per backend WAL statistics

From 190422763353e87f5208e8f6d1aee823eb7860e5 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 07:51:27 +0000
Subject: [PATCH v2 1/3] Extract logic filling pg_stat_get_wal()'s tuple into
 its own routine

This commit adds pg_stat_wal_build_tuple(), a helper routine for
pg_stat_get_wal(), that fills its tuple based on the contents
of PgStat_WalStats.  This will be used in a follow-up commit that uses
the same structures as pg_stat_wal for reporting, but for the PGSTAT_KIND_BACKEND
statistics kind.
---
 src/backend/utils/adt/pgstatfuncs.c | 56 ++++++++++++++++++-----------
 1 file changed, 36 insertions(+), 20 deletions(-)
 100.0% src/backend/utils/adt/

diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 3245f3a8d8..7309f06993 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1560,20 +1560,22 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 }
 
 /*
- * Returns statistics of WAL activity
+ * pg_stat_wal_build_tuple
+ *
+ * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
+ * of wal_stats.
  */
-Datum
-pg_stat_get_wal(PG_FUNCTION_ARGS)
+static Datum
+pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
 {
-#define PG_STAT_GET_WAL_COLS	9
+#define PG_STAT_WAL_COLS	9
 	TupleDesc	tupdesc;
-	Datum		values[PG_STAT_GET_WAL_COLS] = {0};
-	bool		nulls[PG_STAT_GET_WAL_COLS] = {0};
+	Datum		values[PG_STAT_WAL_COLS] = {0};
+	bool		nulls[PG_STAT_WAL_COLS] = {0};
 	char		buf[256];
-	PgStat_WalStats *wal_stats;
 
 	/* Initialise attributes information in the tuple descriptor */
-	tupdesc = CreateTemplateTupleDesc(PG_STAT_GET_WAL_COLS);
+	tupdesc = CreateTemplateTupleDesc(PG_STAT_WAL_COLS);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "wal_records",
 					   INT8OID, -1, 0);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "wal_fpi",
@@ -1595,34 +1597,48 @@ pg_stat_get_wal(PG_FUNCTION_ARGS)
 
 	BlessTupleDesc(tupdesc);
 
-	/* Get statistics about WAL activity */
-	wal_stats = pgstat_fetch_stat_wal();
-
 	/* Fill values and NULLs */
-	values[0] = Int64GetDatum(wal_stats->wal_records);
-	values[1] = Int64GetDatum(wal_stats->wal_fpi);
+	values[0] = Int64GetDatum(wal_stats.wal_records);
+	values[1] = Int64GetDatum(wal_stats.wal_fpi);
 
 	/* Convert to numeric. */
-	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats->wal_bytes);
+	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats.wal_bytes);
 	values[2] = DirectFunctionCall3(numeric_in,
 									CStringGetDatum(buf),
 									ObjectIdGetDatum(0),
 									Int32GetDatum(-1));
 
-	values[3] = Int64GetDatum(wal_stats->wal_buffers_full);
-	values[4] = Int64GetDatum(wal_stats->wal_write);
-	values[5] = Int64GetDatum(wal_stats->wal_sync);
+	values[3] = Int64GetDatum(wal_stats.wal_buffers_full);
+	values[4] = Int64GetDatum(wal_stats.wal_write);
+	values[5] = Int64GetDatum(wal_stats.wal_sync);
 
 	/* Convert counters from microsec to millisec for display */
-	values[6] = Float8GetDatum(((double) wal_stats->wal_write_time) / 1000.0);
-	values[7] = Float8GetDatum(((double) wal_stats->wal_sync_time) / 1000.0);
+	values[6] = Float8GetDatum(((double) wal_stats.wal_write_time) / 1000.0);
+	values[7] = Float8GetDatum(((double) wal_stats.wal_sync_time) / 1000.0);
 
-	values[8] = TimestampTzGetDatum(wal_stats->stat_reset_timestamp);
+	if (wal_stats.stat_reset_timestamp != 0)
+		values[8] = TimestampTzGetDatum(wal_stats.stat_reset_timestamp);
+	else
+		nulls[8] = true;
 
 	/* Returns the record as Datum */
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns statistics of WAL activity
+ */
+Datum
+pg_stat_get_wal(PG_FUNCTION_ARGS)
+{
+	PgStat_WalStats *wal_stats;
+
+	/* Get statistics about WAL activity */
+	wal_stats = pgstat_fetch_stat_wal();
+
+	return (pg_stat_wal_build_tuple(*wal_stats));
+}
+
 /*
  * Returns statistics of SLRU caches.
  */
-- 
2.34.1

From 8477a4931e7dc61a42d8c08ccc13d6f12d7e5707 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 08:44:29 +0000
Subject: [PATCH v2 2/3] PGSTAT_KIND_BACKEND code refactoring

This commit refactors some come related to per backend statistics. It makes
the code more generic or more IO statistics focused as it will be used in a
follow-up commit that will introduce per backend WAL statistics.
---
 src/backend/utils/activity/pgstat.c          |  2 +-
 src/backend/utils/activity/pgstat_backend.c  | 88 +++++++++++++-------
 src/backend/utils/activity/pgstat_io.c       |  8 +-
 src/backend/utils/activity/pgstat_relation.c |  4 +-
 src/backend/utils/adt/pgstatfuncs.c          |  2 +-
 src/include/pgstat.h                         |  6 +-
 src/include/utils/pgstat_internal.h          |  5 +-
 src/tools/pgindent/typedefs.list             |  1 +
 8 files changed, 76 insertions(+), 40 deletions(-)
  85.6% src/backend/utils/activity/
   7.2% src/include/utils/
   4.2% src/include/

diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index 16a03b8ce1..34520535d5 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -370,7 +370,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.shared_size = sizeof(PgStatShared_Backend),
 		.shared_data_off = offsetof(PgStatShared_Backend, stats),
 		.shared_data_len = sizeof(((PgStatShared_Backend *) 0)->stats),
-		.pending_size = sizeof(PgStat_BackendPendingIO),
+		.pending_size = sizeof(PgStat_BackendPending),
 
 		.flush_pending_cb = pgstat_backend_flush_cb,
 		.reset_timestamp_cb = pgstat_backend_reset_timestamp_cb,
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 1f91bfef0a..0ae507160d 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -24,6 +24,12 @@
 
 #include "utils/pgstat_internal.h"
 
+/* flag bits for different types of statistics to flush */
+#define PGSTAT_FLUSH_IO    (1 << 0) /* Flush I/O statistics */
+#define PGSTAT_FLUSH_ALL   (PGSTAT_FLUSH_IO)
+
+static void pgstat_flush_io_entry(PgStat_EntryRef *entry_ref);
+
 /*
  * Returns statistics of a backend by proc number.
  */
@@ -39,23 +45,49 @@ pgstat_fetch_stat_backend(ProcNumber procNumber)
 }
 
 /*
- * Flush out locally pending backend statistics
- *
- * If no stats have been recorded, this function returns false.
+ * Main function to flush backend statistics.
+ * The "stats_to_flush" parameter controls which statistics to flush.
  */
-bool
-pgstat_backend_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
+static bool
+pgstat_flush_backend(PgStat_EntryRef *entry_ref, bool nowait,
+					 bits32 stats_to_flush)
 {
-	PgStatShared_Backend *shbackendioent;
-	PgStat_BackendPendingIO *pendingent;
-	PgStat_BktypeIO *bktype_shstats;
+	if (!pgstat_tracks_backend_bktype(MyBackendType))
+		return false;
+
+	/* Get our own entry_ref if not provided */
+	if (!entry_ref)
+		entry_ref = pgstat_get_entry_ref(PGSTAT_KIND_BACKEND, InvalidOid,
+										 MyProcNumber, false, NULL);
 
 	if (!pgstat_lock_entry(entry_ref, nowait))
 		return false;
 
-	shbackendioent = (PgStatShared_Backend *) entry_ref->shared_stats;
-	bktype_shstats = &shbackendioent->stats.stats;
-	pendingent = (PgStat_BackendPendingIO *) entry_ref->pending;
+	/* Flush requested statistics */
+	if (stats_to_flush & PGSTAT_FLUSH_IO)
+		pgstat_flush_io_entry(entry_ref);
+
+	pgstat_unlock_entry(entry_ref);
+
+	return true;
+}
+
+/*
+ * Flush out locally pending backend IO statistics.
+ * Locking is managed by the caller.
+ */
+static void
+pgstat_flush_io_entry(PgStat_EntryRef *entry_ref)
+{
+	PgStatShared_Backend *shbackendent;
+	PgStat_BackendPending *pendingent;
+	PgStat_BktypeIO *bktype_shstats;
+	PgStat_BackendPendingIO *pending_io;
+
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	pendingent = (PgStat_BackendPending *) entry_ref->pending;
+	bktype_shstats = &shbackendent->stats.io_stats;
+	pending_io = &pendingent->pending_io;
 
 	for (int io_object = 0; io_object < IOOBJECT_NUM_TYPES; io_object++)
 	{
@@ -66,35 +98,35 @@ pgstat_backend_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
 				instr_time	time;
 
 				bktype_shstats->counts[io_object][io_context][io_op] +=
-					pendingent->counts[io_object][io_context][io_op];
+					pending_io->counts[io_object][io_context][io_op];
 
-				time = pendingent->pending_times[io_object][io_context][io_op];
+				time = pending_io->pending_times[io_object][io_context][io_op];
 
 				bktype_shstats->times[io_object][io_context][io_op] +=
 					INSTR_TIME_GET_MICROSEC(time);
 			}
 		}
 	}
+}
 
-	pgstat_unlock_entry(entry_ref);
-
-	return true;
+/*
+ * Flush out locally pending backend statistics
+ *
+ * If no stats have been recorded, this function returns false.
+ */
+bool
+pgstat_backend_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
+{
+	return pgstat_flush_backend(entry_ref, nowait, PGSTAT_FLUSH_ALL);
 }
 
 /*
- * Simpler wrapper of pgstat_backend_flush_cb()
+ * Convenience wrapper to flush I/O statistics.
  */
 void
-pgstat_flush_backend(bool nowait)
+pgstat_backend_flush_io(bool nowait)
 {
-	PgStat_EntryRef *entry_ref;
-
-	if (!pgstat_tracks_backend_bktype(MyBackendType))
-		return;
-
-	entry_ref = pgstat_get_entry_ref(PGSTAT_KIND_BACKEND, InvalidOid,
-									 MyProcNumber, false, NULL);
-	(void) pgstat_backend_flush_cb(entry_ref, nowait);
+	pgstat_flush_backend(NULL, nowait, PGSTAT_FLUSH_IO);
 }
 
 /*
@@ -119,9 +151,9 @@ pgstat_create_backend(ProcNumber procnum)
 }
 
 /*
- * Find or create a local PgStat_BackendPendingIO entry for proc number.
+ * Find or create a local PgStat_BackendPending entry for proc number.
  */
-PgStat_BackendPendingIO *
+PgStat_BackendPending *
 pgstat_prep_backend_pending(ProcNumber procnum)
 {
 	PgStat_EntryRef *entry_ref;
diff --git a/src/backend/utils/activity/pgstat_io.c b/src/backend/utils/activity/pgstat_io.c
index f9a1f91dba..a7445995d3 100644
--- a/src/backend/utils/activity/pgstat_io.c
+++ b/src/backend/utils/activity/pgstat_io.c
@@ -81,10 +81,10 @@ pgstat_count_io_op_n(IOObject io_object, IOContext io_context, IOOp io_op, uint3
 
 	if (pgstat_tracks_backend_bktype(MyBackendType))
 	{
-		PgStat_PendingIO *entry_ref;
+		PgStat_BackendPending *entry_ref;
 
 		entry_ref = pgstat_prep_backend_pending(MyProcNumber);
-		entry_ref->counts[io_object][io_context][io_op] += cnt;
+		entry_ref->pending_io.counts[io_object][io_context][io_op] += cnt;
 	}
 
 	PendingIOStats.counts[io_object][io_context][io_op] += cnt;
@@ -151,10 +151,10 @@ pgstat_count_io_op_time(IOObject io_object, IOContext io_context, IOOp io_op,
 
 		if (pgstat_tracks_backend_bktype(MyBackendType))
 		{
-			PgStat_PendingIO *entry_ref;
+			PgStat_BackendPending *entry_ref;
 
 			entry_ref = pgstat_prep_backend_pending(MyProcNumber);
-			INSTR_TIME_ADD(entry_ref->pending_times[io_object][io_context][io_op],
+			INSTR_TIME_ADD(entry_ref->pending_io.pending_times[io_object][io_context][io_op],
 						   io_time);
 		}
 	}
diff --git a/src/backend/utils/activity/pgstat_relation.c b/src/backend/utils/activity/pgstat_relation.c
index 2cc304f881..6092826479 100644
--- a/src/backend/utils/activity/pgstat_relation.c
+++ b/src/backend/utils/activity/pgstat_relation.c
@@ -264,7 +264,7 @@ pgstat_report_vacuum(Oid tableoid, bool shared,
 	 * VACUUM command has processed all tables and committed.
 	 */
 	pgstat_flush_io(false);
-	pgstat_flush_backend(false);
+	pgstat_backend_flush_io(false);
 }
 
 /*
@@ -351,7 +351,7 @@ pgstat_report_analyze(Relation rel,
 
 	/* see pgstat_report_vacuum() */
 	pgstat_flush_io(false);
-	pgstat_flush_backend(false);
+	pgstat_backend_flush_io(false);
 }
 
 /*
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 7309f06993..8a4340e977 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1544,7 +1544,7 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 	if (bktype == B_INVALID)
 		return (Datum) 0;
 
-	bktype_stats = &backend_stats->stats;
+	bktype_stats = &backend_stats->io_stats;
 
 	/*
 	 * In Assert builds, we can afford an extra loop through all of the
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 0d8427f27d..6631bd2d73 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -381,7 +381,7 @@ typedef PgStat_PendingIO PgStat_BackendPendingIO;
 typedef struct PgStat_Backend
 {
 	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO stats;
+	PgStat_BktypeIO io_stats;
 } PgStat_Backend;
 
 typedef struct PgStat_StatDBEntry
@@ -523,6 +523,10 @@ typedef struct PgStat_PendingWalStats
 	instr_time	wal_sync_time;
 } PgStat_PendingWalStats;
 
+typedef struct PgStat_BackendPending
+{
+	PgStat_BackendPendingIO pending_io;
+} PgStat_BackendPending;
 
 /*
  * Functions in pgstat.c
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 52eb008710..320cd5a842 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -613,9 +613,8 @@ extern void pgstat_archiver_snapshot_cb(void);
  * Functions in pgstat_backend.c
  */
 
-extern void pgstat_flush_backend(bool nowait);
-
-extern PgStat_BackendPendingIO *pgstat_prep_backend_pending(ProcNumber procnum);
+extern void pgstat_backend_flush_io(bool nowait);
+extern PgStat_BackendPending *pgstat_prep_backend_pending(ProcNumber procnum);
 extern bool pgstat_backend_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
 extern void pgstat_backend_reset_timestamp_cb(PgStatShared_Common *header, TimestampTz ts);
 
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 9f83ecf181..f15526236a 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2140,6 +2140,7 @@ PgStatShared_Subscription
 PgStatShared_Wal
 PgStat_ArchiverStats
 PgStat_Backend
+PgStat_BackendPending
 PgStat_BackendPendingIO
 PgStat_BackendSubEntry
 PgStat_BgWriterStats
-- 
2.34.1

From 158dd7d6da4ee39a1743c44ca6385aba33a826e2 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 10:00:00 +0000
Subject: [PATCH v2 3/3] per backend WAL statistics

Now that commit 9aea73fc61 added backend-level statistics to pgstats (and
per backend IO statistics) we can more easily add per backend statistics.

This commit adds per backend WAL statistics using the same layer as pg_stat_wal,
except that it is now possible to know how much WAL activity is happening in each
backend rather than an overall aggregate of all the activity.  A function called
pg_stat_get_backend_wal() is added to access this data depending on the
PID of a backend.

The same limitation as in 9aea73fc61 persists, meaning that Auxiliary processes
are not included in this set of statistics.

XXX: bump catalog version
---
 doc/src/sgml/config.sgml                    |  4 +-
 doc/src/sgml/monitoring.sgml                | 19 ++++
 src/backend/access/transam/xlog.c           | 36 +++++++-
 src/backend/utils/activity/pgstat_backend.c | 99 ++++++++++++++++++++-
 src/backend/utils/activity/pgstat_wal.c     |  6 +-
 src/backend/utils/adt/pgstatfuncs.c         | 78 +++++++++++++---
 src/include/catalog/pg_proc.dat             |  7 ++
 src/include/pgstat.h                        | 37 ++++++--
 src/include/utils/pgstat_internal.h         |  1 +
 src/test/regress/expected/stats.out         | 14 +++
 src/test/regress/sql/stats.sql              |  6 ++
 src/tools/pgindent/typedefs.list            |  2 +
 12 files changed, 279 insertions(+), 30 deletions(-)
   9.9% doc/src/sgml/
   8.6% src/backend/access/transam/
  34.4% src/backend/utils/activity/
  27.8% src/backend/utils/adt/
   4.9% src/include/catalog/
   6.6% src/include/
   3.9% src/test/regress/expected/
   3.0% src/test/regress/sql/

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 8683f0bdf5..8e8478dcb1 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8433,7 +8433,9 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
         measure the overhead of timing on your system.
         I/O timing information is
         displayed in <link linkend="monitoring-pg-stat-wal-view">
-        <structname>pg_stat_wal</structname></link>.
+        <structname>pg_stat_wal</structname></link> and in the output of the
+        <link linkend="pg-stat-get-backend-wal">
+        <function>pg_stat_get_backend_wal()</function></link> function.
         Only superusers and users with the appropriate <literal>SET</literal>
         privilege can change this setting.
        </para>
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index d0d176cc54..84a2d09b76 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -4811,6 +4811,25 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para></entry>
       </row>
 
+      <row>
+       <entry id="pg-stat-get-backend-wal" role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_stat_get_backend_wal</primary>
+        </indexterm>
+        <function>pg_stat_get_backend_wal</function> ( <type>integer</type> )
+        <returnvalue>record</returnvalue>
+       </para>
+       <para>
+        Returns WAL statistics about the backend with the specified
+        process ID. The output fields are exactly the same as the ones in the
+        <structname>pg_stat_wal</structname> view.
+       </para>
+       <para>
+        The function does not return WAL statistics for the checkpointer,
+        the background writer, the startup process and the autovacuum launcher.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index bf3dbda901..0ba9fcb277 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -2058,6 +2058,10 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
 					XLogWrite(WriteRqst, tli, false);
 					LWLockRelease(WALWriteLock);
 					PendingWalStats.wal_buffers_full++;
+
+					if (pgstat_tracks_backend_bktype(MyBackendType))
+						PendingBackendWalStats.wal_buffers_full++;
+
 					TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_DONE();
 				}
 				/* Re-acquire WALBufMappingLock and retry */
@@ -2426,11 +2430,14 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
 			Size		nleft;
 			ssize_t		written;
 			instr_time	start;
+			instr_time	end;
 
 			/* OK to write the page(s) */
 			from = XLogCtl->pages + startidx * (Size) XLOG_BLCKSZ;
 			nbytes = npages * (Size) XLOG_BLCKSZ;
 			nleft = nbytes;
+			/* keep compiler quiet */
+			INSTR_TIME_SET_ZERO(end);
 			do
 			{
 				errno = 0;
@@ -2451,14 +2458,26 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
 				 */
 				if (track_wal_io_timing)
 				{
-					instr_time	end;
-
 					INSTR_TIME_SET_CURRENT(end);
 					INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_write_time, end, start);
 				}
 
 				PendingWalStats.wal_write++;
 
+				if (pgstat_tracks_backend_bktype(MyBackendType))
+				{
+					/*
+					 * We are inside a critical section, so we can't use
+					 * pgstat_prep_pending_entry() and we rely on
+					 * PendingBackendWalStats instead.
+					 */
+					PendingBackendWalStats.wal_write++;
+
+					if (track_wal_io_timing)
+						INSTR_TIME_ACCUM_DIFF(PendingBackendWalStats.wal_write_time,
+											  end, start);
+				}
+
 				if (written <= 0)
 				{
 					char		xlogfname[MAXFNAMELEN];
@@ -8684,8 +8703,11 @@ issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
 {
 	char	   *msg = NULL;
 	instr_time	start;
+	instr_time	end;
 
 	Assert(tli != 0);
+	/* keep compiler quiet */
+	INSTR_TIME_SET_ZERO(end);
 
 	/*
 	 * Quick exit if fsync is disabled or write() has already synced the WAL
@@ -8751,13 +8773,19 @@ issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
 	 */
 	if (track_wal_io_timing)
 	{
-		instr_time	end;
-
 		INSTR_TIME_SET_CURRENT(end);
 		INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_sync_time, end, start);
 	}
 
 	PendingWalStats.wal_sync++;
+
+	if (pgstat_tracks_backend_bktype(MyBackendType))
+	{
+		PendingBackendWalStats.wal_sync++;
+
+		if (track_wal_io_timing)
+			INSTR_TIME_ACCUM_DIFF(PendingBackendWalStats.wal_sync_time, end, start);
+	}
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 0ae507160d..58c41edea4 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -26,9 +26,21 @@
 
 /* flag bits for different types of statistics to flush */
 #define PGSTAT_FLUSH_IO    (1 << 0) /* Flush I/O statistics */
-#define PGSTAT_FLUSH_ALL   (PGSTAT_FLUSH_IO)
+#define PGSTAT_FLUSH_WAL   (1 << 1) /* Flush WAL statistics */
+#define PGSTAT_FLUSH_ALL   (PGSTAT_FLUSH_IO | PGSTAT_FLUSH_WAL)
+
+PgStat_PendingWalStats PendingBackendWalStats = {0};
+
+/*
+ * WAL usage counters saved from pgWalUsage at the previous call to
+ * pgstat_report_wal(). This is used to calculate how much WAL usage
+ * happens between pgstat_report_wal() calls, by subtracting
+ * the previous counters from the current ones.
+ */
+static WalUsage prevBackendWalUsage;
 
 static void pgstat_flush_io_entry(PgStat_EntryRef *entry_ref);
+static void pgstat_flush_wal_entry(PgStat_EntryRef *entry_ref);
 
 /*
  * Returns statistics of a backend by proc number.
@@ -67,6 +79,9 @@ pgstat_flush_backend(PgStat_EntryRef *entry_ref, bool nowait,
 	if (stats_to_flush & PGSTAT_FLUSH_IO)
 		pgstat_flush_io_entry(entry_ref);
 
+	if (stats_to_flush & PGSTAT_FLUSH_WAL)
+		pgstat_flush_wal_entry(entry_ref);
+
 	pgstat_unlock_entry(entry_ref);
 
 	return true;
@@ -109,6 +124,75 @@ pgstat_flush_io_entry(PgStat_EntryRef *entry_ref)
 	}
 }
 
+/*
+ * To determine whether any WAL activity has occurred since last time, not
+ * only the number of generated WAL records but also the numbers of WAL
+ * writes and syncs need to be checked. Because even transaction that
+ * generates no WAL records can write or sync WAL data when flushing the
+ * data pages.
+ */
+static bool
+pgstat_backend_wal_have_pending(void)
+{
+	return pgWalUsage.wal_records != prevBackendWalUsage.wal_records ||
+		PendingBackendWalStats.wal_write != 0 ||
+		PendingBackendWalStats.wal_sync != 0;
+}
+
+/*
+ * Flush out locally pending backend WAL statistics.
+ * Locking is managed by the caller.
+ */
+static void
+pgstat_flush_wal_entry(PgStat_EntryRef *entry_ref)
+{
+	PgStatShared_Backend *shbackendent;
+	PgStat_WalCounters *bktype_shstats;
+	WalUsage	wal_usage_diff = {0};
+
+	/*
+	 * This function can be called even if nothing at all has happened. Avoid
+	 * taking lock for nothing in that case.
+	 */
+	if (!pgstat_backend_wal_have_pending())
+		return;
+
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	bktype_shstats = &shbackendent->stats.wal_stats;
+
+	/*
+	 * We don't update the WAL usage portion of the local WalStats elsewhere.
+	 * Calculate how much WAL usage counters were increased by subtracting the
+	 * previous counters from the current ones.
+	 */
+	WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
+
+#define WALSTAT_ACC(fld, var_to_add) \
+	(bktype_shstats->fld += var_to_add.fld)
+#define WALSTAT_ACC_INSTR_TIME(fld) \
+	(bktype_shstats->fld += INSTR_TIME_GET_MICROSEC(PendingBackendWalStats.fld))
+	WALSTAT_ACC(wal_buffers_full, PendingBackendWalStats);
+	WALSTAT_ACC(wal_write, PendingBackendWalStats);
+	WALSTAT_ACC(wal_sync, PendingBackendWalStats);
+	WALSTAT_ACC(wal_records, wal_usage_diff);
+	WALSTAT_ACC(wal_fpi, wal_usage_diff);
+	WALSTAT_ACC(wal_bytes, wal_usage_diff);
+	WALSTAT_ACC_INSTR_TIME(wal_write_time);
+	WALSTAT_ACC_INSTR_TIME(wal_sync_time);
+#undef WALSTAT_ACC_INSTR_TIME
+#undef WALSTAT_ACC
+
+	/*
+	 * Save the current counters for the subsequent calculation of WAL usage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
+
+	/*
+	 * Clear out the statistics buffer, so it can be re-used.
+	 */
+	MemSet(&PendingBackendWalStats, 0, sizeof(PendingWalStats));
+}
+
 /*
  * Flush out locally pending backend statistics
  *
@@ -129,6 +213,15 @@ pgstat_backend_flush_io(bool nowait)
 	pgstat_flush_backend(NULL, nowait, PGSTAT_FLUSH_IO);
 }
 
+/*
+ * Convenience wrapper to flush WAL statistics.
+ */
+void
+pgstat_backend_flush_wal(bool nowait)
+{
+	pgstat_flush_backend(NULL, nowait, PGSTAT_FLUSH_WAL);
+}
+
 /*
  * Create backend statistics entry for proc number.
  */
@@ -148,10 +241,12 @@ pgstat_create_backend(ProcNumber procnum)
 	 * e.g. if we previously used this proc number.
 	 */
 	memset(&shstatent->stats, 0, sizeof(shstatent->stats));
+
+	prevBackendWalUsage = pgWalUsage;
 }
 
 /*
- * Find or create a local PgStat_BackendPending entry for proc number.
+ * Find or create a local PgStat_BackendPendingIO entry for proc number.
  */
 PgStat_BackendPending *
 pgstat_prep_backend_pending(ProcNumber procnum)
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 18fa6b2936..160ce99a4c 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -55,6 +55,8 @@ pgstat_report_wal(bool force)
 	/* flush wal stats */
 	pgstat_flush_wal(nowait);
 
+	pgstat_backend_flush_wal(nowait);
+
 	/* flush IO stats */
 	pgstat_flush_io(nowait);
 }
@@ -117,9 +119,9 @@ pgstat_wal_flush_cb(bool nowait)
 		return true;
 
 #define WALSTAT_ACC(fld, var_to_add) \
-	(stats_shmem->stats.fld += var_to_add.fld)
+	(stats_shmem->stats.wal_counters.fld += var_to_add.fld)
 #define WALSTAT_ACC_INSTR_TIME(fld) \
-	(stats_shmem->stats.fld += INSTR_TIME_GET_MICROSEC(PendingWalStats.fld))
+	(stats_shmem->stats.wal_counters.fld += INSTR_TIME_GET_MICROSEC(PendingWalStats.fld))
 	WALSTAT_ACC(wal_records, wal_usage_diff);
 	WALSTAT_ACC(wal_fpi, wal_usage_diff);
 	WALSTAT_ACC(wal_bytes, wal_usage_diff);
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 8a4340e977..8d251dfcca 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1562,11 +1562,12 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 /*
  * pg_stat_wal_build_tuple
  *
- * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
- * of wal_stats.
+ * Helper routine for pg_stat_get_wal() and pg_stat_get_backend_wal() returning
+ * one tuple based on the contents of wal_counters.
  */
 static Datum
-pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
+pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
+						TimestampTz stat_reset_timestamp)
 {
 #define PG_STAT_WAL_COLS	9
 	TupleDesc	tupdesc;
@@ -1598,26 +1599,26 @@ pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
 	BlessTupleDesc(tupdesc);
 
 	/* Fill values and NULLs */
-	values[0] = Int64GetDatum(wal_stats.wal_records);
-	values[1] = Int64GetDatum(wal_stats.wal_fpi);
+	values[0] = Int64GetDatum(wal_counters.wal_records);
+	values[1] = Int64GetDatum(wal_counters.wal_fpi);
 
 	/* Convert to numeric. */
-	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats.wal_bytes);
+	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_counters.wal_bytes);
 	values[2] = DirectFunctionCall3(numeric_in,
 									CStringGetDatum(buf),
 									ObjectIdGetDatum(0),
 									Int32GetDatum(-1));
 
-	values[3] = Int64GetDatum(wal_stats.wal_buffers_full);
-	values[4] = Int64GetDatum(wal_stats.wal_write);
-	values[5] = Int64GetDatum(wal_stats.wal_sync);
+	values[3] = Int64GetDatum(wal_counters.wal_buffers_full);
+	values[4] = Int64GetDatum(wal_counters.wal_write);
+	values[5] = Int64GetDatum(wal_counters.wal_sync);
 
 	/* Convert counters from microsec to millisec for display */
-	values[6] = Float8GetDatum(((double) wal_stats.wal_write_time) / 1000.0);
-	values[7] = Float8GetDatum(((double) wal_stats.wal_sync_time) / 1000.0);
+	values[6] = Float8GetDatum(((double) wal_counters.wal_write_time) / 1000.0);
+	values[7] = Float8GetDatum(((double) wal_counters.wal_sync_time) / 1000.0);
 
-	if (wal_stats.stat_reset_timestamp != 0)
-		values[8] = TimestampTzGetDatum(wal_stats.stat_reset_timestamp);
+	if (stat_reset_timestamp != 0)
+		values[8] = TimestampTzGetDatum(stat_reset_timestamp);
 	else
 		nulls[8] = true;
 
@@ -1625,6 +1626,55 @@ pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns WAL statistics for a backend with given PID.
+ */
+Datum
+pg_stat_get_backend_wal(PG_FUNCTION_ARGS)
+{
+	int			pid;
+	PGPROC	   *proc;
+	ProcNumber	procNumber;
+	PgStat_Backend *backend_stats;
+	PgStat_WalCounters bktype_stats;
+	PgBackendStatus *beentry;
+
+	pid = PG_GETARG_INT32(0);
+	proc = BackendPidGetProc(pid);
+
+	/*
+	 * This could be an auxiliary process but these do not report backend
+	 * statistics due to pgstat_tracks_backend_bktype(), so there is no need
+	 * for an extra call to AuxiliaryPidGetProc().
+	 */
+	if (!proc)
+		PG_RETURN_NULL();
+
+	procNumber = GetNumberFromPGProc(proc);
+
+	beentry = pgstat_get_beentry_by_proc_number(procNumber);
+	if (!beentry)
+		PG_RETURN_NULL();
+
+	backend_stats = pgstat_fetch_stat_backend(procNumber);
+	if (!backend_stats)
+		PG_RETURN_NULL();
+
+	/* if PID does not match, leave */
+	if (beentry->st_procpid != pid)
+		PG_RETURN_NULL();
+
+	/* backend may be gone, so recheck in case */
+	if (beentry->st_backendType == B_INVALID)
+		PG_RETURN_NULL();
+
+	bktype_stats = backend_stats->wal_stats;
+
+	/* save tuples with data from this PgStat_BktypeIO */
+	return (pg_stat_wal_build_tuple(bktype_stats, backend_stats->stat_reset_timestamp));
+}
+
+
 /*
  * Returns statistics of WAL activity
  */
@@ -1636,7 +1686,7 @@ pg_stat_get_wal(PG_FUNCTION_ARGS)
 	/* Get statistics about WAL activity */
 	wal_stats = pgstat_fetch_stat_wal();
 
-	return (pg_stat_wal_build_tuple(*wal_stats));
+	return (pg_stat_wal_build_tuple(wal_stats->wal_counters, wal_stats->stat_reset_timestamp));
 }
 
 /*
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index b37e8a6f88..72a5dae4b1 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5929,6 +5929,13 @@
   proargmodes => '{o,o,o,o,o,o,o,o,o}',
   proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,wal_write,wal_sync,wal_write_time,wal_sync_time,stats_reset}',
   prosrc => 'pg_stat_get_wal' },
+{ oid => '8037', descr => 'statistics: backend WAL activity',
+  proname => 'pg_stat_get_backend_wal', provolatile => 'v',
+  proparallel => 'r', prorettype => 'record', proargtypes => 'int4',
+  proallargtypes => '{int4,int8,int8,numeric,int8,int8,int8,float8,float8,timestamptz}',
+  proargmodes => '{i,o,o,o,o,o,o,o,o,o}',
+  proargnames => '{backend_pid,wal_records,wal_fpi,wal_bytes,wal_buffers_full,wal_write,wal_sync,wal_write_time,wal_sync_time,stats_reset}',
+  prosrc => 'pg_stat_get_backend_wal' },
 { oid => '6248', descr => 'statistics: information about WAL prefetching',
   proname => 'pg_stat_get_recovery_prefetch', prorows => '1', proretset => 't',
   provolatile => 'v', prorettype => 'record', proargtypes => '',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 6631bd2d73..045877c5a8 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -378,12 +378,6 @@ typedef struct PgStat_IO
 /* Backend statistics store the same amount of IO data as PGSTAT_KIND_IO */
 typedef PgStat_PendingIO PgStat_BackendPendingIO;
 
-typedef struct PgStat_Backend
-{
-	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO io_stats;
-} PgStat_Backend;
-
 typedef struct PgStat_StatDBEntry
 {
 	PgStat_Counter xact_commit;
@@ -495,7 +489,7 @@ typedef struct PgStat_StatTabEntry
 	PgStat_Counter autoanalyze_count;
 } PgStat_StatTabEntry;
 
-typedef struct PgStat_WalStats
+typedef struct PgStat_WalCounters
 {
 	PgStat_Counter wal_records;
 	PgStat_Counter wal_fpi;
@@ -505,6 +499,11 @@ typedef struct PgStat_WalStats
 	PgStat_Counter wal_sync;
 	PgStat_Counter wal_write_time;
 	PgStat_Counter wal_sync_time;
+} PgStat_WalCounters;
+
+typedef struct PgStat_WalStats
+{
+	PgStat_WalCounters wal_counters;
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
@@ -523,9 +522,21 @@ typedef struct PgStat_PendingWalStats
 	instr_time	wal_sync_time;
 } PgStat_PendingWalStats;
 
+typedef struct PgStat_Backend
+{
+	TimestampTz stat_reset_timestamp;
+	PgStat_BktypeIO io_stats;
+	PgStat_WalCounters wal_stats;
+} PgStat_Backend;
+
 typedef struct PgStat_BackendPending
 {
 	PgStat_BackendPendingIO pending_io;
+
+	/*
+	 * We are not creating one member for PgStat_PendingWalStats. See the
+	 * comment above the PendingBackendWalStats definition as to why.
+	 */
 } PgStat_BackendPending;
 
 /*
@@ -857,5 +868,17 @@ extern PGDLLIMPORT SessionEndType pgStatSessionEndCause;
 /* updated directly by backends and background processes */
 extern PGDLLIMPORT PgStat_PendingWalStats PendingWalStats;
 
