pg_freespacemap question

Am Dienstag, 7. M�rz 2006 15:09 schrieb Tatsuo Ishii:

test=# select * from pg_freespacemap where blockfreebytes = 0;
blockid | relfilenode | reltablespace | reldatabase | relblocknumber | blockfreebytes
---------+-------------+---------------+-------------+----------------+----------------
25 | 2619 | 1663 | 16403 | 0 | 0
63 | 2619 | 1663 | 16384 | 10 | 0
(2 rows)

I've never heard of this thing before but is this column order supposed to make sense?

--
Peter Eisentraut
http://developer.postgresql.org/~petere/

Peter Eisentraut wrote:

Am Dienstag, 7. M�rz 2006 15:09 schrieb Tatsuo Ishii:

test=# select * from pg_freespacemap where blockfreebytes = 0;
blockid | relfilenode | reltablespace | reldatabase | relblocknumber | blockfreebytes
---------+-------------+---------------+-------------+----------------+----------------
25 | 2619 | 1663 | 16403 | 0 | 0
63 | 2619 | 1663 | 16384 | 10 | 0
(2 rows)

I've never heard of this thing before but is this column order supposed to make sense?

I have another question -- why is the view showing relfilenode and
reltablespace? I imagine it should be showing the relation Oid instead.
And what is this "blockid" thing?

--
Alvaro Herrera http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

Peter Eisentraut wrote:

Am Dienstag, 7. März 2006 15:09 schrieb Tatsuo Ishii:

test=# select * from pg_freespacemap where blockfreebytes = 0;
blockid | relfilenode | reltablespace | reldatabase | relblocknumber | blockfreebytes
---------+-------------+---------------+-------------+----------------+----------------
25 | 2619 | 1663 | 16403 | 0 | 0
63 | 2619 | 1663 | 16384 | 10 | 0
(2 rows)

I've never heard of this thing before but is this column order supposed to make sense?

I have another question -- why is the view showing relfilenode and
reltablespace? I imagine it should be showing the relation Oid instead.

I guess that's because FSM keeps those info, not relation oid.

And what is this "blockid" thing?

from README.pg_freespacemap:

blockid | | Id, 1.. max_fsm_pages

BTW, I found the answer to my question myself by reading the source
code: if that's an index, then blockfreebytes is explicitly set to 0.
I suggest that this should be noted in the README and in this case
blockfreebytes is better to set to NULL, rather than 0.
--
Tatsuo Ishii
SRA OSS, Inc. Japan

Tatsuo Ishii <ishii@sraoss.co.jp> writes:

Peter Eisentraut wrote:
I have another question -- why is the view showing relfilenode and
reltablespace? I imagine it should be showing the relation Oid instead.

I guess that's because FSM keeps those info, not relation oid.

Right, which is correct because free space is associated with physical
files not logical relations. (TRUNCATE, CLUSTER, etc will completely
change the freespace situation for a rel, but they don't change its OID.)

I do agree with the comment that the column order seems nonintuitive;
I'd expect database/tablespace/relfilenode/blocknumber, or possibly
tablespace first. The names used for the columns could do with
reconsideration. And I don't see the point of the blockid column at
all.

regards, tom lane

Tatsuo Ishii wrote:

Peter Eisentraut wrote:

Am Dienstag, 7. M�rz 2006 15:09 schrieb Tatsuo Ishii:

test=# select * from pg_freespacemap where blockfreebytes = 0;
blockid | relfilenode | reltablespace | reldatabase | relblocknumber | blockfreebytes
---------+-------------+---------------+-------------+----------------+----------------
25 | 2619 | 1663 | 16403 | 0 | 0
63 | 2619 | 1663 | 16384 | 10 | 0
(2 rows)

I've never heard of this thing before but is this column order supposed to make sense?

I have another question -- why is the view showing relfilenode and
reltablespace? I imagine it should be showing the relation Oid instead.

I guess that's because FSM keeps those info, not relation oid.

And what is this "blockid" thing?

from README.pg_freespacemap:

blockid | | Id, 1.. max_fsm_pages

I put that in as a bit of a sanity check - to see if the view was
picking up all the fsm pages - guess it is a bit redundant now.

