we have already waited for originator to post completion via signal and LWlock
+                 */
+                if (normal_wait == 1) { /*   told to wait on lock */
+                    /*  should this be WARNING (since we can continue) or ERROR (since something bad happened? */
+                    elog(WARNING, "FileCompleteaio: aio unexpectedly in-progress after completion signalled for aiocb at %p LWlock_item at %p LWlock at %p\n"
+						 ,aiocbp ,BAiocbIolockaiocbp ,((struct BAiocbIolock_chain_item*)BAiocbIolockaiocbp)->BAiocbIolock);
+				}
+#endif /*  USE_AIO_SIGEVENT    */
+                max_polls = 256;
+                my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                returnCode = aio_suspend(cblist , 1 , suspend_timeout_P);
+                while (    (returnCode < 0) && (max_polls-- > 0)
+                        && ((EAGAIN == errno) || (EINTR == errno))
+                      ) {
+                    my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                    returnCode = aio_suspend(cblist , 1 , suspend_timeout_P);
+                }
+
+                returnCode = aio_errno = aio_error(my_aiocbp);
+                /*  now return_code is from aio_error  */
+                if (returnCode == 0) {
+                    returnCode = 1;    /*  successful but had to wait */
+                }
+            }
+            if (aio_errno) {
+                elog(LOG, "FileCompleteaio: %d %d", fd, returnCode);
+                returnCode = 0xff; /*  unsuccessful */
+            }
+
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+            /*   if this bkend waiter-locked the waiters-lock exclusive,  then release it now */
+            if (    (normal_wait == 2)  /*   told to suspend     */
+                     /*     there are two places where originator might release the BAiocb waiter-lock :
+                      *      .  here
+                      *      .  in ProcessPendingBAiocbIolocks   (always called)
+                      *      so each case must first check whether lock has already been released.
+                      *      Fortunately there is no possibility that both cases execute simultaneously
+                      *      because they are both performed in main-line (non-thread, non-interrupt) code
+                      *      by same process.
+                      */
+                  && (LWLockHeldByMe((struct LWLock *)(((struct BAiocbIolock_chain_item*)BAiocbIolockaiocbp)->BAiocbIolock))) /* still waiter-locked */
+               ) {
+                    LWLockRelease( (LWLock *)(((struct BAiocbIolock_chain_item*)BAiocbIolockaiocbp)->BAiocbIolock));
+			}
+#endif /*  USE_AIO_SIGEVENT    */
+        } else {
+            if (aio_errno == EINPROGRESS) {
+                do {
+                        max_polls = 256;
+                        my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                        returnCode = aio_cancel(fd, my_aiocbp);
+                        while ((returnCode == AIO_NOTCANCELED) && (max_polls-- > 0)) {
+                            my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                            returnCode = aio_cancel(fd, my_aiocbp);
+                        }
+                    returnCode = aio_errno = aio_error(my_aiocbp);
+                } while (aio_errno == EINPROGRESS);
+                returnCode = 0xff; /*  unsuccessful */
+            }
+            if (returnCode != 0)
+                returnCode = 0xff; /*  unsuccessful */
+        }
+
+	DO_DB(elog(LOG, "FileCompleteaio: %d %d",
+			   fd, returnCode));
+
+	return returnCode;
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 int
 FileRead(File file, char *buffer, int amount)
 {
--- src/backend/storage/lmgr/lwlock.c.orig	2014-06-25 16:37:59.461618894 -0400
+++ src/backend/storage/lmgr/lwlock.c	2014-06-25 18:10:51.192521598 -0400
@@ -45,6 +45,11 @@
 #include "utils/hsearch.h"
 #endif
 
+/*
+ * GUC and GUC-derived parameters
+ */
+extern int      max_async_io_prefetchers; /* Maximum number of backends using asynchronous librt threads to read pages into our buffers */
+extern int		target_prefetch_pages;    /* How many buffers PrefetchBuffer callers should try to stay ahead of their ReadBuffer calls by */
 
 /* We use the ShmemLock spinlock to protect LWLockAssign */
 extern slock_t *ShmemLock;
@@ -231,6 +236,11 @@ NumLWLocks(void)
 	/* bufmgr.c needs two for each shared buffer */
 	numLocks += 2 * NBuffers;
 
+#if defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP) && defined(USE_AIO_SIGEVENT)
+    /* bufmgr needs one for each BAiocb */
+	numLocks += max_async_io_prefetchers * target_prefetch_pages;
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP && USE_AIO_SIGEVENT */
+
 	/* proc.c needs one for each backend or auxiliary process */
 	numLocks += MaxBackends + NUM_AUXILIARY_PROCS;
 
--- src/backend/storage/lmgr/proc.c.orig	2014-06-25 16:37:59.461618894 -0400
+++ src/backend/storage/lmgr/proc.c	2014-06-25 18:10:51.204521645 -0400
@@ -52,6 +52,7 @@
 #include "utils/timeout.h"
 #include "utils/timestamp.h"
 
+extern pid_t this_backend_pid;     /*   pid of this backend */
 
 /* GUC variables */
 int			DeadlockTimeout = 1000;
@@ -361,6 +362,7 @@ InitProcess(void)
 	MyPgXact->xid = InvalidTransactionId;
 	MyPgXact->xmin = InvalidTransactionId;
 	MyProc->pid = MyProcPid;
+        this_backend_pid = getpid();    /*    pid of this backend */
 	/* backendId, databaseId and roleId will be filled in later */
 	MyProc->backendId = InvalidBackendId;
 	MyProc->databaseId = InvalidOid;
--- src/backend/access/heap/heapam.c.orig	2014-06-25 16:37:59.361618874 -0400
+++ src/backend/access/heap/heapam.c	2014-06-25 18:10:51.248521818 -0400
@@ -71,6 +71,28 @@
 #include "utils/syscache.h"
 #include "utils/tqual.h"
 
+#ifdef USE_PREFETCH
+#include <sys/types.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+#include "executor/instrument.h"
+
+extern unsigned int prefetch_dbOid; /*  database oid of relations on which prefetching to be done - 0 means all */
+extern unsigned int prefetch_heap_scans; /*  boolean whether to prefetch non-bitmap heap scans         */
+
+/*  special values for scan->rs_prefetch_target indicating as follows :               */
+#define PREFETCH_MAYBE 0xffffffff      /*   prefetch permitted but not yet in effect  */
+#define PREFETCH_DISABLED 0xfffffffe   /*   prefetch disabled and not permitted       */
+/*  PREFETCH_WRAP_POINT indicates a pretcher who has reached the point where the scan would wrap -
+**  at this point the prefetcher runs on the spot until scan catches up.
+**  This *must* be < maximum valid setting of target_prefetch_pages aka effective_io_concurrency.
+*/
+#define PREFETCH_WRAP_POINT 0x0fffffff
+
+#endif   /* USE_PREFETCH */
+
 
 /* GUC variable */
 bool		synchronize_seqscans = true;
@@ -115,6 +137,10 @@ static XLogRecPtr log_heap_new_cid(Relat
 static HeapTuple ExtractReplicaIdentity(Relation rel, HeapTuple tup, bool key_modified,
 					   bool *copy);
 
+#ifdef USE_PREFETCH
+static void heap_unread_add(HeapScanDesc scan, BlockNumber blockno);
+static void heap_unread_subtract(HeapScanDesc scan, BlockNumber blockno);
+#endif   /* USE_PREFETCH */
 
 /*
  * Each tuple lock mode has a corresponding heavyweight lock, and one or two
@@ -292,9 +318,150 @@ initscan(HeapScanDesc scan, ScanKey key,
 	 * Currently, we don't have a stats counter for bitmap heap scans (but the
 	 * underlying bitmap index scans will be counted).
 	 */
-	if (!scan->rs_bitmapscan)
+#ifdef USE_PREFETCH
+        /*    by default,  no prefetching on any scan  */
+        scan->rs_prefetch_target = PREFETCH_DISABLED;  /*  tentatively disable  */
+        scan->rs_pfchblock = 0; /*  scanner will reset this to be ahead of scan */
+        scan->rs_Unread_Pfetched_base = (BlockNumber *)0; /*  list of prefetched but unread blocknos */
+        scan->rs_Unread_Pfetched_next = 0; /*  next unread blockno */
+        scan->rs_Unread_Pfetched_count = 0; /* number of valid unread blocknos */
+#endif   /* USE_PREFETCH */
+	if (!scan->rs_bitmapscan) {
+
 		pgstat_count_heap_scan(scan->rs_rd);
+#ifdef USE_PREFETCH
+                /*    bitmap scans do their own prefetching -
+                **    for others,  set up prefetching now
+                */
+                if (    prefetch_heap_scans
+                     && (target_prefetch_pages > 0)
+                     &&	(!RelationUsesLocalBuffers(scan->rs_rd))
+                   ) {
+                      /*   prefetch_dbOid may be set to a database Oid to specify only prefetch in that db */
+                      if (    (   (prefetch_dbOid > 0)
+                               && (prefetch_dbOid == scan->rs_rd->rd_node.dbNode)
+                              )
+                          ||  (prefetch_dbOid == 0)
+                         ) {
+                          scan->rs_prefetch_target = PREFETCH_MAYBE;    /*  permitted but let the scan decide */
+                      }
+                      else {
+                      }
 }
+#endif   /* USE_PREFETCH */
+        }
+}
+
+#ifdef USE_PREFETCH
+/* add this blockno to list of prefetched and unread blocknos
+** use the one identified by the (next+count|modulo circumference) index if it is unused,
+** else search for the first available slot if there is one,
+** else error.
+*/
+static void
+heap_unread_add(HeapScanDesc scan, BlockNumber blockno)
+{
+      BlockNumber *available_P;   /*  where to store new blockno */
+      unsigned int Unread_Pfetched_index = scan->rs_Unread_Pfetched_next
+                                         + scan->rs_Unread_Pfetched_count; /* index of next unused slot */
+
+      /*  caller is not supposed to pass InvalidBlockNumber but check anyway */
+      if (blockno != InvalidBlockNumber) {
+
+		  /*  ensure there is some room somewhere   */
+		  if (scan->rs_Unread_Pfetched_count < target_prefetch_pages) {
+
+			  /*  try the "next+count" one */
+			  if (Unread_Pfetched_index >= target_prefetch_pages) {
+				  Unread_Pfetched_index -= target_prefetch_pages;  /* modulo circumference */
+			  }
+			  available_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);   /*  where to store new blockno */
+			  if (*available_P == InvalidBlockNumber) { /* unused */
+				  goto store_blockno;
+			  } else {
+                  /*  slow-search the entire list  */
+				  for (Unread_Pfetched_index = 0; Unread_Pfetched_index < target_prefetch_pages; Unread_Pfetched_index++) {
+					  available_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);   /*  where to store new blockno */
+					  if (*available_P == InvalidBlockNumber) { /* unused */
+                          /*  before storing this blockno,
+                          **  since the next pointer did not locate an unused slot,
+                          **  set it to one which is more likely to be so for the next time
+                          */
+                          scan->rs_Unread_Pfetched_next = Unread_Pfetched_index;
+						  goto store_blockno;
+					  }
+				  }
+			  }
+		  }
+
+          /*  if we reach here,    either there was no available slot
+          **  or we thought there was one and didn't find any  --
+          */
+  		  ereport(NOTICE,
+			  (errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("heap_unread_add overflowed list cannot add blockno %d", blockno)));
+
+  		  return;
+
+    store_blockno:
+		  *available_P = blockno;
+		  scan->rs_Unread_Pfetched_count++;  /*  update count */
+
+	  }
+
+    return;
+}
+
+/* remove specified blockno from list of prefetched and unread blocknos.
+** Usually this will be found at the rs_Unread_Pfetched_next item -
+** else search for it.    If not found,   inore it  -  no error results.
+*/
+static void
+heap_unread_subtract(HeapScanDesc scan, BlockNumber blockno)
+{
+      unsigned int Unread_Pfetched_index = scan->rs_Unread_Pfetched_next; /* index of next unread blockno */
+      BlockNumber *candidate_P;   /*  location of callers blockno - maybe */
+      BlockNumber nextUnreadPfetched;
+
+      /*  caller is not supposed to pass InvalidBlockNumber but check anyway */
+      if (    (blockno != InvalidBlockNumber)
+		   && ( scan->rs_Unread_Pfetched_count > 0 )   /*  if the list is not empty  */
+         ) {
+
+			  /*  take modulo of the circumference.
+			  **  actually rs_Unread_Pfetched_next should never exceed the circumference but check anyway.
+			  */
+			  if (Unread_Pfetched_index >= target_prefetch_pages) {
+				  Unread_Pfetched_index  -= target_prefetch_pages;
+}
+			  candidate_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);
+			  nextUnreadPfetched = *candidate_P;
+
+			  if ( nextUnreadPfetched == blockno ) {
+				  goto remove_blockno;
+			  } else {
+                  /*  slow-search the entire list  */
+				  for (Unread_Pfetched_index = 0; Unread_Pfetched_index < target_prefetch_pages; Unread_Pfetched_index++) {
+					  candidate_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);   /*  where to store new blockno */
+					  if (*candidate_P == blockno) { /* unused */
+						  goto remove_blockno;
+					  }
+				  }
+			  }
+
+    remove_blockno:
+			  *candidate_P = InvalidBlockNumber;
+
+			  scan->rs_Unread_Pfetched_next = (Unread_Pfetched_index+1);  /*  update next pfchd unread */
+			  if (scan->rs_Unread_Pfetched_next >= target_prefetch_pages) {
+					  scan->rs_Unread_Pfetched_next = 0;
+			  }
+			  scan->rs_Unread_Pfetched_count--;  /*  update count */
+	  }
+
+      return;
+}
+#endif   /* USE_PREFETCH */
 
 /*
  * heapgetpage - subroutine for heapgettup()
@@ -304,7 +471,7 @@ initscan(HeapScanDesc scan, ScanKey key,
  * which tuples on the page are visible.
  */
 static void
-heapgetpage(HeapScanDesc scan, BlockNumber page)
+heapgetpage(HeapScanDesc scan, BlockNumber page , BlockNumber prefetchHWM)
 {
 	Buffer		buffer;
 	Snapshot	snapshot;
@@ -314,6 +481,10 @@ heapgetpage(HeapScanDesc scan, BlockNumb
 	OffsetNumber lineoff;
 	ItemId		lpp;
 	bool		all_visible;
+#ifdef USE_PREFETCH
+	int             PrefetchBufferRc;  /*   indicates whether requested prefetch block already in a buffer and if pin count on buffer has been increased */
+#endif   /* USE_PREFETCH */
+
 
 	Assert(page < scan->rs_nblocks);
 
@@ -336,6 +507,98 @@ heapgetpage(HeapScanDesc scan, BlockNumb
 									   RBM_NORMAL, scan->rs_strategy);
 	scan->rs_cblock = page;
 
+#ifdef USE_PREFETCH
+
+        heap_unread_subtract(scan, page);
+
+        /*    maybe prefetch some pages  starting with rs_pfchblock */
+        if (scan->rs_prefetch_target >= 0) {       /*   prefetching enabled on this scan ?                         */
+            int next_block_to_be_read = (page+1);  /*   next block to be read = lowest possible prefetchable block */
+            int num_to_pfch_this_time;             /*   eventually holds the number of blocks to prefetch now      */
+            int prefetchable_range;                /*   size of the area ahead of the current prefetch position    */
+
+            /*  check if prefetcher reached wrap point and the scan has now wrapped */
+            if (  (page == 0) && (scan->rs_prefetch_target == PREFETCH_WRAP_POINT)  ) {
+                scan->rs_prefetch_target = 1;
+                scan->rs_pfchblock = next_block_to_be_read;
+            } else
+            if (scan->rs_pfchblock < next_block_to_be_read) {
+                scan->rs_pfchblock = next_block_to_be_read; /* next block to be prefetched must be ahead of one we just read */
+            }
+
+            /* now we know where we would start prefetching -
+            ** next question   -  if this is a sync scan,  ensure we do not prefetch behind the HWM
+            ** debatable whether to require strict inequality or >=  -   >= works better in practice
+            */
+            if ( (!scan->rs_syncscan) || (scan->rs_pfchblock >= prefetchHWM) ) {
+
+                /* now we know where we will start prefetching -
+                ** next question   -  how many?
+                ** apply two limits :
+                **  1.   target prefetch distance
+                **  2.   number of available blocks ahead of us
+                */
+
+                /*  1.   target prefetch distance   */
+                num_to_pfch_this_time = next_block_to_be_read + scan->rs_prefetch_target; /* page beyond prefetch target */
+                num_to_pfch_this_time -= scan->rs_pfchblock;                              /*  convert to offset        */
+
+                /*   first do prefetching up to our current limit  ...
+                **   highest page number that a scan (pre)-fetches is scan->rs_nblocks-1
+                **   note  -  prefetcher does not wrap a prefetch range -
+                **            instead just stop and then start again if and when main scan wraps
+                */
+                if (scan->rs_pfchblock <= scan->rs_startblock) {  /*  if on second leg towards startblock */
+                    prefetchable_range = ((int)(scan->rs_startblock) - (int)(scan->rs_pfchblock));
+                }
+                else {                                            /*     on first leg towards nblocks     */
+                    prefetchable_range = ((int)(scan->rs_nblocks) - (int)(scan->rs_pfchblock));
+                }
+                if (prefetchable_range > 0) {           /*  if theres a range to prefetch */
+
+                    /*  2.   number of available blocks ahead of us        */
+                    if (num_to_pfch_this_time > prefetchable_range) {
+                        num_to_pfch_this_time = prefetchable_range;
+                    }
+                    while (num_to_pfch_this_time-- > 0) {
+                        PrefetchBufferRc = PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, scan->rs_pfchblock, scan->rs_strategy);
+                        /*  if pin acquired on buffer,  then remember in case of future Discard */
+                        if (PrefetchBufferRc & PREFTCHRC_BUF_PIN_INCREASED) {
+                            heap_unread_add(scan, scan->rs_pfchblock);
+						}
+                        scan->rs_pfchblock++;
+                        /*  if syncscan and requested block was already in buffer pool,
+                        **  this suggests that another scanner is ahead of us and we should advance
+                        */
+                        if ( (scan->rs_syncscan) && (PrefetchBufferRc & PREFTCHRC_BLK_ALREADY_PRESENT) ) {
+                            scan->rs_pfchblock++;
+                            num_to_pfch_this_time--;
+                        }
+                    }
+                }
+                else {
+                    /*   we must not modify scan->rs_pfchblock here
+                    **   because it is needed for possible DiscardBuffer at end of scan  ...
+                    **   ... instead ...
+                    */
+                    scan->rs_prefetch_target = PREFETCH_WRAP_POINT;  /*   mark this prefetcher as waiting to wrap */
+                }
+
+                /*   ...  then adjust prefetching limit : by doubling on each iteration */
+                if (scan->rs_prefetch_target == 0) {
+                    scan->rs_prefetch_target = 1;
+                }
+                else {
+                    scan->rs_prefetch_target *= 2;
+                    if (scan->rs_prefetch_target > target_prefetch_pages) {
+                        scan->rs_prefetch_target = target_prefetch_pages;
+                    }
+                }
+            }
+        }
+#endif   /* USE_PREFETCH */
+
+
 	if (!scan->rs_pageatatime)
 		return;
 
@@ -452,6 +715,10 @@ heapgettup(HeapScanDesc scan,
 	OffsetNumber lineoff;
 	int			linesleft;
 	ItemId		lpp;
+    BlockNumber  prefetchHWM = 0;   /*  HWM of prefetch BlockNum for all participants in same sync scan */
+#ifdef USE_PREFETCH
+    int          ix;
+#endif   /* USE_PREFETCH */
 
 	/*
 	 * calculate next starting lineoff, given scan direction
@@ -470,7 +737,25 @@ heapgettup(HeapScanDesc scan,
 				return;
 			}
 			page = scan->rs_startblock; /* first page */
-			heapgetpage(scan, page);
+#ifdef USE_PREFETCH
+                        /*  decide if we shall do prefetching :  only if :
+                        **   .  prefetching enabled for this scan
+                        **   .  not a bitmap scan (which do their own)
+                        **   .  sufficient number of blocks - at least twice the target_prefetch_pages
+                        */
+                        if (    (!scan->rs_bitmapscan)
+                             && (scan->rs_prefetch_target >= PREFETCH_MAYBE)
+                             && (scan->rs_nblocks > (2*target_prefetch_pages))
+                           )  {
+                                scan->rs_prefetch_target = 1;  /*  do prefetching on forward non-bitmap scan */
+                                scan->rs_Unread_Pfetched_base = (BlockNumber *)palloc(sizeof(BlockNumber)*target_prefetch_pages);
+                                /*  Initialise the list */
+                                for (ix = 0; ix < target_prefetch_pages; ix++) {
+									*(scan->rs_Unread_Pfetched_base + ix) = InvalidBlockNumber;
+								}
+                        }
+#endif   /* USE_PREFETCH */
+			heapgetpage(scan, page, prefetchHWM);
 			lineoff = FirstOffsetNumber;		/* first offnum */
 			scan->rs_inited = true;
 		}
@@ -516,7 +801,7 @@ heapgettup(HeapScanDesc scan,
 				page = scan->rs_startblock - 1;
 			else
 				page = scan->rs_nblocks - 1;
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 		}
 		else
 		{
@@ -557,7 +842,7 @@ heapgettup(HeapScanDesc scan,
 
 		page = ItemPointerGetBlockNumber(&(tuple->t_self));
 		if (page != scan->rs_cblock)
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 
 		/* Since the tuple was previously fetched, needn't lock page here */
 		dp = (Page) BufferGetPage(scan->rs_cbuf);
@@ -660,8 +945,12 @@ heapgettup(HeapScanDesc scan,
 			 * a little bit backwards on every invocation, which is confusing.
 			 * We don't guarantee any specific ordering in general, though.
 			 */
-			if (scan->rs_syncscan)
-				ss_report_location(scan->rs_rd, page);
+			if (scan->rs_syncscan) {
+#ifdef USE_PREFETCH
+				prefetchHWM = scan->rs_pfchblock;
+#endif   /* USE_PREFETCH */
+				ss_report_location(scan->rs_rd, page, &prefetchHWM);
+			}
 		}
 
 		/*
@@ -671,6 +960,22 @@ heapgettup(HeapScanDesc scan,
 		{
 			if (BufferIsValid(scan->rs_cbuf))
 				ReleaseBuffer(scan->rs_cbuf);
+#ifdef USE_PREFETCH
+                        if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                            /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                            while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+                        }
+#endif   /* USE_PREFETCH */
 			scan->rs_cbuf = InvalidBuffer;
 			scan->rs_cblock = InvalidBlockNumber;
 			tuple->t_data = NULL;
@@ -678,7 +983,7 @@ heapgettup(HeapScanDesc scan,
 			return;
 		}
 
-		heapgetpage(scan, page);
+		heapgetpage(scan, page, prefetchHWM);
 
 		LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);
 
@@ -727,6 +1032,10 @@ heapgettup_pagemode(HeapScanDesc scan,
 	OffsetNumber lineoff;
 	int			linesleft;
 	ItemId		lpp;
+    BlockNumber  prefetchHWM = 0;   /*  HWM of prefetch BlockNum for all participants in same sync scan */
+#ifdef USE_PREFETCH
+    int          ix;
+#endif   /* USE_PREFETCH */
 
 	/*
 	 * calculate next starting lineindex, given scan direction
@@ -745,7 +1054,25 @@ heapgettup_pagemode(HeapScanDesc scan,
 				return;
 			}
 			page = scan->rs_startblock; /* first page */
-			heapgetpage(scan, page);
+#ifdef USE_PREFETCH
+                        /*  decide if we shall do prefetching :  only if :
+                        **   .  prefetching enabled for this scan
+                        **   .  not a bitmap scan (which do their own)
+                        **   .  sufficient number of blocks - at least twice the target_prefetch_pages
+                        */
+                        if (    (!scan->rs_bitmapscan)
+                             && (scan->rs_prefetch_target >= PREFETCH_MAYBE)
+                             && (scan->rs_nblocks > (2*target_prefetch_pages))
+                           )  {
+                                scan->rs_prefetch_target = 1;  /*  do prefetching on forward non-bitmap scan */
+                                scan->rs_Unread_Pfetched_base = (BlockNumber *)palloc(sizeof(BlockNumber)*target_prefetch_pages);
+                                /*  Initialise the list */
+                                for (ix = 0; ix < target_prefetch_pages; ix++) {
+									*(scan->rs_Unread_Pfetched_base + ix) = InvalidBlockNumber;
+								}
+                        }
+#endif   /* USE_PREFETCH */
+			heapgetpage(scan, page, prefetchHWM);
 			lineindex = 0;
 			scan->rs_inited = true;
 		}
@@ -788,7 +1115,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 				page = scan->rs_startblock - 1;
 			else
 				page = scan->rs_nblocks - 1;
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 		}
 		else
 		{
@@ -826,7 +1153,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 
 		page = ItemPointerGetBlockNumber(&(tuple->t_self));
 		if (page != scan->rs_cblock)
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 
 		/* Since the tuple was previously fetched, needn't lock page here */
 		dp = (Page) BufferGetPage(scan->rs_cbuf);
@@ -921,8 +1248,12 @@ heapgettup_pagemode(HeapScanDesc scan,
 			 * a little bit backwards on every invocation, which is confusing.
 			 * We don't guarantee any specific ordering in general, though.
 			 */
-			if (scan->rs_syncscan)
-				ss_report_location(scan->rs_rd, page);
+			if (scan->rs_syncscan) {
+#ifdef USE_PREFETCH
+				prefetchHWM = scan->rs_pfchblock;
+#endif   /* USE_PREFETCH */
+				ss_report_location(scan->rs_rd, page, &prefetchHWM);
+			}
 		}
 
 		/*
@@ -932,6 +1263,22 @@ heapgettup_pagemode(HeapScanDesc scan,
 		{
 			if (BufferIsValid(scan->rs_cbuf))
 				ReleaseBuffer(scan->rs_cbuf);
+#ifdef USE_PREFETCH
+                        if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                            /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                             while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+                        }
+#endif   /* USE_PREFETCH */
 			scan->rs_cbuf = InvalidBuffer;
 			scan->rs_cblock = InvalidBlockNumber;
 			tuple->t_data = NULL;
@@ -939,7 +1286,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 			return;
 		}
 
-		heapgetpage(scan, page);
+		heapgetpage(scan, page, prefetchHWM);
 
 		dp = (Page) BufferGetPage(scan->rs_cbuf);
 		lines = scan->rs_ntuples;
@@ -1394,6 +1741,23 @@ void
 heap_rescan(HeapScanDesc scan,
 			ScanKey key)
 {
+
+#ifdef USE_PREFETCH
+         if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                       /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                            while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+         }
+#endif   /* USE_PREFETCH */
 	/*
 	 * unpin scan buffers
 	 */
@@ -1418,6 +1782,23 @@ heap_endscan(HeapScanDesc scan)
 {
 	/* Note: no locking manipulations needed */
 
+
+#ifdef USE_PREFETCH
+         if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                       /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                            while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+         }
+#endif   /* USE_PREFETCH */
 	/*
 	 * unpin scan buffers
 	 */
@@ -1435,6 +1816,12 @@ heap_endscan(HeapScanDesc scan)
 	if (scan->rs_strategy != NULL)
 		FreeAccessStrategy(scan->rs_strategy);
 
+#ifdef USE_PREFETCH
+    if (scan->rs_Unread_Pfetched_base) {
+        pfree(scan->rs_Unread_Pfetched_base);
+    }
+#endif   /* USE_PREFETCH */
+
 	if (scan->rs_temp_snap)
 		UnregisterSnapshot(scan->rs_snapshot);
 
@@ -1464,7 +1851,6 @@ heap_endscan(HeapScanDesc scan)
 #define HEAPDEBUG_3
 #endif   /* !defined(HEAPDEBUGALL) */
 
-
 HeapTuple
 heap_getnext(HeapScanDesc scan, ScanDirection direction)
 {
@@ -6349,6 +6735,25 @@ heap_markpos(HeapScanDesc scan)
 void
 heap_restrpos(HeapScanDesc scan)
 {
+
+
+#ifdef USE_PREFETCH
+                        if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                            /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                             while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+                        }
+#endif   /* USE_PREFETCH */
+
 	/* XXX no amrestrpos checking that ammarkpos called */
 
 	if (!ItemPointerIsValid(&scan->rs_mctid))
--- src/backend/access/heap/syncscan.c.orig	2014-06-25 16:37:59.365618875 -0400
+++ src/backend/access/heap/syncscan.c	2014-06-25 18:10:51.264521880 -0400
@@ -90,6 +90,7 @@ typedef struct ss_scan_location_t
 {
 	RelFileNode relfilenode;	/* identity of a relation */
 	BlockNumber location;		/* last-reported location in the relation */
+	BlockNumber prefetchHWM;	/* high-water-mark of prefetched Blocknum */
 } ss_scan_location_t;
 
 typedef struct ss_lru_item_t
@@ -113,7 +114,7 @@ static ss_scan_locations_t *scan_locatio
 
 /* prototypes for internal functions */
 static BlockNumber ss_search(RelFileNode relfilenode,
-		  BlockNumber location, bool set);
+		  BlockNumber location, bool set , BlockNumber *prefetchHWMp);
 
 
 /*
@@ -160,6 +161,7 @@ SyncScanShmemInit(void)
 			item->location.relfilenode.dbNode = InvalidOid;
 			item->location.relfilenode.relNode = InvalidOid;
 			item->location.location = InvalidBlockNumber;
+			item->location.prefetchHWM = InvalidBlockNumber;
 
 			item->prev = (i > 0) ?
 				(&scan_locations->items[i - 1]) : NULL;
@@ -185,7 +187,7 @@ SyncScanShmemInit(void)
  * data structure.
  */
 static BlockNumber
-ss_search(RelFileNode relfilenode, BlockNumber location, bool set)
+ss_search(RelFileNode relfilenode, BlockNumber location, bool set , BlockNumber *prefetchHWMp)
 {
 	ss_lru_item_t *item;
 
@@ -206,6 +208,22 @@ ss_search(RelFileNode relfilenode, Block
 			{
 				item->location.relfilenode = relfilenode;
 				item->location.location = location;
+                                /*  if prefetch information requested,
+                                **  then reconcile and either update or report back the new HWM.
+                                */
+                                if (prefetchHWMp)
+                                {
+                                    if (   (item->location.prefetchHWM == InvalidBlockNumber)
+                                        || (item->location.prefetchHWM < *prefetchHWMp)
+                                       )
+                                    {
+                                      item->location.prefetchHWM = *prefetchHWMp;
+                                    }
+                                    else
+                                    {
+                                      *prefetchHWMp = item->location.prefetchHWM;
+                                    }
+                                }
 			}
 			else if (set)
 				item->location.location = location;
@@ -252,7 +270,7 @@ ss_get_location(Relation rel, BlockNumbe
 	BlockNumber startloc;
 
 	LWLockAcquire(SyncScanLock, LW_EXCLUSIVE);
-	startloc = ss_search(rel->rd_node, 0, false);
+	startloc = ss_search(rel->rd_node, 0, false , 0);
 	LWLockRelease(SyncScanLock);
 
 	/*
@@ -282,7 +300,7 @@ ss_get_location(Relation rel, BlockNumbe
  * same relfilenode.
  */
 void
-ss_report_location(Relation rel, BlockNumber location)
+ss_report_location(Relation rel, BlockNumber location , BlockNumber *prefetchHWMp)
 {
 #ifdef TRACE_SYNCSCAN
 	if (trace_syncscan)
@@ -306,7 +324,7 @@ ss_report_location(Relation rel, BlockNu
 	{
 		if (LWLockConditionalAcquire(SyncScanLock, LW_EXCLUSIVE))
 		{
-			(void) ss_search(rel->rd_node, location, true);
+			(void) ss_search(rel->rd_node, location, true , prefetchHWMp);
 			LWLockRelease(SyncScanLock);
 		}
 #ifdef TRACE_SYNCSCAN
--- src/backend/access/index/indexam.c.orig	2014-06-25 16:37:59.365618875 -0400
+++ src/backend/access/index/indexam.c	2014-06-25 18:10:51.276521927 -0400
@@ -79,6 +79,55 @@
 #include "utils/tqual.h"
 
 
+#ifdef USE_PREFETCH
+bool BlocknotinBuffer(Buffer buffer, Relation relation, BlockNumber blockNum);
+BlockNumber BlocknumOfBuffer(Buffer buffer);
+void index_mark_or_evict_block(IndexScanDesc scan , BlockNumber blocknumber , int markit);
+
+extern unsigned int prefetch_index_scans; /*  boolean whether to prefetch bitmap heap scans         */
+
+/*  if specified block number is present in the prefetch array,
+**  then either mark it as not to be discarded or evict it according to input param
+*/
+void index_mark_or_evict_block(IndexScanDesc scan , BlockNumber blocknumber , int markit)
+{
+        unsigned short int pfchx , pfchy , pfchz; /*  indexes in BlockIdData array   */
+
+        if (    scan->do_prefetch
+             && ((struct pfch_block_item*)0 != scan->pfch_block_item_list)
+             /* no need to check for scan->pfch_next < prefetch_index_scans
+             ** since we will do nothing if scan->pfch_used == 0
+             */
+           ) {
+            /*  search the prefetch list to find if the block is a member */
+            for (pfchx = 0; pfchx < scan->pfch_used; pfchx++) {
+                if (BlockIdGetBlockNumber(&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)) == blocknumber) {
+                      if (markit) {
+						      /*  mark it as not to be discarded */
+						      ((scan->pfch_block_item_list)+pfchx)->pfch_discard &= ~PREFTCHRC_BUF_PIN_INCREASED;
+					  } else {
+							  /*  shuffle all following the evictee to the left
+							  **  and update next pointer if its element moves
+							  */
+							  pfchy = (scan->pfch_used - 1); /*  current rightmost */
+							  scan->pfch_used = pfchy;
+
+							  while (pfchy > pfchx) {
+								  pfchz = pfchx + 1;
+								  BlockIdCopy((&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)), (&(((scan->pfch_block_item_list)+pfchz)->pfch_blockid)));
+								  ((scan->pfch_block_item_list)+pfchx)->pfch_discard = ((scan->pfch_block_item_list)+pfchz)->pfch_discard;
+								  if (scan->pfch_next == pfchz) {
+									  scan->pfch_next = pfchx;
+								  }
+								  pfchx = pfchz; /* advance */
+							  }
+                      }
+                }
+            }
+        }
+}
+#endif /* USE_PREFETCH */
+
 /* ----------------------------------------------------------------
  *					macros used in index_ routines
  *
@@ -253,6 +302,11 @@ index_beginscan(Relation heapRelation,
 	 */
 	scan->heapRelation = heapRelation;
 	scan->xs_snapshot = snapshot;
+#ifdef USE_PREFETCH
+	scan->do_prefetch = 0;             /*  no prefetching by default  */
+        scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+	scan->pfch_block_item_list = (struct pfch_block_item*)0;
+#endif   /* USE_PREFETCH */
 
 	return scan;
 }
@@ -277,6 +331,11 @@ index_beginscan_bitmap(Relation indexRel
 	 * up by RelationGetIndexScan.
 	 */
 	scan->xs_snapshot = snapshot;
+#ifdef USE_PREFETCH
+	scan->do_prefetch = 0;             /*  no prefetching by default  */
+        scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+	scan->pfch_block_item_list = (struct pfch_block_item*)0;
+#endif   /* USE_PREFETCH */
 
 	return scan;
 }
@@ -311,6 +370,9 @@ index_beginscan_internal(Relation indexR
 									  Int32GetDatum(nkeys),
 									  Int32GetDatum(norderbys)));
 
+	scan->heap_tids_seen = 0;
+	scan->heap_tids_fetched = 0;
+	
 	return scan;
 }
 
@@ -342,6 +404,12 @@ index_rescan(IndexScanDesc scan,
 	/* Release any held pin on a heap page */
 	if (BufferIsValid(scan->xs_cbuf))
 	{
+#ifdef USE_PREFETCH
+                /*   if specified block number is present in the prefetch array,  then evict it */
+                if (scan->do_prefetch) {
+                      index_mark_or_evict_block(scan , BlocknumOfBuffer(scan->xs_cbuf) , 0);
+                }
+#endif   /* USE_PREFETCH */
 		ReleaseBuffer(scan->xs_cbuf);
 		scan->xs_cbuf = InvalidBuffer;
 	}
@@ -373,10 +441,30 @@ index_endscan(IndexScanDesc scan)
 	/* Release any held pin on a heap page */
 	if (BufferIsValid(scan->xs_cbuf))
 	{
+#ifdef USE_PREFETCH
+                /*   if specified block number is present in the prefetch array,  then evict it */
+                if (scan->do_prefetch) {
+                      index_mark_or_evict_block(scan , BlocknumOfBuffer(scan->xs_cbuf) , 0);
+                }
+#endif   /* USE_PREFETCH */
 		ReleaseBuffer(scan->xs_cbuf);
 		scan->xs_cbuf = InvalidBuffer;
 	}
 
+#ifdef USE_PREFETCH
+        /*   discard prefetched but unread buffers */
+        if (    scan->do_prefetch
+             && ((struct pfch_block_item*)0 != scan->pfch_block_item_list)
+           ) {
+            unsigned short int pfchx; /*  index in BlockIdData array   */
+                for (pfchx = 0; pfchx < scan->pfch_used; pfchx++) {
+                  if (((scan->pfch_block_item_list)+pfchx)->pfch_discard) {
+                      DiscardBuffer(scan->heapRelation, MAIN_FORKNUM, BlockIdGetBlockNumber(&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)));
+                  }
+                }
+        }
+#endif   /* USE_PREFETCH */
+
 	/* End the AM's scan */
 	FunctionCall1(procedure, PointerGetDatum(scan));
 
@@ -472,6 +560,12 @@ index_getnext_tid(IndexScanDesc scan, Sc
 		/* ... but first, release any held pin on a heap page */
 		if (BufferIsValid(scan->xs_cbuf))
 		{
+#ifdef USE_PREFETCH
+                    /*   if specified block number is present in the prefetch array,  then evict it */
+                    if (scan->do_prefetch) {
+                          index_mark_or_evict_block(scan , BlocknumOfBuffer(scan->xs_cbuf) , 0);
+                    }
+#endif   /* USE_PREFETCH */
 			ReleaseBuffer(scan->xs_cbuf);
 			scan->xs_cbuf = InvalidBuffer;
 		}
@@ -479,6 +573,11 @@ index_getnext_tid(IndexScanDesc scan, Sc
 	}
 
 	pgstat_count_index_tuples(scan->indexRelation, 1);
+	if (scan->heap_tids_seen++ >= (~0)) {
+		/* Avoid integer overflow */
+		scan->heap_tids_seen = 1;
+		scan->heap_tids_fetched = 0;
+	}
 
 	/* Return the TID of the tuple we found. */
 	return &scan->xs_ctup.t_self;
@@ -502,6 +601,10 @@ index_getnext_tid(IndexScanDesc scan, Sc
  * enough information to do it efficiently in the general case.
  * ----------------
  */
+#if defined(USE_PREFETCH) && defined(AVOID_CATALOG_MIGRATION_FOR_ASYNCIO)
+extern Datum btpeeknexttuple(IndexScanDesc scan);
+#endif /* USE_PREFETCH */
+
 HeapTuple
 index_fetch_heap(IndexScanDesc scan)
 {
@@ -509,16 +612,111 @@ index_fetch_heap(IndexScanDesc scan)
 	bool		all_dead = false;
 	bool		got_heap_tuple;
 
+
+
 	/* We can skip the buffer-switching logic if we're in mid-HOT chain. */
 	if (!scan->xs_continue_hot)
 	{
 		/* Switch to correct buffer if we don't have it already */
 		Buffer		prev_buf = scan->xs_cbuf;
 
+#ifdef USE_PREFETCH
+
+                /*   If the old block is different from new block, then evict old
+                **   block from prefetched array.   It is arguable we should leave it
+                **   in the array because it's likely to remain in the buffer pool
+                **   for a while,  but in that case , if we encounter the block
+                **   again,  prefetching it again does no harm.
+                **   (and note that,  if it's not pinned,  prefetching it will try to
+                **   pin it since prefetch tries to bank a pin for a buffer in the buffer pool).
+                **   therefore it should usually win.
+                */
+                if (    scan->do_prefetch
+                     && ( BufferIsValid(prev_buf) )
+                     && (BlocknotinBuffer(prev_buf,scan->heapRelation,ItemPointerGetBlockNumber(tid)))
+                     && (scan->pfch_next < prefetch_index_scans)  /* ensure there is an entry */
+                        ) {
+                          index_mark_or_evict_block(scan , BlocknumOfBuffer(prev_buf) , 0);
+                }
+
+#endif   /* USE_PREFETCH  */
 		scan->xs_cbuf = ReleaseAndReadBuffer(scan->xs_cbuf,
 											 scan->heapRelation,
 											 ItemPointerGetBlockNumber(tid));
 
+#ifdef USE_PREFETCH
+                /*   If the new block had been prefetched and pinned,
+                **   then mark that it no longer requires to be discarded.
+                **   Of course,  we don't evict the entry,
+                **   because we want to remember that it was recently prefetched.
+                */
+                index_mark_or_evict_block(scan , BlocknumOfBuffer(prev_buf) , 1);
+#endif   /* USE_PREFETCH  */
+
+				scan->heap_tids_fetched++;
+
+#ifdef USE_PREFETCH
+                /*  try prefetching next data block
+                **    (next meaning one containing TIDs from matching keys
+                **     in same index page and different from any block
+                **     we previously prefetched and listed in prefetched array)
+                */
+                {
+                    FmgrInfo   *procedure;
+                    bool	found;             /*  did we find the "next" heap tid in current index page */
+                    int         PrefetchBufferRc;  /*  indicates whether requested prefetch block already in a buffer and if pin count on buffer has been increased */
+
+                    if (scan->do_prefetch) {
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+                        procedure = &scan->indexRelation->rd_aminfo->ampeeknexttuple; /* is incorrect but avoids adding function to catalog */
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                        if (RegProcedureIsValid(scan->indexRelation->rd_am->ampeeknexttuple)) {
+                            GET_SCAN_PROCEDURE(ampeeknexttuple); /* is correct but requires adding function to catalog */
+                        } else {
+                            procedure = 0;
+                        }
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+
+                        if (    procedure          /* does the index access method support peektuple? */
+#ifndef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+                             && procedure->fn_addr /* procedure->fn_addr is non-null only if in catalog */
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                           ) {
+                            int iterations = 1;      /*  how many iterations of prefetching shall we try  -
+                                                     **  if used entries in prefetch list is < target_prefetch_pages
+                                                     **  then 2,  else 1
+                                                     **  this should result in gradually and smoothly increasing up to target_prefetch_pages
+                                                     */
+                            /*  note we trust InitIndexScan verified this scan is forwards only and so set that */
+                            if (scan->pfch_used < target_prefetch_pages) {
+                                iterations = 2;
+                            }
+                            do {
+                                found =  DatumGetBool(
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+                                                         btpeeknexttuple(scan)     /*  pass scan as direct parameter since cant use fmgr because not in catalog */
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                                                         FunctionCall1(procedure, PointerGetDatum(scan)) /* use fmgr to call it because in catalog  */
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                                                     );
+                                if (found) {
+                                    /*    btpeeknexttuple set pfch_next to point to the item in block_item_list to be prefetched */
+                                    PrefetchBufferRc = PrefetchBuffer(scan->heapRelation, MAIN_FORKNUM, BlockIdGetBlockNumber((&((scan->pfch_block_item_list + scan->pfch_next))->pfch_blockid)) , 0);
+                                    /* elog(LOG,"index_fetch_heap prefetched rel %u blockNum %u"
+                                       ,scan->heapRelation->rd_node.relNode ,BlockIdGetBlockNumber(scan->pfch_block_item_list + scan->pfch_next));
+                                    */
+
+                                    /*  if pin acquired on buffer,  then remember in case of future Discard */
+                                    (scan->pfch_block_item_list + scan->pfch_next)->pfch_discard = (PrefetchBufferRc & PREFTCHRC_BUF_PIN_INCREASED);
+
+
+                                }
+                            } while (--iterations > 0);
+                        }
+                    }
+                }
+#endif   /* USE_PREFETCH */
+
 		/*
 		 * Prune page, but only if we weren't already on this page
 		 */
--- src/backend/access/index/genam.c.orig	2014-06-25 16:37:59.365618875 -0400
+++ src/backend/access/index/genam.c	2014-06-25 18:10:51.296522007 -0400
@@ -77,6 +77,12 @@ RelationGetIndexScan(Relation indexRelat
 
 	scan = (IndexScanDesc) palloc(sizeof(IndexScanDescData));
 
+#ifdef USE_PREFETCH
+        scan->do_prefetch = 0;             /*  no prefetching by default  */
+        scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+        scan->pfch_block_item_list = (struct pfch_block_item*)0;
+#endif   /* USE_PREFETCH */
+
 	scan->heapRelation = NULL;	/* may be set later */
 	scan->indexRelation = indexRelation;
 	scan->xs_snapshot = InvalidSnapshot;		/* caller must initialize this */
@@ -139,6 +145,19 @@ RelationGetIndexScan(Relation indexRelat
 void
 IndexScanEnd(IndexScanDesc scan)
 {
+#ifdef USE_PREFETCH
+	if (scan->do_prefetch) {
+		if ( (struct pfch_block_item*)0 != scan->pfch_block_item_list ) {
+			pfree(scan->pfch_block_item_list);
+			scan->pfch_block_item_list = (struct pfch_block_item*)0;
+		}
+		if ( (struct pfch_index_pagelist*)0 != scan->pfch_index_page_list ) {
+			pfree(scan->pfch_index_page_list);
+			scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+		}
+	}
+#endif   /* USE_PREFETCH */
+
 	if (scan->keyData != NULL)
 		pfree(scan->keyData);
 	if (scan->orderByData != NULL)
--- src/backend/access/nbtree/nbtsearch.c.orig	2014-06-25 16:37:59.365618875 -0400
+++ src/backend/access/nbtree/nbtsearch.c	2014-06-25 18:10:51.328522131 -0400
@@ -23,13 +23,18 @@
 #include "utils/lsyscache.h"
 #include "utils/rel.h"
 
+#ifdef USE_PREFETCH
+extern unsigned int prefetch_btree_heaps;  /*  boolean whether to prefetch heap pages in _bt_next for non-bitmap index scans   */
+extern unsigned int prefetch_sequential_index_scans;  /*  boolean whether to prefetch sequential-access non-bitmap index scans  */
+#endif   /* USE_PREFETCH */
 
 static bool _bt_readpage(IndexScanDesc scan, ScanDirection dir,
 			 OffsetNumber offnum);
 static void _bt_saveitem(BTScanOpaque so, int itemIndex,
 			 OffsetNumber offnum, IndexTuple itup);
-static bool _bt_steppage(IndexScanDesc scan, ScanDirection dir);
-static Buffer _bt_walk_left(Relation rel, Buffer buf);
+static bool _bt_steppage(IndexScanDesc scan, ScanDirection dir, 
+			 bool prefetch);
+static Buffer _bt_walk_left(IndexScanDesc scan, Relation rel, Buffer buf);
 static bool _bt_endpoint(IndexScanDesc scan, ScanDirection dir);
 
 
@@ -226,7 +231,11 @@ _bt_moveright(Relation rel,
 				_bt_relbuf(rel, buf);
 
 			/* re-acquire the lock in the right mode, and re-check */
-			buf = _bt_getbuf(rel, blkno, access);
+			buf = _bt_getbuf(rel, blkno, access
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                                        );
 			continue;
 		}
 
@@ -1005,7 +1014,7 @@ _bt_first(IndexScanDesc scan, ScanDirect
 		 * There's no actually-matching data on this page.  Try to advance to
 		 * the next page.  Return false if there's no matching data at all.
 		 */
-		if (!_bt_steppage(scan, dir))
+		if (!_bt_steppage(scan, dir, false))
 			return false;
 	}
 
@@ -1040,6 +1049,8 @@ _bt_next(IndexScanDesc scan, ScanDirecti
 {
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
 	BTScanPosItem *currItem;
+	BlockNumber prevblkno = ItemPointerGetBlockNumber(
+		&scan->xs_ctup.t_self);
 
 	/*
 	 * Advance to next tuple on current page; or if there's no more, try to
@@ -1052,11 +1063,56 @@ _bt_next(IndexScanDesc scan, ScanDirecti
 			/* We must acquire lock before applying _bt_steppage */
 			Assert(BufferIsValid(so->currPos.buf));
 			LockBuffer(so->currPos.buf, BT_READ);
-			if (!_bt_steppage(scan, dir))
+			if (!_bt_steppage(scan, dir, target_prefetch_pages > 0))
 				return false;
 			/* Drop the lock, but not pin, on the new page */
 			LockBuffer(so->currPos.buf, BUFFER_LOCK_UNLOCK);
 		}
+		
+#ifdef USE_PREFETCH
+                /*    consider prefetching */
+		if (prefetch_btree_heaps && (target_prefetch_pages > 0)) {
+			BlockNumber currblkno = ItemPointerGetBlockNumber(
+				&so->currPos.items[so->currPos.itemIndex].heapTid);
+		
+			if (currblkno != prevblkno) {
+				if (so->prefetchBlockCount > 0)
+					so->prefetchBlockCount--;
+			
+				/* If we have heap fetch frequency stats, and it's above ~94%,
+				 * initiate heap prefetches */
+				if (so->currPos.moreRight
+					&& scan->heap_tids_seen > 256 
+					&& ( (scan->heap_tids_seen - scan->heap_tids_seen/16)
+						 <= scan->heap_tids_fetched ) )
+				{
+					bool nonsequential = false;
+				
+					if (so->prefetchItemIndex <= so->currPos.itemIndex)
+						so->prefetchItemIndex = so->currPos.itemIndex + 1;
+					while (    (so->prefetchItemIndex <= so->currPos.lastItem)
+							&& (so->prefetchBlockCount < target_prefetch_pages) )
+					{
+						ItemPointer tid = &so->currPos.items[so->prefetchItemIndex++].heapTid;
+						BlockNumber blkno = ItemPointerGetBlockNumber(tid);
+						if (blkno != so->lastHeapPrefetchBlkno) {  /*  if not a repetition of previous block */
+							/*   start prefetch on next page, providing :
+                            **   EITHER  .  we're reading non-sequentially previously or for this block
+                            **   OR      .  user explicitly specified to prefetch for sequential pattern
+							**   as it may be counterproductive otherwise
+                            */
+							nonsequential = (nonsequential || blkno != (so->lastHeapPrefetchBlkno+1));
+							if (prefetch_sequential_index_scans || nonsequential) {
+								_bt_prefetchbuf(scan->heapRelation, blkno , &scan->pfch_index_page_list );
+							}
+							so->lastHeapPrefetchBlkno = blkno;
+							so->prefetchBlockCount++;
+						}
+					}
+				}
+			}
+		}
+#endif   /* USE_PREFETCH */
 	}
 	else
 	{
@@ -1065,11 +1121,56 @@ _bt_next(IndexScanDesc scan, ScanDirecti
 			/* We must acquire lock before applying _bt_steppage */
 			Assert(BufferIsValid(so->currPos.buf));
 			LockBuffer(so->currPos.buf, BT_READ);
-			if (!_bt_steppage(scan, dir))
+			if (!_bt_steppage(scan, dir, target_prefetch_pages > 0))
 				return false;
 			/* Drop the lock, but not pin, on the new page */
 			LockBuffer(so->currPos.buf, BUFFER_LOCK_UNLOCK);
 		}
+		
+#ifdef USE_PREFETCH
+                /*    consider prefetching */
+		if (prefetch_btree_heaps && (target_prefetch_pages > 0)) {
+			BlockNumber currblkno = ItemPointerGetBlockNumber(
+				&so->currPos.items[so->currPos.itemIndex].heapTid);
+		
+			if (currblkno != prevblkno) {
+				if (so->prefetchBlockCount > 0)
+					so->prefetchBlockCount--;
+			
+				/* If we have heap fetch frequency stats, and it's above ~94%,
+				 * initiate heap prefetches */
+				if (so->currPos.moreLeft
+					&& scan->heap_tids_seen > 256 
+					&& ( (scan->heap_tids_seen - scan->heap_tids_seen/16)
+						 <= scan->heap_tids_fetched ) )
+				{
+					bool nonsequential = false;
+			
+					if (so->prefetchItemIndex >= so->currPos.itemIndex)
+						so->prefetchItemIndex = so->currPos.itemIndex - 1;
+					while (    (so->prefetchItemIndex >= so->currPos.firstItem)
+							&& (so->prefetchBlockCount < target_prefetch_pages) )
+					{
+						ItemPointer tid = &so->currPos.items[so->prefetchItemIndex--].heapTid;
+						BlockNumber blkno = ItemPointerGetBlockNumber(tid);
+						if (blkno != so->lastHeapPrefetchBlkno) {  /*  if not a repetition of previous block */
+							/*   start prefetch on next page, providing :
+                            **   EITHER  .  we're reading non-sequentially previously or for this block
+                            **   OR      .  user explicitly specified to prefetch for sequential pattern
+							**   as it may be counterproductive otherwise
+                            */
+							nonsequential = (nonsequential || blkno != (so->lastHeapPrefetchBlkno+1));
+							if (prefetch_sequential_index_scans || nonsequential) {
+								_bt_prefetchbuf(scan->heapRelation, blkno , &scan->pfch_index_page_list );
+							}
+							so->lastHeapPrefetchBlkno = blkno;
+							so->prefetchBlockCount++;
+						}
+					}
+				}
+			}
+		}
+#endif   /* USE_PREFETCH */
 	}
 
 	/* OK, itemIndex says what to return */
@@ -1119,9 +1220,11 @@ _bt_readpage(IndexScanDesc scan, ScanDir
 	/*
 	 * we must save the page's right-link while scanning it; this tells us
 	 * where to step right to after we're done with these items.  There is no
-	 * corresponding need for the left-link, since splits always go right.
+	 * corresponding need for the left-link, since splits always go right,
+	 * but we need it for back-sequential scan detection.
 	 */
 	so->currPos.nextPage = opaque->btpo_next;
+	so->currPos.prevPage = opaque->btpo_prev;
 
 	/* initialize tuple workspace to empty */
 	so->currPos.nextTupleOffset = 0;
@@ -1156,6 +1259,7 @@ _bt_readpage(IndexScanDesc scan, ScanDir
 		so->currPos.firstItem = 0;
 		so->currPos.lastItem = itemIndex - 1;
 		so->currPos.itemIndex = 0;
+		so->prefetchItemIndex = 0;
 	}
 	else
 	{
@@ -1187,6 +1291,7 @@ _bt_readpage(IndexScanDesc scan, ScanDir
 		so->currPos.firstItem = itemIndex;
 		so->currPos.lastItem = MaxIndexTuplesPerPage - 1;
 		so->currPos.itemIndex = MaxIndexTuplesPerPage - 1;
+		so->prefetchItemIndex = MaxIndexTuplesPerPage - 1;
 	}
 
 	return (so->currPos.firstItem <= so->currPos.lastItem);
@@ -1224,7 +1329,7 @@ _bt_saveitem(BTScanOpaque so, int itemIn
  * locks and pins, set so->currPos.buf to InvalidBuffer, and return FALSE.
  */
 static bool
-_bt_steppage(IndexScanDesc scan, ScanDirection dir)
+_bt_steppage(IndexScanDesc scan, ScanDirection dir, bool prefetch)
 {
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
 	Relation	rel;
@@ -1278,7 +1383,11 @@ _bt_steppage(IndexScanDesc scan, ScanDir
 			/* check for interrupts while we're not holding any buffer lock */
 			CHECK_FOR_INTERRUPTS();
 			/* step right one page */
-			so->currPos.buf = _bt_getbuf(rel, blkno, BT_READ);
+			so->currPos.buf = _bt_getbuf(rel, blkno, BT_READ
+#ifdef USE_PREFETCH
+                                     ,scan->pfch_index_page_list
+#endif /* USE_PREFETCH */
+                                                    );
 			/* check for deleted page */
 			page = BufferGetPage(so->currPos.buf);
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
@@ -1287,9 +1396,22 @@ _bt_steppage(IndexScanDesc scan, ScanDir
 				PredicateLockPage(rel, blkno, scan->xs_snapshot);
 				/* see if there are any matches on this page */
 				/* note that this will clear moreRight if we can stop */
-				if (_bt_readpage(scan, dir, P_FIRSTDATAKEY(opaque)))
+				if (_bt_readpage(scan, dir, P_FIRSTDATAKEY(opaque))) {
+#ifdef USE_PREFETCH
+					if (    prefetch && so->currPos.moreRight
+						/*   start prefetch on next page, providing :
+						**   EITHER  .  we're reading non-sequentially for this block
+						**   OR      .  user explicitly specified to prefetch for sequential pattern
+						**   as it may be counterproductive otherwise
+						*/
+						&& (prefetch_sequential_index_scans || opaque->btpo_next != (blkno+1))
+                                            ) {
+ 						  _bt_prefetchbuf(rel, opaque->btpo_next , &scan->pfch_index_page_list);
+					}
+#endif /* USE_PREFETCH */
 					break;
 			}
+			}
 			/* nope, keep going */
 			blkno = opaque->btpo_next;
 		}
@@ -1317,7 +1439,7 @@ _bt_steppage(IndexScanDesc scan, ScanDir
 			}
 
 			/* Step to next physical page */
-			so->currPos.buf = _bt_walk_left(rel, so->currPos.buf);
+			so->currPos.buf = _bt_walk_left(scan , rel, so->currPos.buf);
 