+/*
+ * Variables in pgstat_backend.c
+ */
+
+/* updated directly by backends and background processes */
+
+/*
+ * WAL pending statistics are incremented inside a critical section
+ * (see XLogWrite()), so we can't use pgstat_prep_pending_entry() and we rely on
+ * PendingBackendWalStats instead.
+ */
+extern PGDLLIMPORT PgStat_PendingWalStats PendingBackendWalStats;
 
 #endif							/* PGSTAT_H */
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 320cd5a842..06681b9102 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -614,6 +614,7 @@ extern void pgstat_archiver_snapshot_cb(void);
  */
 
 extern void pgstat_backend_flush_io(bool nowait);
+extern void pgstat_backend_flush_wal(bool nowait);
 extern PgStat_BackendPending *pgstat_prep_backend_pending(ProcNumber procnum);
 extern bool pgstat_backend_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
 extern void pgstat_backend_reset_timestamp_cb(PgStatShared_Common *header, TimestampTz ts);
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index a0317b7208..cc01fdf274 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -832,6 +832,8 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -851,6 +853,18 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
  t
 (1 row)
 
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+ ?column? 
+----------
+ t
+(1 row)
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 399c72bbcf..28fe0a1a7d 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -423,6 +423,9 @@ SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
 
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 
@@ -435,6 +438,9 @@ CHECKPOINT;
 SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;
 SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
 
+SELECT pg_stat_force_next_flush();
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index f15526236a..b593d8601e 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2159,6 +2159,7 @@ PgStat_KindInfo
 PgStat_LocalState
 PgStat_PendingDroppedStatsItem
 PgStat_PendingIO
+PgStat_PendingBackendWalStats
 PgStat_PendingWalStats
 PgStat_SLRUStats
 PgStat_ShmemControl
@@ -2175,6 +2176,7 @@ PgStat_SubXactStatus
 PgStat_TableCounts
 PgStat_TableStatus
 PgStat_TableXactStatus
+PgStat_WalCounters
 PgStat_WalStats
 PgXmlErrorContext
 PgXmlStrictness
-- 
2.34.1

From eba357e4e2712cd73fdad585f6d8088de0dbaccc Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 08:44:29 +0000
Subject: [PATCH v3] PGSTAT_KIND_BACKEND code refactoring

This commit refactors some come related to per backend statistics. It makes
the code more generic or more IO statistics focused as it will be used in a
follow-up commit that will introduce per backend WAL statistics.
---
 src/include/pgstat.h                         |  6 +-
 src/include/utils/pgstat_internal.h          |  7 +-
 src/backend/utils/activity/pgstat.c          |  2 +-
 src/backend/utils/activity/pgstat_backend.c  | 69 ++++++++++++++------
 src/backend/utils/activity/pgstat_io.c       |  8 +--
 src/backend/utils/activity/pgstat_relation.c |  4 +-
 src/backend/utils/adt/pgstatfuncs.c          |  2 +-
 src/tools/pgindent/typedefs.list             |  1 +
 8 files changed, 68 insertions(+), 31 deletions(-)

diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 0d8427f27d1..6631bd2d730 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -381,7 +381,7 @@ typedef PgStat_PendingIO PgStat_BackendPendingIO;
 typedef struct PgStat_Backend
 {
 	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO stats;
+	PgStat_BktypeIO io_stats;
 } PgStat_Backend;
 
 typedef struct PgStat_StatDBEntry
@@ -523,6 +523,10 @@ typedef struct PgStat_PendingWalStats
 	instr_time	wal_sync_time;
 } PgStat_PendingWalStats;
 
+typedef struct PgStat_BackendPending
+{
+	PgStat_BackendPendingIO pending_io;
+} PgStat_BackendPending;
 
 /*
  * Functions in pgstat.c
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 52eb008710f..4bb8e5c53ab 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -613,9 +613,12 @@ extern void pgstat_archiver_snapshot_cb(void);
  * Functions in pgstat_backend.c
  */
 
-extern void pgstat_flush_backend(bool nowait);
+/* flags for pgstat_flush_backend() */
+#define PGSTAT_BACKEND_FLUSH_IO		(1 << 0)	/* Flush I/O statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO)
 
-extern PgStat_BackendPendingIO *pgstat_prep_backend_pending(ProcNumber procnum);
+extern void pgstat_flush_backend(bool nowait, bits32 flags);
+extern PgStat_BackendPending *pgstat_prep_backend_pending(ProcNumber procnum);
 extern bool pgstat_backend_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
 extern void pgstat_backend_reset_timestamp_cb(PgStatShared_Common *header, TimestampTz ts);
 
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index 16a03b8ce15..34520535d54 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -370,7 +370,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.shared_size = sizeof(PgStatShared_Backend),
 		.shared_data_off = offsetof(PgStatShared_Backend, stats),
 		.shared_data_len = sizeof(((PgStatShared_Backend *) 0)->stats),
-		.pending_size = sizeof(PgStat_BackendPendingIO),
+		.pending_size = sizeof(PgStat_BackendPending),
 
 		.flush_pending_cb = pgstat_backend_flush_cb,
 		.reset_timestamp_cb = pgstat_backend_reset_timestamp_cb,
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 1f91bfef0a3..3972bcf9456 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -39,23 +39,21 @@ pgstat_fetch_stat_backend(ProcNumber procNumber)
 }
 
 /*
- * Flush out locally pending backend statistics
- *
- * If no stats have been recorded, this function returns false.
+ * Flush out locally pending backend IO statistics.  Locking is managed
+ * by the caller.
  */
-bool
-pgstat_backend_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
+static void
+pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 {
-	PgStatShared_Backend *shbackendioent;
-	PgStat_BackendPendingIO *pendingent;
+	PgStatShared_Backend *shbackendent;
+	PgStat_BackendPending *pendingent;
 	PgStat_BktypeIO *bktype_shstats;
+	PgStat_BackendPendingIO *pending_io;
 
-	if (!pgstat_lock_entry(entry_ref, nowait))
-		return false;
-
-	shbackendioent = (PgStatShared_Backend *) entry_ref->shared_stats;
-	bktype_shstats = &shbackendioent->stats.stats;
-	pendingent = (PgStat_BackendPendingIO *) entry_ref->pending;
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	pendingent = (PgStat_BackendPending *) entry_ref->pending;
+	bktype_shstats = &shbackendent->stats.io_stats;
+	pending_io = &pendingent->pending_io;
 
 	for (int io_object = 0; io_object < IOOBJECT_NUM_TYPES; io_object++)
 	{
@@ -66,15 +64,33 @@ pgstat_backend_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
 				instr_time	time;
 
 				bktype_shstats->counts[io_object][io_context][io_op] +=
-					pendingent->counts[io_object][io_context][io_op];
+					pending_io->counts[io_object][io_context][io_op];
 
-				time = pendingent->pending_times[io_object][io_context][io_op];
+				time = pending_io->pending_times[io_object][io_context][io_op];
 
 				bktype_shstats->times[io_object][io_context][io_op] +=
 					INSTR_TIME_GET_MICROSEC(time);
 			}
 		}
 	}
+}
+
+/*
+ * Wrapper routine to flush backend statistics.
+ */
+static bool
+pgstat_flush_backend_entry(PgStat_EntryRef *entry_ref, bool nowait,
+						   bits32 flags)
+{
+	if (!pgstat_tracks_backend_bktype(MyBackendType))
+		return false;
+
+	if (!pgstat_lock_entry(entry_ref, nowait))
+		return false;
+
+	/* Flush requested statistics */
+	if (flags & PGSTAT_BACKEND_FLUSH_IO)
+		pgstat_flush_backend_entry_io(entry_ref);
 
 	pgstat_unlock_entry(entry_ref);
 
@@ -82,10 +98,23 @@ pgstat_backend_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
 }
 
 /*
- * Simpler wrapper of pgstat_backend_flush_cb()
+ * Callback to flush out locally pending backend statistics.
+ *
+ * If no stats have been recorded, this function returns false.
+ */
+bool
+pgstat_backend_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
+{
+	return pgstat_flush_backend_entry(entry_ref, nowait, PGSTAT_BACKEND_FLUSH_ALL);
+}
+
+/*
+ * Flush out locally pending backend statistics
+ *
+ * "flags" parameter controls which statistics to flush.
  */
 void
-pgstat_flush_backend(bool nowait)
+pgstat_flush_backend(bool nowait, bits32 flags)
 {
 	PgStat_EntryRef *entry_ref;
 
@@ -94,7 +123,7 @@ pgstat_flush_backend(bool nowait)
 
 	entry_ref = pgstat_get_entry_ref(PGSTAT_KIND_BACKEND, InvalidOid,
 									 MyProcNumber, false, NULL);
-	(void) pgstat_backend_flush_cb(entry_ref, nowait);
+	(void) pgstat_flush_backend_entry(entry_ref, nowait, flags);
 }
 
 /*
@@ -119,9 +148,9 @@ pgstat_create_backend(ProcNumber procnum)
 }
 
 /*
- * Find or create a local PgStat_BackendPendingIO entry for proc number.
+ * Find or create a local PgStat_BackendPending entry for proc number.
  */
-PgStat_BackendPendingIO *
+PgStat_BackendPending *
 pgstat_prep_backend_pending(ProcNumber procnum)
 {
 	PgStat_EntryRef *entry_ref;
diff --git a/src/backend/utils/activity/pgstat_io.c b/src/backend/utils/activity/pgstat_io.c
index f9a1f91dba8..a7445995d32 100644
--- a/src/backend/utils/activity/pgstat_io.c
+++ b/src/backend/utils/activity/pgstat_io.c
@@ -81,10 +81,10 @@ pgstat_count_io_op_n(IOObject io_object, IOContext io_context, IOOp io_op, uint3
 
 	if (pgstat_tracks_backend_bktype(MyBackendType))
 	{
-		PgStat_PendingIO *entry_ref;
+		PgStat_BackendPending *entry_ref;
 
 		entry_ref = pgstat_prep_backend_pending(MyProcNumber);
-		entry_ref->counts[io_object][io_context][io_op] += cnt;
+		entry_ref->pending_io.counts[io_object][io_context][io_op] += cnt;
 	}
 
 	PendingIOStats.counts[io_object][io_context][io_op] += cnt;
@@ -151,10 +151,10 @@ pgstat_count_io_op_time(IOObject io_object, IOContext io_context, IOOp io_op,
 
 		if (pgstat_tracks_backend_bktype(MyBackendType))
 		{
-			PgStat_PendingIO *entry_ref;
+			PgStat_BackendPending *entry_ref;
 
 			entry_ref = pgstat_prep_backend_pending(MyProcNumber);
-			INSTR_TIME_ADD(entry_ref->pending_times[io_object][io_context][io_op],
+			INSTR_TIME_ADD(entry_ref->pending_io.pending_times[io_object][io_context][io_op],
 						   io_time);
 		}
 	}
diff --git a/src/backend/utils/activity/pgstat_relation.c b/src/backend/utils/activity/pgstat_relation.c
index 2cc304f8812..09247ba0971 100644
--- a/src/backend/utils/activity/pgstat_relation.c
+++ b/src/backend/utils/activity/pgstat_relation.c
@@ -264,7 +264,7 @@ pgstat_report_vacuum(Oid tableoid, bool shared,
 	 * VACUUM command has processed all tables and committed.
 	 */
 	pgstat_flush_io(false);
-	pgstat_flush_backend(false);
+	pgstat_flush_backend(false, PGSTAT_BACKEND_FLUSH_IO);
 }
 
 /*
@@ -351,7 +351,7 @@ pgstat_report_analyze(Relation rel,
 
 	/* see pgstat_report_vacuum() */
 	pgstat_flush_io(false);
-	pgstat_flush_backend(false);
+	pgstat_flush_backend(false, PGSTAT_BACKEND_FLUSH_IO);
 }
 
 /*
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 3245f3a8d8a..5f8d20a406d 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1544,7 +1544,7 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 	if (bktype == B_INVALID)
 		return (Datum) 0;
 
-	bktype_stats = &backend_stats->stats;
+	bktype_stats = &backend_stats->io_stats;
 
 	/*
 	 * In Assert builds, we can afford an extra loop through all of the
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 9f83ecf181f..f15526236a3 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2140,6 +2140,7 @@ PgStatShared_Subscription
 PgStatShared_Wal
 PgStat_ArchiverStats
 PgStat_Backend
+PgStat_BackendPending
 PgStat_BackendPendingIO
 PgStat_BackendSubEntry
 PgStat_BgWriterStats
-- 
2.47.1

From bcdf4eb91f7ff4beeba50433a173ac2650afe432 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 07:51:27 +0000
Subject: [PATCH v3 1/3] Extract logic filling pg_stat_get_wal()'s tuple into
 its own routine

This commit adds pg_stat_wal_build_tuple(), a helper routine for
pg_stat_get_wal(), that fills its tuple based on the contents
of PgStat_WalStats.  This will be used in a follow-up commit that uses
the same structures as pg_stat_wal for reporting, but for the PGSTAT_KIND_BACKEND
statistics kind.
---
 src/backend/utils/adt/pgstatfuncs.c | 56 ++++++++++++++++++-----------
 1 file changed, 36 insertions(+), 20 deletions(-)
 100.0% src/backend/utils/adt/

diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 3245f3a8d8..7309f06993 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1560,20 +1560,22 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 }
 
 /*
- * Returns statistics of WAL activity
+ * pg_stat_wal_build_tuple
+ *
+ * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
+ * of wal_stats.
  */
-Datum
-pg_stat_get_wal(PG_FUNCTION_ARGS)
+static Datum
+pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
 {
-#define PG_STAT_GET_WAL_COLS	9
+#define PG_STAT_WAL_COLS	9
 	TupleDesc	tupdesc;
-	Datum		values[PG_STAT_GET_WAL_COLS] = {0};
-	bool		nulls[PG_STAT_GET_WAL_COLS] = {0};
+	Datum		values[PG_STAT_WAL_COLS] = {0};
+	bool		nulls[PG_STAT_WAL_COLS] = {0};
 	char		buf[256];
-	PgStat_WalStats *wal_stats;
 
 	/* Initialise attributes information in the tuple descriptor */
-	tupdesc = CreateTemplateTupleDesc(PG_STAT_GET_WAL_COLS);
+	tupdesc = CreateTemplateTupleDesc(PG_STAT_WAL_COLS);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "wal_records",
 					   INT8OID, -1, 0);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "wal_fpi",
@@ -1595,34 +1597,48 @@ pg_stat_get_wal(PG_FUNCTION_ARGS)
 
 	BlessTupleDesc(tupdesc);
 
-	/* Get statistics about WAL activity */
-	wal_stats = pgstat_fetch_stat_wal();
-
 	/* Fill values and NULLs */
-	values[0] = Int64GetDatum(wal_stats->wal_records);
-	values[1] = Int64GetDatum(wal_stats->wal_fpi);
+	values[0] = Int64GetDatum(wal_stats.wal_records);
+	values[1] = Int64GetDatum(wal_stats.wal_fpi);
 
 	/* Convert to numeric. */
-	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats->wal_bytes);
+	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats.wal_bytes);
 	values[2] = DirectFunctionCall3(numeric_in,
 									CStringGetDatum(buf),
 									ObjectIdGetDatum(0),
 									Int32GetDatum(-1));
 
-	values[3] = Int64GetDatum(wal_stats->wal_buffers_full);
-	values[4] = Int64GetDatum(wal_stats->wal_write);
-	values[5] = Int64GetDatum(wal_stats->wal_sync);
+	values[3] = Int64GetDatum(wal_stats.wal_buffers_full);
+	values[4] = Int64GetDatum(wal_stats.wal_write);
+	values[5] = Int64GetDatum(wal_stats.wal_sync);
 
 	/* Convert counters from microsec to millisec for display */
-	values[6] = Float8GetDatum(((double) wal_stats->wal_write_time) / 1000.0);
-	values[7] = Float8GetDatum(((double) wal_stats->wal_sync_time) / 1000.0);
+	values[6] = Float8GetDatum(((double) wal_stats.wal_write_time) / 1000.0);
+	values[7] = Float8GetDatum(((double) wal_stats.wal_sync_time) / 1000.0);
 
-	values[8] = TimestampTzGetDatum(wal_stats->stat_reset_timestamp);
+	if (wal_stats.stat_reset_timestamp != 0)
+		values[8] = TimestampTzGetDatum(wal_stats.stat_reset_timestamp);
+	else
+		nulls[8] = true;
 
 	/* Returns the record as Datum */
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns statistics of WAL activity
+ */
+Datum
+pg_stat_get_wal(PG_FUNCTION_ARGS)
+{
+	PgStat_WalStats *wal_stats;
+
+	/* Get statistics about WAL activity */
+	wal_stats = pgstat_fetch_stat_wal();
+
+	return (pg_stat_wal_build_tuple(*wal_stats));
+}
+
 /*
  * Returns statistics of SLRU caches.
  */
-- 
2.34.1

From 9173b59075759fdd88f29bcf2b555e86c823704d Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 08:44:29 +0000
Subject: [PATCH v3 2/3] PGSTAT_KIND_BACKEND code refactoring

This commit refactors some come related to per backend statistics. It makes
the code more generic or more IO statistics focused as it will be used in a
follow-up commit that will introduce per backend WAL statistics.
---
 src/backend/utils/activity/pgstat.c          |  2 +-
 src/backend/utils/activity/pgstat_backend.c  | 70 ++++++++++++++------
 src/backend/utils/activity/pgstat_io.c       |  8 +--
 src/backend/utils/activity/pgstat_relation.c |  4 +-
 src/backend/utils/adt/pgstatfuncs.c          |  2 +-
 src/include/pgstat.h                         |  6 +-
 src/include/utils/pgstat_internal.h          |  7 +-
 src/tools/pgindent/typedefs.list             |  1 +
 8 files changed, 69 insertions(+), 31 deletions(-)
  79.4% src/backend/utils/activity/
  12.9% src/include/utils/
   4.5% src/include/
   3.0% src/

diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index 16a03b8ce1..34520535d5 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -370,7 +370,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.shared_size = sizeof(PgStatShared_Backend),
 		.shared_data_off = offsetof(PgStatShared_Backend, stats),
 		.shared_data_len = sizeof(((PgStatShared_Backend *) 0)->stats),
-		.pending_size = sizeof(PgStat_BackendPendingIO),
+		.pending_size = sizeof(PgStat_BackendPending),
 
 		.flush_pending_cb = pgstat_backend_flush_cb,
 		.reset_timestamp_cb = pgstat_backend_reset_timestamp_cb,
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 1f91bfef0a..ea49208b80 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -39,23 +39,21 @@ pgstat_fetch_stat_backend(ProcNumber procNumber)
 }
 
 /*
- * Flush out locally pending backend statistics
- *
- * If no stats have been recorded, this function returns false.
+ * Flush out locally pending backend IO statistics.  Locking is managed
+ * by the caller.
  */
-bool
-pgstat_backend_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
+static void
+pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 {
-	PgStatShared_Backend *shbackendioent;
-	PgStat_BackendPendingIO *pendingent;
+	PgStatShared_Backend *shbackendent;
+	PgStat_BackendPending *pendingent;
 	PgStat_BktypeIO *bktype_shstats;
+	PgStat_BackendPendingIO *pending_io;
 
-	if (!pgstat_lock_entry(entry_ref, nowait))
-		return false;
-
-	shbackendioent = (PgStatShared_Backend *) entry_ref->shared_stats;
-	bktype_shstats = &shbackendioent->stats.stats;
-	pendingent = (PgStat_BackendPendingIO *) entry_ref->pending;
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	pendingent = (PgStat_BackendPending *) entry_ref->pending;
+	bktype_shstats = &shbackendent->stats.io_stats;
+	pending_io = &pendingent->pending_io;
 
 	for (int io_object = 0; io_object < IOOBJECT_NUM_TYPES; io_object++)
 	{
@@ -66,15 +64,33 @@ pgstat_backend_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
 				instr_time	time;
 
 				bktype_shstats->counts[io_object][io_context][io_op] +=
-					pendingent->counts[io_object][io_context][io_op];
+					pending_io->counts[io_object][io_context][io_op];
 
-				time = pendingent->pending_times[io_object][io_context][io_op];
+				time = pending_io->pending_times[io_object][io_context][io_op];
 
 				bktype_shstats->times[io_object][io_context][io_op] +=
 					INSTR_TIME_GET_MICROSEC(time);
 			}
 		}
 	}
+}
+
+/*
+ * Wrapper routine to flush backend statistics.
+ */
+static bool
+pgstat_flush_backend_entry(PgStat_EntryRef *entry_ref, bool nowait,
+						   bits32 flags)
+{
+	if (!pgstat_tracks_backend_bktype(MyBackendType))
+		return false;
+
+	if (!pgstat_lock_entry(entry_ref, nowait))
+		return false;
+
+	/* Flush requested statistics */
+	if (flags & PGSTAT_BACKEND_FLUSH_IO)
+		pgstat_flush_backend_entry_io(entry_ref);
 
 	pgstat_unlock_entry(entry_ref);
 
@@ -82,10 +98,23 @@ pgstat_backend_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
 }
 
 /*
- * Simpler wrapper of pgstat_backend_flush_cb()
+ * Callback to flush out locally pending backend statistics.
+ *
+ * If no stats have been recorded, this function returns false.
+ */
+bool
+pgstat_backend_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
+{
+	return pgstat_flush_backend_entry(entry_ref, nowait, PGSTAT_BACKEND_FLUSH_ALL);
+}
+
+/*
+ * Flush out locally pending backend statistics
+ *
+ * "flags" parameter controls which statistics to flush.
  */
 void
-pgstat_flush_backend(bool nowait)
+pgstat_flush_backend(bool nowait, bits32 flags)
 {
 	PgStat_EntryRef *entry_ref;
 
@@ -94,7 +123,8 @@ pgstat_flush_backend(bool nowait)
 
 	entry_ref = pgstat_get_entry_ref(PGSTAT_KIND_BACKEND, InvalidOid,
 									 MyProcNumber, false, NULL);
-	(void) pgstat_backend_flush_cb(entry_ref, nowait);
+
+	(void) pgstat_flush_backend_entry(entry_ref, nowait, flags);
 }
 
 /*
@@ -119,9 +149,9 @@ pgstat_create_backend(ProcNumber procnum)
 }
 
 /*
- * Find or create a local PgStat_BackendPendingIO entry for proc number.
+ * Find or create a local PgStat_BackendPending entry for proc number.
  */
-PgStat_BackendPendingIO *
+PgStat_BackendPending *
 pgstat_prep_backend_pending(ProcNumber procnum)
 {
 	PgStat_EntryRef *entry_ref;
diff --git a/src/backend/utils/activity/pgstat_io.c b/src/backend/utils/activity/pgstat_io.c
index f9a1f91dba..a7445995d3 100644
--- a/src/backend/utils/activity/pgstat_io.c
+++ b/src/backend/utils/activity/pgstat_io.c
@@ -81,10 +81,10 @@ pgstat_count_io_op_n(IOObject io_object, IOContext io_context, IOOp io_op, uint3
 
 	if (pgstat_tracks_backend_bktype(MyBackendType))
 	{
-		PgStat_PendingIO *entry_ref;
+		PgStat_BackendPending *entry_ref;
 
 		entry_ref = pgstat_prep_backend_pending(MyProcNumber);
-		entry_ref->counts[io_object][io_context][io_op] += cnt;
+		entry_ref->pending_io.counts[io_object][io_context][io_op] += cnt;
 	}
 
 	PendingIOStats.counts[io_object][io_context][io_op] += cnt;
@@ -151,10 +151,10 @@ pgstat_count_io_op_time(IOObject io_object, IOContext io_context, IOOp io_op,
 
 		if (pgstat_tracks_backend_bktype(MyBackendType))
 		{
-			PgStat_PendingIO *entry_ref;
+			PgStat_BackendPending *entry_ref;
 
 			entry_ref = pgstat_prep_backend_pending(MyProcNumber);
-			INSTR_TIME_ADD(entry_ref->pending_times[io_object][io_context][io_op],
+			INSTR_TIME_ADD(entry_ref->pending_io.pending_times[io_object][io_context][io_op],
 						   io_time);
 		}
 	}
diff --git a/src/backend/utils/activity/pgstat_relation.c b/src/backend/utils/activity/pgstat_relation.c
index 2cc304f881..09247ba097 100644
--- a/src/backend/utils/activity/pgstat_relation.c
+++ b/src/backend/utils/activity/pgstat_relation.c
@@ -264,7 +264,7 @@ pgstat_report_vacuum(Oid tableoid, bool shared,
 	 * VACUUM command has processed all tables and committed.
 	 */
 	pgstat_flush_io(false);
-	pgstat_flush_backend(false);
+	pgstat_flush_backend(false, PGSTAT_BACKEND_FLUSH_IO);
 }
 
 /*
@@ -351,7 +351,7 @@ pgstat_report_analyze(Relation rel,
 
 	/* see pgstat_report_vacuum() */
 	pgstat_flush_io(false);
-	pgstat_flush_backend(false);
+	pgstat_flush_backend(false, PGSTAT_BACKEND_FLUSH_IO);
 }
 
 /*
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 7309f06993..8a4340e977 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1544,7 +1544,7 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 	if (bktype == B_INVALID)
 		return (Datum) 0;
 
-	bktype_stats = &backend_stats->stats;
+	bktype_stats = &backend_stats->io_stats;
 
 	/*
 	 * In Assert builds, we can afford an extra loop through all of the
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 0d8427f27d..6631bd2d73 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -381,7 +381,7 @@ typedef PgStat_PendingIO PgStat_BackendPendingIO;
 typedef struct PgStat_Backend
 {
 	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO stats;
+	PgStat_BktypeIO io_stats;
 } PgStat_Backend;
 
 typedef struct PgStat_StatDBEntry
@@ -523,6 +523,10 @@ typedef struct PgStat_PendingWalStats
 	instr_time	wal_sync_time;
 } PgStat_PendingWalStats;
 
+typedef struct PgStat_BackendPending
+{
+	PgStat_BackendPendingIO pending_io;
+} PgStat_BackendPending;
 
 /*
  * Functions in pgstat.c
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 52eb008710..4bb8e5c53a 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -613,9 +613,12 @@ extern void pgstat_archiver_snapshot_cb(void);
  * Functions in pgstat_backend.c
  */
 
-extern void pgstat_flush_backend(bool nowait);
+/* flags for pgstat_flush_backend() */
+#define PGSTAT_BACKEND_FLUSH_IO		(1 << 0)	/* Flush I/O statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO)
 
-extern PgStat_BackendPendingIO *pgstat_prep_backend_pending(ProcNumber procnum);
+extern void pgstat_flush_backend(bool nowait, bits32 flags);
+extern PgStat_BackendPending *pgstat_prep_backend_pending(ProcNumber procnum);
 extern bool pgstat_backend_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
 extern void pgstat_backend_reset_timestamp_cb(PgStatShared_Common *header, TimestampTz ts);
 
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 9f83ecf181..f15526236a 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2140,6 +2140,7 @@ PgStatShared_Subscription
 PgStatShared_Wal
 PgStat_ArchiverStats
 PgStat_Backend
+PgStat_BackendPending
 PgStat_BackendPendingIO
 PgStat_BackendSubEntry
 PgStat_BgWriterStats
-- 
2.34.1

From b024c89167be988f31dee8330546ed96f43384d5 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 10:00:00 +0000
Subject: [PATCH v3 3/3] per backend WAL statistics

Now that commit 9aea73fc61 added backend-level statistics to pgstats (and
per backend IO statistics) we can more easily add per backend statistics.

This commit adds per backend WAL statistics using the same layer as pg_stat_wal,
except that it is now possible to know how much WAL activity is happening in each
backend rather than an overall aggregate of all the activity.  A function called
pg_stat_get_backend_wal() is added to access this data depending on the
PID of a backend.

The same limitation as in 9aea73fc61 persists, meaning that Auxiliary processes
are not included in this set of statistics.

XXX: bump catalog version
---
 doc/src/sgml/config.sgml                    |  4 +-
 doc/src/sgml/monitoring.sgml                | 19 +++++
 src/backend/access/transam/xlog.c           | 36 ++++++++-
 src/backend/utils/activity/pgstat_backend.c | 86 ++++++++++++++++++++-
 src/backend/utils/activity/pgstat_wal.c     |  6 +-
 src/backend/utils/adt/pgstatfuncs.c         | 78 +++++++++++++++----
 src/include/catalog/pg_proc.dat             |  7 ++
 src/include/pgstat.h                        | 37 +++++++--
 src/include/utils/pgstat_internal.h         |  3 +-
 src/test/regress/expected/stats.out         | 14 ++++
 src/test/regress/sql/stats.sql              |  6 ++
 src/tools/pgindent/typedefs.list            |  2 +
 12 files changed, 268 insertions(+), 30 deletions(-)
  10.1% doc/src/sgml/
   8.8% src/backend/access/transam/
  31.5% src/backend/utils/activity/
  28.3% src/backend/utils/adt/
   5.0% src/include/catalog/
   8.4% src/include/
   4.0% src/test/regress/expected/
   3.0% src/test/regress/sql/

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 8683f0bdf5..8e8478dcb1 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8433,7 +8433,9 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
         measure the overhead of timing on your system.
         I/O timing information is
         displayed in <link linkend="monitoring-pg-stat-wal-view">
-        <structname>pg_stat_wal</structname></link>.
+        <structname>pg_stat_wal</structname></link> and in the output of the
+        <link linkend="pg-stat-get-backend-wal">
+        <function>pg_stat_get_backend_wal()</function></link> function.
         Only superusers and users with the appropriate <literal>SET</literal>
         privilege can change this setting.
        </para>
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index d0d176cc54..84a2d09b76 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -4811,6 +4811,25 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para></entry>
       </row>
 
+      <row>
+       <entry id="pg-stat-get-backend-wal" role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_stat_get_backend_wal</primary>
+        </indexterm>
+        <function>pg_stat_get_backend_wal</function> ( <type>integer</type> )
+        <returnvalue>record</returnvalue>
+       </para>
+       <para>
+        Returns WAL statistics about the backend with the specified
+        process ID. The output fields are exactly the same as the ones in the
+        <structname>pg_stat_wal</structname> view.
+       </para>
+       <para>
+        The function does not return WAL statistics for the checkpointer,
+        the background writer, the startup process and the autovacuum launcher.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index bf3dbda901..0ba9fcb277 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -2058,6 +2058,10 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
 					XLogWrite(WriteRqst, tli, false);
 					LWLockRelease(WALWriteLock);
 					PendingWalStats.wal_buffers_full++;
+
+					if (pgstat_tracks_backend_bktype(MyBackendType))
+						PendingBackendWalStats.wal_buffers_full++;
+
 					TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_DONE();
 				}
 				/* Re-acquire WALBufMappingLock and retry */
@@ -2426,11 +2430,14 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
 			Size		nleft;
 			ssize_t		written;
 			instr_time	start;
+			instr_time	end;
 
 			/* OK to write the page(s) */
 			from = XLogCtl->pages + startidx * (Size) XLOG_BLCKSZ;
 			nbytes = npages * (Size) XLOG_BLCKSZ;
 			nleft = nbytes;
+			/* keep compiler quiet */
+			INSTR_TIME_SET_ZERO(end);
 			do
 			{
 				errno = 0;
@@ -2451,14 +2458,26 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
 				 */
 				if (track_wal_io_timing)
 				{
-					instr_time	end;
-
 					INSTR_TIME_SET_CURRENT(end);
 					INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_write_time, end, start);
 				}
 
 				PendingWalStats.wal_write++;
 
+				if (pgstat_tracks_backend_bktype(MyBackendType))
+				{
+					/*
+					 * We are inside a critical section, so we can't use
+					 * pgstat_prep_pending_entry() and we rely on
+					 * PendingBackendWalStats instead.
+					 */
+					PendingBackendWalStats.wal_write++;
+
+					if (track_wal_io_timing)
+						INSTR_TIME_ACCUM_DIFF(PendingBackendWalStats.wal_write_time,
+											  end, start);
+				}
+
 				if (written <= 0)
 				{
 					char		xlogfname[MAXFNAMELEN];
@@ -8684,8 +8703,11 @@ issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
 {
 	char	   *msg = NULL;
 	instr_time	start;
+	instr_time	end;
 
 	Assert(tli != 0);
+	/* keep compiler quiet */
+	INSTR_TIME_SET_ZERO(end);
 
 	/*
 	 * Quick exit if fsync is disabled or write() has already synced the WAL
@@ -8751,13 +8773,19 @@ issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
 	 */
 	if (track_wal_io_timing)
 	{
-		instr_time	end;
-
 		INSTR_TIME_SET_CURRENT(end);
 		INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_sync_time, end, start);
 	}
 
 	PendingWalStats.wal_sync++;
+
+	if (pgstat_tracks_backend_bktype(MyBackendType))
+	{
+		PendingBackendWalStats.wal_sync++;
+
+		if (track_wal_io_timing)
+			INSTR_TIME_ACCUM_DIFF(PendingBackendWalStats.wal_sync_time, end, start);
+	}
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index ea49208b80..c66cbf70b6 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -24,6 +24,16 @@
 
 #include "utils/pgstat_internal.h"
 
+PgStat_PendingWalStats PendingBackendWalStats = {0};
+
+/*
+ * WAL usage counters saved from pgWalUsage at the previous call to
+ * pgstat_report_wal(). This is used to calculate how much WAL usage
+ * happens between pgstat_report_wal() calls, by subtracting
+ * the previous counters from the current ones.
+ */
+static WalUsage prevBackendWalUsage;
+
 /*
  * Returns statistics of a backend by proc number.
  */
@@ -75,6 +85,75 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 	}
 }
 
+/*
+ * To determine whether any WAL activity has occurred since last time, not
+ * only the number of generated WAL records but also the numbers of WAL
+ * writes and syncs need to be checked. Because even transaction that
+ * generates no WAL records can write or sync WAL data when flushing the
+ * data pages.
+ */
+static bool
+pgstat_backend_wal_have_pending(void)
+{
+	return pgWalUsage.wal_records != prevBackendWalUsage.wal_records ||
+		PendingBackendWalStats.wal_write != 0 ||
+		PendingBackendWalStats.wal_sync != 0;
+}
+
+/*
+ * Flush out locally pending backend WAL statistics.  Locking is managed
+ * by the caller.
+ */
+static void
+pgstat_flush_backend_entry_wal(PgStat_EntryRef *entry_ref)
+{
+	PgStatShared_Backend *shbackendent;
+	PgStat_WalCounters *bktype_shstats;
+	WalUsage	wal_usage_diff = {0};
+
+	/*
+	 * This function can be called even if nothing at all has happened. Avoid
+	 * taking lock for nothing in that case.
+	 */
+	if (!pgstat_backend_wal_have_pending())
+		return;
+
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	bktype_shstats = &shbackendent->stats.wal_stats;
+
+	/*
+	 * We don't update the WAL usage portion of the local WalStats elsewhere.
+	 * Calculate how much WAL usage counters were increased by subtracting the
+	 * previous counters from the current ones.
+	 */
+	WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
+
+#define WALSTAT_ACC(fld, var_to_add) \
+	(bktype_shstats->fld += var_to_add.fld)
+#define WALSTAT_ACC_INSTR_TIME(fld) \
+	(bktype_shstats->fld += INSTR_TIME_GET_MICROSEC(PendingBackendWalStats.fld))
+	WALSTAT_ACC(wal_buffers_full, PendingBackendWalStats);
+	WALSTAT_ACC(wal_write, PendingBackendWalStats);
+	WALSTAT_ACC(wal_sync, PendingBackendWalStats);
+	WALSTAT_ACC(wal_records, wal_usage_diff);
+	WALSTAT_ACC(wal_fpi, wal_usage_diff);
+	WALSTAT_ACC(wal_bytes, wal_usage_diff);
+	WALSTAT_ACC_INSTR_TIME(wal_write_time);
+	WALSTAT_ACC_INSTR_TIME(wal_sync_time);
+#undef WALSTAT_ACC_INSTR_TIME
+#undef WALSTAT_ACC
+
+	/*
+	 * Save the current counters for the subsequent calculation of WAL usage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
+
+	/*
+	 * Clear out the statistics buffer, so it can be re-used.
+	 */
+	MemSet(&PendingBackendWalStats, 0, sizeof(PendingWalStats));
+}
+
 /*
  * Wrapper routine to flush backend statistics.
  */
@@ -92,6 +171,9 @@ pgstat_flush_backend_entry(PgStat_EntryRef *entry_ref, bool nowait,
 	if (flags & PGSTAT_BACKEND_FLUSH_IO)
 		pgstat_flush_backend_entry_io(entry_ref);
 