BTW, I found the answer to my question myself by reading the source
code: if that's an index, then blockfreebytes is explicitly set to 0.
I suggest that this should be noted in the README and in this case
blockfreebytes is better to set to NULL, rather than 0.

Good points! I had not noticed this test case. Probably NULL is better
than zero.

I'll look into making these changes! (good to see people checking the
view out).

Cheers

Mark

Tom Lane wrote:

Tatsuo Ishii <ishii@sraoss.co.jp> writes:

Peter Eisentraut wrote:
I have another question -- why is the view showing relfilenode and
reltablespace? I imagine it should be showing the relation Oid instead.

I guess that's because FSM keeps those info, not relation oid.

Right, which is correct because free space is associated with physical
files not logical relations. (TRUNCATE, CLUSTER, etc will completely
change the freespace situation for a rel, but they don't change its OID.)

I do agree with the comment that the column order seems nonintuitive;
I'd expect database/tablespace/relfilenode/blocknumber, or possibly
tablespace first. The names used for the columns could do with
reconsideration. And I don't see the point of the blockid column at
all.

Tom - agreed, I'll look at making these changes too!

Cheers

Mark

BTW, I found the answer to my question myself by reading the source
code: if that's an index, then blockfreebytes is explicitly set to 0.
I suggest that this should be noted in the README and in this case
blockfreebytes is better to set to NULL, rather than 0.

Good points! I had not noticed this test case. Probably NULL is better
than zero.

Just for curiousity, why FSM gathers info for indexes? I thought FSM
is only good for tables.
--
Tatsuo Ishii
SRA OSS, Inc. Japan

Tatsuo Ishii wrote:

Just for curiousity, why FSM gathers info for indexes? I thought FSM
is only good for tables.

It's part of the implementation of the page-recycling algorithm for
btrees Tom did for 7.4. When a btree page is empty after a vacuum, it's
entered in the free space map. When a page is split, the new page is
taken from the FSM (or the relation is extended if there isn't any.)

That's why the bytes-free number is zero: when a btree page makes it
into the FSM, we are sure it's completely empty.

--
Alvaro Herrera http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

Mark Kirkwood wrote:

Tatsuo Ishii wrote:

BTW, I found the answer to my question myself by reading the source
code: if that's an index, then blockfreebytes is explicitly set to 0.
I suggest that this should be noted in the README and in this case
blockfreebytes is better to set to NULL, rather than 0.

Good points! I had not noticed this test case. Probably NULL is better
than zero.

Would setting it to 'BLCKSZ - (fixed index header stuff)' be better,
since the btree page is empty? (I'll have to read up on how to calculate
the header stuff!).

regards

Mark

Mark Kirkwood <markir@paradise.net.nz> writes:

Good points! I had not noticed this test case. Probably NULL is better

Would setting it to 'BLCKSZ - (fixed index header stuff)' be better,

No, I don't think so, because that will just make it harder to recognize
what's what (remember that BLCKSZ isn't really a constant, and the index
overhead is not the same for all AMs either). The point here is that
for indexes the FSM tracks whole-page availability, not the amount of
free space within pages. So I think NULL is a reasonable representation
of that. Using NULL will make it easy to filter the results if you want
to see only heap-page data or only index-page data, whereas it will be
very hard to do that if the view adopts an ultimately-artificial
convention about the amount of available space on an index page.

regards, tom lane

Tom Lane wrote:

Mark Kirkwood <markir@paradise.net.nz> writes:

Good points! I had not noticed this test case. Probably NULL is better

Would setting it to 'BLCKSZ - (fixed index header stuff)' be better,

No, I don't think so, because that will just make it harder to recognize
what's what (remember that BLCKSZ isn't really a constant, and the index
overhead is not the same for all AMs either). The point here is that
for indexes the FSM tracks whole-page availability, not the amount of
free space within pages. So I think NULL is a reasonable representation
of that. Using NULL will make it easy to filter the results if you want
to see only heap-page data or only index-page data, whereas it will be
very hard to do that if the view adopts an ultimately-artificial
convention about the amount of available space on an index page.