 			/* if we're physically at end of index, return failure */
 			if (so->currPos.buf == InvalidBuffer)
@@ -1332,14 +1454,60 @@ _bt_steppage(IndexScanDesc scan, ScanDir
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 			if (!P_IGNORE(opaque))
 			{
+				/* We must rely on the previously saved prevPage link! */
+				BlockNumber blkno = so->currPos.prevPage;
+				
 				PredicateLockPage(rel, BufferGetBlockNumber(so->currPos.buf), scan->xs_snapshot);
 				/* see if there are any matches on this page */
 				/* note that this will clear moreLeft if we can stop */
-				if (_bt_readpage(scan, dir, PageGetMaxOffsetNumber(page)))
+				if (_bt_readpage(scan, dir, PageGetMaxOffsetNumber(page))) {
+#ifdef USE_PREFETCH
+					if (prefetch && so->currPos.moreLeft) {
+						/* detect back-sequential runs and increase prefetch window blindly 
+						 * downwards 2 blocks at a time. This only works in our favor
+						 * for index-only scans, by merging read requests at the kernel,
+						 * so we want to inflate target_prefetch_pages since merged 
+						 * back-sequential requests are about as expensive as a single one 
+						 */
+						if (scan->xs_want_itup && blkno > 0 && opaque->btpo_prev == (blkno-1)) {
+							BlockNumber backPos;
+							unsigned int back_prefetch_pages = target_prefetch_pages * 16;
+							if (back_prefetch_pages > 64)
+								back_prefetch_pages = 64;
+							
+							if (so->backSeqRun == 0)
+								backPos = (blkno-1);
+							else
+								backPos = so->backSeqPos;
+							so->backSeqRun++;
+							
+							if (backPos > 0 && (blkno - backPos) <= back_prefetch_pages) {
+								_bt_prefetchbuf(rel, backPos-- , &scan->pfch_index_page_list);
+								/* don't start back-seq prefetch too early */
+								if (so->backSeqRun >= back_prefetch_pages
+										&& backPos > 0 
+										&& (blkno - backPos) <= back_prefetch_pages)
+								{
+									_bt_prefetchbuf(rel, backPos-- , &scan->pfch_index_page_list);
+								}
+							}
+							
+							so->backSeqPos = backPos;
+						} else {
+							/* start prefetch on next page */
+							if (so->backSeqRun != 0) {
+								if (opaque->btpo_prev > blkno || opaque->btpo_prev < so->backSeqPos)
+									so->backSeqRun = 0;
+							}
+							_bt_prefetchbuf(rel, opaque->btpo_prev , &scan->pfch_index_page_list);
+						}
+					}
+#endif /* USE_PREFETCH */
 					break;
 			}
 		}
 	}
+	}
 
 	return true;
 }
@@ -1359,7 +1527,7 @@ _bt_steppage(IndexScanDesc scan, ScanDir
  * again if it's important.
  */
 static Buffer
-_bt_walk_left(Relation rel, Buffer buf)
+_bt_walk_left(IndexScanDesc scan, Relation rel, Buffer buf)
 {
 	Page		page;
 	BTPageOpaque opaque;
@@ -1387,7 +1555,11 @@ _bt_walk_left(Relation rel, Buffer buf)
 		_bt_relbuf(rel, buf);
 		/* check for interrupts while we're not holding any buffer lock */
 		CHECK_FOR_INTERRUPTS();
-		buf = _bt_getbuf(rel, blkno, BT_READ);
+		buf = _bt_getbuf(rel, blkno, BT_READ
+#ifdef USE_PREFETCH
+                                     , scan->pfch_index_page_list
+#endif /* USE_PREFETCH */
+                                );
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
@@ -1631,7 +1803,7 @@ _bt_endpoint(IndexScanDesc scan, ScanDir
 		 * There's no actually-matching data on this page.  Try to advance to
 		 * the next page.  Return false if there's no matching data at all.
 		 */
-		if (!_bt_steppage(scan, dir))
+		if (!_bt_steppage(scan, dir, false))
 			return false;
 	}
 
--- src/backend/access/nbtree/nbtinsert.c.orig	2014-06-25 16:37:59.365618875 -0400
+++ src/backend/access/nbtree/nbtinsert.c	2014-06-25 18:10:51.364522273 -0400
@@ -793,7 +793,11 @@ _bt_insertonpg(Relation rel,
 		{
 			Assert(!P_ISLEAF(lpageop));
 
-			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                                );
 			metapg = BufferGetPage(metabuf);
 			metad = BTPageGetMeta(metapg);
 
@@ -972,7 +976,11 @@ _bt_split(Relation rel, Buffer buf, Buff
 	bool		isleaf;
 
 	/* Acquire a new page to split into */
-	rbuf = _bt_getbuf(rel, P_NEW, BT_WRITE);
+	rbuf = _bt_getbuf(rel, P_NEW, BT_WRITE
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                                );
 
 	/*
 	 * origpage is the original page to be split.  leftpage is a temporary
@@ -1175,7 +1183,11 @@ _bt_split(Relation rel, Buffer buf, Buff
 
 	if (!P_RIGHTMOST(oopaque))
 	{
-		sbuf = _bt_getbuf(rel, oopaque->btpo_next, BT_WRITE);
+		sbuf = _bt_getbuf(rel, oopaque->btpo_next, BT_WRITE
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                                );
 		spage = BufferGetPage(sbuf);
 		sopaque = (BTPageOpaque) PageGetSpecialPointer(spage);
 		if (sopaque->btpo_prev != origpagenumber)
@@ -1817,7 +1829,11 @@ _bt_finish_split(Relation rel, Buffer lb
 	Assert(P_INCOMPLETE_SPLIT(lpageop));
 
 	/* Lock right sibling, the one missing the downlink */
-	rbuf = _bt_getbuf(rel, lpageop->btpo_next, BT_WRITE);
+	rbuf = _bt_getbuf(rel, lpageop->btpo_next, BT_WRITE
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                     );
 	rpage = BufferGetPage(rbuf);
 	rpageop = (BTPageOpaque) PageGetSpecialPointer(rpage);
 
@@ -1829,7 +1845,11 @@ _bt_finish_split(Relation rel, Buffer lb
 		BTMetaPageData *metad;
 
 		/* acquire lock on the metapage */
-		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                            );
 		metapg = BufferGetPage(metabuf);
 		metad = BTPageGetMeta(metapg);
 
@@ -1877,7 +1897,11 @@ _bt_getstackbuf(Relation rel, BTStack st
 		Page		page;
 		BTPageOpaque opaque;
 
-		buf = _bt_getbuf(rel, blkno, access);
+		buf = _bt_getbuf(rel, blkno, access
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                        );
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
@@ -2008,12 +2032,20 @@ _bt_newroot(Relation rel, Buffer lbuf, B
 	lopaque = (BTPageOpaque) PageGetSpecialPointer(lpage);
 
 	/* get a new root page */
-	rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE);
+	rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                        );
 	rootpage = BufferGetPage(rootbuf);
 	rootblknum = BufferGetBlockNumber(rootbuf);
 
 	/* acquire lock on the metapage */
-	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                        );
 	metapg = BufferGetPage(metabuf);
 	metad = BTPageGetMeta(metapg);
 
--- src/backend/access/nbtree/nbtpage.c.orig	2014-06-25 16:37:59.365618875 -0400
+++ src/backend/access/nbtree/nbtpage.c	2014-06-25 18:10:51.376522320 -0400
@@ -127,7 +127,11 @@ _bt_getroot(Relation rel, int access)
 		Assert(rootblkno != P_NONE);
 		rootlevel = metad->btm_fastlevel;
 
-		rootbuf = _bt_getbuf(rel, rootblkno, BT_READ);
+		rootbuf = _bt_getbuf(rel, rootblkno, BT_READ
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                            );
 		rootpage = BufferGetPage(rootbuf);
 		rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);
 
@@ -153,7 +157,11 @@ _bt_getroot(Relation rel, int access)
 		rel->rd_amcache = NULL;
 	}
 
-	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
+	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                        );
 	metapg = BufferGetPage(metabuf);
 	metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
 	metad = BTPageGetMeta(metapg);
@@ -209,7 +217,11 @@ _bt_getroot(Relation rel, int access)
 		 * the new root page.  Since this is the first page in the tree, it's
 		 * a leaf as well as the root.
 		 */
-		rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE);
+		rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                            );
 		rootblkno = BufferGetBlockNumber(rootbuf);
 		rootpage = BufferGetPage(rootbuf);
 		rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);
@@ -350,7 +362,11 @@ _bt_gettrueroot(Relation rel)
 		pfree(rel->rd_amcache);
 	rel->rd_amcache = NULL;
 
-	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
+	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                        );
 	metapg = BufferGetPage(metabuf);
 	metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
 	metad = BTPageGetMeta(metapg);
@@ -436,7 +452,11 @@ _bt_getrootheight(Relation rel)
 		Page		metapg;
 		BTPageOpaque metaopaque;
 
-		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
+		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                             );
 		metapg = BufferGetPage(metabuf);
 		metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
 		metad = BTPageGetMeta(metapg);
@@ -561,6 +581,172 @@ _bt_log_reuse_page(Relation rel, BlockNu
 	END_CRIT_SECTION();
 }
 
+#ifdef USE_PREFETCH
+/*
+ *	_bt_prefetchbuf() -- Prefetch a buffer by block number
+ *                           and keep track of prefetched and unread blocknums in pagelist.
+ *   input parms  :
+ *       rel and blockno identify block to be prefetched as usual
+ *       pfch_index_page_list_P points to the pointer anchoring the head of the index page list
+ *             Since the pagelist is a kind of optimization,
+ *             handle palloc failure by quietly omitting the keeping track.
+ */
+void
+_bt_prefetchbuf(Relation rel, BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P)
+{
+
+    int rc = 0;
+    struct pfch_index_pagelist* pfch_index_plp;      /*  pointer to current chunk */
+    struct pfch_index_item* found_item = 0;
+
+		if ((struct pfch_index_pagelist**)0 == pfch_index_page_list_P) {
+			pfch_index_plp = (struct pfch_index_pagelist*)0;
+		} else {
+			pfch_index_plp = *pfch_index_page_list_P;
+		}
+
+    	if (blkno != P_NEW && blkno != P_NONE)
+    	{
+            /* prefetch an existing block of the relation
+            ** but first,  check it has not recently already been prefetched and not yet read
+            */
+            found_item = _bt_find_block(blkno , pfch_index_plp);
+			if ((struct pfch_index_item*)0 == found_item) {  /*  not found */
+
+		        rc = PrefetchBuffer(rel, MAIN_FORKNUM, blkno , 0);
+
+                /*  add the pagenum to the list ,  indicating its discard status
+                **  since it's only an optimization,  ignore failure such as exceeded allowed space
+				*/
+                _bt_add_block( blkno , pfch_index_page_list_P , (uint32)(rc & PREFTCHRC_BUF_PIN_INCREASED));
+
+            }
+	    }
+        return;
+}
+
+/*   _bt_find_block finds the item referencing specified Block in index page list if present
+**   and returns the pointer to the pfch_index_item if found,  or null if not
+*/
+struct pfch_index_item*
+_bt_find_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list)
+{
+
+    struct pfch_index_item* found_item = 0;
+    struct pfch_index_pagelist* pfch_index_plp;      /*  pointer to current chunk */
+    int ix, tx;
+
+		pfch_index_plp = pfch_index_page_list;
+
+		while (     (struct pfch_index_pagelist*)0 != pfch_index_plp
+                &&  ( (struct pfch_index_item*)0 == found_item)
+              ) {
+			ix = 0;
+			tx = pfch_index_plp->pfch_index_item_count;
+			while (     (ix < tx)
+                    &&  ( (struct pfch_index_item*)0 == found_item)
+                  ) {
+				if (pfch_index_plp->pfch_indexid[ix].pfch_blocknum == blkno) {
+					found_item = &pfch_index_plp->pfch_indexid[ix];
+				}
+                ix++;
+			}
+			pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+		}
+
+     return found_item;
+}
+
+/*   _bt_add_block adds the specified Block to the index page list
+**   and returns 0 if successful,  non-zero if not
+*/
+int
+_bt_add_block(BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P , uint32 discard_status)
+{
+    int rc = 1;
+    int ix;
+    struct pfch_index_pagelist* pfch_index_plp;      /*  pointer to current chunk */
+    struct pfch_index_pagelist* pfch_index_page_list_anchor; /*  pointer to first chunk if any  */
+	/*  allow expansion of pagelist to 16 chunks
+	**  which accommodates backwards-sequential index scans
+	**  where the scanner increases target_prefetch_pages by a factor of up to 16
+	**   see code in _bt_steppage
+	**  note - this creates an undesirable weak dependency on this number in _bt_steppage,
+	**         but :
+	**           there is no disaster if the numbers disagree  -  just sub-optimal use of the list
+	**           to implement a proper interface would require that chunks have a variable size
+	**           which would require an extra size variable in each chunk
+	*/
+	int num_chunks = 16;
+
+		if ((struct pfch_index_pagelist**)0 == pfch_index_page_list_P) {
+			pfch_index_page_list_anchor = (struct pfch_index_pagelist*)0;
+		} else {
+			pfch_index_page_list_anchor = *pfch_index_page_list_P;
+		}
+		pfch_index_plp = pfch_index_page_list_anchor;       /* pointer to current chunk */
+
+		while ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+			ix = pfch_index_plp->pfch_index_item_count;
+			if (ix < target_prefetch_pages) {
+				pfch_index_plp->pfch_indexid[ix].pfch_blocknum = blkno;
+				pfch_index_plp->pfch_indexid[ix].pfch_discard = discard_status;
+				pfch_index_plp->pfch_index_item_count = (ix+1);
+                rc = 0;
+				goto stored_pagenum;
+			}
+			pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+			num_chunks--;              /*  keep track of number of chunks */
+		}
+
+		/*   we did not find any free space in existing chunks -
+		**   create new chunk if within our limit and we have a pfch_index_page_list
+		*/
+		if ( (num_chunks > 0) && ((struct pfch_index_pagelist*)0 != pfch_index_page_list_anchor) ) {
+			pfch_index_plp = (struct pfch_index_pagelist*)palloc( sizeof(struct pfch_index_pagelist) + ( (target_prefetch_pages-1) * sizeof(struct pfch_index_item) ) );
+			if ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+				pfch_index_plp->pfch_index_pagelist_next = pfch_index_page_list_anchor;  /* old head of list is next after this */
+				pfch_index_plp->pfch_indexid[0].pfch_blocknum = blkno;
+				pfch_index_plp->pfch_indexid[0].pfch_discard = discard_status;
+				pfch_index_plp->pfch_index_item_count = 1;
+				pfch_index_page_list_P = &pfch_index_plp;   /*  new head of list is new chunk */
+                rc = 0;
+			}
+		}
+
+    stored_pagenum:;
+     return rc;
+}
+
+/*  _bt_subtract_block removes a block from the prefetched-but-unread pagelist if present */
+void
+_bt_subtract_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list)
+{
+    struct pfch_index_pagelist* pfch_index_plp = pfch_index_page_list;
+	if ( (blkno != P_NEW) && (blkno != P_NONE) ) {
+            int ix , jx;
+                while ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+                    ix = pfch_index_plp->pfch_index_item_count;
+                    while (ix-- > 0) {
+                        if (pfch_index_plp->pfch_indexid[ix].pfch_blocknum == blkno) {
+                            /*   move the last item to the curent (now deleted) position and decrement count */
+                            jx = (pfch_index_plp->pfch_index_item_count-1); /*  index of last item ... */
+                            if (jx > ix) {                                  /*  ... is not the current one so move is required */
+                                pfch_index_plp->pfch_indexid[ix].pfch_blocknum = pfch_index_plp->pfch_indexid[jx].pfch_blocknum;
+                                pfch_index_plp->pfch_indexid[ix].pfch_discard = pfch_index_plp->pfch_indexid[jx].pfch_discard;
+                                ix = jx;
+                            }
+                            pfch_index_plp->pfch_index_item_count = ix;
+                            goto done_subtract;
+                        }
+                    }
+                    pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+                }
+        }
+    done_subtract:  return;
+}
+#endif /* USE_PREFETCH */
+
 /*
  *	_bt_getbuf() -- Get a buffer by block number for read or write.
  *
@@ -573,7 +759,11 @@ _bt_log_reuse_page(Relation rel, BlockNu
  *		_bt_checkpage to sanity-check the page (except in P_NEW case).
  */
 Buffer
-_bt_getbuf(Relation rel, BlockNumber blkno, int access)
+_bt_getbuf(Relation rel, BlockNumber blkno, int access
+#ifdef USE_PREFETCH
+                              ,struct pfch_index_pagelist* pfch_index_page_list
+#endif /* USE_PREFETCH */
+          )
 {
 	Buffer		buf;
 
@@ -581,6 +771,12 @@ _bt_getbuf(Relation rel, BlockNumber blk
 	{
 		/* Read an existing block of the relation */
 		buf = ReadBuffer(rel, blkno);
+
+#ifdef USE_PREFETCH
+        /*  if the block is in the prefetched-but-unread pagelist,  remove it */
+        _bt_subtract_block( blkno , pfch_index_page_list);
+#endif /* USE_PREFETCH */
+
 		LockBuffer(buf, access);
 		_bt_checkpage(rel, buf);
 	}
@@ -702,6 +898,10 @@ _bt_getbuf(Relation rel, BlockNumber blk
  * bufmgr when one would do.  However, now it's mainly just a notational
  * convenience.  The only case where it saves work over _bt_relbuf/_bt_getbuf
  * is when the target page is the same one already in the buffer.
+ *
+ * if prefetching of index pages is changed to use this function,
+ * then it should be extended to take the index_page_list as parameter
+ * and call_bt_subtract_block in the same way that _bt_getbuf does.
  */
 Buffer
 _bt_relandgetbuf(Relation rel, Buffer obuf, BlockNumber blkno, int access)
@@ -712,6 +912,7 @@ _bt_relandgetbuf(Relation rel, Buffer ob
 	if (BufferIsValid(obuf))
 		LockBuffer(obuf, BUFFER_LOCK_UNLOCK);
 	buf = ReleaseAndReadBuffer(obuf, rel, blkno);
+
 	LockBuffer(buf, access);
 	_bt_checkpage(rel, buf);
 	return buf;
@@ -965,7 +1166,11 @@ _bt_is_page_halfdead(Relation rel, Block
 	BTPageOpaque opaque;
 	bool		result;
 
-	buf = _bt_getbuf(rel, blk, BT_READ);
+	buf = _bt_getbuf(rel, blk, BT_READ
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                     );
 	page = BufferGetPage(buf);
 	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
@@ -1069,7 +1274,11 @@ _bt_lock_branch_parent(Relation rel, Blo
 				Page		lpage;
 				BTPageOpaque lopaque;
 
-				lbuf = _bt_getbuf(rel, leftsib, BT_READ);
+				lbuf = _bt_getbuf(rel, leftsib, BT_READ
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                                 );
 				lpage = BufferGetPage(lbuf);
 				lopaque = (BTPageOpaque) PageGetSpecialPointer(lpage);
 
@@ -1265,7 +1474,11 @@ _bt_pagedel(Relation rel, Buffer buf)
 					BTPageOpaque lopaque;
 					Page		lpage;
 
-					lbuf = _bt_getbuf(rel, leftsib, BT_READ);
+					lbuf = _bt_getbuf(rel, leftsib, BT_READ
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                                     );
 					lpage = BufferGetPage(lbuf);
 					lopaque = (BTPageOpaque) PageGetSpecialPointer(lpage);
 					/*
@@ -1340,7 +1553,11 @@ _bt_pagedel(Relation rel, Buffer buf)
 		if (!rightsib_empty)
 			break;
 
-		buf = _bt_getbuf(rel, rightsib, BT_WRITE);
+		buf = _bt_getbuf(rel, rightsib, BT_WRITE
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                        );
 	}
 
 	return ndeleted;
@@ -1593,7 +1810,11 @@ _bt_unlink_halfdead_page(Relation rel, B
 		target = topblkno;
 
 		/* fetch the block number of the topmost parent's left sibling */
-		buf = _bt_getbuf(rel, topblkno, BT_READ);
+		buf = _bt_getbuf(rel, topblkno, BT_READ
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                        );
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		leftsib = opaque->btpo_prev;
@@ -1632,7 +1853,11 @@ _bt_unlink_halfdead_page(Relation rel, B
 		LockBuffer(leafbuf, BT_WRITE);
 	if (leftsib != P_NONE)
 	{
-		lbuf = _bt_getbuf(rel, leftsib, BT_WRITE);
+		lbuf = _bt_getbuf(rel, leftsib, BT_WRITE
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                         );
 		page = BufferGetPage(lbuf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		while (P_ISDELETED(opaque) || opaque->btpo_next != target)
@@ -1646,7 +1871,11 @@ _bt_unlink_halfdead_page(Relation rel, B
 					 RelationGetRelationName(rel));
 				return false;
 			}
-			lbuf = _bt_getbuf(rel, leftsib, BT_WRITE);
+			lbuf = _bt_getbuf(rel, leftsib, BT_WRITE
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                              );
 			page = BufferGetPage(lbuf);
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		}
@@ -1701,7 +1930,11 @@ _bt_unlink_halfdead_page(Relation rel, B
 	 * And next write-lock the (current) right sibling.
 	 */
 	rightsib = opaque->btpo_next;
-	rbuf = _bt_getbuf(rel, rightsib, BT_WRITE);
+	rbuf = _bt_getbuf(rel, rightsib, BT_WRITE
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                                );
 	page = BufferGetPage(rbuf);
 	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 	if (opaque->btpo_prev != target)
@@ -1731,7 +1964,11 @@ _bt_unlink_halfdead_page(Relation rel, B
 		if (P_RIGHTMOST(opaque))
 		{
 			/* rightsib will be the only one left on the level */
-			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                                );
 			metapg = BufferGetPage(metabuf);
 			metad = BTPageGetMeta(metapg);
 
--- src/backend/access/nbtree/nbtree.c.orig	2014-06-25 16:37:59.365618875 -0400
+++ src/backend/access/nbtree/nbtree.c	2014-06-25 18:10:51.388522367 -0400
@@ -30,6 +30,18 @@
 #include "tcop/tcopprot.h"
 #include "utils/memutils.h"
 
+#ifdef USE_PREFETCH
+extern unsigned int prefetch_index_scans; /* boolean whether to prefetch non-bitmap index scans also numeric size of pfch_list */
+#endif   /* USE_PREFETCH */
+
+Datum
+btpeeknexttuple(
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+        IndexScanDesc scan
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+        PG_FUNCTION_ARGS
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+);
 
 /* Working state for btbuild and its callback */
 typedef struct
@@ -332,6 +344,78 @@ btgettuple(PG_FUNCTION_ARGS)
 }
 
 /*
+ *	btpeeknexttuple() -- peek at the next tuple different from any blocknum in pfch_block_item_list
+ *                           without reading a new index page
+ *                       and without causing any side-effects such as altering values in control blocks
+ *               if found,     store blocknum in next element of pfch_block_item_list
+ *      This function has no usefulness unless postgresql is compiled with USE_PREFETCH
+ */
+Datum
+btpeeknexttuple(
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+        IndexScanDesc scan
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+        PG_FUNCTION_ARGS
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+)
+{
+#ifndef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+    IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+	BTScanOpaque so = (BTScanOpaque) scan->opaque;
+	bool		res = false;
+	int		itemIndex;		/* current index in items[] */
+
+        
+#ifdef USE_PREFETCH
+        /*
+         * If we've already initialized this scan, we can just advance it in
+         * the appropriate direction.  If we haven't done so yet, bail out
+         */
+        if ( BTScanPosIsValid(so->currPos) ) {
+
+            itemIndex = so->currPos.itemIndex+1;    /*   next item */
+
+            /* This loop handles advancing till we find different data block or end of index page */
+            while (itemIndex <= so->currPos.lastItem) {
+                unsigned short int pfchx; /*  index in BlockIdData array   */
+                for (pfchx = 0; pfchx < scan->pfch_used; pfchx++) {
+                        if (BlockIdEquals((&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)) , &(so->currPos.items[itemIndex].heapTid.ip_blkid))) {
+                             goto block_match;
+                        }
+                }
+
+                /*  if we reach here,  no block in list matched this item  */
+                res = true;
+                /*   set item in prefetch list
+                **   prefer unused entry if there is one,  else overwrite
+                */
+                if (scan->pfch_used < prefetch_index_scans) {
+                    scan->pfch_next = scan->pfch_used;
+                } else {
+                    scan->pfch_next++;
+                    if (scan->pfch_next >= prefetch_index_scans) {
+                        scan->pfch_next = 0;
+                    }
+                }
+
+                BlockIdCopy((&((scan->pfch_block_item_list + scan->pfch_next)->pfch_blockid)) , &(so->currPos.items[itemIndex].heapTid.ip_blkid));
+                if (scan->pfch_used <= scan->pfch_next) {
+                     scan->pfch_used = (scan->pfch_next + 1);
+                }
+
+                goto peek_complete;
+
+      block_match:         itemIndex++;
+            }
+	}
+
+ peek_complete:
+#endif   /* USE_PREFETCH */
+	PG_RETURN_BOOL(res);
+}
+
+/*
  * btgetbitmap() -- gets all matching tuples, and adds them to a bitmap
  */
 Datum
@@ -425,6 +509,12 @@ btbeginscan(PG_FUNCTION_ARGS)
 	so->killedItems = NULL;		/* until needed */
 	so->numKilled = 0;
 
+	so->backSeqRun = 0;
+	so->backSeqPos = 0;
+	so->prefetchItemIndex = 0;
+	so->lastHeapPrefetchBlkno = P_NONE;
+	so->prefetchBlockCount = 0;
+	
 	/*
 	 * We don't know yet whether the scan will be index-only, so we do not
 	 * allocate the tuple workspace arrays until btrescan.  However, we set up
@@ -517,6 +607,23 @@ btendscan(PG_FUNCTION_ARGS)
 	IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
 
+#ifdef USE_PREFETCH
+        struct pfch_index_pagelist* pfch_index_plp;
+        int ix;
+
+	/* discard all prefetched but unread index pages listed in the pagelist */
+        pfch_index_plp = scan->pfch_index_page_list;
+        while ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+                    ix = pfch_index_plp->pfch_index_item_count;
+                    while (ix-- > 0) {
+                        if (pfch_index_plp->pfch_indexid[ix].pfch_discard) {
+                            DiscardBuffer( scan->indexRelation , MAIN_FORKNUM , pfch_index_plp->pfch_indexid[ix].pfch_blocknum);
+                        }
+                    }
+                    pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+        }
+#endif /* USE_PREFETCH */
+
 	/* we aren't holding any read locks, but gotta drop the pins */
 	if (BTScanPosIsValid(so->currPos))
 	{
--- src/backend/nodes/tidbitmap.c.orig	2014-06-25 16:37:59.397618882 -0400
+++ src/backend/nodes/tidbitmap.c	2014-06-25 18:10:51.408522446 -0400
@@ -44,6 +44,9 @@
 #include "nodes/bitmapset.h"
 #include "nodes/tidbitmap.h"
 #include "utils/hsearch.h"
+#ifdef USE_PREFETCH
+extern int	target_prefetch_pages;
+#endif   /* USE_PREFETCH */
 
 /*
  * The maximum number of tuples per page is not large (typically 256 with
@@ -572,7 +575,12 @@ tbm_begin_iterate(TIDBitmap *tbm)
 	 * needs of the TBMIterateResult sub-struct.
 	 */
 	iterator = (TBMIterator *) palloc(sizeof(TBMIterator) +
-								 MAX_TUPLES_PER_PAGE * sizeof(OffsetNumber));
+								 MAX_TUPLES_PER_PAGE * sizeof(OffsetNumber)
+#ifdef USE_PREFETCH
+                                          		      /*  space for remembering every prefetched but unread blockno */
+                                          		      +  (target_prefetch_pages * sizeof(BlockNumber))
+#endif   /* USE_PREFETCH */
+                                         );
 	iterator->tbm = tbm;
 
 	/*
@@ -1020,3 +1028,70 @@ tbm_comparator(const void *left, const v
 		return 1;
 	return 0;
 }
+
+#ifdef USE_PREFETCH
+void
+tbm_zero(TBMIterator *iterator) /* zero list of prefetched and unread blocknos */
+{
+      /* locate the list of prefetched but unread blocknos immediately following the array of offsets
+      ** and note that tbm_begin_iterate allocates space for (1 + MAX_TUPLES_PER_PAGE) offsets  -
+      ** 1 included in struct TBMIterator and MAX_TUPLES_PER_PAGE additional
+      */
+      iterator->output.Unread_Pfetched_base = ((BlockNumber *)(&(iterator->output.offsets[MAX_TUPLES_PER_PAGE+1])));
+      iterator->output.Unread_Pfetched_next = iterator->output.Unread_Pfetched_count = 0;
+}
+
+void
+tbm_add(TBMIterator *iterator, BlockNumber blockno) /* add this blockno to list of prefetched and unread blocknos */
+{
+      unsigned int Unread_Pfetched_index = iterator->output.Unread_Pfetched_next + iterator->output.Unread_Pfetched_count++;
+
+      if (iterator->output.Unread_Pfetched_count > target_prefetch_pages) {
+		ereport(ERROR,
+			(errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("tbm_add overflowed list cannot add blockno %d", blockno)));
+      }
+
+      if (Unread_Pfetched_index >= target_prefetch_pages)
+          Unread_Pfetched_index -= target_prefetch_pages;
+      *(iterator->output.Unread_Pfetched_base + Unread_Pfetched_index) = blockno;
+}
+
+void
+tbm_subtract(TBMIterator *iterator, BlockNumber blockno) /* remove this blockno from list of prefetched and unread blocknos */
+{
+      unsigned int Unread_Pfetched_index = iterator->output.Unread_Pfetched_next++;
+      BlockNumber nextUnreadPfetched;
+
+      /*    make a weak check that the next blockno is the one to be removed,
+      **    although actually in case of disagreement,   we ignore callers blockno and remove next anyway,
+      **    which is really what caller wants
+      */
+      if ( iterator->output.Unread_Pfetched_count == 0 ) {
+		ereport(ERROR,
+			(errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("tbm_subtract empty list cannot subtract blockno %d", blockno)));
+      }
+
+      if (Unread_Pfetched_index >= target_prefetch_pages)
+          Unread_Pfetched_index = 0;
+      nextUnreadPfetched = *(iterator->output.Unread_Pfetched_base + Unread_Pfetched_index);
+      if (   ( nextUnreadPfetched != blockno ) 
+          && ( nextUnreadPfetched != InvalidBlockNumber ) /* dont report it if the block in the list was InvalidBlockNumber */
+         ) {
+		ereport(NOTICE,
+			(errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("tbm_subtract will subtract blockno %d not %d",
+					nextUnreadPfetched, blockno)));
+      }
+      if (iterator->output.Unread_Pfetched_next >= target_prefetch_pages)
+          iterator->output.Unread_Pfetched_next = 0;
+      iterator->output.Unread_Pfetched_count--;
+}
+#endif /* USE_PREFETCH */
+
+TBMIterateResult *
+tbm_locate_IterateResult(TBMIterator *iterator)
+{
+   return &(iterator->output);
+}
--- src/backend/utils/misc/guc.c.orig	2014-06-25 16:37:59.533618908 -0400
+++ src/backend/utils/misc/guc.c	2014-06-25 18:10:51.456522634 -0400
@@ -2259,6 +2259,25 @@ static struct config_int ConfigureNamesI
 	},
 
 	{
+		{"max_async_io_prefetchers",
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+			PGC_USERSET,
+#else
+			PGC_INTERNAL,
+#endif
+			RESOURCES_ASYNCHRONOUS,
+			gettext_noop("Maximum number of background processes concurrently using asynchronous librt threads to prefetch pages into shared memory buffers."),
+		},
+		&max_async_io_prefetchers,
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+	        -1, 0, 8192,      /*  boot val -1 indicates to initialize to something sensible during buf_init */
+#else
+		0, 0, 0,
+#endif
+		NULL, NULL, NULL
+	},
+
+	{
 		{"max_worker_processes",
 			PGC_POSTMASTER,
 			RESOURCES_ASYNCHRONOUS,
--- src/backend/utils/mmgr/aset.c.orig	2014-06-25 16:37:59.533618908 -0400
+++ src/backend/utils/mmgr/aset.c	2014-06-25 18:10:51.484522744 -0400
@@ -733,6 +733,48 @@ AllocSetAlloc(MemoryContext context, Siz
 	 */
 	fidx = AllocSetFreeIndex(size);
 	chunk = set->freelist[fidx];
+#ifdef MEMORY_CONTEXT_CHECKING
+        /*    an instance of segfault caused by a rogue value in set->freelist[fidx]
+        **    has been seen - check for it using crude sanity check based on neighbours :
+        **    if at least one is sufficiently close, then pass,  else fail
+        */
+        if (chunk != 0) {
+            int frx, nrx; /*  frx is index,  nrx is index of failing neighbour for errmsg */
+            for (nrx = -1, frx = 0; (frx < ALLOCSET_NUM_FREELISTS); frx++) {
+                if (   (frx != fidx)     /*  not the chosen one */
+                    && ( ( (unsigned long)(set->freelist[frx]) ) != 0 ) /* not empty */
+                   ) {
+                    if (   ( (unsigned long)chunk < ( ( (unsigned long)(set->freelist[frx]) ) / 2 ) )
+                        && (  ( (unsigned long)(set->freelist[frx]) ) < 0x4000000  )
+               /***     || ( (unsigned long)chunk > ( ( (unsigned long)(set->freelist[frx]) ) * 2 ) )  ***/
+                       ) {
+                       nrx = frx;
+                    } else {
+                       nrx = -1;
+                       break;
+                    }
+                }
+            }
+
+            if (nrx >= 0) {
+                     /*  this must be a rogue value  -  put this list in the garbage */
+                     /*  build message but be careful to avoid recursively triggering same fault */
+                     set->freelist[fidx] = NULL;   /*  mark this list empty */
+                     elog(WARNING, "detected rogue value %p in freelist index %d compared with neighbour %p whose chunksize %d"
+				 , chunk , fidx , set->freelist[nrx] , set->freelist[nrx]->size);
+                     chunk = NULL;
+            }
+        }
+#else /* if not MEMORY_CONTEXT_CHECKING make very simple-minded check*/
+        if ( (chunk != 0) && ( (unsigned long)chunk <  0x40000 ) ) {
+                     /*  this must be a rogue value  -  put this list in the garbage */
+                     /*  build message but be careful to avoid recursively triggering same fault */
+                     set->freelist[fidx] = NULL;   /*  mark this list empty */
+                     elog(WARNING, "detected rogue value %p in freelist index %d"
+				 , chunk , fidx);
+                     chunk = NULL;
+        }
+#endif
 	if (chunk != NULL)
 	{
 		Assert(chunk->size >= size);
--- src/include/executor/instrument.h.orig	2014-06-25 16:37:59.581618917 -0400
+++ src/include/executor/instrument.h	2014-06-25 18:10:51.516522869 -0400
@@ -28,8 +28,18 @@ typedef struct BufferUsage
 	long		local_blks_written;		/* # of local disk blocks written */
 	long		temp_blks_read; /* # of temp blocks read */
 	long		temp_blks_written;		/* # of temp blocks written */
+
 	instr_time	blk_read_time;	/* time spent reading */
 	instr_time	blk_write_time; /* time spent writing */
+
+	long		aio_read_noneed;		/* # of prefetches for which no need for prefetch as block already in buffer pool */
+	long		aio_read_discrd;		/* # of prefetches for which no need for prefetch as block already in buffer pool */
+	long		aio_read_forgot;		/* # of prefetches for which no need for prefetch as block already in buffer pool */
+	long		aio_read_noblok;		/* # of prefetches for which no available BufferAiocb */
+	long		aio_read_failed;		/* # of aio reads for which aio itself failed or the read failed with an errno */
+	long		aio_read_wasted;		/* # of aio reads for which disk block not used */
+	long		aio_read_waited;		/* # of aio reads for which disk block used but had to wait for it */
+	long		aio_read_ontime;		/* # of aio reads for which disk block used and ready on time when requested */
 } BufferUsage;
 
 /* Flag bits included in InstrAlloc's instrument_options bitmask */
--- src/include/storage/bufmgr.h.orig	2014-06-25 16:37:59.593618920 -0400
+++ src/include/storage/bufmgr.h	2014-06-25 18:10:51.536522947 -0400
@@ -41,6 +41,7 @@ typedef enum
 	RBM_ZERO_ON_ERROR,			/* Read, but return an all-zeros page on error */
 	RBM_NORMAL_NO_LOG			/* Don't log page as invalid during WAL
 								 * replay; otherwise same as RBM_NORMAL */
+       ,RBM_NOREAD_FOR_PREFETCH   /* Don't read from disk, don't zero buffer, find buffer only */
 } ReadBufferMode;
 
 /* in globals.c ... this duplicates miscadmin.h */
@@ -57,6 +58,9 @@ extern int	target_prefetch_pages;
 extern PGDLLIMPORT char *BufferBlocks;
 extern PGDLLIMPORT int32 *PrivateRefCount;
 
+/*  in buf_async.c  */;
+extern int      max_async_io_prefetchers; /* Maximum number of backends using asynchronous librt threads to read pages into our buffers */
+
 /* in localbuf.c */
 extern PGDLLIMPORT int NLocBuffer;
 extern PGDLLIMPORT Block *LocalBufferBlockPointers;
@@ -159,9 +163,15 @@ extern PGDLLIMPORT int32 *LocalRefCount;
 #define BufferGetPage(buffer) ((Page)BufferGetBlock(buffer))
 
 /*
- * prototypes for functions in bufmgr.c
+ * prototypes for external functions in bufmgr.c and buf_async.c
  */
-extern void PrefetchBuffer(Relation reln, ForkNumber forkNum,
+extern int PrefetchBuffer(Relation reln, ForkNumber forkNum,
+			   BlockNumber blockNum , BufferAccessStrategy strategy);
+/*   return code  is an int bitmask : */
+#define PREFTCHRC_BUF_PIN_INCREASED 0x01    /*  pin count on buffer has been increased by 1 */
+#define PREFTCHRC_BLK_ALREADY_PRESENT 0x02  /*  block was already present in a buffer       */
+
+extern void DiscardBuffer(Relation reln, ForkNumber forkNum,
 			   BlockNumber blockNum);
 extern Buffer ReadBuffer(Relation reln, BlockNumber blockNum);
 extern Buffer ReadBufferExtended(Relation reln, ForkNumber forkNum,
--- src/include/storage/lwlock.h.orig	2014-06-25 16:37:59.593618920 -0400
+++ src/include/storage/lwlock.h	2014-06-25 18:10:51.552523010 -0400
@@ -55,6 +55,21 @@ typedef struct LWLock
 	/* tail is undefined when head is NULL */
 } LWLock;
 
+
+#if defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP) && defined(USE_AIO_SIGEVENT)
+/*  instantiations of the following structure are embedded in the BufferAiocb defined in buf_internals.h
+ *  but the struct is also needed elsewhere such as in fd.c
+ *  each chain item represents one aio_read whose waiter-lock is held eXclusive by originator
+ *  and will be released by the same originator after the completion signal has been received.
+ *  any other backend may wait Shared on the lock to wait for completion,
+ *  *provided* that the BAiocb is still in use  (at least one dependent on it)
+*/
+struct BAiocbIolock_chain_item {  /*  a simple struct for chaining awaiting-release LWLock ptrs together */
+    struct BAiocbIolock_chain_item volatile * volatile next;    /*  next chain_item                      */
+    LWLock       volatile * volatile BAiocbIolock;    /*  waiter-lock to wait for async I/O to complete  */
+};
+#endif /*  defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP) && defined(USE_AIO_SIGEVENT)  */
+
 /*
  * Prior to PostgreSQL 9.4, every lightweight lock in the system was stored
  * in a single array.  For convenience and for compatibility with past
--- src/include/storage/smgr.h.orig	2014-06-25 16:37:59.593618920 -0400
+++ src/include/storage/smgr.h	2014-06-25 18:10:51.572523089 -0400
@@ -92,6 +92,20 @@ extern void smgrextend(SMgrRelation reln
 		   BlockNumber blocknum, char *buffer, bool skipFsync);
 extern void smgrprefetch(SMgrRelation reln, ForkNumber forknum,
 			 BlockNumber blocknum);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern void smgrinitaio(int max_aio_threads, int max_aio_num);
+extern void smgrstartaio(SMgrRelation reln, ForkNumber forknum,
+			 BlockNumber blocknum , char *aiocbp , int *retcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , void *BAiocbIolockaiocbp
+#endif /* USE_AIO_SIGEVENT  */
+                        );
+extern void smgrcompleteaio( SMgrRelation reln, char *aiocbp , int *inoutcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , void *BAiocbIolockaiocbp
+#endif /* USE_AIO_SIGEVENT  */
+                           );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 extern void smgrread(SMgrRelation reln, ForkNumber forknum,
 		 BlockNumber blocknum, char *buffer);
 extern void smgrwrite(SMgrRelation reln, ForkNumber forknum,
@@ -118,6 +132,19 @@ extern void mdextend(SMgrRelation reln,
 		 BlockNumber blocknum, char *buffer, bool skipFsync);
 extern void mdprefetch(SMgrRelation reln, ForkNumber forknum,
 		   BlockNumber blocknum);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern void mdinitaio(int max_aio_threads, int max_aio_num);
+extern void mdstartaio(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum , char *aiocbp , int *retcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , void *BAiocbIolockaiocbp
+#endif /* USE_AIO_SIGEVENT  */
+                      );
+extern void mdcompleteaio( char *aiocbp , int *inoutcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , void *BAiocbIolockaiocbp
+#endif /* USE_AIO_SIGEVENT  */
+                         );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 extern void mdread(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 	   char *buffer);
 extern void mdwrite(SMgrRelation reln, ForkNumber forknum,
--- src/include/storage/fd.h.orig	2014-06-25 16:37:59.593618920 -0400
+++ src/include/storage/fd.h	2014-06-25 18:10:51.596523183 -0400
@@ -69,6 +69,19 @@ extern File PathNameOpenFile(FileName fi
 extern File OpenTemporaryFile(bool interXact);
 extern void FileClose(File file);
 extern int	FilePrefetch(File file, off_t offset, int amount);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern void FileInitaio(int max_aio_threads, int max_aio_num );
+extern int  FileStartaio(File file, off_t offset, int amount , char *aiocbp
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , void *BAiocbIolockaiocbp
+#endif /* USE_AIO_SIGEVENT  */
+                           );
+extern int  FileCompleteaio( char *aiocbp , int normal_wait
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , void *BAiocbIolockaiocbp
+#endif /* USE_AIO_SIGEVENT  */
+                           );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 extern int	FileRead(File file, char *buffer, int amount);
 extern int	FileWrite(File file, char *buffer, int amount);
 extern int	FileSync(File file);
--- src/include/storage/buf_internals.h.orig	2014-06-25 16:37:59.593618920 -0400
+++ src/include/storage/buf_internals.h	2014-06-25 18:10:51.612523246 -0400
@@ -22,7 +22,9 @@
 #include "storage/smgr.h"
 #include "storage/spin.h"
 #include "utils/relcache.h"
-
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+#include "aio.h"
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * Flags for buffer descriptors
@@ -38,8 +40,23 @@
 #define BM_JUST_DIRTIED			(1 << 5)		/* dirtied since write started */
 #define BM_PIN_COUNT_WAITER		(1 << 6)		/* have waiter for sole pin */
 #define BM_CHECKPOINT_NEEDED	(1 << 7)		/* must write for checkpoint */
-#define BM_PERMANENT			(1 << 8)		/* permanent relation (not
-												 * unlogged) */
+#define BM_PERMANENT			(1 << 8)	/* permanent relation (not unlogged) */
+#define BM_AIO_IN_PROGRESS		(1 << 9)	/* aio in progress    */
+#define BM_AIO_PREFETCH_PIN_BANKED	(1 << 10)	/* pinned when prefetch issued
+                                                        ** and this pin is banked - i.e.
+                                                        ** redeemable by the next use by same task
+                                                        ** note that for any one buffer, a pin can be banked
+                                                        **      by at most one process globally,
+                                                        **      that is,   only one process may bank a pin on the buffer
+                                                        **                 and it may do so only once (may not be stacked)
+                                                        */
+
+/*********
+for asynchronous aio-read prefetching, two golden rules concerning buffer pinning and buffer-header flags must be observed:
+  R1.  a buffer marked as BM_AIO_IN_PROGRESS must be pinned by at least one backend
+  R2.  a buffer marked as BM_AIO_PREFETCH_PIN_BANKED must be pinned by the backend identified by
+               (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio : (-(buf->freeNext))
+*********/
 
 typedef bits16 BufFlags;
 
@@ -140,17 +157,88 @@ typedef struct sbufdesc
 	BufFlags	flags;			/* see bit definitions above */
 	uint16		usage_count;	/* usage counter for clock sweep code */
 	unsigned	refcount;		/* # of backends holding pins on buffer */
-	int			wait_backend_pid;		/* backend PID of pin-count waiter */
+	int		wait_backend_pid;	/*  if     flags & BM_PIN_COUNT_WAITER
+                                                **  then   backend PID of pin-count waiter
+                                                **  else   not set
+                                                */
 
 	slock_t		buf_hdr_lock;	/* protects the above fields */
 
 	int			buf_id;			/* buffer's index number (from 0) */
-	int			freeNext;		/* link in freelist chain */
+        int    	volatile	freeNext;	/* overloaded and much-abused field :
+                                                ** EITHER
+                                                **     if     >= 0
+                                                **     then   link in freelist chain
+                                                **  OR
+                                                **     if     <  0
+                                                **     then    EITHER
+                                                **             if     flags & BM_AIO_IN_PROGRESS
+                                                **             then   negative of (the index of the aiocb in the BufferAiocbs array + 3)
+                                                **             else   if flags & BM_AIO_PREFETCH_PIN_BANKED
+                                                **             then   -(pid of task that issued aio_read and pinned buffer)
+                                                **             else   one of the special values -1 or -2 listed below
+                                                */
 
 	LWLock	   *io_in_progress_lock;	/* to wait for I/O to complete */
 	LWLock	   *content_lock;	/* to lock access to buffer contents */
 } BufferDesc;
 
+/*  structures for control blocks for our implementation of async io */
+
+/*  if USE_AIO_ATOMIC_BUILTIN_COMP_SWAP is not defined,  the following struct is not put into use at runtime
+**  but it is easier to let the compiler find the definition but hide the reference to aiocb
+**  which is the only type it would not understand
+*/
+
+struct BufferAiocb {
+       struct BufferAiocb volatile * volatile BAiocbnext;  /*    next free entry or value of BAIOCB_OCCUPIED means in use  */
+       struct sbufdesc    volatile * volatile BAiocbbufh;  /*    there can be at most one BufferDesc marked BM_AIO_IN_PROGRESS
+                                                           **    and using this BufferAiocb -
+                                                           **    if there is one, BAiocbbufh points to it, else BAiocbbufh is zero
+                                                           **    NOTE  BAiocbbufh should be zero for every BufferAiocb on the free list
+                                                           */
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+       struct aiocb       volatile            BAiocbthis;  /*    the aio library's control block for one async io */
+#ifdef USE_AIO_SIGEVENT                                    /*    signal non-originator waiters instead of letting them poll */
+struct BAiocbIolock_chain_item volatile BAiocbIolockItem;  /*    to wait for async I/O to complete
+                                                            *    BAiocbIolock is waited on only by processes other than
+                                                            *    originator of aio   */
+#endif /*  USE_AIO_SIGEVENT    */
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+       int                volatile  BAiocbDependentCount;  /*    count of tasks who depend on this BufferAiocb
+                                                           **    in the sense that they are waiting for io completion.
+                                                           **    only a Dependent may move the BufferAiocb onto the freelist
+                                                           **    and only when that Dependent is the *only* Dependent (count == 1)
+                                                           **    BAiocbDependentCount is protected by bufferheader spinlock
+                                                           **    and must be updated only when that spinlock is held
+                                                           */
+       pid_t              volatile  pidOfAio;              /*    pid of backend who issued an aio_read using this BAiocb -
+                                                           **    this backend must have pinned the associated buffer.
+                                                           */
+};
+
+#define BAIOCB_OCCUPIED 0x75f1        /*  distinct indicator of a BufferAiocb.BAiocbnext that is NOT on free list */
+#define BAIOCB_FREE 0x7b9d            /*  distinct indicator of a BufferAiocb.BAiocbbufh that IS     on free list */
+
+struct BAiocbAnchor {                 /*  anchor for all control blocks pertaining to aio  */
+       volatile struct BufferAiocb* BufferAiocbs;          /*  aiocbs ...                   */
+       volatile struct BufferAiocb* volatile FreeBAiocbs; /* ... and their free list   */
+};
+
+/*   values for BufCheckAsync input and retcode */
+#define BUF_INTENTION_WANT 		 1  /* wants the buffer, wait for in-progress aio and then pin */
+#define BUF_INTENTION_REJECT_KEEP_PIN 	-1  /* pin already held, do not unpin */
+#define BUF_INTENTION_REJECT_OBTAIN_PIN	-2  /* obtain pin,  caller wants it for same buffer */
+#define BUF_INTENTION_REJECT_FORGET	-3  /* unpin and tell resource owner to forget */
+#define BUF_INTENTION_REJECT_NOADJUST	-4  /* unpin and call ResourceOwnerForgetBuffer */
+#define BUF_INTENTION_REJECT_UNBANK   	-5  /* unpin only if pin banked by caller */
+
+#define BUF_INTENT_RC_CHANGED_TAG	-5
+#define BUF_INTENT_RC_BADPAGE		-4
+#define BUF_INTENT_RC_INVALID_AIO	-3    /*  invalid and aio was in progress */
+#define BUF_INTENT_RC_INVALID_NO_AIO	-1    /*  invalid and no aio was in progress */
+#define BUF_INTENT_RC_VALID		 1
+
 #define BufferDescriptorGetBuffer(bdesc) ((bdesc)->buf_id + 1)
 
 /*
@@ -159,6 +247,7 @@ typedef struct sbufdesc
  */
 #define FREENEXT_END_OF_LIST	(-1)
 #define FREENEXT_NOT_IN_LIST	(-2)
+#define FREENEXT_BAIOCB_ORIGIN	(-3)
 
 /*
  * Macros for acquiring/releasing a shared buffer header's spinlock.
--- src/include/catalog/pg_am.h.orig	2014-06-25 16:37:59.577618917 -0400
+++ src/include/catalog/pg_am.h	2014-06-25 18:10:51.632523324 -0400
@@ -67,6 +67,7 @@ CATALOG(pg_am,2601)
 	regproc		amcanreturn;	/* can indexscan return IndexTuples? */
 	regproc		amcostestimate; /* estimate cost of an indexscan */
 	regproc		amoptions;		/* parse AM-specific parameters */
+	regproc		ampeeknexttuple;	/* peek at the next tuple different from any blocknum in pfch_list without reading a new index page */
 } FormData_pg_am;
 
 /* ----------------
@@ -117,19 +118,19 @@ typedef FormData_pg_am *Form_pg_am;
  * ----------------
  */
 
-DATA(insert OID = 403 (  btree		5 2 t f t t t t t t f t t 0 btinsert btbeginscan btgettuple btgetbitmap btrescan btendscan btmarkpos btrestrpos btbuild btbuildempty btbulkdelete btvacuumcleanup btcanreturn btcostestimate btoptions ));
+DATA(insert OID = 403 (  btree		5 2 t f t t t t t t f t t 0 btinsert btbeginscan btgettuple btgetbitmap btrescan btendscan btmarkpos btrestrpos btbuild btbuildempty btbulkdelete btvacuumcleanup btcanreturn btcostestimate btoptions btpeeknexttuple ));
 DESCR("b-tree index access method");
 #define BTREE_AM_OID 403
-DATA(insert OID = 405 (  hash		1 1 f f t f f f f f f f f 23 hashinsert hashbeginscan hashgettuple hashgetbitmap hashrescan hashendscan hashmarkpos hashrestrpos hashbuild hashbuildempty hashbulkdelete hashvacuumcleanup - hashcostestimate hashoptions ));
+DATA(insert OID = 405 (  hash		1 1 f f t f f f f f f f f 23 hashinsert hashbeginscan hashgettuple hashgetbitmap hashrescan hashendscan hashmarkpos hashrestrpos hashbuild hashbuildempty hashbulkdelete hashvacuumcleanup - hashcostestimate hashoptions - ));
 DESCR("hash index access method");
 #define HASH_AM_OID 405
-DATA(insert OID = 783 (  gist		0 8 f t f f t t f t t t f 0 gistinsert gistbeginscan gistgettuple gistgetbitmap gistrescan gistendscan gistmarkpos gistrestrpos gistbuild gistbuildempty gistbulkdelete gistvacuumcleanup - gistcostestimate gistoptions ));
+DATA(insert OID = 783 (  gist		0 8 f t f f t t f t t t f 0 gistinsert gistbeginscan gistgettuple gistgetbitmap gistrescan gistendscan gistmarkpos gistrestrpos gistbuild gistbuildempty gistbulkdelete gistvacuumcleanup - gistcostestimate gistoptions - ));
 DESCR("GiST index access method");
 #define GIST_AM_OID 783
-DATA(insert OID = 2742 (  gin		0 6 f f f f t t f f t f f 0 gininsert ginbeginscan - gingetbitmap ginrescan ginendscan ginmarkpos ginrestrpos ginbuild ginbuildempty ginbulkdelete ginvacuumcleanup - gincostestimate ginoptions ));
+DATA(insert OID = 2742 (  gin		0 6 f f f f t t f f t f f 0 gininsert ginbeginscan - gingetbitmap ginrescan ginendscan ginmarkpos ginrestrpos ginbuild ginbuildempty ginbulkdelete ginvacuumcleanup - gincostestimate ginoptions - ));
 DESCR("GIN index access method");
 #define GIN_AM_OID 2742
-DATA(insert OID = 4000 (  spgist	0 5 f f f f f t f t f f f 0 spginsert spgbeginscan spggettuple spggetbitmap spgrescan spgendscan spgmarkpos spgrestrpos spgbuild spgbuildempty spgbulkdelete spgvacuumcleanup spgcanreturn spgcostestimate spgoptions ));
+DATA(insert OID = 4000 (  spgist	0 5 f f f f f t f t f f f 0 spginsert spgbeginscan spggettuple spggetbitmap spgrescan spgendscan spgmarkpos spgrestrpos spgbuild spgbuildempty spgbulkdelete spgvacuumcleanup spgcanreturn spgcostestimate spgoptions - ));
 DESCR("SP-GiST index access method");
 #define SPGIST_AM_OID 4000
 
--- src/include/catalog/pg_proc.h.orig	2014-06-25 16:37:59.577618917 -0400
+++ src/include/catalog/pg_proc.h	2014-06-25 18:10:51.676523496 -0400
@@ -536,6 +536,12 @@ DESCR("convert float4 to int4");
 
 DATA(insert OID = 330 (  btgettuple		   PGNSP PGUID 12 1 0 0 0 f f f f t f v 2 0 16 "2281 2281" _null_ _null_ _null_ _null_	btgettuple _null_ _null_ _null_ ));
 DESCR("btree(internal)");
+
+#ifndef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+DATA(insert OID = 3255 (  btpeeknexttuple	   PGNSP PGUID 12 1 0 0 0 f f f f t f v 1 0 16 "2281" _null_ _null_ _null_ _null_ btpeeknexttuple _null_ _null_ _null_ ));
+DESCR("btree(internal)");
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+
 DATA(insert OID = 636 (  btgetbitmap	   PGNSP PGUID 12 1 0 0 0 f f f f t f v 2 0 20 "2281 2281" _null_ _null_ _null_ _null_	btgetbitmap _null_ _null_ _null_ ));
 DESCR("btree(internal)");
 DATA(insert OID = 331 (  btinsert		   PGNSP PGUID 12 1 0 0 0 f f f f t f v 6 0 16 "2281 2281 2281 2281 2281 2281" _null_ _null_ _null_ _null_	btinsert _null_ _null_ _null_ ));
--- src/include/pg_config_manual.h.orig	2014-06-25 16:37:59.589618919 -0400
+++ src/include/pg_config_manual.h	2014-06-25 18:10:51.700523591 -0400
@@ -138,9 +138,17 @@
 /*
  * USE_PREFETCH code should be compiled only if we have a way to implement
  * prefetching.  (This is decoupled from USE_POSIX_FADVISE because there
- * might in future be support for alternative low-level prefetch APIs.)
+ * might in future be support for alternative low-level prefetch APIs  --
+ * -- update October 2013  -- now there is such a new prefetch capability --
+ *   async_io into postgres buffers  -   configuration parameter max_async_io_threads)
  */
-#ifdef USE_POSIX_FADVISE
+#if defined(USE_POSIX_FADVISE) || defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP)
+#define USE_AIO_SIGEVENT 1
+/* AIO_SIGEVENT_SIGNALNUM is the signal used to indicate completion
+ * of an aio operation.  Choose a signal that is not used elsewhere
+ * in postgresql and which can be caught by signal handler.
+*/
+#define AIO_SIGEVENT_SIGNALNUM SIGIO
 #define USE_PREFETCH
 #endif
 
--- src/include/miscadmin.h.orig	2014-06-25 16:37:59.585618918 -0400
+++ src/include/miscadmin.h	2014-06-25 18:10:51.708523623 -0400
@@ -76,6 +76,9 @@
 extern PGDLLIMPORT volatile bool InterruptPending;
 extern PGDLLIMPORT volatile bool QueryCancelPending;
 extern PGDLLIMPORT volatile bool ProcDiePending;
+#if defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP) && defined(USE_AIO_SIGEVENT)
+extern struct BAiocbIolock_chain_item volatile * volatile BAiocbIolock_anchor; /* anchor for chain of awaiting-release LWLock ptrs */
+#endif /*  defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP) && defined(USE_AIO_SIGEVENT)  */
 
 extern volatile bool ClientConnectionLost;
 
@@ -87,13 +90,27 @@ extern PGDLLIMPORT volatile uint32 CritS
 /* in tcop/postgres.c */
 extern void ProcessInterrupts(void);
 
+/* in storage/buffer/buf_init.c */
+extern void ProcessPendingBAiocbIolocks(void);
+
 #ifndef WIN32
 
+#if defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP) && defined(USE_AIO_SIGEVENT)
+#define CHECK_FOR_INTERRUPTS() \
+do { \
+    if (BAiocbIolock_anchor != (struct BAiocbIolock_chain_item*)0) \
+		ProcessPendingBAiocbIolocks(); \
+	if (InterruptPending) \
+		ProcessInterrupts(); \
+} while(0)
+#else /*  not defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP) && defined(USE_AIO_SIGEVENT)  */
+
 #define CHECK_FOR_INTERRUPTS() \
 do { \
 	if (InterruptPending) \
 		ProcessInterrupts(); \
 } while(0)
+#endif /*  defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP) && defined(USE_AIO_SIGEVENT)  */
 #else							/* WIN32 */
 
 #define CHECK_FOR_INTERRUPTS() \
--- src/include/access/nbtree.h.orig	2014-06-25 16:37:59.573618916 -0400
+++ src/include/access/nbtree.h	2014-06-25 18:10:51.716523654 -0400
@@ -19,6 +19,7 @@
 #include "access/sdir.h"
 #include "access/xlog.h"
 #include "access/xlogutils.h"
+#include "access/relscan.h"
 #include "catalog/pg_index.h"
 
 /* There's room for a 16-bit vacuum cycle ID in BTPageOpaqueData */
@@ -524,6 +525,7 @@ typedef struct BTScanPosData
 	Buffer		buf;			/* if valid, the buffer is pinned */
 
 	BlockNumber nextPage;		/* page's right link when we scanned it */
+	BlockNumber prevPage;		/* page's left link when we scanned it */
 
 	/*
 	 * moreLeft and moreRight track whether we think there may be matching
@@ -603,6 +605,15 @@ typedef struct BTScanOpaqueData
 	 */
 	int			markItemIndex;	/* itemIndex, or -1 if not valid */
 