+	if (flags & PGSTAT_BACKEND_FLUSH_WAL)
+		pgstat_flush_backend_entry_wal(entry_ref);
+
 	pgstat_unlock_entry(entry_ref);
 
 	return true;
@@ -146,10 +228,12 @@ pgstat_create_backend(ProcNumber procnum)
 	 * e.g. if we previously used this proc number.
 	 */
 	memset(&shstatent->stats, 0, sizeof(shstatent->stats));
+
+	prevBackendWalUsage = pgWalUsage;
 }
 
 /*
- * Find or create a local PgStat_BackendPending entry for proc number.
+ * Find or create a local PgStat_BackendPendingIO entry for proc number.
  */
 PgStat_BackendPending *
 pgstat_prep_backend_pending(ProcNumber procnum)
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 18fa6b2936..3362764226 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -55,6 +55,8 @@ pgstat_report_wal(bool force)
 	/* flush wal stats */
 	pgstat_flush_wal(nowait);
 
+	pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_WAL);
+
 	/* flush IO stats */
 	pgstat_flush_io(nowait);
 }
@@ -117,9 +119,9 @@ pgstat_wal_flush_cb(bool nowait)
 		return true;
 
 #define WALSTAT_ACC(fld, var_to_add) \
-	(stats_shmem->stats.fld += var_to_add.fld)
+	(stats_shmem->stats.wal_counters.fld += var_to_add.fld)
 #define WALSTAT_ACC_INSTR_TIME(fld) \
-	(stats_shmem->stats.fld += INSTR_TIME_GET_MICROSEC(PendingWalStats.fld))
+	(stats_shmem->stats.wal_counters.fld += INSTR_TIME_GET_MICROSEC(PendingWalStats.fld))
 	WALSTAT_ACC(wal_records, wal_usage_diff);
 	WALSTAT_ACC(wal_fpi, wal_usage_diff);
 	WALSTAT_ACC(wal_bytes, wal_usage_diff);
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 8a4340e977..8d251dfcca 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1562,11 +1562,12 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 /*
  * pg_stat_wal_build_tuple
  *
- * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
- * of wal_stats.
+ * Helper routine for pg_stat_get_wal() and pg_stat_get_backend_wal() returning
+ * one tuple based on the contents of wal_counters.
  */
 static Datum
-pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
+pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
+						TimestampTz stat_reset_timestamp)
 {
 #define PG_STAT_WAL_COLS	9
 	TupleDesc	tupdesc;
@@ -1598,26 +1599,26 @@ pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
 	BlessTupleDesc(tupdesc);
 
 	/* Fill values and NULLs */
-	values[0] = Int64GetDatum(wal_stats.wal_records);
-	values[1] = Int64GetDatum(wal_stats.wal_fpi);
+	values[0] = Int64GetDatum(wal_counters.wal_records);
+	values[1] = Int64GetDatum(wal_counters.wal_fpi);
 
 	/* Convert to numeric. */
-	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats.wal_bytes);
+	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_counters.wal_bytes);
 	values[2] = DirectFunctionCall3(numeric_in,
 									CStringGetDatum(buf),
 									ObjectIdGetDatum(0),
 									Int32GetDatum(-1));
 
-	values[3] = Int64GetDatum(wal_stats.wal_buffers_full);
-	values[4] = Int64GetDatum(wal_stats.wal_write);
-	values[5] = Int64GetDatum(wal_stats.wal_sync);
+	values[3] = Int64GetDatum(wal_counters.wal_buffers_full);
+	values[4] = Int64GetDatum(wal_counters.wal_write);
+	values[5] = Int64GetDatum(wal_counters.wal_sync);
 
 	/* Convert counters from microsec to millisec for display */
-	values[6] = Float8GetDatum(((double) wal_stats.wal_write_time) / 1000.0);
-	values[7] = Float8GetDatum(((double) wal_stats.wal_sync_time) / 1000.0);
+	values[6] = Float8GetDatum(((double) wal_counters.wal_write_time) / 1000.0);
+	values[7] = Float8GetDatum(((double) wal_counters.wal_sync_time) / 1000.0);
 
-	if (wal_stats.stat_reset_timestamp != 0)
-		values[8] = TimestampTzGetDatum(wal_stats.stat_reset_timestamp);
+	if (stat_reset_timestamp != 0)
+		values[8] = TimestampTzGetDatum(stat_reset_timestamp);
 	else
 		nulls[8] = true;
 
@@ -1625,6 +1626,55 @@ pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns WAL statistics for a backend with given PID.
+ */
+Datum
+pg_stat_get_backend_wal(PG_FUNCTION_ARGS)
+{
+	int			pid;
+	PGPROC	   *proc;
+	ProcNumber	procNumber;
+	PgStat_Backend *backend_stats;
+	PgStat_WalCounters bktype_stats;
+	PgBackendStatus *beentry;
+
+	pid = PG_GETARG_INT32(0);
+	proc = BackendPidGetProc(pid);
+
+	/*
+	 * This could be an auxiliary process but these do not report backend
+	 * statistics due to pgstat_tracks_backend_bktype(), so there is no need
+	 * for an extra call to AuxiliaryPidGetProc().
+	 */
+	if (!proc)
+		PG_RETURN_NULL();
+
+	procNumber = GetNumberFromPGProc(proc);
+
+	beentry = pgstat_get_beentry_by_proc_number(procNumber);
+	if (!beentry)
+		PG_RETURN_NULL();
+
+	backend_stats = pgstat_fetch_stat_backend(procNumber);
+	if (!backend_stats)
+		PG_RETURN_NULL();
+
+	/* if PID does not match, leave */
+	if (beentry->st_procpid != pid)
+		PG_RETURN_NULL();
+
+	/* backend may be gone, so recheck in case */
+	if (beentry->st_backendType == B_INVALID)
+		PG_RETURN_NULL();
+
+	bktype_stats = backend_stats->wal_stats;
+
+	/* save tuples with data from this PgStat_BktypeIO */
+	return (pg_stat_wal_build_tuple(bktype_stats, backend_stats->stat_reset_timestamp));
+}
+
+
 /*
  * Returns statistics of WAL activity
  */
@@ -1636,7 +1686,7 @@ pg_stat_get_wal(PG_FUNCTION_ARGS)
 	/* Get statistics about WAL activity */
 	wal_stats = pgstat_fetch_stat_wal();
 
-	return (pg_stat_wal_build_tuple(*wal_stats));
+	return (pg_stat_wal_build_tuple(wal_stats->wal_counters, wal_stats->stat_reset_timestamp));
 }
 
 /*
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index b37e8a6f88..72a5dae4b1 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5929,6 +5929,13 @@
   proargmodes => '{o,o,o,o,o,o,o,o,o}',
   proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,wal_write,wal_sync,wal_write_time,wal_sync_time,stats_reset}',
   prosrc => 'pg_stat_get_wal' },
+{ oid => '8037', descr => 'statistics: backend WAL activity',
+  proname => 'pg_stat_get_backend_wal', provolatile => 'v',
+  proparallel => 'r', prorettype => 'record', proargtypes => 'int4',
+  proallargtypes => '{int4,int8,int8,numeric,int8,int8,int8,float8,float8,timestamptz}',
+  proargmodes => '{i,o,o,o,o,o,o,o,o,o}',
+  proargnames => '{backend_pid,wal_records,wal_fpi,wal_bytes,wal_buffers_full,wal_write,wal_sync,wal_write_time,wal_sync_time,stats_reset}',
+  prosrc => 'pg_stat_get_backend_wal' },
 { oid => '6248', descr => 'statistics: information about WAL prefetching',
   proname => 'pg_stat_get_recovery_prefetch', prorows => '1', proretset => 't',
   provolatile => 'v', prorettype => 'record', proargtypes => '',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 6631bd2d73..045877c5a8 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -378,12 +378,6 @@ typedef struct PgStat_IO
 /* Backend statistics store the same amount of IO data as PGSTAT_KIND_IO */
 typedef PgStat_PendingIO PgStat_BackendPendingIO;
 
-typedef struct PgStat_Backend
-{
-	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO io_stats;
-} PgStat_Backend;
-
 typedef struct PgStat_StatDBEntry
 {
 	PgStat_Counter xact_commit;
@@ -495,7 +489,7 @@ typedef struct PgStat_StatTabEntry
 	PgStat_Counter autoanalyze_count;
 } PgStat_StatTabEntry;
 
-typedef struct PgStat_WalStats
+typedef struct PgStat_WalCounters
 {
 	PgStat_Counter wal_records;
 	PgStat_Counter wal_fpi;
@@ -505,6 +499,11 @@ typedef struct PgStat_WalStats
 	PgStat_Counter wal_sync;
 	PgStat_Counter wal_write_time;
 	PgStat_Counter wal_sync_time;
+} PgStat_WalCounters;
+
+typedef struct PgStat_WalStats
+{
+	PgStat_WalCounters wal_counters;
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
@@ -523,9 +522,21 @@ typedef struct PgStat_PendingWalStats
 	instr_time	wal_sync_time;
 } PgStat_PendingWalStats;
 
+typedef struct PgStat_Backend
+{
+	TimestampTz stat_reset_timestamp;
+	PgStat_BktypeIO io_stats;
+	PgStat_WalCounters wal_stats;
+} PgStat_Backend;
+
 typedef struct PgStat_BackendPending
 {
 	PgStat_BackendPendingIO pending_io;
+
+	/*
+	 * We are not creating one member for PgStat_PendingWalStats. See the
+	 * comment above the PendingBackendWalStats definition as to why.
+	 */
 } PgStat_BackendPending;
 
 /*
@@ -857,5 +868,17 @@ extern PGDLLIMPORT SessionEndType pgStatSessionEndCause;
 /* updated directly by backends and background processes */
 extern PGDLLIMPORT PgStat_PendingWalStats PendingWalStats;
 
+/*
+ * Variables in pgstat_backend.c
+ */
+
+/* updated directly by backends and background processes */
+
+/*
+ * WAL pending statistics are incremented inside a critical section
+ * (see XLogWrite()), so we can't use pgstat_prep_pending_entry() and we rely on
+ * PendingBackendWalStats instead.
+ */
+extern PGDLLIMPORT PgStat_PendingWalStats PendingBackendWalStats;
 
 #endif							/* PGSTAT_H */
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 4bb8e5c53a..05c3e21500 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -615,7 +615,8 @@ extern void pgstat_archiver_snapshot_cb(void);
 
 /* flags for pgstat_flush_backend() */
 #define PGSTAT_BACKEND_FLUSH_IO		(1 << 0)	/* Flush I/O statistics */
-#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO)
+#define PGSTAT_BACKEND_FLUSH_WAL	(1 << 1)	/* Flush WAL statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO | PGSTAT_BACKEND_FLUSH_WAL)
 
 extern void pgstat_flush_backend(bool nowait, bits32 flags);
 extern PgStat_BackendPending *pgstat_prep_backend_pending(ProcNumber procnum);
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index a0317b7208..cc01fdf274 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -832,6 +832,8 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -851,6 +853,18 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
  t
 (1 row)
 
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+ ?column? 
+----------
+ t
+(1 row)
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 399c72bbcf..28fe0a1a7d 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -423,6 +423,9 @@ SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
 
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 
@@ -435,6 +438,9 @@ CHECKPOINT;
 SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;
 SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
 
+SELECT pg_stat_force_next_flush();
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index f15526236a..b593d8601e 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2159,6 +2159,7 @@ PgStat_KindInfo
 PgStat_LocalState
 PgStat_PendingDroppedStatsItem
 PgStat_PendingIO
+PgStat_PendingBackendWalStats
 PgStat_PendingWalStats
 PgStat_SLRUStats
 PgStat_ShmemControl
@@ -2175,6 +2176,7 @@ PgStat_SubXactStatus
 PgStat_TableCounts
 PgStat_TableStatus
 PgStat_TableXactStatus
+PgStat_WalCounters
 PgStat_WalStats
 PgXmlErrorContext
 PgXmlStrictness
-- 
2.34.1

From c424cd02baba3f441768b1f049a5ec6ad11bc3ce Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 07:51:27 +0000
Subject: [PATCH v4 1/2] Extract logic filling pg_stat_get_wal()'s tuple into
 its own routine

This commit adds pg_stat_wal_build_tuple(), a helper routine for
pg_stat_get_wal(), that fills its tuple based on the contents
of PgStat_WalStats.  This will be used in a follow-up commit that uses
the same structures as pg_stat_wal for reporting, but for the PGSTAT_KIND_BACKEND
statistics kind.
---
 src/backend/utils/adt/pgstatfuncs.c | 56 ++++++++++++++++++-----------
 1 file changed, 36 insertions(+), 20 deletions(-)
 100.0% src/backend/utils/adt/

diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 5f8d20a406..8a4340e977 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1560,20 +1560,22 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 }
 
 /*
- * Returns statistics of WAL activity
+ * pg_stat_wal_build_tuple
+ *
+ * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
+ * of wal_stats.
  */
-Datum
-pg_stat_get_wal(PG_FUNCTION_ARGS)
+static Datum
+pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
 {
-#define PG_STAT_GET_WAL_COLS	9
+#define PG_STAT_WAL_COLS	9
 	TupleDesc	tupdesc;
-	Datum		values[PG_STAT_GET_WAL_COLS] = {0};
-	bool		nulls[PG_STAT_GET_WAL_COLS] = {0};
+	Datum		values[PG_STAT_WAL_COLS] = {0};
+	bool		nulls[PG_STAT_WAL_COLS] = {0};
 	char		buf[256];
-	PgStat_WalStats *wal_stats;
 
 	/* Initialise attributes information in the tuple descriptor */
-	tupdesc = CreateTemplateTupleDesc(PG_STAT_GET_WAL_COLS);
+	tupdesc = CreateTemplateTupleDesc(PG_STAT_WAL_COLS);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "wal_records",
 					   INT8OID, -1, 0);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "wal_fpi",
@@ -1595,34 +1597,48 @@ pg_stat_get_wal(PG_FUNCTION_ARGS)
 
 	BlessTupleDesc(tupdesc);
 
-	/* Get statistics about WAL activity */
-	wal_stats = pgstat_fetch_stat_wal();
-
 	/* Fill values and NULLs */
-	values[0] = Int64GetDatum(wal_stats->wal_records);
-	values[1] = Int64GetDatum(wal_stats->wal_fpi);
+	values[0] = Int64GetDatum(wal_stats.wal_records);
+	values[1] = Int64GetDatum(wal_stats.wal_fpi);
 
 	/* Convert to numeric. */
-	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats->wal_bytes);
+	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats.wal_bytes);
 	values[2] = DirectFunctionCall3(numeric_in,
 									CStringGetDatum(buf),
 									ObjectIdGetDatum(0),
 									Int32GetDatum(-1));
 
-	values[3] = Int64GetDatum(wal_stats->wal_buffers_full);
-	values[4] = Int64GetDatum(wal_stats->wal_write);
-	values[5] = Int64GetDatum(wal_stats->wal_sync);
+	values[3] = Int64GetDatum(wal_stats.wal_buffers_full);
+	values[4] = Int64GetDatum(wal_stats.wal_write);
+	values[5] = Int64GetDatum(wal_stats.wal_sync);
 
 	/* Convert counters from microsec to millisec for display */
-	values[6] = Float8GetDatum(((double) wal_stats->wal_write_time) / 1000.0);
-	values[7] = Float8GetDatum(((double) wal_stats->wal_sync_time) / 1000.0);
+	values[6] = Float8GetDatum(((double) wal_stats.wal_write_time) / 1000.0);
+	values[7] = Float8GetDatum(((double) wal_stats.wal_sync_time) / 1000.0);
 
-	values[8] = TimestampTzGetDatum(wal_stats->stat_reset_timestamp);
+	if (wal_stats.stat_reset_timestamp != 0)
+		values[8] = TimestampTzGetDatum(wal_stats.stat_reset_timestamp);
+	else
+		nulls[8] = true;
 
 	/* Returns the record as Datum */
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns statistics of WAL activity
+ */
+Datum
+pg_stat_get_wal(PG_FUNCTION_ARGS)
+{
+	PgStat_WalStats *wal_stats;
+
+	/* Get statistics about WAL activity */
+	wal_stats = pgstat_fetch_stat_wal();
+
+	return (pg_stat_wal_build_tuple(*wal_stats));
+}
+
 /*
  * Returns statistics of SLRU caches.
  */
-- 
2.34.1

From bb1043a1a4240287d8ba94615a6217d1a12f5722 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 10:00:00 +0000
Subject: [PATCH v4 2/2] per backend WAL statistics

Now that commit 9aea73fc61 added backend-level statistics to pgstats (and
per backend IO statistics) we can more easily add per backend statistics.

This commit adds per backend WAL statistics using the same layer as pg_stat_wal,
except that it is now possible to know how much WAL activity is happening in each
backend rather than an overall aggregate of all the activity.  A function called
pg_stat_get_backend_wal() is added to access this data depending on the
PID of a backend.

The same limitation as in 9aea73fc61 persists, meaning that Auxiliary processes
are not included in this set of statistics.

XXX: bump catalog version
---
 doc/src/sgml/config.sgml                    |  4 +-
 doc/src/sgml/monitoring.sgml                | 19 +++++
 src/backend/access/transam/xlog.c           | 36 ++++++++-
 src/backend/utils/activity/pgstat_backend.c | 86 ++++++++++++++++++++-
 src/backend/utils/activity/pgstat_wal.c     |  6 +-
 src/backend/utils/adt/pgstatfuncs.c         | 78 +++++++++++++++----
 src/include/catalog/pg_proc.dat             |  7 ++
 src/include/pgstat.h                        | 59 +++++++++-----
 src/include/utils/pgstat_internal.h         |  3 +-
 src/test/regress/expected/stats.out         | 14 ++++
 src/test/regress/sql/stats.sql              |  6 ++
 src/tools/pgindent/typedefs.list            |  1 +
 12 files changed, 277 insertions(+), 42 deletions(-)
  10.1% doc/src/sgml/
   8.8% src/backend/access/transam/
  31.6% src/backend/utils/activity/
  28.4% src/backend/utils/adt/
   5.0% src/include/catalog/
   8.4% src/include/
   4.0% src/test/regress/expected/
   3.1% src/test/regress/sql/

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index f1ab614575..9193e70a01 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8435,7 +8435,9 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
         measure the overhead of timing on your system.
         I/O timing information is
         displayed in <link linkend="monitoring-pg-stat-wal-view">
-        <structname>pg_stat_wal</structname></link>.
+        <structname>pg_stat_wal</structname></link> and in the output of the
+        <link linkend="pg-stat-get-backend-wal">
+        <function>pg_stat_get_backend_wal()</function></link> function.
         Only superusers and users with the appropriate <literal>SET</literal>
         privilege can change this setting.
        </para>
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index d0d176cc54..84a2d09b76 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -4811,6 +4811,25 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para></entry>
       </row>
 
+      <row>
+       <entry id="pg-stat-get-backend-wal" role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_stat_get_backend_wal</primary>
+        </indexterm>
+        <function>pg_stat_get_backend_wal</function> ( <type>integer</type> )
+        <returnvalue>record</returnvalue>
+       </para>
+       <para>
+        Returns WAL statistics about the backend with the specified
+        process ID. The output fields are exactly the same as the ones in the
+        <structname>pg_stat_wal</structname> view.
+       </para>
+       <para>
+        The function does not return WAL statistics for the checkpointer,
+        the background writer, the startup process and the autovacuum launcher.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index bf3dbda901..0ba9fcb277 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -2058,6 +2058,10 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
 					XLogWrite(WriteRqst, tli, false);
 					LWLockRelease(WALWriteLock);
 					PendingWalStats.wal_buffers_full++;
+
+					if (pgstat_tracks_backend_bktype(MyBackendType))
+						PendingBackendWalStats.wal_buffers_full++;
+
 					TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_DONE();
 				}
 				/* Re-acquire WALBufMappingLock and retry */
@@ -2426,11 +2430,14 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
 			Size		nleft;
 			ssize_t		written;
 			instr_time	start;
+			instr_time	end;
 
 			/* OK to write the page(s) */
 			from = XLogCtl->pages + startidx * (Size) XLOG_BLCKSZ;
 			nbytes = npages * (Size) XLOG_BLCKSZ;
 			nleft = nbytes;
+			/* keep compiler quiet */
+			INSTR_TIME_SET_ZERO(end);
 			do
 			{
 				errno = 0;
@@ -2451,14 +2458,26 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
 				 */
 				if (track_wal_io_timing)
 				{
-					instr_time	end;
-
 					INSTR_TIME_SET_CURRENT(end);
 					INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_write_time, end, start);
 				}
 
 				PendingWalStats.wal_write++;
 
+				if (pgstat_tracks_backend_bktype(MyBackendType))
+				{
+					/*
+					 * We are inside a critical section, so we can't use
+					 * pgstat_prep_pending_entry() and we rely on
+					 * PendingBackendWalStats instead.
+					 */
+					PendingBackendWalStats.wal_write++;
+
+					if (track_wal_io_timing)
+						INSTR_TIME_ACCUM_DIFF(PendingBackendWalStats.wal_write_time,
+											  end, start);
+				}
+
 				if (written <= 0)
 				{
 					char		xlogfname[MAXFNAMELEN];
@@ -8684,8 +8703,11 @@ issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
 {
 	char	   *msg = NULL;
 	instr_time	start;
+	instr_time	end;
 
 	Assert(tli != 0);
+	/* keep compiler quiet */
+	INSTR_TIME_SET_ZERO(end);
 
 	/*
 	 * Quick exit if fsync is disabled or write() has already synced the WAL
@@ -8751,13 +8773,19 @@ issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
 	 */
 	if (track_wal_io_timing)
 	{
-		instr_time	end;
-
 		INSTR_TIME_SET_CURRENT(end);
 		INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_sync_time, end, start);
 	}
 
 	PendingWalStats.wal_sync++;
+
+	if (pgstat_tracks_backend_bktype(MyBackendType))
+	{
+		PendingBackendWalStats.wal_sync++;
+
+		if (track_wal_io_timing)
+			INSTR_TIME_ACCUM_DIFF(PendingBackendWalStats.wal_sync_time, end, start);
+	}
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 207bfa3c27..86c3b106dd 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -24,6 +24,16 @@
 
 #include "utils/pgstat_internal.h"
 
+PgStat_PendingWalStats PendingBackendWalStats = {0};
+
+/*
+ * WAL usage counters saved from pgWalUsage at the previous call to
+ * pgstat_report_wal(). This is used to calculate how much WAL usage
+ * happens between pgstat_report_wal() calls, by subtracting
+ * the previous counters from the current ones.
+ */
+static WalUsage prevBackendWalUsage;
+
 /*
  * Returns statistics of a backend by proc number.
  */
@@ -75,6 +85,75 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 	}
 }
 
+/*
+ * To determine whether any WAL activity has occurred since last time, not
+ * only the number of generated WAL records but also the numbers of WAL
+ * writes and syncs need to be checked. Because even transaction that
+ * generates no WAL records can write or sync WAL data when flushing the
+ * data pages.
+ */
+static bool
+pgstat_backend_wal_have_pending(void)
+{
+	return pgWalUsage.wal_records != prevBackendWalUsage.wal_records ||
+		PendingBackendWalStats.wal_write != 0 ||
+		PendingBackendWalStats.wal_sync != 0;
+}
+
+/*
+ * Flush out locally pending backend WAL statistics.  Locking is managed
+ * by the caller.
+ */
+static void
+pgstat_flush_backend_entry_wal(PgStat_EntryRef *entry_ref)
+{
+	PgStatShared_Backend *shbackendent;
+	PgStat_WalCounters *bktype_shstats;
+	WalUsage	wal_usage_diff = {0};
+
+	/*
+	 * This function can be called even if nothing at all has happened. Avoid
+	 * taking lock for nothing in that case.
+	 */
+	if (!pgstat_backend_wal_have_pending())
+		return;
+
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	bktype_shstats = &shbackendent->stats.wal_stats;
+
+	/*
+	 * We don't update the WAL usage portion of the local WalStats elsewhere.
+	 * Calculate how much WAL usage counters were increased by subtracting the
+	 * previous counters from the current ones.
+	 */
+	WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
+
+#define WALSTAT_ACC(fld, var_to_add) \
+	(bktype_shstats->fld += var_to_add.fld)
+#define WALSTAT_ACC_INSTR_TIME(fld) \
+	(bktype_shstats->fld += INSTR_TIME_GET_MICROSEC(PendingBackendWalStats.fld))
+	WALSTAT_ACC(wal_buffers_full, PendingBackendWalStats);
+	WALSTAT_ACC(wal_write, PendingBackendWalStats);
+	WALSTAT_ACC(wal_sync, PendingBackendWalStats);
+	WALSTAT_ACC(wal_records, wal_usage_diff);
+	WALSTAT_ACC(wal_fpi, wal_usage_diff);
+	WALSTAT_ACC(wal_bytes, wal_usage_diff);
+	WALSTAT_ACC_INSTR_TIME(wal_write_time);
+	WALSTAT_ACC_INSTR_TIME(wal_sync_time);
+#undef WALSTAT_ACC_INSTR_TIME
+#undef WALSTAT_ACC
+
+	/*
+	 * Save the current counters for the subsequent calculation of WAL usage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
+
+	/*
+	 * Clear out the statistics buffer, so it can be re-used.
+	 */
+	MemSet(&PendingBackendWalStats, 0, sizeof(PendingWalStats));
+}
+
 /*
  * Wrapper routine to flush backend statistics.
  */
@@ -92,6 +171,9 @@ pgstat_flush_backend_entry(PgStat_EntryRef *entry_ref, bool nowait,
 	if (flags & PGSTAT_BACKEND_FLUSH_IO)
 		pgstat_flush_backend_entry_io(entry_ref);
 
+	if (flags & PGSTAT_BACKEND_FLUSH_WAL)
+		pgstat_flush_backend_entry_wal(entry_ref);
+
 	pgstat_unlock_entry(entry_ref);
 
 	return true;
@@ -145,10 +227,12 @@ pgstat_create_backend(ProcNumber procnum)
 	 * e.g. if we previously used this proc number.
 	 */
 	memset(&shstatent->stats, 0, sizeof(shstatent->stats));
+
+	prevBackendWalUsage = pgWalUsage;
 }
 
 /*
- * Find or create a local PgStat_BackendPending entry for proc number.
+ * Find or create a local PgStat_BackendPendingIO entry for proc number.
  */
 PgStat_BackendPending *
 pgstat_prep_backend_pending(ProcNumber procnum)
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 18fa6b2936..3362764226 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -55,6 +55,8 @@ pgstat_report_wal(bool force)
 	/* flush wal stats */
 	pgstat_flush_wal(nowait);
 
+	pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_WAL);
+
 	/* flush IO stats */
 	pgstat_flush_io(nowait);
 }
@@ -117,9 +119,9 @@ pgstat_wal_flush_cb(bool nowait)
 		return true;
 
 #define WALSTAT_ACC(fld, var_to_add) \
-	(stats_shmem->stats.fld += var_to_add.fld)
+	(stats_shmem->stats.wal_counters.fld += var_to_add.fld)
 #define WALSTAT_ACC_INSTR_TIME(fld) \
-	(stats_shmem->stats.fld += INSTR_TIME_GET_MICROSEC(PendingWalStats.fld))
+	(stats_shmem->stats.wal_counters.fld += INSTR_TIME_GET_MICROSEC(PendingWalStats.fld))
 	WALSTAT_ACC(wal_records, wal_usage_diff);
 	WALSTAT_ACC(wal_fpi, wal_usage_diff);
 	WALSTAT_ACC(wal_bytes, wal_usage_diff);
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 8a4340e977..8d251dfcca 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1562,11 +1562,12 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 /*
  * pg_stat_wal_build_tuple
  *
- * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
- * of wal_stats.
+ * Helper routine for pg_stat_get_wal() and pg_stat_get_backend_wal() returning
+ * one tuple based on the contents of wal_counters.
  */
 static Datum
-pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
+pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
+						TimestampTz stat_reset_timestamp)
 {
 #define PG_STAT_WAL_COLS	9
 	TupleDesc	tupdesc;
@@ -1598,26 +1599,26 @@ pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
 	BlessTupleDesc(tupdesc);
 
 	/* Fill values and NULLs */
-	values[0] = Int64GetDatum(wal_stats.wal_records);
-	values[1] = Int64GetDatum(wal_stats.wal_fpi);
+	values[0] = Int64GetDatum(wal_counters.wal_records);
+	values[1] = Int64GetDatum(wal_counters.wal_fpi);
 
 	/* Convert to numeric. */
-	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats.wal_bytes);
+	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_counters.wal_bytes);
 	values[2] = DirectFunctionCall3(numeric_in,
 									CStringGetDatum(buf),
 									ObjectIdGetDatum(0),
 									Int32GetDatum(-1));
 
-	values[3] = Int64GetDatum(wal_stats.wal_buffers_full);
-	values[4] = Int64GetDatum(wal_stats.wal_write);
-	values[5] = Int64GetDatum(wal_stats.wal_sync);
+	values[3] = Int64GetDatum(wal_counters.wal_buffers_full);
+	values[4] = Int64GetDatum(wal_counters.wal_write);
+	values[5] = Int64GetDatum(wal_counters.wal_sync);
 
 	/* Convert counters from microsec to millisec for display */
-	values[6] = Float8GetDatum(((double) wal_stats.wal_write_time) / 1000.0);
-	values[7] = Float8GetDatum(((double) wal_stats.wal_sync_time) / 1000.0);
+	values[6] = Float8GetDatum(((double) wal_counters.wal_write_time) / 1000.0);
+	values[7] = Float8GetDatum(((double) wal_counters.wal_sync_time) / 1000.0);
 
-	if (wal_stats.stat_reset_timestamp != 0)
-		values[8] = TimestampTzGetDatum(wal_stats.stat_reset_timestamp);
+	if (stat_reset_timestamp != 0)
+		values[8] = TimestampTzGetDatum(stat_reset_timestamp);
 	else
 		nulls[8] = true;
 
@@ -1625,6 +1626,55 @@ pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns WAL statistics for a backend with given PID.
+ */
+Datum
+pg_stat_get_backend_wal(PG_FUNCTION_ARGS)
+{
+	int			pid;
+	PGPROC	   *proc;
+	ProcNumber	procNumber;
+	PgStat_Backend *backend_stats;
+	PgStat_WalCounters bktype_stats;
+	PgBackendStatus *beentry;
+
+	pid = PG_GETARG_INT32(0);
+	proc = BackendPidGetProc(pid);
+
+	/*
+	 * This could be an auxiliary process but these do not report backend
+	 * statistics due to pgstat_tracks_backend_bktype(), so there is no need
+	 * for an extra call to AuxiliaryPidGetProc().
+	 */
+	if (!proc)
+		PG_RETURN_NULL();
+
+	procNumber = GetNumberFromPGProc(proc);
+
+	beentry = pgstat_get_beentry_by_proc_number(procNumber);
+	if (!beentry)
+		PG_RETURN_NULL();
+
+	backend_stats = pgstat_fetch_stat_backend(procNumber);
+	if (!backend_stats)
+		PG_RETURN_NULL();
+
+	/* if PID does not match, leave */
+	if (beentry->st_procpid != pid)
+		PG_RETURN_NULL();
+
+	/* backend may be gone, so recheck in case */
+	if (beentry->st_backendType == B_INVALID)
+		PG_RETURN_NULL();
+
+	bktype_stats = backend_stats->wal_stats;
+
+	/* save tuples with data from this PgStat_BktypeIO */
+	return (pg_stat_wal_build_tuple(bktype_stats, backend_stats->stat_reset_timestamp));
+}
+
+
 /*
  * Returns statistics of WAL activity
  */
@@ -1636,7 +1686,7 @@ pg_stat_get_wal(PG_FUNCTION_ARGS)
 	/* Get statistics about WAL activity */
 	wal_stats = pgstat_fetch_stat_wal();
 
-	return (pg_stat_wal_build_tuple(*wal_stats));
+	return (pg_stat_wal_build_tuple(wal_stats->wal_counters, wal_stats->stat_reset_timestamp));
 }
 
 /*
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index b37e8a6f88..72a5dae4b1 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5929,6 +5929,13 @@
   proargmodes => '{o,o,o,o,o,o,o,o,o}',
   proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,wal_write,wal_sync,wal_write_time,wal_sync_time,stats_reset}',
   prosrc => 'pg_stat_get_wal' },
+{ oid => '8037', descr => 'statistics: backend WAL activity',
+  proname => 'pg_stat_get_backend_wal', provolatile => 'v',
+  proparallel => 'r', prorettype => 'record', proargtypes => 'int4',
+  proallargtypes => '{int4,int8,int8,numeric,int8,int8,int8,float8,float8,timestamptz}',
+  proargmodes => '{i,o,o,o,o,o,o,o,o,o}',
+  proargnames => '{backend_pid,wal_records,wal_fpi,wal_bytes,wal_buffers_full,wal_write,wal_sync,wal_write_time,wal_sync_time,stats_reset}',
+  prosrc => 'pg_stat_get_backend_wal' },
 { oid => '6248', descr => 'statistics: information about WAL prefetching',
   proname => 'pg_stat_get_recovery_prefetch', prorows => '1', proretset => 't',
   provolatile => 'v', prorettype => 'record', proargtypes => '',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 6475889c58..dd9cbbe103 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -375,24 +375,6 @@ typedef struct PgStat_IO
 	PgStat_BktypeIO stats[BACKEND_NUM_TYPES];
 } PgStat_IO;
 
-typedef struct PgStat_Backend
-{
-	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO io_stats;
-} PgStat_Backend;
-
-/* ---------
- * PgStat_BackendPending	Non-flushed backend stats.
- * ---------
- */
-typedef struct PgStat_BackendPending
-{
-	/*
-	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
-	 */
-	PgStat_PendingIO pending_io;
-} PgStat_BackendPending;
-
 typedef struct PgStat_StatDBEntry
 {
 	PgStat_Counter xact_commit;
@@ -504,7 +486,7 @@ typedef struct PgStat_StatTabEntry
 	PgStat_Counter autoanalyze_count;
 } PgStat_StatTabEntry;
 
-typedef struct PgStat_WalStats
+typedef struct PgStat_WalCounters
 {
 	PgStat_Counter wal_records;
 	PgStat_Counter wal_fpi;
@@ -514,6 +496,11 @@ typedef struct PgStat_WalStats
 	PgStat_Counter wal_sync;
 	PgStat_Counter wal_write_time;
 	PgStat_Counter wal_sync_time;
+} PgStat_WalCounters;
+
+typedef struct PgStat_WalStats
+{
+	PgStat_WalCounters wal_counters;
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
@@ -532,6 +519,28 @@ typedef struct PgStat_PendingWalStats
 	instr_time	wal_sync_time;
 } PgStat_PendingWalStats;
 
+/* ---------
+ * PgStat_BackendPending	Non-flushed backend stats.
+ * ---------
+ */
+typedef struct PgStat_BackendPending
+{
+	/*
+	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
+	 */
+	PgStat_PendingIO pending_io;
+	/*
+	 * We are not creating one member for PgStat_PendingWalStats. See the
+	 * comment above the PendingBackendWalStats definition as to why.
+	 */
+} PgStat_BackendPending;
+
+typedef struct PgStat_Backend
+{
+	TimestampTz stat_reset_timestamp;
+	PgStat_BktypeIO io_stats;
+	PgStat_WalCounters wal_stats;
+} PgStat_Backend;
 
 /*
  * Functions in pgstat.c
@@ -862,5 +871,17 @@ extern PGDLLIMPORT SessionEndType pgStatSessionEndCause;
 /* updated directly by backends and background processes */
 extern PGDLLIMPORT PgStat_PendingWalStats PendingWalStats;
 
+/*
+ * Variables in pgstat_backend.c
+ */
+
+/* updated directly by backends and background processes */
+
+/*
+ * WAL pending statistics are incremented inside a critical section
+ * (see XLogWrite()), so we can't use pgstat_prep_pending_entry() and we rely on
+ * PendingBackendWalStats instead.
+ */
+extern PGDLLIMPORT PgStat_PendingWalStats PendingBackendWalStats;
 
 #endif							/* PGSTAT_H */
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 4bb8e5c53a..05c3e21500 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -615,7 +615,8 @@ extern void pgstat_archiver_snapshot_cb(void);
 
 /* flags for pgstat_flush_backend() */
 #define PGSTAT_BACKEND_FLUSH_IO		(1 << 0)	/* Flush I/O statistics */
-#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO)
+#define PGSTAT_BACKEND_FLUSH_WAL	(1 << 1)	/* Flush WAL statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO | PGSTAT_BACKEND_FLUSH_WAL)
 
 extern void pgstat_flush_backend(bool nowait, bits32 flags);
 extern PgStat_BackendPending *pgstat_prep_backend_pending(ProcNumber procnum);
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index a0317b7208..cc01fdf274 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -832,6 +832,8 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -851,6 +853,18 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
  t
 (1 row)
 
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+ ?column? 
+----------
+ t
+(1 row)
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 399c72bbcf..28fe0a1a7d 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -423,6 +423,9 @@ SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
 
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 
@@ -435,6 +438,9 @@ CHECKPOINT;
 SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;
 SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
 
+SELECT pg_stat_force_next_flush();
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index eb93debe10..faca109b80 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2174,6 +2174,7 @@ PgStat_SubXactStatus
 PgStat_TableCounts
 PgStat_TableStatus
 PgStat_TableXactStatus
+PgStat_WalCounters
 PgStat_WalStats
 PgXmlErrorContext
 PgXmlStrictness
-- 
2.34.1

From 327160f7f96bcdc1c3779d25bb0228f25a4c2504 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Thu, 16 Jan 2025 08:51:09 +0000
Subject: [PATCH v5 1/4] Add allow_critical_section to PgStat_KindInfo for
 pgstats kinds

This new field controls if a variable-numbered stats kind is allowed
to allocate memory in contexts while in a critical section.