Right - after suggesting it I realized that coding the different index
overhead for each possible AM would have been ... difficult :-). A patch
is attached to implement the NULL free bytes and other recommendations:

1/ Index free bytes set to NULL
2/ Comment added to the README briefly mentioning the index business
3/ Columns reordered more logically
4/ 'Blockid' column removed
5/ Free bytes column renamed to just 'bytes' instead of 'blockfreebytes'

Now 5/ was only hinted at, but seemed worth doing while I was there
(hopefully I haven't made it too terse now....).

cheers

Mark

Mark,

I have tried your patches and it worked great. Thanks.
--
Tatsuo Ishii
SRA OSS, Inc. Japan

BTW, I noticed difference of outputs from pg_freespacemap and
pgstattuple.

I ran pgbench and inspected "accounts" table by using these tools.

pg_freespacemap:
sum of bytes: 250712

pgstattuple:
free_space: 354880

Shouldn't they be identical?
--
Tatsuo Ishii
SRA OSS, Inc. Japan

Tatsuo Ishii wrote:

BTW, I noticed difference of outputs from pg_freespacemap and
pgstattuple.

I ran pgbench and inspected "accounts" table by using these tools.

pg_freespacemap:
sum of bytes: 250712

pgstattuple:
free_space: 354880

Shouldn't they be identical?

I would have thought so - unless there are not enough pages left in the
FSM...

pg_freespacemap is reporting on what gets into the FSM - so provided I
haven't put a bug in there somewhere (!) - we need to look at how VACUUM
reports free space to the FSM....

cheers

Mark

Tatsuo Ishii wrote:

BTW, I noticed difference of outputs from pg_freespacemap and
pgstattuple.

I ran pgbench and inspected "accounts" table by using these tools.

pg_freespacemap:
sum of bytes: 250712

pgstattuple:
free_space: 354880

Shouldn't they be identical?

No, because (a) pgbench vacuums at the start of the run not the end,
and (b) vacuum/fsm disregard pages with "uselessly small" amounts of
free space (less than the average tuple size, IIRC).

I do notice a rather serious shortcoming of pg_freespacemap in its
current incarnation, which is that it *only* shows you the per-page free
space data, and not any of the information that would let you determine
what the FSM is doing to filter the raw data. The per-relation
avgRequest and lastPageCount fields would be interesting for instance.
Perhaps there should be a second view with one row per relation to
carry the appropriate data.

regards, tom lane

Tom Lane wrote:

Tatsuo Ishii wrote:

BTW, I noticed difference of outputs from pg_freespacemap and
pgstattuple.

I ran pgbench and inspected "accounts" table by using these tools.

pg_freespacemap:
sum of bytes: 250712

pgstattuple:
free_space: 354880

Shouldn't they be identical?

vacuum/fsm disregard pages with "uselessly small" amounts of
free space (less than the average tuple size, IIRC).

Ah - that what I was seeing! Thanks.

I do notice a rather serious shortcoming of pg_freespacemap in its
current incarnation, which is that it *only* shows you the per-page free
space data, and not any of the information that would let you determine
what the FSM is doing to filter the raw data. The per-relation
avgRequest and lastPageCount fields would be interesting for instance.
Perhaps there should be a second view with one row per relation to
carry the appropriate data.

Ok - I did wonder about 2 views, but was unsure if the per-relation
stuff was interesting. Given that it looks like it is interesting, I'll
see about getting a second view going.

Cheers

Mark

Tatsuo Ishii wrote:

BTW, I noticed difference of outputs from pg_freespacemap and
pgstattuple.

I ran pgbench and inspected "accounts" table by using these tools.

pg_freespacemap:
sum of bytes: 250712

pgstattuple:
free_space: 354880

Shouldn't they be identical?

No, because (a) pgbench vacuums at the start of the run not the end,

I ran VACUUM after pbench run and still got the differece.

and (b) vacuum/fsm disregard pages with "uselessly small" amounts of
free space (less than the average tuple size, IIRC).