+	/* prefetch logic state */
+	unsigned int	backSeqRun;	/* number of back-sequential pages in a run */
+	BlockNumber		backSeqPos;	/* blkid last prefetched in back-sequential 
+				          		   runs */
+	BlockNumber		lastHeapPrefetchBlkno;	/* blkid last prefetched from heap */
+	int				prefetchItemIndex; /* item index within currPos last
+					                      fetched by heap prefetch */
+	int				prefetchBlockCount; /* number of prefetched heap blocks */
+	
 	/* keep these last in struct for efficiency */
 	BTScanPosData currPos;		/* current position data */
 	BTScanPosData markPos;		/* marked position, if any */
@@ -655,7 +666,17 @@ extern Buffer _bt_getroot(Relation rel,
 extern Buffer _bt_gettrueroot(Relation rel);
 extern int	_bt_getrootheight(Relation rel);
 extern void _bt_checkpage(Relation rel, Buffer buf);
-extern Buffer _bt_getbuf(Relation rel, BlockNumber blkno, int access);
+extern Buffer _bt_getbuf(Relation rel, BlockNumber blkno, int access
+#ifdef USE_PREFETCH
+                                      , struct pfch_index_pagelist* pfch_index_page_list
+#endif /* USE_PREFETCH */
+                         );
+#ifdef USE_PREFETCH
+extern void _bt_prefetchbuf(Relation rel, BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P);
+extern struct pfch_index_item* _bt_find_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list);
+extern int _bt_add_block(BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P , uint32 discard_status);
+extern void _bt_subtract_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list);
+#endif /* USE_PREFETCH */
 extern Buffer _bt_relandgetbuf(Relation rel, Buffer obuf,
 				 BlockNumber blkno, int access);
 extern void _bt_relbuf(Relation rel, Buffer buf);
--- src/include/access/heapam.h.orig	2014-06-25 16:37:59.573618916 -0400
+++ src/include/access/heapam.h	2014-06-25 18:10:51.728523701 -0400
@@ -175,7 +175,7 @@ extern void heap_page_prune_execute(Buff
 extern void heap_get_root_tuples(Page page, OffsetNumber *root_offsets);
 
 /* in heap/syncscan.c */
-extern void ss_report_location(Relation rel, BlockNumber location);
+extern void ss_report_location(Relation rel, BlockNumber location , BlockNumber *prefetchHWMp);
 extern BlockNumber ss_get_location(Relation rel, BlockNumber relnblocks);
 extern void SyncScanShmemInit(void);
 extern Size SyncScanShmemSize(void);
--- src/include/access/relscan.h.orig	2014-06-25 16:37:59.573618916 -0400
+++ src/include/access/relscan.h	2014-06-25 18:10:51.740523748 -0400
@@ -44,6 +44,24 @@ typedef struct HeapScanDescData
 	bool		rs_inited;		/* false = scan not init'd yet */
 	HeapTupleData rs_ctup;		/* current tuple in scan, if any */
 	BlockNumber rs_cblock;		/* current block # in scan, if any */
+#ifdef USE_PREFETCH
+	int	    rs_prefetch_target; /* target distance (numblocks) for prefetch to reach beyond main scan */
+	BlockNumber rs_pfchblock;	/* next block # to be prefetched in scan, if any */
+
+        /*   Unread_Pfetched is a "mostly" circular list of recently prefetched blocknos of size target_prefetch_pages
+        **   the index of the first unread block is held in Unread_Pfetched_next
+        **         and is advanced when a block is read
+        **   the count of number of unread blocks is in Unread_Pfetched_count (and this subset can wrap around)
+        **   "mostly" means that there may be gaps caused by storing entries for blocks which do not need to be discarded -
+        **   these are indicated by blockno = InvalidBlockNumber,  and these slots are reused when found.
+        */
+        BlockNumber *rs_Unread_Pfetched_base;   /*  where the list of prefetched but unread blocknos starts */
+        unsigned int rs_Unread_Pfetched_next;   /*  where the next unread blockno probably is relative to start --
+                                                **  this is only a hint which may be temporarily stale.
+                                                */
+        unsigned int rs_Unread_Pfetched_count;  /*  number of valid unread blocknos in list */
+#endif   /* USE_PREFETCH */
+
 	Buffer		rs_cbuf;		/* current buffer in scan, if any */
 	/* NB: if rs_cbuf is not InvalidBuffer, we hold a pin on that buffer */
 	ItemPointerData rs_mctid;	/* marked scan position, if any */
@@ -55,6 +73,27 @@ typedef struct HeapScanDescData
 	OffsetNumber rs_vistuples[MaxHeapTuplesPerPage];	/* their offsets */
 }	HeapScanDescData;
 
+/* pfch_index_items track prefetched and unread index pages -   chunks of blocknumbers are chained in singly-linked list from scan->pfch_index_item_list */
+struct pfch_index_item {                              /*  index-relation BlockIds which we will/have prefetched */
+       BlockNumber         pfch_blocknum;    /* Blocknum which we will/have prefetched */
+       uint32              pfch_discard;     /* whether block is to be discarded when scan is closed */
+};
+
+struct pfch_block_item {
+       struct BlockIdData  pfch_blockid;     /* BlockId which we will/have prefetched */
+       uint32              pfch_discard;     /* whether block is to be discarded when scan is closed */
+};
+
+/* pfch_index_page_items track prefetched and unread index pages -
+** chunks of blocknumbers are chained backwards (newest first,  oldest last)
+** in singly-linked list from scan->pfch_index_item_list
+*/
+struct pfch_index_pagelist {                          /*  index-relation BlockIds which we will/have prefetched */
+       struct pfch_index_pagelist* pfch_index_pagelist_next;  /*  pointer to next chunk if any */
+       unsigned int    pfch_index_item_count;         /*  number of used entries in this chunk */
+       struct pfch_index_item pfch_indexid[1];        /*  in-line list of Blocknums which we will/have prefetched and whether to be discarded */
+};
+
 /*
  * We use the same IndexScanDescData structure for both amgettuple-based
  * and amgetbitmap-based index scans.  Some fields are only relevant in
@@ -75,8 +114,15 @@ typedef struct IndexScanDescData
 	/* signaling to index AM about killing index tuples */
 	bool		kill_prior_tuple;		/* last-returned tuple is dead */
 	bool		ignore_killed_tuples;	/* do not return killed entries */
-	bool		xactStartedInRecovery;	/* prevents killing/seeing killed
-										 * tuples */
+	bool		xactStartedInRecovery;	/* prevents killing/seeing killed tuples */
+										 
+#ifdef USE_PREFETCH
+        struct pfch_index_pagelist* pfch_index_page_list;  /* array of index-relation BlockIds which we will/have prefetched */
+        struct pfch_block_item* pfch_block_item_list;   /* array of heap-relation BlockIds which we will/have prefetched */
+        unsigned short int     pfch_used;	/*  number of used elements in BlockIdData array   */
+        unsigned short int     pfch_next;	/*  next element for prefetch in BlockIdData array */
+	int             do_prefetch;    /*  should I prefetch ? */
+#endif   /* USE_PREFETCH */
 
 	/* index access method's private state */
 	void	   *opaque;			/* access-method-specific info */
@@ -91,6 +137,10 @@ typedef struct IndexScanDescData
 	/* NB: if xs_cbuf is not InvalidBuffer, we hold a pin on that buffer */
 	bool		xs_recheck;		/* T means scan keys must be rechecked */
 
+	/* heap fetch statistics for read-ahead logic */
+	unsigned int heap_tids_seen;
+	unsigned int heap_tids_fetched;
+
 	/* state data for traversing HOT chains in index_getnext */
 	bool		xs_continue_hot;	/* T if must keep walking HOT chain */
 }	IndexScanDescData;
--- src/include/nodes/tidbitmap.h.orig	2014-06-25 16:37:59.585618918 -0400
+++ src/include/nodes/tidbitmap.h	2014-06-25 18:10:51.752523795 -0400
@@ -41,6 +41,16 @@ typedef struct
 	int			ntuples;		/* -1 indicates lossy result */
 	bool		recheck;		/* should the tuples be rechecked? */
 	/* Note: recheck is always true if ntuples < 0 */
+#ifdef USE_PREFETCH
+        /*   Unread_Pfetched is a circular list of recently prefetched blocknos of size target_prefetch_pages
+        **   the index of the first unread block is held in Unread_Pfetched_next
+        **         and is advanced when a block is read
+        **   the count of number of unread blocks is in Unread_Pfetched_count (and this subset can wrap around)
+        */
+        BlockNumber *Unread_Pfetched_base;   /*  where the list of prefetched but unread blocknos starts */
+        unsigned int Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+        unsigned int Unread_Pfetched_count;  /*  number of valid unread blocknos in list */
+#endif   /* USE_PREFETCH */
 	OffsetNumber offsets[1];	/* VARIABLE LENGTH ARRAY */
 } TBMIterateResult;				/* VARIABLE LENGTH STRUCT */
 
@@ -62,5 +72,8 @@ extern bool tbm_is_empty(const TIDBitmap
 extern TBMIterator *tbm_begin_iterate(TIDBitmap *tbm);
 extern TBMIterateResult *tbm_iterate(TBMIterator *iterator);
 extern void tbm_end_iterate(TBMIterator *iterator);
-
+extern void tbm_zero(TBMIterator *iterator); /* zero list of prefetched and unread blocknos */
+extern void tbm_add(TBMIterator *iterator, BlockNumber blockno); /* add this blockno to list of prefetched and unread blocknos */
+extern void tbm_subtract(TBMIterator *iterator, BlockNumber blockno); /* remove this blockno from list of prefetched and unread blocknos */
+extern TBMIterateResult *tbm_locate_IterateResult(TBMIterator *iterator); /* locate the TBMIterateResult of an iterator */
 #endif   /* TIDBITMAP_H */
--- src/include/utils/rel.h.orig	2014-06-25 16:37:59.601618921 -0400
+++ src/include/utils/rel.h	2014-06-25 18:10:51.760523826 -0400
@@ -61,6 +61,7 @@ typedef struct RelationAmInfo
 	FmgrInfo	ammarkpos;
 	FmgrInfo	amrestrpos;
 	FmgrInfo	amcanreturn;
+	FmgrInfo	ampeeknexttuple;	/* peek at the next tuple different from any blocknum in pfch_list without reading a new index page */
 } RelationAmInfo;
 
 
--- src/include/pg_config.h.in.orig	2014-06-25 16:37:59.589618919 -0400
+++ src/include/pg_config.h.in	2014-06-25 18:10:51.768523858 -0400
@@ -1,4 +1,4 @@
-/* src/include/pg_config.h.in.  Generated from configure.in by autoheader.  */
+/* src/include/pg_config.h.in.  Generated from - by autoheader.  */
 
 /* Define to the type of arg 1 of 'accept' */
 #undef ACCEPT_TYPE_ARG1
@@ -751,6 +751,10 @@
 /* Define to the appropriate snprintf format for unsigned 64-bit ints. */
 #undef UINT64_FORMAT
 
+/* Define to select librt-style async io and the gcc atomic compare_and_swap.
+   */
+#undef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+
 /* Define to 1 to build with assertion checks. (--enable-cassert) */
 #undef USE_ASSERT_CHECKING
 

--- configure.in.orig	2014-08-18 14:10:36.737016150 -0400
+++ configure.in	2014-08-19 16:56:12.631193389 -0400
@@ -1763,6 +1763,12 @@ operating system;  use --disable-thread-
 fi
 fi
 
+#  test whether this system has both the librt-style async io and the gcc atomic compare_and_swap
+#      and test operation  of the latter.
+PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP
+if test x"$pgac_cv_aio_atomic_builtin_comp_swap" = x"yes"; then
+      AC_DEFINE(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP, 1, [Define to select librt-style async io and the gcc atomic compare_and_swap.])
+fi
 
 # Select semaphore implementation type.
 if test "$PORTNAME" != "win32"; then
--- contrib/pg_prewarm/pg_prewarm.c.orig	2014-08-18 14:10:36.757016243 -0400
+++ contrib/pg_prewarm/pg_prewarm.c	2014-08-19 16:56:12.947194521 -0400
@@ -159,7 +159,7 @@ pg_prewarm(PG_FUNCTION_ARGS)
 		 */
 		for (block = first_block; block <= last_block; ++block)
 		{
-			PrefetchBuffer(rel, forkNumber, block);
+			PrefetchBuffer(rel, forkNumber, block, 0);
 			++blocks_done;
 		}
 #else
--- contrib/pg_stat_statements/pg_stat_statements--1.3.sql.orig	2014-08-19 10:17:32.814616339 -0400
+++ contrib/pg_stat_statements/pg_stat_statements--1.3.sql	2014-08-19 16:56:12.995194693 -0400
@@ -0,0 +1,52 @@
+/* contrib/pg_stat_statements/pg_stat_statements--1.3.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION pg_stat_statements VERSION '1.3'" to load this file. \quit
+
+-- Register functions.
+CREATE FUNCTION pg_stat_statements_reset()
+RETURNS void
+AS 'MODULE_PATHNAME'
+LANGUAGE C;
+
+CREATE FUNCTION pg_stat_statements(IN showtext boolean,
+    OUT userid oid,
+    OUT dbid oid,
+    OUT queryid bigint,
+    OUT query text,
+    OUT calls int8,
+    OUT total_time float8,
+    OUT rows int8,
+    OUT shared_blks_hit int8,
+    OUT shared_blks_read int8,
+    OUT shared_blks_dirtied int8,
+    OUT shared_blks_written int8,
+    OUT local_blks_hit int8,
+    OUT local_blks_read int8,
+    OUT local_blks_dirtied int8,
+    OUT local_blks_written int8,
+    OUT temp_blks_read int8,
+    OUT temp_blks_written int8,
+    OUT blk_read_time float8,
+    OUT blk_write_time float8
+   , OUT aio_read_noneed int8
+   , OUT aio_read_discrd int8
+   , OUT aio_read_forgot int8
+   , OUT aio_read_noblok int8
+   , OUT aio_read_failed int8
+   , OUT aio_read_wasted int8
+   , OUT aio_read_waited int8
+   , OUT aio_read_ontime int8
+)
+RETURNS SETOF record
+AS 'MODULE_PATHNAME', 'pg_stat_statements_1_3'
+LANGUAGE C STRICT VOLATILE;
+
+-- Register a view on the function for ease of use.
+CREATE VIEW pg_stat_statements AS
+  SELECT * FROM pg_stat_statements(true);
+
+GRANT SELECT ON pg_stat_statements TO PUBLIC;
+
+-- Don't want this to be available to non-superusers.
+REVOKE ALL ON FUNCTION pg_stat_statements_reset() FROM PUBLIC;
--- contrib/pg_stat_statements/Makefile.orig	2014-08-18 14:10:36.757016243 -0400
+++ contrib/pg_stat_statements/Makefile	2014-08-19 16:56:13.007194736 -0400
@@ -4,7 +4,8 @@ MODULE_big = pg_stat_statements
 OBJS = pg_stat_statements.o $(WIN32RES)
 
 EXTENSION = pg_stat_statements
-DATA = pg_stat_statements--1.2.sql pg_stat_statements--1.1--1.2.sql \
+DATA = pg_stat_statements--1.3.sql pg_stat_statements--1.2--1.3.sql \
+	pg_stat_statements--1.2.sql pg_stat_statements--1.1--1.2.sql \
 	pg_stat_statements--1.0--1.1.sql pg_stat_statements--unpackaged--1.0.sql
 PGFILEDESC = "pg_stat_statements - execution statistics of SQL statements"
 
--- contrib/pg_stat_statements/pg_stat_statements.c.orig	2014-08-18 14:10:36.757016243 -0400
+++ contrib/pg_stat_statements/pg_stat_statements.c	2014-08-19 16:56:13.051194894 -0400
@@ -116,6 +116,7 @@ typedef enum pgssVersion
 	PGSS_V1_0 = 0,
 	PGSS_V1_1,
 	PGSS_V1_2
+	,PGSS_V1_3
 } pgssVersion;
 
 /*
@@ -147,6 +148,16 @@ typedef struct Counters
 	int64		local_blks_written;		/* # of local disk blocks written */
 	int64		temp_blks_read; /* # of temp blocks read */
 	int64		temp_blks_written;		/* # of temp blocks written */
+
+	int64		aio_read_noneed;		/* # of prefetches for which no need for prefetch as block already in buffer pool  */
+	int64		aio_read_discrd;		/* # of prefetches for which buffer not subsequently read and therefore discarded  */
+	int64		aio_read_forgot;		/* # of prefetches for which buffer not subsequently read and then forgotten about */
+	int64		aio_read_noblok;		/* # of prefetches for which no available BufferAiocb  control block               */
+	int64		aio_read_failed;		/* # of aio reads for which aio itself failed or the read failed with an errno     */
+	int64		aio_read_wasted;		/* # of aio reads for which in-progress aio cancelled and disk block not used      */
+	int64		aio_read_waited;		/* # of aio reads for which disk block used but had to wait for it                 */
+	int64		aio_read_ontime;		/* # of aio reads for which disk block used and ready on time when requested       */
+
 	double		blk_read_time;	/* time spent reading, in msec */
 	double		blk_write_time; /* time spent writing, in msec */
 	double		usage;			/* usage factor */
@@ -274,6 +285,7 @@ void		_PG_fini(void);
 
 PG_FUNCTION_INFO_V1(pg_stat_statements_reset);
 PG_FUNCTION_INFO_V1(pg_stat_statements_1_2);
+PG_FUNCTION_INFO_V1(pg_stat_statements_1_3);
 PG_FUNCTION_INFO_V1(pg_stat_statements);
 
 static void pgss_shmem_startup(void);
@@ -1025,7 +1037,25 @@ pgss_ProcessUtility(Node *parsetree, con
 		bufusage.temp_blks_read =
 			pgBufferUsage.temp_blks_read - bufusage_start.temp_blks_read;
 		bufusage.temp_blks_written =
-			pgBufferUsage.temp_blks_written - bufusage_start.temp_blks_written;
+			pgBufferUsage.temp_blks_written - bufusage.temp_blks_written;
+
+		bufusage.aio_read_noneed =
+			pgBufferUsage.aio_read_noneed - bufusage.aio_read_noneed;
+		bufusage.aio_read_discrd =
+			pgBufferUsage.aio_read_discrd - bufusage.aio_read_discrd;
+		bufusage.aio_read_forgot =
+			pgBufferUsage.aio_read_forgot - bufusage.aio_read_forgot;
+		bufusage.aio_read_noblok =
+			pgBufferUsage.aio_read_noblok - bufusage.aio_read_noblok;
+		bufusage.aio_read_failed =
+			pgBufferUsage.aio_read_failed - bufusage.aio_read_failed;
+		bufusage.aio_read_wasted =
+			pgBufferUsage.aio_read_wasted - bufusage.aio_read_wasted;
+		bufusage.aio_read_waited =
+			pgBufferUsage.aio_read_waited - bufusage.aio_read_waited;
+		bufusage.aio_read_ontime =
+			pgBufferUsage.aio_read_ontime - bufusage.aio_read_ontime;
+
 		bufusage.blk_read_time = pgBufferUsage.blk_read_time;
 		INSTR_TIME_SUBTRACT(bufusage.blk_read_time, bufusage_start.blk_read_time);
 		bufusage.blk_write_time = pgBufferUsage.blk_write_time;
@@ -1040,6 +1070,7 @@ pgss_ProcessUtility(Node *parsetree, con
 				   rows,
 				   &bufusage,
 				   NULL);
+
 	}
 	else
 	{
@@ -1223,6 +1254,16 @@ pgss_store(const char *query, uint32 que
 		e->counters.local_blks_written += bufusage->local_blks_written;
 		e->counters.temp_blks_read += bufusage->temp_blks_read;
 		e->counters.temp_blks_written += bufusage->temp_blks_written;
+
+		e->counters.aio_read_noneed     += bufusage->aio_read_noneed;
+		e->counters.aio_read_discrd     += bufusage->aio_read_discrd;
+		e->counters.aio_read_forgot     += bufusage->aio_read_forgot;
+		e->counters.aio_read_noblok     += bufusage->aio_read_noblok;
+		e->counters.aio_read_failed     += bufusage->aio_read_failed;
+		e->counters.aio_read_wasted     += bufusage->aio_read_wasted;
+		e->counters.aio_read_waited     += bufusage->aio_read_waited;
+		e->counters.aio_read_ontime     += bufusage->aio_read_ontime;
+
 		e->counters.blk_read_time += INSTR_TIME_GET_MILLISEC(bufusage->blk_read_time);
 		e->counters.blk_write_time += INSTR_TIME_GET_MILLISEC(bufusage->blk_write_time);
 		e->counters.usage += USAGE_EXEC(total_time);
@@ -1256,7 +1297,8 @@ pg_stat_statements_reset(PG_FUNCTION_ARG
 #define PG_STAT_STATEMENTS_COLS_V1_0	14
 #define PG_STAT_STATEMENTS_COLS_V1_1	18
 #define PG_STAT_STATEMENTS_COLS_V1_2	19
-#define PG_STAT_STATEMENTS_COLS			19		/* maximum of above */
+#define PG_STAT_STATEMENTS_COLS_V1_3	27
+#define PG_STAT_STATEMENTS_COLS			27		/* maximum of above */
 
 /*
  * Retrieve statement statistics.
@@ -1269,6 +1311,16 @@ pg_stat_statements_reset(PG_FUNCTION_ARG
  * function.  Unfortunately we weren't bright enough to do that for 1.1.
  */
 Datum
+pg_stat_statements_1_3(PG_FUNCTION_ARGS)
+{
+	bool		showtext = PG_GETARG_BOOL(0);
+
+	pg_stat_statements_internal(fcinfo, PGSS_V1_3, showtext);
+
+	return (Datum) 0;
+}
+
+Datum
 pg_stat_statements_1_2(PG_FUNCTION_ARGS)
 {
 	bool		showtext = PG_GETARG_BOOL(0);
@@ -1357,6 +1409,10 @@ pg_stat_statements_internal(FunctionCall
 			if (api_version != PGSS_V1_2)
 				elog(ERROR, "incorrect number of output arguments");
 			break;
+		case PG_STAT_STATEMENTS_COLS_V1_3:
+			if (api_version != PGSS_V1_3)
+				elog(ERROR, "incorrect number of output arguments");
+			break;
 		default:
 			elog(ERROR, "incorrect number of output arguments");
 	}
@@ -1533,11 +1589,24 @@ pg_stat_statements_internal(FunctionCall
 		{
 			values[i++] = Float8GetDatumFast(tmp.blk_read_time);
 			values[i++] = Float8GetDatumFast(tmp.blk_write_time);
+
+			if (api_version >= PGSS_V1_3)
+			{
+				values[i++] = Int64GetDatumFast(tmp.aio_read_noneed);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_discrd);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_forgot);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_noblok);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_failed);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_wasted);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_waited);
+				values[i++] = Int64GetDatumFast(tmp.aio_read_ontime);
+			}
 		}
 
 		Assert(i == (api_version == PGSS_V1_0 ? PG_STAT_STATEMENTS_COLS_V1_0 :
 					 api_version == PGSS_V1_1 ? PG_STAT_STATEMENTS_COLS_V1_1 :
 					 api_version == PGSS_V1_2 ? PG_STAT_STATEMENTS_COLS_V1_2 :
+					 api_version == PGSS_V1_3 ? PG_STAT_STATEMENTS_COLS_V1_3 :
 					 -1 /* fail if you forget to update this assert */ ));
 
 		tuplestore_putvalues(tupstore, tupdesc, values, nulls);
--- contrib/pg_stat_statements/pg_stat_statements--1.2--1.3.sql.orig	2014-08-19 10:17:32.814616339 -0400
+++ contrib/pg_stat_statements/pg_stat_statements--1.2--1.3.sql	2014-08-19 16:56:13.079194994 -0400
@@ -0,0 +1,51 @@
+/* contrib/pg_stat_statements/pg_stat_statements--1.2--1.3.sql */
+
+-- complain if script is sourced in psql, rather than via ALTER EXTENSION
+\echo Use "ALTER EXTENSION pg_stat_statements UPDATE TO '1.3'" to load this file. \quit
+
+/* First we have to remove them from the extension */
+ALTER EXTENSION pg_stat_statements DROP VIEW pg_stat_statements;
+ALTER EXTENSION pg_stat_statements DROP FUNCTION pg_stat_statements();
+
+/* Then we can drop them */
+DROP VIEW pg_stat_statements;
+DROP FUNCTION pg_stat_statements();
+
+/* Now redefine */
+CREATE FUNCTION pg_stat_statements(IN showtext boolean,
+    OUT userid oid,
+    OUT dbid oid,
+    OUT queryid bigint,
+    OUT query text,
+    OUT calls int8,
+    OUT total_time float8,
+    OUT rows int8,
+    OUT shared_blks_hit int8,
+    OUT shared_blks_read int8,
+    OUT shared_blks_dirtied int8,
+    OUT shared_blks_written int8,
+    OUT local_blks_hit int8,
+    OUT local_blks_read int8,
+    OUT local_blks_dirtied int8,
+    OUT local_blks_written int8,
+    OUT temp_blks_read int8,
+    OUT temp_blks_written int8,
+    OUT blk_read_time float8,
+    OUT blk_write_time float8
+   , OUT aio_read_noneed int8
+   , OUT aio_read_discrd int8
+   , OUT aio_read_forgot int8
+   , OUT aio_read_noblok int8
+   , OUT aio_read_failed int8
+   , OUT aio_read_wasted int8
+   , OUT aio_read_waited int8
+   , OUT aio_read_ontime int8
+)
+RETURNS SETOF record
+AS 'MODULE_PATHNAME', 'pg_stat_statements_1_3'
+LANGUAGE C STRICT VOLATILE;
+
+CREATE VIEW pg_stat_statements AS
+  SELECT * FROM pg_stat_statements(true);
+
+GRANT SELECT ON pg_stat_statements TO PUBLIC;
--- postgresql-prefetching-asyncio.README.orig	2014-08-19 10:17:32.814616339 -0400
+++ postgresql-prefetching-asyncio.README	2014-08-19 16:56:13.175195339 -0400
@@ -0,0 +1,618 @@
+Postgresql  --   Extended Prefetching using Asynchronous IO
+============================================================
+
+Postgresql currently (9.3.4) provides a limited prefetching capability
+using posix_fadvise to give hints to the Operating System kernel
+about which pages it expects to read in the near future.
+This capability is used only during the heap-scan phase of bitmap-index scans.
+It is controlled via the effective_io_concurrency configuration parameter.
+
+This capability is now extended in two ways :
+   .   use asynchronous IO into Postgresql shared buffers as an
+       alternative to posix_fadvise
+   .   Implement prefetching in other types of scan :
+            .  non-bitmap (i.e. simple) index scans - index pages
+                     currently only for B-tree indexes.
+                    (developed by Claudio Freire <klaussfreire(at)gmail(dot)com>)
+            .  non-bitmap (i.e. simple) index scans - heap pages
+                          currently only for B-tree indexes.
+            .  simple heap scans
+
+Posix asynchronous IO is chosen as the function library for asynchronous IO,
+since this is well supported and also fits very well with the model of
+the prefetching process,  particularly as regards checking for completion
+of an asynchronous read.    On linux,   Posix asynchronous IO is provided
+in the librt library.    librt uses independently-schedulable threads to
+achieve the asynchronicity,   rather than kernel functionality.
+
+In this implementation,  use of asynchronous IO is limited to prefetching
+while performing one of the three types of scan
+            .  B-tree bitmap index scan - heap pages    (as already exists)
+            .  B-tree non-bitmap (i.e. simple) index scans - index and heap pages
+            .  simple heap scans
+on permanent relations.   It is not used on temporary tables nor for writes.
+
+The advantages of Posix asynchronous IO into shared buffers
+compared to posix_fadvise are :
+   .   Beneficial for non-sequential access patterns as well as sequential
+   .   No restriction on the kinds of IO which can be used
+       (other kinds of asynchronous IO impose restrictions such as
+        buffer alignment,  use of non-buffered IO).
+   .   Does not interfere with standard linux kernel read-ahead functionality.
+       (It has been stated in 
+ www.postgresql.org/message-id/CAGTBQpbu2M=-M7NUr6DWr0K8gUVmXVhwKohB-Cnj7kYS1AhH4A@mail.gmail.com
+       that :
+          "the kernel stops doing read-ahead when a call to posix_fadvise comes.
+           I noticed the performance hit, and checked the kernel's code.
+           It effectively changes the prediction mode from sequential to fadvise,
+           negating the (assumed) kernel's prefetch logic")
+   .   When the read request is issued after a prefetch has completed,
+       no delay associated with a kernel call to copy the page from
+       kernel page buffers into the Postgresql shared buffer,
+       since it is already there.
+       Also,   in a memory-constrained environment,   there is a greater
+       probability that the prefetched page will "stick" in memory
+       since the linux kernel victimizes the filesystem page cache in preference
+       to swapping out user process pages.
+   .   Statistics on prefetch success can be gathered (see "Statistics" below)
+       which helps the administrator to tune the prefetching settings.
+
+These benefits are most likely to be obtained in a system whose usage profile
+(e.g. from iostat)  shows:
+     .   high IO wait from mostly-read activity
+     .   disk access pattern is not entirely sequential
+         (so kernel readahead can't predict it but postgresql can)
+     .   sufficient spare idle CPU to run the librt pthreads
+         or,  stated another way,    the CPU subsystem is relatively powerful
+         compared to the disk subsystem.
+In such ideal conditions,  and with a workload with plenty of index scans,
+around 10% - 20% improvement in throughput has been achieved.
+In an admittedly extreme environment measured by this author,    with a workload
+consisting of 8 client applications each running similar complex queries
+(same query structure but different predicates and constants),
+including 2 Bitmap Index Scans and 17 non-bitmap index scans,
+on a dual-core Intel laptop (4 hyperthreads) with the database on a single
+USB3-attached 500GB disk drive, and no part of the database in filesystem buffers
+initially,  (filesystem freshly mounted),  comparing unpatched build
+using posix_fadvise with effective_io_concurrency 4 against same build patched
+with async IO and effective_io_concurrency 4 and max_async_io_prefetchers 32,
+elapse time repeatably improved from around 640-670 seconds to around 530-550 seconds,
+a 17% - 18% improvement. 
+
+The disadvantages of Posix asynchronous IO compared to posix_fadvise are:
+     .   probably higher CPU utilization:
+         Firstly, the extra work performed by the librt threads adds CPU
+         overhead, and secondly, if the asynchronous prefetching is effective,
+         then it will deliver better (greater) overlap of CPU with IO, which
+         will reduce elapsed times and hence increase CPU utilization percentage
+         still more (during that shorter elapsed time).
+     .   more context switching,  because of the additional threads.
+
+
+Statistics:
+___________
+
+A number of additional statistics relating to effectiveness of asynchronous IO
+are provided as an extension of the existing pg_stat_statements loadable module.
+Refer to the appendix "Additional Supplied Modules" in the current
+PostgreSQL Documentation for details of this module.
+
+The following additional statistics are provided for asynchronous IO prefetching:
+
+    . aio_read_noneed  :   number of prefetches for which no need for prefetch as block already in buffer pool
+    . aio_read_discrd  :   number of prefetches for which buffer not subsequently read and therefore discarded
+    . aio_read_forgot  :   number of prefetches for which buffer not subsequently read and then forgotten about
+    . aio_read_noblok  :   number of prefetches for which no available BufferAiocb  control block
+    . aio_read_failed  :   number of aio reads for which aio itself failed or the read failed with an errno
+    . aio_read_wasted  :   number of aio reads for which in-progress aio cancelled and disk block not used
+    . aio_read_waited  :   number of aio reads for which disk block used but had to wait for it
+    . aio_read_ontime  :   number of aio reads for which disk block used and ready on time when requested
+
+Some of these are (hopefully) self-explanatory.    Some additional notes:
+
+    . aio_read_discrd and aio_read_forgot  :
+                    prefetch was wasted work since the buffer was not subsequently read
+                    The discrd case indicates that the scanner realized this and discarded the buffer,
+                    whereas the forgot case indicates that the scanner did not realize it,
+                    which should not normally occur.
+                    A high number in either suggests lowering effective_io_concurrency.
+
+    . aio_read_noblok  :   
+                    Any significant number in relation to all the other numbers indicates that
+                    max_async_io_prefetchers should be increased.
+
+    . aio_read_waited  :
+                    The page was prefetched but the asynchronous read had not completed by the time the
+                    scanner requested to read it.     causes extra overhead in waiting and indicates
+                    prefetching is not providing much if any benefit.
+                    The disk subsystem may be underpowered/overloaded in relation to the available CPU power.
+
+    . aio_read_ontime  :
+                    The page was prefetched and the asynchronous read had completed by the time the
+                    scanner requested to read it.     Optimal behaviour.      If this number if large
+                    in relation to all the other numbers except (possibly) aio_read_noneed,
+                    then prefetching is working well.
+
+To create the extension with support for these additional statistics, use the following syntax:
+     CREATE EXTENSION pg_stat_statements VERSION '1.3'
+or,  if you run the new code against an existing database which already has the extension
+( see installation and migration below ),  you can 
+     ALTER EXTENSION pg_stat_statements UPDATE TO '1.3'
+
+A suggested set of commands for displaying these statistics might be :
+
+ /*  OPTIONALLY */ DROP extension pg_stat_statements;
+                   CREATE extension pg_stat_statements VERSION '1.3';
+ /*  run your workload   */
+                  select userid , dbid , substring(query from 1 for 24) , calls , total_time , rows , shared_blks_read , blk_read_time , blk_write_time \
+                    , aio_read_noneed , aio_read_noblok , aio_read_failed , aio_read_wasted , aio_read_waited , aio_read_ontime , aio_read_forgot       \
+                      from pg_stat_statements where shared_blks_read > 0;
+
+
+Installation and Build Configuration:
+_____________________________________
+
+1. First -  a prerequsite:
+#  as well as requiring all the usual package build tools such as gcc , make etc,
+#  as described in the instructions for building postgresql,
+#  the following is required :
+    gnu autoconf at version 2.69 :
+# run the following command
+autoconf -V
+# it *must* return
+autoconf (GNU Autoconf) 2.69
+
+2. If you don't have it or it is a different version,
+then you must obtain version 2.69 (which is the current version)
+from your distribution provider or from the gnu software download site.
+
+3. Also you must have the source tree for postgresql version 9.4 (development version).
+#   all the following commands assume your current working directory is the top of the source tree.
+
+4. cd to top of source tree :
+#   check it appears to be a postgresql source tree
+ls -ld configure.in src
+#   should show both the file and the directory
+grep PostgreSQL COPYRIGHT
+#   should show PostgreSQL Database Management System
+
+5. Apply the patch :
+patch -b -p0 -i <patch_file_path>
+#   should report no errors, 48 files patched (see list at bottom of this README)
+#   and all hunks applied
+#  check the patch was appplied to configure.in
+ls -ld configure.in.orig configure.in
+#   should show both files
+
+6. Rebuild the configure script with the patched configure.in :
+mv configure configure.orig;
+autoconf configure.in >configure;echo "rc= $? from autoconf"; chmod +x configure;
+ls -lrt configure.orig configure;
+
+7. run the new configure script :
+#   if you have run configure before,
+#   then you may first want to save existing config.status and config.log if they exist,
+#   and then specify same configure flags and options as you specified before.
+#   the patch does not alter or extend the set of configure options
+#   if unsure,   run ./configure --help
+#   if still unsure,   run ./configure
+./configure <other configure options as desired>
+
+
+
+8. now check that configure decided that this environment supports asynchronous IO :
+grep USE_AIO_ATOMIC_BUILTIN_COMP_SWAP src/include/pg_config.h
+#  it should show
+#define USE_AIO_ATOMIC_BUILTIN_COMP_SWAP 1
+#  if not,  apparently your environment does not support asynch IO  -
+#  the config.log will show how it came to that conclusion,
+#  also check for :
+#    . a librt.so somewhere in the loader's library path (probably under /lib , /lib64 , or /usr)
+#    . your gcc must support the atomic compare_and_swap __sync_bool_compare_and_swap built-in function
+#  do not proceed without this define being set.
+
+9. do you want to use the new code on an existing cluster
+   that was created using the same code base but without the patch?
+   If so then run this nasty-looking command :
+   (cut-and-paste it into a terminal window or a shell-script file)
+   Otherwise continue to step 10.
+   see Migration note below for explanation.
+###############################################################################################
+   fl=src/Makefile.global; typeset -i bkx=0; while [[ $bkx < 200 ]]; do {
+       bkfl="${fl}.bak${bkx}"; if [[ -a ${bkfl} ]]; then ((bkx=bkx+1)); else break; fi;
+   }; done;
+   if [[ -a ${bkfl} ]]; then echo "sorry cannot find a backup name for $fl";
+   elif [[ -a $fl ]]; then {
+       mv $fl $bkfl && {
+          sed -e "/^CFLAGS =/ s/\$/ -DAVOID_CATALOG_MIGRATION_FOR_ASYNCIO/" $bkfl > $fl;
+          str="diff -w $bkfl $fl";echo "$str"; eval "$str";
+       };
+   };
+   else echo "ooopppss $fl is missing";
+   fi;
+###############################################################################################
+# it should report something like
+diff -w Makefile.global.bak0 Makefile.global
+222c222
+< CFLAGS = XXXX
+---
+> CFLAGS = XXXX -DAVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+#   where XXXX is some set of flags
+
+
+10. now run the rest of the build process as usual  -
+    follow instructions in file INSTALL if that file exists,
+    else e.g. run
+make && make install
+
+If the build fails with the following error:
+undefined reference to `aio_init'
+Then edit the following file
+src/include/pg_config_manual.h
+and add the following line at the bottom:
+
+#define DONT_HAVE_AIO_INIT
+
+and then run
+make clean && make && make install
+See notes to section Runtime Configuration below for more information on this.
+
+If you would like to use the sigevent mechanism for signalling completion
+of asynchronous io to non-originating backends,  instead of the polling method,
+(see section Checking AIO Completion below)
+then add these lines to src/include/pg_config_manual.h
+
+#define USE_AIO_SIGEVENT 1
+#define AIO_SIGEVENT_SIGNALNUM SIGIO  /* or signal num of your choice */
+
+Here's the context
+
+/*
+ * USE_PREFETCH code should be compiled only if we have a way to implement
+ * prefetching.  (This is decoupled from USE_POSIX_FADVISE because there
+ * might in future be support for alternative low-level prefetch APIs  --
+ * -- update October 2013  -- now there is such a new prefetch capability --
+ *   async_io into postgres buffers  -   configuration parameter max_async_io_threads)
+ */
+#if defined(USE_POSIX_FADVISE) || defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP)
+#define USE_AIO_SIGEVENT 1
+/* AIO_SIGEVENT_SIGNALNUM is the signal used to indicate completion
+ * of an aio operation.  Choose a signal that is not used elsewhere
+ * in postgresql and which can be caught by signal handler.
+*/
+#define AIO_SIGEVENT_SIGNALNUM SIGIO
+#define USE_PREFETCH
+#endif
+
+
+
+
+Migration , Runtime Configuration, and Use:
+___________________________________________
+
+
+Database Migration:
+___________________
+
+The new prefetching code for non-bitmap index scans introduces a new btree-index
+function named btpeeknexttuple.    The correct way to add such a function involves
+also adding it to the catalog as an internal function in pg_proc.
+However,  this results in the new built code considering an existing database to be
+incompatible,  i.e requiring backup on the old code and restore on the new.
+This is normal behaviour for migration to a new version of postgresql,  and is
+also a valid way of migrating a database for use with this asynchronous IO feature,
+but in this case it may be inconvenient.
+
+As an alternative,    the new code may be compiled with the macro define
+AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+which does what it says by not altering the catalog.   The patched build can then
+be run against an existing database cluster initdb'd using the unpatched build.
+
+There are no known ill-effects of so doing,  but :
+     .  in any case,  it is strongly suggested to make a backup of any precious database
+        before accessing it with a patched build
+     .  be aware that if this asynchronous IO feature is eventually released as part of postgresql,
+        migration will probably be required anyway.
+
+This option to avoid catalog migration is intended as a convenience for a quick test,
+and also makes it easier to obtain performance comparisons on the same database.
+
+
+
+Runtime Configuration:
+______________________
+
+One new configuration parameter settable in postgresql.conf and
+in any other way as described in the postgresql documentation :
+
+max_async_io_prefetchers
+  Maximum number of background processes concurrently using asynchronous
+  librt threads to prefetch pages into shared memory buffers
+
+This number can be thought of as the maximum number
+of librt threads concurrently active,   each working on a list of
+from 1 to target_prefetch_pages pages ( see notes 1 and 2 ).
+
+In practice,    this number simply controls how many prefetch requests in total
+may be active concurrently :
+        max_async_io_prefetchers * target_prefetch_pages ( see note 1)
+
+default is max_connections/6
+and recall that the default for max_connections is 100
+
+
+note 1  a number based on effective_io_concurrency and approximately n * ln(n)
+        where n is effective_io_concurrency
+
+note 2  Provided that the gnu extension to Posix AIO which provides the
+aio_init() function is present,   then aio_init() is called
+to set the librt maximum number of threads to max_async_io_prefetchers,
+and to set the maximum number of concurrent aio read requests to the product of
+        max_async_io_prefetchers * target_prefetch_pages
+
+
+As well as this regular configuration parameter,
+there are several other parameters that can be set via environment variable.
+The reason why they are environment vars rather than regular configuration parameters
+is that it is not expected that they should need to be set,   but they may be useful :
+                variable name         values                  default        meaning
+   PG_TRY_PREFETCHING_FOR_BITMAP      [Y|N]                    Y         whether to prefetch bitmap heap scans
+   PG_TRY_PREFETCHING_FOR_ISCAN       [Y|N|integer[,[N|Y]]]   256,N      whether to prefetch  non-bitmap index scans
+                                                                    also numeric size of list of prefetched blocks
+                                                                    also whether to prefetch forward-sequential-pattern index pages
+   PG_TRY_PREFETCHING_FOR_BTREE       [Y|N]                    Y         whether to prefetch heap pages in non-bitmap index scans
+   PG_TRY_PREFETCHING_FOR_HEAP        [Y|N]                    N         whether to prefetch relation (un-indexed) heap scans
+
+
+The setting for PG_TRY_PREFETCHING_FOR_ISCAN is a litle complicated.
+It can be set to Y or N to control prefetching of  non-bitmap index scans;
+But in addition it can be set to an integer,   which both implies Y
+and also sets the size of a list used to remember prefetched but unread heap pages.
+This list is an optimization used to avoid re-prefetching and maximise the potential
+set of prefetchable blocks indexed by one index page.
+And if set to an integer,  this integer may be followed by either ,Y or ,N
+to specify to prefetch index pages which are being accessed forward-sequentially.
+It has been found that prefetching is not of great benefit for this access pattern,
+and so it is not the default,  but also does no harm (provided sufficient CPU capacity).
+
+
+
+Usage :
+______
+
+
+There are no changes in usage other than as noted under Configuration and Statistics.
+However,   in order to assess benefit from this feature,   it will be useful to
+understand the query access plans of your workload using EXPLAIN.    Before doing that,
+make sure that statistics are up to date using ANALYZE.
+
+
+
+Internals:
+__________
+
+
+Internal changes span two areas and the interface between them :
+
+ .  buffer manager layer
+ .  programming interface for scanner to call buffer manager
+ .  scanner layer
+
+ .  buffer manager layer
+    ____________________
+
+    changes comprise :
+       .   allocating,  pinning , unpinning buffers
+            this is complex and discussed briefly below in "Buffer Management"
+       .   acquiring and releasing a BufferAiocb, the control block
+            associated with a single aio_read,  and checking for its completion
+            a new file,  backend/storage/buffer/buf_async.c, provides three new functions,
+                  BufStartAsync        BufReleaseAsync            BufCheckAsync
+            which handle this.
+       .   calling librt asynch io functions
+            this follows the example of all other filesystem interfaces
+            and is straightforward.    
+            two new functions are provided in fd.c:
+                   FileStartaio        FileCompleteaio
+            and corresponding interfaces in smgr.c
+
+ .  programming interface for scanner to call buffer manager
+    ________________________________________________________
+       . calling interface for existing function PrefetchBuffer is modified :
+           .  one new argument,   BufferAccessStrategy strategy
+           .  now returns an int return code which indicates :
+                     whether pin count on buffer has been increased by 1
+                     whether block was already present in a buffer
+       .  new function DiscardBuffer
+           .  discard buffer used for a previously prefetched page
+                 which scanner decides it does not want to read.
+           .  same arguments as for PrefetchBuffer except for omission of BufferAccessStrategy
+           .  note - this is different from the existing function ReleaseBuffer
+                     in that ReleaseBuffer takes a buffer_descriptor as argument
+                     for a buffer which has been read, but has similar purpose.
+
+ .  scanner layer
+    _____________
+        common to all scanners is that the scanner which wishes to prefetch must do two things:
+          .  decide which pages to prefetch and call PrefetchBuffer to prefetch them
+                 nodeBitmapHeapscan already does this (but note one extra argument on PrefetchBuffer)
+          .  remember which pages it has prefetched in some list (actual or conceptual,  e.g. a page range),
+                 removing each page from this list if and when it subsequently reads the page.
+          .  at end of scan,  call DiscardBuffer for every remembered (i.e. prefetched not unread) page
+       how this list of prefetched pages is implemented varies for each of the three scanners and four scan types:
+            .  bitmap index scan - heap pages
+            .  non-bitmap (i.e. simple) index scans - index pages
+            .  non-bitmap (i.e. simple) index scans - heap pages
+            .  simple heap scans
+       The consequences of forgetting to call DiscardBuffer on a prefetched but unread page are:
+            .   counted in aio_read_forgot  (see "Statistics" above)
+            .   may incur an annoying but harmless warning in the pg_log "Buffer Leak ... "
+                  (the buffer is released at commit)
+       This does sometimes happen ...
+     
+
+
+Buffer Management
+_________________
+
+With async io,   PrefetchBuffer must allocate and pin a buffer,  which is relatively straightforward,
+but also every other part of buffer manager must know about the possibility that a buffer may be in
+a state of async_io_in_progress state and be prepared to determine the possible completion.
+That is,  one backend BK1 may start the io but another BK2 may try to read it before BK1 does.
+Posix Asynchronous IO provides a means for waiting on this or another task's read if in progress,
+namely aio_suspend(),  which this extension uses.    Therefore,  although StartBufferIO and TerminateBufferIO
+are called as part of asynchronous prefetching,   their role is limited to maintaining the buffer descriptor flags,
+and they do not track the asynchronous IO itself.    Instead,   asynchronous IOs are tracked in
+a separate set of shared control blocks,  the BufferAiocb list -
+refer to     include/storage/buf_internals.h
+Checking asynchronous io status is handled in  backend/storage/buffer/buf_async.c BufCheckAsync function.
+Read the commentary for this function for more details.
+
+Checking AIO Completion
+_______________________
+There is a difference in how completion is checked depending on whether the backend doing the checking is :
+    .   the same as the backend which started the asynch io (the "originator")
+    .   a different backend ("non-originator")
+The "originator" case is most common and also simplest  -
+      FileCompleteaio simply issues the appriopriate aio_xxxx calls and suspends if not complete.
+The "non-originator" case is more complex and three methods are currently designed and implemented :
+    .  polling the aiocb for completion
+          This is currently the default method,  and the simplest,
+          but suffers from the CPU overhead of frequent polling.
+    .  /*  discarded approach ....     LWlock method
+          use of LWlocks and sigevent to cause a signal to be delivered to the originator.
+          The originator locks eXclusive at aio start and releases after delivery of the signal.
+          The non-originator locks Shared and releases.
+       */
+       The above approach was discarded because :
+        A. it relied on the associated signal being delivered for every originated aio --
+            but it turns out that the kernel does not always deliver every
+           requested signal.     This resulted in gradual depletion of aiocbs.
+        B. Furthermore,  use of LWlocks for synchronization,   although being consistent
+           with synchronous IO, prevent timely posting of waiters, because the LWLockRelease
+           call cannot be safely performed inside the signal handler and must be deferred to
+           the next CHECK_INTERRUPTS(), which might be delayed if e.g. the backend is itself
+           waiting on some LWlock.
+    .  /*  alternative approach ...  */   sigevent + sigsuspend method
+       This method also uses sigevent to cause a signal to be delivered to the originator.
+       However, to overcome the problems associated with the discarded LWlock method,
+       signals are also used for wait/post:
+        A. Instead of the LWlock mechanism,  waiters will wait by 
+           chaining themselves onto a chain of PGPROCs anchored in the BAiocb,
+           and then calling sigsuspend() to wait to be posted by originator.
+        B. Each backend will keep track of which aios it has originated,
+           and,   whenever any sigevent is delivered,   check all outstanding aios
+           which it originated prior to and including the one associated with the sigevent.
+           For each completed aio,  it will run the chain of waiter PROCs
+           and unchain and post each one by simple kill().
+           The same signal number can be used for this as for the sigevent,
+           by default SIGIO.
+        C. Note that although this method does not depend on every requested signal
+                being delivered,  it *does* rely on the following :
+                for *every* originated aio_read,   initiated at time T say,
+                at least one sigevent shall be delivered to originator at some time T+ > T
+
+
+Pinning and unpinning of buffers is the most complex aspect of asynch io prefetching,
+and the logic is spread throughout BufStartAsync , BufCheckAsync , and many functions in bufmgr.c.
+When a backend BK2 requests ReadBuffer of a page for which asynch read is in progress,
+buffer manager has to determine which backend BK1 pinned this buffer during previous PrefetchBuffer,
+and for example must not be re-pinned a second time if BK2 is BK1.
+Information concerning which backend initiated the prefetch is held in the BufferAiocb.
+
+The trickiest case concerns the scenario in which :
+   .  BK1 initiates prefetch and acquires a pin
+   .  BK2 possibly waits for completion and then reads the buffer,  and perhaps later on
+         releases it by ReleaseBuffer.
+   .  Since the asynchronous IO is no longer in progress,     there is no longer any
+         BufferAiocb associated with it.    Yet buffer manager must remember that BK1 holds a
+         "prefetch" pin, i.e. a pin which must not be repeated if and when BK1 finally issues ReadBuffer.
+   .  The solution to this problem is to invent the concept of a "banked" pin,
+      which is a pin obtained when prefetch was issued,   identied as in "banked" status only if and when
+      the associated asynchronous IO terminates,  and redeemable by the next use by same task,
+      either by ReadBuffer or DiscardBuffer.
+      The pid of the backend which holds a banked pin on a buffer (there can be at most one such backend)
+      is stored in the buffer descriptor.
+      This is done without increasing size of the buffer descriptor,  which is important since
+      there may be a very large number of these.     This does overload the relevant field in the descriptor.
+      Refer to include/storage/buf_internals.h for more details
+      and search for BM_AIO_PREFETCH_PIN_BANKED in storage/buffer/bufmgr.c and  backend/storage/buffer/buf_async.c
+
+______________________________________________________________________________
+The following 46 files are changed in this feature (output of the patch command) :
+
+patching file configure.in
+patching file contrib/pg_prewarm/pg_prewarm.c
+patching file contrib/pg_stat_statements/pg_stat_statements--1.3.sql
+patching file contrib/pg_stat_statements/Makefile
+patching file contrib/pg_stat_statements/pg_stat_statements.c
+patching file contrib/pg_stat_statements/pg_stat_statements--1.2--1.3.sql
+patching file postgresql-prefetching-asyncio.README
+patching file config/c-library.m4
+patching file src/backend/postmaster/postmaster.c
+patching file src/backend/executor/nodeBitmapHeapscan.c
+patching file src/backend/executor/nodeIndexscan.c
+patching file src/backend/executor/instrument.c
+patching file src/backend/storage/buffer/Makefile
+patching file src/backend/storage/buffer/bufmgr.c
+patching file src/backend/storage/buffer/buf_async.c
+patching file src/backend/storage/buffer/buf_init.c
+patching file src/backend/storage/smgr/md.c
+patching file src/backend/storage/smgr/smgr.c
+patching file src/backend/storage/file/fd.c
+patching file src/backend/storage/lmgr/proc.c
+patching file src/backend/access/heap/heapam.c
+patching file src/backend/access/heap/syncscan.c
+patching file src/backend/access/index/indexam.c
+patching file src/backend/access/index/genam.c
+patching file src/backend/access/nbtree/nbtsearch.c
+patching file src/backend/access/nbtree/nbtinsert.c
+patching file src/backend/access/nbtree/nbtpage.c
+patching file src/backend/access/nbtree/nbtree.c
+patching file src/backend/nodes/tidbitmap.c
+patching file src/backend/utils/misc/guc.c
+patching file src/backend/utils/mmgr/aset.c
+patching file src/include/executor/instrument.h
+patching file src/include/storage/bufmgr.h
+patching file src/include/storage/proc.h
+patching file src/include/storage/smgr.h
+patching file src/include/storage/fd.h
+patching file src/include/storage/buf_internals.h
+patching file src/include/catalog/pg_am.h
+patching file src/include/catalog/pg_proc.h
+patching file src/include/pg_config_manual.h
+patching file src/include/access/nbtree.h
+patching file src/include/access/heapam.h
+patching file src/include/access/relscan.h
+patching file src/include/nodes/tidbitmap.h
+patching file src/include/utils/rel.h
+patching file src/include/pg_config.h.in
+
+
+Future Possibilities:
+____________________
+
+There are several possible extensions of this feature :
+   .   Extend prefetching of index scans to types of index
+       other than B-tree.
+       This should be fairly straightforward,  but requires some
+       good base of benchmarkable workloads to prove the value.
+   .   Investigate why asynchronous IO prefetching does not greatly
+       improve sequential relation heap scans and possibly find how to
+       achieve a benefit.
+   .   Build knowledge of asycnhronous IO prefetching into the
+       Query Planner costing.
+       This is far from straightforward.    The Postgresql Query Planner's
+       costing model is based on resource consumption rather than elapsed time.
+       Use of asynchronous IO prefetching is intended to improve elapsed time
+       as the expense of (probably) higher resource consumption.
+       Although Costing understands about the reduced cost of reading buffered
+       blocks, it does not take asynchronicity or overlap of CPU with disk
+       into account.  A naive approach might be to try to tweak the Query
+       Planner's Cost Constant configuration parameters
+       such as seq_page_cost , random_page_cost
+       but this is hazardous as explained in the Documentation.
+
+
+
+John Lumby,  johnlumby(at)hotmail(dot)com
--- config/c-library.m4.orig	2014-08-18 14:10:36.733016131 -0400
+++ config/c-library.m4	2014-08-19 16:56:13.243195582 -0400
@@ -367,3 +367,152 @@ if test "$pgac_cv_type_locale_t" = 'yes
   AC_DEFINE(LOCALE_T_IN_XLOCALE, 1,
             [Define to 1 if `locale_t' requires <xlocale.h>.])
 fi])])# PGAC_HEADER_XLOCALE
+
+
+# PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP
+# ---------------------------------------
+# test whether this system has both the librt-style async io and the gcc atomic compare_and_swap
+#      and test operation of both,
+#      including verifying that aio_error can retrieve completion status
+#      of aio_read issued by a different process
+#
+AC_DEFUN([PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP],
+[AC_MSG_CHECKING([whether have both librt-style async io and the gcc atomic compare_and_swap])
+AC_CACHE_VAL(pgac_cv_aio_atomic_builtin_comp_swap,
+pgac_save_LIBS=$LIBS
+LIBS=" -lrt $pgac_save_LIBS"
+[AC_TRY_RUN([#include <stdio.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+#include <fcntl.h>
+#include "aio.h"
+#include <errno.h>
+
+char *shmem;
+
+/*  returns rc of aio_read or -1 if some error */
+int
+processA(void)
+{
+	int fd , rc;
+	struct aiocb *aiocbp = (struct aiocb *) shmem;
+	char *buf = shmem + sizeof(struct aiocb);
+
+	rc = fd = open("configure", O_RDONLY );
+	if (fd != -1) {
+
+            memset(aiocbp, 0, sizeof(struct aiocb));
+            aiocbp->aio_fildes = fd;
+            aiocbp->aio_offset = 0;
+            aiocbp->aio_buf = buf;
+            aiocbp->aio_nbytes = 8;
+            aiocbp->aio_reqprio = 0;
+            aiocbp->aio_sigevent.sigev_notify = SIGEV_NONE;
+
+            rc = aio_read(aiocbp);
+	}
+        return rc;
+}
+
+/*  returns result of aio_error  -  0 if io completed successfully */
+int
+processB(void)
+{
+	struct aiocb *aiocbp = (struct aiocb *) shmem;
+	const struct aiocb * const pl[1] = { aiocbp };
+	int rv;
+	int	returnCode;
+        struct timespec my_timeout = { 0 , 10000 };
+        int max_iters , max_polls;
+
+	rv = aio_error(aiocbp);
+        max_iters = 100;
+        while ( (max_iters-- > 0) && (rv == EINPROGRESS) ) {
+                max_polls = 256;
+                my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                returnCode = aio_suspend(pl , 1 , &my_timeout);
+                while ((returnCode < 0) && (EAGAIN == errno) && (max_polls-- > 0)) {
+                    my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                    returnCode = aio_suspend(pl , 1 , &my_timeout);
+                }
+                rv = aio_error(aiocbp);
+        }
+
+	return rv;
+}
+
+int main (int argc, char *argv[])
+ {
+   int rc;
+   int pidB;
+   int child_status;
+   struct aiocb volatile * first_aiocb;
+   struct aiocb volatile * second_aiocb;
+   struct aiocb volatile * my_aiocbp = (struct aiocb *)20000008;
+
+   first_aiocb = (struct aiocb *)20000008;
+   second_aiocb = (struct aiocb *)40000008;
+
+   /*  first test  --  __sync_bool_compare_and_swap
+   **  set zero as success if two comp-swaps both worked as expected -
+   **  first compares equal and swaps,  second compares unequal
+   */
+   rc = (__sync_bool_compare_and_swap (&my_aiocbp, first_aiocb, second_aiocb));
+   if (rc) {
+      rc = (__sync_bool_compare_and_swap (&my_aiocbp, first_aiocb, second_aiocb));
+   } else {
+      rc = -1;
+   }
+
+   if (rc == 0) {
+       /*  second test  --  process A start aio_read
+       **  and process B checks completion by polling
+       */
+        rc = -1; /* pessimistic */
+
+	shmem = mmap(NULL, sizeof(struct aiocb) + 2048,
+				 PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS,
+				 -1, 0);
+	if (shmem != MAP_FAILED) {
+            
+            /*
+             * Start the I/O request in parent process, then fork and try to wait
+             * for it to finish from the child process.
+             */
+            rc = processA();
+            if (rc >= 0) {
+
+                rc = pidB = fork();
+                if (pidB != -1) {
+                    if (pidB != 0) {
+                        /* parent */
+                        wait (&child_status);
+                        if (WIFEXITED(child_status)) {
+                            rc = WEXITSTATUS(child_status);
+                        }
+                    } else {
+                        /* child */
+                        rc = processB();
+                        exit(rc);
+                    }
+                }
+            }
+        }
+   }
+
+   return rc;
+}],
+[pgac_cv_aio_atomic_builtin_comp_swap=yes],
+[pgac_cv_aio_atomic_builtin_comp_swap=no],
+[pgac_cv_aio_atomic_builtin_comp_swap=cross])
+])dnl AC_CACHE_VAL
+AC_MSG_RESULT([$pgac_cv_aio_atomic_builtin_comp_swap])
+if test x"$pgac_cv_aio_atomic_builtin_comp_swap" != x"yes"; then
+LIBS=$pgac_save_LIBS
+fi
+])# PGAC_FUNC_AIO_ATOMIC_BUILTIN_COMP_SWAP
--- src/backend/postmaster/postmaster.c.orig	2014-08-18 14:10:36.925017033 -0400
+++ src/backend/postmaster/postmaster.c	2014-08-19 16:56:13.315195840 -0400
@@ -123,6 +123,11 @@
 #include "storage/spin.h"
 #endif
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+void ReportFreeBAiocbs(void);
+int CountInuseBAiocbs(void);
+extern int hwmBufferAiocbs;         /*  high water mark of in-use  BufferAiocbs in pool           */
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * Possible types of a backend. Beyond being the possible bkend_type values in
@@ -1489,9 +1494,15 @@ ServerLoop(void)
 	fd_set		readmask;
 	int			nSockets;
 	time_t		now,
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                           count_baiocb_time,
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 				last_touch_time;
 
 	last_touch_time = time(NULL);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        count_baiocb_time = time(NULL);
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 	nSockets = initMasks(&readmask);
 