Allocating memory in contexts while in a critical section isn't usually allowed,
but we make an exception.

It means that there's a theoretical possibility to  run out of memory while
allocating the memory, which leads to a PANIC.

The memory allocation here are small (pending entries or entry reference) so
that's unlikely to happen in practice.

This is useful for a following commit that will track WAL statistics while in
a critical section (while in XLogWrite() for example).
---
 src/backend/utils/activity/pgstat.c           | 19 +++++++++++++++++++
 src/backend/utils/activity/pgstat_shmem.c     | 13 +++++++++++++
 src/include/utils/pgstat_internal.h           |  3 +++
 .../injection_points/injection_stats.c        |  1 +
 4 files changed, 36 insertions(+)
  89.9% src/backend/utils/activity/
   7.6% src/include/utils/

diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index 34520535d54..a942e04bb97 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -284,6 +284,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 
 		.fixed_amount = false,
 		.write_to_file = true,
+		.allow_critical_section = false,
 		/* so pg_stat_database entries can be seen in all databases */
 		.accessed_across_databases = true,
 
@@ -301,6 +302,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 
 		.fixed_amount = false,
 		.write_to_file = true,
+		.allow_critical_section = false,
 
 		.shared_size = sizeof(PgStatShared_Relation),
 		.shared_data_off = offsetof(PgStatShared_Relation, stats),
@@ -316,6 +318,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 
 		.fixed_amount = false,
 		.write_to_file = true,
+		.allow_critical_section = false,
 
 		.shared_size = sizeof(PgStatShared_Function),
 		.shared_data_off = offsetof(PgStatShared_Function, stats),
@@ -330,6 +333,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 
 		.fixed_amount = false,
 		.write_to_file = true,
+		.allow_critical_section = false,
 
 		.accessed_across_databases = true,
 
@@ -347,6 +351,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 
 		.fixed_amount = false,
 		.write_to_file = true,
+		.allow_critical_section = false,
 		/* so pg_stat_subscription_stats entries can be seen in all databases */
 		.accessed_across_databases = true,
 
@@ -364,6 +369,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 
 		.fixed_amount = false,
 		.write_to_file = false,
+		.allow_critical_section = false,
 
 		.accessed_across_databases = true,
 
@@ -1316,7 +1322,20 @@ pgstat_prep_pending_entry(PgStat_Kind kind, Oid dboid, uint64 objid, bool *creat
 
 		Assert(entrysize != (size_t) -1);
 
+		/*
+		 * That could be done within a critical section, which isn't usually
+		 * allowed, but we make an exception. It means that there's a
+		 * theoretical possibility that you run out of memory while preparing
+		 * the entry, which leads to a PANIC. Fortunately the pending entry is
+		 * small so that's unlikely to happen in practice.
+		 */
+		if (pgstat_get_kind_info(kind)->allow_critical_section)
+			MemoryContextAllowInCriticalSection(pgStatPendingContext, true);
+
 		entry_ref->pending = MemoryContextAllocZero(pgStatPendingContext, entrysize);
+
+		MemoryContextAllowInCriticalSection(pgStatPendingContext, false);
+
 		dlist_push_tail(&pgStatPending, &entry_ref->pending_node);
 	}
 
diff --git a/src/backend/utils/activity/pgstat_shmem.c b/src/backend/utils/activity/pgstat_shmem.c
index 342586397d6..cb1ad330bd4 100644
--- a/src/backend/utils/activity/pgstat_shmem.c
+++ b/src/backend/utils/activity/pgstat_shmem.c
@@ -402,9 +402,22 @@ pgstat_get_entry_ref_cached(PgStat_HashKey key, PgStat_EntryRef **entry_ref_p)
 	{
 		PgStat_EntryRef *entry_ref;
 
+		/*
+		 * That could be done within a critical section, which isn't usually
+		 * allowed, but we make an exception. It means that there's a
+		 * theoretical possibility that you run out of memory while creating
+		 * the entry ref, which leads to a PANIC. Fortunately the pending
+		 * entry is small so that's unlikely to happen in practice.
+		 */
+		if (pgstat_get_kind_info(key.kind)->allow_critical_section)
+			MemoryContextAllowInCriticalSection(pgStatSharedRefContext, true);
+
 		cache_entry->entry_ref = entry_ref =
 			MemoryContextAlloc(pgStatSharedRefContext,
 							   sizeof(PgStat_EntryRef));
+
+		MemoryContextAllowInCriticalSection(pgStatSharedRefContext, false);
+
 		entry_ref->shared_stats = NULL;
 		entry_ref->shared_entry = NULL;
 		entry_ref->pending = NULL;
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 4bb8e5c53ab..0b21603c863 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -216,6 +216,9 @@ typedef struct PgStat_KindInfo
 	/* Should stats be written to the on-disk stats file? */
 	bool		write_to_file:1;
 
+	/* For variable-numbered stats: allow memory context in critical section */
+	bool		allow_critical_section:1;
+
 	/*
 	 * The size of an entry in the shared stats hash table (pointed to by
 	 * PgStatShared_HashEntry->body).  For fixed-numbered statistics, this is
diff --git a/src/test/modules/injection_points/injection_stats.c b/src/test/modules/injection_points/injection_stats.c
index 5db62bca66f..873dc88aace 100644
--- a/src/test/modules/injection_points/injection_stats.c
+++ b/src/test/modules/injection_points/injection_stats.c
@@ -40,6 +40,7 @@ static const PgStat_KindInfo injection_stats = {
 	.name = "injection_points",
 	.fixed_amount = false,		/* Bounded by the number of points */
 	.write_to_file = true,
+	.allow_critical_section = false,
 
 	/* Injection points are system-wide */
 	.accessed_across_databases = true,
-- 
2.34.1

From 0ff81e01d4292e575270a4bd380ebb0505c57e11 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 07:51:27 +0000
Subject: [PATCH v5 2/4] Extract logic filling pg_stat_get_wal()'s tuple into
 its own routine

This commit adds pg_stat_wal_build_tuple(), a helper routine for
pg_stat_get_wal(), that fills its tuple based on the contents
of PgStat_WalStats.  This will be used in a follow-up commit that uses
the same structures as pg_stat_wal for reporting, but for the PGSTAT_KIND_BACKEND
statistics kind.
---
 src/backend/utils/adt/pgstatfuncs.c | 56 ++++++++++++++++++-----------
 1 file changed, 36 insertions(+), 20 deletions(-)
 100.0% src/backend/utils/adt/

diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 0f5e0a9778d..0442be03304 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1604,20 +1604,22 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 }
 
 /*
- * Returns statistics of WAL activity
+ * pg_stat_wal_build_tuple
+ *
+ * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
+ * of wal_stats.
  */
-Datum
-pg_stat_get_wal(PG_FUNCTION_ARGS)
+static Datum
+pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
 {
-#define PG_STAT_GET_WAL_COLS	9
+#define PG_STAT_WAL_COLS	9
 	TupleDesc	tupdesc;
-	Datum		values[PG_STAT_GET_WAL_COLS] = {0};
-	bool		nulls[PG_STAT_GET_WAL_COLS] = {0};
+	Datum		values[PG_STAT_WAL_COLS] = {0};
+	bool		nulls[PG_STAT_WAL_COLS] = {0};
 	char		buf[256];
-	PgStat_WalStats *wal_stats;
 
 	/* Initialise attributes information in the tuple descriptor */
-	tupdesc = CreateTemplateTupleDesc(PG_STAT_GET_WAL_COLS);
+	tupdesc = CreateTemplateTupleDesc(PG_STAT_WAL_COLS);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "wal_records",
 					   INT8OID, -1, 0);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "wal_fpi",
@@ -1639,34 +1641,48 @@ pg_stat_get_wal(PG_FUNCTION_ARGS)
 
 	BlessTupleDesc(tupdesc);
 
-	/* Get statistics about WAL activity */
-	wal_stats = pgstat_fetch_stat_wal();
-
 	/* Fill values and NULLs */
-	values[0] = Int64GetDatum(wal_stats->wal_records);
-	values[1] = Int64GetDatum(wal_stats->wal_fpi);
+	values[0] = Int64GetDatum(wal_stats.wal_records);
+	values[1] = Int64GetDatum(wal_stats.wal_fpi);
 
 	/* Convert to numeric. */
-	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats->wal_bytes);
+	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats.wal_bytes);
 	values[2] = DirectFunctionCall3(numeric_in,
 									CStringGetDatum(buf),
 									ObjectIdGetDatum(0),
 									Int32GetDatum(-1));
 
-	values[3] = Int64GetDatum(wal_stats->wal_buffers_full);
-	values[4] = Int64GetDatum(wal_stats->wal_write);
-	values[5] = Int64GetDatum(wal_stats->wal_sync);
+	values[3] = Int64GetDatum(wal_stats.wal_buffers_full);
+	values[4] = Int64GetDatum(wal_stats.wal_write);
+	values[5] = Int64GetDatum(wal_stats.wal_sync);
 
 	/* Convert counters from microsec to millisec for display */
-	values[6] = Float8GetDatum(((double) wal_stats->wal_write_time) / 1000.0);
-	values[7] = Float8GetDatum(((double) wal_stats->wal_sync_time) / 1000.0);
+	values[6] = Float8GetDatum(((double) wal_stats.wal_write_time) / 1000.0);
+	values[7] = Float8GetDatum(((double) wal_stats.wal_sync_time) / 1000.0);
 
-	values[8] = TimestampTzGetDatum(wal_stats->stat_reset_timestamp);
+	if (wal_stats.stat_reset_timestamp != 0)
+		values[8] = TimestampTzGetDatum(wal_stats.stat_reset_timestamp);
+	else
+		nulls[8] = true;
 
 	/* Returns the record as Datum */
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns statistics of WAL activity
+ */
+Datum
+pg_stat_get_wal(PG_FUNCTION_ARGS)
+{
+	PgStat_WalStats *wal_stats;
+
+	/* Get statistics about WAL activity */
+	wal_stats = pgstat_fetch_stat_wal();
+
+	return (pg_stat_wal_build_tuple(*wal_stats));
+}
+
 /*
  * Returns statistics of SLRU caches.
  */
-- 
2.34.1

From 383d186b1699ac74a53f2fea85ced5abc10cd558 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Thu, 16 Jan 2025 15:06:01 +0000
Subject: [PATCH v5 3/4] Adding a new PgStat_WalCounters struct

This new struct contains only the counters related to the WAL statistics.
This will be used in a follow-up commit that uses the same structures but
for the PGSTAT_KIND_BACKEND statistics kind.
---
 src/backend/utils/activity/pgstat_wal.c |  4 ++--
 src/backend/utils/adt/pgstatfuncs.c     | 28 +++++++++++++------------
 src/include/pgstat.h                    |  7 ++++++-
 src/tools/pgindent/typedefs.list        |  1 +
 4 files changed, 24 insertions(+), 16 deletions(-)
  14.1% src/backend/utils/activity/
  79.8% src/backend/utils/adt/
   5.0% src/include/

diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 18fa6b2936a..bfc06178a68 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -117,9 +117,9 @@ pgstat_wal_flush_cb(bool nowait)
 		return true;
 
 #define WALSTAT_ACC(fld, var_to_add) \
-	(stats_shmem->stats.fld += var_to_add.fld)
+	(stats_shmem->stats.wal_counters.fld += var_to_add.fld)
 #define WALSTAT_ACC_INSTR_TIME(fld) \
-	(stats_shmem->stats.fld += INSTR_TIME_GET_MICROSEC(PendingWalStats.fld))
+	(stats_shmem->stats.wal_counters.fld += INSTR_TIME_GET_MICROSEC(PendingWalStats.fld))
 	WALSTAT_ACC(wal_records, wal_usage_diff);
 	WALSTAT_ACC(wal_fpi, wal_usage_diff);
 	WALSTAT_ACC(wal_bytes, wal_usage_diff);
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 0442be03304..97510d48eef 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1607,10 +1607,11 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
  * pg_stat_wal_build_tuple
  *
  * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
- * of wal_stats.
+ * of wal_counters.
  */
 static Datum
-pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
+pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
+						TimestampTz stat_reset_timestamp)
 {
 #define PG_STAT_WAL_COLS	9
 	TupleDesc	tupdesc;
@@ -1642,26 +1643,26 @@ pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
 	BlessTupleDesc(tupdesc);
 
 	/* Fill values and NULLs */
-	values[0] = Int64GetDatum(wal_stats.wal_records);
-	values[1] = Int64GetDatum(wal_stats.wal_fpi);
+	values[0] = Int64GetDatum(wal_counters.wal_records);
+	values[1] = Int64GetDatum(wal_counters.wal_fpi);
 
 	/* Convert to numeric. */
-	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats.wal_bytes);
+	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_counters.wal_bytes);
 	values[2] = DirectFunctionCall3(numeric_in,
 									CStringGetDatum(buf),
 									ObjectIdGetDatum(0),
 									Int32GetDatum(-1));
 
-	values[3] = Int64GetDatum(wal_stats.wal_buffers_full);
-	values[4] = Int64GetDatum(wal_stats.wal_write);
-	values[5] = Int64GetDatum(wal_stats.wal_sync);
+	values[3] = Int64GetDatum(wal_counters.wal_buffers_full);
+	values[4] = Int64GetDatum(wal_counters.wal_write);
+	values[5] = Int64GetDatum(wal_counters.wal_sync);
 
 	/* Convert counters from microsec to millisec for display */
-	values[6] = Float8GetDatum(((double) wal_stats.wal_write_time) / 1000.0);
-	values[7] = Float8GetDatum(((double) wal_stats.wal_sync_time) / 1000.0);
+	values[6] = Float8GetDatum(((double) wal_counters.wal_write_time) / 1000.0);
+	values[7] = Float8GetDatum(((double) wal_counters.wal_sync_time) / 1000.0);
 
-	if (wal_stats.stat_reset_timestamp != 0)
-		values[8] = TimestampTzGetDatum(wal_stats.stat_reset_timestamp);
+	if (stat_reset_timestamp != 0)
+		values[8] = TimestampTzGetDatum(stat_reset_timestamp);
 	else
 		nulls[8] = true;
 
@@ -1680,7 +1681,8 @@ pg_stat_get_wal(PG_FUNCTION_ARGS)
 	/* Get statistics about WAL activity */
 	wal_stats = pgstat_fetch_stat_wal();
 
-	return (pg_stat_wal_build_tuple(*wal_stats));
+	return (pg_stat_wal_build_tuple(wal_stats->wal_counters,
+									wal_stats->stat_reset_timestamp));
 }
 
 /*
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index a878402f502..bb8e0044a47 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -463,7 +463,7 @@ typedef struct PgStat_StatTabEntry
 	PgStat_Counter autoanalyze_count;
 } PgStat_StatTabEntry;
 
-typedef struct PgStat_WalStats
+typedef struct PgStat_WalCounters
 {
 	PgStat_Counter wal_records;
 	PgStat_Counter wal_fpi;
@@ -473,6 +473,11 @@ typedef struct PgStat_WalStats
 	PgStat_Counter wal_sync;
 	PgStat_Counter wal_write_time;
 	PgStat_Counter wal_sync_time;
+} PgStat_WalCounters;
+
+typedef struct PgStat_WalStats
+{
+	PgStat_WalCounters wal_counters;
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 94dc956ae8c..d8b6623ba9c 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2176,6 +2176,7 @@ PgStat_SubXactStatus
 PgStat_TableCounts
 PgStat_TableStatus
 PgStat_TableXactStatus
+PgStat_WalCounters
 PgStat_WalStats
 PgXmlErrorContext
 PgXmlStrictness
-- 
2.34.1

From 094bc124baea085a721f0013da290f3b92f96607 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 10:00:00 +0000
Subject: [PATCH v5 4/4] per backend WAL statistics

Now that commit 9aea73fc61 added backend-level statistics to pgstats (and
per backend IO statistics) we can more easily add per backend statistics.

This commit adds per backend WAL statistics using the same layer as pg_stat_wal,
except that it is now possible to know how much WAL activity is happening in each
backend rather than an overall aggregate of all the activity.  A function called
pg_stat_get_backend_wal() is added to access this data depending on the
PID of a backend.

Note that allow_critical_section has been enabled for the PGSTAT_KIND_BACKEND
stats kind as we are now tracking variable-numbered stats in critical section(s)
(XLogWrite() for example).

The same limitation as in 9aea73fc61 persists, meaning that Auxiliary processes
are not included in this set of statistics.

XXX: bump catalog version
---
 doc/src/sgml/config.sgml                    |  4 +-
 doc/src/sgml/monitoring.sgml                | 19 +++++++
 src/backend/access/transam/xlog.c           | 44 +++++++++++++--
 src/backend/utils/activity/pgstat.c         |  2 +-
 src/backend/utils/activity/pgstat_backend.c | 59 +++++++++++++++++++++
 src/backend/utils/activity/pgstat_wal.c     |  2 +
 src/backend/utils/adt/pgstatfuncs.c         | 53 +++++++++++++++++-
 src/include/catalog/pg_proc.dat             |  7 +++
 src/include/pgstat.h                        | 43 ++++++++-------
 src/include/utils/pgstat_internal.h         |  3 +-
 src/test/regress/expected/stats.out         | 14 +++++
 src/test/regress/sql/stats.sql              |  6 +++
 12 files changed, 227 insertions(+), 29 deletions(-)
  14.2% doc/src/sgml/
  14.9% src/backend/access/transam/
  28.8% src/backend/utils/activity/
  19.7% src/backend/utils/adt/
   7.0% src/include/catalog/
   5.1% src/include/
   5.6% src/test/regress/expected/
   4.3% src/test/regress/sql/

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 3f41a17b1fe..075cb4fe1a8 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8292,7 +8292,9 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
         measure the overhead of timing on your system.
         I/O timing information is
         displayed in <link linkend="monitoring-pg-stat-wal-view">
-        <structname>pg_stat_wal</structname></link>.
+        <structname>pg_stat_wal</structname></link> and in the output of the
+        <link linkend="pg-stat-get-backend-wal">
+        <function>pg_stat_get_backend_wal()</function></link> function.
         Only superusers and users with the appropriate <literal>SET</literal>
         privilege can change this setting.
        </para>
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index e5888fae2b5..8fdf7d13f21 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -4824,6 +4824,25 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para></entry>
       </row>
 
+      <row>
+       <entry id="pg-stat-get-backend-wal" role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_stat_get_backend_wal</primary>
+        </indexterm>
+        <function>pg_stat_get_backend_wal</function> ( <type>integer</type> )
+        <returnvalue>record</returnvalue>
+       </para>
+       <para>
+        Returns WAL statistics about the backend with the specified
+        process ID. The output fields are exactly the same as the ones in the
+        <structname>pg_stat_wal</structname> view.
+       </para>
+       <para>
+        The function does not return WAL statistics for the checkpointer,
+        the background writer, the startup process and the autovacuum launcher.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index bf3dbda901d..d09a079bcbb 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -96,6 +96,7 @@
 #include "utils/guc_hooks.h"
 #include "utils/guc_tables.h"
 #include "utils/injection_point.h"
+#include "utils/pgstat_internal.h"
 #include "utils/ps_status.h"
 #include "utils/relmapper.h"
 #include "utils/snapmgr.h"
@@ -2058,6 +2059,15 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
 					XLogWrite(WriteRqst, tli, false);
 					LWLockRelease(WALWriteLock);
 					PendingWalStats.wal_buffers_full++;
+
+					if (pgstat_tracks_backend_bktype(MyBackendType))
+					{
+						PgStat_BackendPending *entry_ref;
+
+						entry_ref = pgstat_prep_backend_pending(MyProcNumber);
+						entry_ref->pending_wal.wal_buffers_full++;
+					}
+
 					TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_DONE();
 				}
 				/* Re-acquire WALBufMappingLock and retry */
@@ -2426,11 +2436,14 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
 			Size		nleft;
 			ssize_t		written;
 			instr_time	start;
+			instr_time	end;
 
 			/* OK to write the page(s) */
 			from = XLogCtl->pages + startidx * (Size) XLOG_BLCKSZ;
 			nbytes = npages * (Size) XLOG_BLCKSZ;
 			nleft = nbytes;
+			/* keep compiler quiet */
+			INSTR_TIME_SET_ZERO(end);
 			do
 			{
 				errno = 0;
@@ -2451,14 +2464,24 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
 				 */
 				if (track_wal_io_timing)
 				{
-					instr_time	end;
-
 					INSTR_TIME_SET_CURRENT(end);
 					INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_write_time, end, start);
 				}
 
 				PendingWalStats.wal_write++;
 
+				if (pgstat_tracks_backend_bktype(MyBackendType))
+				{
+					PgStat_BackendPending *entry_ref;
+
+					entry_ref = pgstat_prep_backend_pending(MyProcNumber);
+					entry_ref->pending_wal.wal_write++;
+
+					if (track_wal_io_timing)
+						INSTR_TIME_ACCUM_DIFF(entry_ref->pending_wal.wal_write_time,
+											  end, start);
+				}
+
 				if (written <= 0)
 				{
 					char		xlogfname[MAXFNAMELEN];
@@ -8684,8 +8707,11 @@ issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
 {
 	char	   *msg = NULL;
 	instr_time	start;
+	instr_time	end;
 
 	Assert(tli != 0);
+	/* keep compiler quiet */
+	INSTR_TIME_SET_ZERO(end);
 
 	/*
 	 * Quick exit if fsync is disabled or write() has already synced the WAL
@@ -8751,13 +8777,23 @@ issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
 	 */
 	if (track_wal_io_timing)
 	{
-		instr_time	end;
-
 		INSTR_TIME_SET_CURRENT(end);
 		INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_sync_time, end, start);
 	}
 
 	PendingWalStats.wal_sync++;
+
+	if (pgstat_tracks_backend_bktype(MyBackendType))
+	{
+		PgStat_BackendPending *entry_ref;
+
+		entry_ref = pgstat_prep_backend_pending(MyProcNumber);
+		entry_ref->pending_wal.wal_sync++;
+
+		if (track_wal_io_timing)
+			INSTR_TIME_ACCUM_DIFF(entry_ref->pending_wal.wal_sync_time,
+								  end, start);
+	}
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index a942e04bb97..65b2f687816 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -369,7 +369,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 
 		.fixed_amount = false,
 		.write_to_file = false,
-		.allow_critical_section = false,
+		.allow_critical_section = true,
 
 		.accessed_across_databases = true,
 
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 79e4d0a3053..94762b12f8d 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -24,6 +24,14 @@
 
 #include "utils/pgstat_internal.h"
 
+/*
+ * WAL usage counters saved from pgWalUsage at the previous call to
+ * pgstat_report_wal(). This is used to calculate how much WAL usage
+ * happens between pgstat_report_wal() calls, by subtracting
+ * the previous counters from the current ones.
+ */
+static WalUsage prevBackendWalUsage;
+
 /*
  * Returns statistics of a backend by proc number.
  */
@@ -77,6 +85,52 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 	}
 }
 
+/*
+ * Flush out locally pending backend WAL statistics.  Locking is managed
+ * by the caller.
+ */
+static void
+pgstat_flush_backend_entry_wal(PgStat_EntryRef *entry_ref)
+{
+	PgStatShared_Backend *shbackendent;
+	PgStat_BackendPending *pendingent;
+	PgStat_WalCounters *bktype_shstats;
+	PgStat_PendingWalStats pending_wal;
+	WalUsage	wal_usage_diff = {0};
+
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	pendingent = (PgStat_BackendPending *) entry_ref->pending;
+	bktype_shstats = &shbackendent->stats.wal_stats;
+	pending_wal = pendingent->pending_wal;
+
+	/*
+	 * We don't update the WAL usage portion of the local WalStats elsewhere.
+	 * Calculate how much WAL usage counters were increased by subtracting the
+	 * previous counters from the current ones.
+	 */
+	WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
+
+#define WALSTAT_ACC(fld, var_to_add) \
+	(bktype_shstats->fld += var_to_add.fld)
+#define WALSTAT_ACC_INSTR_TIME(fld) \
+	(bktype_shstats->fld += INSTR_TIME_GET_MICROSEC(pending_wal.fld))
+	WALSTAT_ACC(wal_buffers_full, pending_wal);
+	WALSTAT_ACC(wal_write, pending_wal);
+	WALSTAT_ACC(wal_sync, pending_wal);
+	WALSTAT_ACC(wal_records, wal_usage_diff);
+	WALSTAT_ACC(wal_fpi, wal_usage_diff);
+	WALSTAT_ACC(wal_bytes, wal_usage_diff);
+	WALSTAT_ACC_INSTR_TIME(wal_write_time);
+	WALSTAT_ACC_INSTR_TIME(wal_sync_time);
+#undef WALSTAT_ACC_INSTR_TIME
+#undef WALSTAT_ACC
+
+	/*
+	 * Save the current counters for the subsequent calculation of WAL usage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
+}
+
 /*
  * Wrapper routine to flush backend statistics.
  */
@@ -94,6 +148,9 @@ pgstat_flush_backend_entry(PgStat_EntryRef *entry_ref, bool nowait,
 	if (flags & PGSTAT_BACKEND_FLUSH_IO)
 		pgstat_flush_backend_entry_io(entry_ref);
 
+	if (flags & PGSTAT_BACKEND_FLUSH_WAL)
+		pgstat_flush_backend_entry_wal(entry_ref);
+
 	pgstat_unlock_entry(entry_ref);
 
 	return true;
@@ -147,6 +204,8 @@ pgstat_create_backend(ProcNumber procnum)
 	 * e.g. if we previously used this proc number.
 	 */
 	memset(&shstatent->stats, 0, sizeof(shstatent->stats));
+
+	prevBackendWalUsage = pgWalUsage;
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index bfc06178a68..33627642261 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -55,6 +55,8 @@ pgstat_report_wal(bool force)
 	/* flush wal stats */
 	pgstat_flush_wal(nowait);
 
+	pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_WAL);
+
 	/* flush IO stats */
 	pgstat_flush_io(nowait);
 }
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 97510d48eef..52bfda4f923 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1606,8 +1606,8 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 /*
  * pg_stat_wal_build_tuple
  *
- * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
- * of wal_counters.
+ * Helper routine for pg_stat_get_wal() and pg_stat_get_backend_wal () returning
+ * one tuple based on the contents of wal_counters.
  */
 static Datum
 pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
@@ -1670,6 +1670,55 @@ pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns WAL statistics for a backend with given PID.
+ */
+Datum
+pg_stat_get_backend_wal(PG_FUNCTION_ARGS)
+{
+	int			pid;
+	PGPROC	   *proc;
+	ProcNumber	procNumber;
+	PgStat_Backend *backend_stats;
+	PgStat_WalCounters bktype_stats;
+	PgBackendStatus *beentry;
+
+	pid = PG_GETARG_INT32(0);
+	proc = BackendPidGetProc(pid);
+
+	/*
+	 * This could be an auxiliary process but these do not report backend
+	 * statistics due to pgstat_tracks_backend_bktype(), so there is no need
+	 * for an extra call to AuxiliaryPidGetProc().
+	 */
+	if (!proc)
+		PG_RETURN_NULL();
+
+	procNumber = GetNumberFromPGProc(proc);
+
+	beentry = pgstat_get_beentry_by_proc_number(procNumber);
+	if (!beentry)
+		PG_RETURN_NULL();
+
+	backend_stats = pgstat_fetch_stat_backend(procNumber);
+	if (!backend_stats)
+		PG_RETURN_NULL();
+
+	/* if PID does not match, leave */
+	if (beentry->st_procpid != pid)
+		PG_RETURN_NULL();
+
+	/* backend may be gone, so recheck in case */
+	if (beentry->st_backendType == B_INVALID)
+		PG_RETURN_NULL();
+
+	bktype_stats = backend_stats->wal_stats;
+
+	/* save tuples with data from this PgStat_BktypeIO */
+	return (pg_stat_wal_build_tuple(bktype_stats, backend_stats->stat_reset_timestamp));
+}
+
+
 /*
  * Returns statistics of WAL activity
  */
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index ba02ba53b29..9d57c69c152 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5929,6 +5929,13 @@
   proargmodes => '{o,o,o,o,o,o,o,o,o}',
   proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,wal_write,wal_sync,wal_write_time,wal_sync_time,stats_reset}',
   prosrc => 'pg_stat_get_wal' },
+{ oid => '8037', descr => 'statistics: backend WAL activity',
+  proname => 'pg_stat_get_backend_wal', provolatile => 'v',
+  proparallel => 'r', prorettype => 'record', proargtypes => 'int4',
+  proallargtypes => '{int4,int8,int8,numeric,int8,int8,int8,float8,float8,timestamptz}',
+  proargmodes => '{i,o,o,o,o,o,o,o,o,o}',
+  proargnames => '{backend_pid,wal_records,wal_fpi,wal_bytes,wal_buffers_full,wal_write,wal_sync,wal_write_time,wal_sync_time,stats_reset}',
+  prosrc => 'pg_stat_get_backend_wal' },
 { oid => '6248', descr => 'statistics: information about WAL prefetching',
   proname => 'pg_stat_get_recovery_prefetch', prorows => '1', proretset => 't',
   provolatile => 'v', prorettype => 'record', proargtypes => '',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index bb8e0044a47..821104047a0 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -334,24 +334,6 @@ typedef struct PgStat_IO
 	PgStat_BktypeIO stats[BACKEND_NUM_TYPES];
 } PgStat_IO;
 
-typedef struct PgStat_Backend
-{
-	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO io_stats;
-} PgStat_Backend;
-
-/* ---------
- * PgStat_BackendPending	Non-flushed backend stats.
- * ---------
- */
-typedef struct PgStat_BackendPending
-{
-	/*
-	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
-	 */
-	PgStat_PendingIO pending_io;
-} PgStat_BackendPending;
-
 typedef struct PgStat_StatDBEntry
 {
 	PgStat_Counter xact_commit;
@@ -496,6 +478,29 @@ typedef struct PgStat_PendingWalStats
 	instr_time	wal_sync_time;
 } PgStat_PendingWalStats;
 
+/* ---------
+ * PgStat_BackendPending	Non-flushed backend stats.
+ * ---------
+ */
+typedef struct PgStat_BackendPending
+{
+	/*
+	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
+	 */
+	PgStat_PendingIO pending_io;
+
+	/*
+	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_WAL.
+	 */
+	PgStat_PendingWalStats pending_wal;
+} PgStat_BackendPending;
+
+typedef struct PgStat_Backend
+{
+	TimestampTz stat_reset_timestamp;
+	PgStat_BktypeIO io_stats;
+	PgStat_WalCounters wal_stats;
+} PgStat_Backend;
 
 /*
  * Functions in pgstat.c
@@ -826,6 +831,4 @@ extern PGDLLIMPORT SessionEndType pgStatSessionEndCause;
 
 /* updated directly by backends and background processes */
 extern PGDLLIMPORT PgStat_PendingWalStats PendingWalStats;
-
-
 #endif							/* PGSTAT_H */
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 0b21603c863..5a284d8c94a 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -618,7 +618,8 @@ extern void pgstat_archiver_snapshot_cb(void);
 
 /* flags for pgstat_flush_backend() */
 #define PGSTAT_BACKEND_FLUSH_IO		(1 << 0)	/* Flush I/O statistics */
-#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO)
+#define PGSTAT_BACKEND_FLUSH_WAL	(1 << 1)	/* Flush WAL statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO | PGSTAT_BACKEND_FLUSH_WAL)
 
 extern void pgstat_flush_backend(bool nowait, bits32 flags);
 extern PgStat_BackendPending *pgstat_prep_backend_pending(ProcNumber procnum);
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index a0317b7208e..cc01fdf2741 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -832,6 +832,8 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -851,6 +853,18 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
  t
 (1 row)
 
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+ ?column? 
+----------
+ t
+(1 row)
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 399c72bbcf7..28fe0a1a7d0 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -423,6 +423,9 @@ SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
 
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 
@@ -435,6 +438,9 @@ CHECKPOINT;
 SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;
 SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
 
+SELECT pg_stat_force_next_flush();
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
-- 
2.34.1

From 4f2a197b086dd64a2723a8491cc66453012743e9 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 07:51:27 +0000
Subject: [PATCH v6 1/3] Extract logic filling pg_stat_get_wal()'s tuple into
 its own routine