That sounds strange to me. Each record of accounts tables is actually
exactly same, i.e fixed size. So it should be possible that UPDATE
reuses any free spaces made by previous UPDATE. If FSM neglects those
free spaces "because they are uselessly small", then the unrecycled
pages are getting grow even if they are regulary VACUUMed, no?

I do notice a rather serious shortcoming of pg_freespacemap in its
current incarnation, which is that it *only* shows you the per-page free
space data, and not any of the information that would let you determine
what the FSM is doing to filter the raw data. The per-relation
avgRequest and lastPageCount fields would be interesting for instance.
Perhaps there should be a second view with one row per relation to
carry the appropriate data.

--
Tatsuo Ishii
SRA OSS, Inc. Japan

Tatsuo Ishii <ishii@sraoss.co.jp> writes:

That sounds strange to me. Each record of accounts tables is actually
exactly same, i.e fixed size. So it should be possible that UPDATE
reuses any free spaces made by previous UPDATE. If FSM neglects those
free spaces "because they are uselessly small", then the unrecycled
pages are getting grow even if they are regulary VACUUMed, no?

The point here is that if tuples require 50 bytes, and there are 20
bytes free on a page, pgstattuple counts 20 free bytes while FSM
ignores the page. Recording that space in the FSM will not improve
matters, it'll just risk pushing out FSM records for pages that do
have useful amounts of free space.

regards, tom lane

The point here is that if tuples require 50 bytes, and there are 20
bytes free on a page, pgstattuple counts 20 free bytes while FSM
ignores the page. Recording that space in the FSM will not improve
matters, it'll just risk pushing out FSM records for pages that do
have useful amounts of free space.

Maybe an overloaded pgstattuple function that allows you to request FSM
behavior?

Chris

Christopher Kings-Lynne wrote:

The point here is that if tuples require 50 bytes, and there are 20
bytes free on a page, pgstattuple counts 20 free bytes while FSM
ignores the page. Recording that space in the FSM will not improve
matters, it'll just risk pushing out FSM records for pages that do
have useful amounts of free space.

Maybe an overloaded pgstattuple function that allows you to request FSM
behavior?

That's a nice idea - could also do equivalently by adding an extra
column "usable_free_space" or some such, and calculating this using FSM
logic.

Cheers

Mark

Mark Kirkwood wrote:

Christopher Kings-Lynne wrote:

The point here is that if tuples require 50 bytes, and there are 20
bytes free on a page, pgstattuple counts 20 free bytes while FSM
ignores the page. Recording that space in the FSM will not improve
matters, it'll just risk pushing out FSM records for pages that do
have useful amounts of free space.

Maybe an overloaded pgstattuple function that allows you to request FSM
behavior?

That's a nice idea - could also do equivalently by adding an extra
column "usable_free_space" or some such, and calculating this using FSM
logic.

The current pgstattuple function scans the whole table, so I don't think
this is a good idea. Re: the overloaded function, I think the behaviors
are different enough to merit a separate function, with a different
name.

--
Alvaro Herrera http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.

Mark Kirkwood wrote:

Tom Lane wrote:

I do notice a rather serious shortcoming of pg_freespacemap in its
current incarnation, which is that it *only* shows you the per-page free
space data, and not any of the information that would let you determine
what the FSM is doing to filter the raw data. The per-relation
avgRequest and lastPageCount fields would be interesting for instance.
Perhaps there should be a second view with one row per relation to
carry the appropriate data.

Ok - I did wonder about 2 views, but was unsure if the per-relation
stuff was interesting. Given that it looks like it is interesting, I'll
see about getting a second view going.

This patch implements the second view for FSM relations. I have renamed
the functions and views to be:

pg_freespacemap_relations
pg_freespacemap_pages

This patch depends on the previous one (which was called simply
'pg_freespacemap.patch').

Cheers

Mark

Patch applied. Thanks.