@@ -1650,6 +1661,19 @@ ServerLoop(void)
 			last_touch_time = now;
 		}
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                /*   maintain the hwm of used baiocbs every 10 seconds  */
+		if ((now - count_baiocb_time) >= 10)
+		{
+                        int inuseBufferAiocbs;         /*  current in-use  BufferAiocbs in pool */
+                        inuseBufferAiocbs = CountInuseBAiocbs();
+                        if (inuseBufferAiocbs > hwmBufferAiocbs) {
+			    hwmBufferAiocbs = inuseBufferAiocbs;
+			}
+			count_baiocb_time = now;
+		}
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 		/*
 		 * If we already sent SIGQUIT to children and they are slow to shut
 		 * down, it's time to send them SIGKILL.  This doesn't happen
@@ -3440,6 +3464,9 @@ PostmasterStateMachine(void)
 						signal_child(PgStatPID, SIGQUIT);
 				}
 			}
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                        ReportFreeBAiocbs();
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 		}
 	}
 
--- src/backend/executor/nodeBitmapHeapscan.c.orig	2014-08-18 14:10:36.869016769 -0400
+++ src/backend/executor/nodeBitmapHeapscan.c	2014-08-19 16:56:13.375196054 -0400
@@ -34,6 +34,8 @@
  *		ExecEndBitmapHeapScan		releases all storage.
  */
 #include "postgres.h"
+#include <sys/types.h> 
+#include <unistd.h>
 
 #include "access/relscan.h"
 #include "access/transam.h"
@@ -47,6 +49,10 @@
 #include "utils/snapmgr.h"
 #include "utils/tqual.h"
 
+#ifdef USE_PREFETCH
+extern unsigned int prefetch_dbOid; /*  database oid of relations on which prefetching to be done - 0 means all */
+extern unsigned int prefetch_bitmap_scans; /*  boolean whether to prefetch bitmap heap scans         */
+#endif   /* USE_PREFETCH */
 
 static TupleTableSlot *BitmapHeapNext(BitmapHeapScanState *node);
 static void bitgetpage(HeapScanDesc scan, TBMIterateResult *tbmres);
@@ -111,10 +117,21 @@ BitmapHeapNext(BitmapHeapScanState *node
 		node->tbmres = tbmres = NULL;
 
 #ifdef USE_PREFETCH
-		if (target_prefetch_pages > 0)
-		{
+		if (    prefetch_bitmap_scans
+                     && (target_prefetch_pages > 0)
+                     && (    (   (prefetch_dbOid > 0)
+                               && (prefetch_dbOid == scan->rs_rd->rd_node.dbNode)
+                             )
+                         ||  (prefetch_dbOid == 0)
+                        )
+                        /* sufficient number of blocks - at least twice the target_prefetch_pages */
+                     && (scan->rs_nblocks > (2*target_prefetch_pages))
+                   ) {
 			node->prefetch_iterator = prefetch_iterator = tbm_begin_iterate(tbm);
 			node->prefetch_pages = 0;
+                        if (prefetch_iterator) {
+                          tbm_zero(prefetch_iterator);  /* zero list of prefetched and unread blocknos */
+                        }
 			node->prefetch_target = -1;
 		}
 #endif   /* USE_PREFETCH */
@@ -138,12 +155,14 @@ BitmapHeapNext(BitmapHeapScanState *node
 			}
 
 #ifdef USE_PREFETCH
+                        if (prefetch_iterator) {
 			if (node->prefetch_pages > 0)
 			{
 				/* The main iterator has closed the distance by one page */
 				node->prefetch_pages--;
+                                tbm_subtract(prefetch_iterator, tbmres->blockno); /* remove this blockno from list of prefetched and unread blocknos */
 			}
-			else if (prefetch_iterator)
+                            else
 			{
 				/* Do not let the prefetch iterator get behind the main one */
 				TBMIterateResult *tbmpre = tbm_iterate(prefetch_iterator);
@@ -151,6 +170,7 @@ BitmapHeapNext(BitmapHeapScanState *node
 				if (tbmpre == NULL || tbmpre->blockno != tbmres->blockno)
 					elog(ERROR, "prefetch and main iterators are out of sync");
 			}
+                        }
 #endif   /* USE_PREFETCH */
 
 			/*
@@ -239,16 +259,26 @@ BitmapHeapNext(BitmapHeapScanState *node
 			while (node->prefetch_pages < node->prefetch_target)
 			{
 				TBMIterateResult *tbmpre = tbm_iterate(prefetch_iterator);
+                                int             PrefetchBufferRc; /*  return value from PrefetchBuffer  - refer to bufmgr.h */
+
 
 				if (tbmpre == NULL)
 				{
 					/* No more pages to prefetch */
-					tbm_end_iterate(prefetch_iterator);
-					node->prefetch_iterator = prefetch_iterator = NULL;
+                                        /* let ExecEndBitmapHeapScan terminate the prefetch_iterator
+				        **	tbm_end_iterate(prefetch_iterator);
+					**      node->prefetch_iterator = NULL;
+                                        */
+                                        prefetch_iterator = NULL;
 					break;
 				}
 				node->prefetch_pages++;
-				PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, tbmpre->blockno);
+				PrefetchBufferRc = PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, tbmpre->blockno , 0);
+                                /*  add this blockno to list of prefetched and unread blocknos
+                                **  if pin count did not increase then indicate so in the Unread_Pfetched list
+                                */
+                                tbm_add(prefetch_iterator
+                                   ,( (PrefetchBufferRc & PREFTCHRC_BUF_PIN_INCREASED) ? tbmpre->blockno : InvalidBlockNumber ) );
 			}
 		}
 #endif   /* USE_PREFETCH */
@@ -482,12 +512,31 @@ ExecEndBitmapHeapScan(BitmapHeapScanStat
 {
 	Relation	relation;
 	HeapScanDesc scanDesc;
+	TBMIterator *prefetch_iterator;
 
 	/*
 	 * extract information from the node
 	 */
 	relation = node->ss.ss_currentRelation;
 	scanDesc = node->ss.ss_currentScanDesc;
+	prefetch_iterator = node->prefetch_iterator;
+
+#ifdef USE_PREFETCH
+        /*  before any other cleanup,  discard any prefetched but unread buffers  */
+        if (prefetch_iterator != NULL) {
+            TBMIterateResult *tbmpre = tbm_locate_IterateResult(prefetch_iterator);
+            BlockNumber *Unread_Pfetched_base = tbmpre->Unread_Pfetched_base;
+            unsigned int Unread_Pfetched_next = tbmpre->Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+            unsigned int Unread_Pfetched_count = tbmpre->Unread_Pfetched_count;
+
+            while ((Unread_Pfetched_count--) > 0) {
+                DiscardBuffer( scanDesc->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                Unread_Pfetched_next++;
+                if (Unread_Pfetched_next >= target_prefetch_pages)
+                    Unread_Pfetched_next = 0;
+            }
+        }
+#endif   /* USE_PREFETCH */
 
 	/*
 	 * Free the exprcontext
--- src/backend/executor/nodeIndexscan.c.orig	2014-08-18 14:10:36.869016769 -0400
+++ src/backend/executor/nodeIndexscan.c	2014-08-19 16:56:13.391196111 -0400
@@ -35,8 +35,13 @@
 #include "utils/rel.h"
 
 
+
 static TupleTableSlot *IndexNext(IndexScanState *node);
 
+#ifdef USE_PREFETCH
+extern unsigned int prefetch_dbOid; /*  database oid of relations on which prefetching to be done - 0 means all */
+extern unsigned int prefetch_index_scans; /*  boolean whether to prefetch bitmap heap scans         */
+#endif   /* USE_PREFETCH */
 
 /* ----------------------------------------------------------------
  *		IndexNext
@@ -418,7 +423,12 @@ ExecEndIndexScan(IndexScanState *node)
 	 * close the index relation (no-op if we didn't open it)
 	 */
 	if (indexScanDesc)
+        {
 		index_endscan(indexScanDesc);
+
+        /*  note  -  at this point all scan controlblock resources have been freed by IndexScanEnd called by index_endscan */
+
+        }
 	if (indexRelationDesc)
 		index_close(indexRelationDesc, NoLock);
 
@@ -609,6 +619,33 @@ ExecInitIndexScan(IndexScan *node, EStat
 											   indexstate->iss_NumScanKeys,
 											 indexstate->iss_NumOrderByKeys);
 
+#ifdef USE_PREFETCH
+        /*  initialize prefetching   */
+                indexstate->iss_ScanDesc->pfch_index_page_list =  (struct pfch_index_pagelist*)0;
+                indexstate->iss_ScanDesc->pfch_block_item_list = (struct pfch_block_item*)0;
+		if (    prefetch_index_scans
+			 && (target_prefetch_pages > 0)
+			 &&	(!RelationUsesLocalBuffers(indexstate->iss_ScanDesc->heapRelation)) /* I think this must always be true for an indexed heap ? */
+			 && (    (   (prefetch_dbOid > 0)
+					   && (prefetch_dbOid == indexstate->iss_ScanDesc->heapRelation->rd_node.dbNode)
+					 )
+				 ||  (prefetch_dbOid == 0)
+				)
+		   ) {
+			indexstate->iss_ScanDesc->pfch_index_page_list = palloc( sizeof(struct pfch_index_pagelist) + ( (target_prefetch_pages-1) * sizeof(struct pfch_index_item) ) );
+			indexstate->iss_ScanDesc->pfch_block_item_list = palloc( prefetch_index_scans * sizeof(struct pfch_block_item) );
+			if (     ( (struct pfch_index_pagelist*)0 != indexstate->iss_ScanDesc->pfch_index_page_list )
+                  && ( (struct pfch_block_item*)0 != indexstate->iss_ScanDesc->pfch_block_item_list )
+               ) {
+                          indexstate->iss_ScanDesc->pfch_used = 0;
+                          indexstate->iss_ScanDesc->pfch_next = prefetch_index_scans; /* ensure first entry is at index 0 */
+                          indexstate->iss_ScanDesc->pfch_index_page_list->pfch_index_pagelist_next = (struct pfch_index_pagelist*)0;
+                          indexstate->iss_ScanDesc->pfch_index_page_list->pfch_index_item_count = 0;
+                          indexstate->iss_ScanDesc->do_prefetch = 1;
+            }
+		}
+#endif   /* USE_PREFETCH */
+
 	/*
 	 * If no run-time keys to calculate, go ahead and pass the scankeys to the
 	 * index AM.
--- src/backend/executor/instrument.c.orig	2014-08-18 14:10:36.869016769 -0400
+++ src/backend/executor/instrument.c	2014-08-19 16:56:13.415196197 -0400
@@ -41,6 +41,14 @@ InstrAlloc(int n, int instrument_options
 		{
 			instr[i].need_bufusage = need_buffers;
 			instr[i].need_timer = need_timer;
+			instr[i].bufusage_start.aio_read_noneed = 0;
+			instr[i].bufusage_start.aio_read_discrd = 0;
+			instr[i].bufusage_start.aio_read_forgot = 0;
+			instr[i].bufusage_start.aio_read_noblok = 0;
+			instr[i].bufusage_start.aio_read_failed = 0;
+			instr[i].bufusage_start.aio_read_wasted = 0;
+			instr[i].bufusage_start.aio_read_waited = 0;
+			instr[i].bufusage_start.aio_read_ontime = 0;
 		}
 	}
 
@@ -143,6 +151,16 @@ BufferUsageAccumDiff(BufferUsage *dst,
 	dst->local_blks_written += add->local_blks_written - sub->local_blks_written;
 	dst->temp_blks_read += add->temp_blks_read - sub->temp_blks_read;
 	dst->temp_blks_written += add->temp_blks_written - sub->temp_blks_written;
+
+	dst->aio_read_noneed       += add->aio_read_noneed - sub->aio_read_noneed;
+	dst->aio_read_discrd       += add->aio_read_discrd - sub->aio_read_discrd;
+	dst->aio_read_forgot       += add->aio_read_forgot - sub->aio_read_forgot;
+	dst->aio_read_noblok       += add->aio_read_noblok - sub->aio_read_noblok;
+	dst->aio_read_failed       += add->aio_read_failed - sub->aio_read_failed;
+	dst->aio_read_wasted       += add->aio_read_wasted - sub->aio_read_wasted;
+	dst->aio_read_waited       += add->aio_read_waited - sub->aio_read_waited;
+	dst->aio_read_ontime       += add->aio_read_ontime - sub->aio_read_ontime;
+
 	INSTR_TIME_ACCUM_DIFF(dst->blk_read_time,
 						  add->blk_read_time, sub->blk_read_time);
 	INSTR_TIME_ACCUM_DIFF(dst->blk_write_time,
--- src/backend/storage/buffer/Makefile.orig	2014-08-18 14:10:36.933017070 -0400
+++ src/backend/storage/buffer/Makefile	2014-08-19 16:56:13.471196400 -0400
@@ -12,6 +12,6 @@ subdir = src/backend/storage/buffer
 top_builddir = ../../../..
 include $(top_builddir)/src/Makefile.global
 
-OBJS = buf_table.o buf_init.o bufmgr.o freelist.o localbuf.o
+OBJS = buf_table.o buf_init.o bufmgr.o freelist.o localbuf.o buf_async.o
 
 include $(top_srcdir)/src/backend/common.mk
--- src/backend/storage/buffer/bufmgr.c.orig	2014-08-18 14:10:36.937017089 -0400
+++ src/backend/storage/buffer/bufmgr.c	2014-08-19 16:56:13.515196557 -0400
@@ -29,7 +29,7 @@
  *		buf_table.c -- manages the buffer lookup table
  */
 #include "postgres.h"
-
+#include <sys/types.h> 
 #include <sys/file.h>
 #include <unistd.h>
 
@@ -50,7 +50,6 @@
 #include "utils/resowner_private.h"
 #include "utils/timestamp.h"
 
-
 /* Note: these two macros only work on shared buffers, not local ones! */
 #define BufHdrGetBlock(bufHdr)	((Block) (BufferBlocks + ((Size) (bufHdr)->buf_id) * BLCKSZ))
 #define BufferGetLSN(bufHdr)	(PageGetLSN(BufHdrGetBlock(bufHdr)))
@@ -63,6 +62,17 @@
 #define BUF_WRITTEN				0x01
 #define BUF_REUSABLE			0x02
 
+extern volatile struct BAiocbAnchor *BAiocbAnchr;  /*  anchor for all control blocks pertaining to aio  */
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern int numBufferAiocbs;        /*  total number of  BufferAiocbs in pool  */
+#if defined(USE_PREFETCH) && defined(USE_AIO_SIGEVENT)
+extern long num_startedaio;
+extern long num_cancelledaio;
+extern long num_signalledaio;                  /* count of number of signals delivered - counted by handlaiosignal */
+extern struct BAiocbIolock_chain_item volatile * volatile BAiocbIolock_anchor; /* anchor for chain of awaiting-release LWLock ptrs */
+#endif /* defined(USE_PREFETCH) && defined(USE_AIO_SIGEVENT)  */
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 #define DROP_RELS_BSEARCH_THRESHOLD		20
 
 /* GUC variables */
@@ -78,26 +88,33 @@ bool		track_io_timing = false;
  */
 int			target_prefetch_pages = 0;
 
-/* local state for StartBufferIO and related functions */
+/* local state for StartBufferIO and related functions
+**  but ONLY for synchronous IO  -  not altered for aio
+*/
 static volatile BufferDesc *InProgressBuf = NULL;
 static bool IsForInput;
+pid_t this_backend_pid = 0;    /*    pid of this backend */
 
 /* local state for LockBufferForCleanup */
 static volatile BufferDesc *PinCountWaitBuf = NULL;
 
-
-static Buffer ReadBuffer_common(SMgrRelation reln, char relpersistence,
+extern int
+BufStartAsync(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy);
+extern int
+BufCheckAsync(SMgrRelation caller_smgr, Relation caller_reln, volatile BufferDesc *buf_desc, int intention
+        ,BufferAccessStrategy strategy , int index_for_aio , bool spinLockHeld , LWLockId PartitionLock );
+Buffer ReadBuffer_common(SMgrRelation reln, char relpersistence,
 				  ForkNumber forkNum, BlockNumber blockNum,
 				  ReadBufferMode mode, BufferAccessStrategy strategy,
-				  bool *hit);
-static bool PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy);
-static void PinBuffer_Locked(volatile BufferDesc *buf);
-static void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner);
+				  bool *hit , int index_for_aio);
+bool PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy);
+void PinBuffer_Locked(volatile BufferDesc *buf);
+void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner);
 static void BufferSync(int flags);
 static int	SyncOneBuffer(int buf_id, bool skip_recently_used);
 static void WaitIO(volatile BufferDesc *buf);
-static bool StartBufferIO(volatile BufferDesc *buf, bool forInput);
-static void TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
+static bool StartBufferIO(volatile BufferDesc *buf, bool forInput , int index_for_aio );
+void TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
 				  int set_flag_bits);
 static void shared_buffer_write_error_callback(void *arg);
 static void local_buffer_write_error_callback(void *arg);
@@ -106,25 +123,67 @@ static volatile BufferDesc *BufferAlloc(
 			ForkNumber forkNum,
 			BlockNumber blockNum,
 			BufferAccessStrategy strategy,
-			bool *foundPtr);
+			int *foundPtr , int index_for_aio );
 static void FlushBuffer(volatile BufferDesc *buf, SMgrRelation reln);
 static void AtProcExit_Buffers(int code, Datum arg);
 static void CheckForBufferLeaks(void);
 static int	rnode_comparator(const void *p1, const void *p2);
 
+BlockNumber BlocknumOfBuffer(Buffer buffer);
+bool BlocknotinBuffer(Buffer buffer, Relation relation, BlockNumber blockNum);
 
 /*
  * PrefetchBuffer -- initiate asynchronous read of a block of a relation
  *
- * This is named by analogy to ReadBuffer but doesn't actually allocate a
- * buffer.  Instead it tries to ensure that a future ReadBuffer for the given
- * block will not be delayed by the I/O.  Prefetching is optional.
+ * This is named by analogy to ReadBuffer but allocates a buffer only if using asynchronous I/O.
+ * Its purpose  is to try to ensure that a future ReadBuffer for the given block
+ * will not be delayed by the I/O.  Prefetching is optional.
  * No-op if prefetching isn't compiled in.
- */
-void
-PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
-{
+ *
+ * Originally the prefetch simply called posix_fadvise() to recommend read-ahead into kernel page cache.
+ * Extended to provide an alternative of issuing an asynchronous aio_read() to read into a buffer.
+ * This extension has an implication on how this bufmgr component manages concurrent requests
+ * for the same disk block.
+ *
+ * Synchronous IO (read()) does not provide a means for waiting on another task's read if in progress,
+ * and bufmgr implements its own scheme in StartBufferIO, WaitIO, and TerminateBufferIO.
+ *
+ * Asynchronous IO (aio_read()) provides a means for waiting on this or another task's read if in progress,
+ * namely aio_suspend(),  which this extension uses.    Therefore,  although StartBufferIO and TerminateBufferIO
+ * are called as part of asynchronous prefetching,   their role is limited to maintaining the buffer desc flags,
+ * and they do not track the asynchronous IO itself.    Instead,   asynchronous IOs are tracked in 
+ * a separate set of shared control blocks,  the BufferAiocb list -
+ *   refer to     include/storage/buf_internals.h and storage/buffer/buf_init.c
+ *
+ * Another implication of asynchronous IO concerns buffer pinning.
+ * The buffer used for the prefetch is pinned before aio_read is issued.
+ * It is expected that the same task (and possibly others) will later ask to read the page
+ * and eventually release and unpin the buffer.
+ * However,  if the task which issued the aio_read later decides not to read the page,
+ * and return code indicates delta_pin_count > 0 (see below)
+ * it *must* instead issue a DiscardBuffer() (see function later in this file)
+ * so that its pin is released.
+ * Therefore,  each client which uses the PrefetchBuffer service must either always read all
+ * prefetched pages,  or keep track of prefetched pages and discard unread ones at end of scan.
+ *
+ * return code:   is an int bitmask defined in bufmgr.h
+        PREFTCHRC_BUF_PIN_INCREASED 0x01      pin count on buffer has been increased by 1
+        PREFTCHRC_BLK_ALREADY_PRESENT 0x02    block was already present in a buffer
+ *
+ * PREFTCHRC_BLK_ALREADY_PRESENT is a hint to caller that the prefetch may be unnecessary
+ */
+int
+PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy)
+{
+	Buffer		buf_id; /* indicates buffer containing the requested block  */
+        int             PrefetchBufferRc = 0; /*  return value as described above  */
+        int             PinCountOnEntry = 0;  /*  pin count on entry           */
+        int             PinCountdelta = 0;    /*  pin count delta increase     */
+
+
 #ifdef USE_PREFETCH
+
+	buf_id = -1;
 	Assert(RelationIsValid(reln));
 	Assert(BlockNumberIsValid(blockNum));
 
@@ -147,7 +206,12 @@ PrefetchBuffer(Relation reln, ForkNumber
 		BufferTag	newTag;		/* identity of requested block */
 		uint32		newHash;	/* hash value for newTag */
 		LWLock	   *newPartitionLock;	/* buffer partition lock for it */
-		int			buf_id;
+        int         BufStartAsyncrc = -1;  /*  retcode from BufStartAsync :
+                                                       **        0 if started successfully (which implies buffer was newly pinned )
+                                                       **       -1 if failed for some reason
+                                                       **        1+PrivateRefCount if we found desired buffer in buffer pool
+                                                       **  and we set it likewise if we find buffer in buffer pool
+                                                       */
 
 		/* create a tag so we can lookup the buffer */
 		INIT_BUFFERTAG(newTag, reln->rd_smgr->smgr_rnode.node,
@@ -159,28 +223,121 @@ PrefetchBuffer(Relation reln, ForkNumber
 
 		/* see if the block is in the buffer pool already */
 		LWLockAcquire(newPartitionLock, LW_SHARED);
-		buf_id = BufTableLookup(&newTag, newHash);
+		buf_id = (Buffer)BufTableLookup(&newTag, newHash);
+                if (buf_id >= 0) {
+                    PinCountOnEntry = PrivateRefCount[buf_id];  /*  pin count on entry               */
+                    BufStartAsyncrc = 1 + PinCountOnEntry; /* indicate this backends pin count - see above comment */
+                    PrefetchBufferRc = PREFTCHRC_BLK_ALREADY_PRESENT;       /* indicate buffer present */
+                } else {
+                    PrefetchBufferRc = 0;                                   /* indicate buffer not present */
+                }
 		LWLockRelease(newPartitionLock);
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+     not_in_buffers:
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 		/* If not in buffers, initiate prefetch */
-		if (buf_id < 0)
+		if (buf_id < 0) {
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                    /*    try using async aio_read with a buffer */
+                    BufStartAsyncrc = BufStartAsync( reln, forkNum, blockNum , strategy );
+                    if (BufStartAsyncrc < 0) {
+                            pgBufferUsage.aio_read_noblok++;
+                    }
+#else /* not USE_AIO_ATOMIC_BUILTIN_COMP_SWAP so try the alternative that does not read the block into a postgresql buffer */
 			smgrprefetch(reln->rd_smgr, forkNum, blockNum);
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+		}
 
-		/*
-		 * If the block *is* in buffers, we do nothing.  This is not really
-		 * ideal: the block might be just about to be evicted, which would be
-		 * stupid since we know we are going to need it soon.  But the only
-		 * easy answer is to bump the usage_count, which does not seem like a
-		 * great solution: when the caller does ultimately touch the block,
-		 * usage_count would get bumped again, resulting in too much
-		 * favoritism for blocks that are involved in a prefetch sequence. A
-		 * real fix would involve some additional per-buffer state, and it's
-		 * not clear that there's enough of a problem to justify that.
+        if (  (buf_id >= 0) || (BufStartAsyncrc >= 1)  ) {
+                        /* The block *is* in buffers.    */
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                        pgBufferUsage.aio_read_noneed++;
+#ifndef USE_PREFETCH_BUT_DONT_PIN_ALREADY_PRESENT /* jury is out on whether the following wins but it ought to ...  */
+                        /*
+                        ** If this backend already had pinned it,
+                        ** or another backend had banked a pin on it,
+                        ** or there is an IO in progress,
+                        ** or it is not marked valid,
+                        ** then do nothing.
+                        ** Otherwise pin it and mark the buffer's pin as banked by this backend.
+                        ** Note  -  it may or not be pinned by another backend -
+                        **          it is ok for us to bank a pin on it
+                        **          *provided* the other backend did not bank its pin.
+                        **          The reason for this is that the banked-pin indicator is global -
+                        **          it can identify at most one process.
+                        */
+                        /* pgBufferUsage.aio_read_wasted++;  overload counter - only for debugging */
+                        if (BufStartAsyncrc == 1) {            /*   not pinned by me  */
+                              /*  pgBufferUsage.aio_read_wasted++;  overload counter - only for debugging */
+                              /*  note   -   all we can say with certainty is that the buffer is not pinned by me
+                              **             we cannot be sure that it is still in buffer pool
+                              **             so must go through the entire locking and searching all over again ...
 		 */
+                            LWLockAcquire(newPartitionLock, LW_SHARED);
+                            buf_id = (Buffer)BufTableLookup(&newTag, newHash);
+                            /* If in buffers, proceed */
+                            if (buf_id >= 0) {
+                                /*  since the block is now present,
+                                **  save the current pin count to ensure final delta is calculated correctly
+                                */
+                                PinCountOnEntry = PrivateRefCount[buf_id];  /*  pin count on entry               */
+                                if ( PinCountOnEntry == 0) { /*  paranoid check it's still not pinned by me */
+                                    volatile        BufferDesc *buf_desc;
+
+                                    buf_desc = &BufferDescriptors[buf_id];        /* found buffer descriptor */
+                                    LockBufHdr(buf_desc);
+                                    if (    (buf_desc->flags & BM_VALID)           /* buffer is valid        */
+                                         && (!(buf_desc->flags & (BM_IO_IN_PROGRESS|BM_AIO_IN_PROGRESS|BM_AIO_PREFETCH_PIN_BANKED))) /* buffer is not any of ... */
+                                       ) {
+                                        buf_desc->flags |= BM_AIO_PREFETCH_PIN_BANKED; /* bank the pin for next use by this task */
+                                        /*  note  - we can call PinBuffer_Locked with the BM_AIO_PREFETCH_PIN_BANKED flag set because it is not yet pinned by me */
+                                        buf_desc->freeNext = -(this_backend_pid);       /* remember which pid banked it */
+                                        /*  pgBufferUsage.aio_read_wasted--;      overload counter - not wasted after all - only for debugging */
+
+                                        /* Make sure we will have room to remember the buffer pin */
+                                        ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
+
+                                        PinBuffer_Locked(buf_desc);
+	}
+                                    else {
+                                        UnlockBufHdr(buf_desc);
+                                    }
+                                }
+                            }
+                            LWLockRelease(newPartitionLock);
+                            /*  although unlikely,  maybe it was evicted while we were puttering about  */
+                            if (buf_id < 0) {
+                                pgBufferUsage.aio_read_noneed--;   /*   back out the accounting */
+                                goto not_in_buffers;               /*   and try again           */
 	}
-#endif   /* USE_PREFETCH */
 }
+#endif /*  USE_PREFETCH_BUT_DONT_PIN_ALREADY_PRESENT */
 
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+		}
+
+		if (buf_id >= 0) {
+			PinCountdelta = PrivateRefCount[buf_id] - PinCountOnEntry;  /*  pin count delta increase    */
+			if (  (PinCountdelta < 0) || (PinCountdelta > 1) ) {
+				  elog(ERROR,
+						 "PrefetchBuffer #%d : incremented pin count by %d on bufdesc %p refcount %u localpins %d\n"
+								  ,(buf_id+1) , PinCountdelta , &BufferDescriptors[buf_id] ,BufferDescriptors[buf_id].refcount , PrivateRefCount[buf_id]);
+}
+		} else
+		if (BufStartAsyncrc == 0) {  /* aio started successfully (which implies buffer was newly pinned ) */
+			PinCountdelta = 1;
+		}
+
+		/*  set final PrefetchBufferRc according to previous value */
+		PrefetchBufferRc |= PinCountdelta;  /* set the PREFTCHRC_BUF_PIN_INCREASED bit */
+	}
+
+#endif   /* USE_PREFETCH */
+
+	return PrefetchBufferRc; /*  return value as described above */
+}
 
 /*
  * ReadBuffer -- a shorthand for ReadBufferExtended, for reading from main
@@ -253,7 +410,7 @@ ReadBufferExtended(Relation reln, ForkNu
 	 */
 	pgstat_count_buffer_read(reln);
 	buf = ReadBuffer_common(reln->rd_smgr, reln->rd_rel->relpersistence,
-							forkNum, blockNum, mode, strategy, &hit);
+							forkNum, blockNum, mode, strategy, &hit , 0);
 	if (hit)
 		pgstat_count_buffer_hit(reln);
 	return buf;
@@ -281,7 +438,7 @@ ReadBufferWithoutRelcache(RelFileNode rn
 	Assert(InRecovery);
 
 	return ReadBuffer_common(smgr, RELPERSISTENCE_PERMANENT, forkNum, blockNum,
-							 mode, strategy, &hit);
+							 mode, strategy, &hit , 0);
 }
 
 
@@ -289,15 +446,18 @@ ReadBufferWithoutRelcache(RelFileNode rn
  * ReadBuffer_common -- common logic for all ReadBuffer variants
  *
  * *hit is set to true if the request was satisfied from shared buffer cache.
+ * index_for_aio ,  if -ve , is negative of ( index of the aiocb in the BufferAiocbs array + 3 )
+ *     which is passed through to StartBufferIO
  */
-static Buffer
+Buffer
 ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 				  BlockNumber blockNum, ReadBufferMode mode,
-				  BufferAccessStrategy strategy, bool *hit)
+				  BufferAccessStrategy strategy, bool *hit , int index_for_aio )
 {
 	volatile BufferDesc *bufHdr;
 	Block		bufBlock;
 	bool		found;
+        int             allocrc;             /*  retcode from BufferAlloc */
 	bool		isExtend;
 	bool		isLocalBuf = SmgrIsTemp(smgr);
 
@@ -329,16 +489,40 @@ ReadBuffer_common(SMgrRelation smgr, cha
 	}
 	else
 	{
+                allocrc = mode; /* pass mode to BufferAlloc since it must not wait for async io if RBM_NOREAD_FOR_PREFETCH */
 		/*
 		 * lookup the buffer.  IO_IN_PROGRESS is set if the requested block is
 		 * not currently in memory.
 		 */
 		bufHdr = BufferAlloc(smgr, relpersistence, forkNum, blockNum,
-							 strategy, &found);
-		if (found)
-			pgBufferUsage.shared_blks_hit++;
+							 strategy, &allocrc , index_for_aio );
+		if (allocrc < 0) {
+                    if (mode == RBM_ZERO_ON_ERROR || zero_damaged_pages)
+                    {
+                        ereport(WARNING,
+                                (errcode(ERRCODE_DATA_CORRUPTED),
+                                 errmsg("invalid page header in block %u of relation %s; zeroing out page",
+                                        blockNum,
+                                        relpath(smgr->smgr_rnode, forkNum))));
+                        bufBlock = BufHdrGetBlock(bufHdr);
+                        MemSet((char *) bufBlock, 0, BLCKSZ);
+                    }
 		else
+                      ereport(ERROR,
+                              (errcode(ERRCODE_DATA_CORRUPTED),
+                               errmsg("invalid page header in block %u of relation %s",
+                                      blockNum,
+                                      relpath(smgr->smgr_rnode, forkNum))));
+                        found = true;
+                }
+		else if (allocrc > 0) {
+			pgBufferUsage.shared_blks_hit++;
+                        found = true;
+                }
+		else {
 			pgBufferUsage.shared_blks_read++;
+                        found = false;
+                }
 	}
 
 	/* At this point we do NOT hold any locks. */
@@ -411,7 +595,7 @@ ReadBuffer_common(SMgrRelation smgr, cha
 				Assert(bufHdr->flags & BM_VALID);
 				bufHdr->flags &= ~BM_VALID;
 				UnlockBufHdr(bufHdr);
-			} while (!StartBufferIO(bufHdr, true));
+			} while (!StartBufferIO(bufHdr, true, 0));
 		}
 	}
 
@@ -431,6 +615,7 @@ ReadBuffer_common(SMgrRelation smgr, cha
 
 	bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);
 
+        if (mode != RBM_NOREAD_FOR_PREFETCH) {
 	if (isExtend)
 	{
 		/* new buffers are zero-filled */
@@ -500,6 +685,7 @@ ReadBuffer_common(SMgrRelation smgr, cha
 	VacuumPageMiss++;
 	if (VacuumCostActive)
 		VacuumCostBalance += VacuumCostPageMiss;
+	}
 
 	TRACE_POSTGRESQL_BUFFER_READ_DONE(forkNum, blockNum,
 									  smgr->smgr_rnode.node.spcNode,
@@ -521,21 +707,39 @@ ReadBuffer_common(SMgrRelation smgr, cha
  * the default strategy.  The selected buffer's usage_count is advanced when
  * using the default strategy, but otherwise possibly not (see PinBuffer).
  *
- * The returned buffer is pinned and is already marked as holding the
- * desired page.  If it already did have the desired page, *foundPtr is
- * set TRUE.  Otherwise, *foundPtr is set FALSE and the buffer is marked
+ * index_for_aio is an input parm which,  if non-zero,  identifies a BufferAiocb
+ * acquired by caller and to be used for any StartBufferIO performed by this routine.
+ * In this case,  if block not found in buffer pool and we allocate a new buffer,
+ * then we must maintain the spinlock on the buffer and pass it back to caller.
+ *
+ * foundPtr is input and output :
+ *  . input   -  indicates the read-buffer mode  ( see bufmgr.h )
+ *  . output  -  indicates the status of the buffer - see below
+ *
+ * Except for the case of RBM_NOREAD_FOR_PREFETCH and buffer is found,
+ * the returned buffer is pinned and is already marked as holding the
+ * desired page.
+ *  If it already did have the desired page and page content is valid,
+ *  *foundPtr is set to 1
+ *  If it already did have the desired page and mode is RBM_NOREAD_FOR_PREFETCH
+ *    and StartBufferIO returned false
+ *    (meaning it could not initialise the buffer for aio)
+ *  *foundPtr is set to 2
+ *  If it already did have the desired page but page content is invalid,
+ *  *foundPtr is set to -1
+ *   this can happen only if the buffer was read by an async read
+ *   and the aio is still in progress or pinned by the issuer of the startaio.
+ *  Otherwise, *foundPtr is set to 0 and the buffer is marked
  * as IO_IN_PROGRESS; ReadBuffer will now need to do I/O to fill it.
  *
- * *foundPtr is actually redundant with the buffer's BM_VALID flag, but
- * we keep it for simplicity in ReadBuffer.
- *
- * No locks are held either at entry or exit.
+ * No locks are held either at entry or exit EXCEPT for case noted above
+ * of passing an empty buffer back to async io caller ( index_for_aio set ).
  */
 static volatile BufferDesc *
 BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 			BlockNumber blockNum,
 			BufferAccessStrategy strategy,
-			bool *foundPtr)
+			int *foundPtr , int index_for_aio )
 {
 	BufferTag	newTag;			/* identity of requested block */
 	uint32		newHash;		/* hash value for newTag */
@@ -547,6 +751,13 @@ BufferAlloc(SMgrRelation smgr, char relp
 	int			buf_id;
 	volatile BufferDesc *buf;
 	bool		valid;
+        int             IntentionBufferrc;      /* retcode from BufCheckAsync */
+        bool            StartBufferIOrc;        /* retcode from StartBufferIO */
+        ReadBufferMode mode;
+
+
+        mode = *foundPtr;
+        *foundPtr = 0;
 
 	/* create a tag so we can lookup the buffer */
 	INIT_BUFFERTAG(newTag, smgr->smgr_rnode.node, forkNum, blockNum);
@@ -561,21 +772,53 @@ BufferAlloc(SMgrRelation smgr, char relp
 	if (buf_id >= 0)
 	{
 		/*
-		 * Found it.  Now, pin the buffer so no one can steal it from the
-		 * buffer pool, and check to see if the correct data has been loaded
-		 * into the buffer.
+		 * Found it.
 		 */
+                *foundPtr = 1;
 		buf = &BufferDescriptors[buf_id];
 
-		valid = PinBuffer(buf, strategy);
-
-		/* Can release the mapping lock as soon as we've pinned it */
+                /*   If prefetch mode,  then return immediately indicating found,
+                **   and NOTE in this case only,  we did not pin buffer.
+                **   In theory we might try to check whether the buffer is valid,  io in progress,  etc
+                **   but in practice it is simpler to abandon the prefetch if the buffer exists
+                */
+                if (mode == RBM_NOREAD_FOR_PREFETCH) {
+                    /* release the mapping lock and return    */
 		LWLockRelease(newPartitionLock);
+                } else {
+                    /*   note that the current request is for same tag as the one associated with the aio -
+                    **   so simply complete the aio and we have our buffer.
+                    **         If an aio was started on this buffer,
+                    **         check complete and wait for it if not.
+                    **         And,  if aio had been started,  then the task
+                    **         which issued the start aio already pinned it for this read,
+                    **         so if that task was me and the aio was successful,
+                    **         pass the current pin to this read without dropping and re-acquiring.
+                    **         this is all done by BufCheckAsync
+                    */
+                    IntentionBufferrc = BufCheckAsync(smgr , 0 , buf, BUF_INTENTION_WANT , strategy , index_for_aio , false , newPartitionLock );
 
-		*foundPtr = TRUE;
+                    /*       check to see if the correct data has been loaded into the buffer.  */
+                    valid = (IntentionBufferrc == BUF_INTENT_RC_VALID);
 
-		if (!valid)
-		{
+                    /*  check for serious IO errors   */
+                    if (!valid) {
+                        if (    (IntentionBufferrc != BUF_INTENT_RC_INVALID_NO_AIO)
+                             && (IntentionBufferrc != BUF_INTENT_RC_INVALID_AIO)
+                           ) {
+                            *foundPtr = -1;  /*  inform caller of serious error */
+                        }
+                        else
+                        if (IntentionBufferrc == BUF_INTENT_RC_INVALID_AIO) {
+                            goto proceed_with_not_found;  /*  yes,  I know,  a goto ... think of it as a break out of the if */
+                        }
+                     }
+
+                    /* BufCheckAsync pinned the buffer            */
+                    /* so can now release the mapping lock               */
+                    LWLockRelease(newPartitionLock);
+
+                    if (!valid) {
 			/*
 			 * We can only get here if (a) someone else is still reading in
 			 * the page, or (b) a previous read attempt failed.  We have to
@@ -583,19 +826,21 @@ BufferAlloc(SMgrRelation smgr, char relp
 			 * own read attempt if the page is still not BM_VALID.
 			 * StartBufferIO does it all.
 			 */
-			if (StartBufferIO(buf, true))
+                        if (StartBufferIO(buf, true, index_for_aio))
 			{
 				/*
 				 * If we get here, previous attempts to read the buffer must
 				 * have failed ... but we shall bravely try again.
 				 */
-				*foundPtr = FALSE;
+							*foundPtr = 0;
+						}
 			}
 		}
 
 		return buf;
 	}
 
+  proceed_with_not_found:
 	/*
 	 * Didn't find it in the buffer pool.  We'll have to initialize a new
 	 * buffer.  Remember to unlock the mapping lock while doing the work.
@@ -620,8 +865,10 @@ BufferAlloc(SMgrRelation smgr, char relp
 		/* Must copy buffer flags while we still hold the spinlock */
 		oldFlags = buf->flags;
 
-		/* Pin the buffer and then release the buffer spinlock */
-		PinBuffer_Locked(buf);
+        /*         If an aio was started on this buffer,
+        **         check complete and cancel it if not.
+        */
+        BufCheckAsync(smgr , 0 , buf, BUF_INTENTION_REJECT_OBTAIN_PIN , 0 , index_for_aio, true , 0 );
 
 		/* Now it's safe to release the freelist lock */
 		if (lock_held)
@@ -792,13 +1039,18 @@ BufferAlloc(SMgrRelation smgr, char relp
 				 * then set up our own read attempt if the page is still not
 				 * BM_VALID.  StartBufferIO does it all.
 				 */
-				if (StartBufferIO(buf, true))
+                                StartBufferIOrc = StartBufferIO(buf, true , index_for_aio); /* retcode from StartBufferIO */
+				if (StartBufferIOrc)
 				{
 					/*
 					 * If we get here, previous attempts to read the buffer
 					 * must have failed ... but we shall bravely try again.
 					 */
-					*foundPtr = FALSE;
+					*foundPtr = 0;
+                                } else
+                                if (mode == RBM_NOREAD_FOR_PREFETCH) {
+					UnpinBuffer(buf, true); /* must unpin if RBM_NOREAD_FOR_PREFETCH and found */
+                                        *foundPtr = 2;  /*  inform BufStartAsync that buffer must not be used */
 				}
 			}
 
@@ -861,10 +1113,17 @@ BufferAlloc(SMgrRelation smgr, char relp
 	 * lock.  If StartBufferIO returns false, then someone else managed to
 	 * read it before we did, so there's nothing left for BufferAlloc() to do.
 	 */
-	if (StartBufferIO(buf, true))
-		*foundPtr = FALSE;
-	else
-		*foundPtr = TRUE;
+        StartBufferIOrc = StartBufferIO(buf, true , index_for_aio); /* retcode from StartBufferIO */
+        if (StartBufferIOrc) {
+		*foundPtr = 0;
+        } else {
+                if (mode == RBM_NOREAD_FOR_PREFETCH) {
+                    UnpinBuffer(buf, true); /* must unpin if RBM_NOREAD_FOR_PREFETCH and found */
+                    *foundPtr = 2;  /*  inform BufStartAsync that buffer must not be used */
+                } else {
+                    *foundPtr = 1;
+                }
+        }
 
 	return buf;
 }
@@ -971,6 +1230,10 @@ retry:
 	/*
 	 * Insert the buffer at the head of the list of free buffers.
 	 */
+        /*   avoid confusing freelist with strange-looking freeNext */
+        if (buf->freeNext <= FREENEXT_BAIOCB_ORIGIN) { /* means was used for aiocb index */
+            buf->freeNext = FREENEXT_NOT_IN_LIST;
+        }
 	StrategyFreeBuffer(buf);
 }
 
@@ -1023,6 +1286,56 @@ MarkBufferDirty(Buffer buffer)
 	UnlockBufHdr(bufHdr);
 }
 
+/*  return the blocknum of the block in a buffer if it is valid
+**  if a shared buffer,  it must be pinned
+*/
+BlockNumber
+BlocknumOfBuffer(Buffer buffer)
+{
+	volatile BufferDesc *bufHdr;
+        BlockNumber rc = 0;
+
+	if (BufferIsValid(buffer)) {
+            if  (BufferIsLocal(buffer)) {
+                bufHdr = &LocalBufferDescriptors[-buffer - 1];
+            } else {
+                bufHdr = &BufferDescriptors[buffer - 1];
+            }
+
+            rc = bufHdr->tag.blockNum;
+        }
+
+        return rc;
+}
+
+/*  report whether specified buffer contains same or different block
+**  if a shared buffer,  it must be pinned
+*/
+bool
+BlocknotinBuffer(Buffer buffer,
+					 Relation relation,
+					 BlockNumber blockNum)
+{
+	volatile BufferDesc *bufHdr;
+        bool rc = false;
+
+	if (BufferIsValid(buffer)) {
+            if  (BufferIsLocal(buffer)) {
+                bufHdr = &LocalBufferDescriptors[-buffer - 1];
+            } else {
+                bufHdr = &BufferDescriptors[buffer - 1];
+            }
+
+            rc =
+                  (   (bufHdr->tag.blockNum != blockNum)
+                   || (!(RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) ))
+                   || (bufHdr->tag.forkNum != MAIN_FORKNUM)
+                  );
+        }
+
+        return rc;
+}
+
 /*
  * ReleaseAndReadBuffer -- combine ReleaseBuffer() and ReadBuffer()
  *
@@ -1041,18 +1354,18 @@ ReleaseAndReadBuffer(Buffer buffer,
 					 Relation relation,
 					 BlockNumber blockNum)
 {
-	ForkNumber	forkNum = MAIN_FORKNUM;
 	volatile BufferDesc *bufHdr;
+        bool isDifferentBlock;   /*       requesting different block from that already in buffer ? */
 
 	if (BufferIsValid(buffer))
 	{
+	    /* if a shared buff, we have pin, so it's ok to examine tag without spinlock */
+            isDifferentBlock = BlocknotinBuffer(buffer,relation,blockNum); /*  requesting different block from that already in buffer ? */
 		if (BufferIsLocal(buffer))
 		{
 			Assert(LocalRefCount[-buffer - 1] > 0);
 			bufHdr = &LocalBufferDescriptors[-buffer - 1];
-			if (bufHdr->tag.blockNum == blockNum &&
-				RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
-				bufHdr->tag.forkNum == forkNum)
+			if (!isDifferentBlock)
 				return buffer;
 			ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
 			LocalRefCount[-buffer - 1]--;
@@ -1061,12 +1374,12 @@ ReleaseAndReadBuffer(Buffer buffer,
 		{
 			Assert(PrivateRefCount[buffer - 1] > 0);
 			bufHdr = &BufferDescriptors[buffer - 1];
-			/* we have pin, so it's ok to examine tag without spinlock */
-			if (bufHdr->tag.blockNum == blockNum &&
-				RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
-				bufHdr->tag.forkNum == forkNum)
+                        BufCheckAsync(0 , relation , bufHdr , ( isDifferentBlock ? BUF_INTENTION_REJECT_FORGET
+                                                                                 : BUF_INTENTION_REJECT_KEEP_PIN )
+                                                            , 0 , 0 , false , 0 ); /* end any IO and maybe unpin */
+			if (!isDifferentBlock) {
 				return buffer;
-			UnpinBuffer(bufHdr, true);
+                        }
 		}
 	}
 
@@ -1091,11 +1404,12 @@ ReleaseAndReadBuffer(Buffer buffer,
  * Returns TRUE if buffer is BM_VALID, else FALSE.  This provision allows
  * some callers to avoid an extra spinlock cycle.
  */
-static bool
+bool
 PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy)
 {
 	int			b = buf->buf_id;
 	bool		result;
+        bool       pin_already_banked_by_me = 0;  /* buffer is already pinned by me and redeemable */
 
 	if (PrivateRefCount[b] == 0)
 	{
@@ -1117,12 +1431,34 @@ PinBuffer(volatile BufferDesc *buf, Buff
 	else
 	{
 		/* If we previously pinned the buffer, it must surely be valid */
+                /* Errr  -   is that really true  ???    I don't think so  :
+                ** what if I pin,  try an IO,  in progress,  then mistakenly pin again
 		result = true;
+                */
+		LockBufHdr(buf);
+                pin_already_banked_by_me = (    (PrivateRefCount[b] > 0) && (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                                             && (   (    (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                                                     : (-(buf->freeNext))  )  == this_backend_pid )
+                                           );
+                if (pin_already_banked_by_me) {
+                        elog(LOG, "PinBuffer : on buffer %d with banked pin rel=%s, blockNum=%u, with flags %X refcount=%u"
+                       ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                       ,buf->tag.blockNum, buf->flags, buf->refcount);
+                    buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;
+                    if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                        buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
 	}
+                }
+		result = (buf->flags & BM_VALID) != 0;
+		UnlockBufHdr(buf);
+	}
+
+        if (!pin_already_banked_by_me) {
 	PrivateRefCount[b]++;
 	Assert(PrivateRefCount[b] > 0);
 	ResourceOwnerRememberBuffer(CurrentResourceOwner,
 								BufferDescriptorGetBuffer(buf));
+        }
 	return result;
 }
 
@@ -1139,19 +1475,36 @@ PinBuffer(volatile BufferDesc *buf, Buff
  * to save a spin lock/unlock cycle, because we need to pin a buffer before
  * its state can change under us.
  */
-static void
+void
 PinBuffer_Locked(volatile BufferDesc *buf)
 {
 	int			b = buf->buf_id;
+        bool       pin_already_banked_by_me;  /* buffer is already pinned by me and redeemable */
 
-	if (PrivateRefCount[b] == 0)
+        pin_already_banked_by_me = (    (PrivateRefCount[b] > 0) && (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                                     && (   (    (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                                             : (-(buf->freeNext))  )  == this_backend_pid )
+                             );
+	if (PrivateRefCount[b] == 0) {
 		buf->refcount++;
+        }
+        if (pin_already_banked_by_me) {
+                        elog(LOG, "PinBuffer_Locked : on buffer %d with banked pin rel=%s, blockNum=%u, with flags %X refcount=%u"
+                       ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                       ,buf->tag.blockNum, buf->flags, buf->refcount);
+            buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;
+                    if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                        buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+                    }
+        }
 	UnlockBufHdr(buf);
+        if (!pin_already_banked_by_me) {
 	PrivateRefCount[b]++;
 	Assert(PrivateRefCount[b] > 0);
 	ResourceOwnerRememberBuffer(CurrentResourceOwner,
 								BufferDescriptorGetBuffer(buf));
 }
+}
 
 /*
  * UnpinBuffer -- make buffer available for replacement.
@@ -1161,29 +1514,68 @@ PinBuffer_Locked(volatile BufferDesc *bu
  * Most but not all callers want CurrentResourceOwner to be adjusted.
  * Those that don't should pass fixOwner = FALSE.
  */
-static void
+void
 UnpinBuffer(volatile BufferDesc *buf, bool fixOwner)
 {
+
 	int			b = buf->buf_id;
+        bool       pin_already_banked_by_me;  /* buffer is already pinned by me and redeemable */
 
-	if (fixOwner)
+	if (fixOwner) {
 		ResourceOwnerForgetBuffer(CurrentResourceOwner,
 								  BufferDescriptorGetBuffer(buf));
+        }
 
 	Assert(PrivateRefCount[b] > 0);
 	PrivateRefCount[b]--;
 	if (PrivateRefCount[b] == 0)
 	{
+
 		/* I'd better not still hold any locks on the buffer */
 		Assert(!LWLockHeldByMe(buf->content_lock));
 		Assert(!LWLockHeldByMe(buf->io_in_progress_lock));
 
 		LockBufHdr(buf);
 
+		/* this backend has released last pin - buffer should not have pin banked by me
+                ** and if AIO in progress then there should be another backend pin
+                */
+                pin_already_banked_by_me = (       (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                                             &&    (   (    (buf->flags & BM_AIO_IN_PROGRESS)
+                                                         ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                         : (-(buf->freeNext))
+                                                       )  == this_backend_pid )
+                                           );
+                if (pin_already_banked_by_me) {
+                        /*  this is a strange situation  -  caller had a banked pin (which callers are supposed not to know about)
+                        **                                  but either discovered it had it or has over-counted how many pins it has
+                        */
+                        buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;   /*   redeem the pin although it is now of no use since about to release */
+                        if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                            buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+                        }
+
+                        /*     temporarily suppress logging error to avoid performance degradation -
+                        **     either this task really does not need the buffer in which case the error is harmless
+                        **     or a more severe error will be detected later (possibly immediately below)
+                        elog(LOG, "UnpinBuffer :  released last this-backend pin on buffer %d rel=%s, blockNum=%u, but had banked pin flags %X refcount=%u"
+                       ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                       ,buf->tag.blockNum, buf->flags, buf->refcount);
+                        */
+                }
+
 		/* Decrement the shared reference count */
 		Assert(buf->refcount > 0);
 		buf->refcount--;
 
+                if (   (buf->refcount == 0) && (buf->flags & BM_AIO_IN_PROGRESS)  ) {
+
+                        elog(ERROR, "UnpinBuffer :  released last any-backend pin on buffer %d rel=%s, blockNum=%u, but AIO in progress flags %X refcount=%u"
+                            ,buf->buf_id,relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum)
+                            ,buf->tag.blockNum, buf->flags, buf->refcount);
+                }
+
+
 		/* Support LockBufferForCleanup() */
 		if ((buf->flags & BM_PIN_COUNT_WAITER) &&
 			buf->refcount == 1)
@@ -1658,6 +2050,7 @@ SyncOneBuffer(int buf_id, bool skip_rece
 	volatile BufferDesc *bufHdr = &BufferDescriptors[buf_id];
 	int			result = 0;
 
+
 	/*
 	 * Check whether buffer needs writing.
 	 *
@@ -1724,6 +2117,16 @@ void
 InitBufferPoolBackend(void)
 {
 	on_shmem_exit(AtProcExit_Buffers, 0);
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        /*   init the aio subsystem max number of threads and max number of requests
+        **   max number of threads   <-->  max_async_io_prefetchers
+        **   max number of requests  <-->  numBufferAiocbs = (target_prefetch_pages*max_async_io_prefetchers)
+        **   there is no return code so we just hope.
+        */
+        smgrinitaio(max_async_io_prefetchers , numBufferAiocbs);
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 }
 
 /*
@@ -1736,6 +2139,11 @@ AtProcExit_Buffers(int code, Datum arg)
 	AbortBufferIO();
 	UnlockBuffers();
 
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on BAiocb lock instead of polling */
+    elog(LOG,"backend %d started %ld aios , cancelled %ld , signalled %ld\n"
+           ,this_backend_pid,num_startedaio,num_cancelledaio ,num_signalledaio);
+#endif /*  USE_AIO_SIGEVENT    */
+
 	CheckForBufferLeaks();
 
 	/* localbuf.c needs a chance too */
@@ -1779,6 +2187,8 @@ PrintBufferLeakWarning(Buffer buffer)
 	char	   *path;
 	BackendId	backend;
 
+
+
 	Assert(BufferIsValid(buffer));
 	if (BufferIsLocal(buffer))
 	{
@@ -1789,12 +2199,28 @@ PrintBufferLeakWarning(Buffer buffer)
 	else
 	{
 		buf = &BufferDescriptors[buffer - 1];
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                /*   If reason that this buffer is pinned
+                **   is that it was prefetched with async_io
+                **   and never read or discarded, then omit the
+                **   warning,  because this is expected in some
+                **   cases when a scan is closed abnormally.
+                **   Note that the buffer will be released soon by our caller.
+                */
+                if (buf->flags & BM_AIO_PREFETCH_PIN_BANKED) {
+                    pgBufferUsage.aio_read_forgot++; /* account for it */
+                    return;
+                }
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 		loccount = PrivateRefCount[buffer - 1];
 		backend = InvalidBackendId;
 	}
 
+/* #if defined(DO_ISCAN_DISCARD_BUFFER) && defined(DO_HEAPSCAN_DISCARD_BUFFER) not yet working safely 130530 */
 	/* theoretically we should lock the bufhdr here */
 	path = relpathbackend(buf->tag.rnode, backend, buf->tag.forkNum);
+
+
 	elog(WARNING,
 		 "buffer refcount leak: [%03d] "
 		 "(rel=%s, blockNum=%u, flags=0x%x, refcount=%u %d)",
@@ -1802,6 +2228,7 @@ PrintBufferLeakWarning(Buffer buffer)
 		 buf->tag.blockNum, buf->flags,
 		 buf->refcount, loccount);
 	pfree(path);
+/* #endif defined(DO_ISCAN_DISCARD_BUFFER) && defined(DO_HEAPSCAN_DISCARD_BUFFER) not yet working safely 130530 */
 }
 
 /*
@@ -1918,7 +2345,7 @@ FlushBuffer(volatile BufferDesc *buf, SM
 	 * false, then someone else flushed the buffer before we could, so we need
 	 * not do anything.
 	 */
-	if (!StartBufferIO(buf, false))
+	if (!StartBufferIO(buf, false, 0))
 		return;
 
 	/* Setup error traceback support for ereport() */
@@ -2502,6 +2929,70 @@ FlushDatabaseBuffers(Oid dbid)
 	}
 }
 