This commit adds pg_stat_wal_build_tuple(), a helper routine for
pg_stat_get_wal(), that fills its tuple based on the contents
of PgStat_WalStats.  This will be used in a follow-up commit that uses
the same structures as pg_stat_wal for reporting, but for the PGSTAT_KIND_BACKEND
statistics kind.
---
 src/backend/utils/adt/pgstatfuncs.c | 56 ++++++++++++++++++-----------
 1 file changed, 36 insertions(+), 20 deletions(-)
 100.0% src/backend/utils/adt/

diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 0f5e0a9778d..0442be03304 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1604,20 +1604,22 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 }
 
 /*
- * Returns statistics of WAL activity
+ * pg_stat_wal_build_tuple
+ *
+ * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
+ * of wal_stats.
  */
-Datum
-pg_stat_get_wal(PG_FUNCTION_ARGS)
+static Datum
+pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
 {
-#define PG_STAT_GET_WAL_COLS	9
+#define PG_STAT_WAL_COLS	9
 	TupleDesc	tupdesc;
-	Datum		values[PG_STAT_GET_WAL_COLS] = {0};
-	bool		nulls[PG_STAT_GET_WAL_COLS] = {0};
+	Datum		values[PG_STAT_WAL_COLS] = {0};
+	bool		nulls[PG_STAT_WAL_COLS] = {0};
 	char		buf[256];
-	PgStat_WalStats *wal_stats;
 
 	/* Initialise attributes information in the tuple descriptor */
-	tupdesc = CreateTemplateTupleDesc(PG_STAT_GET_WAL_COLS);
+	tupdesc = CreateTemplateTupleDesc(PG_STAT_WAL_COLS);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "wal_records",
 					   INT8OID, -1, 0);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "wal_fpi",
@@ -1639,34 +1641,48 @@ pg_stat_get_wal(PG_FUNCTION_ARGS)
 
 	BlessTupleDesc(tupdesc);
 
-	/* Get statistics about WAL activity */
-	wal_stats = pgstat_fetch_stat_wal();
-
 	/* Fill values and NULLs */
-	values[0] = Int64GetDatum(wal_stats->wal_records);
-	values[1] = Int64GetDatum(wal_stats->wal_fpi);
+	values[0] = Int64GetDatum(wal_stats.wal_records);
+	values[1] = Int64GetDatum(wal_stats.wal_fpi);
 
 	/* Convert to numeric. */
-	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats->wal_bytes);
+	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats.wal_bytes);
 	values[2] = DirectFunctionCall3(numeric_in,
 									CStringGetDatum(buf),
 									ObjectIdGetDatum(0),
 									Int32GetDatum(-1));
 
-	values[3] = Int64GetDatum(wal_stats->wal_buffers_full);
-	values[4] = Int64GetDatum(wal_stats->wal_write);
-	values[5] = Int64GetDatum(wal_stats->wal_sync);
+	values[3] = Int64GetDatum(wal_stats.wal_buffers_full);
+	values[4] = Int64GetDatum(wal_stats.wal_write);
+	values[5] = Int64GetDatum(wal_stats.wal_sync);
 
 	/* Convert counters from microsec to millisec for display */
-	values[6] = Float8GetDatum(((double) wal_stats->wal_write_time) / 1000.0);
-	values[7] = Float8GetDatum(((double) wal_stats->wal_sync_time) / 1000.0);
+	values[6] = Float8GetDatum(((double) wal_stats.wal_write_time) / 1000.0);
+	values[7] = Float8GetDatum(((double) wal_stats.wal_sync_time) / 1000.0);
 
-	values[8] = TimestampTzGetDatum(wal_stats->stat_reset_timestamp);
+	if (wal_stats.stat_reset_timestamp != 0)
+		values[8] = TimestampTzGetDatum(wal_stats.stat_reset_timestamp);
+	else
+		nulls[8] = true;
 
 	/* Returns the record as Datum */
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns statistics of WAL activity
+ */
+Datum
+pg_stat_get_wal(PG_FUNCTION_ARGS)
+{
+	PgStat_WalStats *wal_stats;
+
+	/* Get statistics about WAL activity */
+	wal_stats = pgstat_fetch_stat_wal();
+
+	return (pg_stat_wal_build_tuple(*wal_stats));
+}
+
 /*
  * Returns statistics of SLRU caches.
  */
-- 
2.34.1

From 49b4d7f60c30dfa53d7994b05ce012782f449190 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Thu, 16 Jan 2025 15:06:01 +0000
Subject: [PATCH v6 2/3] Adding a new PgStat_WalCounters struct

This new struct contains only the counters related to the WAL statistics.
This will be used in a follow-up commit that uses the same structures but
for the PGSTAT_KIND_BACKEND statistics kind.
---
 src/backend/utils/activity/pgstat_wal.c |  4 ++--
 src/backend/utils/adt/pgstatfuncs.c     | 28 +++++++++++++------------
 src/include/pgstat.h                    |  7 ++++++-
 src/tools/pgindent/typedefs.list        |  1 +
 4 files changed, 24 insertions(+), 16 deletions(-)
  14.1% src/backend/utils/activity/
  79.8% src/backend/utils/adt/
   5.0% src/include/

diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 18fa6b2936a..bfc06178a68 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -117,9 +117,9 @@ pgstat_wal_flush_cb(bool nowait)
 		return true;
 
 #define WALSTAT_ACC(fld, var_to_add) \
-	(stats_shmem->stats.fld += var_to_add.fld)
+	(stats_shmem->stats.wal_counters.fld += var_to_add.fld)
 #define WALSTAT_ACC_INSTR_TIME(fld) \
-	(stats_shmem->stats.fld += INSTR_TIME_GET_MICROSEC(PendingWalStats.fld))
+	(stats_shmem->stats.wal_counters.fld += INSTR_TIME_GET_MICROSEC(PendingWalStats.fld))
 	WALSTAT_ACC(wal_records, wal_usage_diff);
 	WALSTAT_ACC(wal_fpi, wal_usage_diff);
 	WALSTAT_ACC(wal_bytes, wal_usage_diff);
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 0442be03304..97510d48eef 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1607,10 +1607,11 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
  * pg_stat_wal_build_tuple
  *
  * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
- * of wal_stats.
+ * of wal_counters.
  */
 static Datum
-pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
+pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
+						TimestampTz stat_reset_timestamp)
 {
 #define PG_STAT_WAL_COLS	9
 	TupleDesc	tupdesc;
@@ -1642,26 +1643,26 @@ pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
 	BlessTupleDesc(tupdesc);
 
 	/* Fill values and NULLs */
-	values[0] = Int64GetDatum(wal_stats.wal_records);
-	values[1] = Int64GetDatum(wal_stats.wal_fpi);
+	values[0] = Int64GetDatum(wal_counters.wal_records);
+	values[1] = Int64GetDatum(wal_counters.wal_fpi);
 
 	/* Convert to numeric. */
-	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats.wal_bytes);
+	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_counters.wal_bytes);
 	values[2] = DirectFunctionCall3(numeric_in,
 									CStringGetDatum(buf),
 									ObjectIdGetDatum(0),
 									Int32GetDatum(-1));
 
-	values[3] = Int64GetDatum(wal_stats.wal_buffers_full);
-	values[4] = Int64GetDatum(wal_stats.wal_write);
-	values[5] = Int64GetDatum(wal_stats.wal_sync);
+	values[3] = Int64GetDatum(wal_counters.wal_buffers_full);
+	values[4] = Int64GetDatum(wal_counters.wal_write);
+	values[5] = Int64GetDatum(wal_counters.wal_sync);
 
 	/* Convert counters from microsec to millisec for display */
-	values[6] = Float8GetDatum(((double) wal_stats.wal_write_time) / 1000.0);
-	values[7] = Float8GetDatum(((double) wal_stats.wal_sync_time) / 1000.0);
+	values[6] = Float8GetDatum(((double) wal_counters.wal_write_time) / 1000.0);
+	values[7] = Float8GetDatum(((double) wal_counters.wal_sync_time) / 1000.0);
 
-	if (wal_stats.stat_reset_timestamp != 0)
-		values[8] = TimestampTzGetDatum(wal_stats.stat_reset_timestamp);
+	if (stat_reset_timestamp != 0)
+		values[8] = TimestampTzGetDatum(stat_reset_timestamp);
 	else
 		nulls[8] = true;
 
@@ -1680,7 +1681,8 @@ pg_stat_get_wal(PG_FUNCTION_ARGS)
 	/* Get statistics about WAL activity */
 	wal_stats = pgstat_fetch_stat_wal();
 
-	return (pg_stat_wal_build_tuple(*wal_stats));
+	return (pg_stat_wal_build_tuple(wal_stats->wal_counters,
+									wal_stats->stat_reset_timestamp));
 }
 
 /*
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index d0d45150977..9bbba883685 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -467,7 +467,7 @@ typedef struct PgStat_StatTabEntry
 	PgStat_Counter autoanalyze_count;
 } PgStat_StatTabEntry;
 
-typedef struct PgStat_WalStats
+typedef struct PgStat_WalCounters
 {
 	PgStat_Counter wal_records;
 	PgStat_Counter wal_fpi;
@@ -477,6 +477,11 @@ typedef struct PgStat_WalStats
 	PgStat_Counter wal_sync;
 	PgStat_Counter wal_write_time;
 	PgStat_Counter wal_sync_time;
+} PgStat_WalCounters;
+
+typedef struct PgStat_WalStats
+{
+	PgStat_WalCounters wal_counters;
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index d5aa5c295ae..d752e626cb0 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2178,6 +2178,7 @@ PgStat_SubXactStatus
 PgStat_TableCounts
 PgStat_TableStatus
 PgStat_TableXactStatus
+PgStat_WalCounters
 PgStat_WalStats
 PgXmlErrorContext
 PgXmlStrictness
-- 
2.34.1

From c00c545ed8da5c37d5590018df30b633b32b2ed7 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 10:00:00 +0000
Subject: [PATCH v6 3/3] per backend WAL statistics

Now that commit 9aea73fc61 added backend-level statistics to pgstats (and
per backend IO statistics) we can more easily add per backend statistics.

This commit adds per backend WAL statistics using the same layer as pg_stat_wal,
except that it is now possible to know how much WAL activity is happening in each
backend rather than an overall aggregate of all the activity.  A function called
pg_stat_get_backend_wal() is added to access this data depending on the
PID of a backend.

The same limitation as in 9aea73fc61 persists, meaning that Auxiliary processes
are not included in this set of statistics.

XXX: bump catalog version
---
 doc/src/sgml/config.sgml                    |  4 +-
 doc/src/sgml/monitoring.sgml                | 19 +++++
 src/backend/access/transam/xlog.c           | 35 +++++++-
 src/backend/utils/activity/pgstat_backend.c | 91 ++++++++++++++++++++-
 src/backend/utils/activity/pgstat_wal.c     |  2 +
 src/backend/utils/adt/pgstatfuncs.c         | 53 +++++++++++-
 src/include/catalog/pg_proc.dat             |  7 ++
 src/include/pgstat.h                        | 48 +++++++----
 src/include/utils/pgstat_internal.h         |  3 +-
 src/test/regress/expected/stats.out         | 14 ++++
 src/test/regress/sql/stats.sql              |  6 ++
 11 files changed, 253 insertions(+), 29 deletions(-)
  12.5% doc/src/sgml/
  11.2% src/backend/access/transam/
  37.5% src/backend/utils/activity/
  17.3% src/backend/utils/adt/
   6.1% src/include/catalog/
   6.3% src/include/
   5.0% src/test/regress/expected/
   3.8% src/test/regress/sql/

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index a8866292d46..5a48b427609 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8292,7 +8292,9 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
         measure the overhead of timing on your system.
         I/O timing information is
         displayed in <link linkend="monitoring-pg-stat-wal-view">
-        <structname>pg_stat_wal</structname></link>.
+        <structname>pg_stat_wal</structname></link> and in the output of the
+        <link linkend="pg-stat-get-backend-wal">
+        <function>pg_stat_get_backend_wal()</function></link> function.
         Only superusers and users with the appropriate <literal>SET</literal>
         privilege can change this setting.
        </para>
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index e5888fae2b5..8fdf7d13f21 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -4824,6 +4824,25 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para></entry>
       </row>
 
+      <row>
+       <entry id="pg-stat-get-backend-wal" role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_stat_get_backend_wal</primary>
+        </indexterm>
+        <function>pg_stat_get_backend_wal</function> ( <type>integer</type> )
+        <returnvalue>record</returnvalue>
+       </para>
+       <para>
+        Returns WAL statistics about the backend with the specified
+        process ID. The output fields are exactly the same as the ones in the
+        <structname>pg_stat_wal</structname> view.
+       </para>
+       <para>
+        The function does not return WAL statistics for the checkpointer,
+        the background writer, the startup process and the autovacuum launcher.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index bf3dbda901d..f7c5b1e5e3b 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -2058,6 +2058,11 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
 					XLogWrite(WriteRqst, tli, false);
 					LWLockRelease(WALWriteLock);
 					PendingWalStats.wal_buffers_full++;
+
+					/* Add the per-backend related statistic */
+					if (pgstat_tracks_backend_bktype(MyBackendType))
+						PendingBackendStats.pending_wal.wal_buffers_full++;
+
 					TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_DONE();
 				}
 				/* Re-acquire WALBufMappingLock and retry */
@@ -2426,11 +2431,14 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
 			Size		nleft;
 			ssize_t		written;
 			instr_time	start;
+			instr_time	end;
 
 			/* OK to write the page(s) */
 			from = XLogCtl->pages + startidx * (Size) XLOG_BLCKSZ;
 			nbytes = npages * (Size) XLOG_BLCKSZ;
 			nleft = nbytes;
+			/* keep compiler quiet */
+			INSTR_TIME_SET_ZERO(end);
 			do
 			{
 				errno = 0;
@@ -2451,14 +2459,22 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
 				 */
 				if (track_wal_io_timing)
 				{
-					instr_time	end;
-
 					INSTR_TIME_SET_CURRENT(end);
 					INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_write_time, end, start);
 				}
 
 				PendingWalStats.wal_write++;
 
+				/* Add the per-backend related statistic */
+				if (pgstat_tracks_backend_bktype(MyBackendType))
+				{
+					PendingBackendStats.pending_wal.wal_write++;
+
+					if (track_wal_io_timing)
+						INSTR_TIME_ACCUM_DIFF(PendingBackendStats.pending_wal.wal_write_time,
+											  end, start);
+				}
+
 				if (written <= 0)
 				{
 					char		xlogfname[MAXFNAMELEN];
@@ -8684,8 +8700,11 @@ issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
 {
 	char	   *msg = NULL;
 	instr_time	start;
+	instr_time	end;
 
 	Assert(tli != 0);
+	/* keep compiler quiet */
+	INSTR_TIME_SET_ZERO(end);
 
 	/*
 	 * Quick exit if fsync is disabled or write() has already synced the WAL
@@ -8751,13 +8770,21 @@ issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
 	 */
 	if (track_wal_io_timing)
 	{
-		instr_time	end;
-
 		INSTR_TIME_SET_CURRENT(end);
 		INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_sync_time, end, start);
 	}
 
 	PendingWalStats.wal_sync++;
+
+	/* Add the per-backend related statistic */
+	if (pgstat_tracks_backend_bktype(MyBackendType))
+	{
+		PendingBackendStats.pending_wal.wal_sync++;
+
+		if (track_wal_io_timing)
+			INSTR_TIME_ACCUM_DIFF(PendingBackendStats.pending_wal.wal_sync_time,
+								  end, start);
+	}
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index bcf9e4b1487..90fb5f72bc1 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -33,11 +33,18 @@
  * reported within critical sections so we use static memory in order to avoid
  * memory allocation.
  */
-static PgStat_BackendPending PendingBackendStats;
+PgStat_BackendPending PendingBackendStats;
 
 /*
- * Utility routines to report I/O stats for backends, kept here to avoid
- * exposing PendingBackendStats to the outside world.
+ * WAL usage counters saved from pgWalUsage at the previous call to
+ * pgstat_report_wal(). This is used to calculate how much WAL usage
+ * happens between pgstat_report_wal() calls, by subtracting
+ * the previous counters from the current ones.
+ */
+static WalUsage prevBackendWalUsage;
+
+/*
+ * Utility routines to report I/O stats for backends.
  */
 void
 pgstat_count_backend_io_op_time(IOObject io_object, IOContext io_context,
@@ -131,6 +138,79 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 	MemSet(&PendingBackendStats.pending_io, 0, sizeof(PgStat_PendingIO));
 }
 
+/*
+ * To determine whether any WAL activity has occurred since last time, not
+ * only the number of generated WAL records but also the numbers of WAL
+ * writes and syncs need to be checked. Because even transaction that
+ * generates no WAL records can write or sync WAL data when flushing the
+ * data pages.
+ */
+static bool
+pgstat_backend_wal_have_pending(void)
+{
+	PgStat_PendingWalStats pending_wal;
+
+	pending_wal = PendingBackendStats.pending_wal;
+
+	return pgWalUsage.wal_records != prevBackendWalUsage.wal_records ||
+		pending_wal.wal_write != 0 || pending_wal.wal_sync != 0;
+}
+
+/*
+ * Flush out locally pending backend WAL statistics.  Locking is managed
+ * by the caller.
+ */
+static void
+pgstat_flush_backend_entry_wal(PgStat_EntryRef *entry_ref)
+{
+	PgStatShared_Backend *shbackendent;
+	PgStat_WalCounters *bktype_shstats;
+	PgStat_PendingWalStats pending_wal = PendingBackendStats.pending_wal;
+	WalUsage	wal_usage_diff = {0};
+
+	/*
+	 * This function can be called even if nothing at all has happened. Avoid
+	 * taking lock for nothing in that case.
+	 */
+	if (!pgstat_backend_wal_have_pending())
+		return;
+
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	bktype_shstats = &shbackendent->stats.wal_stats;
+
+	/*
+	 * We don't update the WAL usage portion of the local WalStats elsewhere.
+	 * Calculate how much WAL usage counters were increased by subtracting the
+	 * previous counters from the current ones.
+	 */
+	WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
+
+#define WALSTAT_ACC(fld, var_to_add) \
+	(bktype_shstats->fld += var_to_add.fld)
+#define WALSTAT_ACC_INSTR_TIME(fld) \
+   (bktype_shstats->fld += INSTR_TIME_GET_MICROSEC(pending_wal.fld))
+	WALSTAT_ACC(wal_buffers_full, pending_wal);
+	WALSTAT_ACC(wal_write, pending_wal);
+	WALSTAT_ACC(wal_sync, pending_wal);
+	WALSTAT_ACC(wal_records, wal_usage_diff);
+	WALSTAT_ACC(wal_fpi, wal_usage_diff);
+	WALSTAT_ACC(wal_bytes, wal_usage_diff);
+	WALSTAT_ACC_INSTR_TIME(wal_write_time);
+	WALSTAT_ACC_INSTR_TIME(wal_sync_time);
+#undef WALSTAT_ACC_INSTR_TIME
+#undef WALSTAT_ACC
+
+	/*
+	 * Save the current counters for the subsequent calculation of WAL usage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
+
+	/*
+	 * Clear out the statistics buffer, so it can be re-used.
+	 */
+	MemSet(&PendingBackendStats.pending_wal, 0, sizeof(PendingWalStats));
+}
+
 /*
  * Flush out locally pending backend statistics
  *
@@ -158,6 +238,9 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 	if (flags & PGSTAT_BACKEND_FLUSH_IO)
 		pgstat_flush_backend_entry_io(entry_ref);
 
+	if (flags & PGSTAT_BACKEND_FLUSH_WAL)
+		pgstat_flush_backend_entry_wal(entry_ref);
+
 	pgstat_unlock_entry(entry_ref);
 
 	return false;
@@ -205,6 +288,8 @@ pgstat_create_backend(ProcNumber procnum)
 	pgstat_unlock_entry(entry_ref);
 
 	MemSet(&PendingBackendStats, 0, sizeof(PgStat_BackendPending));
+
+	prevBackendWalUsage = pgWalUsage;
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index bfc06178a68..33627642261 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -55,6 +55,8 @@ pgstat_report_wal(bool force)
 	/* flush wal stats */
 	pgstat_flush_wal(nowait);
 
+	pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_WAL);
+
 	/* flush IO stats */
 	pgstat_flush_io(nowait);
 }
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 97510d48eef..6da98bafbf2 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1606,8 +1606,8 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 /*
  * pg_stat_wal_build_tuple
  *
- * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
- * of wal_counters.
+ * Helper routine for pg_stat_get_wal() and pg_stat_get_backend_wal() returning
+ * one tuple based on the contents of wal_counters.
  */
 static Datum
 pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
@@ -1670,6 +1670,55 @@ pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns WAL statistics for a backend with given PID.
+ */
+Datum
+pg_stat_get_backend_wal(PG_FUNCTION_ARGS)
+{
+	int			pid;
+	PGPROC	   *proc;
+	ProcNumber	procNumber;
+	PgStat_Backend *backend_stats;
+	PgStat_WalCounters bktype_stats;
+	PgBackendStatus *beentry;
+
+	pid = PG_GETARG_INT32(0);
+	proc = BackendPidGetProc(pid);
+
+	/*
+	 * This could be an auxiliary process but these do not report backend
+	 * statistics due to pgstat_tracks_backend_bktype(), so there is no need
+	 * for an extra call to AuxiliaryPidGetProc().
+	 */
+	if (!proc)
+		PG_RETURN_NULL();
+
+	procNumber = GetNumberFromPGProc(proc);
+
+	beentry = pgstat_get_beentry_by_proc_number(procNumber);
+	if (!beentry)
+		PG_RETURN_NULL();
+
+	backend_stats = pgstat_fetch_stat_backend(procNumber);
+	if (!backend_stats)
+		PG_RETURN_NULL();
+
+	/* if PID does not match, leave */
+	if (beentry->st_procpid != pid)
+		PG_RETURN_NULL();
+
+	/* backend may be gone, so recheck in case */
+	if (beentry->st_backendType == B_INVALID)
+		PG_RETURN_NULL();
+
+	bktype_stats = backend_stats->wal_stats;
+
+	/* save tuples with data from this PgStat_BktypeIO */
+	return (pg_stat_wal_build_tuple(bktype_stats, backend_stats->stat_reset_timestamp));
+}
+
+
 /*
  * Returns statistics of WAL activity
  */
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 18560755d26..ca60af1e434 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5929,6 +5929,13 @@
   proargmodes => '{o,o,o,o,o,o,o,o,o}',
   proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,wal_write,wal_sync,wal_write_time,wal_sync_time,stats_reset}',
   prosrc => 'pg_stat_get_wal' },
+{ oid => '8037', descr => 'statistics: backend WAL activity',
+  proname => 'pg_stat_get_backend_wal', provolatile => 'v',
+  proparallel => 'r', prorettype => 'record', proargtypes => 'int4',
+  proallargtypes => '{int4,int8,int8,numeric,int8,int8,int8,float8,float8,timestamptz}',
+  proargmodes => '{i,o,o,o,o,o,o,o,o,o}',
+  proargnames => '{backend_pid,wal_records,wal_fpi,wal_bytes,wal_buffers_full,wal_write,wal_sync,wal_write_time,wal_sync_time,stats_reset}',
+  prosrc => 'pg_stat_get_backend_wal' },
 { oid => '6248', descr => 'statistics: information about WAL prefetching',
   proname => 'pg_stat_get_recovery_prefetch', prorows => '1', proretset => 't',
   provolatile => 'v', prorettype => 'record', proargtypes => '',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 9bbba883685..27a0053ea21 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -338,24 +338,6 @@ typedef struct PgStat_IO
 	PgStat_BktypeIO stats[BACKEND_NUM_TYPES];
 } PgStat_IO;
 
-typedef struct PgStat_Backend
-{
-	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO io_stats;
-} PgStat_Backend;
-
-/* ---------
- * PgStat_BackendPending	Non-flushed backend stats.
- * ---------
- */
-typedef struct PgStat_BackendPending
-{
-	/*
-	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
-	 */
-	PgStat_PendingIO pending_io;
-} PgStat_BackendPending;
-
 typedef struct PgStat_StatDBEntry
 {
 	PgStat_Counter xact_commit;
@@ -500,6 +482,30 @@ typedef struct PgStat_PendingWalStats
 	instr_time	wal_sync_time;
 } PgStat_PendingWalStats;
 
+/* ---------
+ * PgStat_BackendPending	Non-flushed backend stats.
+ * ---------
+ */
+typedef struct PgStat_BackendPending
+{
+	/*
+	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
+	 */
+	PgStat_PendingIO pending_io;
+
+	/*
+	 * Backend statistics store the same amount of WAL data as
+	 * PGSTAT_KIND_WAL.
+	 */
+	PgStat_PendingWalStats pending_wal;
+} PgStat_BackendPending;
+
+typedef struct PgStat_Backend
+{
+	TimestampTz stat_reset_timestamp;
+	PgStat_BktypeIO io_stats;
+	PgStat_WalCounters wal_stats;
+} PgStat_Backend;
 
 /*
  * Functions in pgstat.c
@@ -840,5 +846,11 @@ extern PGDLLIMPORT SessionEndType pgStatSessionEndCause;
 /* updated directly by backends and background processes */
 extern PGDLLIMPORT PgStat_PendingWalStats PendingWalStats;
 
+/*
+ * Variables in pgstat_backend.c
+ */
+
+/* updated directly by backends */
+extern PGDLLIMPORT PgStat_BackendPending PendingBackendStats;
 
 #endif							/* PGSTAT_H */
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index a3d39d2b725..6155a3fc983 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -622,7 +622,8 @@ extern void pgstat_archiver_snapshot_cb(void);
 
 /* flags for pgstat_flush_backend() */
 #define PGSTAT_BACKEND_FLUSH_IO		(1 << 0)	/* Flush I/O statistics */
-#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO)
+#define PGSTAT_BACKEND_FLUSH_WAL   (1 << 1) /* Flush WAL statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL   (PGSTAT_BACKEND_FLUSH_IO | PGSTAT_BACKEND_FLUSH_WAL)
 
 extern bool pgstat_flush_backend(bool nowait, bits32 flags);
 extern bool pgstat_backend_flush_cb(bool nowait);
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index a0317b7208e..cc01fdf2741 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -832,6 +832,8 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -851,6 +853,18 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
  t
 (1 row)
 
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+ ?column? 
+----------
+ t
+(1 row)
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 399c72bbcf7..28fe0a1a7d0 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -423,6 +423,9 @@ SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
 
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 
@@ -435,6 +438,9 @@ CHECKPOINT;
 SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;
 SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
 
+SELECT pg_stat_force_next_flush();
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
-- 
2.34.1

From 3e54a2f32b79238cefdd946e2fc017de0883c182 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 17 Feb 2025 07:17:49 +0000
Subject: [PATCH v7 1/4] Remove wal_[sync|write][_time] from pg_stat_wal

a051e71e28a added this information into pg_stat_io (with more details and
granularity), so there is no need to keep it in pg_stat_wal. This also
allows to remove PendingWalStats and simplifies up coming commits related
to per backend WAL statistics.
---
 doc/src/sgml/config.sgml                |  4 +-
 doc/src/sgml/monitoring.sgml            | 98 +++++++++++--------------
 doc/src/sgml/wal.sgml                   |  9 ++-
 src/backend/access/transam/xlog.c       | 27 -------
 src/backend/catalog/system_views.sql    |  4 -
 src/backend/utils/activity/pgstat_wal.c | 24 +-----
 src/backend/utils/adt/pgstatfuncs.c     | 20 +----
 src/include/catalog/pg_proc.dat         |  6 +-
 src/include/pgstat.h                    | 28 -------
 src/test/regress/expected/rules.out     |  6 +-
 src/tools/pgindent/typedefs.list        |  1 -
 11 files changed, 57 insertions(+), 170 deletions(-)
  56.1% doc/src/sgml/
   6.6% src/backend/access/transam/
  11.6% src/backend/utils/activity/
  11.0% src/backend/utils/adt/
   3.4% src/include/catalog/
   7.7% src/include/
   3.3% src/

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 336630ce417..9fff9048551 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8341,8 +8341,8 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
         You can use the <application>pg_test_timing</application> tool to
         measure the overhead of timing on your system.
         I/O timing information is
-        displayed in <link linkend="monitoring-pg-stat-wal-view">
-        <structname>pg_stat_wal</structname></link>.
+        displayed in <link linkend="monitoring-pg-stat-io-view">
+        <structname>pg_stat_io</structname></link> for the wal <literal>object</literal>.
         Only superusers and users with the appropriate <literal>SET</literal>
         privilege can change this setting.
        </para>
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 928a6eb64b0..e645591ce53 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -2930,6 +2930,47 @@ description | Waiting for a newly initialized WAL file to reach durable storage
    writer</literal>.
   </para>
 
+  <para>
+   For the <literal>wal</literal> <structfield>object</structfield>:
+   <itemizedlist>
+    <listitem>
+     <para>
+      <structfield>writes</structfield> is the number of times WAL buffers were
+       written out to disk via <function>XLogWrite</function> request (See <xref linkend="wal-configuration"/>
+       for more information about the internal WAL function <function>XLogWrite</function>).
+     </para>
+    </listitem>
+    <listitem>
+     <para>
+      <structfield>fsyncs</structfield> is the number of times WAL files were
+       synced to disk via <function>issue_xlog_fsync</function> request (if <xref linkend="guc-fsync"/>
+       is <literal>on</literal> and <xref linkend="guc-wal-sync-method"/> is either
+       <literal>fdatasync</literal>, <literal>fsync</literal> or <literal>fsync_writethrough</literal>,
+       otherwise zero) (See <xref linkend="wal-configuration"/> for more information about
+       the internal WAL function <function>issue_xlog_fsync</function>).
+     </para>
+    </listitem>
+    <listitem>
+     <para>
+      <structfield>write_time</structfield> is the total amount of time spent writing
+       WAL buffers to disk via <function>XLogWrite</function> request (if <xref linkend="guc-track-wal-io-timing"/>
+       is enabled, otherwise zero). This includes the sync time when <varname>wal_sync_method</varname>
+       is either <literal>open_datasync</literal> or <literal>open_sync</literal>.
+     </para>
+    </listitem>
+    <listitem>
+     <para>
+      <structfield>sync_time</structfield> is the total amount of time spent syncing
+       WAL files to disk via <function>issue_xlog_fsync</function> request (if
+       <varname>track_wal_io_timing</varname> is enabled, <varname>fsync</varname> is
+       <literal>on</literal>, and <varname>wal_sync_method</varname> is either
+       <literal>fdatasync</literal>, <literal>fsync</literal> or <literal>fsync_writethrough</literal>,
+       otherwise zero).
+     </para>
+    </listitem>
+   </itemizedlist>
+  </para>
+
   <para>
    <structname>pg_stat_io</structname> can be used to inform database tuning.
    For example:
@@ -3255,63 +3296,6 @@ description | Waiting for a newly initialized WAL file to reach durable storage
       </para></entry>
      </row>
 
-     <row>
-      <entry role="catalog_table_entry"><para role="column_definition">
-       <structfield>wal_write</structfield> <type>bigint</type>
-      </para>
-      <para>
-       Number of times WAL buffers were written out to disk via
-       <function>XLogWrite</function> request.
-       See <xref linkend="wal-configuration"/> for more information about
-       the internal WAL function <function>XLogWrite</function>.
-      </para></entry>
-     </row>
-
-     <row>
-      <entry role="catalog_table_entry"><para role="column_definition">
-       <structfield>wal_sync</structfield> <type>bigint</type>
-      </para>
-      <para>
-       Number of times WAL files were synced to disk via
-       <function>issue_xlog_fsync</function> request
-       (if <xref linkend="guc-fsync"/> is <literal>on</literal> and
-       <xref linkend="guc-wal-sync-method"/> is either
-       <literal>fdatasync</literal>, <literal>fsync</literal> or
-       <literal>fsync_writethrough</literal>, otherwise zero).
-       See <xref linkend="wal-configuration"/> for more information about
-       the internal WAL function <function>issue_xlog_fsync</function>.
-      </para></entry>
-     </row>
-
-     <row>
-      <entry role="catalog_table_entry"><para role="column_definition">
-       <structfield>wal_write_time</structfield> <type>double precision</type>
-      </para>
-      <para>
-       Total amount of time spent writing WAL buffers to disk via
-       <function>XLogWrite</function> request, in milliseconds
-       (if <xref linkend="guc-track-wal-io-timing"/> is enabled,
-       otherwise zero).  This includes the sync time when
-       <varname>wal_sync_method</varname> is either
-       <literal>open_datasync</literal> or <literal>open_sync</literal>.
-      </para></entry>
-     </row>
-
-     <row>
-      <entry role="catalog_table_entry"><para role="column_definition">
-       <structfield>wal_sync_time</structfield> <type>double precision</type>
-      </para>
-      <para>
-       Total amount of time spent syncing WAL files to disk via
-       <function>issue_xlog_fsync</function> request, in milliseconds
-       (if <varname>track_wal_io_timing</varname> is enabled,
-       <varname>fsync</varname> is <literal>on</literal>, and
-       <varname>wal_sync_method</varname> is either
-       <literal>fdatasync</literal>, <literal>fsync</literal> or
-       <literal>fsync_writethrough</literal>, otherwise zero).
-      </para></entry>
-     </row>
-
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>stats_reset</structfield> <type>timestamp with time zone</type>
diff --git a/doc/src/sgml/wal.sgml b/doc/src/sgml/wal.sgml
index b908720adea..bb4892413fc 100644
--- a/doc/src/sgml/wal.sgml
+++ b/doc/src/sgml/wal.sgml
@@ -813,8 +813,8 @@
    When <xref linkend="guc-track-wal-io-timing"/> is enabled, the total
    amounts of time <function>XLogWrite</function> writes and
    <function>issue_xlog_fsync</function> syncs WAL data to disk are counted as
-   <literal>wal_write_time</literal> and <literal>wal_sync_time</literal> in
-   <xref linkend="pg-stat-wal-view"/>, respectively.
+   <literal>write_time</literal> and <literal>sync_time</literal> in
+   <xref linkend="pg-stat-io-view"/> for the wal <literal>object</literal>, respectively.
    <function>XLogWrite</function> is normally called by
    <function>XLogInsertRecord</function> (when there is no space for the new
    record in WAL buffers), <function>XLogFlush</function> and the WAL writer,
@@ -832,8 +832,9 @@
    of the setting of <varname>track_wal_io_timing</varname>, the number
    of times <function>XLogWrite</function> writes and
    <function>issue_xlog_fsync</function> syncs WAL data to disk are also
-   counted as <literal>wal_write</literal> and <literal>wal_sync</literal>
-   in <structname>pg_stat_wal</structname>, respectively.
+   counted as <literal>write</literal> and <literal>sync</literal>
+   in <structname>pg_stat_io</structname> for the <literal>wal</literal> object
+   respectively.
   </para>
 
   <para>
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 25a5c605404..020b99d402c 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -2448,20 +2448,6 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
 				pgstat_count_io_op_time(IOOBJECT_WAL, IOCONTEXT_NORMAL,
 										IOOP_WRITE, start, 1, written);
 
-				/*
-				 * Increment the I/O timing and the number of times WAL data
-				 * were written out to disk.
-				 */
-				if (track_wal_io_timing)
-				{
-					instr_time	end;
-
-					INSTR_TIME_SET_CURRENT(end);
-					INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_write_time, end, start);
-				}
-
-				PendingWalStats.wal_write++;
-
 				if (written <= 0)
 				{
 					char		xlogfname[MAXFNAMELEN];
@@ -8767,21 +8753,8 @@ issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
 
 	pgstat_report_wait_end();
 