---------------------------------------------------------------------------

Mark Kirkwood wrote:

Tom Lane wrote:

Mark Kirkwood <markir@paradise.net.nz> writes:

Good points! I had not noticed this test case. Probably NULL is better

Would setting it to 'BLCKSZ - (fixed index header stuff)' be better,

No, I don't think so, because that will just make it harder to recognize
what's what (remember that BLCKSZ isn't really a constant, and the index
overhead is not the same for all AMs either). The point here is that
for indexes the FSM tracks whole-page availability, not the amount of
free space within pages. So I think NULL is a reasonable representation
of that. Using NULL will make it easy to filter the results if you want
to see only heap-page data or only index-page data, whereas it will be
very hard to do that if the view adopts an ultimately-artificial
convention about the amount of available space on an index page.

Right - after suggesting it I realized that coding the different index
overhead for each possible AM would have been ... difficult :-). A patch
is attached to implement the NULL free bytes and other recommendations:

1/ Index free bytes set to NULL
2/ Comment added to the README briefly mentioning the index business
3/ Columns reordered more logically
4/ 'Blockid' column removed
5/ Free bytes column renamed to just 'bytes' instead of 'blockfreebytes'

Now 5/ was only hinted at, but seemed worth doing while I was there
(hopefully I haven't made it too terse now....).

cheers

Mark

Index: pg_freespacemap.c
===================================================================
RCS file: /projects/cvsroot/pgsql/contrib/pg_freespacemap/pg_freespacemap.c,v
retrieving revision 1.2
diff -c -r1.2 pg_freespacemap.c
*** pg_freespacemap.c	14 Feb 2006 15:03:59 -0000	1.2
--- pg_freespacemap.c	9 Mar 2006 03:38:10 -0000
***************
*** 12,18 ****
#include "storage/freespace.h"
#include "utils/relcache.h"

! #define NUM_FREESPACE_PAGES_ELEM 6

#if defined(WIN32) || defined(__CYGWIN__)
/* Need DLLIMPORT for some things that are not so marked in main headers */
--- 12,18 ----
#include "storage/freespace.h"
#include "utils/relcache.h"

! #define NUM_FREESPACE_PAGES_ELEM 5

#if defined(WIN32) || defined(__CYGWIN__)
/* Need DLLIMPORT for some things that are not so marked in main headers */
***************
*** 29,40 ****
typedef struct
{

- uint32 blockid;
- uint32 relfilenode;
uint32 reltablespace;
uint32 reldatabase;
uint32 relblocknumber;
! uint32 blockfreebytes;

} FreeSpacePagesRec;

--- 29,40 ----
typedef struct
{

uint32 reltablespace;
uint32 reldatabase;
+ uint32 relfilenode;
uint32 relblocknumber;
! uint32 bytes;
! bool isindex;

} FreeSpacePagesRec;

***************
*** 91,107 ****

/* Construct a tuple to return. */
tupledesc = CreateTemplateTupleDesc(NUM_FREESPACE_PAGES_ELEM, false);
! TupleDescInitEntry(tupledesc, (AttrNumber) 1, "blockid",
! INT4OID, -1, 0);
! TupleDescInitEntry(tupledesc, (AttrNumber) 2, "relfilenode",
OIDOID, -1, 0);
! TupleDescInitEntry(tupledesc, (AttrNumber) 3, "reltablespace",
OIDOID, -1, 0);
! TupleDescInitEntry(tupledesc, (AttrNumber) 4, "reldatabase",
OIDOID, -1, 0);
! TupleDescInitEntry(tupledesc, (AttrNumber) 5, "relblocknumber",
INT8OID, -1, 0);
! TupleDescInitEntry(tupledesc, (AttrNumber) 6, "blockfreebytes",
INT4OID, -1, 0);

/* Generate attribute metadata needed later to produce tuples */
--- 91,105 ----

/* Construct a tuple to return. */
tupledesc = CreateTemplateTupleDesc(NUM_FREESPACE_PAGES_ELEM, false);
! TupleDescInitEntry(tupledesc, (AttrNumber) 1, "reltablespace",
OIDOID, -1, 0);
! TupleDescInitEntry(tupledesc, (AttrNumber) 2, "reldatabase",
OIDOID, -1, 0);
! TupleDescInitEntry(tupledesc, (AttrNumber) 3, "relfilenode",
OIDOID, -1, 0);
! TupleDescInitEntry(tupledesc, (AttrNumber) 4, "relblocknumber",
INT8OID, -1, 0);
! TupleDescInitEntry(tupledesc, (AttrNumber) 5, "bytes",
INT4OID, -1, 0);