+#ifdef USE_PREFETCH
+/*
+ * DiscardBuffer -- discard shared buffer used for a previously
+ *                  prefetched but unread block of a relation
+ *
+ * If the buffer is found and pinned with a banked pin,  then :
+ *      .  if AIO in progress, terminate AIO without waiting
+ *      .  if AIO had already completed successfully,
+ *         then mark buffer valid (in case someone else wants it)
+ *      .  redeem the banked pin and unpin it.
+ *
+ * This function is similar in purpose to ReleaseBuffer (below)
+ * but sufficiently different that it is a separate function.
+ * Two important differences are :
+ *   .   caller identifies buffer by blocknumber,  not buffer number
+ *   .   we unpin buffer *only* if the pin is banked,
+ *                      *never* if pinned but not banked.
+ *       This is essential as caller may perform a sequence of
+ *  SCAN1   . PrefetchBuffer   (and remember block was prefetched)
+ *  SCAN2   . ReadBuffer       (but fails to connect this read to the prefetch by SCAN1)
+ *  SCAN1   . DiscardBuffer    (SCAN1 terminates early)
+ *  SCAN2   . access tuples in buffer
+ *        Clearly the Discard *must not* unpin the buffer since SCAN2 needs it!
+ *
+ *
+ * caller may pass InvalidBlockNumber as blockNum to mean do nothing
+ */
+void
+DiscardBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
+{
+        BufferTag	newTag;		   /* identity of requested block */
+        uint32		newHash;	   /* hash value for newTag */
+        LWLockId	newPartitionLock;  /* buffer partition lock for it */
+        Buffer		buf_id;
+        volatile        BufferDesc *buf_desc;
+
+    if (!SmgrIsTemp(reln->rd_smgr)) {
+	Assert(RelationIsValid(reln));
+	if (BlockNumberIsValid(blockNum)) {
+
+            /* create a tag so we can lookup the buffer */
+            INIT_BUFFERTAG(newTag, reln->rd_smgr->smgr_rnode.node,
+                                       forkNum, blockNum);
+
+            /* determine its hash code and partition lock ID */
+            newHash = BufTableHashCode(&newTag);
+            newPartitionLock = BufMappingPartitionLock(newHash);
+
+            /* see if the block is in the buffer pool already */
+            LWLockAcquire(newPartitionLock, LW_SHARED);
+            buf_id = BufTableLookup(&newTag, newHash);
+            LWLockRelease(newPartitionLock);
+
+            /* If in buffers, proceed */
+            if (buf_id >= 0) {
+                buf_desc = &BufferDescriptors[buf_id];        /* found buffer descriptor */
+                BufCheckAsync(0 , reln, buf_desc , BUF_INTENTION_REJECT_UNBANK , 0 , 0 , false , 0); /* end the IO and unpin if banked */
+                pgBufferUsage.aio_read_discrd++; /* account for it */
+            }
+        }
+    }
+}
+#endif   /* USE_PREFETCH */
+
 /*
  * ReleaseBuffer -- release the pin on a buffer
  */
@@ -2510,26 +3001,23 @@ ReleaseBuffer(Buffer buffer)
 {
 	volatile BufferDesc *bufHdr;
 
+
 	if (!BufferIsValid(buffer))
 		elog(ERROR, "bad buffer ID: %d", buffer);
 
-	ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
 
 	if (BufferIsLocal(buffer))
 	{
+                ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
 		Assert(LocalRefCount[-buffer - 1] > 0);
 		LocalRefCount[-buffer - 1]--;
 		return;
 	}
-
-	bufHdr = &BufferDescriptors[buffer - 1];
-
-	Assert(PrivateRefCount[buffer - 1] > 0);
-
-	if (PrivateRefCount[buffer - 1] > 1)
-		PrivateRefCount[buffer - 1]--;
 	else
-		UnpinBuffer(bufHdr, false);
+        {
+                bufHdr = &BufferDescriptors[buffer - 1];
+                BufCheckAsync(0 , 0 , bufHdr , BUF_INTENTION_REJECT_NOADJUST , 0 , 0 , false , 0 );
+        }
 }
 
 /*
@@ -2555,14 +3043,41 @@ UnlockReleaseBuffer(Buffer buffer)
 void
 IncrBufferRefCount(Buffer buffer)
 {
+        bool       pin_already_banked_by_me = false;  /* buffer is already pinned by me and redeemable */
+        volatile BufferDesc *buf;                     /* descriptor for a shared buffer */
+
 	Assert(BufferIsPinned(buffer));
+
+        if (!(BufferIsLocal(buffer))) {
+                buf = &BufferDescriptors[buffer - 1];
+		LockBufHdr(buf);
+                pin_already_banked_by_me =
+                      (    (buf->flags & BM_AIO_PREFETCH_PIN_BANKED)
+                        && (   (    (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio )
+                                                                      : (-(buf->freeNext))  )  == this_backend_pid )
+                      );
+        }
+
+        if (!pin_already_banked_by_me) {
 	ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
 	ResourceOwnerRememberBuffer(CurrentResourceOwner, buffer);
+        }
+
 	if (BufferIsLocal(buffer))
 		LocalRefCount[-buffer - 1]++;
-	else
+	else {
+                if (pin_already_banked_by_me) {
+                    buf->flags &= ~BM_AIO_PREFETCH_PIN_BANKED;
+                    if (!(buf->flags & BM_AIO_IN_PROGRESS)) {
+                        buf->freeNext = FREENEXT_NOT_IN_LIST;      /*  forget the bank client */
+                    }
+                }
+		UnlockBufHdr(buf);
+                if (!pin_already_banked_by_me) {
 		PrivateRefCount[buffer - 1]++;
 }
+        }
+}
 
 /*
  * MarkBufferDirtyHint
@@ -2984,61 +3499,138 @@ WaitIO(volatile BufferDesc *buf)
  *
  * In some scenarios there are race conditions in which multiple backends
  * could attempt the same I/O operation concurrently.  If someone else
- * has already started I/O on this buffer then we will block on the
+ * has already started synchronous I/O on this buffer then we will block on the
  * io_in_progress lock until he's done.
  *
+ * if an async io is in progress and we are doing synchronous io,
+ * then readbuffer uses call to smgrcompleteaio to wait,
+ * and so we treat this request as if no io in progress
+ *
  * Input operations are only attempted on buffers that are not BM_VALID,
  * and output operations only on buffers that are BM_VALID and BM_DIRTY,
  * so we can always tell if the work is already done.
  *
+ * index_for_aio is an input parm which,  if non-zero,  identifies a BufferAiocb
+ * acquired by caller and to be attached to the buffer header for use with async io
+ *
  * Returns TRUE if we successfully marked the buffer as I/O busy,
  * FALSE if someone else already did the work.
  */
 static bool
-StartBufferIO(volatile BufferDesc *buf, bool forInput)
+StartBufferIO(volatile BufferDesc *buf, bool forInput , int index_for_aio )
 {
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        struct BufferAiocb volatile * BAiocb = (struct BufferAiocb*)0;  /*  BufferAiocb for use with aio */
+#endif   /* USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+ 
+        if (!index_for_aio)
 	Assert(!InProgressBuf);
 
 	for (;;)
 	{
+                if (!index_for_aio) {
 		/*
 		 * Grab the io_in_progress lock so that other processes can wait for
 		 * me to finish the I/O.
 		 */
 		LWLockAcquire(buf->io_in_progress_lock, LW_EXCLUSIVE);
+                }
 
 		LockBufHdr(buf);
 
-		if (!(buf->flags & BM_IO_IN_PROGRESS))
+                /*     the following test is intended to distinguish between :
+                **      .   buffer which : 
+                **           .     has io in progress
+                **             AND is not associated with a current aio
+                **      .   not the above
+                **     Here,  "recent" means an aio marked by buf->freeNext <= FREENEXT_BAIOCB_ORIGIN but no longer in progress -
+                **          this situation arises when the aio has just been cancelled and this process now wishes to recycle the buffer.
+                **          In this case,  the first such would-be recycler (i.e. me) must :
+                **             . avoid waiting for the cancelled aio to complete
+                **             . if not myself doing async read, then assume responsibility for posting other future readbuffers.
+                */
+		if (    (buf->flags & BM_AIO_IN_PROGRESS)
+             || (!(buf->flags & BM_IO_IN_PROGRESS))
+           )
 			break;
 
 		/*
-		 * The only way BM_IO_IN_PROGRESS could be set when the io_in_progress
+		 * The only way BM_IO_IN_PROGRESS without AIO in progress could be set when the io_in_progress
 		 * lock isn't held is if the process doing the I/O is recovering from
 		 * an error (see AbortBufferIO).  If that's the case, we must wait for
 		 * him to get unwedged.
 		 */
 		UnlockBufHdr(buf);
+                if (!index_for_aio) {
 		LWLockRelease(buf->io_in_progress_lock);
+                }
 		WaitIO(buf);
 	}
 
-	/* Once we get here, there is definitely no I/O active on this buffer */
-
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+	/* Once we get here, there is definitely no synchronous I/O active on this buffer
+        ** but if being asked to attach a BufferAiocb to the buf header,
+        ** then we must also check if there is any async io currently
+        ** in progress or pinned started by a different task.
+        */
+        if (index_for_aio) {
+            BAiocb = (BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext);
+            if (    (buf->flags & (BM_AIO_IN_PROGRESS|BM_AIO_PREFETCH_PIN_BANKED))
+                 && (buf->freeNext <= FREENEXT_BAIOCB_ORIGIN)
+                 && (BAiocb->pidOfAio != this_backend_pid)
+               ) {
+                    /* someone else already doing async I/O */
+                    UnlockBufHdr(buf);
+                    return false;
+            }
+	}
+#endif   /* USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 	if (forInput ? (buf->flags & BM_VALID) : !(buf->flags & BM_DIRTY))
 	{
 		/* someone else already did the I/O */
 		UnlockBufHdr(buf);
+                if (!index_for_aio) {
 		LWLockRelease(buf->io_in_progress_lock);
+                }
 		return false;
 	}
 
 	buf->flags |= BM_IO_IN_PROGRESS;
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        if (index_for_aio) {
+            BAiocb = (BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - index_for_aio);
+            /*   insist that no other buffer is using this BufferAiocb for async IO */
+            if (BAiocb->BAiocbbufh == (struct sbufdesc *)0) {
+                BAiocb->BAiocbbufh = buf;
+            }
+            if (BAiocb->BAiocbbufh != buf) {
+                               ereport(ERROR,
+                                    (errcode(ERRCODE_INTERNAL_ERROR),
+                                     errmsg("AIO control block %p to be used by %p already in use by %p"
+                                              ,BAiocb ,buf , BAiocb->BAiocbbufh)));
+            }
+            /*   note - there is no need to register self as an dependent of BAiocb
+            **   as we shall not unlock buf_desc before we free the BAiocb
+            */
+
+            buf->flags |= BM_AIO_IN_PROGRESS;
+            buf->freeNext = index_for_aio;
+            /*  at this point,  this buffer appears to have an in-progress aio_read,
+            **  and any other task which is able to look inside the buffer might try waiting on that aio -
+            **  except we have not yet issued the aio!   So we must keep the buffer header locked
+            **  from here all the way back to the BufStartAsync caller
+            */
+        } else {
+#endif   /* USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 	UnlockBufHdr(buf);
 
 	InProgressBuf = buf;
 	IsForInput = forInput;
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        }
+#endif   /* USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 	return true;
 }
@@ -3048,7 +3640,7 @@ StartBufferIO(volatile BufferDesc *buf,
  *	(Assumptions)
  *	My process is executing IO for the buffer
  *	BM_IO_IN_PROGRESS bit is set for the buffer
- *	We hold the buffer's io_in_progress lock
+ *	if no async IO in progress,  then We hold the buffer's io_in_progress lock
  *	The buffer is Pinned
  *
  * If clear_dirty is TRUE and BM_JUST_DIRTIED is not set, we clear the
@@ -3060,26 +3652,32 @@ StartBufferIO(volatile BufferDesc *buf,
  * BM_IO_ERROR in a failure case.  For successful completion it could
  * be 0, or BM_VALID if we just finished reading in the page.
  */
-static void
+void
 TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
 				  int set_flag_bits)
 {
-	Assert(buf == InProgressBuf);
+        int flags_on_entry;
 
 	LockBufHdr(buf);
 
+        flags_on_entry = buf->flags;
+
+        if (! (flags_on_entry & BM_AIO_IN_PROGRESS) )
+            Assert( buf == InProgressBuf );
+
 	Assert(buf->flags & BM_IO_IN_PROGRESS);
-	buf->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR);
+	buf->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR | BM_AIO_IN_PROGRESS);
 	if (clear_dirty && !(buf->flags & BM_JUST_DIRTIED))
 		buf->flags &= ~(BM_DIRTY | BM_CHECKPOINT_NEEDED);
 	buf->flags |= set_flag_bits;
 
 	UnlockBufHdr(buf);
 
+        if (! (flags_on_entry & BM_AIO_IN_PROGRESS) ) {
 	InProgressBuf = NULL;
-
 	LWLockRelease(buf->io_in_progress_lock);
 }
+}
 
 /*
  * AbortBufferIO: Clean up any active buffer I/O after an error.
--- src/backend/storage/buffer/buf_async.c.orig	2014-08-19 10:17:32.822616336 -0400
+++ src/backend/storage/buffer/buf_async.c	2014-08-19 16:56:13.535196628 -0400
@@ -0,0 +1,1203 @@
+/*-------------------------------------------------------------------------
+ *
+ * buf_async.c
+ *	  buffer manager asynchronous disk read routines
+ *
+ * Portions Copyright (c) 1996-2012, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/storage/buffer/buf_async.c
+ *
+ *-------------------------------------------------------------------------
+ */
+/*
+ * Principal entry points:
+ *
+ * BufStartAsync() -- start an asynchronous read of a block into a buffer and
+ *	 pin it so that no one can destroy it while this process is using it.
+ *
+ * BufCheckAsync() -- check completion of an asynchronous read
+ *       and either claim buffer or discard it
+ *
+ * Private helper
+ *
+ * BufReleaseAsync() -- release the BAiocb resources used for an asynchronous read
+ *
+ * See also these files:
+ *		bufmgr.c -- main buffer manager functions
+ *		buf_init.c -- initialisation of resources
+ */
+#include "postgres.h"
+#include <sys/types.h> 
+#include <sys/file.h>
+#include <unistd.h>
+#include <sched.h>
+
+#include "catalog/catalog.h"
+#include "common/relpath.h"
+#include "executor/instrument.h"
+#include "miscadmin.h"
+#include "pg_trace.h"
+#include "pgstat.h"
+#include "postmaster/bgwriter.h"
+#include "storage/buf_internals.h"
+#include "storage/bufmgr.h"
+#include "storage/ipc.h"
+#include "storage/proc.h"
+#include "storage/smgr.h"
+#include "storage/standby.h"
+#include "utils/rel.h"
+#include "utils/resowner_private.h"
+
+/*
+ * GUC parameters
+ */
+int      max_async_io_prefetchers; /* Maximum number of backends using asynchronous librt threads to read pages into our buffers */
+
+#if defined(USE_AIO_SIGEVENT)
+long num_startedaio = 0;
+long num_cancelledaio = 0;
+long num_signalledaio = 0;                  /* count of number of signals delivered - counted by handlaiosignal */
+unsigned long BAiocbaioOrdinal = 0;    /* ordinal of most recent aio originated by me */
+#endif /*  USE_AIO_SIGEVENT   && defined(COUNT_QUEUED_SIGNALS)  */
+
+/* Note: these two macros only work on shared buffers, not local ones! */
+#define BufHdrGetBlock(bufHdr)	((Block) (BufferBlocks + ((Size) (bufHdr)->buf_id) * BLCKSZ))
+#define BufferGetLSN(bufHdr)	(PageGetLSN(BufHdrGetBlock(bufHdr)))
+
+extern volatile struct BAiocbAnchor *BAiocbAnchr;  /*  anchor for all control blocks pertaining to aio  */
+extern int numBufferAiocbs;        /*  total number of  BufferAiocbs in pool  */
+extern int maxGetBAiocbTries;      /*   max times we will try to get a free BufferAiocb */
+extern int maxRelBAiocbTries;      /*   max times we will try to release a BufferAiocb back to freelist */
+extern pid_t this_backend_pid;     /*   pid of this backend */
+
+extern bool PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy);
+extern void PinBuffer_Locked(volatile BufferDesc *buf);
+extern Buffer ReadBuffer_common(SMgrRelation reln, char relpersistence,
+				  ForkNumber forkNum, BlockNumber blockNum,
+				  ReadBufferMode mode, BufferAccessStrategy strategy,
+				  bool *hit , int index_for_aio);
+extern void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner);
+extern void TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
+				  int set_flag_bits);
+int BufCheckAsync(SMgrRelation caller_smgr, Relation caller_reln, volatile BufferDesc *buf_desc
+  ,int intention , BufferAccessStrategy strategy , int index_for_aio , bool spinLockHeld , LWLockId PartitionLock );
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+int BufStartAsync(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy);
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on signal instead of polling */
+/*   PoorMans atomics forward decls */
+static int PoorMLock(volatile int *intP);
+static int PoorMUnLock(volatile int *intP);
+static int PoorMIncr(volatile int *intP);
+static int PoorMDecr(volatile int *intP);
+void BufPostAioWaiters(struct BufferAiocb volatile * BAiocb);
+void BufHandlSigAsync(int sigsent, siginfo_t * siginfP, void *ucontext);
+int BufAWaitAioCompletion (struct BufferAiocb *BAiocb);
+#endif  /* USE_AIO_SIGEVENT    */
+
+static struct BufferAiocb volatile * cachedBAiocb = (struct BufferAiocb*)0;  /*  one cached BufferAiocb for use with aio */
+
+/*  BufReleaseAsync releases a BufferAiocb and returns 0 if successful else non-zero
+**  it *must* be called :
+**     EITHER with a valid  BAiocb->BAiocbbufh -> buf_desc
+**            and that buf_desc must be spin-locked
+**     OR     with BAiocb->BAiocbbufh == 0
+*/
+static int
+BufReleaseAsync(struct BufferAiocb volatile * BAiocb)
+{
+        int    LockTries;         /*  max times we will try to release the BufferAiocb */
+        volatile struct BufferAiocb *BufferAiocbs;
+        struct BufferAiocb volatile * oldFreeBAiocb;  /*  old value of FreeBAiocbs */
+
+        int failed = 1; /* by end of this function, non-zero  will indicate if we failed to return the BAiocb */
+
+
+        if (    ( BAiocb == (struct BufferAiocb*)0 )
+             || ( BAiocb == (struct BufferAiocb*)BAIOCB_OCCUPIED )
+             || ( ((unsigned long)BAiocb) & 0x1 )
+           ) {
+                          elog(ERROR,
+                                 "AIO control block corruption on release of aiocb %p - invalid BAiocb"
+                                          ,BAiocb);
+        }
+        else 
+        if (   (0 == BAiocb->BAiocbDependentCount)     /*  no dependents  */
+            && ((struct BufferAiocb*)BAIOCB_OCCUPIED == BAiocb->BAiocbnext)  /*  not already on freelist */
+           ) {
+
+            if ((struct sbufdesc*)0 != BAiocb->BAiocbbufh) { /*  if a buffer was attached */
+                volatile        BufferDesc *buf_desc = BAiocb->BAiocbbufh;
+
+                /*  spinlock held so instead of TerminateBufferIO(buf, false , 0); ... */
+                if (buf_desc->flags & BM_AIO_PREFETCH_PIN_BANKED) { /* if a pid banked the pin */
+                    buf_desc->freeNext = -(BAiocb->pidOfAio);       /* then remember which pid */
+                }
+                else if (buf_desc->freeNext <= FREENEXT_BAIOCB_ORIGIN) {
+                    buf_desc->freeNext = FREENEXT_NOT_IN_LIST;   /* disconnect BufferAiocb from buf_desc */
+                }
+                buf_desc->flags &= ~BM_AIO_IN_PROGRESS;
+            }
+            
+            BAiocb->BAiocbbufh = (struct sbufdesc *)0; /* disconnect buf_desc from BufferAiocb */
+            BAiocb->pidOfAio = 0;                      /*  clean */
+            LockTries = maxRelBAiocbTries;         /*  max times we will try to release the BufferAiocb */
+            do {
+                register long long int dividend , remainder;
+
+                /*      retrieve old value of FreeBAiocbs  */
+                BAiocb->BAiocbnext = oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs;  /*  old value of FreeBAiocbs */
+
+                /*  this is a volatile value unprotected by any lock,  so must validate it;
+                **  safest is to verify that it is identical to one of the BufferAiocbs
+                **  to do so,  verify by direct division that its address offset from first control block 
+                **  is an integral multiple of the control block size
+                **  that lies within the range [ 0 , (numBufferAiocbs-1) ]
+                */
+                BufferAiocbs = BAiocbAnchr->BufferAiocbs;
+                remainder = ((long long int)oldFreeBAiocb - (long long int)BufferAiocbs)
+                            % (long long int)(sizeof(struct BufferAiocb));
+                failed = (int)remainder;
+                if (!failed) {
+                    dividend = ((long long int)oldFreeBAiocb - (long long int)BufferAiocbs)
+                               / (long long int)(sizeof(struct BufferAiocb));
+                     failed = ( (dividend < 0 ) || ( dividend >= numBufferAiocbs) );
+                     if (!failed) {
+                         if (__sync_bool_compare_and_swap (&(BAiocbAnchr->FreeBAiocbs), oldFreeBAiocb, BAiocb)) {
+                            LockTries = 0;   /*  end the do loop  */
+
+                            goto cheering;   /*  cant simply break because then failed would be set incorrectly */
+                         }
+                    }
+                }
+                /*  if we reach here, we have failed and failed is set to -1 */
+
+       cheering: ;
+
+                if ( LockTries > 1 ) {
+                    sched_yield();    /*   yield to another process,  (hopefully a backend) */
+                }
+            } while  (LockTries-- > 0);
+
+            if (failed) {
+#ifdef LOG_RELBAIOCB_DEPLETION
+                          elog(LOG,
+                                "BufReleaseAsync:AIO control block %p unreleased after tries= %d\n"
+                                       ,BAiocb,maxRelBAiocbTries);
+#endif  /* LOG_RELBAIOCB_DEPLETION */
+            }
+
+        }
+              else
+                          elog(LOG,
+                                "BufReleaseAsync:AIO control block %p either has dependents= %d or is already on freelist %p or has no buf_header %p\n"
+                                       ,BAiocb , BAiocb->BAiocbDependentCount , BAiocb->BAiocbnext , BAiocb->BAiocbbufh);
+        return failed;
+}
+
+/*  try using asynchronous aio_read to prefetch into a buffer
+**  return code :
+**        0 if started successfully
+**       -1 if failed for some reason
+**        1+PrivateRefCount if we found desired buffer in buffer pool
+**
+**  There is a harmless race condition here :
+**  two different backends may both arrive here simultaneously
+**  to prefetch the same buffer.    This is not unlikely when a syncscan is in progress.
+**  .  One will acquire the buffer and issue the smgrstartaio
+**  .  Other will find the buffer on return from  ReadBuffer_common with hit = true
+**  Only the first task has a pin on the buffer since ReadBuffer_common knows not to get a pin
+**  on a found buffer in prefetch mode.
+**  Therefore  -   the second task must simply abandon the prefetch if it finds the buffer in the buffer pool.
+**
+**  if we fail to acquire a BAiocb because of concurrent theft from freelist by other backend,
+**  retry up to maxGetBAiocbTries times provided that there actually was at least one BAiocb on the freelist.
+*/
+int
+BufStartAsync(Relation reln, ForkNumber forkNum, BlockNumber blockNum , BufferAccessStrategy strategy) {
+
+        int retcode = -1;
+        struct BufferAiocb volatile * prevBAiocb;  /*  previous BufferAiocb in cachedchain */
+        struct BufferAiocb volatile * BAiocb = (struct BufferAiocb*)0;  /*  BufferAiocb for use with aio */
+        int  smgrstartaio_rc = -1;           /*  retcode from smgrstartaio */
+        bool do_unpin_buffer = false;        /* unpin must be deferred until after buffer descriptor is unlocked */
+        Buffer		buf_id;
+        bool		hit = false;
+        volatile        BufferDesc *buf_desc = (BufferDesc *)0;
+
+        int    LockTries;         /*  max times we will try to get a free BufferAiocb */
+
+        struct BufferAiocb volatile * oldFreeBAiocb;  /*  old value of FreeBAiocbs */
+        struct BufferAiocb volatile * newFreeBAiocb;  /*  new value of FreeBAiocbs */
+
+
+    /*  return immediately if no async io resources */
+    if (numBufferAiocbs > 0) {
+        buf_id = (Buffer)0;
+
+        if ( (struct BAiocbAnchor *)0 != BAiocbAnchr ) {
+
+            volatile struct BufferAiocb *BufferAiocbs;
+
+            prevBAiocb = (struct BufferAiocb*)(&cachedBAiocb); /*  prev points to BAiocb */
+            BAiocb = cachedBAiocb;      /* tentatively try the first if there is one */
+            if ((struct BufferAiocb*)0 != BAiocb) {  /* any usable cached BufferAiocb ? */
+                prevBAiocb->BAiocbnext = BAiocb->BAiocbnext;  /* unchain from cached chain */
+                BAiocb->BAiocbnext = (struct BufferAiocb*)BAIOCB_OCCUPIED; /* mark as not on freelist */
+                BAiocb->BAiocbDependentCount = 0;  /* no dependent yet */
+                BAiocb->BAiocbbufh = (struct sbufdesc*)0;
+                BAiocb->pidOfAio = 0;
+            } else {
+
+                LockTries = maxGetBAiocbTries;         /*  max times we will try to get a free BufferAiocb */
+                do {
+                    register long long int dividend = -1 , remainder;
+                    /*  check if we have a free BufferAiocb */
+
+                    oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs;  /*  old value of FreeBAiocbs */
+
+                    /*  check if we have a free BufferAiocb */
+
+                    /*  BAiocbAnchr->FreeBAiocbs is a volatile value unprotected by any lock,
+                    **  and use of compare-and-swap to add and remove items from the list has
+                    **  two potential pitfalls,    both relating to the fact that we must
+                    **  access data de-referenced from this pointer before the compare-and-swap.
+                    **  1)  The value we load may be corrupt,  e.g. mixture of bytes from
+                    **      two different values,   so must validate it;
+                    **      safest is to verify that it is identical to one of the BufferAiocbs.
+                    **      to do so,  verify by direct division that its address offset from
+                    **      first control block is an integral multiple of the control block size
+                    **      that lies within the range [ 0 , (numBufferAiocbs-1) ]
+                    **      Thus we completely prevent this pitfall.
+                    **  2)  The content of the item's next pointer may have changed between the
+                    **      time we de-reference it and the time of the compare-and-swap.
+                    **      Thus even though the compare-and-swap succeeds,   we might set the
+                    **      new head of the freelist to an invalid value  (either a free item
+                    **      that is not the first in the free chain  -  resulting only in
+                    **      loss of the orphaned free items, or,  much worse, an in-use item).
+                    **      In practice this is extremely unlikely because of the implied huge delay
+                    **      in this window interval in this (current) process.    Here are two scenarios:
+                    **      legend:
+                    **         P0  -  this (current) process,  P1,  P2 , ... other processes
+                    **         content of freelist shown as BAiocbAnchr->FreeBAiocbs -> first item -> 2nd item ...
+                    **         @[X] means address of X
+                    **         |      timeline of window of exposure to problems
+                    **      successive lines in chronological order                                       content of freelist
+                    **        2.1    P0           access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I0]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **               P0          access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I1]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1        access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I0]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1       access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I1]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1        swap-remove I0,  place I1 at head of list                F -> I1 -> I2 -> I3 ...
+                    **         |           P2     access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I1]  F -> I1 -> I2 -> I3 ...
+                    **         |           P2    access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I2]  F -> I1 -> I2 -> I3 ...
+                    **         |           P2     swap-remove I1,  place I2 at head of list                F -> I2 -> I3 ...
+                    **         |        P1       complete aio,  replace I0 at head of list                 F -> I0 -> I2 -> I3 ...
+                    **               P0        swap-remove I0,  place I1 at head of list                   F -> I1 IS IN USE !! CORRUPT !!
+                    **               compare-and-swap succeeded but the value of newFreeBAiocb was stale
+                    **               and had become in-use during the window.
+                    **        2.2    P0           access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I0]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **               P0          access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I1]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1        access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I0]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1       access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I1]  F -> I0 -> I1 -> I2 -> I3 ...
+                    **         |        P1        swap-remove I0,  place I1 at head of list                F -> I1 -> I2 -> I3 ...
+                    **         |           P2     access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I1]  F -> I1 -> I2 -> I3 ...
+                    **         |           P2    access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I2]  F -> I1 -> I2 -> I3 ...
+                    **         |           P2     swap-remove I1,  place I2 at head of list                F -> I2 -> I3 ...
+                    **         |              P3  access oldFreeBAiocb = BAiocbAnchr->FreeBAiocbs = @[I2]  F -> I2 -> I3 ...
+                    **         |              P3 access newFreeBAiocb = oldFreeBAiocb->BAiocbnext = @[I3]  F -> I2 -> I3 ...
+                    **         |              P3  swap-remove I2,  place I3 at head of list                F -> I3 ...
+                    **         |           P2    complete aio,  replace I1 at head of list                 F -> I1 -> I3 ...
+                    **         |              P3 complete aio,  replace I2 at head of list                 F -> I2 -> I1 -> I3 ...
+                    **         |        P1       complete aio,  replace I0 at head of list                 F -> I0 -> I2 -> I1 -> I3 ...
+                    **               P0        swap-remove I0,  place I1 at head of list                   F -> I1 -> I3 ... ! I2 is orphaned !
+                    **               compare-and-swap succeeded but the value of newFreeBAiocb was stale
+                    **               and had moved further down the free list during the window.
+                    **      Unfortunately, we cannot prevent this pitfall but we can detect it (after the fact),
+                    **      by checking that the next pointer of the item we have just removed for our use still points to the same item.
+                    **      This test is not subject to any timing or uncertainty since :
+                    **       .  The fact that the compare-and-swap succeeded implies that the item we removed
+                    **          was defintely on the freelist (at the head) when it was removed,
+                    **          and therefore cannot be in use,  and therefore its next pointer is no longer volatile.
+                    **       .  Although pointers of the anchor and items on the freelist are volatile,
+                    **          the addresses of items never change -  they are in an allocated array and never move.
+                    **      E.g. in the above two scenarios,   the test is that I0.next still -> I1,
+                    **      and this is true if and only if the second item on the freelist is
+                    **      still the same at the end of the window as it was at the start of the window.
+                    **      Note that we do not insist that it did not change during the window,
+                    **           only that it is still the correct new head of freelist.
+                    **      If this test fails,  we abort immediately as the subsystem is damaged and cannot be repaired.
+                    **      Note that at least one aio must have been issued *and* completed during the window
+                    **           for this to occur,  and since the window is just one single machine instruction,
+                    **           it is very unlikely in practice.
+                    */
+                    BufferAiocbs = BAiocbAnchr->BufferAiocbs;
+                    remainder = ((long long int)oldFreeBAiocb - (long long int)BufferAiocbs)
+                                % (long long int)(sizeof(struct BufferAiocb));
+                    if (remainder == 0) {
+                        dividend = ((long long int)oldFreeBAiocb - (long long int)BufferAiocbs)
+                                    / (long long int)(sizeof(struct BufferAiocb));
+                    }
+                    if (    (remainder == 0)
+                         && ( (dividend >= 0 ) && ( dividend < numBufferAiocbs) )
+                       )
+                    {
+                        newFreeBAiocb = oldFreeBAiocb->BAiocbnext; /*  tentative new value is second on free list */
+                        /*   Here we are in the exposure window referred to in the above comments,
+                        **   so moving along rapidly ...
+                        */
+                        if (__sync_bool_compare_and_swap (&(BAiocbAnchr->FreeBAiocbs), oldFreeBAiocb, newFreeBAiocb)) {   /*  did we get it ? */
+                                /*   We have successfully swapped head of freelist pointed to by oldFreeBAiocb off the list;
+                                **   Here we check that the item we just placed at head of freelist, pointed to by newFreeBAiocb,
+                                **   is the right one
+                                **
+                                **   also check that the BAiocb we have acquired was not in use
+                                **   i.e. that scenario 2.1 above did not occur just before our compare-and-swap
+                                **   The test is that the BAiocb is not in use.
+                                **
+                                **  in one hypothetical case,
+                                **  we can be certain that there is no corruption -
+                                **  the case where newFreeBAiocb == 0 and oldFreeBAiocb->BAiocbnext != BAIOCB_OCCUPIED -
+                                **  i.e. we have set the freelist to empty but we have a baiocb chained from ours.
+                                **  in this case our comp_swap removed all BAiocbs from the list (including ours)
+                                **  so the others chained from ours are either orphaned (no harm done)
+                                **  or in use by another backend and will eventually be returned (fine).
+                                */
+                                if ((struct BufferAiocb *)0 == newFreeBAiocb) {
+                                    if ((struct BufferAiocb *)BAIOCB_OCCUPIED == oldFreeBAiocb->BAiocbnext) {
+                                        goto baiocb_corruption;
+                                    } else if ((struct BufferAiocb *)0 != oldFreeBAiocb->BAiocbnext) {
+                                      elog(LOG,
+                                         "AIO control block inconsistency on acquiring aiocb %p - its next free %p may be orphaned (no corruption has occurred)"
+                                         	,oldFreeBAiocb , oldFreeBAiocb->BAiocbnext);
+                                    }
+                                } else {
+                                    /*  case of newFreeBAiocb not null  -  so must check more carefully ... */
+                                    remainder = ((long long int)newFreeBAiocb - (long long int)BufferAiocbs)
+                                                % (long long int)(sizeof(struct BufferAiocb));
+                                    dividend = ((long long int)newFreeBAiocb - (long long int)BufferAiocbs)
+                                                / (long long int)(sizeof(struct BufferAiocb));
+
+                                    if (    (newFreeBAiocb != oldFreeBAiocb->BAiocbnext)
+                                         || (remainder != 0)
+                                         || ( (dividend < 0 ) || ( dividend >= numBufferAiocbs) )
+                                       ) {
+                                        goto baiocb_corruption;
+                                    }
+                                }
+                                BAiocb = oldFreeBAiocb;
+                                BAiocb->BAiocbnext = (struct BufferAiocb*)BAIOCB_OCCUPIED; /* mark as not on freelist */
+                                BAiocb->BAiocbDependentCount = 0;  /* no dependent yet */
+                                BAiocb->BAiocbbufh = (struct sbufdesc*)0;
+                                BAiocb->pidOfAio = 0;
+
+                                LockTries = 0;   /*  end the do loop  */
+
+                        }
+                    }
+
+                    if ( LockTries > 1 ) {
+                        sched_yield();    /*   yield to another process,  (hopefully a backend) */
+                    }
+                } while (     ((struct BufferAiocb*)0 == BAiocb)            /*  did not get a BAiocb    */
+                           && ((struct BufferAiocb*)0 != oldFreeBAiocb)     /*  there was a free BAiocb */
+                           && (LockTries-- > 0)                             /*  told to retry           */
+                        );
+            }
+        }
+
+        if ( BAiocb != (struct BufferAiocb*)0 ) {
+            /*  try an async io    */
+            BAiocb->BAiocbthis.aio_fildes = -1; /* necessary to ensure any thief realizes aio not yet started */
+            BAiocb->pidOfAio = this_backend_pid;
+
+            /*  now try to acquire a buffer :
+            **  note -   ReadBuffer_common returns hit=true if the block is found in the buffer pool,
+            **            in which case there is no need to prefetch.
+            **  otherwise ReadBuffer_common pins returned buffer and calls StartBufferIO
+            **           and StartBufferIO :
+            **      . sets buf_desc->freeNext to negative of ( index of the aiocb in the BufferAiocbs array + 3 )
+            **      . sets  BAiocb->BAiocbbufh -> buf_desc
+            **           and in this case the buffer spinlock is held.
+            **           This is essential as no other task must issue any intention with respect
+            **           to the buffer until we have started the aio_read.
+            **  Also note that ReadBuffer_common handles enlarging the ResourceOwner buffer list as needed
+            **       so I dont need to do that
+            */
+            buf_id = ReadBuffer_common(reln->rd_smgr, reln->rd_rel->relpersistence,
+                                        forkNum, blockNum
+                                       ,RBM_NOREAD_FOR_PREFETCH  /*  tells ReadBuffer not to do any read,  just alloc buf */
+                                       ,strategy , &hit , (FREENEXT_BAIOCB_ORIGIN - (BAiocb - (BAiocbAnchr->BufferAiocbs))));
+            buf_desc = &BufferDescriptors[buf_id-1];    /* find buffer descriptor */
+
+            /*  normally hit will be false as presumably it was not in the pool
+            **  when our caller looked - but it could be there now ...
+            */
+            if (hit) {
+                   /*   see earlier comments  -  we must abandon the prefetch */
+                   retcode = 1 + PrivateRefCount[buf_id];
+                   BAiocb->BAiocbbufh = (struct sbufdesc *)0; /* indicator that we must release the BAiocb before exiting */
+            } else
+            if (  (buf_id > 0) && ((BufferDesc *)0 != buf_desc) && (buf_desc == BAiocb->BAiocbbufh)  ) {
+                   /*   buff descriptor header lock should be held.
+                   **   However,  just to be safe ,   now validate that
+                   **   we are still the owner and no other task already stole it.
+                   */
+
+                   buf_desc->flags &= ~BM_AIO_PREFETCH_PIN_BANKED; /* ensure no banked pin */
+                   /*  there should not be any other pid waiting on this buffer
+                   **  so check both of BM_VALID and BM_PIN_COUNT_WAITER are not set
+                   */
+                   if (    ( !(buf_desc->flags & (BM_VALID|BM_PIN_COUNT_WAITER) ) )
+                        && (buf_desc->flags & BM_AIO_IN_PROGRESS)
+                        && ((FREENEXT_BAIOCB_ORIGIN - (BAiocb - (BAiocbAnchr->BufferAiocbs))) == buf_desc->freeNext) /* it is still mine */
+                        && (-1 == BAiocb->BAiocbthis.aio_fildes)   /* no thief stole it */
+                        && (0 == BAiocb->BAiocbDependentCount)     /* no dependent */
+                     ) {
+                        /*   we have an empty buffer for our use */
+
+                        BAiocb->BAiocbthis.aio_buf = (void *)(BufHdrGetBlock(buf_desc)); /* Location of actual buffer.  */
+
+                        /*   note - there is no need to register self as an dependent of BAiocb
+                        **   as we shall not unlock buf_desc before we free the BAiocb
+                        */
+
+                        /*   smgrstartaio retcode is returned in smgrstartaio_rc -
+                        **   it indicates whether started or not
+                        */
+#ifdef USE_AIO_SIGEVENT
+                        BAiocb->BAiocbaioOrdinal = BAiocbaioOrdinal;
+#endif /*  USE_AIO_SIGEVENT    */
+                        smgrstartaio(reln->rd_smgr, forkNum, blockNum , (char *)&(BAiocb->BAiocbthis) , &smgrstartaio_rc
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on signal instead of polling */
+                                    ,BAiocbaioOrdinal
+#endif /*  USE_AIO_SIGEVENT    */
+                                    );
+#if defined(USE_AIO_SIGEVENT)
+                        BAiocbaioOrdinal++;    /* ordinal of most recent aio originated by me */
+#endif /*  USE_AIO_SIGEVENT    */
+
+                        if (smgrstartaio_rc == 0) {
+                            retcode = 0;
+                            buf_desc->flags |= BM_AIO_PREFETCH_PIN_BANKED; /* bank the pin for next use by this task */
+                            /* we did not register register self as an dependent of BAiocb so no need to unregister  */
+                        } else {
+                            /*  failed - return the BufferAiocb to the free list  */
+                            do_unpin_buffer = true;    /* unpin must be deferred until after buffer descriptor is unlocked */
+                            /*  spinlock held so instead of TerminateBufferIO(buf_desc, false , 0); ... */
+                            buf_desc->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR | BM_AIO_IN_PROGRESS | BM_AIO_PREFETCH_PIN_BANKED | BM_VALID);
+                            /* we did not register register self as an dependent of BAiocb so no need to unregister  */
+
+                            /*  return the BufferAiocb to the free list  */
+                            if (
+                                   BufReleaseAsync(BAiocb)
+                               ) {         /*  failed ? */
+                                BAiocb->BAiocbnext = cachedBAiocb;    /* then ...       */
+                                cachedBAiocb = BAiocb;                /*  ... cache it  */
+                            }
+
+                            pgBufferUsage.aio_read_failed++;
+                            smgrstartaio_rc = 1;  /*   to distinguish from aio not even attempted */
+                        }
+                   }
+                   else {
+                            /*  buffer was stolen or in use by other task - return the BufferAiocb to the free list  */
+                            do_unpin_buffer = true;    /* unpin must be deferred until after buffer descriptor is unlocked */
+                   }
+
+                   UnlockBufHdr(buf_desc);
+                   if (do_unpin_buffer) {
+                        if (smgrstartaio_rc >= 0) { /*  if  aio was attempted */
+                            TerminateBufferIO(buf_desc, false , 0);
+                        }
+                        UnpinBuffer(buf_desc, true);
+                   }
+            }
+            else {
+                BAiocb->BAiocbbufh = (struct sbufdesc *)0; /* indicator that we must release the BAiocb before exiting */
+            }
+
+            if ((struct sbufdesc*)0 == BAiocb->BAiocbbufh) { /*  we did not associate a buffer */
+                                                             /*  so return the BufferAiocb to the free list  */
+                            if (
+                                   BufReleaseAsync(BAiocb)
+                               ) {         /*  failed ? */
+                                BAiocb->BAiocbnext = cachedBAiocb;    /* then ...       */
+                                cachedBAiocb = BAiocb;                /*  ... cache it  */
+                            }
+            }
+        }
+    }
+
+    return retcode;
+
+    baiocb_corruption:;
+         elog(PANIC,
+              "AIO control block corruption on acquiring aiocb %p - its next free %p conflicts with new freelist pointer %p which may be invalid (corruption may have occurred)"
+                                                    ,oldFreeBAiocb , oldFreeBAiocb->BAiocbnext , newFreeBAiocb);
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+/*
+ * BufCheckAsync --      act upon caller's intention regarding a shared buffer,
+ *                       primarily in connection with any async io in progress on the buffer.
+ *     class  subvalue   intention has two main classes and some subvalues within those :
+ *      +ve      1            .   want    <=>  caller wants the buffer,
+ *                                             wait for in-progress aio and then always pin
+ *      -ve                   .   reject  <=>  caller does not want the buffer,
+ *                                             if there are no dependents,  then cancel the aio
+ *              -1, -2 , -3 , ... (see below)        and then optionally unpin
+ *                             Used when there may have been a previous fetch or prefetch.
+ *
+ * buffer is assumed to be an existing member of the shared buffer pool
+ *    as returned by BufTableLookup.
+ * if AIO in progress,  then :
+ *      .  terminate AIO, waiting for completion if +ve intention, else without waiting
+ *      .  if the AIO had already completed successfully,   then mark buffer valid
+ *      .  pin/unpin as requested
+ *
+ * +ve intention indicates that buffer must be pinned :
+ *   if the strategy parameter is null,  then use the PinBuffer_Locked optimization
+ *   to pin and unlock in one operation.   But always update buffer usage count.
+ *
+ * -ve intention indicates whether and how to unpin :
+ *   BUF_INTENTION_REJECT_KEEP_PIN 	-1   pin already held, do not unpin, (caller wants to keep it)
+ *   BUF_INTENTION_REJECT_OBTAIN_PIN	-2   obtain pin,  caller wants it for same buffer
+ *   BUF_INTENTION_REJECT_FORGET	-3   unpin and tell resource owner to forget
+ *   BUF_INTENTION_REJECT_NOADJUST	-4   unpin and call ResourceOwnerForgetBuffer myself
+ *                                           instead of telling UnpinBuffer to adjust CurrentResource owner
+ *                                           (quirky simulation of ReleaseBuffer logic)
+ *   BUF_INTENTION_REJECT_UNBANK   	-5   unpin only if pin banked by caller
+ *   The behaviour for the -ve case is based on that of ReleaseBuffer, adding handling of async io.
+ *
+ * pin/unpin action must take account of whether this backend hold a "disposable" pin on the particular buffer.
+ * A "disposable" pin is a pin acquired by buffer manager without caller knowing, such as :
+ *      when required to safeguard an async AIO  -  pin can be held across multiple bufmgr calls
+ *      when required to safeguard waiting for an async AIO  -  pin acquired and released within this function
+ * if a disposable pin is held,   then :
+ *      if a new pin is requested,  the disposable pin must be retained (redeemed) and any flags relating to it unset
+ *      if an unpin is requested,   then :
+ *              if    either no AIO in progress or this backend did not initiate the AIO
+ *              then  the disposable pin must be dropped (redeemed) and any flags relating to it unset
+ *              else  log warning and do nothing
+ *  i.e. in either case,   there is no longer a disposable pin after this function has completed.
+ *       Note that if    intention is BUF_INTENTION_REJECT_UNBANK,
+ *                 then caller expects there to be a disposable banked pin
+ *                      and if there isn't one,  we do nothing
+ *                 for all other intentions,  if there is no disposable pin,   we pin/unpin normally.
+ *
+ * index_for_aio indicates the BAiocb to be used for next aio (see PrefetchBuffer)
+ * BufFreelistLockHeld indicates whether freelistlock is held
+ * spinLockHeld indicates whether buffer header spinlock is held
+ * PartitionLock is the  buffer partition lock to be used
+ *
+ * return code (meaningful ONLY if intention is +ve) indicates validity of buffer :
+ *  -1    buffer is invalid and failed PageHeaderIsValid check
+ *   0    buffer is not valid
+ *   1    buffer is valid
+ *   2    buffer is valid but tag changed  -  (so content does not match the relation block that caller expects)
+ */
+int
+BufCheckAsync(SMgrRelation caller_smgr, Relation caller_reln, BufferDesc volatile * buf_desc, int intention , BufferAccessStrategy strategy , int index_for_aio , bool spinLockHeld , LWLockId PartitionLock )
+{
+
+        int retcode = BUF_INTENT_RC_INVALID_NO_AIO;
+        bool valid = false;
+
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        int smgrcompleteaio_rc;          /*  retcode from smgrcompleteaio */
+        SMgrRelation smgr = caller_smgr;
+        int    		BAiocbDependentCount_after_aio_finished = -1;  /*  for debugging  -  can be printed in gdb */
+	    BufferTag origTag = buf_desc->tag;	/* original identity of selected buffer */
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+        BufFlags	flags_on_entry;  /*  for debugging  -  can be printed in gdb */
+        int    		freeNext_on_entry;  /*  for debugging  -  can be printed in gdb */
+        bool       disposable_pin = false;            /* this backend had a disposable pin on entry or pins the buffer while waiting for aio_read to complete */
+        bool       pin_already_banked_by_me;          /* buffer is already pinned by me and redeemable */
+
+        int aio_successful = -1;         /*  did the aio_read succeed ?  -1 = no aio,  0 unsuccessful , 1 successful */
+        int local_intention = intention; /*  copy of intention which in one special case below may be set differently to intention */
+
+            if (!spinLockHeld) {
+                /*  lock buffer header    */
+                LockBufHdr(buf_desc);
+            }
+
+	    flags_on_entry = buf_desc->flags;
+	    freeNext_on_entry = buf_desc->freeNext;
+            pin_already_banked_by_me =
+                      (    (flags_on_entry & BM_AIO_PREFETCH_PIN_BANKED)
+                        && (   (    (flags_on_entry & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - freeNext_on_entry))->pidOfAio )
+                                                                      : (-(freeNext_on_entry))  )  == this_backend_pid )
+                      );
+
+            if (pin_already_banked_by_me) {
+                if (PrivateRefCount[buf_desc->buf_id] == 0) {  /*  but do we actually have a pin ?? */
+                    /*   this is an anomalous situation   -  somehow our disposable pin was lost without us noticing
+                    **   if AIO is in progress and we started it,
+                    **   then this is disastrous  -   two backends might both issue IO on same buffer
+                    **   otherwise,  it is harmless,  and simply means we have no disposable pin,
+                    **               but we must update flags to "notice" the fact now
+                    */
+                    if (flags_on_entry & BM_AIO_IN_PROGRESS) {
+                            elog(ERROR, "BufCheckAsync : AIO control block issuer of aio_read lost pin with BM_AIO_IN_PROGRESS on buffer %d rel=%s, blockNum=%u, flags 0x%X refcount=%u intention= %d"
+                                ,buf_desc->buf_id,relpathbackend(buf_desc->tag.rnode, InvalidBackendId, buf_desc->tag.forkNum)
+                           ,buf_desc->tag.blockNum, flags_on_entry, buf_desc->refcount ,intention);
+                    } else {
+                            elog(LOG, "BufCheckAsync : AIO control block issuer of aio_read lost pin on buffer %d rel=%s, blockNum=%u, with flags 0x%X refcount=%u intention= %d"
+                               ,buf_desc->buf_id,relpathbackend(buf_desc->tag.rnode, InvalidBackendId, buf_desc->tag.forkNum)
+                               ,buf_desc->tag.blockNum, flags_on_entry, buf_desc->refcount ,intention);
+							buf_desc->flags &= ~BM_AIO_PREFETCH_PIN_BANKED; /*   redeem the banked pin  */
+							/*   since AIO not in progress,  disconnect the buffer from banked pin */
+							buf_desc->freeNext = FREENEXT_NOT_IN_LIST;      /*   and forget the bank client */
+							pin_already_banked_by_me = false;
+                    }
+                } else {
+                    disposable_pin = true;
+                }
+            }
+
+            /*  the case of BUF_INTENTION_REJECT_UNBANK is handled specially :
+            **    if this backend has a banked pin,  then proceed just as for BUF_INTENTION_REJECT_FORGET
+            **    else the call is a no-op  --  unlock buf header and return immediately
+            */
+
+            if (intention == BUF_INTENTION_REJECT_UNBANK) {
+                if (pin_already_banked_by_me) {
+                    local_intention = BUF_INTENTION_REJECT_FORGET;
+                } else {
+                    goto unlock_buf_header;  /*  code following the unlock will do nothing since local_intention still set to BUF_INTENTION_REJECT_UNBANK */
+                }
+            }
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+            /*       we do not expect that BM_AIO_IN_PROGRESS is set without freeNext identifying the BAiocb */
+            if ( (buf_desc->flags & BM_AIO_IN_PROGRESS) && (buf_desc->freeNext == FREENEXT_NOT_IN_LIST) ) {
+
+					elog(ERROR, "BufCheckAsync : found BM_AIO_IN_PROGRESS without a BAiocb on buffer %d rel=%s, blockNum=%u, flags %X refcount=%u"
+						,buf_desc->buf_id,relpathbackend(buf_desc->tag.rnode, InvalidBackendId, buf_desc->tag.forkNum)
+						,buf_desc->tag.blockNum, buf_desc->flags, buf_desc->refcount);
+                }
+            /*     check whether aio in progress  */
+            if  (    ( (struct BAiocbAnchor *)0 != BAiocbAnchr )
+                  && (buf_desc->flags & BM_AIO_IN_PROGRESS)
+                  && (buf_desc->freeNext <= FREENEXT_BAIOCB_ORIGIN)                       /*  has a valid BAiocb */
+                  && ((FREENEXT_BAIOCB_ORIGIN - buf_desc->freeNext) < numBufferAiocbs)    /*  double-check */
+                ) {        /* this is aio   */
+                    struct BufferAiocb volatile * BAiocb = (BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf_desc->freeNext); /*  BufferAiocb associated with this aio */
+                    if ((struct BufferAiocb*)BAIOCB_OCCUPIED == BAiocb->BAiocbnext) { /*  ensure BAiocb is occupied */
+                        aio_successful = 0;       /*  tentatively the aio_read did not succeed   */
+                        retcode = BUF_INTENT_RC_INVALID_AIO;
+
+                        if (smgr == NULL) {
+                            if (caller_reln == NULL) {
+                                smgr = smgropen(buf_desc->tag.rnode, InvalidBackendId);
+                            } else {
+                                smgr = caller_reln->rd_smgr;
+                            }
+                        }
+
+                        /*  assert that this AIO is not using the same BufferAiocb as the one caller asked us to use */
+                        if ((index_for_aio < 0) && (index_for_aio == buf_desc->freeNext)) {
+                                   ereport(ERROR,
+                                        (errcode(ERRCODE_INTERNAL_ERROR),
+                                         errmsg("AIO control block index %d to be used by %p already in use by %p"
+                                                  ,index_for_aio, buf_desc, BAiocb->BAiocbbufh)));
+                        }
+
+                        /*    Call smgrcompleteaio only if either we want buffer or there are no dependents.
+                        **    In the other case of reject and there are dependents,
+                        **    then one of them will do it.
+                        */
+                        if (   (local_intention > 0) || (0 == BAiocb->BAiocbDependentCount)  ) {
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on signal instead of polling */
+                            int cancelled = 0;                 /* is io to be cancelled ? */
+#endif /*  USE_AIO_SIGEVENT    */
+                            if (local_intention > 0) {
+                                /*  wait for in-progress aio and then pin
+                                **  OR  if I did not issue the aio and do not have a pin
+                                **  then pin now before waiting to ensure the buffer does not become unpinned while I wait
+                                **  we may potentially wait for io to complete
+                                **  so release buf header lock so that others may also wait here
+                                */
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on signal instead of polling */
+                                smgrcompleteaio_rc = 16;
+                                while (!(PoorMIncr(&(BAiocb->BAiocbDependentCount)))) { /* register self as dependent  */
+                                    if (smgrcompleteaio_rc--) {
+                                        elog(LOG, "BufCheckAsync : yielding %d to increment BAiocb %p dependent",smgrcompleteaio_rc,BAiocb);
+                                    }
+                                    sched_yield();
+                                }
+#else
+                                BAiocb->BAiocbDependentCount++; /* register self as dependent  */
+#endif /* USE_AIO_SIGEVENT  */
+                                if (PrivateRefCount[buf_desc->buf_id] == 0) {   /* if this buffer not pinned by me */
+                                    disposable_pin = true; /* this backend has pinned the buffer while waiting for aio_read to complete */
+                                    PinBuffer_Locked(buf_desc);
+                                } else {
+                                    UnlockBufHdr(buf_desc);
+                                }
+                                LWLockRelease(PartitionLock);
+
+                                smgrcompleteaio_rc = 1   /*  tell smgrcompleteaio to wait  */
+                                                   + ( BAiocb->pidOfAio == this_backend_pid ); /*  and whether I initiated the aio */
+                            } else {
+                                smgrcompleteaio_rc = 0;   /*  tell smgrcompleteaio to cancel */
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on signal instead of polling */
+            /*   because aio is to be cancelled , remember to post suspended waiters after, because no signal will be sent */
+                                cancelled = 1;
+#endif /*  USE_AIO_SIGEVENT    */
+                            }
+
+                            smgrcompleteaio( smgr , (char *)&(BAiocb->BAiocbthis) , &smgrcompleteaio_rc
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on signal instead of polling */
+                                           , ((int (*)(char *))(&BufAWaitAioCompletion))
+                                           , ((char *)BAiocb)
+#endif /*  USE_AIO_SIGEVENT    */
+                                           );
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on signal instead of polling */
+            /*   if aio was cancelled , then post suspended waiters because no signal will be sent */
+                            if (cancelled) {
+                                BufPostAioWaiters(BAiocb);
+                            }
+#endif /*  USE_AIO_SIGEVENT    */
+                            if ( (smgrcompleteaio_rc == 0) || (smgrcompleteaio_rc == 1) ) {
+                                  aio_successful = 1;
+                            }
+
+                            /*   statistics  */
+                            if (local_intention > 0) {
+                                if (smgrcompleteaio_rc == 0) {
+                                    /*    completed successfully and did not have to wait  */
+                                    pgBufferUsage.aio_read_ontime++;
+                                } else if (smgrcompleteaio_rc == 1) {
+                                    /*    completed successfully and did have to wait  */
+                                    pgBufferUsage.aio_read_waited++;
+                                } else {
+                                    /*  bad news   -   read failed and so buffer not usable
+                                    **  the buffer is still pinned so unpin and proceed with "not found" case
+                                    */
+                                    pgBufferUsage.aio_read_failed++;
+                                }
+
+                                /*  regain locks and handle the validity of the buffer and intention regarding it    */
+                                LWLockAcquire(PartitionLock, LW_SHARED);
+                                LockBufHdr(buf_desc);
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on signal instead of polling */
+                                smgrcompleteaio_rc = 16;
+                                while (!(PoorMDecr(&(BAiocb->BAiocbDependentCount)))) { /* unregister self as dependent  */
+                                    if (smgrcompleteaio_rc--) {
+                                        elog(LOG, "BufCheckAsync : yielding %d to decrement BAiocb %p dependent",smgrcompleteaio_rc,BAiocb);
+                                    }
+                                    sched_yield();
+                                }
+#else
+                                BAiocb->BAiocbDependentCount--; /* unregister self as dependent  */
+#endif /* USE_AIO_SIGEVENT  */
+                            } else {
+                                    pgBufferUsage.aio_read_wasted++;  /*   regardless of whether aio_successful */
+                            }
+
+
+                            if (local_intention > 0) {
+                                /*  verify the buffer is still ours and has same identity
+                                **  There is one slightly tricky point here -
+                                **  if there are other dependents,   then each of them will perform this same check
+                                **  this is unavoidable as the correct setting of retcode and the BM_VALID flag
+                                **  is required by each dependent,  so we may not leave it to the last one to do it.
+                                **  It should not do any harm and easier to let them all do it than try to avoid.
+                                */
+                                if ((FREENEXT_BAIOCB_ORIGIN - (BAiocb - (BAiocbAnchr->BufferAiocbs))) == buf_desc->freeNext) { /* it is still mine */
+
+                                    if (aio_successful) {
+                                        /* validate page header.   If valid,  then mark the buffer as valid */
+                                        if (PageIsVerified((Page)(BufHdrGetBlock(buf_desc)) , ((BAiocb->BAiocbthis).aio_offset/BLCKSZ))) {
+                                            buf_desc->flags |= BM_VALID;
+                                            if (BUFFERTAGS_EQUAL(origTag , buf_desc->tag)) {
+                                                retcode = BUF_INTENT_RC_VALID;
+                                            } else {
+                                                retcode = BUF_INTENT_RC_CHANGED_TAG;
+                                            }
+                                        } else {
+                                            retcode = BUF_INTENT_RC_BADPAGE;
+                                        }
+                                    }
+                                }
+                            }
+
+                            BAiocbDependentCount_after_aio_finished = BAiocb->BAiocbDependentCount;
+
+                            /*  if no dependents,   then disconnect the BAiocb and update buffer header */
+                            if (BAiocbDependentCount_after_aio_finished == 0 ) {
+
+                                /*  return the BufferAiocb to the free list  */
+                                buf_desc->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR | BM_AIO_IN_PROGRESS);
+
+                                if (
+                                           BufReleaseAsync(BAiocb)
+                                   ) {        /*  failed ? */
+                                    BAiocb->BAiocbnext = cachedBAiocb;   /* then ...       */
+                                    cachedBAiocb = BAiocb;               /*  ... cache it  */
+                                }
+                            }
+
+                        }
+                    }
+            }         /* end this is aio   */
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+            /*  At this point,  buffer header spinlock is held in all cases.
+            **  1.  it was either acquired by caller and held on entry or we acquired at early in this function
+            **  2.  if  (local_intention <= 0) it was never released
+            **  3.  if  (local_intention >  0) it was released and re-acquired around the call to smgrstartaio
+            */
+            /*  note whether buffer is valid before unlocking spinlock */
+            valid = ((buf_desc->flags & BM_VALID) != 0);
+
+            /*  if there was a disposable pin on entry to this function (i.e. marked in buffer flags)
+            **  then unmark it  -  refer to prologue comments talking about :
+            **    if a disposable pin is held,   then :
+            **     ...
+            **    i.e. in either case,   there is no longer a disposable pin after this function has completed.
+            */
+            if (pin_already_banked_by_me) {
+                        buf_desc->flags &= ~BM_AIO_PREFETCH_PIN_BANKED; /*   redeem the banked pin  */
+                        /*   if AIO not in progress,  then disconnect the buffer from BAiocb and/or banked pin */
+                        if (!(buf_desc->flags & BM_AIO_IN_PROGRESS)) {
+                            buf_desc->freeNext = FREENEXT_NOT_IN_LIST;      /*   and forget the bank client */
+                        }
+                        /********** for debugging   *****************
+                        else elog(LOG, "BufCheckAsync : found BM_AIO_IN_PROGRESS when redeeming banked pin on buffer %d rel=%s, blockNum=%u, flags %X refcount=%u"
+                       ,buf_desc->buf_id,relpathbackend(buf_desc->tag.rnode, InvalidBackendId, buf_desc->tag.forkNum)
+                       ,buf_desc->tag.blockNum, buf_desc->flags, buf_desc->refcount);
+                        ********** for debugging     *****************/
+            }
+
+            /*  If we are to obtain new pin, then use pin optimization  -  pin and unlock.
+            **  However,   if the caller is the same backend who issued the aio_read,
+            **  then he ought to have obtained the pin at that time and must not acquire
+            **  a "second" one since this is logically the same read -  he would have obtained
+            **  a single pin if using synchronous read and we emulate that behaviour.
+            **  Its important to understand that the caller is not aware that he already obtained a pin -
+            **  because calling PrefetchBuffer did not imply a pin -
+            **  so we must track that via the pidOfAio field in the BAiocb.
+            **  And to add one further complication :
+            **      we assume that although PrefetchBuffer pinned the buffer,
+            **      it did not increment the usage count.
+            **      (because it called PinBuffer_Locked which does not do that)
+            **      so in this case,   we must increment the usage count without double-pinning.
+            **      yes its ugly  -  and theres a goto!
+            */
+            if (   (local_intention > 0)
+                || (local_intention == BUF_INTENTION_REJECT_OBTAIN_PIN)
+               ) {
+
+                /* Make sure we will have room to remember the buffer pin */
+                ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
+
+                /*    here we really want a version of PinBuffer_Locked which updates usage count ... */
+                if (   (PrivateRefCount[buf_desc->buf_id] == 0) /*   if this buffer not previously pinned by me */
+                    || pin_already_banked_by_me                 /*   or I had a disposable pin on entry */
+                   ) {
+                    if (strategy == NULL)
+                    {
+                            if (buf_desc->usage_count < BM_MAX_USAGE_COUNT)
+                                    buf_desc->usage_count++;
+                    }
+                    else
+                    {
+                            if (buf_desc->usage_count == 0)
+                                    buf_desc->usage_count = 1;
+                    }
+                }
+
+                /*  now pin buffer unless we have a disposable */
+                if (!disposable_pin) { /* this backend neither banked pin for aio nor pinned the buffer while waiting for aio_read to complete */
+                    PinBuffer_Locked(buf_desc);
+                    goto unlocked_it;
+                }
+                else
+                /*    if this task previously issued the aio or pinned the buffer while waiting for aio_read to complete
+                **       and aio was unsuccessful,    then release the pin
+                */
+                if (     disposable_pin
+                      && (aio_successful == 0)       /*  aio_read failed ? */
+                   ) {
+                    UnlockBufHdr(buf_desc);    /*  release spinlock since UnpinBuffer acquires it -  annoying but infrequent case */
+                    UnpinBuffer(buf_desc, true);
+                    goto unlocked_it;
+                }
+            }
+
+    unlock_buf_header:
+            UnlockBufHdr(buf_desc);
+    unlocked_it:
+
+            /*   now do any requested pin (if not done immediately above) or unpin/forget  */
+            if (local_intention == BUF_INTENTION_REJECT_KEEP_PIN) {
+            /*   the caller is supposed to hold a pin already so there should be nothing to do ... */
+                if (PrivateRefCount[buf_desc->buf_id] == 0) {
+                    elog(LOG, "request to keep pin on unpinned buffer %d",buf_desc->buf_id);
+
+                    valid = PinBuffer(buf_desc, strategy);
+                }
+            }
+            else
+            if (   (   (local_intention == BUF_INTENTION_REJECT_FORGET)
+                    || (local_intention == BUF_INTENTION_REJECT_NOADJUST)
+                   )
+                && (PrivateRefCount[buf_desc->buf_id] > 0) /*   if this buffer was previously pinned by me ... */
+               )  {
+
+                    if (local_intention == BUF_INTENTION_REJECT_FORGET) {
+                        UnpinBuffer(buf_desc, true); /*  ... then release the pin                   */
+                    } else
+                    if (local_intention == BUF_INTENTION_REJECT_NOADJUST) {
+                        /*   following code moved from ReleaseBuffer :
+                        **   not sure why we can't simply UnpinBuffer(buf_desc, true)
+                        **   but better leave it the way it was
+                        */
+                        ResourceOwnerForgetBuffer(CurrentResourceOwner, BufferDescriptorGetBuffer(buf_desc));
+                        if (PrivateRefCount[buf_desc->buf_id] > 1) {
+                            PrivateRefCount[buf_desc->buf_id]--;
+                        } else {
+                            UnpinBuffer(buf_desc, false);
+                        }
+                    }
+            }
+
+            /*    if retcode has not been set to one of the unusual conditions
+            **        namely failed header validity or tag changed
+            **    then the setting of valid takes precedence
+            **    over whatever retcode may be currently set to.
+            */
+            if ( ( (retcode == BUF_INTENT_RC_INVALID_NO_AIO) || (retcode == BUF_INTENT_RC_INVALID_AIO) ) && valid) {
+                   retcode = BUF_INTENT_RC_VALID;
+            } else
+            if ((retcode == BUF_INTENT_RC_VALID) && (!valid)) {
+                   if (aio_successful == -1) { /* aio not attempted */
+                       retcode = BUF_INTENT_RC_INVALID_NO_AIO;
+                   } else {
+                       retcode = BUF_INTENT_RC_INVALID_AIO;
+                   }
+            }
+
+            return retcode;
+}
+
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on signal instead of polling */
+
+/*   PoorMans Atomic functions :
+**   these functions operate on a signed int as follows :
+**   suppose current value of int is v
+**       .  lock       if v >  0 then compswap int to -v else fail
+**       .  unlock     if v <  0 then compswap int to -v (i.e. abs(v)) else fail
+**       .  increment  if v >= 0 then compswap int to (v+1) else fail
+**       .  decrement  if v >  0 then compswap int to (v-1) else fail
+**
+**   thus the lock prevents int from being incremented or decremented
+**
+**   in all cases :
+**      there is no retrying within PoorMan,  caller must do so if desired
+**      return return code from either test as described above or from compswap
+**              ( non-zero <=> success , 0 <=> fail )
+**      although not policed,  caller who gets lock should release it very soon after
+**                             and should not wait in any way while lock is held.
+*/
+
+static int PoorMLock(volatile int *intP) {
+     int rc = *intP;
+     if (rc > 0) {
+         rc = (__sync_bool_compare_and_swap (intP, rc, (-rc))); /* swap to -ve */
+     } else {
+         rc = 0; /* fail */
+     }
+     return rc;
+}
+
+static int PoorMUnLock(volatile int *intP) {
+     int rc = *intP;
+     if (rc < 0) {
+         rc = (__sync_bool_compare_and_swap (intP, rc, (-rc))); /* swap to +ve */
+     } else {
+         rc = 0; /* fail */
+     }
+     return rc;
+}
+
+static int PoorMIncr(volatile int *intP) {
+     int rc = *intP;
+     if (rc >= 0) {
+         rc = (__sync_bool_compare_and_swap (intP, rc, (rc+1))); /* swap to v+1 */
+     } else {
+         rc = 0; /* fail */
+     }
+     return rc;
+}
+
+static int PoorMDecr(volatile int *intP) {
+     int rc = *intP;
+     if (rc > 0) {
+         rc = (__sync_bool_compare_and_swap (intP, rc, (rc-1))); /* swap to v-1 */
+     } else {
+         rc = 0; /* fail */
+     }
+     return rc;
+}
+
+/*  the following functions break the smgr / md / fd layering by calling aio_error directly
+**  but this is a lesser evil than locating them in fd.c since they refer heavily to
+**  buffer-manager async-io signalling/completion constructs.
+*/
+#include "aio.h"
+/*  post backends waiting on BAiocb
+**  may be called either from signal-handler context or from mainline context
+*/
+void
+BufPostAioWaiters(struct BufferAiocb volatile * BAiocb)
+{
+      int cs_rc;
+	  volatile struct PGPROC *BAiocbWaiterProc;	    /* backend waiting for aio completion */
+	  volatile struct PGPROC *BAiocbWaiterProcnext;	/* next backend waiting for aio completion */
+
+          cs_rc = PoorMLock( &(BAiocb->BAiocbDependentCount) ); /* lock the BAiocb dependents */
+          if (cs_rc) { /* locked?  note locked implies BAiocbDependentCount was > 0 */
+              /*  check whether actually complete and get status.
+              **  The correct method is to call smgrcompleteaio
+              **  but that function might wait which we must not do,
+              **  so we must call aio_error directly and bypass the layers.
+              */
+              cs_rc = aio_error((const struct aiocb *)(&(BAiocb->BAiocbthis)));
+              if (cs_rc != EINPROGRESS) { /* any rc other than EINPROGRESS implies it is not in progress */
+                  /*  run the chain and wake up any waiters */
+                  /*  note - waiter may be me,  in which case omit sending another signal.
+                  */
+                  BAiocbWaiterProc = BAiocb->BAiocbWaiterchain;          /* current head of chain */
+                  while (BAiocbWaiterProc != (struct PGPROC *)0) {        /*  not reached end */
+                      BAiocbWaiterProcnext = BAiocbWaiterProc->BAiocbWaiterLink;
+                      cs_rc = (__sync_bool_compare_and_swap (&(BAiocb->BAiocbWaiterchain), BAiocbWaiterProc, BAiocbWaiterProcnext)); /* swap it off chain */
+                      if (cs_rc) {
+                          if (this_backend_pid != BAiocbWaiterProc->pid) {
+                              kill(BAiocbWaiterProc->pid , AIO_SIGEVENT_SIGNALNUM);
+                          }
+                          BAiocbWaiterProc = BAiocb->BAiocbWaiterchain;   /* new head of chain */
+                      } else {
+                          elog(LOG, "BufPostAioWaiters : my pid %d unable to remove waiter pid %d from BAiocb %p dept count= %d"
+                                    ,this_backend_pid,BAiocbWaiterProc->pid,BAiocb,BAiocb->BAiocbDependentCount);
+                          break; /* abandon ship if we could not swap it off cleanly */
+                      }
+                  }
+              }
+              /*  whatever else we achieve,  we *must* unlock the BAiocbDependentCount
+              **  it would be a bug if the following PoorMUnLock fails
+              **  since no other process can modify it while locked,
+              **  but we must check that.
+              */
+              cs_rc = PoorMUnLock( &(BAiocb->BAiocbDependentCount) ); /* unlock the BAiocb dependents */
+              while (!cs_rc) {
+                  sched_yield();
+                  cs_rc = PoorMUnLock( &(BAiocb->BAiocbDependentCount) ); /* unlock the BAiocb dependents */
+              }
+          }
+}
+/*   handle the AIO SIGNAL sent by kernel and by other backend for a completion.
+**    .  if sent by other backend,  then do nothing as the sole purpose is to wake me up
+**    .  if sent by kernel as a result of a sigqueue by aio completion, then :
+**         for each BAiocb,
+**           poor-man lock the BAiocb against change in dependents (including being released),
+**           check if io is complete (successful or otherwise)
+**           then check if there are any waiters and if so signal them.
+**           Note  -  logically,   each backend should operate on only BAiocbs associated with
+**                    ios which that same backend originated;
+**                    However,   there is no reason to insist on being originated by me,
+**                    and a potential benefit from not doing so in that we may
+**                    post a waiter sooner,  so simply check all.
+**           Retort - experimentally it seems that only posting waiters waiting on aio I originated is better
+*/
+void BufHandlSigAsync(int sigsent, siginfo_t * siginfP, void *ucontext)
+{
+      volatile struct BufferAiocb *BufferAiocbs;
+	  volatile struct BufferAiocb *BAiocb;	/*  this BAiocb */
+      int ix;
+
+      if (sigsent == AIO_SIGEVENT_SIGNALNUM) {                                     /*  related to aio completion ... */
+          if ( (siginfP != (siginfo_t *)0) && (siginfP->si_code == SI_ASYNCIO) ) { /* ... from sigqueue */
+              BufferAiocbs = BAiocbAnchr->BufferAiocbs;    /*  address base of the BAiocbs */
+              __sync_fetch_and_add (&num_signalledaio, 1); /*  accounting */
+              for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                  BAiocb = (BufferAiocbs+ix);	/*  this BAiocb */
+#ifndef AIO_PRIMISCUOUS_POSTING
+                  /*  post only if this aio was originated by me
+                  **  no later than the one to which this signal applies
+                  */
+                  if ( (BAiocb->pidOfAio == this_backend_pid) && (BAiocb->BAiocbaioOrdinal <= (unsigned long)(siginfP->si_value.sival_ptr)) )
+#endif /*  AIO_PRIMISCUOUS_POSTING */
+                  BufPostAioWaiters(BAiocb);
+              }
+          }
+      }
+}
+
+/*  function to await completion if non-originator
+*   called as a callback from FileCompleteaio
+*   returns the aio_error return code from completion
+*/
+extern sigset_t blockaiosighandlrmask; /* defined in buf_init.c */
+int BufAWaitAioCompletion (struct BufferAiocb *BAiocb) /* pointer to BAIOcb  */ 
+{
+
+    int onwaitchain = 0;              /*  is my PROC on the waiter chain ? */
+	int	aio_errno;
+    struct aiocb volatile * my_aiocbp = &(BAiocb->BAiocbthis);
+    struct PGPROC volatile ** BAiocbWaiterchainP = &(BAiocb->BAiocbWaiterchain); /* anchor of chain of backends waiting for aio completion */
+    struct PGPROC * FirstWaiterProc; /* PROC of first backend waiting for aio completion */
+    sigset_t tempmask;
+    volatile struct PGPROC *BAiocbWaiterProc;	    /* backend waiting for aio completion */
+	volatile struct PGPROC *BAiocbWaiterProcnext;	/* next backend waiting for aio completion */
+
+        sigprocmask(SIG_BLOCK,&blockaiosighandlrmask,&tempmask); /*  temporarily block signal AIO_SIGEVENT_SIGNALNUM */
+        aio_errno = aio_error((const struct aiocb *)my_aiocbp);
+        while (aio_errno == EINPROGRESS) {              /*  check still in progress */
+            if (!onwaitchain) {
+                /*  place self on chain of waiters  */
+                FirstWaiterProc = (struct PGPROC *)(*BAiocbWaiterchainP);  /*  dereference current head of chain */
+                MyProc->BAiocbWaiterLink = FirstWaiterProc;            /*  and attach this current chain to mine */
+                onwaitchain = __sync_bool_compare_and_swap (BAiocbWaiterchainP, FirstWaiterProc, MyProc); /* swap it onto chain */
+                if (onwaitchain) {                               /*   successfully swapped onto chain */
+                    sigsuspend(&tempmask);                       /*   unblock signal AIO_SIGEVENT_SIGNALNUM and wait for it */
+                } else {
+                    /*  failed to place myself on waiter chain so yield and try again */
+                    sigprocmask(SIG_SETMASK,&tempmask,0);            /*  normal mask allows AIO_SIGEVENT_SIGNALNUM */
+                    sched_yield();
+                    sigprocmask(SIG_BLOCK,&blockaiosighandlrmask,&tempmask); /*  temporarily block signal AIO_SIGEVENT_SIGNALNUM again */
+                }
+            }
+            aio_errno = aio_error((const struct aiocb *)my_aiocbp);                             /* and check once again */
+        }
+        sigprocmask(SIG_SETMASK,&tempmask,0);            /*  restore original mask which allows AIO_SIGEVENT_SIGNALNUM */
+
+        /*  ensure my PROC is no longer on wait chain -
+        *   signal-handler may or may not have done so.
+        *   note there is no race condition with sig handler
+        *        because sig handler swaps the PROC off the waiter chain *before* signalling me
+        */
+        while (onwaitchain) {                               /*   successfully swapped onto chain */
+            onwaitchain = 0;              /*  tentatively my PROC is no longer on the waiter chain */
+            BAiocbWaiterProc = BAiocb->BAiocbWaiterchain;          /* current head of chain */
+            while (BAiocbWaiterProc != (struct PGPROC *)0) {        /*  not reached end */
+                BAiocbWaiterProcnext = BAiocbWaiterProc->BAiocbWaiterLink;
+                if (BAiocbWaiterProc == MyProc) {  /*  is this me ? */
+                    onwaitchain = ( (__sync_bool_compare_and_swap (&(BAiocb->BAiocbWaiterchain), BAiocbWaiterProc, BAiocbWaiterProcnext)) /* swap it off chain */
+                                    ? 0  /*  my PROC is no longer on the waiter chain */
+                                    : 1  /*  my PROC is on the waiter chain */
+                    );
+                }
+                BAiocbWaiterProc = BAiocbWaiterProcnext;
+            }
+        }
+
+        return aio_errno;
+}
+#endif /*  USE_AIO_SIGEVENT    */
--- src/backend/storage/buffer/buf_init.c.orig	2014-08-18 14:10:36.933017070 -0400
+++ src/backend/storage/buffer/buf_init.c	2014-08-19 16:56:13.567196743 -0400
@@ -13,15 +13,122 @@
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
+#include <sys/types.h> 
+#include <unistd.h>
 