-	/*
-	 * Increment the I/O timing and the number of times WAL files were synced.
-	 */
-	if (track_wal_io_timing)
-	{
-		instr_time	end;
-
-		INSTR_TIME_SET_CURRENT(end);
-		INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_sync_time, end, start);
-	}
-
 	pgstat_count_io_op_time(IOOBJECT_WAL, IOCONTEXT_NORMAL, IOOP_FSYNC,
 							start, 1, 0);
-
-	PendingWalStats.wal_sync++;
 }
 
 /*
diff --git a/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index eff0990957e..a4d2cfdcaf5 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -1189,10 +1189,6 @@ CREATE VIEW pg_stat_wal AS
         w.wal_fpi,
         w.wal_bytes,
         w.wal_buffers_full,
-        w.wal_write,
-        w.wal_sync,
-        w.wal_write_time,
-        w.wal_sync_time,
         w.stats_reset
     FROM pg_stat_get_wal() w;
 
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index c1ca65eb991..4dc41a4a590 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -21,8 +21,6 @@
 #include "utils/pgstat_internal.h"
 
 
-PgStat_PendingWalStats PendingWalStats = {0};
-
 /*
  * WAL usage counters saved from pgWalUsage at the previous call to
  * pgstat_report_wal(). This is used to calculate how much WAL usage
@@ -118,17 +116,10 @@ pgstat_wal_flush_cb(bool nowait)
 
 #define WALSTAT_ACC(fld, var_to_add) \
 	(stats_shmem->stats.fld += var_to_add.fld)
-#define WALSTAT_ACC_INSTR_TIME(fld) \
-	(stats_shmem->stats.fld += INSTR_TIME_GET_MICROSEC(PendingWalStats.fld))
 	WALSTAT_ACC(wal_records, wal_usage_diff);
 	WALSTAT_ACC(wal_fpi, wal_usage_diff);
 	WALSTAT_ACC(wal_bytes, wal_usage_diff);
 	WALSTAT_ACC(wal_buffers_full, wal_usage_diff);
-	WALSTAT_ACC(wal_write, PendingWalStats);
-	WALSTAT_ACC(wal_sync, PendingWalStats);
-	WALSTAT_ACC_INSTR_TIME(wal_write_time);
-	WALSTAT_ACC_INSTR_TIME(wal_sync_time);
-#undef WALSTAT_ACC_INSTR_TIME
 #undef WALSTAT_ACC
 
 	LWLockRelease(&stats_shmem->lock);
@@ -138,11 +129,6 @@ pgstat_wal_flush_cb(bool nowait)
 	 */
 	prevWalUsage = pgWalUsage;
 
-	/*
-	 * Clear out the statistics buffer, so it can be re-used.
-	 */
-	MemSet(&PendingWalStats, 0, sizeof(PendingWalStats));
-
 	return false;
 }
 
@@ -158,18 +144,12 @@ pgstat_wal_init_backend_cb(void)
 }
 
 /*
- * To determine whether any WAL activity has occurred since last time, not
- * only the number of generated WAL records but also the numbers of WAL
- * writes and syncs need to be checked. Because even transaction that
- * generates no WAL records can write or sync WAL data when flushing the
- * data pages.
+ * To determine whether WAL usage happened.
  */
 bool
 pgstat_wal_have_pending_cb(void)
 {
-	return pgWalUsage.wal_records != prevWalUsage.wal_records ||
-		PendingWalStats.wal_write != 0 ||
-		PendingWalStats.wal_sync != 0;
+	return pgWalUsage.wal_records != prevWalUsage.wal_records;
 }
 
 void
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index e9096a88492..68e16e52ab6 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1637,7 +1637,7 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 Datum
 pg_stat_get_wal(PG_FUNCTION_ARGS)
 {
-#define PG_STAT_GET_WAL_COLS	9
+#define PG_STAT_GET_WAL_COLS	5
 	TupleDesc	tupdesc;
 	Datum		values[PG_STAT_GET_WAL_COLS] = {0};
 	bool		nulls[PG_STAT_GET_WAL_COLS] = {0};
@@ -1654,15 +1654,7 @@ pg_stat_get_wal(PG_FUNCTION_ARGS)
 					   NUMERICOID, -1, 0);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 4, "wal_buffers_full",
 					   INT8OID, -1, 0);
-	TupleDescInitEntry(tupdesc, (AttrNumber) 5, "wal_write",
-					   INT8OID, -1, 0);
-	TupleDescInitEntry(tupdesc, (AttrNumber) 6, "wal_sync",
-					   INT8OID, -1, 0);
-	TupleDescInitEntry(tupdesc, (AttrNumber) 7, "wal_write_time",
-					   FLOAT8OID, -1, 0);
-	TupleDescInitEntry(tupdesc, (AttrNumber) 8, "wal_sync_time",
-					   FLOAT8OID, -1, 0);
-	TupleDescInitEntry(tupdesc, (AttrNumber) 9, "stats_reset",
+	TupleDescInitEntry(tupdesc, (AttrNumber) 5, "stats_reset",
 					   TIMESTAMPTZOID, -1, 0);
 
 	BlessTupleDesc(tupdesc);
@@ -1682,14 +1674,8 @@ pg_stat_get_wal(PG_FUNCTION_ARGS)
 									Int32GetDatum(-1));
 
 	values[3] = Int64GetDatum(wal_stats->wal_buffers_full);
-	values[4] = Int64GetDatum(wal_stats->wal_write);
-	values[5] = Int64GetDatum(wal_stats->wal_sync);
-
-	/* Convert counters from microsec to millisec for display */
-	values[6] = Float8GetDatum(((double) wal_stats->wal_write_time) / 1000.0);
-	values[7] = Float8GetDatum(((double) wal_stats->wal_sync_time) / 1000.0);
 
-	values[8] = TimestampTzGetDatum(wal_stats->stat_reset_timestamp);
+	values[4] = TimestampTzGetDatum(wal_stats->stat_reset_timestamp);
 
 	/* Returns the record as Datum */
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 9e803d610d7..1e1626964e3 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5950,9 +5950,9 @@
 { oid => '1136', descr => 'statistics: information about WAL activity',
   proname => 'pg_stat_get_wal', proisstrict => 'f', provolatile => 's',
   proparallel => 'r', prorettype => 'record', proargtypes => '',
-  proallargtypes => '{int8,int8,numeric,int8,int8,int8,float8,float8,timestamptz}',
-  proargmodes => '{o,o,o,o,o,o,o,o,o}',
-  proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,wal_write,wal_sync,wal_write_time,wal_sync_time,stats_reset}',
+  proallargtypes => '{int8,int8,numeric,int8,timestamptz}',
+  proargmodes => '{o,o,o,o,o}',
+  proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
   prosrc => 'pg_stat_get_wal' },
 { oid => '6248', descr => 'statistics: information about WAL prefetching',
   proname => 'pg_stat_get_recovery_prefetch', prorows => '1', proretset => 't',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 53f2a8458e6..a3a341cc604 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -480,28 +480,9 @@ typedef struct PgStat_WalStats
 	PgStat_Counter wal_fpi;
 	uint64		wal_bytes;
 	PgStat_Counter wal_buffers_full;
-	PgStat_Counter wal_write;
-	PgStat_Counter wal_sync;
-	PgStat_Counter wal_write_time;
-	PgStat_Counter wal_sync_time;
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
-/*
- * This struct stores wal-related durations as instr_time, which makes it
- * cheaper and easier to accumulate them, by not requiring type
- * conversions. During stats flush instr_time will be converted into
- * microseconds.
- */
-typedef struct PgStat_PendingWalStats
-{
-	PgStat_Counter wal_write;
-	PgStat_Counter wal_sync;
-	instr_time	wal_write_time;
-	instr_time	wal_sync_time;
-} PgStat_PendingWalStats;
-
-
 /*
  * Functions in pgstat.c
  */
@@ -834,13 +815,4 @@ extern PGDLLIMPORT PgStat_Counter pgStatTransactionIdleTime;
 /* updated by the traffic cop and in errfinish() */
 extern PGDLLIMPORT SessionEndType pgStatSessionEndCause;
 
-
-/*
- * Variables in pgstat_wal.c
- */
-
-/* updated directly by backends and background processes */
-extern PGDLLIMPORT PgStat_PendingWalStats PendingWalStats;
-
-
 #endif							/* PGSTAT_H */
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index 5baba8d39ff..62f69ac20b2 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -2259,12 +2259,8 @@ pg_stat_wal| SELECT wal_records,
     wal_fpi,
     wal_bytes,
     wal_buffers_full,
-    wal_write,
-    wal_sync,
-    wal_write_time,
-    wal_sync_time,
     stats_reset
-   FROM pg_stat_get_wal() w(wal_records, wal_fpi, wal_bytes, wal_buffers_full, wal_write, wal_sync, wal_write_time, wal_sync_time, stats_reset);
+   FROM pg_stat_get_wal() w(wal_records, wal_fpi, wal_bytes, wal_buffers_full, stats_reset);
 pg_stat_wal_receiver| SELECT pid,
     status,
     receive_start_lsn,
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index bce4214503d..740777127e9 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2163,7 +2163,6 @@ PgStat_KindInfo
 PgStat_LocalState
 PgStat_PendingDroppedStatsItem
 PgStat_PendingIO
-PgStat_PendingWalStats
 PgStat_SLRUStats
 PgStat_ShmemControl
 PgStat_Snapshot
-- 
2.34.1

From c516da1bd0730c274d892999f8c390138dd3c8aa Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 07:51:27 +0000
Subject: [PATCH v7 2/4] Extract logic filling pg_stat_get_wal()'s tuple into
 its own routine

This commit adds pg_stat_wal_build_tuple(), a helper routine for
pg_stat_get_wal(), that fills its tuple based on the contents
of PgStat_WalStats.  This will be used in a follow-up commit that uses
the same structures as pg_stat_wal for reporting, but for the PGSTAT_KIND_BACKEND
statistics kind.
---
 src/backend/utils/adt/pgstatfuncs.c | 48 +++++++++++++++++++----------
 1 file changed, 32 insertions(+), 16 deletions(-)
 100.0% src/backend/utils/adt/

diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 68e16e52ab6..620d60a0938 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1632,20 +1632,22 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 }
 
 /*
- * Returns statistics of WAL activity
+ * pg_stat_wal_build_tuple
+ *
+ * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
+ * of wal_stats.
  */
-Datum
-pg_stat_get_wal(PG_FUNCTION_ARGS)
+static Datum
+pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
 {
-#define PG_STAT_GET_WAL_COLS	5
+#define PG_STAT_WAL_COLS	5
 	TupleDesc	tupdesc;
-	Datum		values[PG_STAT_GET_WAL_COLS] = {0};
-	bool		nulls[PG_STAT_GET_WAL_COLS] = {0};
+	Datum		values[PG_STAT_WAL_COLS] = {0};
+	bool		nulls[PG_STAT_WAL_COLS] = {0};
 	char		buf[256];
-	PgStat_WalStats *wal_stats;
 
 	/* Initialise attributes information in the tuple descriptor */
-	tupdesc = CreateTemplateTupleDesc(PG_STAT_GET_WAL_COLS);
+	tupdesc = CreateTemplateTupleDesc(PG_STAT_WAL_COLS);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "wal_records",
 					   INT8OID, -1, 0);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "wal_fpi",
@@ -1659,28 +1661,42 @@ pg_stat_get_wal(PG_FUNCTION_ARGS)
 
 	BlessTupleDesc(tupdesc);
 
-	/* Get statistics about WAL activity */
-	wal_stats = pgstat_fetch_stat_wal();
-
 	/* Fill values and NULLs */
-	values[0] = Int64GetDatum(wal_stats->wal_records);
-	values[1] = Int64GetDatum(wal_stats->wal_fpi);
+	values[0] = Int64GetDatum(wal_stats.wal_records);
+	values[1] = Int64GetDatum(wal_stats.wal_fpi);
 
 	/* Convert to numeric. */
-	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats->wal_bytes);
+	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats.wal_bytes);
 	values[2] = DirectFunctionCall3(numeric_in,
 									CStringGetDatum(buf),
 									ObjectIdGetDatum(0),
 									Int32GetDatum(-1));
 
-	values[3] = Int64GetDatum(wal_stats->wal_buffers_full);
+	values[3] = Int64GetDatum(wal_stats.wal_buffers_full);
 
-	values[4] = TimestampTzGetDatum(wal_stats->stat_reset_timestamp);
+	if (wal_stats.stat_reset_timestamp != 0)
+		values[4] = TimestampTzGetDatum(wal_stats.stat_reset_timestamp);
+	else
+		nulls[4] = true;
 
 	/* Returns the record as Datum */
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns statistics of WAL activity
+ */
+Datum
+pg_stat_get_wal(PG_FUNCTION_ARGS)
+{
+	PgStat_WalStats *wal_stats;
+
+	/* Get statistics about WAL activity */
+	wal_stats = pgstat_fetch_stat_wal();
+
+	return (pg_stat_wal_build_tuple(*wal_stats));
+}
+
 /*
  * Returns statistics of SLRU caches.
  */
-- 
2.34.1

From a33621c2a2468c9e908622ed9940d15b19c2d843 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Thu, 16 Jan 2025 15:06:01 +0000
Subject: [PATCH v7 3/4] Adding a new PgStat_WalCounters struct

This new struct contains only the counters related to the WAL statistics.
This will be used in a follow-up commit that uses the same structures but
for the PGSTAT_KIND_BACKEND statistics kind.
---
 src/backend/utils/activity/pgstat_wal.c |  2 +-
 src/backend/utils/adt/pgstatfuncs.c     | 20 +++++++++++---------
 src/include/pgstat.h                    |  7 ++++++-
 src/tools/pgindent/typedefs.list        |  1 +
 4 files changed, 19 insertions(+), 11 deletions(-)
   8.5% src/backend/utils/activity/
  82.0% src/backend/utils/adt/
   7.8% src/include/

diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 4dc41a4a590..6d024872701 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -115,7 +115,7 @@ pgstat_wal_flush_cb(bool nowait)
 		return true;
 
 #define WALSTAT_ACC(fld, var_to_add) \
-	(stats_shmem->stats.fld += var_to_add.fld)
+	(stats_shmem->stats.wal_counters.fld += var_to_add.fld)
 	WALSTAT_ACC(wal_records, wal_usage_diff);
 	WALSTAT_ACC(wal_fpi, wal_usage_diff);
 	WALSTAT_ACC(wal_bytes, wal_usage_diff);
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 620d60a0938..9de14ffd449 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1635,10 +1635,11 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
  * pg_stat_wal_build_tuple
  *
  * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
- * of wal_stats.
+ * of wal_counters.
  */
 static Datum
-pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
+pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
+						TimestampTz stat_reset_timestamp)
 {
 #define PG_STAT_WAL_COLS	5
 	TupleDesc	tupdesc;
@@ -1662,20 +1663,20 @@ pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
 	BlessTupleDesc(tupdesc);
 
 	/* Fill values and NULLs */
-	values[0] = Int64GetDatum(wal_stats.wal_records);
-	values[1] = Int64GetDatum(wal_stats.wal_fpi);
+	values[0] = Int64GetDatum(wal_counters.wal_records);
+	values[1] = Int64GetDatum(wal_counters.wal_fpi);
 
 	/* Convert to numeric. */
-	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats.wal_bytes);
+	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_counters.wal_bytes);
 	values[2] = DirectFunctionCall3(numeric_in,
 									CStringGetDatum(buf),
 									ObjectIdGetDatum(0),
 									Int32GetDatum(-1));
 
-	values[3] = Int64GetDatum(wal_stats.wal_buffers_full);
+	values[3] = Int64GetDatum(wal_counters.wal_buffers_full);
 
-	if (wal_stats.stat_reset_timestamp != 0)
-		values[4] = TimestampTzGetDatum(wal_stats.stat_reset_timestamp);
+	if (stat_reset_timestamp != 0)
+		values[4] = TimestampTzGetDatum(stat_reset_timestamp);
 	else
 		nulls[4] = true;
 
@@ -1694,7 +1695,8 @@ pg_stat_get_wal(PG_FUNCTION_ARGS)
 	/* Get statistics about WAL activity */
 	wal_stats = pgstat_fetch_stat_wal();
 
-	return (pg_stat_wal_build_tuple(*wal_stats));
+	return (pg_stat_wal_build_tuple(wal_stats->wal_counters,
+									wal_stats->stat_reset_timestamp));
 }
 
 /*
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index a3a341cc604..459a7cb328e 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -474,12 +474,17 @@ typedef struct PgStat_StatTabEntry
 	PgStat_Counter total_autoanalyze_time;
 } PgStat_StatTabEntry;
 
-typedef struct PgStat_WalStats
+typedef struct PgStat_WalCounters
 {
 	PgStat_Counter wal_records;
 	PgStat_Counter wal_fpi;
 	uint64		wal_bytes;
 	PgStat_Counter wal_buffers_full;
+} PgStat_WalCounters;
+
+typedef struct PgStat_WalStats
+{
+	PgStat_WalCounters wal_counters;
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 740777127e9..77b47a6df7e 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2178,6 +2178,7 @@ PgStat_SubXactStatus
 PgStat_TableCounts
 PgStat_TableStatus
 PgStat_TableXactStatus
+PgStat_WalCounters
 PgStat_WalStats
 PgXmlErrorContext
 PgXmlStrictness
-- 
2.34.1

From 7665e01b6cf0b6bae3d277ce150387aff15e8f0f Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 10:00:00 +0000
Subject: [PATCH v7 4/4] per backend WAL statistics

Now that commit 9aea73fc61 added backend-level statistics to pgstats (and
per backend IO statistics) we can more easily add per backend statistics.

This commit adds per backend WAL statistics using the same layer as pg_stat_wal,
except that it is now possible to know how much WAL activity is happening in each
backend rather than an overall aggregate of all the activity.  A function called
pg_stat_get_backend_wal() is added to access this data depending on the
PID of a backend.

The same limitation as in 9aea73fc61 persists, meaning that Auxiliary processes
are not included in this set of statistics.

XXX: bump catalog version
---
 doc/src/sgml/monitoring.sgml                | 19 ++++++
 src/backend/utils/activity/pgstat_backend.c | 64 +++++++++++++++++++++
 src/backend/utils/activity/pgstat_wal.c     |  2 +
 src/backend/utils/adt/pgstatfuncs.c         | 52 ++++++++++++++++-
 src/include/catalog/pg_proc.dat             |  7 +++
 src/include/pgstat.h                        | 13 +++--
 src/include/utils/pgstat_internal.h         |  3 +-
 src/test/regress/expected/stats.out         | 14 +++++
 src/test/regress/sql/stats.sql              |  6 ++
 9 files changed, 171 insertions(+), 9 deletions(-)
  14.0% doc/src/sgml/
  34.5% src/backend/utils/activity/
  25.9% src/backend/utils/adt/
   7.7% src/include/catalog/
   3.9% src/include/utils/
   7.4% src/test/regress/expected/
   5.6% src/test/regress/sql/

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index e645591ce53..bd3fc5bd4c6 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -4859,6 +4859,25 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para></entry>
       </row>
 
+      <row>
+       <entry id="pg-stat-get-backend-wal" role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_stat_get_backend_wal</primary>
+        </indexterm>
+        <function>pg_stat_get_backend_wal</function> ( <type>integer</type> )
+        <returnvalue>record</returnvalue>
+       </para>
+       <para>
+        Returns WAL statistics about the backend with the specified
+        process ID. The output fields are exactly the same as the ones in the
+        <structname>pg_stat_wal</structname> view.
+       </para>
+       <para>
+        The function does not return WAL statistics for the checkpointer,
+        the background writer, the startup process and the autovacuum launcher.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 4a667e7019c..cf3fac678c2 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -35,6 +35,14 @@
  */
 static PgStat_BackendPending PendingBackendStats;
 
+/*
+ * WAL usage counters saved from pgWalUsage at the previous call to
+ * pgstat_report_wal(). This is used to calculate how much WAL usage
+ * happens between pgstat_report_wal() calls, by subtracting
+ * the previous counters from the current ones.
+ */
+static WalUsage prevBackendWalUsage;
+
 /*
  * Utility routines to report I/O stats for backends, kept here to avoid
  * exposing PendingBackendStats to the outside world.
@@ -131,6 +139,57 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 	MemSet(&PendingBackendStats.pending_io, 0, sizeof(PgStat_PendingIO));
 }
 
+/*
+ * To determine whether WAL usage happened.
+ */
+static bool
+pgstat_backend_wal_have_pending(void)
+{
+	return pgWalUsage.wal_records != prevBackendWalUsage.wal_records;
+}
+
+/*
+ * Flush out locally pending backend WAL statistics.  Locking is managed
+ * by the caller.
+ */
+static void
+pgstat_flush_backend_entry_wal(PgStat_EntryRef *entry_ref)
+{
+	PgStatShared_Backend *shbackendent;
+	PgStat_WalCounters *bktype_shstats;
+	WalUsage	wal_usage_diff = {0};
+
+	/*
+	 * This function can be called even if nothing at all has happened. Avoid
+	 * taking lock for nothing in that case.
+	 */
+	if (!pgstat_backend_wal_have_pending())
+		return;
+
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	bktype_shstats = &shbackendent->stats.wal_counters;
+
+	/*
+	 * We don't update the WAL usage portion of the local WalStats elsewhere.
+	 * Calculate how much WAL usage counters were increased by subtracting the
+	 * previous counters from the current ones.
+	 */
+	WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
+
+#define WALSTAT_ACC(fld, var_to_add) \
+	(bktype_shstats->fld += var_to_add.fld)
+	WALSTAT_ACC(wal_buffers_full, wal_usage_diff);
+	WALSTAT_ACC(wal_records, wal_usage_diff);
+	WALSTAT_ACC(wal_fpi, wal_usage_diff);
+	WALSTAT_ACC(wal_bytes, wal_usage_diff);
+#undef WALSTAT_ACC
+
+	/*
+	 * Save the current counters for the subsequent calculation of WAL usage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
+}
+
 /*
  * Flush out locally pending backend statistics
  *
@@ -158,6 +217,9 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 	if (flags & PGSTAT_BACKEND_FLUSH_IO)
 		pgstat_flush_backend_entry_io(entry_ref);
 
+	if (flags & PGSTAT_BACKEND_FLUSH_WAL)
+		pgstat_flush_backend_entry_wal(entry_ref);
+
 	pgstat_unlock_entry(entry_ref);
 
 	return false;
@@ -205,6 +267,8 @@ pgstat_create_backend(ProcNumber procnum)
 	pgstat_unlock_entry(entry_ref);
 
 	MemSet(&PendingBackendStats, 0, sizeof(PgStat_BackendPending));
+
+	prevBackendWalUsage = pgWalUsage;
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 6d024872701..f268a610fb8 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -53,6 +53,8 @@ pgstat_report_wal(bool force)
 	/* flush wal stats */
 	pgstat_flush_wal(nowait);
 
+	pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_WAL);
+
 	/* flush IO stats */
 	pgstat_flush_io(nowait);
 }
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 9de14ffd449..6676245651c 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1634,8 +1634,8 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 /*
  * pg_stat_wal_build_tuple
  *
- * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
- * of wal_counters.
+ * Helper routine for pg_stat_get_wal() and pg_stat_get_backend_wal() returning
+ * one tuple based on the contents of wal_counters.
  */
 static Datum
 pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
@@ -1684,6 +1684,54 @@ pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns WAL statistics for a backend with given PID.
+ */
+Datum
+pg_stat_get_backend_wal(PG_FUNCTION_ARGS)
+{
+	int			pid;
+	PGPROC	   *proc;
+	ProcNumber	procNumber;
+	PgStat_Backend *backend_stats;
+	PgStat_WalCounters bktype_stats;
+	PgBackendStatus *beentry;
+
+	pid = PG_GETARG_INT32(0);
+	proc = BackendPidGetProc(pid);
+
+	/*
+	 * This could be an auxiliary process but these do not report backend
+	 * statistics due to pgstat_tracks_backend_bktype(), so there is no need
+	 * for an extra call to AuxiliaryPidGetProc().
+	 */
+	if (!proc)
+		PG_RETURN_NULL();
+
+	procNumber = GetNumberFromPGProc(proc);
+
+	beentry = pgstat_get_beentry_by_proc_number(procNumber);
+	if (!beentry)
+		PG_RETURN_NULL();
+
+	backend_stats = pgstat_fetch_stat_backend(procNumber);
+	if (!backend_stats)
+		PG_RETURN_NULL();
+
+	/* if PID does not match, leave */
+	if (beentry->st_procpid != pid)
+		PG_RETURN_NULL();
+
+	/* backend may be gone, so recheck in case */
+	if (beentry->st_backendType == B_INVALID)
+		PG_RETURN_NULL();
+
+	bktype_stats = backend_stats->wal_counters;
+
+	/* save tuples with data from this PgStat_WalCounters */
+	return (pg_stat_wal_build_tuple(bktype_stats, backend_stats->stat_reset_timestamp));
+}
+
 /*
  * Returns statistics of WAL activity
  */
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 1e1626964e3..97108a1f4c4 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5954,6 +5954,13 @@
   proargmodes => '{o,o,o,o,o}',
   proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
   prosrc => 'pg_stat_get_wal' },
+{ oid => '8037', descr => 'statistics: backend WAL activity',
+  proname => 'pg_stat_get_backend_wal', provolatile => 'v',
+  proparallel => 'r', prorettype => 'record', proargtypes => 'int4',
+  proallargtypes => '{int4,int8,int8,numeric,int8,timestamptz}',
+  proargmodes => '{i,o,o,o,o,o}',
+  proargnames => '{backend_pid,wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
+  prosrc => 'pg_stat_get_backend_wal' },
 { oid => '6248', descr => 'statistics: information about WAL prefetching',
   proname => 'pg_stat_get_recovery_prefetch', prorows => '1', proretset => 't',
   provolatile => 'v', prorettype => 'record', proargtypes => '',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 459a7cb328e..6b282be4d5b 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -340,12 +340,6 @@ typedef struct PgStat_IO
 	PgStat_BktypeIO stats[BACKEND_NUM_TYPES];
 } PgStat_IO;
 
-typedef struct PgStat_Backend
-{
-	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO io_stats;
-} PgStat_Backend;
-
 /* ---------
  * PgStat_BackendPending	Non-flushed backend stats.
  * ---------
@@ -488,6 +482,13 @@ typedef struct PgStat_WalStats
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
+typedef struct PgStat_Backend
+{
+	TimestampTz stat_reset_timestamp;
+	PgStat_BktypeIO io_stats;
+	PgStat_WalCounters wal_counters;
+} PgStat_Backend;
+
 /*
  * Functions in pgstat.c
  */
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 06dcea3f0dc..2385839d83f 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -622,7 +622,8 @@ extern void pgstat_archiver_snapshot_cb(void);
 
 /* flags for pgstat_flush_backend() */
 #define PGSTAT_BACKEND_FLUSH_IO		(1 << 0)	/* Flush I/O statistics */
-#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO)
+#define PGSTAT_BACKEND_FLUSH_WAL   (1 << 1) /* Flush WAL statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL   (PGSTAT_BACKEND_FLUSH_IO | PGSTAT_BACKEND_FLUSH_WAL)
 
 extern bool pgstat_flush_backend(bool nowait, bits32 flags);
 extern bool pgstat_backend_flush_cb(bool nowait);
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index 7d91f047bb3..ec056cd00cf 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -832,6 +832,8 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -851,6 +853,18 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
  t
 (1 row)
 
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+ ?column? 
+----------
+ t
+(1 row)
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 11628ebc8a1..92e2ec46a6f 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -423,6 +423,9 @@ SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
 
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 
@@ -435,6 +438,9 @@ CHECKPOINT;
 SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;
 SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
 
+SELECT pg_stat_force_next_flush();
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
-- 
2.34.1

From e4cebfe49be300d37ddf54dd58a44c73c642f4ba Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 10:00:00 +0000
Subject: [PATCH v8 3/3] per backend WAL statistics

Now that commit 9aea73fc61 added backend-level statistics to pgstats (and
per backend IO statistics) we can more easily add per backend statistics.

This commit adds per backend WAL statistics using the same layer as pg_stat_wal,
except that it is now possible to know how much WAL activity is happening in each
backend rather than an overall aggregate of all the activity.  A function called
pg_stat_get_backend_wal() is added to access this data depending on the
PID of a backend.

The same limitation as in 9aea73fc61 persists, meaning that Auxiliary processes
are not included in this set of statistics.

XXX: bump catalog version
---
 doc/src/sgml/monitoring.sgml                | 19 ++++++
 src/backend/utils/activity/pgstat_backend.c | 64 +++++++++++++++++++++
 src/backend/utils/activity/pgstat_wal.c     |  2 +
 src/backend/utils/adt/pgstatfuncs.c         | 52 ++++++++++++++++-
 src/include/catalog/pg_proc.dat             |  7 +++
 src/include/pgstat.h                        | 13 +++--
 src/include/utils/pgstat_internal.h         |  3 +-
 src/test/regress/expected/stats.out         | 14 +++++
 src/test/regress/sql/stats.sql              |  6 ++
 9 files changed, 171 insertions(+), 9 deletions(-)
  14.0% doc/src/sgml/
  34.5% src/backend/utils/activity/
  25.9% src/backend/utils/adt/
   7.7% src/include/catalog/
   3.9% src/include/utils/
   7.4% src/test/regress/expected/
   5.6% src/test/regress/sql/

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 3dfd059b7ee..6b5c9b23c3a 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -4856,6 +4856,25 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para></entry>
       </row>
 
+      <row>
+       <entry id="pg-stat-get-backend-wal" role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_stat_get_backend_wal</primary>
+        </indexterm>
+        <function>pg_stat_get_backend_wal</function> ( <type>integer</type> )
+        <returnvalue>record</returnvalue>
+       </para>
+       <para>
+        Returns WAL statistics about the backend with the specified
+        process ID. The output fields are exactly the same as the ones in the
+        <structname>pg_stat_wal</structname> view.
+       </para>
+       <para>
+        The function does not return WAL statistics for the checkpointer,
+        the background writer, the startup process and the autovacuum launcher.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 4a667e7019c..cf3fac678c2 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -35,6 +35,14 @@
  */
 static PgStat_BackendPending PendingBackendStats;
 
+/*
+ * WAL usage counters saved from pgWalUsage at the previous call to
+ * pgstat_report_wal(). This is used to calculate how much WAL usage
+ * happens between pgstat_report_wal() calls, by subtracting
+ * the previous counters from the current ones.
+ */
+static WalUsage prevBackendWalUsage;
+
 /*
  * Utility routines to report I/O stats for backends, kept here to avoid
  * exposing PendingBackendStats to the outside world.
@@ -131,6 +139,57 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 	MemSet(&PendingBackendStats.pending_io, 0, sizeof(PgStat_PendingIO));
 }
 
+/*
+ * To determine whether WAL usage happened.
+ */
+static bool
+pgstat_backend_wal_have_pending(void)
+{
+	return pgWalUsage.wal_records != prevBackendWalUsage.wal_records;
+}
+
+/*
+ * Flush out locally pending backend WAL statistics.  Locking is managed
+ * by the caller.
+ */
+static void
+pgstat_flush_backend_entry_wal(PgStat_EntryRef *entry_ref)
+{
+	PgStatShared_Backend *shbackendent;
+	PgStat_WalCounters *bktype_shstats;
+	WalUsage	wal_usage_diff = {0};
+
+	/*
+	 * This function can be called even if nothing at all has happened. Avoid
+	 * taking lock for nothing in that case.
+	 */
+	if (!pgstat_backend_wal_have_pending())
+		return;
+
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	bktype_shstats = &shbackendent->stats.wal_counters;
+
+	/*
+	 * We don't update the WAL usage portion of the local WalStats elsewhere.
+	 * Calculate how much WAL usage counters were increased by subtracting the
+	 * previous counters from the current ones.
+	 */
+	WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
+
+#define WALSTAT_ACC(fld, var_to_add) \
+	(bktype_shstats->fld += var_to_add.fld)
+	WALSTAT_ACC(wal_buffers_full, wal_usage_diff);
+	WALSTAT_ACC(wal_records, wal_usage_diff);
+	WALSTAT_ACC(wal_fpi, wal_usage_diff);
+	WALSTAT_ACC(wal_bytes, wal_usage_diff);
+#undef WALSTAT_ACC
+
+	/*
+	 * Save the current counters for the subsequent calculation of WAL usage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
+}
+
 /*
  * Flush out locally pending backend statistics
  *
@@ -158,6 +217,9 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 	if (flags & PGSTAT_BACKEND_FLUSH_IO)
 		pgstat_flush_backend_entry_io(entry_ref);
 
+	if (flags & PGSTAT_BACKEND_FLUSH_WAL)
+		pgstat_flush_backend_entry_wal(entry_ref);
+
 	pgstat_unlock_entry(entry_ref);
 
 	return false;
@@ -205,6 +267,8 @@ pgstat_create_backend(ProcNumber procnum)
 	pgstat_unlock_entry(entry_ref);
 
 	MemSet(&PendingBackendStats, 0, sizeof(PgStat_BackendPending));
+
+	prevBackendWalUsage = pgWalUsage;
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 6d024872701..f268a610fb8 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -53,6 +53,8 @@ pgstat_report_wal(bool force)
 	/* flush wal stats */
 	pgstat_flush_wal(nowait);
 
+	pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_WAL);
+
 	/* flush IO stats */
 	pgstat_flush_io(nowait);
 }
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 9de14ffd449..6676245651c 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1634,8 +1634,8 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 /*
  * pg_stat_wal_build_tuple
  *
- * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
- * of wal_counters.
+ * Helper routine for pg_stat_get_wal() and pg_stat_get_backend_wal() returning
+ * one tuple based on the contents of wal_counters.
  */
 static Datum
 pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
@@ -1684,6 +1684,54 @@ pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns WAL statistics for a backend with given PID.
+ */
+Datum
+pg_stat_get_backend_wal(PG_FUNCTION_ARGS)
+{
+	int			pid;
+	PGPROC	   *proc;
+	ProcNumber	procNumber;
+	PgStat_Backend *backend_stats;
+	PgStat_WalCounters bktype_stats;
+	PgBackendStatus *beentry;
+
+	pid = PG_GETARG_INT32(0);
+	proc = BackendPidGetProc(pid);
+
+	/*
+	 * This could be an auxiliary process but these do not report backend
+	 * statistics due to pgstat_tracks_backend_bktype(), so there is no need
+	 * for an extra call to AuxiliaryPidGetProc().
+	 */
+	if (!proc)
+		PG_RETURN_NULL();
+
+	procNumber = GetNumberFromPGProc(proc);
+
+	beentry = pgstat_get_beentry_by_proc_number(procNumber);
+	if (!beentry)
+		PG_RETURN_NULL();
+
+	backend_stats = pgstat_fetch_stat_backend(procNumber);
+	if (!backend_stats)
+		PG_RETURN_NULL();
+
+	/* if PID does not match, leave */
+	if (beentry->st_procpid != pid)
+		PG_RETURN_NULL();
+
+	/* backend may be gone, so recheck in case */
+	if (beentry->st_backendType == B_INVALID)
+		PG_RETURN_NULL();
+
+	bktype_stats = backend_stats->wal_counters;
+
+	/* save tuples with data from this PgStat_WalCounters */
+	return (pg_stat_wal_build_tuple(bktype_stats, backend_stats->stat_reset_timestamp));
+}
+
 /*
  * Returns statistics of WAL activity
  */
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index af9546de23d..399fccb1afb 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5954,6 +5954,13 @@
   proargmodes => '{o,o,o,o,o}',
   proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
   prosrc => 'pg_stat_get_wal' },
+{ oid => '8037', descr => 'statistics: backend WAL activity',
+  proname => 'pg_stat_get_backend_wal', provolatile => 'v',
+  proparallel => 'r', prorettype => 'record', proargtypes => 'int4',
+  proallargtypes => '{int4,int8,int8,numeric,int8,timestamptz}',
+  proargmodes => '{i,o,o,o,o,o}',
+  proargnames => '{backend_pid,wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
+  prosrc => 'pg_stat_get_backend_wal' },
 { oid => '6248', descr => 'statistics: information about WAL prefetching',
   proname => 'pg_stat_get_recovery_prefetch', prorows => '1', proretset => 't',
   provolatile => 'v', prorettype => 'record', proargtypes => '',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 438736870b3..8a706ddabab 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -340,12 +340,6 @@ typedef struct PgStat_IO
 	PgStat_BktypeIO stats[BACKEND_NUM_TYPES];
 } PgStat_IO;
 