/* Generate attribute metadata needed later to produce tuples */
***************
*** 129,135 ****
fctx->values[2] = (char *) palloc(3 * sizeof(uint32) + 1);
fctx->values[3] = (char *) palloc(3 * sizeof(uint32) + 1);
fctx->values[4] = (char *) palloc(3 * sizeof(uint32) + 1);
- fctx->values[5] = (char *) palloc(3 * sizeof(uint32) + 1);

/* Return to original context when allocating transient memory */
--- 127,132 ----
***************
*** 158,169 ****
for (nPages = 0; nPages < fsmrel->storedPages; nPages++)
{

- fctx->record[i].blockid = i;
- fctx->record[i].relfilenode = fsmrel->key.relNode;
fctx->record[i].reltablespace = fsmrel->key.spcNode;
fctx->record[i].reldatabase = fsmrel->key.dbNode;
fctx->record[i].relblocknumber = IndexFSMPageGetPageNum(page);
! fctx->record[i].blockfreebytes = 0; /* index.*/

page++;
i++;
--- 155,166 ----
for (nPages = 0; nPages < fsmrel->storedPages; nPages++)
{

fctx->record[i].reltablespace = fsmrel->key.spcNode;
fctx->record[i].reldatabase = fsmrel->key.dbNode;
+ fctx->record[i].relfilenode = fsmrel->key.relNode;
fctx->record[i].relblocknumber = IndexFSMPageGetPageNum(page);
! fctx->record[i].bytes = 0;
! fctx->record[i].isindex = true;

page++;
i++;
***************
*** 178,189 ****

for (nPages = 0; nPages < fsmrel->storedPages; nPages++)
{
- fctx->record[i].blockid = i;
- fctx->record[i].relfilenode = fsmrel->key.relNode;
fctx->record[i].reltablespace = fsmrel->key.spcNode;
fctx->record[i].reldatabase = fsmrel->key.dbNode;
fctx->record[i].relblocknumber = FSMPageGetPageNum(page);
! fctx->record[i].blockfreebytes = FSMPageGetSpace(page);

page++;
i++;
--- 175,186 ----

for (nPages = 0; nPages < fsmrel->storedPages; nPages++)
{
fctx->record[i].reltablespace = fsmrel->key.spcNode;
fctx->record[i].reldatabase = fsmrel->key.dbNode;
+ fctx->record[i].relfilenode = fsmrel->key.relNode;
fctx->record[i].relblocknumber = FSMPageGetPageNum(page);
! fctx->record[i].bytes = FSMPageGetSpace(page);
! fctx->record[i].isindex = false;

page++;
i++;
***************
*** 209,227 ****
if (funcctx->call_cntr < funcctx->max_calls)
{
uint32 i = funcctx->call_cntr;

- sprintf(fctx->values[0], "%u", fctx->record[i].blockid);
- sprintf(fctx->values[1], "%u", fctx->record[i].relfilenode);
- sprintf(fctx->values[2], "%u", fctx->record[i].reltablespace);
- sprintf(fctx->values[3], "%u", fctx->record[i].reldatabase);
- sprintf(fctx->values[4], "%u", fctx->record[i].relblocknumber);
- sprintf(fctx->values[5], "%u", fctx->record[i].blockfreebytes);

/* Build and return the tuple. */
! tuple = BuildTupleFromCStrings(funcctx->attinmeta, fctx->values);
result = HeapTupleGetDatum(tuple);

--- 206,246 ----
if (funcctx->call_cntr < funcctx->max_calls)
{
uint32		i = funcctx->call_cntr;
+ 		char		*values[NUM_FREESPACE_PAGES_ELEM];
+ 		int			j;