 #include "storage/bufmgr.h"
 #include "storage/buf_internals.h"
-
+#include <stdlib.h> /* for getenv() */
+#include <errno.h> /* for strtoul() */
+#if defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP) && defined(USE_AIO_SIGEVENT)
+#include <sched.h>
+#endif /*  defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP) && defined(USE_AIO_SIGEVENT)  */
 
 BufferDesc *BufferDescriptors;
 char	   *BufferBlocks;
-int32	   *PrivateRefCount;
+int32	   *PrivateRefCount;       /*  array of counts per buffer of how many times this task has pinned this buffer */
+
+volatile struct BAiocbAnchor *BAiocbAnchr = (struct BAiocbAnchor *)0;  /*  anchor for all control blocks pertaining to aio  */
+
+int CountInuseBAiocbs(void);     /*  keep compiler happy */
+void ReportFreeBAiocbs(void);    /*  keep compiler happy */
+
+extern int	MaxConnections;  /*  max number of client connections which postmaster will allow  */
+int numBufferAiocbs = 0;         /*  total number of  BufferAiocbs in pool (0 <=> no async io) */
+int hwmBufferAiocbs = 0;         /*  high water mark of in-use  BufferAiocbs in pool
+                                 **  (not required to be accurate, kindly maintained for us somehow by postmaster)
+                                 */
 
+#ifdef USE_PREFETCH
+unsigned int prefetch_dbOid = 0; /*  database oid of relations on which prefetching to be done - 0 means all */
+unsigned int prefetch_bitmap_scans = 1; /*  boolean whether to prefetch bitmap heap scans        */
+unsigned int prefetch_heap_scans = 0;   /*  boolean whether to prefetch non-bitmap heap scans    */
+unsigned int prefetch_sequential_index_scans = 0;  /*  boolean whether to prefetch sequential-access non-bitmap index scans  */
+unsigned int prefetch_index_scans = 256;  /*  boolean whether to prefetch non-bitmap index scans also numeric size of pfch_list  */
+unsigned int prefetch_btree_heaps = 1;  /*  boolean whether to prefetch heap pages in _bt_next for non-bitmap index scans   */
+#endif /* USE_PREFETCH */
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+int maxGetBAiocbTries = 1;       /*  max times we will try to get a free BufferAiocb */
+int maxRelBAiocbTries = 1;       /*  max times we will try to release a BufferAiocb back to freelist */
+
+/*  locking protocol for manipulating the BufferAiocbs and FreeBAiocbs list :
+**    1.    ownership of a BufferAiocb :
+**          to gain ownership of a BufferAiocb, a task must
+**          EITHER    remove it from FreeBAiocbs (it is now temporary owner and no other task can find it)
+**                    if decision is to attach it to a buffer descriptor header, then
+**                       .   lock the buffer descriptor header
+**                       .   check  NOT flags & BM_AIO_IN_PROGRESS
+**                       .   attach to buffer descriptor header
+**                       .   increment the BufferAiocb.dependent_count
+**                       .   unlock the buffer descriptor header
+**                     and ownership scope is from lock to unlock
+***          OR        locate it by dereferencing the pointer in a buffer descriptor,
+**                    in which case :
+**                       .   lock the buffer descriptor header
+**                       .   check  flags & BM_AIO_IN_PROGRESS
+**                       .   increment the BufferAiocb.dependent_count
+**                       .   if decision is to return to FreeBAiocbs,
+**                           then   (with buffer descriptor header still locked)
+**                                  .  turn off BM_AIO_IN_PROGRESS
+**                       .   IF        the BufferAiocb.dependent_count == 1 (I am sole dependent)
+**                       .   THEN
+**                       .       .  decrement the BufferAiocb.dependent_count
+**                               .  return to FreeBAiocbs (see below)
+**                       .   unlock the buffer descriptor header
+**                     and ownership scope is from lock to either return to FreeBAiocbs or unlock
+**    2.    adding and removing from FreeBAiocbs :
+**      two alternative methods - controlled by conditional macro definition LOCK_BAIOCB_FOR_GET_REL
+**       2.1 LOCK_BAIOCB_FOR_GET_REL is defined - use a lock
+**          .   lock BufFreelistLock exclusive
+**          .   add / remove from FreeBAiocbs
+**          .   unlock BufFreelistLock exclusive
+**          advantage of this method  -  never fails to add or remove
+**       2.2  LOCK_BAIOCB_FOR_GET_REL is not defined - use compare_and_swap
+**          .   retrieve the current Freelist pointer and validate
+**          .   compare_and_swap on/off the FreeBAiocb list
+**          .   unlock BufFreelistLock exclusive
+**          advantage of this method  -  never waits
+**          to avoid losing a FreeBAiocbs,   save it in a process-local cache and reuse
+*/
+#ifdef USE_AIO_SIGEVENT                                    /*    signal non-originator waiters instead of letting them poll */
+extern long num_startedaio;
+extern long num_signalledaio;
+
+/*  structures relating to keeping track of aios originated by this backend */
+/*  BAiocbIostatusArray is a circular array of aio operations past and present;
+ *  each item is a 32-bit unsigned int container consisting of two fields
+ *    .   unsigned short status - one of three states :
+ *        .   not_in_progress          no aio in progress,  io lock not held
+ *        .   locked_in_progress          aio in progress,  io lock held eXclusive
+ *        .   locked_pending_release      aio completion signalled,  io lock held eXclusive
+ *    .   unsigned short BAiocb_index - the array index of the associated BAiocb when io lock held
+ *
+ *  The active items in the array correspond to all aio_reads issued by this backend
+ *  since the earliest io-locked aio.    BAiocbIostatusEarliest indexes this entry.
+ *  Every aio operation originated by this backend is assigned a monotonic-increasing sequence number
+ *  which,   modulo the size of the array,  identifies the array item representing that aio.
+*/
+unsigned int volatile * BAiocbIostatusArray = (unsigned int *)0;
+unsigned long BAiocbNextaioOrdinal = 0;           /*  ordinal number of each aio in the sequence  */
+unsigned long BAiocbEarliestiolockedOrdinal = 0;  /*  ordinal number of earliest io-locked aio */
+unsigned long BAiocbIostatusDimension = 0;        /*  number of items in array */
+
+  /*  BufHandlSigAsync is invoked whenever io completion signal is issued.
+  *   siginfP->si_value.sival_ptr contains the address of the LWlock
+  *   on which non-originator waiters wait and which we are to release
+  */
+void BufHandlSigAsync(int sigsent, siginfo_t * siginfP, void *ucontext);
+sigset_t blockaiosighandlrmask;     /*  sig mask to block AIO signal temporarily */
+#endif /*  USE_AIO_SIGEVENT    */
+#else   /*  not USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+        /*   this dummy structure is to ensure that references to these fields in other bufmgr runtime code
+        **   that is not conditionally ifdefd on USE_AIO_ATOMIC_BUILTIN_COMP_SWAP compiles and runs correctly
+        */
+        struct BAiocbAnchor dummy_BAiocbAnchr = { (struct BufferAiocb*)0 , (struct BufferAiocb*)0 };
+int maxGetBAiocbTries = -1;       /*  max times we will try to get a free BufferAiocb */
+int maxRelBAiocbTries = -1;       /*  max times we will try to release a BufferAiocb back to freelist */
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * Data Structures:
@@ -73,7 +180,20 @@ void
 InitBufferPool(void)
 {
 	bool		foundBufs,
-				foundDescs;
+				foundDescs
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+              , foundAiocbs
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+          ;
+#if defined(USE_PREFETCH) || defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP)
+        char *envvarpointer = (char *)0;  /*  might point to an environment variable string */
+        char *charptr;
+#endif /* USE_PREFETCH */
+
+#ifdef USE_AIO_SIGEVENT                                    /*    signal waiters instead of letting them poll */
+/*    signal environment to invoke BufHandlSigAsync    */
+struct sigaction chsigaction , olsigAIOaction;
+#endif /*  USE_AIO_SIGEVENT    */
 
 	BufferDescriptors = (BufferDesc *)
 		ShmemInitStruct("Buffer Descriptors",
@@ -83,6 +203,146 @@ InitBufferPool(void)
 		ShmemInitStruct("Buffer Blocks",
 						NBuffers * (Size) BLCKSZ, &foundBufs);
 
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+        BAiocbAnchr = (struct BAiocbAnchor *)0; /*  anchor for all control blocks pertaining to aio  */
+        if (max_async_io_prefetchers < 0) {  /*  negative value indicates to initialize to something sensible during buf_init */
+            max_async_io_prefetchers = MaxConnections/6;  /*  default allows for average of MaxConnections/6 concurrent prefetchers  - reasonable ??? */
+        }
+
+        if ((target_prefetch_pages > 0) && (max_async_io_prefetchers > 0)) {
+            int ix;
+            volatile struct BufferAiocb *BufferAiocbs;
+            volatile struct BufferAiocb * volatile FreeBAiocbs;
+
+            numBufferAiocbs = (target_prefetch_pages*max_async_io_prefetchers);  /*  target_prefetch_pages per prefetcher */
+            BAiocbAnchr = (struct BAiocbAnchor *)
+	        	ShmemInitStruct("Buffer Aiocbs",
+                          sizeof(struct BAiocbAnchor) + (numBufferAiocbs * sizeof(struct BufferAiocb)), &foundAiocbs);
+            if (BAiocbAnchr) {
+                BufferAiocbs = BAiocbAnchr->BufferAiocbs = (struct BufferAiocb*)(((char *)BAiocbAnchr) + sizeof(struct BAiocbAnchor));
+                FreeBAiocbs = (struct BufferAiocb*)0;
+                for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                    (BufferAiocbs+ix)->BAiocbnext = FreeBAiocbs;   /*  init the free list,  last one -> 0  */
+                    (BufferAiocbs+ix)->BAiocbbufh = (struct sbufdesc*)0;
+                    (BufferAiocbs+ix)->BAiocbDependentCount = 0;
+                    (BufferAiocbs+ix)->pidOfAio = 0;
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on signal instead of polling */
+			        (BufferAiocbs+ix)->BAiocbWaiterchain = (struct PGPROC *)0; /* initially not on a chain */
+#endif /*  USE_AIO_SIGEVENT    */
+                    FreeBAiocbs = (BufferAiocbs+ix);
+
+                }
+                BAiocbAnchr->FreeBAiocbs = FreeBAiocbs;
+                envvarpointer = getenv("PG_MAX_GET_BAIOCB_TRIES");
+                if (    (envvarpointer != (char *)0)
+                     && (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   )
+                   ) {
+                      maxGetBAiocbTries = strtol(envvarpointer, 0, 10);
+                }
+                envvarpointer = getenv("PG_MAX_REL_BAIOCB_TRIES");
+                if (    (envvarpointer != (char *)0)
+                     && (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   )
+                   ) {
+                      maxRelBAiocbTries = strtol(envvarpointer, 0, 10);
+                }
+
+#ifdef USE_AIO_SIGEVENT                                    /*    signal waiters instead of letting them poll */
+				/*    set our signal environment to invoke BufHandlSigAsync    */
+				sigfillset(&blockaiosighandlrmask);
+				chsigaction.sa_sigaction = (void(*)(int, siginfo_t *, void *ucontext))&BufHandlSigAsync; /*  call for siginfo */
+				chsigaction.sa_mask = blockaiosighandlrmask;
+				chsigaction.sa_flags = SA_SIGINFO; /*  call for siginfo */
+				sigaction(AIO_SIGEVENT_SIGNALNUM, &chsigaction, &olsigAIOaction);
+#endif /*  USE_AIO_SIGEVENT    */
+            }
+        }
+#else   /*  not USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+        /*   this dummy structure is to ensure that references to these fields in other bufmgr runtime code
+        **   that is not conditionally ifdefd on USE_AIO_ATOMIC_BUILTIN_COMP_SWAP compiles and runs correctly
+        */
+        BAiocbAnchr = &dummy_BAiocbAnchr;
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+
+#ifdef USE_PREFETCH
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_BITMAP");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_bitmap_scans = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+                 prefetch_bitmap_scans = 1;
+             }
+        }
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_ISCAN");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_index_scans = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+                 prefetch_index_scans = 1;
+             } else
+             if (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   ) {
+                 prefetch_index_scans = strtol(envvarpointer, &charptr, 10);
+                 if (charptr && (',' == *charptr)) {   /*  optional sequential prefetch in index scans */
+					 charptr++;        /*   following the comma ... */
+					 if ( ('Y' == *charptr) || ('y' == *charptr) || ('1' == *charptr) ) {
+                         prefetch_sequential_index_scans = 1;
+					 }
+				 }
+             }
+             /*  if prefeching for ISCAN,  then we require size of pfch_list to be at least target_prefetch_pages */
+             if (   (prefetch_index_scans > 0)
+                 && (prefetch_index_scans < target_prefetch_pages)
+                ) {
+                 prefetch_index_scans = target_prefetch_pages;
+             }
+        }
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_BTREE");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_btree_heaps = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+                 prefetch_btree_heaps = 1;
+             }
+        }
+        envvarpointer = getenv("PG_TRY_PREFETCHING_FOR_HEAP");
+        if (envvarpointer != (char *)0) {
+             if (    ('N' == *envvarpointer) || ('n' == *envvarpointer)   ) {
+                 prefetch_heap_scans = 0;
+             } else
+             if (    ('Y' == *envvarpointer) || ('y' == *envvarpointer)   ) {
+              prefetch_heap_scans = 1;
+             }
+        }
+        envvarpointer = getenv("PG_PREFETCH_DBOID");
+        if (    (envvarpointer != (char *)0)
+             && (   ('1' <= *envvarpointer) && ('9' >= *envvarpointer)   )
+           ) {
+              errno = 0;   /*  required in order to distinguish error from 0 */
+              prefetch_dbOid = (unsigned int)strtoul((const char *)envvarpointer, 0, 10);
+              if (errno) {
+                  prefetch_dbOid = 0;
+              }
+        }
+        elog(LOG, "prefetching initialised with target_prefetch_pages= %d "
+                  ", max_async_io_prefetchers= %d implying aio concurrency= %d "
+                  ", prefetching_for_bitmap= %s "
+                  ", prefetching_for_heap= %s "
+                  ", prefetching_for_iscan= %d with sequential_index_page_prefetching= %s "
+                  ", prefetching_for_btree= %s"
+                   ,target_prefetch_pages ,max_async_io_prefetchers ,numBufferAiocbs
+                   ,(prefetch_bitmap_scans ? "Y" : "N")
+                   ,(prefetch_heap_scans ? "Y" : "N")
+                   ,prefetch_index_scans
+                   ,(prefetch_sequential_index_scans ? "Y" : "N")
+                   ,(prefetch_btree_heaps ? "Y" : "N")
+            );
+#endif /* USE_PREFETCH */
+
+
 	if (foundDescs || foundBufs)
 	{
 		/* both should be present or neither */
@@ -176,3 +436,82 @@ BufferShmemSize(void)
 
 	return size;
 }
+
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*     imprecise count of number of in-use BAiocbs at any time
+ *     we scan the array read-only without latching so are subject to unstable result
+ *     (but since the array is in well-known contiguous storage,
+ *     we are not subject to segmentation violation)
+ *     This function may be called at any time and just does its best
+ *     return the count of what we counted.
+ */
+int
+CountInuseBAiocbs(void)
+{
+        volatile struct BufferAiocb *BAiocb;
+        int count = 0;
+        int ix;
+
+
+        if (BAiocbAnchr != (struct BAiocbAnchor *)0 ) {
+            BAiocb = BAiocbAnchr->BufferAiocbs;             /*   start of list */
+
+            for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                    if ((struct BufferAiocb*)BAIOCB_OCCUPIED == (BAiocb+ix)->BAiocbnext) {   /* not on freelist ? */
+                        count++;
+                    }
+            }
+        }
+        return count;
+}
+
+/*
+ * report how many free BAiocbs at shutdown
+ * DO NOT call this while backends are actively working!!
+ * this report is useful when compare_and_swap method used (see above)
+ * as it can be used to deduce how many BAiocbs were in process-local caches -
+ * (original_number_on_freelist_at_startup - this_reported_number_at_shutdown)
+ */
+void
+ReportFreeBAiocbs(void)
+{
+        volatile struct BufferAiocb *BAiocb;
+        volatile struct BufferAiocb *BufferAiocbs;
+        int count = 0;
+        int fx , ix;
+
+
+        if (BAiocbAnchr != (struct BAiocbAnchor *)0 ) {
+            BAiocb = BAiocbAnchr->FreeBAiocbs;             /*   start of free list */
+            BufferAiocbs = BAiocbAnchr->BufferAiocbs;
+
+            for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                    (BufferAiocbs+ix)->BAiocbDependentCount = 0;  /* use this as marker for finding it on freelist */
+            }
+            for (fx = (numBufferAiocbs-1);  ( (fx>=0) &&  ( BAiocb != (struct BufferAiocb*)0 ) );  fx--) {
+                    
+                    /*  check if it is a valid BufferAiocb */
+                    for (ix = (numBufferAiocbs-1);  ix>=0;  ix--) {
+                        if ((BufferAiocbs+ix) == BAiocb) { /*  is it this one ? */
+                             break;
+                        }
+                    }
+                    if (ix >= 0) {
+                        if (BAiocb->BAiocbDependentCount) {   /* seen it already ? */
+                            elog(LOG, "ReportFreeBAiocbs closed cycle on AIO control block freelist %p"
+                                          ,BAiocb);
+                            fx = 0; /* give up at this point */
+                        }
+                        BAiocb->BAiocbDependentCount = 1;  /* use this as marker for finding it on freelist */
+                        count++;
+                        BAiocb = BAiocb->BAiocbnext;
+                    } else {
+                        elog(LOG, "ReportFreeBAiocbs invalid item on AIO control block freelist %p"
+                                          ,BAiocb);
+                        fx = 0; /* give up at this point */
+                    }
+            }
+        }
+        elog(LOG, "ReportFreeBAiocbs AIO control block list : poolsize= %d  in-use-hwm= %d  final-free= %d" ,numBufferAiocbs , hwmBufferAiocbs , count);
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
--- src/backend/storage/smgr/md.c.orig	2014-08-18 14:10:36.941017108 -0400
+++ src/backend/storage/smgr/md.c	2014-08-19 16:56:13.603196872 -0400
@@ -664,6 +664,80 @@ mdprefetch(SMgrRelation reln, ForkNumber
 }
 
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*
+ *	mdinitaio() --  init the aio subsystem max number of threads and max number of requests
+ */
+void
+mdinitaio(int max_aio_threads, int max_aio_num)
+{
+     FileInitaio( max_aio_threads, max_aio_num );
+}
+
+/*
+ *	mdstartaio() -- start aio read of the specified block of a relation
+ */
+void
+mdstartaio(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum , char *aiocbp , int *retcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , unsigned long BAiocbaioOrdinal
+#endif /*  USE_AIO_SIGEVENT    */
+ )
+{
+#ifdef USE_PREFETCH
+	off_t		seekpos;
+	MdfdVec    *v;
+        int local_retcode;
+
+	v = _mdfd_getseg(reln, forknum, blocknum, false, EXTENSION_FAIL);
+
+	seekpos = (off_t) BLCKSZ *(blocknum % ((BlockNumber) RELSEG_SIZE));
+
+	Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+
+	local_retcode = FileStartaio(v->mdfd_vfd, seekpos, BLCKSZ , aiocbp
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , BAiocbaioOrdinal
+#endif /*  USE_AIO_SIGEVENT    */
+                                   );
+	if (retcode) {
+            *retcode = local_retcode;
+        }
+#endif   /* USE_PREFETCH */
+}
+
+
+/*
+ *	mdcompleteaio() -- complete aio read of the specified block of a relation
+ *      on entry, *inoutcode should indicate :
+ *           .  non-0  <=>   check if complete and wait if not
+ *           .  0      <=>   cancel io immediately
+ */
+void
+mdcompleteaio( char *aiocbp , int *inoutcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , int (*BufAWaitAioCompletion)(char *)  /*  function to await completion if non-originator */
+                                , char *BAiocbP /* pointer to BAIOcb  */
+#endif /*  USE_AIO_SIGEVENT    */
+             )
+{
+#ifdef USE_PREFETCH
+        int local_retcode;
+
+	local_retcode = FileCompleteaio(aiocbp, (inoutcode ? *inoutcode : 0)
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , BufAWaitAioCompletion
+                                , BAiocbP /* pointer to BAIOcb  */
+#endif /*  USE_AIO_SIGEVENT    */
+                                       );
+	if (inoutcode) {
+            *inoutcode = local_retcode;
+        }
+#endif   /* USE_PREFETCH */
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
+
 /*
  *	mdread() -- Read the specified block from a relation.
  */
--- src/backend/storage/smgr/smgr.c.orig	2014-08-18 14:10:36.941017108 -0400
+++ src/backend/storage/smgr/smgr.c	2014-08-19 16:56:13.623196943 -0400
@@ -49,6 +49,21 @@ typedef struct f_smgr
 						 BlockNumber blocknum, char *buffer, bool skipFsync);
 	void		(*smgr_prefetch) (SMgrRelation reln, ForkNumber forknum,
 											  BlockNumber blocknum);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+	void		(*smgr_initaio) (int max_aio_threads, int max_aio_num);
+	void		(*smgr_startaio) (SMgrRelation reln, ForkNumber forknum,
+											  BlockNumber blocknum , char *aiocbp , int *retcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , unsigned long BAiocbaioOrdinal
+#endif /*  USE_AIO_SIGEVENT    */
+                                 );
+	void		(*smgr_completeaio) ( char *aiocbp , int *inoutcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , int (*BufAWaitAioCompletion)(char *)  /*  function to await completion if non-originator */
+                                , char *BAiocbP /* pointer to BAIOcb  */
+#endif /*  USE_AIO_SIGEVENT    */
+                                    );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 	void		(*smgr_read) (SMgrRelation reln, ForkNumber forknum,
 										  BlockNumber blocknum, char *buffer);
 	void		(*smgr_write) (SMgrRelation reln, ForkNumber forknum,
@@ -66,7 +81,11 @@ typedef struct f_smgr
 static const f_smgr smgrsw[] = {
 	/* magnetic disk */
 	{mdinit, NULL, mdclose, mdcreate, mdexists, mdunlink, mdextend,
-		mdprefetch, mdread, mdwrite, mdnblocks, mdtruncate, mdimmedsync,
+		mdprefetch
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+                        ,mdinitaio, mdstartaio, mdcompleteaio
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+              , mdread, mdwrite, mdnblocks, mdtruncate, mdimmedsync,
 		mdpreckpt, mdsync, mdpostckpt
 	}
 };
@@ -612,6 +631,53 @@ smgrprefetch(SMgrRelation reln, ForkNumb
 	(*(smgrsw[reln->smgr_which].smgr_prefetch)) (reln, forknum, blocknum);
 }
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*
+ *	smgrinitaio() -- initialize the aio subsystem max number of threads and max number of requests
+ */
+void
+smgrinitaio(int max_aio_threads, int max_aio_num)
+{
+	(*(smgrsw[0].smgr_initaio)) ( max_aio_threads, max_aio_num );
+}
+
+/*
+ *	smgrstartaio() -- Initiate aio read of the specified block of a relation.
+ */
+void
+smgrstartaio(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum , char *aiocbp , int *retcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , unsigned long BAiocbaioOrdinal
+#endif /*  USE_AIO_SIGEVENT    */
+             )
+{
+	(*(smgrsw[reln->smgr_which].smgr_startaio)) (reln, forknum, blocknum , aiocbp , retcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , BAiocbaioOrdinal
+#endif /*  USE_AIO_SIGEVENT    */
+                                                );
+}
+
+/*
+ *	smgrcompleteaio() -- Complete aio read of the specified block of a relation.
+ */
+void
+smgrcompleteaio(SMgrRelation reln,  char *aiocbp , int *inoutcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , int (*BufAWaitAioCompletion)(char *)  /*  function to await completion if non-originator */
+                                , char *BAiocbP /* pointer to BAIOcb  */
+#endif /*  USE_AIO_SIGEVENT    */
+               )
+{
+	(*(smgrsw[reln->smgr_which].smgr_completeaio)) ( aiocbp , inoutcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , BufAWaitAioCompletion
+                                , BAiocbP
+#endif /*  USE_AIO_SIGEVENT    */
+                                                   );
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 /*
  *	smgrread() -- read a particular block from a relation into the supplied
  *				  buffer.
--- src/backend/storage/file/fd.c.orig	2014-08-18 14:10:36.937017089 -0400
+++ src/backend/storage/file/fd.c	2014-08-19 16:56:13.655197058 -0400
@@ -77,6 +77,9 @@
 #include "utils/guc.h"
 #include "utils/resowner_private.h"
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+#include "aio.h"
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * We must leave some file descriptors free for system(), the dynamic loader,
@@ -123,6 +126,10 @@ int			max_files_per_process = 1000;
  */
 int			max_safe_fds = 32;	/* default if not changed */
 
+#ifdef USE_AIO_SIGEVENT                                    /*    signal non-originator waiters instead of letting them poll */
+extern long num_startedaio;
+extern long num_cancelledaio;
+#endif /*  USE_AIO_SIGEVENT    */
 
 /* Debugging.... */
 
@@ -1239,6 +1246,10 @@ FileClose(File file)
  * We could add an implementation using libaio in the future; but note that
  * this API is inappropriate for libaio, which wants to have a buffer provided
  * to read into.
+ * Also note that a new, different implementation of asynchronous prefetch
+ * using librt,  not libaio,  is provided by the two functions following this one,
+ * FileStartaio and FileCompleteaio.   These also require to have a buffer provided
+ * to read into,  which the new async_io support provides.
  */
 int
 FilePrefetch(File file, off_t offset, int amount)
@@ -1266,6 +1277,207 @@ FilePrefetch(File file, off_t offset, in
 #endif
 }
 
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+/*
+ * FileInitaio - initialize the aio subsystem max number of threads and max number of requests
+ *  input parms
+ *  max_aio_threads;    maximum number of threads
+ *  max_aio_num;        maximum number of concurrent aio read requests
+ *
+ *  on linux, man page for the librt implemenation of aio_init() says :
+ *         This function is a GNU extension.
+ *  If your posix aio does not have it,   then add the following line to 
+ *        src/include/pg_config_manual.h
+ *    #define DONT_HAVE_AIO_INIT
+ *  to render it as a no-op
+ */
+void
+FileInitaio(int max_aio_threads, int max_aio_num )
+{
+#ifndef DONT_HAVE_AIO_INIT
+    struct aioinit aioinit_struct;  /*  structure to pass to aio_init */
+
+    aioinit_struct.aio_threads = max_aio_threads; /*     maximum number of threads */
+    aioinit_struct.aio_num = max_aio_num;         /*     maximum number of concurrent aio read requests */
+    aioinit_struct.aio_idle_time = 1;             /*     we dont want to alter this but aio_init does not ignore it so set to the default */
+    aio_init(&aioinit_struct);
+#endif  /* ndef DONT_HAVE_AIO_INIT */
+    return;
+}
+
+/*
+ * FileStartaio - initiate asynchronous read of a given range of the file.
+ * The logical seek position is unaffected.
+ *
+ * use standard posix aio (librt)
+ *  ASSUME   BufferAiocb.aio_buf already set to -> buffer by caller
+ *  return 0 if successfully started,  else non-zero
+ */
+int
+FileStartaio(File file, off_t offset, int amount , char *aiocbp
+#ifdef USE_AIO_SIGEVENT                                    /*    signal  non-originator waiters instead of letting them poll */
+            , unsigned long BAiocbaioOrdinal               /*    ordinal number of this aio in backend's sequence  */
+#endif /*  USE_AIO_SIGEVENT    */
+                      )
+{
+	int	returnCode;
+        struct aiocb *my_aiocbp = (struct aiocb *)aiocbp;
+
+	Assert(FileIsValid(file));
+
+	DO_DB(elog(LOG, "FileStartaio: %d (%s) " INT64_FORMAT " %d",
+			   file, VfdCache[file].fileName,
+			   (int64) offset, amount));
+
+	returnCode = FileAccess(file);
+	if (returnCode >= 0) {
+
+            my_aiocbp->aio_fildes = VfdCache[file].fd;
+            my_aiocbp->aio_lio_opcode = LIO_READ;
+            my_aiocbp->aio_nbytes = amount;
+            my_aiocbp->aio_offset = offset;
+#ifdef USE_AIO_SIGEVENT                                    /*    signal non-originator waiters instead of letting them poll */
+            my_aiocbp->aio_sigevent.sigev_notify = SIGEV_SIGNAL;
+            my_aiocbp->aio_sigevent.sigev_signo = AIO_SIGEVENT_SIGNALNUM;
+            my_aiocbp->aio_sigevent.sigev_value.sival_ptr = (void *)BAiocbaioOrdinal; /*  ordinal number of this aio */
+#endif /*  USE_AIO_SIGEVENT    */
+            returnCode = aio_read(my_aiocbp);
+#ifdef USE_AIO_SIGEVENT                                    /*    signal non-originator waiters instead of letting them poll */
+            if (returnCode == 0) {
+				num_startedaio++;
+            }
+#endif /*  USE_AIO_SIGEVENT    */
+        }
+
+	return returnCode;
+}
+
+/*
+ * FileCompleteaio - complete asynchronous aio read
+ * normal_wait indicates whether to cancel or wait -
+ *                -1 <=> check - dont wait or cancel but report whether complete
+ *                 0 <=> cancel
+ *                 1 <=> wait by polling the aiocb or waiting on signal i.e. non-originator
+ *                 2 <=> wait by suspending on the aiocb                i.e. originator
+ *
+ * use standard posix aio (librt)
+ *  return 0 if successfull and did not have to wait,
+ *         1 if successfull and had to wait,
+ *    else x'ff'
+ */
+int
+FileCompleteaio( char *aiocbp , int normal_wait
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on signal instead of polling */
+            , int (*BufAWaitAioCompletion)(char *)  /*  function to await completion if non-originator */
+            , char *BAiocbP /* pointer to BAIOcb  */
+#endif /*  USE_AIO_SIGEVENT    */
+ )
+{
+	int	returnCode;
+	int	aio_errno;
+        struct aiocb *my_aiocbp = (struct aiocb *)aiocbp;
+        const struct aiocb const*cblist[1];
+        int fd;
+        struct timespec my_timeout = { 0 , 10000 };
+        struct timespec *suspend_timeout_P; /*  the timeout actually used depending on normal_wait */
+        int max_polls;
+
+        fd = my_aiocbp->aio_fildes;
+        cblist[0] = my_aiocbp;
+
+        returnCode = aio_errno = aio_error(my_aiocbp);
+        /* note that aio_error returns 0 if op already completed successfully */
+
+        /*  first handle normal case of waiting for op to complete  */
+        if (normal_wait > 0) {
+            /*   if told not to poll,   then specify no timeout  */
+            suspend_timeout_P = ( (normal_wait == 1) ? &my_timeout : (struct timespec *)0 );
+
+            while (aio_errno == EINPROGRESS) {
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on signal instead of polling */
+                if (normal_wait == 1) { /*   told to wait on signal */
+                    returnCode = aio_errno = (*BufAWaitAioCompletion)(BAiocbP); /*  wait for completion */
+				} else {
+#endif /*  USE_AIO_SIGEVENT    */
+                    max_polls = 256;
+                    my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                    returnCode = aio_suspend(cblist , 1 , suspend_timeout_P);
+                    while (    (returnCode < 0) && (max_polls-- > 0)
+                            && ((EAGAIN == errno) || (EINTR == errno))
+                          ) {
+                        my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+                        returnCode = aio_suspend(cblist , 1 , suspend_timeout_P);
+                    }
+
+                    returnCode = aio_errno = aio_error(my_aiocbp);
+
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on signal instead of polling */
+                }
+#endif /*  USE_AIO_SIGEVENT    */
+                /*  now return_code is from aio_error  */
+                if (returnCode == 0) {
+                    returnCode = 1;    /*  successful but had to wait */
+                }
+            }
+            if (aio_errno) {
+                elog(LOG, "FileCompleteaio: normal_wait= %d fd= %d aio_errno= %d", normal_wait, fd, aio_errno);
+                returnCode = 0xff; /*  unsuccessful */
+            }
+
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on signal instead of polling */
+            /*   if this bkend is the originator, and therefore has the responsibility
+            **   to post suspended waiters,  then do so now.
+            **   140814 : Actually best not to do so :
+            **   because  mutually redundant with the same functionality in BufHandlSigAsync.
+            **   at one time we thought that it was necessary to do so here in the mistaken belief
+            **   that there could be cases where
+            **          . an aio is started 
+            **          . no signal is subsequently delivered until after the *next* aio is started
+            **   but in fact we have found that,  after any aio_read is started,
+            **   at least one signal will be delivered after it completes
+            **   even if no subequent aio_read is started.
+            if (    (normal_wait == 2)  **   originator   **
+                     **     there are several places where originator might post suspended waiters :
+                      *      .  BufCheckAsync if cancelling aio
+                      *      .  here
+                      *      .  in BufHandlSigAsync  ( called if signal is delivered )
+                      *      The last two appear to be mutually redundant,
+                      *      but signals are not always delivered
+                      *      Fortunately it does no harm if both cases execute simultaneously
+                      *      because each serializes on the waiters list
+                      **
+               ) {
+                    BufPostAioWaiters(BAiocbaiolockP);
+            }
+            */
+#endif /*  USE_AIO_SIGEVENT    */
+        } else {                    /* check or cancel */
+            if (normal_wait == 0) { /* cancel */
+				if (aio_errno == EINPROGRESS) {
+					do {
+							max_polls = 256;
+							my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+							returnCode = aio_cancel(fd, my_aiocbp);
+							while ((returnCode == AIO_NOTCANCELED) && (max_polls-- > 0)) {
+								my_timeout.tv_sec = 0; my_timeout.tv_nsec = 10000;
+								returnCode = aio_cancel(fd, my_aiocbp);
+							}
+						returnCode = aio_errno = aio_error(my_aiocbp);
+					} while (aio_errno == EINPROGRESS);
+					returnCode = 0xff; /*  unsuccessful */
+				}
+			}
+            if (returnCode != 0)
+                returnCode = 0xff; /*  unsuccessful */
+        }
+
+	DO_DB(elog(LOG, "FileCompleteaio: %d %d",
+			   fd, returnCode));
+
+	return returnCode;
+}
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+
 int
 FileRead(File file, char *buffer, int amount)
 {
--- src/backend/storage/lmgr/proc.c.orig	2014-08-18 14:10:36.941017108 -0400
+++ src/backend/storage/lmgr/proc.c	2014-08-19 16:56:13.695197201 -0400
@@ -52,6 +52,7 @@
 #include "utils/timeout.h"
 #include "utils/timestamp.h"
 
+extern pid_t this_backend_pid;     /*   pid of this backend */
 
 /* GUC variables */
 int			DeadlockTimeout = 1000;
@@ -361,6 +362,7 @@ InitProcess(void)
 	MyPgXact->xid = InvalidTransactionId;
 	MyPgXact->xmin = InvalidTransactionId;
 	MyProc->pid = MyProcPid;
+        this_backend_pid = getpid();    /*    pid of this backend */
 	/* backendId, databaseId and roleId will be filled in later */
 	MyProc->backendId = InvalidBackendId;
 	MyProc->databaseId = InvalidOid;
@@ -536,6 +538,9 @@ InitAuxiliaryProcess(void)
 	MyProc->lwWaiting = false;
 	MyProc->lwWaitMode = 0;
 	MyProc->lwWaitLink = NULL;
+#if defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP) && defined(USE_AIO_SIGEVENT)
+	MyProc->BAiocbWaiterLink = NULL;
+#endif
 	MyProc->waitLock = NULL;
 	MyProc->waitProcLock = NULL;
 #ifdef USE_ASSERT_CHECKING
--- src/backend/access/heap/heapam.c.orig	2014-08-18 14:10:36.841016638 -0400
+++ src/backend/access/heap/heapam.c	2014-08-19 16:56:13.795197558 -0400
@@ -71,6 +71,28 @@
 #include "utils/syscache.h"
 #include "utils/tqual.h"
 
+#ifdef USE_PREFETCH
+#include <sys/types.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+#include "executor/instrument.h"
+
+extern unsigned int prefetch_dbOid; /*  database oid of relations on which prefetching to be done - 0 means all */
+extern unsigned int prefetch_heap_scans; /*  boolean whether to prefetch non-bitmap heap scans         */
+
+/*  special values for scan->rs_prefetch_target indicating as follows :               */
+#define PREFETCH_MAYBE 0xffffffff      /*   prefetch permitted but not yet in effect  */
+#define PREFETCH_DISABLED 0xfffffffe   /*   prefetch disabled and not permitted       */
+/*  PREFETCH_WRAP_POINT indicates a pretcher who has reached the point where the scan would wrap -
+**  at this point the prefetcher runs on the spot until scan catches up.
+**  This *must* be < maximum valid setting of target_prefetch_pages aka effective_io_concurrency.
+*/
+#define PREFETCH_WRAP_POINT 0x0fffffff
+
+#endif   /* USE_PREFETCH */
+
 
 /* GUC variable */
 bool		synchronize_seqscans = true;
@@ -115,6 +137,10 @@ static XLogRecPtr log_heap_new_cid(Relat
 static HeapTuple ExtractReplicaIdentity(Relation rel, HeapTuple tup, bool key_modified,
 					   bool *copy);
 
+#ifdef USE_PREFETCH
+static void heap_unread_add(HeapScanDesc scan, BlockNumber blockno);
+static void heap_unread_subtract(HeapScanDesc scan, BlockNumber blockno);
+#endif   /* USE_PREFETCH */
 
 /*
  * Each tuple lock mode has a corresponding heavyweight lock, and one or two
@@ -292,9 +318,150 @@ initscan(HeapScanDesc scan, ScanKey key,
 	 * Currently, we don't have a stats counter for bitmap heap scans (but the
 	 * underlying bitmap index scans will be counted).
 	 */
-	if (!scan->rs_bitmapscan)
+#ifdef USE_PREFETCH
+        /*    by default,  no prefetching on any scan  */
+        scan->rs_prefetch_target = PREFETCH_DISABLED;  /*  tentatively disable  */
+        scan->rs_pfchblock = 0; /*  scanner will reset this to be ahead of scan */
+        scan->rs_Unread_Pfetched_base = (BlockNumber *)0; /*  list of prefetched but unread blocknos */
+        scan->rs_Unread_Pfetched_next = 0; /*  next unread blockno */
+        scan->rs_Unread_Pfetched_count = 0; /* number of valid unread blocknos */
+#endif   /* USE_PREFETCH */
+	if (!scan->rs_bitmapscan) {
+
 		pgstat_count_heap_scan(scan->rs_rd);
+#ifdef USE_PREFETCH
+                /*    bitmap scans do their own prefetching -
+                **    for others,  set up prefetching now
+                */
+                if (    prefetch_heap_scans
+                     && (target_prefetch_pages > 0)
+                     &&	(!RelationUsesLocalBuffers(scan->rs_rd))
+                   ) {
+                      /*   prefetch_dbOid may be set to a database Oid to specify only prefetch in that db */
+                      if (    (   (prefetch_dbOid > 0)
+                               && (prefetch_dbOid == scan->rs_rd->rd_node.dbNode)
+                              )
+                          ||  (prefetch_dbOid == 0)
+                         ) {
+                          scan->rs_prefetch_target = PREFETCH_MAYBE;    /*  permitted but let the scan decide */
+                      }
+                      else {
+                      }
 }
+#endif   /* USE_PREFETCH */
+        }
+}
+
+#ifdef USE_PREFETCH
+/* add this blockno to list of prefetched and unread blocknos
+** use the one identified by the (next+count|modulo circumference) index if it is unused,
+** else search for the first available slot if there is one,
+** else error.
+*/
+static void
+heap_unread_add(HeapScanDesc scan, BlockNumber blockno)
+{
+      BlockNumber *available_P;   /*  where to store new blockno */
+      unsigned int Unread_Pfetched_index = scan->rs_Unread_Pfetched_next
+                                         + scan->rs_Unread_Pfetched_count; /* index of next unused slot */
+
+      /*  caller is not supposed to pass InvalidBlockNumber but check anyway */
+      if (blockno != InvalidBlockNumber) {
+
+		  /*  ensure there is some room somewhere   */
+		  if (scan->rs_Unread_Pfetched_count < target_prefetch_pages) {
+
+			  /*  try the "next+count" one */
+			  if (Unread_Pfetched_index >= target_prefetch_pages) {
+				  Unread_Pfetched_index -= target_prefetch_pages;  /* modulo circumference */
+			  }
+			  available_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);   /*  where to store new blockno */
+			  if (*available_P == InvalidBlockNumber) { /* unused */
+				  goto store_blockno;
+			  } else {
+                  /*  slow-search the entire list  */
+				  for (Unread_Pfetched_index = 0; Unread_Pfetched_index < target_prefetch_pages; Unread_Pfetched_index++) {
+					  available_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);   /*  where to store new blockno */
+					  if (*available_P == InvalidBlockNumber) { /* unused */
+                          /*  before storing this blockno,
+                          **  since the next pointer did not locate an unused slot,
+                          **  set it to one which is more likely to be so for the next time
+                          */
+                          scan->rs_Unread_Pfetched_next = Unread_Pfetched_index;
+						  goto store_blockno;
+					  }
+				  }
+			  }
+		  }
+
+          /*  if we reach here,    either there was no available slot
+          **  or we thought there was one and didn't find any  --
+          */
+  		  ereport(NOTICE,
+			  (errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("heap_unread_add overflowed list cannot add blockno %d", blockno)));
+
+  		  return;
+
+    store_blockno:
+		  *available_P = blockno;
+		  scan->rs_Unread_Pfetched_count++;  /*  update count */
+
+	  }
+
+    return;
+}
+
+/* remove specified blockno from list of prefetched and unread blocknos.
+** Usually this will be found at the rs_Unread_Pfetched_next item -
+** else search for it.    If not found,   inore it  -  no error results.
+*/
+static void
+heap_unread_subtract(HeapScanDesc scan, BlockNumber blockno)
+{
+      unsigned int Unread_Pfetched_index = scan->rs_Unread_Pfetched_next; /* index of next unread blockno */
+      BlockNumber *candidate_P;   /*  location of callers blockno - maybe */
+      BlockNumber nextUnreadPfetched;
+
+      /*  caller is not supposed to pass InvalidBlockNumber but check anyway */
+      if (    (blockno != InvalidBlockNumber)
+		   && ( scan->rs_Unread_Pfetched_count > 0 )   /*  if the list is not empty  */
+         ) {
+
+			  /*  take modulo of the circumference.
+			  **  actually rs_Unread_Pfetched_next should never exceed the circumference but check anyway.
+			  */
+			  if (Unread_Pfetched_index >= target_prefetch_pages) {
+				  Unread_Pfetched_index  -= target_prefetch_pages;
+}
+			  candidate_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);
+			  nextUnreadPfetched = *candidate_P;
+
+			  if ( nextUnreadPfetched == blockno ) {
+				  goto remove_blockno;
+			  } else {
+                  /*  slow-search the entire list  */
+				  for (Unread_Pfetched_index = 0; Unread_Pfetched_index < target_prefetch_pages; Unread_Pfetched_index++) {
+					  candidate_P = (scan->rs_Unread_Pfetched_base + Unread_Pfetched_index);   /*  where to store new blockno */
+					  if (*candidate_P == blockno) { /* unused */
+						  goto remove_blockno;
+					  }
+				  }
+			  }
+
+    remove_blockno:
+			  *candidate_P = InvalidBlockNumber;
+
+			  scan->rs_Unread_Pfetched_next = (Unread_Pfetched_index+1);  /*  update next pfchd unread */
+			  if (scan->rs_Unread_Pfetched_next >= target_prefetch_pages) {
+					  scan->rs_Unread_Pfetched_next = 0;
+			  }
+			  scan->rs_Unread_Pfetched_count--;  /*  update count */
+	  }
+
+      return;
+}
+#endif   /* USE_PREFETCH */
 