-typedef struct PgStat_Backend
-{
-	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO io_stats;
-} PgStat_Backend;
-
 /* ---------
  * PgStat_BackendPending	Non-flushed backend stats.
  * ---------
@@ -488,6 +482,13 @@ typedef struct PgStat_WalStats
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
+typedef struct PgStat_Backend
+{
+	TimestampTz stat_reset_timestamp;
+	PgStat_BktypeIO io_stats;
+	PgStat_WalCounters wal_counters;
+} PgStat_Backend;
+
 /*
  * Functions in pgstat.c
  */
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 06dcea3f0dc..2385839d83f 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -622,7 +622,8 @@ extern void pgstat_archiver_snapshot_cb(void);
 
 /* flags for pgstat_flush_backend() */
 #define PGSTAT_BACKEND_FLUSH_IO		(1 << 0)	/* Flush I/O statistics */
-#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO)
+#define PGSTAT_BACKEND_FLUSH_WAL   (1 << 1) /* Flush WAL statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL   (PGSTAT_BACKEND_FLUSH_IO | PGSTAT_BACKEND_FLUSH_WAL)
 
 extern bool pgstat_flush_backend(bool nowait, bits32 flags);
 extern bool pgstat_backend_flush_cb(bool nowait);
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index 093e6368dbb..b3c303c98cb 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -832,6 +832,8 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -851,6 +853,18 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
  t
 (1 row)
 
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+ ?column? 
+----------
+ t
+(1 row)
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 0a44e14d9f4..ad3f7b7e66a 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -423,6 +423,9 @@ SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
 
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 
@@ -435,6 +438,9 @@ CHECKPOINT;
 SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;
 SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
 
+SELECT pg_stat_force_next_flush();
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
-- 
2.34.1

From a791f83c453e5e94d3508cde526b6d992512dc14 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Thu, 16 Jan 2025 15:06:01 +0000
Subject: [PATCH v8 2/3] Adding a new PgStat_WalCounters struct

This new struct contains only the counters related to the WAL statistics.
This will be used in a follow-up commit that uses the same structures but
for the PGSTAT_KIND_BACKEND statistics kind.
---
 src/backend/utils/activity/pgstat_wal.c |  2 +-
 src/backend/utils/adt/pgstatfuncs.c     | 20 +++++++++++---------
 src/include/pgstat.h                    |  7 ++++++-
 src/tools/pgindent/typedefs.list        |  1 +
 4 files changed, 19 insertions(+), 11 deletions(-)
   8.5% src/backend/utils/activity/
  82.0% src/backend/utils/adt/
   7.8% src/include/

diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 4dc41a4a590..6d024872701 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -115,7 +115,7 @@ pgstat_wal_flush_cb(bool nowait)
 		return true;
 
 #define WALSTAT_ACC(fld, var_to_add) \
-	(stats_shmem->stats.fld += var_to_add.fld)
+	(stats_shmem->stats.wal_counters.fld += var_to_add.fld)
 	WALSTAT_ACC(wal_records, wal_usage_diff);
 	WALSTAT_ACC(wal_fpi, wal_usage_diff);
 	WALSTAT_ACC(wal_bytes, wal_usage_diff);
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 620d60a0938..9de14ffd449 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1635,10 +1635,11 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
  * pg_stat_wal_build_tuple
  *
  * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
- * of wal_stats.
+ * of wal_counters.
  */
 static Datum
-pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
+pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
+						TimestampTz stat_reset_timestamp)
 {
 #define PG_STAT_WAL_COLS	5
 	TupleDesc	tupdesc;
@@ -1662,20 +1663,20 @@ pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
 	BlessTupleDesc(tupdesc);
 
 	/* Fill values and NULLs */
-	values[0] = Int64GetDatum(wal_stats.wal_records);
-	values[1] = Int64GetDatum(wal_stats.wal_fpi);
+	values[0] = Int64GetDatum(wal_counters.wal_records);
+	values[1] = Int64GetDatum(wal_counters.wal_fpi);
 
 	/* Convert to numeric. */
-	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats.wal_bytes);
+	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_counters.wal_bytes);
 	values[2] = DirectFunctionCall3(numeric_in,
 									CStringGetDatum(buf),
 									ObjectIdGetDatum(0),
 									Int32GetDatum(-1));
 
-	values[3] = Int64GetDatum(wal_stats.wal_buffers_full);
+	values[3] = Int64GetDatum(wal_counters.wal_buffers_full);
 
-	if (wal_stats.stat_reset_timestamp != 0)
-		values[4] = TimestampTzGetDatum(wal_stats.stat_reset_timestamp);
+	if (stat_reset_timestamp != 0)
+		values[4] = TimestampTzGetDatum(stat_reset_timestamp);
 	else
 		nulls[4] = true;
 
@@ -1694,7 +1695,8 @@ pg_stat_get_wal(PG_FUNCTION_ARGS)
 	/* Get statistics about WAL activity */
 	wal_stats = pgstat_fetch_stat_wal();
 
-	return (pg_stat_wal_build_tuple(*wal_stats));
+	return (pg_stat_wal_build_tuple(wal_stats->wal_counters,
+									wal_stats->stat_reset_timestamp));
 }
 
 /*
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index fc651d03cf9..438736870b3 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -474,12 +474,17 @@ typedef struct PgStat_StatTabEntry
 	PgStat_Counter total_autoanalyze_time;
 } PgStat_StatTabEntry;
 
-typedef struct PgStat_WalStats
+typedef struct PgStat_WalCounters
 {
 	PgStat_Counter wal_records;
 	PgStat_Counter wal_fpi;
 	uint64		wal_bytes;
 	PgStat_Counter wal_buffers_full;
+} PgStat_WalCounters;
+
+typedef struct PgStat_WalStats
+{
+	PgStat_WalCounters wal_counters;
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index e3e09a2207e..19d510c9ec3 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2188,6 +2188,7 @@ PgStat_SubXactStatus
 PgStat_TableCounts
 PgStat_TableStatus
 PgStat_TableXactStatus
+PgStat_WalCounters
 PgStat_WalStats
 PgXmlErrorContext
 PgXmlStrictness
-- 
2.34.1

From a4a1dce54b8542c74e39117e77b2f1d005341501 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 07:51:27 +0000
Subject: [PATCH v8 1/3] Extract logic filling pg_stat_get_wal()'s tuple into
 its own routine

This commit adds pg_stat_wal_build_tuple(), a helper routine for
pg_stat_get_wal(), that fills its tuple based on the contents
of PgStat_WalStats.  This will be used in a follow-up commit that uses
the same structures as pg_stat_wal for reporting, but for the PGSTAT_KIND_BACKEND
statistics kind.
---
 src/backend/utils/adt/pgstatfuncs.c | 48 +++++++++++++++++++----------
 1 file changed, 32 insertions(+), 16 deletions(-)
 100.0% src/backend/utils/adt/

diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 68e16e52ab6..620d60a0938 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1632,20 +1632,22 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 }
 
 /*
- * Returns statistics of WAL activity
+ * pg_stat_wal_build_tuple
+ *
+ * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
+ * of wal_stats.
  */
-Datum
-pg_stat_get_wal(PG_FUNCTION_ARGS)
+static Datum
+pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
 {
-#define PG_STAT_GET_WAL_COLS	5
+#define PG_STAT_WAL_COLS	5
 	TupleDesc	tupdesc;
-	Datum		values[PG_STAT_GET_WAL_COLS] = {0};
-	bool		nulls[PG_STAT_GET_WAL_COLS] = {0};
+	Datum		values[PG_STAT_WAL_COLS] = {0};
+	bool		nulls[PG_STAT_WAL_COLS] = {0};
 	char		buf[256];
-	PgStat_WalStats *wal_stats;
 
 	/* Initialise attributes information in the tuple descriptor */
-	tupdesc = CreateTemplateTupleDesc(PG_STAT_GET_WAL_COLS);
+	tupdesc = CreateTemplateTupleDesc(PG_STAT_WAL_COLS);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "wal_records",
 					   INT8OID, -1, 0);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "wal_fpi",
@@ -1659,28 +1661,42 @@ pg_stat_get_wal(PG_FUNCTION_ARGS)
 
 	BlessTupleDesc(tupdesc);
 
-	/* Get statistics about WAL activity */
-	wal_stats = pgstat_fetch_stat_wal();
-
 	/* Fill values and NULLs */
-	values[0] = Int64GetDatum(wal_stats->wal_records);
-	values[1] = Int64GetDatum(wal_stats->wal_fpi);
+	values[0] = Int64GetDatum(wal_stats.wal_records);
+	values[1] = Int64GetDatum(wal_stats.wal_fpi);
 
 	/* Convert to numeric. */
-	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats->wal_bytes);
+	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats.wal_bytes);
 	values[2] = DirectFunctionCall3(numeric_in,
 									CStringGetDatum(buf),
 									ObjectIdGetDatum(0),
 									Int32GetDatum(-1));
 
-	values[3] = Int64GetDatum(wal_stats->wal_buffers_full);
+	values[3] = Int64GetDatum(wal_stats.wal_buffers_full);
 
-	values[4] = TimestampTzGetDatum(wal_stats->stat_reset_timestamp);
+	if (wal_stats.stat_reset_timestamp != 0)
+		values[4] = TimestampTzGetDatum(wal_stats.stat_reset_timestamp);
+	else
+		nulls[4] = true;
 
 	/* Returns the record as Datum */
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns statistics of WAL activity
+ */
+Datum
+pg_stat_get_wal(PG_FUNCTION_ARGS)
+{
+	PgStat_WalStats *wal_stats;
+
+	/* Get statistics about WAL activity */
+	wal_stats = pgstat_fetch_stat_wal();
+
+	return (pg_stat_wal_build_tuple(*wal_stats));
+}
+
 /*
  * Returns statistics of SLRU caches.
  */
-- 
2.34.1

From ee2fb0f389161c8f706923aa58e227ab6a2c623f Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Tue, 25 Feb 2025 08:22:50 +0000
Subject: [PATCH v9 1/5] make pgstat_flush_wal() static

pgstat_flush_wal() is used only in pgstat_wal.c so make it static.
---
 src/backend/utils/activity/pgstat_wal.c | 3 ++-
 src/include/utils/pgstat_internal.h     | 2 --
 2 files changed, 2 insertions(+), 3 deletions(-)
  57.6% src/backend/utils/activity/
  42.3% src/include/utils/

diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 4dc41a4a590..b702891ed46 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -29,6 +29,7 @@
  */
 static WalUsage prevWalUsage;
 
+static void pgstat_flush_wal(bool nowait);
 
 /*
  * Calculate how much WAL usage counters have increased and update
@@ -72,7 +73,7 @@ pgstat_fetch_stat_wal(void)
 /*
  * Simple wrapper of pgstat_wal_flush_cb()
  */
-void
+static void
 pgstat_flush_wal(bool nowait)
 {
 	(void) pgstat_wal_flush_cb(nowait);
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 06dcea3f0dc..36d228e3558 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -748,8 +748,6 @@ extern void pgstat_slru_snapshot_cb(void);
  * Functions in pgstat_wal.c
  */
 
-extern void pgstat_flush_wal(bool nowait);
-
 extern void pgstat_wal_init_backend_cb(void);
 extern bool pgstat_wal_have_pending_cb(void);
 extern bool pgstat_wal_flush_cb(bool nowait);
-- 
2.34.1

From f2c3f83228e0cb1909cdd43f732ecaa377e0ebeb Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 07:51:27 +0000
Subject: [PATCH v9 2/5] Extract logic filling pg_stat_get_wal()'s tuple into
 its own routine

This commit adds pg_stat_wal_build_tuple(), a helper routine for
pg_stat_get_wal(), that fills its tuple based on the contents
of PgStat_WalStats.  This will be used in a follow-up commit that uses
the same structures as pg_stat_wal for reporting, but for the PGSTAT_KIND_BACKEND
statistics kind.
---
 src/backend/utils/adt/pgstatfuncs.c | 48 +++++++++++++++++++----------
 1 file changed, 32 insertions(+), 16 deletions(-)
 100.0% src/backend/utils/adt/

diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 68e16e52ab6..620d60a0938 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1632,20 +1632,22 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 }
 
 /*
- * Returns statistics of WAL activity
+ * pg_stat_wal_build_tuple
+ *
+ * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
+ * of wal_stats.
  */
-Datum
-pg_stat_get_wal(PG_FUNCTION_ARGS)
+static Datum
+pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
 {
-#define PG_STAT_GET_WAL_COLS	5
+#define PG_STAT_WAL_COLS	5
 	TupleDesc	tupdesc;
-	Datum		values[PG_STAT_GET_WAL_COLS] = {0};
-	bool		nulls[PG_STAT_GET_WAL_COLS] = {0};
+	Datum		values[PG_STAT_WAL_COLS] = {0};
+	bool		nulls[PG_STAT_WAL_COLS] = {0};
 	char		buf[256];
-	PgStat_WalStats *wal_stats;
 
 	/* Initialise attributes information in the tuple descriptor */
-	tupdesc = CreateTemplateTupleDesc(PG_STAT_GET_WAL_COLS);
+	tupdesc = CreateTemplateTupleDesc(PG_STAT_WAL_COLS);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "wal_records",
 					   INT8OID, -1, 0);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "wal_fpi",
@@ -1659,28 +1661,42 @@ pg_stat_get_wal(PG_FUNCTION_ARGS)
 
 	BlessTupleDesc(tupdesc);
 
-	/* Get statistics about WAL activity */
-	wal_stats = pgstat_fetch_stat_wal();
-
 	/* Fill values and NULLs */
-	values[0] = Int64GetDatum(wal_stats->wal_records);
-	values[1] = Int64GetDatum(wal_stats->wal_fpi);
+	values[0] = Int64GetDatum(wal_stats.wal_records);
+	values[1] = Int64GetDatum(wal_stats.wal_fpi);
 
 	/* Convert to numeric. */
-	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats->wal_bytes);
+	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats.wal_bytes);
 	values[2] = DirectFunctionCall3(numeric_in,
 									CStringGetDatum(buf),
 									ObjectIdGetDatum(0),
 									Int32GetDatum(-1));
 
-	values[3] = Int64GetDatum(wal_stats->wal_buffers_full);
+	values[3] = Int64GetDatum(wal_stats.wal_buffers_full);
 
-	values[4] = TimestampTzGetDatum(wal_stats->stat_reset_timestamp);
+	if (wal_stats.stat_reset_timestamp != 0)
+		values[4] = TimestampTzGetDatum(wal_stats.stat_reset_timestamp);
+	else
+		nulls[4] = true;
 
 	/* Returns the record as Datum */
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns statistics of WAL activity
+ */
+Datum
+pg_stat_get_wal(PG_FUNCTION_ARGS)
+{
+	PgStat_WalStats *wal_stats;
+
+	/* Get statistics about WAL activity */
+	wal_stats = pgstat_fetch_stat_wal();
+
+	return (pg_stat_wal_build_tuple(*wal_stats));
+}
+
 /*
  * Returns statistics of SLRU caches.
  */
-- 
2.34.1

From 5a9a7f138128b8b36c566ab5c44eb3a37c357f88 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Thu, 16 Jan 2025 15:06:01 +0000
Subject: [PATCH v9 3/5] Adding a new PgStat_WalCounters struct

This new struct contains only the counters related to the WAL statistics.
This will be used in a follow-up commit that uses the same structures but
for the PGSTAT_KIND_BACKEND statistics kind.
---
 src/backend/utils/activity/pgstat_wal.c |  2 +-
 src/backend/utils/adt/pgstatfuncs.c     | 20 +++++++++++---------
 src/include/pgstat.h                    |  7 ++++++-
 src/tools/pgindent/typedefs.list        |  1 +
 4 files changed, 19 insertions(+), 11 deletions(-)
   8.5% src/backend/utils/activity/
  82.0% src/backend/utils/adt/
   7.8% src/include/

diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index b702891ed46..830b51a7b93 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -116,7 +116,7 @@ pgstat_wal_flush_cb(bool nowait)
 		return true;
 
 #define WALSTAT_ACC(fld, var_to_add) \
-	(stats_shmem->stats.fld += var_to_add.fld)
+	(stats_shmem->stats.wal_counters.fld += var_to_add.fld)
 	WALSTAT_ACC(wal_records, wal_usage_diff);
 	WALSTAT_ACC(wal_fpi, wal_usage_diff);
 	WALSTAT_ACC(wal_bytes, wal_usage_diff);
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 620d60a0938..9de14ffd449 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1635,10 +1635,11 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
  * pg_stat_wal_build_tuple
  *
  * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
- * of wal_stats.
+ * of wal_counters.
  */
 static Datum
-pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
+pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
+						TimestampTz stat_reset_timestamp)
 {
 #define PG_STAT_WAL_COLS	5
 	TupleDesc	tupdesc;
@@ -1662,20 +1663,20 @@ pg_stat_wal_build_tuple(PgStat_WalStats wal_stats)
 	BlessTupleDesc(tupdesc);
 
 	/* Fill values and NULLs */
-	values[0] = Int64GetDatum(wal_stats.wal_records);
-	values[1] = Int64GetDatum(wal_stats.wal_fpi);
+	values[0] = Int64GetDatum(wal_counters.wal_records);
+	values[1] = Int64GetDatum(wal_counters.wal_fpi);
 
 	/* Convert to numeric. */
-	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_stats.wal_bytes);
+	snprintf(buf, sizeof buf, UINT64_FORMAT, wal_counters.wal_bytes);
 	values[2] = DirectFunctionCall3(numeric_in,
 									CStringGetDatum(buf),
 									ObjectIdGetDatum(0),
 									Int32GetDatum(-1));
 
-	values[3] = Int64GetDatum(wal_stats.wal_buffers_full);
+	values[3] = Int64GetDatum(wal_counters.wal_buffers_full);
 
-	if (wal_stats.stat_reset_timestamp != 0)
-		values[4] = TimestampTzGetDatum(wal_stats.stat_reset_timestamp);
+	if (stat_reset_timestamp != 0)
+		values[4] = TimestampTzGetDatum(stat_reset_timestamp);
 	else
 		nulls[4] = true;
 
@@ -1694,7 +1695,8 @@ pg_stat_get_wal(PG_FUNCTION_ARGS)
 	/* Get statistics about WAL activity */
 	wal_stats = pgstat_fetch_stat_wal();
 
-	return (pg_stat_wal_build_tuple(*wal_stats));
+	return (pg_stat_wal_build_tuple(wal_stats->wal_counters,
+									wal_stats->stat_reset_timestamp));
 }
 
 /*
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index fc651d03cf9..438736870b3 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -474,12 +474,17 @@ typedef struct PgStat_StatTabEntry
 	PgStat_Counter total_autoanalyze_time;
 } PgStat_StatTabEntry;
 
-typedef struct PgStat_WalStats
+typedef struct PgStat_WalCounters
 {
 	PgStat_Counter wal_records;
 	PgStat_Counter wal_fpi;
 	uint64		wal_bytes;
 	PgStat_Counter wal_buffers_full;
+} PgStat_WalCounters;
+
+typedef struct PgStat_WalStats
+{
+	PgStat_WalCounters wal_counters;
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index e3e09a2207e..19d510c9ec3 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2188,6 +2188,7 @@ PgStat_SubXactStatus
 PgStat_TableCounts
 PgStat_TableStatus
 PgStat_TableXactStatus
+PgStat_WalCounters
 PgStat_WalStats
 PgXmlErrorContext
 PgXmlStrictness
-- 
2.34.1

From ff26611c1f74cfc23b13e4607dcf7494cc807ae5 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Tue, 25 Feb 2025 09:03:55 +0000
Subject: [PATCH v9 4/5] Add the pg_stat_get_backend_stats() helper for
 pg_stat_get_backend_io()

This commit adds pg_stat_get_backend_stats(), a helper routine for
pg_stat_get_backend_io(), that returns the backend stats based on a pid passed
as an argument.

This will be used in a follow-up commit that uses the same logic to return the
per backend WAL stats.
---
 src/backend/utils/activity/pgstat_backend.c | 52 +++++++++++++++++++++
 src/backend/utils/adt/pgstatfuncs.c         | 30 +-----------
 src/include/pgstat.h                        |  1 +
 3 files changed, 54 insertions(+), 29 deletions(-)
  58.1% src/backend/utils/activity/
  38.0% src/backend/utils/adt/
   3.7% src/include/

diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 4a667e7019c..3284d97fa95 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -25,6 +25,8 @@
 #include "postgres.h"
 
 #include "storage/bufmgr.h"
+#include "storage/proc.h"
+#include "storage/procarray.h"
 #include "utils/memutils.h"
 #include "utils/pgstat_internal.h"
 
@@ -81,6 +83,56 @@ pgstat_fetch_stat_backend(ProcNumber procNumber)
 	return backend_entry;
 }
 
+/*
+ * Returns statistics of a backend by pid.
+ *
+ * It adds extra checks as compared to pgstat_fetch_stat_backend() to ensure
+ * that the backend is not gone. Also, if not NULL, bktype is populated as
+ * pg_stat_get_backend_io() needs it.
+ */
+PgStat_Backend *
+pg_stat_get_backend_stats(int pid, BackendType *bktype)
+{
+
+	PGPROC	   *proc;
+	PgBackendStatus *beentry;
+	ProcNumber	procNumber;
+	PgStat_Backend *backend_stats;
+
+	proc = BackendPidGetProc(pid);
+
+	/*
+	 * This could be an auxiliary process but these do not report backend
+	 * statistics due to pgstat_tracks_backend_bktype(), so there is no need
+	 * for an extra call to AuxiliaryPidGetProc().
+	 */
+	if (!proc)
+		return NULL;
+
+	procNumber = GetNumberFromPGProc(proc);
+
+	beentry = pgstat_get_beentry_by_proc_number(procNumber);
+	if (!beentry)
+		return NULL;
+
+	backend_stats = pgstat_fetch_stat_backend(procNumber);
+	if (!backend_stats)
+		return NULL;
+
+	/* if PID does not match, leave */
+	if (beentry->st_procpid != pid)
+		return NULL;
+
+	/* backend may be gone, so recheck in case */
+	if (beentry->st_backendType == B_INVALID)
+		return NULL;
+
+	if (bktype)
+		*bktype = beentry->st_backendType;
+
+	return backend_stats;
+}
+
 /*
  * Flush out locally pending backend IO statistics.  Locking is managed
  * by the caller.
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 9de14ffd449..13c91515480 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1576,46 +1576,18 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 	ReturnSetInfo *rsinfo;
 	BackendType bktype;
 	int			pid;
-	PGPROC	   *proc;
-	ProcNumber	procNumber;
 	PgStat_Backend *backend_stats;
 	PgStat_BktypeIO *bktype_stats;
-	PgBackendStatus *beentry;
 
 	InitMaterializedSRF(fcinfo, 0);
 	rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
 
 	pid = PG_GETARG_INT32(0);
-	proc = BackendPidGetProc(pid);
-
-	/*
-	 * This could be an auxiliary process but these do not report backend
-	 * statistics due to pgstat_tracks_backend_bktype(), so there is no need
-	 * for an extra call to AuxiliaryPidGetProc().
-	 */
-	if (!proc)
-		return (Datum) 0;
-
-	procNumber = GetNumberFromPGProc(proc);
+	backend_stats = pg_stat_get_backend_stats(pid, &bktype);
 
-	beentry = pgstat_get_beentry_by_proc_number(procNumber);
-	if (!beentry)
-		return (Datum) 0;
-
-	backend_stats = pgstat_fetch_stat_backend(procNumber);
 	if (!backend_stats)
 		return (Datum) 0;
 
-	bktype = beentry->st_backendType;
-
-	/* if PID does not match, leave */
-	if (beentry->st_procpid != pid)
-		return (Datum) 0;
-
-	/* backend may be gone, so recheck in case */
-	if (bktype == B_INVALID)
-		return (Datum) 0;
-
 	bktype_stats = &backend_stats->io_stats;
 
 	/*
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 438736870b3..3b71c3d4ed6 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -542,6 +542,7 @@ extern void pgstat_count_backend_io_op(IOObject io_object,
 									   IOOp io_op, uint32 cnt,
 									   uint64 bytes);
 extern PgStat_Backend *pgstat_fetch_stat_backend(ProcNumber procNumber);
+extern PgStat_Backend *pg_stat_get_backend_stats(int pid, BackendType *bktype);
 extern bool pgstat_tracks_backend_bktype(BackendType bktype);
 extern void pgstat_create_backend(ProcNumber procnum);
 
-- 
2.34.1

From 84b2c89c0f1a719ddfe4cf87fc0f994e21667335 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 10:00:00 +0000
Subject: [PATCH v9 5/5] per backend WAL statistics

Now that commit 9aea73fc61 added backend-level statistics to pgstats (and
per backend IO statistics) we can more easily add per backend statistics.

This commit adds per backend WAL statistics using the same layer as pg_stat_wal,
except that it is now possible to know how much WAL activity is happening in each
backend rather than an overall aggregate of all the activity.  A function called
pg_stat_get_backend_wal() is added to access this data depending on the
PID of a backend.

The same limitation as in 9aea73fc61 persists, meaning that Auxiliary processes
are not included in this set of statistics.

XXX: bump catalog version
---
 doc/src/sgml/monitoring.sgml                | 19 ++++++
 src/backend/utils/activity/pgstat_backend.c | 64 +++++++++++++++++++++
 src/backend/utils/activity/pgstat_wal.c     |  2 +
 src/backend/utils/adt/pgstatfuncs.c         | 26 ++++++++-
 src/include/catalog/pg_proc.dat             |  7 +++
 src/include/pgstat.h                        | 13 +++--
 src/include/utils/pgstat_internal.h         |  3 +-
 src/test/regress/expected/stats.out         | 14 +++++
 src/test/regress/sql/stats.sql              |  6 ++
 9 files changed, 145 insertions(+), 9 deletions(-)
  15.9% doc/src/sgml/
  39.4% src/backend/utils/activity/
  15.5% src/backend/utils/adt/
   8.8% src/include/catalog/
   4.5% src/include/utils/
   8.4% src/test/regress/expected/
   6.4% src/test/regress/sql/

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 3dfd059b7ee..6b5c9b23c3a 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -4856,6 +4856,25 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para></entry>
       </row>
 
+      <row>
+       <entry id="pg-stat-get-backend-wal" role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_stat_get_backend_wal</primary>
+        </indexterm>
+        <function>pg_stat_get_backend_wal</function> ( <type>integer</type> )
+        <returnvalue>record</returnvalue>
+       </para>
+       <para>
+        Returns WAL statistics about the backend with the specified
+        process ID. The output fields are exactly the same as the ones in the
+        <structname>pg_stat_wal</structname> view.
+       </para>
+       <para>
+        The function does not return WAL statistics for the checkpointer,
+        the background writer, the startup process and the autovacuum launcher.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 3284d97fa95..b49e8539324 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -37,6 +37,14 @@
  */
 static PgStat_BackendPending PendingBackendStats;
 
+/*
+ * WAL usage counters saved from pgWalUsage at the previous call to
+ * pgstat_report_wal(). This is used to calculate how much WAL usage
+ * happens between pgstat_report_wal() calls, by subtracting
+ * the previous counters from the current ones.
+ */
+static WalUsage prevBackendWalUsage;
+
 /*
  * Utility routines to report I/O stats for backends, kept here to avoid
  * exposing PendingBackendStats to the outside world.
@@ -183,6 +191,57 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 	MemSet(&PendingBackendStats.pending_io, 0, sizeof(PgStat_PendingIO));
 }
 
+/*
+ * To determine whether WAL usage happened.
+ */
+static bool
+pgstat_backend_wal_have_pending(void)
+{
+	return pgWalUsage.wal_records != prevBackendWalUsage.wal_records;
+}
+
+/*
+ * Flush out locally pending backend WAL statistics.  Locking is managed
+ * by the caller.
+ */
+static void
+pgstat_flush_backend_entry_wal(PgStat_EntryRef *entry_ref)
+{
+	PgStatShared_Backend *shbackendent;
+	PgStat_WalCounters *bktype_shstats;
+	WalUsage	wal_usage_diff = {0};
+
+	/*
+	 * This function can be called even if nothing at all has happened. Avoid
+	 * taking lock for nothing in that case.
+	 */
+	if (!pgstat_backend_wal_have_pending())
+		return;
+
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	bktype_shstats = &shbackendent->stats.wal_counters;
+
+	/*
+	 * We don't update the WAL usage portion of the local WalStats elsewhere.
+	 * Calculate how much WAL usage counters were increased by subtracting the
+	 * previous counters from the current ones.
+	 */
+	WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
+
+#define WALSTAT_ACC(fld, var_to_add) \
+	(bktype_shstats->fld += var_to_add.fld)
+	WALSTAT_ACC(wal_buffers_full, wal_usage_diff);
+	WALSTAT_ACC(wal_records, wal_usage_diff);
+	WALSTAT_ACC(wal_fpi, wal_usage_diff);
+	WALSTAT_ACC(wal_bytes, wal_usage_diff);
+#undef WALSTAT_ACC
+
+	/*
+	 * Save the current counters for the subsequent calculation of WAL usage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
+}
+
 /*
  * Flush out locally pending backend statistics
  *
@@ -210,6 +269,9 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 	if (flags & PGSTAT_BACKEND_FLUSH_IO)
 		pgstat_flush_backend_entry_io(entry_ref);
 
+	if (flags & PGSTAT_BACKEND_FLUSH_WAL)
+		pgstat_flush_backend_entry_wal(entry_ref);
+
 	pgstat_unlock_entry(entry_ref);
 
 	return false;
@@ -257,6 +319,8 @@ pgstat_create_backend(ProcNumber procnum)
 	pgstat_unlock_entry(entry_ref);
 
 	MemSet(&PendingBackendStats, 0, sizeof(PgStat_BackendPending));
+
+	prevBackendWalUsage = pgWalUsage;
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 830b51a7b93..5051fa596b4 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -54,6 +54,8 @@ pgstat_report_wal(bool force)
 	/* flush wal stats */
 	pgstat_flush_wal(nowait);
 
+	pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_WAL);
+
 	/* flush IO stats */
 	pgstat_flush_io(nowait);
 }
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 13c91515480..4fca5b26fde 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1606,8 +1606,8 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 /*
  * pg_stat_wal_build_tuple
  *
- * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
- * of wal_counters.
+ * Helper routine for pg_stat_get_wal() and pg_stat_get_backend_wal() returning
+ * one tuple based on the contents of wal_counters.
  */
 static Datum
 pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
@@ -1656,6 +1656,28 @@ pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns WAL statistics for a backend with given PID.
+ */
+Datum
+pg_stat_get_backend_wal(PG_FUNCTION_ARGS)
+{
+	int			pid;
+	PgStat_Backend *backend_stats;
+	PgStat_WalCounters bktype_stats;
+
+	pid = PG_GETARG_INT32(0);
+	backend_stats = pg_stat_get_backend_stats(pid, NULL);
+
+	if (!backend_stats)
+		PG_RETURN_NULL();
+
+	bktype_stats = backend_stats->wal_counters;
+
+	/* save tuples with data from this PgStat_WalCounters */
+	return (pg_stat_wal_build_tuple(bktype_stats, backend_stats->stat_reset_timestamp));
+}
+
 /*
  * Returns statistics of WAL activity
  */
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index af9546de23d..399fccb1afb 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5954,6 +5954,13 @@
   proargmodes => '{o,o,o,o,o}',
   proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
   prosrc => 'pg_stat_get_wal' },
+{ oid => '8037', descr => 'statistics: backend WAL activity',
+  proname => 'pg_stat_get_backend_wal', provolatile => 'v',
+  proparallel => 'r', prorettype => 'record', proargtypes => 'int4',
+  proallargtypes => '{int4,int8,int8,numeric,int8,timestamptz}',
+  proargmodes => '{i,o,o,o,o,o}',
+  proargnames => '{backend_pid,wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
+  prosrc => 'pg_stat_get_backend_wal' },
 { oid => '6248', descr => 'statistics: information about WAL prefetching',
   proname => 'pg_stat_get_recovery_prefetch', prorows => '1', proretset => 't',
   provolatile => 'v', prorettype => 'record', proargtypes => '',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 3b71c3d4ed6..2665887b1e2 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -340,12 +340,6 @@ typedef struct PgStat_IO
 	PgStat_BktypeIO stats[BACKEND_NUM_TYPES];
 } PgStat_IO;
 
-typedef struct PgStat_Backend
-{
-	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO io_stats;
-} PgStat_Backend;
-
 /* ---------
  * PgStat_BackendPending	Non-flushed backend stats.
  * ---------
@@ -488,6 +482,13 @@ typedef struct PgStat_WalStats
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
+typedef struct PgStat_Backend
+{
+	TimestampTz stat_reset_timestamp;
+	PgStat_BktypeIO io_stats;
+	PgStat_WalCounters wal_counters;
+} PgStat_Backend;
+
 /*
  * Functions in pgstat.c
  */
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 36d228e3558..d5557e6e998 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -622,7 +622,8 @@ extern void pgstat_archiver_snapshot_cb(void);
 
 /* flags for pgstat_flush_backend() */
 #define PGSTAT_BACKEND_FLUSH_IO		(1 << 0)	/* Flush I/O statistics */
-#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO)
+#define PGSTAT_BACKEND_FLUSH_WAL   (1 << 1) /* Flush WAL statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL   (PGSTAT_BACKEND_FLUSH_IO | PGSTAT_BACKEND_FLUSH_WAL)
 
 extern bool pgstat_flush_backend(bool nowait, bits32 flags);
 extern bool pgstat_backend_flush_cb(bool nowait);
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index 093e6368dbb..b3c303c98cb 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -832,6 +832,8 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -851,6 +853,18 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
  t
 (1 row)
 
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+ ?column? 
+----------
+ t
+(1 row)
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 0a44e14d9f4..ad3f7b7e66a 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -423,6 +423,9 @@ SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
 
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 
@@ -435,6 +438,9 @@ CHECKPOINT;
 SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;
 SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
 
+SELECT pg_stat_force_next_flush();
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
-- 
2.34.1

From ba78796701d3d4228342465e5d08429398ebdc9e Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 07:51:27 +0000
Subject: [PATCH v10 1/3] Extract logic filling pg_stat_get_wal()'s tuple into
 its own routine

This commit adds pg_stat_wal_build_tuple(), a helper routine for
pg_stat_get_wal(), that fills its tuple based on the contents
of PgStat_WalStats.  This will be used in a follow-up commit that uses
the same structures as pg_stat_wal for reporting, but for the PGSTAT_KIND_BACKEND
statistics kind.
---
 src/backend/utils/adt/pgstatfuncs.c | 44 ++++++++++++++++++++---------
 1 file changed, 30 insertions(+), 14 deletions(-)
 100.0% src/backend/utils/adt/

diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 0ea41299e07..9de14ffd449 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1632,21 +1632,23 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 }
 
 /*
- * Returns statistics of WAL activity
+ * pg_stat_wal_build_tuple
+ *
+ * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
+ * of wal_counters.
  */
-Datum
-pg_stat_get_wal(PG_FUNCTION_ARGS)
+static Datum
+pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
+						TimestampTz stat_reset_timestamp)
 {
-#define PG_STAT_GET_WAL_COLS	5
+#define PG_STAT_WAL_COLS	5
 	TupleDesc	tupdesc;
-	Datum		values[PG_STAT_GET_WAL_COLS] = {0};
-	bool		nulls[PG_STAT_GET_WAL_COLS] = {0};
+	Datum		values[PG_STAT_WAL_COLS] = {0};
+	bool		nulls[PG_STAT_WAL_COLS] = {0};
 	char		buf[256];
-	PgStat_WalStats *wal_stats;
-	PgStat_WalCounters wal_counters;
 
 	/* Initialise attributes information in the tuple descriptor */
-	tupdesc = CreateTemplateTupleDesc(PG_STAT_GET_WAL_COLS);
+	tupdesc = CreateTemplateTupleDesc(PG_STAT_WAL_COLS);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "wal_records",
 					   INT8OID, -1, 0);
 	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "wal_fpi",
@@ -1660,10 +1662,6 @@ pg_stat_get_wal(PG_FUNCTION_ARGS)
 
 	BlessTupleDesc(tupdesc);
 
-	/* Get statistics about WAL activity */
-	wal_stats = pgstat_fetch_stat_wal();
-	wal_counters = wal_stats->wal_counters;
-
 	/* Fill values and NULLs */
 	values[0] = Int64GetDatum(wal_counters.wal_records);
 	values[1] = Int64GetDatum(wal_counters.wal_fpi);
@@ -1677,12 +1675,30 @@ pg_stat_get_wal(PG_FUNCTION_ARGS)
 
 	values[3] = Int64GetDatum(wal_counters.wal_buffers_full);
 
-	values[4] = TimestampTzGetDatum(wal_stats->stat_reset_timestamp);
+	if (stat_reset_timestamp != 0)
+		values[4] = TimestampTzGetDatum(stat_reset_timestamp);
+	else
+		nulls[4] = true;
 
 	/* Returns the record as Datum */
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns statistics of WAL activity
+ */
+Datum
+pg_stat_get_wal(PG_FUNCTION_ARGS)
+{
+	PgStat_WalStats *wal_stats;
+
+	/* Get statistics about WAL activity */
+	wal_stats = pgstat_fetch_stat_wal();
+
+	return (pg_stat_wal_build_tuple(wal_stats->wal_counters,
+									wal_stats->stat_reset_timestamp));
+}
+
 /*
  * Returns statistics of SLRU caches.
  */
-- 
2.34.1

From f145c27a9aef12499bee907fbb6121be3aeca9ca Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Tue, 25 Feb 2025 09:03:55 +0000
Subject: [PATCH v10 2/3] Add the pg_stat_get_backend_stats() helper for
 pg_stat_get_backend_io()

This commit adds pg_stat_get_backend_stats(), a helper routine for
pg_stat_get_backend_io(), that returns the backend stats based on a pid passed
as an argument.