+ 		/*
+ 		 * Use a temporary values array, initially pointing to fctx->values,
+ 		 * so it can be reassigned w/o losing the storage for subsequent
+ 		 * calls.
+ 		 */
+ 		for (j = 0; j < NUM_FREESPACE_PAGES_ELEM; j++)
+ 		{
+ 			values[j] = fctx->values[j];
+ 		}
+ 
+ 
+ 		sprintf(values[0], "%u", fctx->record[i].reltablespace);
+ 		sprintf(values[1], "%u", fctx->record[i].reldatabase);
+ 		sprintf(values[2], "%u", fctx->record[i].relfilenode);
+ 		sprintf(values[3], "%u", fctx->record[i].relblocknumber);

+ 		/*
+ 		 * Set (free) bytes to NULL for an index relation.
+ 		 */
+ 		if (fctx->record[i].isindex == true)
+ 		{
+ 			values[4] = NULL;
+ 		}
+ 		else
+ 		{
+ 			sprintf(values[4], "%u", fctx->record[i].bytes);
+ 		}

/* Build and return the tuple. */
! tuple = BuildTupleFromCStrings(funcctx->attinmeta, values);
result = HeapTupleGetDatum(tuple);

Index: pg_freespacemap.sql.in
===================================================================
RCS file: /projects/cvsroot/pgsql/contrib/pg_freespacemap/pg_freespacemap.sql.in,v
retrieving revision 1.2
diff -c -r1.2 pg_freespacemap.sql.in
*** pg_freespacemap.sql.in	27 Feb 2006 16:09:48 -0000	1.2
--- pg_freespacemap.sql.in	9 Mar 2006 03:42:15 -0000
***************
*** 11,17 ****
-- Create a view for convenient access.
CREATE VIEW pg_freespacemap AS
SELECT P.* FROM pg_freespacemap() AS P
!  	(blockid int4, relfilenode oid, reltablespace oid, reldatabase oid, relblocknumber int8, blockfreebytes int4);

-- Don't want these to be available at public.
REVOKE ALL ON FUNCTION pg_freespacemap() FROM PUBLIC;
--- 11,17 ----
-- Create a view for convenient access.
CREATE VIEW pg_freespacemap AS
SELECT P.* FROM pg_freespacemap() AS P
!  	(reltablespace oid, reldatabase oid, relfilenode oid, relblocknumber int8, bytes int4);

-- Don't want these to be available at public.
REVOKE ALL ON FUNCTION pg_freespacemap() FROM PUBLIC;
Index: README.pg_freespacemap
===================================================================
RCS file: /projects/cvsroot/pgsql/contrib/pg_freespacemap/README.pg_freespacemap,v
retrieving revision 1.1
diff -c -r1.1 README.pg_freespacemap
*** README.pg_freespacemap	12 Feb 2006 03:55:53 -0000	1.1
--- README.pg_freespacemap	9 Mar 2006 03:43:16 -0000
***************
*** 34,45 ****

Column | references | Description
----------------+----------------------+------------------------------------
- blockid | | Id, 1.. max_fsm_pages
- relfilenode | pg_class.relfilenode | Refilenode of the relation.
reltablespace | pg_tablespace.oid | Tablespace oid of the relation.
reldatabase | pg_database.oid | Database for the relation.
relblocknumber | | Offset of the page in the relation.
! blockfreebytes | | Free bytes in the block/page.

There is one row for each page in the free space map.
--- 34,45 ----

Column     |  references          | Description
----------------+----------------------+------------------------------------
reltablespace  | pg_tablespace.oid    | Tablespace oid of the relation.
reldatabase    | pg_database.oid      | Database for the relation.
+    relfilenode    | pg_class.relfilenode | Refilenode of the relation.
relblocknumber |                      | Offset of the page in the relation.
!    bytes          |                      | Free bytes in the block/page, or NULL
!                   |                      | for an index page (see below).

There is one row for each page in the free space map.
***************
*** 47,52 ****
--- 47,55 ----
Because the map is shared by all the databases, there are pages from
relations not belonging to the current database.