 /*
  * heapgetpage - subroutine for heapgettup()
@@ -304,7 +471,7 @@ initscan(HeapScanDesc scan, ScanKey key,
  * which tuples on the page are visible.
  */
 static void
-heapgetpage(HeapScanDesc scan, BlockNumber page)
+heapgetpage(HeapScanDesc scan, BlockNumber page , BlockNumber prefetchHWM)
 {
 	Buffer		buffer;
 	Snapshot	snapshot;
@@ -314,6 +481,10 @@ heapgetpage(HeapScanDesc scan, BlockNumb
 	OffsetNumber lineoff;
 	ItemId		lpp;
 	bool		all_visible;
+#ifdef USE_PREFETCH
+	int             PrefetchBufferRc;  /*   indicates whether requested prefetch block already in a buffer and if pin count on buffer has been increased */
+#endif   /* USE_PREFETCH */
+
 
 	Assert(page < scan->rs_nblocks);
 
@@ -336,6 +507,98 @@ heapgetpage(HeapScanDesc scan, BlockNumb
 									   RBM_NORMAL, scan->rs_strategy);
 	scan->rs_cblock = page;
 
+#ifdef USE_PREFETCH
+
+        heap_unread_subtract(scan, page);
+
+        /*    maybe prefetch some pages  starting with rs_pfchblock */
+        if (scan->rs_prefetch_target >= 0) {       /*   prefetching enabled on this scan ?                         */
+            int next_block_to_be_read = (page+1);  /*   next block to be read = lowest possible prefetchable block */
+            int num_to_pfch_this_time;             /*   eventually holds the number of blocks to prefetch now      */
+            int prefetchable_range;                /*   size of the area ahead of the current prefetch position    */
+
+            /*  check if prefetcher reached wrap point and the scan has now wrapped */
+            if (  (page == 0) && (scan->rs_prefetch_target == PREFETCH_WRAP_POINT)  ) {
+                scan->rs_prefetch_target = 1;
+                scan->rs_pfchblock = next_block_to_be_read;
+            } else
+            if (scan->rs_pfchblock < next_block_to_be_read) {
+                scan->rs_pfchblock = next_block_to_be_read; /* next block to be prefetched must be ahead of one we just read */
+            }
+
+            /* now we know where we would start prefetching -
+            ** next question   -  if this is a sync scan,  ensure we do not prefetch behind the HWM
+            ** debatable whether to require strict inequality or >=  -   >= works better in practice
+            */
+            if ( (!scan->rs_syncscan) || (scan->rs_pfchblock >= prefetchHWM) ) {
+
+                /* now we know where we will start prefetching -
+                ** next question   -  how many?
+                ** apply two limits :
+                **  1.   target prefetch distance
+                **  2.   number of available blocks ahead of us
+                */
+
+                /*  1.   target prefetch distance   */
+                num_to_pfch_this_time = next_block_to_be_read + scan->rs_prefetch_target; /* page beyond prefetch target */
+                num_to_pfch_this_time -= scan->rs_pfchblock;                              /*  convert to offset        */
+
+                /*   first do prefetching up to our current limit  ...
+                **   highest page number that a scan (pre)-fetches is scan->rs_nblocks-1
+                **   note  -  prefetcher does not wrap a prefetch range -
+                **            instead just stop and then start again if and when main scan wraps
+                */
+                if (scan->rs_pfchblock <= scan->rs_startblock) {  /*  if on second leg towards startblock */
+                    prefetchable_range = ((int)(scan->rs_startblock) - (int)(scan->rs_pfchblock));
+                }
+                else {                                            /*     on first leg towards nblocks     */
+                    prefetchable_range = ((int)(scan->rs_nblocks) - (int)(scan->rs_pfchblock));
+                }
+                if (prefetchable_range > 0) {           /*  if theres a range to prefetch */
+
+                    /*  2.   number of available blocks ahead of us        */
+                    if (num_to_pfch_this_time > prefetchable_range) {
+                        num_to_pfch_this_time = prefetchable_range;
+                    }
+                    while (num_to_pfch_this_time-- > 0) {
+                        PrefetchBufferRc = PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, scan->rs_pfchblock, scan->rs_strategy);
+                        /*  if pin acquired on buffer,  then remember in case of future Discard */
+                        if (PrefetchBufferRc & PREFTCHRC_BUF_PIN_INCREASED) {
+                            heap_unread_add(scan, scan->rs_pfchblock);
+						}
+                        scan->rs_pfchblock++;
+                        /*  if syncscan and requested block was already in buffer pool,
+                        **  this suggests that another scanner is ahead of us and we should advance
+                        */
+                        if ( (scan->rs_syncscan) && (PrefetchBufferRc & PREFTCHRC_BLK_ALREADY_PRESENT) ) {
+                            scan->rs_pfchblock++;
+                            num_to_pfch_this_time--;
+                        }
+                    }
+                }
+                else {
+                    /*   we must not modify scan->rs_pfchblock here
+                    **   because it is needed for possible DiscardBuffer at end of scan  ...
+                    **   ... instead ...
+                    */
+                    scan->rs_prefetch_target = PREFETCH_WRAP_POINT;  /*   mark this prefetcher as waiting to wrap */
+                }
+
+                /*   ...  then adjust prefetching limit : by doubling on each iteration */
+                if (scan->rs_prefetch_target == 0) {
+                    scan->rs_prefetch_target = 1;
+                }
+                else {
+                    scan->rs_prefetch_target *= 2;
+                    if (scan->rs_prefetch_target > target_prefetch_pages) {
+                        scan->rs_prefetch_target = target_prefetch_pages;
+                    }
+                }
+            }
+        }
+#endif   /* USE_PREFETCH */
+
+
 	if (!scan->rs_pageatatime)
 		return;
 
@@ -452,6 +715,10 @@ heapgettup(HeapScanDesc scan,
 	OffsetNumber lineoff;
 	int			linesleft;
 	ItemId		lpp;
+    BlockNumber  prefetchHWM = 0;   /*  HWM of prefetch BlockNum for all participants in same sync scan */
+#ifdef USE_PREFETCH
+    int          ix;
+#endif   /* USE_PREFETCH */
 
 	/*
 	 * calculate next starting lineoff, given scan direction
@@ -470,7 +737,25 @@ heapgettup(HeapScanDesc scan,
 				return;
 			}
 			page = scan->rs_startblock; /* first page */
-			heapgetpage(scan, page);
+#ifdef USE_PREFETCH
+                        /*  decide if we shall do prefetching :  only if :
+                        **   .  prefetching enabled for this scan
+                        **   .  not a bitmap scan (which do their own)
+                        **   .  sufficient number of blocks - at least twice the target_prefetch_pages
+                        */
+                        if (    (!scan->rs_bitmapscan)
+                             && (scan->rs_prefetch_target >= PREFETCH_MAYBE)
+                             && (scan->rs_nblocks > (2*target_prefetch_pages))
+                           )  {
+                                scan->rs_prefetch_target = 1;  /*  do prefetching on forward non-bitmap scan */
+                                scan->rs_Unread_Pfetched_base = (BlockNumber *)palloc(sizeof(BlockNumber)*target_prefetch_pages);
+                                /*  Initialise the list */
+                                for (ix = 0; ix < target_prefetch_pages; ix++) {
+									*(scan->rs_Unread_Pfetched_base + ix) = InvalidBlockNumber;
+								}
+                        }
+#endif   /* USE_PREFETCH */
+			heapgetpage(scan, page, prefetchHWM);
 			lineoff = FirstOffsetNumber;		/* first offnum */
 			scan->rs_inited = true;
 		}
@@ -516,7 +801,7 @@ heapgettup(HeapScanDesc scan,
 				page = scan->rs_startblock - 1;
 			else
 				page = scan->rs_nblocks - 1;
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 		}
 		else
 		{
@@ -557,7 +842,7 @@ heapgettup(HeapScanDesc scan,
 
 		page = ItemPointerGetBlockNumber(&(tuple->t_self));
 		if (page != scan->rs_cblock)
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 
 		/* Since the tuple was previously fetched, needn't lock page here */
 		dp = (Page) BufferGetPage(scan->rs_cbuf);
@@ -660,8 +945,12 @@ heapgettup(HeapScanDesc scan,
 			 * a little bit backwards on every invocation, which is confusing.
 			 * We don't guarantee any specific ordering in general, though.
 			 */
-			if (scan->rs_syncscan)
-				ss_report_location(scan->rs_rd, page);
+			if (scan->rs_syncscan) {
+#ifdef USE_PREFETCH
+				prefetchHWM = scan->rs_pfchblock;
+#endif   /* USE_PREFETCH */
+				ss_report_location(scan->rs_rd, page, &prefetchHWM);
+			}
 		}
 
 		/*
@@ -671,6 +960,22 @@ heapgettup(HeapScanDesc scan,
 		{
 			if (BufferIsValid(scan->rs_cbuf))
 				ReleaseBuffer(scan->rs_cbuf);
+#ifdef USE_PREFETCH
+                        if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                            /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                            while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+                        }
+#endif   /* USE_PREFETCH */
 			scan->rs_cbuf = InvalidBuffer;
 			scan->rs_cblock = InvalidBlockNumber;
 			tuple->t_data = NULL;
@@ -678,7 +983,7 @@ heapgettup(HeapScanDesc scan,
 			return;
 		}
 
-		heapgetpage(scan, page);
+		heapgetpage(scan, page, prefetchHWM);
 
 		LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);
 
@@ -727,6 +1032,10 @@ heapgettup_pagemode(HeapScanDesc scan,
 	OffsetNumber lineoff;
 	int			linesleft;
 	ItemId		lpp;
+    BlockNumber  prefetchHWM = 0;   /*  HWM of prefetch BlockNum for all participants in same sync scan */
+#ifdef USE_PREFETCH
+    int          ix;
+#endif   /* USE_PREFETCH */
 
 	/*
 	 * calculate next starting lineindex, given scan direction
@@ -745,7 +1054,25 @@ heapgettup_pagemode(HeapScanDesc scan,
 				return;
 			}
 			page = scan->rs_startblock; /* first page */
-			heapgetpage(scan, page);
+#ifdef USE_PREFETCH
+                        /*  decide if we shall do prefetching :  only if :
+                        **   .  prefetching enabled for this scan
+                        **   .  not a bitmap scan (which do their own)
+                        **   .  sufficient number of blocks - at least twice the target_prefetch_pages
+                        */
+                        if (    (!scan->rs_bitmapscan)
+                             && (scan->rs_prefetch_target >= PREFETCH_MAYBE)
+                             && (scan->rs_nblocks > (2*target_prefetch_pages))
+                           )  {
+                                scan->rs_prefetch_target = 1;  /*  do prefetching on forward non-bitmap scan */
+                                scan->rs_Unread_Pfetched_base = (BlockNumber *)palloc(sizeof(BlockNumber)*target_prefetch_pages);
+                                /*  Initialise the list */
+                                for (ix = 0; ix < target_prefetch_pages; ix++) {
+									*(scan->rs_Unread_Pfetched_base + ix) = InvalidBlockNumber;
+								}
+                        }
+#endif   /* USE_PREFETCH */
+			heapgetpage(scan, page, prefetchHWM);
 			lineindex = 0;
 			scan->rs_inited = true;
 		}
@@ -788,7 +1115,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 				page = scan->rs_startblock - 1;
 			else
 				page = scan->rs_nblocks - 1;
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 		}
 		else
 		{
@@ -826,7 +1153,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 
 		page = ItemPointerGetBlockNumber(&(tuple->t_self));
 		if (page != scan->rs_cblock)
-			heapgetpage(scan, page);
+			heapgetpage(scan, page, prefetchHWM);
 
 		/* Since the tuple was previously fetched, needn't lock page here */
 		dp = (Page) BufferGetPage(scan->rs_cbuf);
@@ -921,8 +1248,12 @@ heapgettup_pagemode(HeapScanDesc scan,
 			 * a little bit backwards on every invocation, which is confusing.
 			 * We don't guarantee any specific ordering in general, though.
 			 */
-			if (scan->rs_syncscan)
-				ss_report_location(scan->rs_rd, page);
+			if (scan->rs_syncscan) {
+#ifdef USE_PREFETCH
+				prefetchHWM = scan->rs_pfchblock;
+#endif   /* USE_PREFETCH */
+				ss_report_location(scan->rs_rd, page, &prefetchHWM);
+			}
 		}
 
 		/*
@@ -932,6 +1263,22 @@ heapgettup_pagemode(HeapScanDesc scan,
 		{
 			if (BufferIsValid(scan->rs_cbuf))
 				ReleaseBuffer(scan->rs_cbuf);
+#ifdef USE_PREFETCH
+                        if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                            /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                             while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+                        }
+#endif   /* USE_PREFETCH */
 			scan->rs_cbuf = InvalidBuffer;
 			scan->rs_cblock = InvalidBlockNumber;
 			tuple->t_data = NULL;
@@ -939,7 +1286,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 			return;
 		}
 
-		heapgetpage(scan, page);
+		heapgetpage(scan, page, prefetchHWM);
 
 		dp = (Page) BufferGetPage(scan->rs_cbuf);
 		lines = scan->rs_ntuples;
@@ -1394,6 +1741,23 @@ void
 heap_rescan(HeapScanDesc scan,
 			ScanKey key)
 {
+
+#ifdef USE_PREFETCH
+         if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                       /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                            while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+         }
+#endif   /* USE_PREFETCH */
 	/*
 	 * unpin scan buffers
 	 */
@@ -1418,6 +1782,23 @@ heap_endscan(HeapScanDesc scan)
 {
 	/* Note: no locking manipulations needed */
 
+
+#ifdef USE_PREFETCH
+         if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                       /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                            while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+         }
+#endif   /* USE_PREFETCH */
 	/*
 	 * unpin scan buffers
 	 */
@@ -1435,6 +1816,12 @@ heap_endscan(HeapScanDesc scan)
 	if (scan->rs_strategy != NULL)
 		FreeAccessStrategy(scan->rs_strategy);
 
+#ifdef USE_PREFETCH
+    if (scan->rs_Unread_Pfetched_base) {
+        pfree(scan->rs_Unread_Pfetched_base);
+    }
+#endif   /* USE_PREFETCH */
+
 	if (scan->rs_temp_snap)
 		UnregisterSnapshot(scan->rs_snapshot);
 
@@ -1464,7 +1851,6 @@ heap_endscan(HeapScanDesc scan)
 #define HEAPDEBUG_3
 #endif   /* !defined(HEAPDEBUGALL) */
 
-
 HeapTuple
 heap_getnext(HeapScanDesc scan, ScanDirection direction)
 {
@@ -6364,6 +6750,25 @@ heap_markpos(HeapScanDesc scan)
 void
 heap_restrpos(HeapScanDesc scan)
 {
+
+
+#ifdef USE_PREFETCH
+                        if (  (scan->rs_pfchblock > 0) && (scan->rs_cblock != InvalidBlockNumber)  ) {
+                            BlockNumber *Unread_Pfetched_base = scan->rs_Unread_Pfetched_base;
+                            unsigned int Unread_Pfetched_next = scan->rs_Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+                            unsigned int Unread_Pfetched_count = scan->rs_Unread_Pfetched_count;
+
+                            /*  we have prefetched but not read the range from cblock+1 to pfchblock-1 if that is non-empty */
+                             while ((Unread_Pfetched_count--) > 0) {
+                                DiscardBuffer( scan->rs_rd, MAIN_FORKNUM, *(Unread_Pfetched_base+Unread_Pfetched_next));
+                                heap_unread_subtract(scan, *(Unread_Pfetched_base+Unread_Pfetched_next)); /* remove this blockno from list of prefetched and unread blocknos */
+                                Unread_Pfetched_next++;
+                                if (Unread_Pfetched_next >= target_prefetch_pages)
+                                    Unread_Pfetched_next = 0;
+                            }
+                        }
+#endif   /* USE_PREFETCH */
+
 	/* XXX no amrestrpos checking that ammarkpos called */
 
 	if (!ItemPointerIsValid(&scan->rs_mctid))
--- src/backend/access/heap/syncscan.c.orig	2014-08-18 14:10:36.841016638 -0400
+++ src/backend/access/heap/syncscan.c	2014-08-19 16:56:13.835197702 -0400
@@ -90,6 +90,7 @@ typedef struct ss_scan_location_t
 {
 	RelFileNode relfilenode;	/* identity of a relation */
 	BlockNumber location;		/* last-reported location in the relation */
+	BlockNumber prefetchHWM;	/* high-water-mark of prefetched Blocknum */
 } ss_scan_location_t;
 
 typedef struct ss_lru_item_t
@@ -113,7 +114,7 @@ static ss_scan_locations_t *scan_locatio
 
 /* prototypes for internal functions */
 static BlockNumber ss_search(RelFileNode relfilenode,
-		  BlockNumber location, bool set);
+		  BlockNumber location, bool set , BlockNumber *prefetchHWMp);
 
 
 /*
@@ -160,6 +161,7 @@ SyncScanShmemInit(void)
 			item->location.relfilenode.dbNode = InvalidOid;
 			item->location.relfilenode.relNode = InvalidOid;
 			item->location.location = InvalidBlockNumber;
+			item->location.prefetchHWM = InvalidBlockNumber;
 
 			item->prev = (i > 0) ?
 				(&scan_locations->items[i - 1]) : NULL;
@@ -185,7 +187,7 @@ SyncScanShmemInit(void)
  * data structure.
  */
 static BlockNumber
-ss_search(RelFileNode relfilenode, BlockNumber location, bool set)
+ss_search(RelFileNode relfilenode, BlockNumber location, bool set , BlockNumber *prefetchHWMp)
 {
 	ss_lru_item_t *item;
 
@@ -206,6 +208,22 @@ ss_search(RelFileNode relfilenode, Block
 			{
 				item->location.relfilenode = relfilenode;
 				item->location.location = location;
+                                /*  if prefetch information requested,
+                                **  then reconcile and either update or report back the new HWM.
+                                */
+                                if (prefetchHWMp)
+                                {
+                                    if (   (item->location.prefetchHWM == InvalidBlockNumber)
+                                        || (item->location.prefetchHWM < *prefetchHWMp)
+                                       )
+                                    {
+                                      item->location.prefetchHWM = *prefetchHWMp;
+                                    }
+                                    else
+                                    {
+                                      *prefetchHWMp = item->location.prefetchHWM;
+                                    }
+                                }
 			}
 			else if (set)
 				item->location.location = location;
@@ -252,7 +270,7 @@ ss_get_location(Relation rel, BlockNumbe
 	BlockNumber startloc;
 
 	LWLockAcquire(SyncScanLock, LW_EXCLUSIVE);
-	startloc = ss_search(rel->rd_node, 0, false);
+	startloc = ss_search(rel->rd_node, 0, false , 0);
 	LWLockRelease(SyncScanLock);
 
 	/*
@@ -282,7 +300,7 @@ ss_get_location(Relation rel, BlockNumbe
  * same relfilenode.
  */
 void
-ss_report_location(Relation rel, BlockNumber location)
+ss_report_location(Relation rel, BlockNumber location , BlockNumber *prefetchHWMp)
 {
 #ifdef TRACE_SYNCSCAN
 	if (trace_syncscan)
@@ -306,7 +324,7 @@ ss_report_location(Relation rel, BlockNu
 	{
 		if (LWLockConditionalAcquire(SyncScanLock, LW_EXCLUSIVE))
 		{
-			(void) ss_search(rel->rd_node, location, true);
+			(void) ss_search(rel->rd_node, location, true , prefetchHWMp);
 			LWLockRelease(SyncScanLock);
 		}
 #ifdef TRACE_SYNCSCAN
--- src/backend/access/index/indexam.c.orig	2014-08-18 14:10:36.845016657 -0400
+++ src/backend/access/index/indexam.c	2014-08-19 16:56:13.855197774 -0400
@@ -79,6 +79,55 @@
 #include "utils/tqual.h"
 
 
+#ifdef USE_PREFETCH
+bool BlocknotinBuffer(Buffer buffer, Relation relation, BlockNumber blockNum);
+BlockNumber BlocknumOfBuffer(Buffer buffer);
+void index_mark_or_evict_block(IndexScanDesc scan , BlockNumber blocknumber , int markit);
+
+extern unsigned int prefetch_index_scans; /*  boolean whether to prefetch bitmap heap scans         */
+
+/*  if specified block number is present in the prefetch array,
+**  then either mark it as not to be discarded or evict it according to input param
+*/
+void index_mark_or_evict_block(IndexScanDesc scan , BlockNumber blocknumber , int markit)
+{
+        unsigned short int pfchx , pfchy , pfchz; /*  indexes in BlockIdData array   */
+
+        if (    scan->do_prefetch
+             && ((struct pfch_block_item*)0 != scan->pfch_block_item_list)
+             /* no need to check for scan->pfch_next < prefetch_index_scans
+             ** since we will do nothing if scan->pfch_used == 0
+             */
+           ) {
+            /*  search the prefetch list to find if the block is a member */
+            for (pfchx = 0; pfchx < scan->pfch_used; pfchx++) {
+                if (BlockIdGetBlockNumber(&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)) == blocknumber) {
+                      if (markit) {
+						      /*  mark it as not to be discarded */
+						      ((scan->pfch_block_item_list)+pfchx)->pfch_discard &= ~PREFTCHRC_BUF_PIN_INCREASED;
+					  } else {
+							  /*  shuffle all following the evictee to the left
+							  **  and update next pointer if its element moves
+							  */
+							  pfchy = (scan->pfch_used - 1); /*  current rightmost */
+							  scan->pfch_used = pfchy;
+
+							  while (pfchy > pfchx) {
+								  pfchz = pfchx + 1;
+								  BlockIdCopy((&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)), (&(((scan->pfch_block_item_list)+pfchz)->pfch_blockid)));
+								  ((scan->pfch_block_item_list)+pfchx)->pfch_discard = ((scan->pfch_block_item_list)+pfchz)->pfch_discard;
+								  if (scan->pfch_next == pfchz) {
+									  scan->pfch_next = pfchx;
+								  }
+								  pfchx = pfchz; /* advance */
+							  }
+                      }
+                }
+            }
+        }
+}
+#endif /* USE_PREFETCH */
+
 /* ----------------------------------------------------------------
  *					macros used in index_ routines
  *
@@ -253,6 +302,11 @@ index_beginscan(Relation heapRelation,
 	 */
 	scan->heapRelation = heapRelation;
 	scan->xs_snapshot = snapshot;
+#ifdef USE_PREFETCH
+	scan->do_prefetch = 0;             /*  no prefetching by default  */
+        scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+	scan->pfch_block_item_list = (struct pfch_block_item*)0;
+#endif   /* USE_PREFETCH */
 
 	return scan;
 }
@@ -277,6 +331,11 @@ index_beginscan_bitmap(Relation indexRel
 	 * up by RelationGetIndexScan.
 	 */
 	scan->xs_snapshot = snapshot;
+#ifdef USE_PREFETCH
+	scan->do_prefetch = 0;             /*  no prefetching by default  */
+        scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+	scan->pfch_block_item_list = (struct pfch_block_item*)0;
+#endif   /* USE_PREFETCH */
 
 	return scan;
 }
@@ -311,6 +370,9 @@ index_beginscan_internal(Relation indexR
 									  Int32GetDatum(nkeys),
 									  Int32GetDatum(norderbys)));
 
+	scan->heap_tids_seen = 0;
+	scan->heap_tids_fetched = 0;
+	
 	return scan;
 }
 
@@ -342,6 +404,12 @@ index_rescan(IndexScanDesc scan,
 	/* Release any held pin on a heap page */
 	if (BufferIsValid(scan->xs_cbuf))
 	{
+#ifdef USE_PREFETCH
+                /*   if specified block number is present in the prefetch array,  then evict it */
+                if (scan->do_prefetch) {
+                      index_mark_or_evict_block(scan , BlocknumOfBuffer(scan->xs_cbuf) , 0);
+                }
+#endif   /* USE_PREFETCH */
 		ReleaseBuffer(scan->xs_cbuf);
 		scan->xs_cbuf = InvalidBuffer;
 	}
@@ -373,10 +441,30 @@ index_endscan(IndexScanDesc scan)
 	/* Release any held pin on a heap page */
 	if (BufferIsValid(scan->xs_cbuf))
 	{
+#ifdef USE_PREFETCH
+                /*   if specified block number is present in the prefetch array,  then evict it */
+                if (scan->do_prefetch) {
+                      index_mark_or_evict_block(scan , BlocknumOfBuffer(scan->xs_cbuf) , 0);
+                }
+#endif   /* USE_PREFETCH */
 		ReleaseBuffer(scan->xs_cbuf);
 		scan->xs_cbuf = InvalidBuffer;
 	}
 
+#ifdef USE_PREFETCH
+        /*   discard prefetched but unread buffers */
+        if (    scan->do_prefetch
+             && ((struct pfch_block_item*)0 != scan->pfch_block_item_list)
+           ) {
+            unsigned short int pfchx; /*  index in BlockIdData array   */
+                for (pfchx = 0; pfchx < scan->pfch_used; pfchx++) {
+                  if (((scan->pfch_block_item_list)+pfchx)->pfch_discard) {
+                      DiscardBuffer(scan->heapRelation, MAIN_FORKNUM, BlockIdGetBlockNumber(&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)));
+                  }
+                }
+        }
+#endif   /* USE_PREFETCH */
+
 	/* End the AM's scan */
 	FunctionCall1(procedure, PointerGetDatum(scan));
 
@@ -472,6 +560,12 @@ index_getnext_tid(IndexScanDesc scan, Sc
 		/* ... but first, release any held pin on a heap page */
 		if (BufferIsValid(scan->xs_cbuf))
 		{
+#ifdef USE_PREFETCH
+                    /*   if specified block number is present in the prefetch array,  then evict it */
+                    if (scan->do_prefetch) {
+                          index_mark_or_evict_block(scan , BlocknumOfBuffer(scan->xs_cbuf) , 0);
+                    }
+#endif   /* USE_PREFETCH */
 			ReleaseBuffer(scan->xs_cbuf);
 			scan->xs_cbuf = InvalidBuffer;
 		}
@@ -479,6 +573,11 @@ index_getnext_tid(IndexScanDesc scan, Sc
 	}
 
 	pgstat_count_index_tuples(scan->indexRelation, 1);
+	if (scan->heap_tids_seen++ >= (~0)) {
+		/* Avoid integer overflow */
+		scan->heap_tids_seen = 1;
+		scan->heap_tids_fetched = 0;
+	}
 
 	/* Return the TID of the tuple we found. */
 	return &scan->xs_ctup.t_self;
@@ -502,6 +601,10 @@ index_getnext_tid(IndexScanDesc scan, Sc
  * enough information to do it efficiently in the general case.
  * ----------------
  */
+#if defined(USE_PREFETCH) && defined(AVOID_CATALOG_MIGRATION_FOR_ASYNCIO)
+extern Datum btpeeknexttuple(IndexScanDesc scan);
+#endif /* USE_PREFETCH */
+
 HeapTuple
 index_fetch_heap(IndexScanDesc scan)
 {
@@ -509,16 +612,111 @@ index_fetch_heap(IndexScanDesc scan)
 	bool		all_dead = false;
 	bool		got_heap_tuple;
 
+
+
 	/* We can skip the buffer-switching logic if we're in mid-HOT chain. */
 	if (!scan->xs_continue_hot)
 	{
 		/* Switch to correct buffer if we don't have it already */
 		Buffer		prev_buf = scan->xs_cbuf;
 
+#ifdef USE_PREFETCH
+
+                /*   If the old block is different from new block, then evict old
+                **   block from prefetched array.   It is arguable we should leave it
+                **   in the array because it's likely to remain in the buffer pool
+                **   for a while,  but in that case , if we encounter the block
+                **   again,  prefetching it again does no harm.
+                **   (and note that,  if it's not pinned,  prefetching it will try to
+                **   pin it since prefetch tries to bank a pin for a buffer in the buffer pool).
+                **   therefore it should usually win.
+                */
+                if (    scan->do_prefetch
+                     && ( BufferIsValid(prev_buf) )
+                     && (BlocknotinBuffer(prev_buf,scan->heapRelation,ItemPointerGetBlockNumber(tid)))
+                     && (scan->pfch_next < prefetch_index_scans)  /* ensure there is an entry */
+                        ) {
+                          index_mark_or_evict_block(scan , BlocknumOfBuffer(prev_buf) , 0);
+                }
+
+#endif   /* USE_PREFETCH  */
 		scan->xs_cbuf = ReleaseAndReadBuffer(scan->xs_cbuf,
 											 scan->heapRelation,
 											 ItemPointerGetBlockNumber(tid));
 
+#ifdef USE_PREFETCH
+                /*   If the new block had been prefetched and pinned,
+                **   then mark that it no longer requires to be discarded.
+                **   Of course,  we don't evict the entry,
+                **   because we want to remember that it was recently prefetched.
+                */
+                index_mark_or_evict_block(scan , BlocknumOfBuffer(prev_buf) , 1);
+#endif   /* USE_PREFETCH  */
+
+				scan->heap_tids_fetched++;
+
+#ifdef USE_PREFETCH
+                /*  try prefetching next data block
+                **    (next meaning one containing TIDs from matching keys
+                **     in same index page and different from any block
+                **     we previously prefetched and listed in prefetched array)
+                */
+                {
+                    FmgrInfo   *procedure;
+                    bool	found;             /*  did we find the "next" heap tid in current index page */
+                    int         PrefetchBufferRc;  /*  indicates whether requested prefetch block already in a buffer and if pin count on buffer has been increased */
+
+                    if (scan->do_prefetch) {
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+                        procedure = &scan->indexRelation->rd_aminfo->ampeeknexttuple; /* is incorrect but avoids adding function to catalog */
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                        if (RegProcedureIsValid(scan->indexRelation->rd_am->ampeeknexttuple)) {
+                            GET_SCAN_PROCEDURE(ampeeknexttuple); /* is correct but requires adding function to catalog */
+                        } else {
+                            procedure = 0;
+                        }
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+
+                        if (    procedure          /* does the index access method support peektuple? */
+#ifndef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+                             && procedure->fn_addr /* procedure->fn_addr is non-null only if in catalog */
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                           ) {
+                            int iterations = 1;      /*  how many iterations of prefetching shall we try  -
+                                                     **  if used entries in prefetch list is < target_prefetch_pages
+                                                     **  then 2,  else 1
+                                                     **  this should result in gradually and smoothly increasing up to target_prefetch_pages
+                                                     */
+                            /*  note we trust InitIndexScan verified this scan is forwards only and so set that */
+                            if (scan->pfch_used < target_prefetch_pages) {
+                                iterations = 2;
+                            }
+                            do {
+                                found =  DatumGetBool(
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+                                                         btpeeknexttuple(scan)     /*  pass scan as direct parameter since cant use fmgr because not in catalog */
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                                                         FunctionCall1(procedure, PointerGetDatum(scan)) /* use fmgr to call it because in catalog  */
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+                                                     );
+                                if (found) {
+                                    /*    btpeeknexttuple set pfch_next to point to the item in block_item_list to be prefetched */
+                                    PrefetchBufferRc = PrefetchBuffer(scan->heapRelation, MAIN_FORKNUM, BlockIdGetBlockNumber((&((scan->pfch_block_item_list + scan->pfch_next))->pfch_blockid)) , 0);
+                                    /* elog(LOG,"index_fetch_heap prefetched rel %u blockNum %u"
+                                       ,scan->heapRelation->rd_node.relNode ,BlockIdGetBlockNumber(scan->pfch_block_item_list + scan->pfch_next));
+                                    */
+
+                                    /*  if pin acquired on buffer,  then remember in case of future Discard */
+                                    (scan->pfch_block_item_list + scan->pfch_next)->pfch_discard = (PrefetchBufferRc & PREFTCHRC_BUF_PIN_INCREASED);
+
+
+                                }
+                            } while (--iterations > 0);
+                        }
+                    }
+                }
+#endif   /* USE_PREFETCH */
+
 		/*
 		 * Prune page, but only if we weren't already on this page
 		 */
--- src/backend/access/index/genam.c.orig	2014-08-18 14:10:36.845016657 -0400
+++ src/backend/access/index/genam.c	2014-08-19 16:56:13.883197874 -0400
@@ -77,6 +77,12 @@ RelationGetIndexScan(Relation indexRelat
 
 	scan = (IndexScanDesc) palloc(sizeof(IndexScanDescData));
 
+#ifdef USE_PREFETCH
+        scan->do_prefetch = 0;             /*  no prefetching by default  */
+        scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+        scan->pfch_block_item_list = (struct pfch_block_item*)0;
+#endif   /* USE_PREFETCH */
+
 	scan->heapRelation = NULL;	/* may be set later */
 	scan->indexRelation = indexRelation;
 	scan->xs_snapshot = InvalidSnapshot;		/* caller must initialize this */
@@ -139,6 +145,19 @@ RelationGetIndexScan(Relation indexRelat
 void
 IndexScanEnd(IndexScanDesc scan)
 {
+#ifdef USE_PREFETCH
+	if (scan->do_prefetch) {
+		if ( (struct pfch_block_item*)0 != scan->pfch_block_item_list ) {
+			pfree(scan->pfch_block_item_list);
+			scan->pfch_block_item_list = (struct pfch_block_item*)0;
+		}
+		if ( (struct pfch_index_pagelist*)0 != scan->pfch_index_page_list ) {
+			pfree(scan->pfch_index_page_list);
+			scan->pfch_index_page_list = (struct pfch_index_pagelist*)0;
+		}
+	}
+#endif   /* USE_PREFETCH */
+
 	if (scan->keyData != NULL)
 		pfree(scan->keyData);
 	if (scan->orderByData != NULL)
--- src/backend/access/nbtree/nbtsearch.c.orig	2014-08-18 14:10:36.845016657 -0400
+++ src/backend/access/nbtree/nbtsearch.c	2014-08-19 16:56:13.911197974 -0400
@@ -23,13 +23,18 @@
 #include "utils/lsyscache.h"
 #include "utils/rel.h"
 
+#ifdef USE_PREFETCH
+extern unsigned int prefetch_btree_heaps;  /*  boolean whether to prefetch heap pages in _bt_next for non-bitmap index scans   */
+extern unsigned int prefetch_sequential_index_scans;  /*  boolean whether to prefetch sequential-access non-bitmap index scans  */
+#endif   /* USE_PREFETCH */
 
 static bool _bt_readpage(IndexScanDesc scan, ScanDirection dir,
 			 OffsetNumber offnum);
 static void _bt_saveitem(BTScanOpaque so, int itemIndex,
 			 OffsetNumber offnum, IndexTuple itup);
-static bool _bt_steppage(IndexScanDesc scan, ScanDirection dir);
-static Buffer _bt_walk_left(Relation rel, Buffer buf);
+static bool _bt_steppage(IndexScanDesc scan, ScanDirection dir, 
+			 bool prefetch);
+static Buffer _bt_walk_left(IndexScanDesc scan, Relation rel, Buffer buf);
 static bool _bt_endpoint(IndexScanDesc scan, ScanDirection dir);
 
 
@@ -226,7 +231,11 @@ _bt_moveright(Relation rel,
 				_bt_relbuf(rel, buf);
 
 			/* re-acquire the lock in the right mode, and re-check */
-			buf = _bt_getbuf(rel, blkno, access);
+			buf = _bt_getbuf(rel, blkno, access
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                                        );
 			continue;
 		}
 
@@ -1005,7 +1014,7 @@ _bt_first(IndexScanDesc scan, ScanDirect
 		 * There's no actually-matching data on this page.  Try to advance to
 		 * the next page.  Return false if there's no matching data at all.
 		 */
-		if (!_bt_steppage(scan, dir))
+		if (!_bt_steppage(scan, dir, false))
 			return false;
 	}
 
@@ -1040,6 +1049,8 @@ _bt_next(IndexScanDesc scan, ScanDirecti
 {
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
 	BTScanPosItem *currItem;
+	BlockNumber prevblkno = ItemPointerGetBlockNumber(
+		&scan->xs_ctup.t_self);
 
 	/*
 	 * Advance to next tuple on current page; or if there's no more, try to
@@ -1052,11 +1063,56 @@ _bt_next(IndexScanDesc scan, ScanDirecti
 			/* We must acquire lock before applying _bt_steppage */
 			Assert(BufferIsValid(so->currPos.buf));
 			LockBuffer(so->currPos.buf, BT_READ);
-			if (!_bt_steppage(scan, dir))
+			if (!_bt_steppage(scan, dir, target_prefetch_pages > 0))
 				return false;
 			/* Drop the lock, but not pin, on the new page */
 			LockBuffer(so->currPos.buf, BUFFER_LOCK_UNLOCK);
 		}
+		
+#ifdef USE_PREFETCH
+                /*    consider prefetching */
+		if (prefetch_btree_heaps && (target_prefetch_pages > 0)) {
+			BlockNumber currblkno = ItemPointerGetBlockNumber(
+				&so->currPos.items[so->currPos.itemIndex].heapTid);
+		
+			if (currblkno != prevblkno) {
+				if (so->prefetchBlockCount > 0)
+					so->prefetchBlockCount--;
+			
+				/* If we have heap fetch frequency stats, and it's above ~94%,
+				 * initiate heap prefetches */
+				if (so->currPos.moreRight
+					&& scan->heap_tids_seen > 256 
+					&& ( (scan->heap_tids_seen - scan->heap_tids_seen/16)
+						 <= scan->heap_tids_fetched ) )
+				{
+					bool nonsequential = false;
+				
+					if (so->prefetchItemIndex <= so->currPos.itemIndex)
+						so->prefetchItemIndex = so->currPos.itemIndex + 1;
+					while (    (so->prefetchItemIndex <= so->currPos.lastItem)
+							&& (so->prefetchBlockCount < target_prefetch_pages) )
+					{
+						ItemPointer tid = &so->currPos.items[so->prefetchItemIndex++].heapTid;
+						BlockNumber blkno = ItemPointerGetBlockNumber(tid);
+						if (blkno != so->lastHeapPrefetchBlkno) {  /*  if not a repetition of previous block */
+							/*   start prefetch on next page, providing :
+                            **   EITHER  .  we're reading non-sequentially previously or for this block
+                            **   OR      .  user explicitly specified to prefetch for sequential pattern
+							**   as it may be counterproductive otherwise
+                            */
+							nonsequential = (nonsequential || blkno != (so->lastHeapPrefetchBlkno+1));
+							if (prefetch_sequential_index_scans || nonsequential) {
+								_bt_prefetchbuf(scan->heapRelation, blkno , &scan->pfch_index_page_list );
+							}
+							so->lastHeapPrefetchBlkno = blkno;
+							so->prefetchBlockCount++;
+						}
+					}
+				}
+			}
+		}
+#endif   /* USE_PREFETCH */
 	}
 	else
 	{
@@ -1065,11 +1121,56 @@ _bt_next(IndexScanDesc scan, ScanDirecti
 			/* We must acquire lock before applying _bt_steppage */
 			Assert(BufferIsValid(so->currPos.buf));
 			LockBuffer(so->currPos.buf, BT_READ);
-			if (!_bt_steppage(scan, dir))
+			if (!_bt_steppage(scan, dir, target_prefetch_pages > 0))
 				return false;
 			/* Drop the lock, but not pin, on the new page */
 			LockBuffer(so->currPos.buf, BUFFER_LOCK_UNLOCK);
 		}
+		
+#ifdef USE_PREFETCH
+                /*    consider prefetching */
+		if (prefetch_btree_heaps && (target_prefetch_pages > 0)) {
+			BlockNumber currblkno = ItemPointerGetBlockNumber(
+				&so->currPos.items[so->currPos.itemIndex].heapTid);
+		
+			if (currblkno != prevblkno) {
+				if (so->prefetchBlockCount > 0)
+					so->prefetchBlockCount--;
+			
+				/* If we have heap fetch frequency stats, and it's above ~94%,
+				 * initiate heap prefetches */
+				if (so->currPos.moreLeft
+					&& scan->heap_tids_seen > 256 
+					&& ( (scan->heap_tids_seen - scan->heap_tids_seen/16)
+						 <= scan->heap_tids_fetched ) )
+				{
+					bool nonsequential = false;
+			
+					if (so->prefetchItemIndex >= so->currPos.itemIndex)
+						so->prefetchItemIndex = so->currPos.itemIndex - 1;
+					while (    (so->prefetchItemIndex >= so->currPos.firstItem)
+							&& (so->prefetchBlockCount < target_prefetch_pages) )
+					{
+						ItemPointer tid = &so->currPos.items[so->prefetchItemIndex--].heapTid;
+						BlockNumber blkno = ItemPointerGetBlockNumber(tid);
+						if (blkno != so->lastHeapPrefetchBlkno) {  /*  if not a repetition of previous block */
+							/*   start prefetch on next page, providing :
+                            **   EITHER  .  we're reading non-sequentially previously or for this block
+                            **   OR      .  user explicitly specified to prefetch for sequential pattern
+							**   as it may be counterproductive otherwise
+                            */
+							nonsequential = (nonsequential || blkno != (so->lastHeapPrefetchBlkno+1));
+							if (prefetch_sequential_index_scans || nonsequential) {
+								_bt_prefetchbuf(scan->heapRelation, blkno , &scan->pfch_index_page_list );
+							}
+							so->lastHeapPrefetchBlkno = blkno;
+							so->prefetchBlockCount++;
+						}
+					}
+				}
+			}
+		}
+#endif   /* USE_PREFETCH */
 	}
 
 	/* OK, itemIndex says what to return */
@@ -1119,9 +1220,11 @@ _bt_readpage(IndexScanDesc scan, ScanDir
 	/*
 	 * we must save the page's right-link while scanning it; this tells us
 	 * where to step right to after we're done with these items.  There is no
-	 * corresponding need for the left-link, since splits always go right.
+	 * corresponding need for the left-link, since splits always go right,
+	 * but we need it for back-sequential scan detection.
 	 */
 	so->currPos.nextPage = opaque->btpo_next;
+	so->currPos.prevPage = opaque->btpo_prev;
 
 	/* initialize tuple workspace to empty */
 	so->currPos.nextTupleOffset = 0;
@@ -1156,6 +1259,7 @@ _bt_readpage(IndexScanDesc scan, ScanDir
 		so->currPos.firstItem = 0;
 		so->currPos.lastItem = itemIndex - 1;
 		so->currPos.itemIndex = 0;
+		so->prefetchItemIndex = 0;
 	}
 	else
 	{
@@ -1187,6 +1291,7 @@ _bt_readpage(IndexScanDesc scan, ScanDir
 		so->currPos.firstItem = itemIndex;
 		so->currPos.lastItem = MaxIndexTuplesPerPage - 1;
 		so->currPos.itemIndex = MaxIndexTuplesPerPage - 1;
+		so->prefetchItemIndex = MaxIndexTuplesPerPage - 1;
 	}
 
 	return (so->currPos.firstItem <= so->currPos.lastItem);
@@ -1224,7 +1329,7 @@ _bt_saveitem(BTScanOpaque so, int itemIn
  * locks and pins, set so->currPos.buf to InvalidBuffer, and return FALSE.
  */
 static bool
-_bt_steppage(IndexScanDesc scan, ScanDirection dir)
+_bt_steppage(IndexScanDesc scan, ScanDirection dir, bool prefetch)
 {
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
 	Relation	rel;
@@ -1278,7 +1383,11 @@ _bt_steppage(IndexScanDesc scan, ScanDir
 			/* check for interrupts while we're not holding any buffer lock */
 			CHECK_FOR_INTERRUPTS();
 			/* step right one page */
-			so->currPos.buf = _bt_getbuf(rel, blkno, BT_READ);
+			so->currPos.buf = _bt_getbuf(rel, blkno, BT_READ
+#ifdef USE_PREFETCH
+                                     ,scan->pfch_index_page_list
+#endif /* USE_PREFETCH */
+                                                    );
 			/* check for deleted page */
 			page = BufferGetPage(so->currPos.buf);
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
@@ -1287,9 +1396,22 @@ _bt_steppage(IndexScanDesc scan, ScanDir
 				PredicateLockPage(rel, blkno, scan->xs_snapshot);
 				/* see if there are any matches on this page */
 				/* note that this will clear moreRight if we can stop */
-				if (_bt_readpage(scan, dir, P_FIRSTDATAKEY(opaque)))
+				if (_bt_readpage(scan, dir, P_FIRSTDATAKEY(opaque))) {
+#ifdef USE_PREFETCH
+					if (    prefetch && so->currPos.moreRight
+						/*   start prefetch on next page, providing :
+						**   EITHER  .  we're reading non-sequentially for this block
+						**   OR      .  user explicitly specified to prefetch for sequential pattern
+						**   as it may be counterproductive otherwise
+						*/
+						&& (prefetch_sequential_index_scans || opaque->btpo_next != (blkno+1))
+                                            ) {
+ 						  _bt_prefetchbuf(rel, opaque->btpo_next , &scan->pfch_index_page_list);
+					}
+#endif /* USE_PREFETCH */
 					break;
 			}
+			}
 			/* nope, keep going */
 			blkno = opaque->btpo_next;
 		}
@@ -1317,7 +1439,7 @@ _bt_steppage(IndexScanDesc scan, ScanDir
 			}
 
 			/* Step to next physical page */
-			so->currPos.buf = _bt_walk_left(rel, so->currPos.buf);
+			so->currPos.buf = _bt_walk_left(scan , rel, so->currPos.buf);
 
 			/* if we're physically at end of index, return failure */
 			if (so->currPos.buf == InvalidBuffer)
@@ -1332,14 +1454,60 @@ _bt_steppage(IndexScanDesc scan, ScanDir
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 			if (!P_IGNORE(opaque))
 			{
+				/* We must rely on the previously saved prevPage link! */
+				BlockNumber blkno = so->currPos.prevPage;
+				
 				PredicateLockPage(rel, BufferGetBlockNumber(so->currPos.buf), scan->xs_snapshot);
 				/* see if there are any matches on this page */
 				/* note that this will clear moreLeft if we can stop */
-				if (_bt_readpage(scan, dir, PageGetMaxOffsetNumber(page)))
+				if (_bt_readpage(scan, dir, PageGetMaxOffsetNumber(page))) {
+#ifdef USE_PREFETCH
+					if (prefetch && so->currPos.moreLeft) {
+						/* detect back-sequential runs and increase prefetch window blindly 
+						 * downwards 2 blocks at a time. This only works in our favor
+						 * for index-only scans, by merging read requests at the kernel,
+						 * so we want to inflate target_prefetch_pages since merged 
+						 * back-sequential requests are about as expensive as a single one 
+						 */
+						if (scan->xs_want_itup && blkno > 0 && opaque->btpo_prev == (blkno-1)) {
+							BlockNumber backPos;
+							unsigned int back_prefetch_pages = target_prefetch_pages * 16;
+							if (back_prefetch_pages > 64)
+								back_prefetch_pages = 64;
+							
+							if (so->backSeqRun == 0)
+								backPos = (blkno-1);
+							else
+								backPos = so->backSeqPos;
+							so->backSeqRun++;
+							
+							if (backPos > 0 && (blkno - backPos) <= back_prefetch_pages) {
+								_bt_prefetchbuf(rel, backPos-- , &scan->pfch_index_page_list);
+								/* don't start back-seq prefetch too early */
+								if (so->backSeqRun >= back_prefetch_pages
+										&& backPos > 0 
+										&& (blkno - backPos) <= back_prefetch_pages)
+								{
+									_bt_prefetchbuf(rel, backPos-- , &scan->pfch_index_page_list);
+								}
+							}
+							
+							so->backSeqPos = backPos;
+						} else {
+							/* start prefetch on next page */
+							if (so->backSeqRun != 0) {
+								if (opaque->btpo_prev > blkno || opaque->btpo_prev < so->backSeqPos)
+									so->backSeqRun = 0;
+							}
+							_bt_prefetchbuf(rel, opaque->btpo_prev , &scan->pfch_index_page_list);
+						}
+					}
+#endif /* USE_PREFETCH */
 					break;
 			}
 		}
 	}
+	}
 
 	return true;
 }
@@ -1359,7 +1527,7 @@ _bt_steppage(IndexScanDesc scan, ScanDir
  * again if it's important.
  */
 static Buffer
-_bt_walk_left(Relation rel, Buffer buf)
+_bt_walk_left(IndexScanDesc scan, Relation rel, Buffer buf)
 {
 	Page		page;
 	BTPageOpaque opaque;
@@ -1387,7 +1555,11 @@ _bt_walk_left(Relation rel, Buffer buf)
 		_bt_relbuf(rel, buf);
 		/* check for interrupts while we're not holding any buffer lock */
 		CHECK_FOR_INTERRUPTS();
-		buf = _bt_getbuf(rel, blkno, BT_READ);
+		buf = _bt_getbuf(rel, blkno, BT_READ
+#ifdef USE_PREFETCH
+                                     , scan->pfch_index_page_list
+#endif /* USE_PREFETCH */
+                                );
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
@@ -1631,7 +1803,7 @@ _bt_endpoint(IndexScanDesc scan, ScanDir
 		 * There's no actually-matching data on this page.  Try to advance to
 		 * the next page.  Return false if there's no matching data at all.
 		 */
-		if (!_bt_steppage(scan, dir))
+		if (!_bt_steppage(scan, dir, false))
 			return false;
 	}
 
--- src/backend/access/nbtree/nbtinsert.c.orig	2014-08-18 14:10:36.845016657 -0400
+++ src/backend/access/nbtree/nbtinsert.c	2014-08-19 16:56:13.947198104 -0400
@@ -793,7 +793,11 @@ _bt_insertonpg(Relation rel,
 		{
 			Assert(!P_ISLEAF(lpageop));
 
-			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                                );
 			metapg = BufferGetPage(metabuf);
 			metad = BTPageGetMeta(metapg);
 
@@ -972,7 +976,11 @@ _bt_split(Relation rel, Buffer buf, Buff
 	bool		isleaf;
 
 	/* Acquire a new page to split into */
-	rbuf = _bt_getbuf(rel, P_NEW, BT_WRITE);
+	rbuf = _bt_getbuf(rel, P_NEW, BT_WRITE
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                                );
 
 	/*
 	 * origpage is the original page to be split.  leftpage is a temporary
@@ -1175,7 +1183,11 @@ _bt_split(Relation rel, Buffer buf, Buff
 
 	if (!P_RIGHTMOST(oopaque))
 	{
-		sbuf = _bt_getbuf(rel, oopaque->btpo_next, BT_WRITE);
+		sbuf = _bt_getbuf(rel, oopaque->btpo_next, BT_WRITE
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                                );
 		spage = BufferGetPage(sbuf);
 		sopaque = (BTPageOpaque) PageGetSpecialPointer(spage);
 		if (sopaque->btpo_prev != origpagenumber)
@@ -1817,7 +1829,11 @@ _bt_finish_split(Relation rel, Buffer lb
 	Assert(P_INCOMPLETE_SPLIT(lpageop));
 
 	/* Lock right sibling, the one missing the downlink */
-	rbuf = _bt_getbuf(rel, lpageop->btpo_next, BT_WRITE);
+	rbuf = _bt_getbuf(rel, lpageop->btpo_next, BT_WRITE
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                     );
 	rpage = BufferGetPage(rbuf);
 	rpageop = (BTPageOpaque) PageGetSpecialPointer(rpage);
 
@@ -1829,7 +1845,11 @@ _bt_finish_split(Relation rel, Buffer lb
 		BTMetaPageData *metad;
 
 		/* acquire lock on the metapage */
-		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                            );
 		metapg = BufferGetPage(metabuf);
 		metad = BTPageGetMeta(metapg);
 
@@ -1877,7 +1897,11 @@ _bt_getstackbuf(Relation rel, BTStack st
 		Page		page;
 		BTPageOpaque opaque;
 
-		buf = _bt_getbuf(rel, blkno, access);
+		buf = _bt_getbuf(rel, blkno, access
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                        );
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
@@ -2008,12 +2032,20 @@ _bt_newroot(Relation rel, Buffer lbuf, B
 	lopaque = (BTPageOpaque) PageGetSpecialPointer(lpage);
 
 	/* get a new root page */
-	rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE);
+	rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                        );
 	rootpage = BufferGetPage(rootbuf);
 	rootblknum = BufferGetBlockNumber(rootbuf);
 
 	/* acquire lock on the metapage */
-	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                        );
 	metapg = BufferGetPage(metabuf);
 	metad = BTPageGetMeta(metapg);
 
--- src/backend/access/nbtree/nbtpage.c.orig	2014-08-18 14:10:36.845016657 -0400
+++ src/backend/access/nbtree/nbtpage.c	2014-08-19 16:56:13.967198176 -0400
@@ -127,7 +127,11 @@ _bt_getroot(Relation rel, int access)
 		Assert(rootblkno != P_NONE);
 		rootlevel = metad->btm_fastlevel;
 
-		rootbuf = _bt_getbuf(rel, rootblkno, BT_READ);
+		rootbuf = _bt_getbuf(rel, rootblkno, BT_READ
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                            );
 		rootpage = BufferGetPage(rootbuf);
 		rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);
 
@@ -153,7 +157,11 @@ _bt_getroot(Relation rel, int access)
 		rel->rd_amcache = NULL;
 	}
 
-	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
+	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                        );
 	metapg = BufferGetPage(metabuf);
 	metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
 	metad = BTPageGetMeta(metapg);
@@ -209,7 +217,11 @@ _bt_getroot(Relation rel, int access)
 		 * the new root page.  Since this is the first page in the tree, it's
 		 * a leaf as well as the root.
 		 */
-		rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE);
+		rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                            );
 		rootblkno = BufferGetBlockNumber(rootbuf);
 		rootpage = BufferGetPage(rootbuf);
 		rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);
@@ -350,7 +362,11 @@ _bt_gettrueroot(Relation rel)
 		pfree(rel->rd_amcache);
 	rel->rd_amcache = NULL;
 
-	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
+	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                        );
 	metapg = BufferGetPage(metabuf);
 	metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
 	metad = BTPageGetMeta(metapg);
@@ -436,7 +452,11 @@ _bt_getrootheight(Relation rel)
 		Page		metapg;
 		BTPageOpaque metaopaque;
 
-		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
+		metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                             );
 		metapg = BufferGetPage(metabuf);
 		metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
 		metad = BTPageGetMeta(metapg);
@@ -561,6 +581,172 @@ _bt_log_reuse_page(Relation rel, BlockNu
 	END_CRIT_SECTION();
 }
 
+#ifdef USE_PREFETCH
+/*
+ *	_bt_prefetchbuf() -- Prefetch a buffer by block number
+ *                           and keep track of prefetched and unread blocknums in pagelist.
+ *   input parms  :
+ *       rel and blockno identify block to be prefetched as usual
+ *       pfch_index_page_list_P points to the pointer anchoring the head of the index page list
+ *             Since the pagelist is a kind of optimization,
+ *             handle palloc failure by quietly omitting the keeping track.
+ */
+void
+_bt_prefetchbuf(Relation rel, BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P)
+{
+
+    int rc = 0;
+    struct pfch_index_pagelist* pfch_index_plp;      /*  pointer to current chunk */
+    struct pfch_index_item* found_item = 0;
+
+		if ((struct pfch_index_pagelist**)0 == pfch_index_page_list_P) {
+			pfch_index_plp = (struct pfch_index_pagelist*)0;
+		} else {
+			pfch_index_plp = *pfch_index_page_list_P;
+		}
+
+    	if (blkno != P_NEW && blkno != P_NONE)
+    	{
+            /* prefetch an existing block of the relation
+            ** but first,  check it has not recently already been prefetched and not yet read
+            */
+            found_item = _bt_find_block(blkno , pfch_index_plp);
+			if ((struct pfch_index_item*)0 == found_item) {  /*  not found */
+
+		        rc = PrefetchBuffer(rel, MAIN_FORKNUM, blkno , 0);
+
+                /*  add the pagenum to the list ,  indicating its discard status
+                **  since it's only an optimization,  ignore failure such as exceeded allowed space
+				*/
+                _bt_add_block( blkno , pfch_index_page_list_P , (uint32)(rc & PREFTCHRC_BUF_PIN_INCREASED));
+
+            }
+	    }
+        return;
+}
+
+/*   _bt_find_block finds the item referencing specified Block in index page list if present
+**   and returns the pointer to the pfch_index_item if found,  or null if not
+*/
+struct pfch_index_item*
+_bt_find_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list)
+{
+
+    struct pfch_index_item* found_item = 0;
+    struct pfch_index_pagelist* pfch_index_plp;      /*  pointer to current chunk */
+    int ix, tx;
+
+		pfch_index_plp = pfch_index_page_list;
+
+		while (     (struct pfch_index_pagelist*)0 != pfch_index_plp
+                &&  ( (struct pfch_index_item*)0 == found_item)
+              ) {
+			ix = 0;
+			tx = pfch_index_plp->pfch_index_item_count;
+			while (     (ix < tx)
+                    &&  ( (struct pfch_index_item*)0 == found_item)
+                  ) {
+				if (pfch_index_plp->pfch_indexid[ix].pfch_blocknum == blkno) {
+					found_item = &pfch_index_plp->pfch_indexid[ix];
+				}
+                ix++;
+			}
+			pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+		}
+
+     return found_item;
+}
+
+/*   _bt_add_block adds the specified Block to the index page list
+**   and returns 0 if successful,  non-zero if not
+*/
+int
+_bt_add_block(BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P , uint32 discard_status)
+{
+    int rc = 1;
+    int ix;
+    struct pfch_index_pagelist* pfch_index_plp;      /*  pointer to current chunk */
+    struct pfch_index_pagelist* pfch_index_page_list_anchor; /*  pointer to first chunk if any  */
+	/*  allow expansion of pagelist to 16 chunks
+	**  which accommodates backwards-sequential index scans
+	**  where the scanner increases target_prefetch_pages by a factor of up to 16
+	**   see code in _bt_steppage
+	**  note - this creates an undesirable weak dependency on this number in _bt_steppage,
+	**         but :
+	**           there is no disaster if the numbers disagree  -  just sub-optimal use of the list
+	**           to implement a proper interface would require that chunks have a variable size
+	**           which would require an extra size variable in each chunk
+	*/
+	int num_chunks = 16;
+
+		if ((struct pfch_index_pagelist**)0 == pfch_index_page_list_P) {
+			pfch_index_page_list_anchor = (struct pfch_index_pagelist*)0;
+		} else {
+			pfch_index_page_list_anchor = *pfch_index_page_list_P;
+		}
+		pfch_index_plp = pfch_index_page_list_anchor;       /* pointer to current chunk */
+
+		while ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+			ix = pfch_index_plp->pfch_index_item_count;
+			if (ix < target_prefetch_pages) {
+				pfch_index_plp->pfch_indexid[ix].pfch_blocknum = blkno;
+				pfch_index_plp->pfch_indexid[ix].pfch_discard = discard_status;
+				pfch_index_plp->pfch_index_item_count = (ix+1);
+                rc = 0;
+				goto stored_pagenum;
+			}
+			pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+			num_chunks--;              /*  keep track of number of chunks */
+		}
+
+		/*   we did not find any free space in existing chunks -
+		**   create new chunk if within our limit and we have a pfch_index_page_list
+		*/
+		if ( (num_chunks > 0) && ((struct pfch_index_pagelist*)0 != pfch_index_page_list_anchor) ) {
+			pfch_index_plp = (struct pfch_index_pagelist*)palloc( sizeof(struct pfch_index_pagelist) + ( (target_prefetch_pages-1) * sizeof(struct pfch_index_item) ) );
+			if ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+				pfch_index_plp->pfch_index_pagelist_next = pfch_index_page_list_anchor;  /* old head of list is next after this */
+				pfch_index_plp->pfch_indexid[0].pfch_blocknum = blkno;
+				pfch_index_plp->pfch_indexid[0].pfch_discard = discard_status;
+				pfch_index_plp->pfch_index_item_count = 1;
+				pfch_index_page_list_P = &pfch_index_plp;   /*  new head of list is new chunk */
+                rc = 0;
+			}
+		}
+
+    stored_pagenum:;
+     return rc;
+}
+
+/*  _bt_subtract_block removes a block from the prefetched-but-unread pagelist if present */
+void
+_bt_subtract_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list)
+{
+    struct pfch_index_pagelist* pfch_index_plp = pfch_index_page_list;
+	if ( (blkno != P_NEW) && (blkno != P_NONE) ) {
+            int ix , jx;
+                while ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+                    ix = pfch_index_plp->pfch_index_item_count;
+                    while (ix-- > 0) {
+                        if (pfch_index_plp->pfch_indexid[ix].pfch_blocknum == blkno) {
+                            /*   move the last item to the curent (now deleted) position and decrement count */
+                            jx = (pfch_index_plp->pfch_index_item_count-1); /*  index of last item ... */
+                            if (jx > ix) {                                  /*  ... is not the current one so move is required */
+                                pfch_index_plp->pfch_indexid[ix].pfch_blocknum = pfch_index_plp->pfch_indexid[jx].pfch_blocknum;
+                                pfch_index_plp->pfch_indexid[ix].pfch_discard = pfch_index_plp->pfch_indexid[jx].pfch_discard;
+                                ix = jx;
+                            }
+                            pfch_index_plp->pfch_index_item_count = ix;
+                            goto done_subtract;
+                        }
+                    }
+                    pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+                }
+        }
+    done_subtract:  return;
+}
+#endif /* USE_PREFETCH */
+
 /*
  *	_bt_getbuf() -- Get a buffer by block number for read or write.
  *
@@ -573,7 +759,11 @@ _bt_log_reuse_page(Relation rel, BlockNu
  *		_bt_checkpage to sanity-check the page (except in P_NEW case).
  */
 Buffer
-_bt_getbuf(Relation rel, BlockNumber blkno, int access)
+_bt_getbuf(Relation rel, BlockNumber blkno, int access
+#ifdef USE_PREFETCH
+                              ,struct pfch_index_pagelist* pfch_index_page_list
+#endif /* USE_PREFETCH */
+          )
 {
 	Buffer		buf;
 
@@ -581,6 +771,12 @@ _bt_getbuf(Relation rel, BlockNumber blk
 	{
 		/* Read an existing block of the relation */
 		buf = ReadBuffer(rel, blkno);
+
+#ifdef USE_PREFETCH
+        /*  if the block is in the prefetched-but-unread pagelist,  remove it */
+        _bt_subtract_block( blkno , pfch_index_page_list);
+#endif /* USE_PREFETCH */
+
 		LockBuffer(buf, access);
 		_bt_checkpage(rel, buf);
 	}
@@ -702,6 +898,10 @@ _bt_getbuf(Relation rel, BlockNumber blk
  * bufmgr when one would do.  However, now it's mainly just a notational
  * convenience.  The only case where it saves work over _bt_relbuf/_bt_getbuf
  * is when the target page is the same one already in the buffer.
+ *
+ * if prefetching of index pages is changed to use this function,
+ * then it should be extended to take the index_page_list as parameter
+ * and call_bt_subtract_block in the same way that _bt_getbuf does.
  */
 Buffer
 _bt_relandgetbuf(Relation rel, Buffer obuf, BlockNumber blkno, int access)
@@ -712,6 +912,7 @@ _bt_relandgetbuf(Relation rel, Buffer ob
 	if (BufferIsValid(obuf))
 		LockBuffer(obuf, BUFFER_LOCK_UNLOCK);
 	buf = ReleaseAndReadBuffer(obuf, rel, blkno);
+
 	LockBuffer(buf, access);
 	_bt_checkpage(rel, buf);
 	return buf;
@@ -965,7 +1166,11 @@ _bt_is_page_halfdead(Relation rel, Block
 	BTPageOpaque opaque;
 	bool		result;
 
-	buf = _bt_getbuf(rel, blk, BT_READ);
+	buf = _bt_getbuf(rel, blk, BT_READ
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                     );
 	page = BufferGetPage(buf);
 	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
@@ -1069,7 +1274,11 @@ _bt_lock_branch_parent(Relation rel, Blo
 				Page		lpage;
 				BTPageOpaque lopaque;
 
-				lbuf = _bt_getbuf(rel, leftsib, BT_READ);
+				lbuf = _bt_getbuf(rel, leftsib, BT_READ
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                                 );
 				lpage = BufferGetPage(lbuf);
 				lopaque = (BTPageOpaque) PageGetSpecialPointer(lpage);
 
@@ -1265,7 +1474,11 @@ _bt_pagedel(Relation rel, Buffer buf)
 					BTPageOpaque lopaque;
 					Page		lpage;
 
-					lbuf = _bt_getbuf(rel, leftsib, BT_READ);
+					lbuf = _bt_getbuf(rel, leftsib, BT_READ
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                                     );
 					lpage = BufferGetPage(lbuf);
 					lopaque = (BTPageOpaque) PageGetSpecialPointer(lpage);
 					/*
@@ -1340,7 +1553,11 @@ _bt_pagedel(Relation rel, Buffer buf)
 		if (!rightsib_empty)
 			break;
 
-		buf = _bt_getbuf(rel, rightsib, BT_WRITE);
+		buf = _bt_getbuf(rel, rightsib, BT_WRITE
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                        );
 	}
 
 	return ndeleted;
@@ -1593,7 +1810,11 @@ _bt_unlink_halfdead_page(Relation rel, B
 		target = topblkno;
 
 		/* fetch the block number of the topmost parent's left sibling */
-		buf = _bt_getbuf(rel, topblkno, BT_READ);
+		buf = _bt_getbuf(rel, topblkno, BT_READ
+#ifdef USE_PREFETCH
+                                    , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                        );
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		leftsib = opaque->btpo_prev;
@@ -1632,7 +1853,11 @@ _bt_unlink_halfdead_page(Relation rel, B
 		LockBuffer(leafbuf, BT_WRITE);
 	if (leftsib != P_NONE)
 	{
-		lbuf = _bt_getbuf(rel, leftsib, BT_WRITE);
+		lbuf = _bt_getbuf(rel, leftsib, BT_WRITE
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                         );
 		page = BufferGetPage(lbuf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		while (P_ISDELETED(opaque) || opaque->btpo_next != target)
@@ -1646,7 +1871,11 @@ _bt_unlink_halfdead_page(Relation rel, B
 					 RelationGetRelationName(rel));
 				return false;
 			}
-			lbuf = _bt_getbuf(rel, leftsib, BT_WRITE);
+			lbuf = _bt_getbuf(rel, leftsib, BT_WRITE
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                              );
 			page = BufferGetPage(lbuf);
 			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		}
@@ -1701,7 +1930,11 @@ _bt_unlink_halfdead_page(Relation rel, B
 	 * And next write-lock the (current) right sibling.
 	 */
 	rightsib = opaque->btpo_next;
-	rbuf = _bt_getbuf(rel, rightsib, BT_WRITE);
+	rbuf = _bt_getbuf(rel, rightsib, BT_WRITE
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                                );
 	page = BufferGetPage(rbuf);
 	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 	if (opaque->btpo_prev != target)
@@ -1731,7 +1964,11 @@ _bt_unlink_halfdead_page(Relation rel, B
 		if (P_RIGHTMOST(opaque))
 		{
 			/* rightsib will be the only one left on the level */
-			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
+			metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE
+#ifdef USE_PREFETCH
+                                     , (struct pfch_index_pagelist*)0
+#endif /* USE_PREFETCH */
+                                );
 			metapg = BufferGetPage(metabuf);
 			metad = BTPageGetMeta(metapg);
 
--- src/backend/access/nbtree/nbtree.c.orig	2014-08-18 14:10:36.845016657 -0400
+++ src/backend/access/nbtree/nbtree.c	2014-08-19 16:56:13.979198218 -0400
@@ -29,6 +29,18 @@
 #include "tcop/tcopprot.h"
 #include "utils/memutils.h"
 
+#ifdef USE_PREFETCH
+extern unsigned int prefetch_index_scans; /* boolean whether to prefetch non-bitmap index scans also numeric size of pfch_list */
+#endif   /* USE_PREFETCH */
+
+Datum
+btpeeknexttuple(
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+        IndexScanDesc scan
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+        PG_FUNCTION_ARGS
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+);
 
 /* Working state for btbuild and its callback */
 typedef struct
@@ -324,6 +336,78 @@ btgettuple(PG_FUNCTION_ARGS)
 }
 
 /*
+ *	btpeeknexttuple() -- peek at the next tuple different from any blocknum in pfch_block_item_list
+ *                           without reading a new index page
+ *                       and without causing any side-effects such as altering values in control blocks
+ *               if found,     store blocknum in next element of pfch_block_item_list
+ *      This function has no usefulness unless postgresql is compiled with USE_PREFETCH
+ */
+Datum
+btpeeknexttuple(
+#ifdef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+        IndexScanDesc scan
+#else /*  not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+        PG_FUNCTION_ARGS
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+)
+{
+#ifndef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+    IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+	BTScanOpaque so = (BTScanOpaque) scan->opaque;
+	bool		res = false;
+	int		itemIndex;		/* current index in items[] */
+
+        
+#ifdef USE_PREFETCH
+        /*
+         * If we've already initialized this scan, we can just advance it in
+         * the appropriate direction.  If we haven't done so yet, bail out
+         */
+        if ( BTScanPosIsValid(so->currPos) ) {
+
+            itemIndex = so->currPos.itemIndex+1;    /*   next item */
+
+            /* This loop handles advancing till we find different data block or end of index page */
+            while (itemIndex <= so->currPos.lastItem) {
+                unsigned short int pfchx; /*  index in BlockIdData array   */
+                for (pfchx = 0; pfchx < scan->pfch_used; pfchx++) {
+                        if (BlockIdEquals((&(((scan->pfch_block_item_list)+pfchx)->pfch_blockid)) , &(so->currPos.items[itemIndex].heapTid.ip_blkid))) {
+                             goto block_match;
+                        }
+                }
+
+                /*  if we reach here,  no block in list matched this item  */
+                res = true;
+                /*   set item in prefetch list
+                **   prefer unused entry if there is one,  else overwrite
+                */
+                if (scan->pfch_used < prefetch_index_scans) {
+                    scan->pfch_next = scan->pfch_used;
+                } else {
+                    scan->pfch_next++;
+                    if (scan->pfch_next >= prefetch_index_scans) {
+                        scan->pfch_next = 0;
+                    }
+                }
+
+                BlockIdCopy((&((scan->pfch_block_item_list + scan->pfch_next)->pfch_blockid)) , &(so->currPos.items[itemIndex].heapTid.ip_blkid));
+                if (scan->pfch_used <= scan->pfch_next) {
+                     scan->pfch_used = (scan->pfch_next + 1);
+                }
+
+                goto peek_complete;
+
+      block_match:         itemIndex++;
+            }
+	}
+
+ peek_complete:
+#endif   /* USE_PREFETCH */
+	PG_RETURN_BOOL(res);
+}
+
+/*
  * btgetbitmap() -- gets all matching tuples, and adds them to a bitmap
  */
 Datum
@@ -417,6 +501,12 @@ btbeginscan(PG_FUNCTION_ARGS)
 	so->killedItems = NULL;		/* until needed */
 	so->numKilled = 0;
 
+	so->backSeqRun = 0;
+	so->backSeqPos = 0;
+	so->prefetchItemIndex = 0;
+	so->lastHeapPrefetchBlkno = P_NONE;
+	so->prefetchBlockCount = 0;
+	
 	/*
 	 * We don't know yet whether the scan will be index-only, so we do not
 	 * allocate the tuple workspace arrays until btrescan.  However, we set up
@@ -509,6 +599,23 @@ btendscan(PG_FUNCTION_ARGS)
 	IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
 
+#ifdef USE_PREFETCH
+        struct pfch_index_pagelist* pfch_index_plp;
+        int ix;
+
+	/* discard all prefetched but unread index pages listed in the pagelist */
+        pfch_index_plp = scan->pfch_index_page_list;
+        while ( (struct pfch_index_pagelist*)0 != pfch_index_plp ) {
+                    ix = pfch_index_plp->pfch_index_item_count;
+                    while (ix-- > 0) {
+                        if (pfch_index_plp->pfch_indexid[ix].pfch_discard) {
+                            DiscardBuffer( scan->indexRelation , MAIN_FORKNUM , pfch_index_plp->pfch_indexid[ix].pfch_blocknum);
+                        }
+                    }
+                    pfch_index_plp = pfch_index_plp->pfch_index_pagelist_next;
+        }
+#endif /* USE_PREFETCH */
+
 	/* we aren't holding any read locks, but gotta drop the pins */
 	if (BTScanPosIsValid(so->currPos))
 	{
--- src/backend/nodes/tidbitmap.c.orig	2014-08-18 14:10:36.877016807 -0400
+++ src/backend/nodes/tidbitmap.c	2014-08-19 16:56:14.063198519 -0400
@@ -44,6 +44,9 @@
 #include "nodes/bitmapset.h"
 #include "nodes/tidbitmap.h"
 #include "utils/hsearch.h"
+#ifdef USE_PREFETCH
+extern int	target_prefetch_pages;
+#endif   /* USE_PREFETCH */
 
 /*
  * The maximum number of tuples per page is not large (typically 256 with
@@ -572,7 +575,12 @@ tbm_begin_iterate(TIDBitmap *tbm)
 	 * needs of the TBMIterateResult sub-struct.
 	 */
 	iterator = (TBMIterator *) palloc(sizeof(TBMIterator) +
-								 MAX_TUPLES_PER_PAGE * sizeof(OffsetNumber));
+								 MAX_TUPLES_PER_PAGE * sizeof(OffsetNumber)
+#ifdef USE_PREFETCH
+                                          		      /*  space for remembering every prefetched but unread blockno */
+                                          		      +  (target_prefetch_pages * sizeof(BlockNumber))
+#endif   /* USE_PREFETCH */
+                                         );
 	iterator->tbm = tbm;
 
 	/*
@@ -1020,3 +1028,70 @@ tbm_comparator(const void *left, const v
 		return 1;
 	return 0;
 }
+
+#ifdef USE_PREFETCH
+void
+tbm_zero(TBMIterator *iterator) /* zero list of prefetched and unread blocknos */
+{
+      /* locate the list of prefetched but unread blocknos immediately following the array of offsets
+      ** and note that tbm_begin_iterate allocates space for (1 + MAX_TUPLES_PER_PAGE) offsets  -
+      ** 1 included in struct TBMIterator and MAX_TUPLES_PER_PAGE additional
+      */
+      iterator->output.Unread_Pfetched_base = ((BlockNumber *)(&(iterator->output.offsets[MAX_TUPLES_PER_PAGE+1])));
+      iterator->output.Unread_Pfetched_next = iterator->output.Unread_Pfetched_count = 0;
+}
+
+void
+tbm_add(TBMIterator *iterator, BlockNumber blockno) /* add this blockno to list of prefetched and unread blocknos */
+{
+      unsigned int Unread_Pfetched_index = iterator->output.Unread_Pfetched_next + iterator->output.Unread_Pfetched_count++;
+
+      if (iterator->output.Unread_Pfetched_count > target_prefetch_pages) {
+		ereport(ERROR,
+			(errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("tbm_add overflowed list cannot add blockno %d", blockno)));
+      }
+
+      if (Unread_Pfetched_index >= target_prefetch_pages)
+          Unread_Pfetched_index -= target_prefetch_pages;
+      *(iterator->output.Unread_Pfetched_base + Unread_Pfetched_index) = blockno;
+}
+
+void
+tbm_subtract(TBMIterator *iterator, BlockNumber blockno) /* remove this blockno from list of prefetched and unread blocknos */
+{
+      unsigned int Unread_Pfetched_index = iterator->output.Unread_Pfetched_next++;
+      BlockNumber nextUnreadPfetched;
+
+      /*    make a weak check that the next blockno is the one to be removed,
+      **    although actually in case of disagreement,   we ignore callers blockno and remove next anyway,
+      **    which is really what caller wants
+      */
+      if ( iterator->output.Unread_Pfetched_count == 0 ) {
+		ereport(ERROR,
+			(errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("tbm_subtract empty list cannot subtract blockno %d", blockno)));
+      }
+
+      if (Unread_Pfetched_index >= target_prefetch_pages)
+          Unread_Pfetched_index = 0;
+      nextUnreadPfetched = *(iterator->output.Unread_Pfetched_base + Unread_Pfetched_index);
+      if (   ( nextUnreadPfetched != blockno ) 
+          && ( nextUnreadPfetched != InvalidBlockNumber ) /* dont report it if the block in the list was InvalidBlockNumber */
+         ) {
+		ereport(NOTICE,
+			(errcode(ERRCODE_INTERNAL_ERROR),
+                                errmsg("tbm_subtract will subtract blockno %d not %d",
+					nextUnreadPfetched, blockno)));
+      }
+      if (iterator->output.Unread_Pfetched_next >= target_prefetch_pages)
+          iterator->output.Unread_Pfetched_next = 0;
+      iterator->output.Unread_Pfetched_count--;
+}
+#endif /* USE_PREFETCH */
+
+TBMIterateResult *
+tbm_locate_IterateResult(TBMIterator *iterator)
+{
+   return &(iterator->output);
+}
--- src/backend/utils/misc/guc.c.orig	2014-08-18 14:10:37.009017427 -0400
+++ src/backend/utils/misc/guc.c	2014-08-19 16:56:14.119198719 -0400
@@ -2258,6 +2258,25 @@ static struct config_int ConfigureNamesI
 	},
 
 	{
+		{"max_async_io_prefetchers",
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+			PGC_USERSET,
+#else
+			PGC_INTERNAL,
+#endif
+			RESOURCES_ASYNCHRONOUS,
+			gettext_noop("Maximum number of background processes concurrently using asynchronous librt threads to prefetch pages into shared memory buffers."),
+		},
+		&max_async_io_prefetchers,
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+	        -1, 0, 8192,      /*  boot val -1 indicates to initialize to something sensible during buf_init */
+#else
+		0, 0, 0,
+#endif
+		NULL, NULL, NULL
+	},
+
+	{
 		{"max_worker_processes",
 			PGC_POSTMASTER,
 			RESOURCES_ASYNCHRONOUS,
--- src/backend/utils/mmgr/aset.c.orig	2014-08-18 14:10:37.009017427 -0400
+++ src/backend/utils/mmgr/aset.c	2014-08-19 16:56:14.179198935 -0400
@@ -733,6 +733,48 @@ AllocSetAlloc(MemoryContext context, Siz
 	 */
 	fidx = AllocSetFreeIndex(size);
 	chunk = set->freelist[fidx];
+#ifdef MEMORY_CONTEXT_CHECKING
+        /*    an instance of segfault caused by a rogue value in set->freelist[fidx]
+        **    has been seen - check for it using crude sanity check based on neighbours :
+        **    if at least one is sufficiently close, then pass,  else fail
+        */
+        if (chunk != 0) {
+            int frx, nrx; /*  frx is index,  nrx is index of failing neighbour for errmsg */
+            for (nrx = -1, frx = 0; (frx < ALLOCSET_NUM_FREELISTS); frx++) {
+                if (   (frx != fidx)     /*  not the chosen one */
+                    && ( ( (unsigned long)(set->freelist[frx]) ) != 0 ) /* not empty */
+                   ) {
+                    if (   ( (unsigned long)chunk < ( ( (unsigned long)(set->freelist[frx]) ) / 2 ) )
+                        && (  ( (unsigned long)(set->freelist[frx]) ) < 0x4000000  )
+               /***     || ( (unsigned long)chunk > ( ( (unsigned long)(set->freelist[frx]) ) * 2 ) )  ***/
+                       ) {
+                       nrx = frx;
+                    } else {
+                       nrx = -1;
+                       break;
+                    }
+                }
+            }
+
+            if (nrx >= 0) {
+                     /*  this must be a rogue value  -  put this list in the garbage */
+                     /*  build message but be careful to avoid recursively triggering same fault */
+                     set->freelist[fidx] = NULL;   /*  mark this list empty */
+                     elog(WARNING, "detected rogue value %p in freelist index %d compared with neighbour %p whose chunksize %d"
+				 , chunk , fidx , set->freelist[nrx] , set->freelist[nrx]->size);
+                     chunk = NULL;
+            }
+        }
+#else /* if not MEMORY_CONTEXT_CHECKING make very simple-minded check*/
+        if ( (chunk != 0) && ( (unsigned long)chunk <  0x40000 ) ) {
+                     /*  this must be a rogue value  -  put this list in the garbage */
+                     /*  build message but be careful to avoid recursively triggering same fault */
+                     set->freelist[fidx] = NULL;   /*  mark this list empty */
+                     elog(WARNING, "detected rogue value %p in freelist index %d"
+				 , chunk , fidx);
+                     chunk = NULL;
+        }
+#endif
 	if (chunk != NULL)
 	{
 		Assert(chunk->size >= size);
--- src/include/executor/instrument.h.orig	2014-08-18 14:10:37.057017653 -0400
+++ src/include/executor/instrument.h	2014-08-19 16:56:14.427199824 -0400
@@ -28,8 +28,18 @@ typedef struct BufferUsage
 	long		local_blks_written;		/* # of local disk blocks written */
 	long		temp_blks_read; /* # of temp blocks read */
 	long		temp_blks_written;		/* # of temp blocks written */
+
 	instr_time	blk_read_time;	/* time spent reading */
 	instr_time	blk_write_time; /* time spent writing */
+
+	long		aio_read_noneed;		/* # of prefetches for which no need for prefetch as block already in buffer pool */
+	long		aio_read_discrd;		/* # of prefetches for which no need for prefetch as block already in buffer pool */
+	long		aio_read_forgot;		/* # of prefetches for which no need for prefetch as block already in buffer pool */
+	long		aio_read_noblok;		/* # of prefetches for which no available BufferAiocb */
+	long		aio_read_failed;		/* # of aio reads for which aio itself failed or the read failed with an errno */
+	long		aio_read_wasted;		/* # of aio reads for which disk block not used */
+	long		aio_read_waited;		/* # of aio reads for which disk block used but had to wait for it */
+	long		aio_read_ontime;		/* # of aio reads for which disk block used and ready on time when requested */
 } BufferUsage;
 
 /* Flag bits included in InstrAlloc's instrument_options bitmask */
--- src/include/storage/bufmgr.h.orig	2014-08-18 14:10:37.065017690 -0400
+++ src/include/storage/bufmgr.h	2014-08-19 16:56:14.455199924 -0400
@@ -41,6 +41,7 @@ typedef enum
 	RBM_ZERO_ON_ERROR,			/* Read, but return an all-zeros page on error */
 	RBM_NORMAL_NO_LOG			/* Don't log page as invalid during WAL
 								 * replay; otherwise same as RBM_NORMAL */
+       ,RBM_NOREAD_FOR_PREFETCH   /* Don't read from disk, don't zero buffer, find buffer only */
 } ReadBufferMode;
 
 /* in globals.c ... this duplicates miscadmin.h */
@@ -57,6 +58,9 @@ extern int	target_prefetch_pages;
 extern PGDLLIMPORT char *BufferBlocks;
 extern PGDLLIMPORT int32 *PrivateRefCount;
 
+/*  in buf_async.c  */;
+extern int      max_async_io_prefetchers; /* Maximum number of backends using asynchronous librt threads to read pages into our buffers */
+
 /* in localbuf.c */
 extern PGDLLIMPORT int NLocBuffer;
 extern PGDLLIMPORT Block *LocalBufferBlockPointers;
@@ -159,9 +163,15 @@ extern PGDLLIMPORT int32 *LocalRefCount;
 #define BufferGetPage(buffer) ((Page)BufferGetBlock(buffer))
 
 /*
- * prototypes for functions in bufmgr.c
+ * prototypes for external functions in bufmgr.c and buf_async.c
  */
-extern void PrefetchBuffer(Relation reln, ForkNumber forkNum,
+extern int PrefetchBuffer(Relation reln, ForkNumber forkNum,
+			   BlockNumber blockNum , BufferAccessStrategy strategy);
+/*   return code  is an int bitmask : */
+#define PREFTCHRC_BUF_PIN_INCREASED 0x01    /*  pin count on buffer has been increased by 1 */
+#define PREFTCHRC_BLK_ALREADY_PRESENT 0x02  /*  block was already present in a buffer       */
+
+extern void DiscardBuffer(Relation reln, ForkNumber forkNum,
 			   BlockNumber blockNum);
 extern Buffer ReadBuffer(Relation reln, BlockNumber blockNum);
 extern Buffer ReadBufferExtended(Relation reln, ForkNumber forkNum,
--- src/include/storage/proc.h.orig	2014-08-18 14:10:37.065017690 -0400
+++ src/include/storage/proc.h	2014-08-19 16:56:14.487200038 -0400
@@ -106,6 +106,10 @@ struct PGPROC
 	uint8		lwWaitMode;		/* lwlock mode being waited for */
 	struct PGPROC *lwWaitLink;	/* next waiter for same LW lock */
 
+#if defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP) && defined(USE_AIO_SIGEVENT)
+	struct PGPROC *BAiocbWaiterLink;	/*  chain of backends waiting for aio completion */
+#endif
+
 	/* Info about lock the process is currently waiting for, if any. */
 	/* waitLock and waitProcLock are NULL if not currently waiting. */
 	LOCK	   *waitLock;		/* Lock object we're sleeping on ... */
--- src/include/storage/smgr.h.orig	2014-08-18 14:10:37.069017709 -0400
+++ src/include/storage/smgr.h	2014-08-19 16:56:14.507200109 -0400
@@ -92,6 +92,21 @@ extern void smgrextend(SMgrRelation reln
 		   BlockNumber blocknum, char *buffer, bool skipFsync);
 extern void smgrprefetch(SMgrRelation reln, ForkNumber forknum,
 			 BlockNumber blocknum);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern void smgrinitaio(int max_aio_threads, int max_aio_num);
+extern void smgrstartaio(SMgrRelation reln, ForkNumber forknum,
+			 BlockNumber blocknum , char *aiocbp , int *retcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , unsigned long BAiocbaioOrdinal
+#endif /* USE_AIO_SIGEVENT  */
+                        );
+extern void smgrcompleteaio( SMgrRelation reln, char *aiocbp , int *inoutcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , int (*BufAWaitAioCompletion)(char *)  /*  function to await completion if non-originator */
+                                , char *BAiocbP /* pointer to BAIOcb  */
+#endif /* USE_AIO_SIGEVENT  */
+                           );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 extern void smgrread(SMgrRelation reln, ForkNumber forknum,
 		 BlockNumber blocknum, char *buffer);
 extern void smgrwrite(SMgrRelation reln, ForkNumber forknum,
@@ -118,6 +133,20 @@ extern void mdextend(SMgrRelation reln,
 		 BlockNumber blocknum, char *buffer, bool skipFsync);
 extern void mdprefetch(SMgrRelation reln, ForkNumber forknum,
 		   BlockNumber blocknum);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern void mdinitaio(int max_aio_threads, int max_aio_num);
+extern void mdstartaio(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum , char *aiocbp , int *retcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , unsigned long BAiocbaioOrdinal
+#endif /* USE_AIO_SIGEVENT  */
+                      );
+extern void mdcompleteaio( char *aiocbp , int *inoutcode
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , int (*BufAWaitAioCompletion)(char *)  /*  function to await completion if non-originator */
+                                , char *BAiocbP /* pointer to BAIOcb  */
+#endif /* USE_AIO_SIGEVENT  */
+                         );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 extern void mdread(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 	   char *buffer);
 extern void mdwrite(SMgrRelation reln, ForkNumber forknum,
--- src/include/storage/fd.h.orig	2014-08-18 14:10:37.065017690 -0400
+++ src/include/storage/fd.h	2014-08-19 16:56:14.547200253 -0400
@@ -69,6 +69,20 @@ extern File PathNameOpenFile(FileName fi
 extern File OpenTemporaryFile(bool interXact);
 extern void FileClose(File file);
 extern int	FilePrefetch(File file, off_t offset, int amount);
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+extern void FileInitaio(int max_aio_threads, int max_aio_num );
+extern int  FileStartaio(File file, off_t offset, int amount , char *aiocbp
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , unsigned long BAiocbaioOrdinal /* ordinal number of this aio in backend's sequence  */
+#endif /* USE_AIO_SIGEVENT  */
+                           );
+extern int  FileCompleteaio( char *aiocbp , int normal_wait
+#ifdef USE_AIO_SIGEVENT     /*    non-originator waiters wait on lock instead of polling */
+                                , int (*BufAWaitAioCompletion)(char *)  /*  function to await completion if non-originator */
+                                , char *BAiocbP /* pointer to BAIOcb  */
+#endif /* USE_AIO_SIGEVENT  */
+                           );
+#endif  /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 extern int	FileRead(File file, char *buffer, int amount);
 extern int	FileWrite(File file, char *buffer, int amount);
 extern int	FileSync(File file);
--- src/include/storage/buf_internals.h.orig	2014-08-18 14:10:37.065017690 -0400
+++ src/include/storage/buf_internals.h	2014-08-19 16:56:14.579200367 -0400
@@ -22,7 +22,9 @@
 #include "storage/smgr.h"
 #include "storage/spin.h"
 #include "utils/relcache.h"
-
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+#include "aio.h"
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
 
 /*
  * Flags for buffer descriptors
@@ -38,8 +40,23 @@
 #define BM_JUST_DIRTIED			(1 << 5)		/* dirtied since write started */
 #define BM_PIN_COUNT_WAITER		(1 << 6)		/* have waiter for sole pin */
 #define BM_CHECKPOINT_NEEDED	(1 << 7)		/* must write for checkpoint */
-#define BM_PERMANENT			(1 << 8)		/* permanent relation (not
-												 * unlogged) */
+#define BM_PERMANENT			(1 << 8)	/* permanent relation (not unlogged) */
+#define BM_AIO_IN_PROGRESS		(1 << 9)	/* aio in progress    */
+#define BM_AIO_PREFETCH_PIN_BANKED	(1 << 10)	/* pinned when prefetch issued
+                                                        ** and this pin is banked - i.e.
+                                                        ** redeemable by the next use by same task
+                                                        ** note that for any one buffer, a pin can be banked
+                                                        **      by at most one process globally,
+                                                        **      that is,   only one process may bank a pin on the buffer
+                                                        **                 and it may do so only once (may not be stacked)
+                                                        */
+
+/*********
+for asynchronous aio-read prefetching, two golden rules concerning buffer pinning and buffer-header flags must be observed:
+  R1.  a buffer marked as BM_AIO_IN_PROGRESS must be pinned by at least one backend
+  R2.  a buffer marked as BM_AIO_PREFETCH_PIN_BANKED must be pinned by the backend identified by
+               (buf->flags & BM_AIO_IN_PROGRESS) ? ( ((BAiocbAnchr->BufferAiocbs)+(FREENEXT_BAIOCB_ORIGIN - buf->freeNext))->pidOfAio : (-(buf->freeNext))
+*********/
 
 typedef bits16 BufFlags;
 
@@ -140,17 +157,87 @@ typedef struct sbufdesc
 	BufFlags	flags;			/* see bit definitions above */
 	uint16		usage_count;	/* usage counter for clock sweep code */
 	unsigned	refcount;		/* # of backends holding pins on buffer */
-	int			wait_backend_pid;		/* backend PID of pin-count waiter */
+	int		wait_backend_pid;	/*  if     flags & BM_PIN_COUNT_WAITER
+                                                **  then   backend PID of pin-count waiter
+                                                **  else   not set
+                                                */
 
 	slock_t		buf_hdr_lock;	/* protects the above fields */
-
 	int			buf_id;			/* buffer's index number (from 0) */
-	int			freeNext;		/* link in freelist chain */
+        int    	volatile	freeNext;	/* overloaded and much-abused field :
+                                                ** EITHER
+                                                **     if     >= 0
+                                                **     then   link in freelist chain
+                                                **  OR
+                                                **     if     <  0
+                                                **     then    EITHER
+                                                **             if     flags & BM_AIO_IN_PROGRESS
+                                                **             then   negative of (the index of the aiocb in the BufferAiocbs array + 3)
+                                                **             else   if flags & BM_AIO_PREFETCH_PIN_BANKED
+                                                **             then   -(pid of task that issued aio_read and pinned buffer)
+                                                **             else   one of the special values -1 or -2 listed below
+                                                */
 
 	LWLock	   *io_in_progress_lock;	/* to wait for I/O to complete */
 	LWLock	   *content_lock;	/* to lock access to buffer contents */
 } BufferDesc;
 
+/*  structures for control blocks for our implementation of async io */
+
+/*  if USE_AIO_ATOMIC_BUILTIN_COMP_SWAP is not defined,  the following struct is not put into use at runtime
+**  but it is easier to let the compiler find the definition but hide the reference to aiocb
+**  which is the only type it would not understand
+*/
+
+struct BufferAiocb {
+       struct BufferAiocb volatile * volatile BAiocbnext;  /*    next free entry or value of BAIOCB_OCCUPIED means in use  */
+       struct sbufdesc    volatile * volatile BAiocbbufh;  /*    there can be at most one BufferDesc marked BM_AIO_IN_PROGRESS
+                                                           **    and using this BufferAiocb -
+                                                           **    if there is one, BAiocbbufh points to it, else BAiocbbufh is zero
+                                                           **    NOTE  BAiocbbufh should be zero for every BufferAiocb on the free list
+                                                           */
+#ifdef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+       struct aiocb       volatile            BAiocbthis;  /*    the aio library's control block for one async io */
+#ifdef USE_AIO_SIGEVENT
+       struct PGPROC  volatile * BAiocbWaiterchain;        /*    chain of backends waiting for aio completion */
+       unsigned long BAiocbaioOrdinal;                     /*    ordinal of most recent aio originated by me */
+#endif  /* USE_AIO_SIGEVENT */
+#endif /*  USE_AIO_ATOMIC_BUILTIN_COMP_SWAP */
+       int                volatile  BAiocbDependentCount;  /*    count of tasks who depend on this BufferAiocb
+                                                           **    in the sense that they are waiting for io completion.
+                                                           **    only a Dependent may move the BufferAiocb onto the freelist
+                                                           **    and only when that Dependent is the *only* Dependent (count == 1)
+                                                           **    BAiocbDependentCount is protected by bufferheader spinlock
+                                                           **    and must be updated only when that spinlock is held
+                                                           **    negative value indicates briefly locked (may not be updated)
+                                                           */
+       pid_t              volatile  pidOfAio;              /*    pid of backend who issued an aio_read using this BAiocb -
+                                                           **    this backend must have pinned the associated buffer.
+                                                           */
+};
+
+#define BAIOCB_OCCUPIED 0x75f1        /*  distinct indicator of a BufferAiocb.BAiocbnext that is NOT on free list */
+#define BAIOCB_FREE 0x7b9d            /*  distinct indicator of a BufferAiocb.BAiocbbufh that IS     on free list */
+
+struct BAiocbAnchor {                 /*  anchor for all control blocks pertaining to aio  */
+       volatile struct BufferAiocb* BufferAiocbs;          /*  aiocbs ...                   */
+       volatile struct BufferAiocb* volatile FreeBAiocbs; /* ... and their free list   */
+};
+
+/*   values for BufCheckAsync input and retcode */
+#define BUF_INTENTION_WANT 		 1  /* wants the buffer, wait for in-progress aio and then pin */
+#define BUF_INTENTION_REJECT_KEEP_PIN 	-1  /* pin already held, do not unpin */
+#define BUF_INTENTION_REJECT_OBTAIN_PIN	-2  /* obtain pin,  caller wants it for same buffer */
+#define BUF_INTENTION_REJECT_FORGET	-3  /* unpin and tell resource owner to forget */
+#define BUF_INTENTION_REJECT_NOADJUST	-4  /* unpin and call ResourceOwnerForgetBuffer */
+#define BUF_INTENTION_REJECT_UNBANK   	-5  /* unpin only if pin banked by caller */
+
+#define BUF_INTENT_RC_CHANGED_TAG	-5
+#define BUF_INTENT_RC_BADPAGE		-4
+#define BUF_INTENT_RC_INVALID_AIO	-3    /*  invalid and aio was in progress */
+#define BUF_INTENT_RC_INVALID_NO_AIO	-1    /*  invalid and no aio was in progress */
+#define BUF_INTENT_RC_VALID		 1
+
 #define BufferDescriptorGetBuffer(bdesc) ((bdesc)->buf_id + 1)
 
 /*
@@ -159,6 +246,7 @@ typedef struct sbufdesc
  */
 #define FREENEXT_END_OF_LIST	(-1)
 #define FREENEXT_NOT_IN_LIST	(-2)
+#define FREENEXT_BAIOCB_ORIGIN	(-3)
 
 /*
  * Macros for acquiring/releasing a shared buffer header's spinlock.
--- src/include/catalog/pg_am.h.orig	2014-08-18 14:10:37.049017615 -0400
+++ src/include/catalog/pg_am.h	2014-08-19 16:56:14.611200482 -0400
@@ -67,6 +67,7 @@ CATALOG(pg_am,2601)
 	regproc		amcanreturn;	/* can indexscan return IndexTuples? */
 	regproc		amcostestimate; /* estimate cost of an indexscan */
 	regproc		amoptions;		/* parse AM-specific parameters */
+	regproc		ampeeknexttuple;	/* peek at the next tuple different from any blocknum in pfch_list without reading a new index page */
 } FormData_pg_am;
 
 /* ----------------
@@ -117,19 +118,19 @@ typedef FormData_pg_am *Form_pg_am;
  * ----------------
  */
 
-DATA(insert OID = 403 (  btree		5 2 t f t t t t t t f t t 0 btinsert btbeginscan btgettuple btgetbitmap btrescan btendscan btmarkpos btrestrpos btbuild btbuildempty btbulkdelete btvacuumcleanup btcanreturn btcostestimate btoptions ));
+DATA(insert OID = 403 (  btree		5 2 t f t t t t t t f t t 0 btinsert btbeginscan btgettuple btgetbitmap btrescan btendscan btmarkpos btrestrpos btbuild btbuildempty btbulkdelete btvacuumcleanup btcanreturn btcostestimate btoptions btpeeknexttuple ));
 DESCR("b-tree index access method");
 #define BTREE_AM_OID 403
-DATA(insert OID = 405 (  hash		1 1 f f t f f f f f f f f 23 hashinsert hashbeginscan hashgettuple hashgetbitmap hashrescan hashendscan hashmarkpos hashrestrpos hashbuild hashbuildempty hashbulkdelete hashvacuumcleanup - hashcostestimate hashoptions ));
+DATA(insert OID = 405 (  hash		1 1 f f t f f f f f f f f 23 hashinsert hashbeginscan hashgettuple hashgetbitmap hashrescan hashendscan hashmarkpos hashrestrpos hashbuild hashbuildempty hashbulkdelete hashvacuumcleanup - hashcostestimate hashoptions - ));
 DESCR("hash index access method");
 #define HASH_AM_OID 405
-DATA(insert OID = 783 (  gist		0 8 f t f f t t f t t t f 0 gistinsert gistbeginscan gistgettuple gistgetbitmap gistrescan gistendscan gistmarkpos gistrestrpos gistbuild gistbuildempty gistbulkdelete gistvacuumcleanup - gistcostestimate gistoptions ));
+DATA(insert OID = 783 (  gist		0 8 f t f f t t f t t t f 0 gistinsert gistbeginscan gistgettuple gistgetbitmap gistrescan gistendscan gistmarkpos gistrestrpos gistbuild gistbuildempty gistbulkdelete gistvacuumcleanup - gistcostestimate gistoptions - ));
 DESCR("GiST index access method");
 #define GIST_AM_OID 783
-DATA(insert OID = 2742 (  gin		0 6 f f f f t t f f t f f 0 gininsert ginbeginscan - gingetbitmap ginrescan ginendscan ginmarkpos ginrestrpos ginbuild ginbuildempty ginbulkdelete ginvacuumcleanup - gincostestimate ginoptions ));
+DATA(insert OID = 2742 (  gin		0 6 f f f f t t f f t f f 0 gininsert ginbeginscan - gingetbitmap ginrescan ginendscan ginmarkpos ginrestrpos ginbuild ginbuildempty ginbulkdelete ginvacuumcleanup - gincostestimate ginoptions - ));
 DESCR("GIN index access method");
 #define GIN_AM_OID 2742
-DATA(insert OID = 4000 (  spgist	0 5 f f f f f t f t f f f 0 spginsert spgbeginscan spggettuple spggetbitmap spgrescan spgendscan spgmarkpos spgrestrpos spgbuild spgbuildempty spgbulkdelete spgvacuumcleanup spgcanreturn spgcostestimate spgoptions ));
+DATA(insert OID = 4000 (  spgist	0 5 f f f f f t f t f f f 0 spginsert spgbeginscan spggettuple spggetbitmap spgrescan spgendscan spgmarkpos spgrestrpos spgbuild spgbuildempty spgbulkdelete spgvacuumcleanup spgcanreturn spgcostestimate spgoptions - ));
 DESCR("SP-GiST index access method");
 #define SPGIST_AM_OID 4000
 
--- src/include/catalog/pg_proc.h.orig	2014-08-18 14:10:37.053017634 -0400
+++ src/include/catalog/pg_proc.h	2014-08-19 16:56:14.667200683 -0400
@@ -536,6 +536,12 @@ DESCR("convert float4 to int4");
 
 DATA(insert OID = 330 (  btgettuple		   PGNSP PGUID 12 1 0 0 0 f f f f t f v 2 0 16 "2281 2281" _null_ _null_ _null_ _null_	btgettuple _null_ _null_ _null_ ));
 DESCR("btree(internal)");
+
+#ifndef AVOID_CATALOG_MIGRATION_FOR_ASYNCIO
+DATA(insert OID = 3256 (  btpeeknexttuple	   PGNSP PGUID 12 1 0 0 0 f f f f t f v 1 0 16 "2281" _null_ _null_ _null_ _null_ btpeeknexttuple _null_ _null_ _null_ ));
+DESCR("btree(internal)");
+#endif /* not AVOID_CATALOG_MIGRATION_FOR_ASYNCIO */
+
 DATA(insert OID = 636 (  btgetbitmap	   PGNSP PGUID 12 1 0 0 0 f f f f t f v 2 0 20 "2281 2281" _null_ _null_ _null_ _null_	btgetbitmap _null_ _null_ _null_ ));
 DESCR("btree(internal)");
 DATA(insert OID = 331 (  btinsert		   PGNSP PGUID 12 1 0 0 0 f f f f t f v 6 0 16 "2281 2281 2281 2281 2281 2281" _null_ _null_ _null_ _null_	btinsert _null_ _null_ _null_ ));
--- src/include/pg_config_manual.h.orig	2014-08-18 14:10:37.061017672 -0400
+++ src/include/pg_config_manual.h	2014-08-19 16:56:14.723200883 -0400
@@ -138,9 +138,11 @@
 /*
  * USE_PREFETCH code should be compiled only if we have a way to implement
  * prefetching.  (This is decoupled from USE_POSIX_FADVISE because there
- * might in future be support for alternative low-level prefetch APIs.)
+ * might in future be support for alternative low-level prefetch APIs  --
+ * -- update October 2013  -- now there is such a new prefetch capability --
+ *   async_io into postgres buffers  -   configuration parameter max_async_io_threads)
  */
-#ifdef USE_POSIX_FADVISE
+#if defined(USE_POSIX_FADVISE) || defined(USE_AIO_ATOMIC_BUILTIN_COMP_SWAP)
 #define USE_PREFETCH
 #endif
 
--- src/include/access/nbtree.h.orig	2014-08-18 14:10:37.045017596 -0400
+++ src/include/access/nbtree.h	2014-08-19 16:56:14.759201013 -0400
@@ -19,6 +19,7 @@
 #include "access/sdir.h"
 #include "access/xlog.h"
 #include "access/xlogutils.h"
+#include "access/relscan.h"
 #include "catalog/pg_index.h"
 
 /* There's room for a 16-bit vacuum cycle ID in BTPageOpaqueData */
@@ -524,6 +525,7 @@ typedef struct BTScanPosData
 	Buffer		buf;			/* if valid, the buffer is pinned */
 
 	BlockNumber nextPage;		/* page's right link when we scanned it */
+	BlockNumber prevPage;		/* page's left link when we scanned it */
 
 	/*
 	 * moreLeft and moreRight track whether we think there may be matching
@@ -603,6 +605,15 @@ typedef struct BTScanOpaqueData
 	 */
 	int			markItemIndex;	/* itemIndex, or -1 if not valid */
 
+	/* prefetch logic state */
+	unsigned int	backSeqRun;	/* number of back-sequential pages in a run */
+	BlockNumber		backSeqPos;	/* blkid last prefetched in back-sequential 
+				          		   runs */
+	BlockNumber		lastHeapPrefetchBlkno;	/* blkid last prefetched from heap */
+	int				prefetchItemIndex; /* item index within currPos last
+					                      fetched by heap prefetch */
+	int				prefetchBlockCount; /* number of prefetched heap blocks */
+	
 	/* keep these last in struct for efficiency */
 	BTScanPosData currPos;		/* current position data */
 	BTScanPosData markPos;		/* marked position, if any */
@@ -655,7 +666,17 @@ extern Buffer _bt_getroot(Relation rel,
 extern Buffer _bt_gettrueroot(Relation rel);
 extern int	_bt_getrootheight(Relation rel);
 extern void _bt_checkpage(Relation rel, Buffer buf);
-extern Buffer _bt_getbuf(Relation rel, BlockNumber blkno, int access);
+extern Buffer _bt_getbuf(Relation rel, BlockNumber blkno, int access
+#ifdef USE_PREFETCH
+                                      , struct pfch_index_pagelist* pfch_index_page_list
+#endif /* USE_PREFETCH */
+                         );
+#ifdef USE_PREFETCH
+extern void _bt_prefetchbuf(Relation rel, BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P);
+extern struct pfch_index_item* _bt_find_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list);
+extern int _bt_add_block(BlockNumber blkno , struct pfch_index_pagelist** pfch_index_page_list_P , uint32 discard_status);
+extern void _bt_subtract_block(BlockNumber blkno , struct pfch_index_pagelist* pfch_index_page_list);
+#endif /* USE_PREFETCH */
 extern Buffer _bt_relandgetbuf(Relation rel, Buffer obuf,
 				 BlockNumber blkno, int access);
 extern void _bt_relbuf(Relation rel, Buffer buf);
--- src/include/access/heapam.h.orig	2014-08-18 14:10:37.045017596 -0400
+++ src/include/access/heapam.h	2014-08-19 16:56:14.787201113 -0400
@@ -175,7 +175,7 @@ extern void heap_page_prune_execute(Buff
 extern void heap_get_root_tuples(Page page, OffsetNumber *root_offsets);
 
 /* in heap/syncscan.c */
-extern void ss_report_location(Relation rel, BlockNumber location);
+extern void ss_report_location(Relation rel, BlockNumber location , BlockNumber *prefetchHWMp);
 extern BlockNumber ss_get_location(Relation rel, BlockNumber relnblocks);
 extern void SyncScanShmemInit(void);
 extern Size SyncScanShmemSize(void);
--- src/include/access/relscan.h.orig	2014-08-18 14:10:37.049017615 -0400
+++ src/include/access/relscan.h	2014-08-19 16:56:14.823201242 -0400
@@ -44,6 +44,24 @@ typedef struct HeapScanDescData
 	bool		rs_inited;		/* false = scan not init'd yet */
 	HeapTupleData rs_ctup;		/* current tuple in scan, if any */
 	BlockNumber rs_cblock;		/* current block # in scan, if any */
+#ifdef USE_PREFETCH
+	int	    rs_prefetch_target; /* target distance (numblocks) for prefetch to reach beyond main scan */
+	BlockNumber rs_pfchblock;	/* next block # to be prefetched in scan, if any */
+
+        /*   Unread_Pfetched is a "mostly" circular list of recently prefetched blocknos of size target_prefetch_pages
+        **   the index of the first unread block is held in Unread_Pfetched_next
+        **         and is advanced when a block is read
+        **   the count of number of unread blocks is in Unread_Pfetched_count (and this subset can wrap around)
+        **   "mostly" means that there may be gaps caused by storing entries for blocks which do not need to be discarded -
+        **   these are indicated by blockno = InvalidBlockNumber,  and these slots are reused when found.
+        */
+        BlockNumber *rs_Unread_Pfetched_base;   /*  where the list of prefetched but unread blocknos starts */
+        unsigned int rs_Unread_Pfetched_next;   /*  where the next unread blockno probably is relative to start --
+                                                **  this is only a hint which may be temporarily stale.
+                                                */
+        unsigned int rs_Unread_Pfetched_count;  /*  number of valid unread blocknos in list */
+#endif   /* USE_PREFETCH */
+
 	Buffer		rs_cbuf;		/* current buffer in scan, if any */
 	/* NB: if rs_cbuf is not InvalidBuffer, we hold a pin on that buffer */
 	ItemPointerData rs_mctid;	/* marked scan position, if any */
@@ -55,6 +73,27 @@ typedef struct HeapScanDescData
 	OffsetNumber rs_vistuples[MaxHeapTuplesPerPage];	/* their offsets */
 }	HeapScanDescData;
 
+/* pfch_index_items track prefetched and unread index pages -   chunks of blocknumbers are chained in singly-linked list from scan->pfch_index_item_list */
+struct pfch_index_item {                              /*  index-relation BlockIds which we will/have prefetched */
+       BlockNumber         pfch_blocknum;    /* Blocknum which we will/have prefetched */
+       uint32              pfch_discard;     /* whether block is to be discarded when scan is closed */
+};
+
+struct pfch_block_item {
+       struct BlockIdData  pfch_blockid;     /* BlockId which we will/have prefetched */
+       uint32              pfch_discard;     /* whether block is to be discarded when scan is closed */
+};
+
+/* pfch_index_page_items track prefetched and unread index pages -
+** chunks of blocknumbers are chained backwards (newest first,  oldest last)
+** in singly-linked list from scan->pfch_index_item_list
+*/
+struct pfch_index_pagelist {                          /*  index-relation BlockIds which we will/have prefetched */
+       struct pfch_index_pagelist* pfch_index_pagelist_next;  /*  pointer to next chunk if any */
+       unsigned int    pfch_index_item_count;         /*  number of used entries in this chunk */
+       struct pfch_index_item pfch_indexid[1];        /*  in-line list of Blocknums which we will/have prefetched and whether to be discarded */
+};
+
 /*
  * We use the same IndexScanDescData structure for both amgettuple-based
  * and amgetbitmap-based index scans.  Some fields are only relevant in
@@ -75,8 +114,15 @@ typedef struct IndexScanDescData
 	/* signaling to index AM about killing index tuples */
 	bool		kill_prior_tuple;		/* last-returned tuple is dead */
 	bool		ignore_killed_tuples;	/* do not return killed entries */
-	bool		xactStartedInRecovery;	/* prevents killing/seeing killed
-										 * tuples */
+	bool		xactStartedInRecovery;	/* prevents killing/seeing killed tuples */
+										 
+#ifdef USE_PREFETCH
+        struct pfch_index_pagelist* pfch_index_page_list;  /* array of index-relation BlockIds which we will/have prefetched */
+        struct pfch_block_item* pfch_block_item_list;   /* array of heap-relation BlockIds which we will/have prefetched */
+        unsigned short int     pfch_used;	/*  number of used elements in BlockIdData array   */
+        unsigned short int     pfch_next;	/*  next element for prefetch in BlockIdData array */
+	int             do_prefetch;    /*  should I prefetch ? */
+#endif   /* USE_PREFETCH */
 
 	/* index access method's private state */
 	void	   *opaque;			/* access-method-specific info */
@@ -91,6 +137,10 @@ typedef struct IndexScanDescData
 	/* NB: if xs_cbuf is not InvalidBuffer, we hold a pin on that buffer */
 	bool		xs_recheck;		/* T means scan keys must be rechecked */
 
+	/* heap fetch statistics for read-ahead logic */
+	unsigned int heap_tids_seen;
+	unsigned int heap_tids_fetched;
+
 	/* state data for traversing HOT chains in index_getnext */
 	bool		xs_continue_hot;	/* T if must keep walking HOT chain */
 }	IndexScanDescData;
--- src/include/nodes/tidbitmap.h.orig	2014-08-18 14:10:37.061017672 -0400
+++ src/include/nodes/tidbitmap.h	2014-08-19 16:56:14.863201385 -0400
@@ -41,6 +41,16 @@ typedef struct
 	int			ntuples;		/* -1 indicates lossy result */
 	bool		recheck;		/* should the tuples be rechecked? */
 	/* Note: recheck is always true if ntuples < 0 */
+#ifdef USE_PREFETCH
+        /*   Unread_Pfetched is a circular list of recently prefetched blocknos of size target_prefetch_pages
+        **   the index of the first unread block is held in Unread_Pfetched_next
+        **         and is advanced when a block is read
+        **   the count of number of unread blocks is in Unread_Pfetched_count (and this subset can wrap around)
+        */
+        BlockNumber *Unread_Pfetched_base;   /*  where the list of prefetched but unread blocknos starts */
+        unsigned int Unread_Pfetched_next;   /*  where the next unread blockno is relative to start */
+        unsigned int Unread_Pfetched_count;  /*  number of valid unread blocknos in list */
+#endif   /* USE_PREFETCH */
 	OffsetNumber offsets[1];	/* VARIABLE LENGTH ARRAY */
 } TBMIterateResult;				/* VARIABLE LENGTH STRUCT */
 
@@ -62,5 +72,8 @@ extern bool tbm_is_empty(const TIDBitmap
 extern TBMIterator *tbm_begin_iterate(TIDBitmap *tbm);
 extern TBMIterateResult *tbm_iterate(TBMIterator *iterator);
 extern void tbm_end_iterate(TBMIterator *iterator);
-
+extern void tbm_zero(TBMIterator *iterator); /* zero list of prefetched and unread blocknos */
+extern void tbm_add(TBMIterator *iterator, BlockNumber blockno); /* add this blockno to list of prefetched and unread blocknos */
+extern void tbm_subtract(TBMIterator *iterator, BlockNumber blockno); /* remove this blockno from list of prefetched and unread blocknos */
+extern TBMIterateResult *tbm_locate_IterateResult(TBMIterator *iterator); /* locate the TBMIterateResult of an iterator */
 #endif   /* TIDBITMAP_H */
--- src/include/utils/rel.h.orig	2014-08-18 14:10:37.069017709 -0400
+++ src/include/utils/rel.h	2014-08-19 16:56:14.891201485 -0400
@@ -61,6 +61,7 @@ typedef struct RelationAmInfo
 	FmgrInfo	ammarkpos;
 	FmgrInfo	amrestrpos;
 	FmgrInfo	amcanreturn;
+	FmgrInfo	ampeeknexttuple;	/* peek at the next tuple different from any blocknum in pfch_list without reading a new index page */
 } RelationAmInfo;
 
 
--- src/include/pg_config.h.in.orig	2014-08-18 14:10:37.061017672 -0400
+++ src/include/pg_config.h.in	2014-08-19 16:56:14.915201571 -0400
@@ -1,4 +1,4 @@
-/* src/include/pg_config.h.in.  Generated from configure.in by autoheader.  */
+/* src/include/pg_config.h.in.  Generated from - by autoheader.  */
 
 /* Define to the type of arg 1 of 'accept' */
 #undef ACCEPT_TYPE_ARG1
@@ -747,6 +747,10 @@
 /* Define to the appropriate snprintf format for unsigned 64-bit ints. */
 #undef UINT64_FORMAT
 
+/* Define to select librt-style async io and the gcc atomic compare_and_swap.
+   */
+#undef USE_AIO_ATOMIC_BUILTIN_COMP_SWAP
+
 /* Define to 1 to build with assertion checks. (--enable-cassert) */
 #undef USE_ASSERT_CHECKING