+   The free space map can contain pages for btree indexes if they were emptied 
+   by a vacuum process. The bytes field is set to NULL in this case.
+ 
When the pg_freespacemap view is accessed, internal free space map locks are
taken, and a copy of the map data is made for the view to display. 
This ensures that the view produces a consistent set of results, while not 
***************
*** 58,91 ****
-------------

regression=# \d pg_freespacemap
! View "public.pg_freespacemap"
Column | Type | Modifiers
! ---------------+---------+-----------
! blockid | integer |
! relfilenode | oid |
reltablespace | oid |
reldatabase | oid |
relblocknumber | bigint |
! blockfreebytes | integer |
View definition:
! SELECT p.blockid, p.relfilenode, p.reltablespace, p.reldatabase, p.relblocknumber, p.blockfreebytes
! FROM pg_freespacemap() p(blockid integer, relfilenode oid, reltablespace oid, reldatabase oid, relblocknumber bigint, blockfreebytes integer);

! regression=# SELECT c.relname, m.relblocknumber, m.blockfreebytes
FROM pg_freespacemap m INNER JOIN pg_class c
ON c.relfilenode = m.relfilenode LIMIT 10;
! relname | relblocknumber | blockfreebytes
! ------------------------+----------------+----------------
! sql_features | 5 | 2696
! sql_implementation_info | 0 | 7104
! sql_languages | 0 | 8016
! sql_packages | 0 | 7376
! sql_sizing | 0 | 6032
! pg_authid | 0 | 7424
! pg_toast_2618 | 13 | 4588
! pg_toast_2618 | 12 | 1680
! pg_toast_2618 | 10 | 1436
! pg_toast_2618 | 7 | 1136
(10 rows)

regression=# 
--- 61,93 ----
-------------

regression=# \d pg_freespacemap
! View "public.pg_freespacemap"
Column | Type | Modifiers
! ----------------+---------+-----------
reltablespace | oid |
reldatabase | oid |
+ relfilenode | oid |
relblocknumber | bigint |
! bytes | integer |
View definition:
! SELECT p.reltablespace, p.reldatabase, p.relfilenode, p.relblocknumber, p.bytes
! FROM pg_freespacemap() p(reltablespace oid, reldatabase oid, relfilenode oid, relblocknumber bigint, bytes integer);

! regression=# SELECT c.relname, m.relblocknumber, m.bytes
FROM pg_freespacemap m INNER JOIN pg_class c
ON c.relfilenode = m.relfilenode LIMIT 10;
! relname | relblocknumber | bytes
! ------------------------+----------------+--------
! sql_features | 5 | 2696
! sql_implementation_info | 0 | 7104
! sql_languages | 0 | 8016
! sql_packages | 0 | 7376
! sql_sizing | 0 | 6032
! pg_authid | 0 | 7424
! pg_toast_2618 | 13 | 4588
! pg_toast_2618 | 12 | 1680
! pg_toast_2618 | 10 | 1436
! pg_toast_2618 | 7 | 1136
(10 rows)

regression=#

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--
Bruce Momjian http://candle.pha.pa.us
EnterpriseDB http://www.enterprisedb.com

+ If your life is a hard drive, Christ can be your backup. +

Patch applied. Thanks.

---------------------------------------------------------------------------

Mark Kirkwood wrote:

Mark Kirkwood wrote:

Tom Lane wrote:

I do notice a rather serious shortcoming of pg_freespacemap in its
current incarnation, which is that it *only* shows you the per-page free
space data, and not any of the information that would let you determine
what the FSM is doing to filter the raw data. The per-relation
avgRequest and lastPageCount fields would be interesting for instance.
Perhaps there should be a second view with one row per relation to
carry the appropriate data.

Ok - I did wonder about 2 views, but was unsure if the per-relation
stuff was interesting. Given that it looks like it is interesting, I'll
see about getting a second view going.

This patch implements the second view for FSM relations. I have renamed
the functions and views to be:

pg_freespacemap_relations
pg_freespacemap_pages

This patch depends on the previous one (which was called simply
'pg_freespacemap.patch').

Cheers

Mark

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--
Bruce Momjian http://candle.pha.pa.us
EnterpriseDB http://www.enterprisedb.com

+ If your life is a hard drive, Christ can be your backup. +