This will be used in a follow-up commit that uses the same logic to return the
per backend WAL stats.
---
 src/backend/utils/activity/pgstat_backend.c | 52 +++++++++++++++++++++
 src/backend/utils/adt/pgstatfuncs.c         | 30 +-----------
 src/include/pgstat.h                        |  1 +
 3 files changed, 54 insertions(+), 29 deletions(-)
  58.1% src/backend/utils/activity/
  38.0% src/backend/utils/adt/
   3.7% src/include/

diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 338da73a9a9..6f17e1ff39f 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -26,6 +26,8 @@
 
 #include "access/xlog.h"
 #include "storage/bufmgr.h"
+#include "storage/proc.h"
+#include "storage/procarray.h"
 #include "utils/memutils.h"
 #include "utils/pgstat_internal.h"
 
@@ -82,6 +84,56 @@ pgstat_fetch_stat_backend(ProcNumber procNumber)
 	return backend_entry;
 }
 
+/*
+ * Returns statistics of a backend by pid.
+ *
+ * It adds extra checks as compared to pgstat_fetch_stat_backend() to ensure
+ * that the backend is not gone. Also, if not NULL, bktype is populated as
+ * pg_stat_get_backend_io() needs it.
+ */
+PgStat_Backend *
+pg_stat_get_backend_stats(int pid, BackendType *bktype)
+{
+
+	PGPROC	   *proc;
+	PgBackendStatus *beentry;
+	ProcNumber	procNumber;
+	PgStat_Backend *backend_stats;
+
+	proc = BackendPidGetProc(pid);
+
+	/*
+	 * This could be an auxiliary process but these do not report backend
+	 * statistics due to pgstat_tracks_backend_bktype(), so there is no need
+	 * for an extra call to AuxiliaryPidGetProc().
+	 */
+	if (!proc)
+		return NULL;
+
+	procNumber = GetNumberFromPGProc(proc);
+
+	beentry = pgstat_get_beentry_by_proc_number(procNumber);
+	if (!beentry)
+		return NULL;
+
+	backend_stats = pgstat_fetch_stat_backend(procNumber);
+	if (!backend_stats)
+		return NULL;
+
+	/* if PID does not match, leave */
+	if (beentry->st_procpid != pid)
+		return NULL;
+
+	/* backend may be gone, so recheck in case */
+	if (beentry->st_backendType == B_INVALID)
+		return NULL;
+
+	if (bktype)
+		*bktype = beentry->st_backendType;
+
+	return backend_stats;
+}
+
 /*
  * Flush out locally pending backend IO statistics.  Locking is managed
  * by the caller.
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 9de14ffd449..13c91515480 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1576,46 +1576,18 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 	ReturnSetInfo *rsinfo;
 	BackendType bktype;
 	int			pid;
-	PGPROC	   *proc;
-	ProcNumber	procNumber;
 	PgStat_Backend *backend_stats;
 	PgStat_BktypeIO *bktype_stats;
-	PgBackendStatus *beentry;
 
 	InitMaterializedSRF(fcinfo, 0);
 	rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
 
 	pid = PG_GETARG_INT32(0);
-	proc = BackendPidGetProc(pid);
-
-	/*
-	 * This could be an auxiliary process but these do not report backend
-	 * statistics due to pgstat_tracks_backend_bktype(), so there is no need
-	 * for an extra call to AuxiliaryPidGetProc().
-	 */
-	if (!proc)
-		return (Datum) 0;
-
-	procNumber = GetNumberFromPGProc(proc);
+	backend_stats = pg_stat_get_backend_stats(pid, &bktype);
 
-	beentry = pgstat_get_beentry_by_proc_number(procNumber);
-	if (!beentry)
-		return (Datum) 0;
-
-	backend_stats = pgstat_fetch_stat_backend(procNumber);
 	if (!backend_stats)
 		return (Datum) 0;
 
-	bktype = beentry->st_backendType;
-
-	/* if PID does not match, leave */
-	if (beentry->st_procpid != pid)
-		return (Datum) 0;
-
-	/* backend may be gone, so recheck in case */
-	if (bktype == B_INVALID)
-		return (Datum) 0;
-
 	bktype_stats = &backend_stats->io_stats;
 
 	/*
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 67656264b62..e8e7d95b334 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -554,6 +554,7 @@ extern void pgstat_count_backend_io_op(IOObject io_object,
 									   IOOp io_op, uint32 cnt,
 									   uint64 bytes);
 extern PgStat_Backend *pgstat_fetch_stat_backend(ProcNumber procNumber);
+extern PgStat_Backend *pg_stat_get_backend_stats(int pid, BackendType *bktype);
 extern bool pgstat_tracks_backend_bktype(BackendType bktype);
 extern void pgstat_create_backend(ProcNumber procnum);
 
-- 
2.34.1

From 3a3d2d36b377338c61dfaf00d257f990e248465c Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 10:00:00 +0000
Subject: [PATCH v10 3/3] per backend WAL statistics

Now that commit 9aea73fc61 added backend-level statistics to pgstats (and
per backend IO statistics) we can more easily add per backend statistics.

This commit adds per backend WAL statistics using the same layer as pg_stat_wal,
except that it is now possible to know how much WAL activity is happening in each
backend rather than an overall aggregate of all the activity.  A function called
pg_stat_get_backend_wal() is added to access this data depending on the
PID of a backend.

The same limitation as in 9aea73fc61 persists, meaning that Auxiliary processes
are not included in this set of statistics.

XXX: bump catalog version
---
 doc/src/sgml/monitoring.sgml                | 19 ++++++
 src/backend/utils/activity/pgstat_backend.c | 64 +++++++++++++++++++++
 src/backend/utils/activity/pgstat_wal.c     |  1 +
 src/backend/utils/adt/pgstatfuncs.c         | 26 ++++++++-
 src/include/catalog/pg_proc.dat             |  7 +++
 src/include/pgstat.h                        | 13 +++--
 src/include/utils/pgstat_internal.h         |  3 +-
 src/test/regress/expected/stats.out         | 14 +++++
 src/test/regress/sql/stats.sql              |  6 ++
 9 files changed, 144 insertions(+), 9 deletions(-)
  16.0% doc/src/sgml/
  39.4% src/backend/utils/activity/
  15.5% src/backend/utils/adt/
   8.8% src/include/catalog/
   4.5% src/include/utils/
   8.4% src/test/regress/expected/
   6.4% src/test/regress/sql/

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 9178f1d34ef..f4c37c811ba 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -4860,6 +4860,25 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para></entry>
       </row>
 
+      <row>
+       <entry id="pg-stat-get-backend-wal" role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_stat_get_backend_wal</primary>
+        </indexterm>
+        <function>pg_stat_get_backend_wal</function> ( <type>integer</type> )
+        <returnvalue>record</returnvalue>
+       </para>
+       <para>
+        Returns WAL statistics about the backend with the specified
+        process ID. The output fields are exactly the same as the ones in the
+        <structname>pg_stat_wal</structname> view.
+       </para>
+       <para>
+        The function does not return WAL statistics for the checkpointer,
+        the background writer, the startup process and the autovacuum launcher.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 6f17e1ff39f..c39b10fb3f2 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -38,6 +38,14 @@
  */
 static PgStat_BackendPending PendingBackendStats;
 
+/*
+ * WAL usage counters saved from pgWalUsage at the previous call to
+ * pgstat_report_wal(). This is used to calculate how much WAL usage
+ * happens between pgstat_report_wal() calls, by subtracting
+ * the previous counters from the current ones.
+ */
+static WalUsage prevBackendWalUsage;
+
 /*
  * Utility routines to report I/O stats for backends, kept here to avoid
  * exposing PendingBackendStats to the outside world.
@@ -184,6 +192,57 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 	MemSet(&PendingBackendStats.pending_io, 0, sizeof(PgStat_PendingIO));
 }
 
+/*
+ * To determine whether WAL usage happened.
+ */
+static bool
+pgstat_backend_wal_have_pending(void)
+{
+	return pgWalUsage.wal_records != prevBackendWalUsage.wal_records;
+}
+
+/*
+ * Flush out locally pending backend WAL statistics.  Locking is managed
+ * by the caller.
+ */
+static void
+pgstat_flush_backend_entry_wal(PgStat_EntryRef *entry_ref)
+{
+	PgStatShared_Backend *shbackendent;
+	PgStat_WalCounters *bktype_shstats;
+	WalUsage	wal_usage_diff = {0};
+
+	/*
+	 * This function can be called even if nothing at all has happened. Avoid
+	 * taking lock for nothing in that case.
+	 */
+	if (!pgstat_backend_wal_have_pending())
+		return;
+
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	bktype_shstats = &shbackendent->stats.wal_counters;
+
+	/*
+	 * We don't update the WAL usage portion of the local WalStats elsewhere.
+	 * Calculate how much WAL usage counters were increased by subtracting the
+	 * previous counters from the current ones.
+	 */
+	WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
+
+#define WALSTAT_ACC(fld, var_to_add) \
+	(bktype_shstats->fld += var_to_add.fld)
+	WALSTAT_ACC(wal_buffers_full, wal_usage_diff);
+	WALSTAT_ACC(wal_records, wal_usage_diff);
+	WALSTAT_ACC(wal_fpi, wal_usage_diff);
+	WALSTAT_ACC(wal_bytes, wal_usage_diff);
+#undef WALSTAT_ACC
+
+	/*
+	 * Save the current counters for the subsequent calculation of WAL usage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
+}
+
 /*
  * Flush out locally pending backend statistics
  *
@@ -211,6 +270,9 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 	if (flags & PGSTAT_BACKEND_FLUSH_IO)
 		pgstat_flush_backend_entry_io(entry_ref);
 
+	if (flags & PGSTAT_BACKEND_FLUSH_WAL)
+		pgstat_flush_backend_entry_wal(entry_ref);
+
 	pgstat_unlock_entry(entry_ref);
 
 	return false;
@@ -258,6 +320,8 @@ pgstat_create_backend(ProcNumber procnum)
 	pgstat_unlock_entry(entry_ref);
 
 	MemSet(&PendingBackendStats, 0, sizeof(PgStat_BackendPending));
+
+	prevBackendWalUsage = pgWalUsage;
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 943be0cbeef..c1c2e6dc386 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -52,6 +52,7 @@ pgstat_report_wal(bool force)
 
 	/* flush wal stats */
 	(void) pgstat_wal_flush_cb(nowait);
+	pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_WAL);
 
 	/* flush IO stats */
 	pgstat_flush_io(nowait);
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 13c91515480..4fca5b26fde 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1606,8 +1606,8 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 /*
  * pg_stat_wal_build_tuple
  *
- * Helper routine for pg_stat_get_wal() returning one tuple based on the contents
- * of wal_counters.
+ * Helper routine for pg_stat_get_wal() and pg_stat_get_backend_wal() returning
+ * one tuple based on the contents of wal_counters.
  */
 static Datum
 pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
@@ -1656,6 +1656,28 @@ pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns WAL statistics for a backend with given PID.
+ */
+Datum
+pg_stat_get_backend_wal(PG_FUNCTION_ARGS)
+{
+	int			pid;
+	PgStat_Backend *backend_stats;
+	PgStat_WalCounters bktype_stats;
+
+	pid = PG_GETARG_INT32(0);
+	backend_stats = pg_stat_get_backend_stats(pid, NULL);
+
+	if (!backend_stats)
+		PG_RETURN_NULL();
+
+	bktype_stats = backend_stats->wal_counters;
+
+	/* save tuples with data from this PgStat_WalCounters */
+	return (pg_stat_wal_build_tuple(bktype_stats, backend_stats->stat_reset_timestamp));
+}
+
 /*
  * Returns statistics of WAL activity
  */
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 1c1d96e0c7e..ed3eb823d5c 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5954,6 +5954,13 @@
   proargmodes => '{o,o,o,o,o}',
   proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
   prosrc => 'pg_stat_get_wal' },
+{ oid => '8037', descr => 'statistics: backend WAL activity',
+  proname => 'pg_stat_get_backend_wal', provolatile => 'v',
+  proparallel => 'r', prorettype => 'record', proargtypes => 'int4',
+  proallargtypes => '{int4,int8,int8,numeric,int8,timestamptz}',
+  proargmodes => '{i,o,o,o,o,o}',
+  proargnames => '{backend_pid,wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
+  prosrc => 'pg_stat_get_backend_wal' },
 { oid => '6248', descr => 'statistics: information about WAL prefetching',
   proname => 'pg_stat_get_recovery_prefetch', prorows => '1', proretset => 't',
   provolatile => 'v', prorettype => 'record', proargtypes => '',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index e8e7d95b334..aaddb7acdf6 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -340,12 +340,6 @@ typedef struct PgStat_IO
 	PgStat_BktypeIO stats[BACKEND_NUM_TYPES];
 } PgStat_IO;
 
-typedef struct PgStat_Backend
-{
-	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO io_stats;
-} PgStat_Backend;
-
 /* ---------
  * PgStat_BackendPending	Non-flushed backend stats.
  * ---------
@@ -500,6 +494,13 @@ typedef struct PgStat_WalStats
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
+typedef struct PgStat_Backend
+{
+	TimestampTz stat_reset_timestamp;
+	PgStat_BktypeIO io_stats;
+	PgStat_WalCounters wal_counters;
+} PgStat_Backend;
+
 /*
  * Functions in pgstat.c
  */
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 36d228e3558..d5557e6e998 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -622,7 +622,8 @@ extern void pgstat_archiver_snapshot_cb(void);
 
 /* flags for pgstat_flush_backend() */
 #define PGSTAT_BACKEND_FLUSH_IO		(1 << 0)	/* Flush I/O statistics */
-#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO)
+#define PGSTAT_BACKEND_FLUSH_WAL   (1 << 1) /* Flush WAL statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL   (PGSTAT_BACKEND_FLUSH_IO | PGSTAT_BACKEND_FLUSH_WAL)
 
 extern bool pgstat_flush_backend(bool nowait, bits32 flags);
 extern bool pgstat_backend_flush_cb(bool nowait);
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index 093e6368dbb..b3c303c98cb 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -832,6 +832,8 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -851,6 +853,18 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
  t
 (1 row)
 
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+ ?column? 
+----------
+ t
+(1 row)
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 0a44e14d9f4..ad3f7b7e66a 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -423,6 +423,9 @@ SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
 
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 
@@ -435,6 +438,9 @@ CHECKPOINT;
 SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;
 SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
 
+SELECT pg_stat_force_next_flush();
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
-- 
2.34.1

From a98f27ccecd62867ed7aeb9be166488fffb4304d Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Tue, 25 Feb 2025 09:03:55 +0000
Subject: [PATCH v11 1/2] Add the pg_stat_get_backend_stats() helper for
 pg_stat_get_backend_io()

This commit adds pg_stat_get_backend_stats(), a helper routine for
pg_stat_get_backend_io(), that returns the backend stats based on a pid passed
as an argument.

This will be used in a follow-up commit that uses the same logic to return the
per backend WAL stats.
---
 src/backend/utils/activity/pgstat_backend.c | 52 +++++++++++++++++++++
 src/backend/utils/adt/pgstatfuncs.c         | 30 +-----------
 src/include/pgstat.h                        |  1 +
 3 files changed, 54 insertions(+), 29 deletions(-)
  58.1% src/backend/utils/activity/
  38.0% src/backend/utils/adt/
   3.7% src/include/

diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 338da73a9a9..6f17e1ff39f 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -26,6 +26,8 @@
 
 #include "access/xlog.h"
 #include "storage/bufmgr.h"
+#include "storage/proc.h"
+#include "storage/procarray.h"
 #include "utils/memutils.h"
 #include "utils/pgstat_internal.h"
 
@@ -82,6 +84,56 @@ pgstat_fetch_stat_backend(ProcNumber procNumber)
 	return backend_entry;
 }
 
+/*
+ * Returns statistics of a backend by pid.
+ *
+ * It adds extra checks as compared to pgstat_fetch_stat_backend() to ensure
+ * that the backend is not gone. Also, if not NULL, bktype is populated as
+ * pg_stat_get_backend_io() needs it.
+ */
+PgStat_Backend *
+pg_stat_get_backend_stats(int pid, BackendType *bktype)
+{
+
+	PGPROC	   *proc;
+	PgBackendStatus *beentry;
+	ProcNumber	procNumber;
+	PgStat_Backend *backend_stats;
+
+	proc = BackendPidGetProc(pid);
+
+	/*
+	 * This could be an auxiliary process but these do not report backend
+	 * statistics due to pgstat_tracks_backend_bktype(), so there is no need
+	 * for an extra call to AuxiliaryPidGetProc().
+	 */
+	if (!proc)
+		return NULL;
+
+	procNumber = GetNumberFromPGProc(proc);
+
+	beentry = pgstat_get_beentry_by_proc_number(procNumber);
+	if (!beentry)
+		return NULL;
+
+	backend_stats = pgstat_fetch_stat_backend(procNumber);
+	if (!backend_stats)
+		return NULL;
+
+	/* if PID does not match, leave */
+	if (beentry->st_procpid != pid)
+		return NULL;
+
+	/* backend may be gone, so recheck in case */
+	if (beentry->st_backendType == B_INVALID)
+		return NULL;
+
+	if (bktype)
+		*bktype = beentry->st_backendType;
+
+	return backend_stats;
+}
+
 /*
  * Flush out locally pending backend IO statistics.  Locking is managed
  * by the caller.
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index efb6d0032af..ea91c8fc9d5 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1576,46 +1576,18 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 	ReturnSetInfo *rsinfo;
 	BackendType bktype;
 	int			pid;
-	PGPROC	   *proc;
-	ProcNumber	procNumber;
 	PgStat_Backend *backend_stats;
 	PgStat_BktypeIO *bktype_stats;
-	PgBackendStatus *beentry;
 
 	InitMaterializedSRF(fcinfo, 0);
 	rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
 
 	pid = PG_GETARG_INT32(0);
-	proc = BackendPidGetProc(pid);
-
-	/*
-	 * This could be an auxiliary process but these do not report backend
-	 * statistics due to pgstat_tracks_backend_bktype(), so there is no need
-	 * for an extra call to AuxiliaryPidGetProc().
-	 */
-	if (!proc)
-		return (Datum) 0;
-
-	procNumber = GetNumberFromPGProc(proc);
+	backend_stats = pg_stat_get_backend_stats(pid, &bktype);
 
-	beentry = pgstat_get_beentry_by_proc_number(procNumber);
-	if (!beentry)
-		return (Datum) 0;
-
-	backend_stats = pgstat_fetch_stat_backend(procNumber);
 	if (!backend_stats)
 		return (Datum) 0;
 
-	bktype = beentry->st_backendType;
-
-	/* if PID does not match, leave */
-	if (beentry->st_procpid != pid)
-		return (Datum) 0;
-
-	/* backend may be gone, so recheck in case */
-	if (bktype == B_INVALID)
-		return (Datum) 0;
-
 	bktype_stats = &backend_stats->io_stats;
 
 	/*
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 67656264b62..e8e7d95b334 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -554,6 +554,7 @@ extern void pgstat_count_backend_io_op(IOObject io_object,
 									   IOOp io_op, uint32 cnt,
 									   uint64 bytes);
 extern PgStat_Backend *pgstat_fetch_stat_backend(ProcNumber procNumber);
+extern PgStat_Backend *pg_stat_get_backend_stats(int pid, BackendType *bktype);
 extern bool pgstat_tracks_backend_bktype(BackendType bktype);
 extern void pgstat_create_backend(ProcNumber procnum);
 
-- 
2.34.1

From 9fe4e1ba92e5f270fdb5654142d13c3397dec993 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 10:00:00 +0000
Subject: [PATCH v11 2/2] per backend WAL statistics

Now that commit 9aea73fc61 added backend-level statistics to pgstats (and
per backend IO statistics) we can more easily add per backend statistics.

This commit adds per backend WAL statistics using the same layer as pg_stat_wal,
except that it is now possible to know how much WAL activity is happening in each
backend rather than an overall aggregate of all the activity.  A function called
pg_stat_get_backend_wal() is added to access this data depending on the
PID of a backend.

The same limitation as in 9aea73fc61 persists, meaning that Auxiliary processes
are not included in this set of statistics.

XXX: bump catalog version
---
 doc/src/sgml/monitoring.sgml                | 19 ++++++
 src/backend/utils/activity/pgstat_backend.c | 64 +++++++++++++++++++++
 src/backend/utils/activity/pgstat_wal.c     |  1 +
 src/backend/utils/adt/pgstatfuncs.c         | 26 ++++++++-
 src/include/catalog/pg_proc.dat             |  7 +++
 src/include/pgstat.h                        | 37 ++++++------
 src/include/utils/pgstat_internal.h         |  3 +-
 src/test/regress/expected/stats.out         | 14 +++++
 src/test/regress/sql/stats.sql              |  6 ++
 9 files changed, 156 insertions(+), 21 deletions(-)
  16.0% doc/src/sgml/
  39.4% src/backend/utils/activity/
  15.5% src/backend/utils/adt/
   8.8% src/include/catalog/
   4.5% src/include/utils/
   8.4% src/test/regress/expected/
   6.4% src/test/regress/sql/

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 9178f1d34ef..f4c37c811ba 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -4860,6 +4860,25 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para></entry>
       </row>
 
+      <row>
+       <entry id="pg-stat-get-backend-wal" role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_stat_get_backend_wal</primary>
+        </indexterm>
+        <function>pg_stat_get_backend_wal</function> ( <type>integer</type> )
+        <returnvalue>record</returnvalue>
+       </para>
+       <para>
+        Returns WAL statistics about the backend with the specified
+        process ID. The output fields are exactly the same as the ones in the
+        <structname>pg_stat_wal</structname> view.
+       </para>
+       <para>
+        The function does not return WAL statistics for the checkpointer,
+        the background writer, the startup process and the autovacuum launcher.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 6f17e1ff39f..c39b10fb3f2 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -38,6 +38,14 @@
  */
 static PgStat_BackendPending PendingBackendStats;
 
+/*
+ * WAL usage counters saved from pgWalUsage at the previous call to
+ * pgstat_report_wal(). This is used to calculate how much WAL usage
+ * happens between pgstat_report_wal() calls, by subtracting
+ * the previous counters from the current ones.
+ */
+static WalUsage prevBackendWalUsage;
+
 /*
  * Utility routines to report I/O stats for backends, kept here to avoid
  * exposing PendingBackendStats to the outside world.
@@ -184,6 +192,57 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 	MemSet(&PendingBackendStats.pending_io, 0, sizeof(PgStat_PendingIO));
 }
 
+/*
+ * To determine whether WAL usage happened.
+ */
+static bool
+pgstat_backend_wal_have_pending(void)
+{
+	return pgWalUsage.wal_records != prevBackendWalUsage.wal_records;
+}
+
+/*
+ * Flush out locally pending backend WAL statistics.  Locking is managed
+ * by the caller.
+ */
+static void
+pgstat_flush_backend_entry_wal(PgStat_EntryRef *entry_ref)
+{
+	PgStatShared_Backend *shbackendent;
+	PgStat_WalCounters *bktype_shstats;
+	WalUsage	wal_usage_diff = {0};
+
+	/*
+	 * This function can be called even if nothing at all has happened. Avoid
+	 * taking lock for nothing in that case.
+	 */
+	if (!pgstat_backend_wal_have_pending())
+		return;
+
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	bktype_shstats = &shbackendent->stats.wal_counters;
+
+	/*
+	 * We don't update the WAL usage portion of the local WalStats elsewhere.
+	 * Calculate how much WAL usage counters were increased by subtracting the
+	 * previous counters from the current ones.
+	 */
+	WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
+
+#define WALSTAT_ACC(fld, var_to_add) \
+	(bktype_shstats->fld += var_to_add.fld)
+	WALSTAT_ACC(wal_buffers_full, wal_usage_diff);
+	WALSTAT_ACC(wal_records, wal_usage_diff);
+	WALSTAT_ACC(wal_fpi, wal_usage_diff);
+	WALSTAT_ACC(wal_bytes, wal_usage_diff);
+#undef WALSTAT_ACC
+
+	/*
+	 * Save the current counters for the subsequent calculation of WAL usage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
+}
+
 /*
  * Flush out locally pending backend statistics
  *
@@ -211,6 +270,9 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 	if (flags & PGSTAT_BACKEND_FLUSH_IO)
 		pgstat_flush_backend_entry_io(entry_ref);
 
+	if (flags & PGSTAT_BACKEND_FLUSH_WAL)
+		pgstat_flush_backend_entry_wal(entry_ref);
+
 	pgstat_unlock_entry(entry_ref);
 
 	return false;
@@ -258,6 +320,8 @@ pgstat_create_backend(ProcNumber procnum)
 	pgstat_unlock_entry(entry_ref);
 
 	MemSet(&PendingBackendStats, 0, sizeof(PgStat_BackendPending));
+
+	prevBackendWalUsage = pgWalUsage;
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 943be0cbeef..c1c2e6dc386 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -52,6 +52,7 @@ pgstat_report_wal(bool force)
 
 	/* flush wal stats */
 	(void) pgstat_wal_flush_cb(nowait);
+	pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_WAL);
 
 	/* flush IO stats */
 	pgstat_flush_io(nowait);
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index ea91c8fc9d5..164a0bbb3d9 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1606,8 +1606,8 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 /*
  * pg_stat_wal_build_tuple
  *
- * Helper routine for pg_stat_get_wal() returning one tuple based on the
- * contents of wal_counters.
+ * Helper routine for pg_stat_get_wal() and pg_stat_get_backend_wal()
+ * returning one tuple based on the contents of wal_counters.
  */
 static Datum
 pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
@@ -1656,6 +1656,28 @@ pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns WAL statistics for a backend with given PID.
+ */
+Datum
+pg_stat_get_backend_wal(PG_FUNCTION_ARGS)
+{
+	int			pid;
+	PgStat_Backend *backend_stats;
+	PgStat_WalCounters bktype_stats;
+
+	pid = PG_GETARG_INT32(0);
+	backend_stats = pg_stat_get_backend_stats(pid, NULL);
+
+	if (!backend_stats)
+		PG_RETURN_NULL();
+
+	bktype_stats = backend_stats->wal_counters;
+
+	/* save tuples with data from this PgStat_WalCounters */
+	return (pg_stat_wal_build_tuple(bktype_stats, backend_stats->stat_reset_timestamp));
+}
+
 /*
  * Returns statistics of WAL activity
  */
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index cd9422d0bac..3e35f8b8e99 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5954,6 +5954,13 @@
   proargmodes => '{o,o,o,o,o}',
   proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
   prosrc => 'pg_stat_get_wal' },
+{ oid => '8037', descr => 'statistics: backend WAL activity',
+  proname => 'pg_stat_get_backend_wal', provolatile => 'v',
+  proparallel => 'r', prorettype => 'record', proargtypes => 'int4',
+  proallargtypes => '{int4,int8,int8,numeric,int8,timestamptz}',
+  proargmodes => '{i,o,o,o,o,o}',
+  proargnames => '{backend_pid,wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
+  prosrc => 'pg_stat_get_backend_wal' },
 { oid => '6248', descr => 'statistics: information about WAL prefetching',
   proname => 'pg_stat_get_recovery_prefetch', prorows => '1', proretset => 't',
   provolatile => 'v', prorettype => 'record', proargtypes => '',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index e8e7d95b334..6409ea23c84 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -340,24 +340,6 @@ typedef struct PgStat_IO
 	PgStat_BktypeIO stats[BACKEND_NUM_TYPES];
 } PgStat_IO;
 
-typedef struct PgStat_Backend
-{
-	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO io_stats;
-} PgStat_Backend;
-
-/* ---------
- * PgStat_BackendPending	Non-flushed backend stats.
- * ---------
- */
-typedef struct PgStat_BackendPending
-{
-	/*
-	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
-	 */
-	PgStat_PendingIO pending_io;
-} PgStat_BackendPending;
-
 typedef struct PgStat_StatDBEntry
 {
 	PgStat_Counter xact_commit;
@@ -500,6 +482,25 @@ typedef struct PgStat_WalStats
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
+typedef struct PgStat_Backend
+{
+	TimestampTz stat_reset_timestamp;
+	PgStat_BktypeIO io_stats;
+	PgStat_WalCounters wal_counters;
+} PgStat_Backend;
+
+/* ---------
+ * PgStat_BackendPending	Non-flushed backend stats.
+ * ---------
+ */
+typedef struct PgStat_BackendPending
+{
+	/*
+	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
+	 */
+	PgStat_PendingIO pending_io;
+} PgStat_BackendPending;
+
 /*
  * Functions in pgstat.c
  */
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 36d228e3558..d5557e6e998 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -622,7 +622,8 @@ extern void pgstat_archiver_snapshot_cb(void);
 
 /* flags for pgstat_flush_backend() */
 #define PGSTAT_BACKEND_FLUSH_IO		(1 << 0)	/* Flush I/O statistics */
-#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO)
+#define PGSTAT_BACKEND_FLUSH_WAL   (1 << 1) /* Flush WAL statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL   (PGSTAT_BACKEND_FLUSH_IO | PGSTAT_BACKEND_FLUSH_WAL)
 
 extern bool pgstat_flush_backend(bool nowait, bits32 flags);
 extern bool pgstat_backend_flush_cb(bool nowait);
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index 093e6368dbb..b3c303c98cb 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -832,6 +832,8 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -851,6 +853,18 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
  t
 (1 row)
 
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+ ?column? 
+----------
+ t
+(1 row)
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 0a44e14d9f4..ad3f7b7e66a 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -423,6 +423,9 @@ SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
 
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 
@@ -435,6 +438,9 @@ CHECKPOINT;
 SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;
 SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
 
+SELECT pg_stat_force_next_flush();
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
-- 
2.34.1

From 42fd2860e3f06d525936187f46aad06892073078 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 10:00:00 +0000
Subject: [PATCH v12] per backend WAL statistics

Now that commit 9aea73fc61 added backend-level statistics to pgstats (and
per backend IO statistics) we can more easily add per backend statistics.

This commit adds per backend WAL statistics using the same layer as pg_stat_wal,
except that it is now possible to know how much WAL activity is happening in each
backend rather than an overall aggregate of all the activity.  A function called
pg_stat_get_backend_wal() is added to access this data depending on the
PID of a backend.

The same limitation as in 9aea73fc61 persists, meaning that Auxiliary processes
are not included in this set of statistics.

XXX: bump catalog version
XXX: bump of stats file format not required, as backend stats do not
persist on disk.
---
 src/include/catalog/pg_proc.dat             |  7 +++
 src/include/pgstat.h                        | 37 ++++++------
 src/include/utils/pgstat_internal.h         |  3 +-
 src/backend/utils/activity/pgstat_backend.c | 64 +++++++++++++++++++++
 src/backend/utils/activity/pgstat_wal.c     |  1 +
 src/backend/utils/adt/pgstatfuncs.c         | 26 ++++++++-
 src/test/regress/expected/stats.out         | 14 +++++
 src/test/regress/sql/stats.sql              |  6 ++
 doc/src/sgml/monitoring.sgml                | 19 ++++++
 9 files changed, 156 insertions(+), 21 deletions(-)

diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index cd9422d0bacf..3e35f8b8e99a 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5954,6 +5954,13 @@
   proargmodes => '{o,o,o,o,o}',
   proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
   prosrc => 'pg_stat_get_wal' },
+{ oid => '8037', descr => 'statistics: backend WAL activity',
+  proname => 'pg_stat_get_backend_wal', provolatile => 'v',
+  proparallel => 'r', prorettype => 'record', proargtypes => 'int4',
+  proallargtypes => '{int4,int8,int8,numeric,int8,timestamptz}',
+  proargmodes => '{i,o,o,o,o,o}',
+  proargnames => '{backend_pid,wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
+  prosrc => 'pg_stat_get_backend_wal' },
 { oid => '6248', descr => 'statistics: information about WAL prefetching',
   proname => 'pg_stat_get_recovery_prefetch', prorows => '1', proretset => 't',
   provolatile => 'v', prorettype => 'record', proargtypes => '',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 4aad10b0b6d5..06359b9157d2 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -340,24 +340,6 @@ typedef struct PgStat_IO
 	PgStat_BktypeIO stats[BACKEND_NUM_TYPES];
 } PgStat_IO;
 
-typedef struct PgStat_Backend
-{
-	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO io_stats;
-} PgStat_Backend;
-
-/* ---------
- * PgStat_BackendPending	Non-flushed backend stats.
- * ---------
- */
-typedef struct PgStat_BackendPending
-{
-	/*
-	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
-	 */
-	PgStat_PendingIO pending_io;
-} PgStat_BackendPending;
-
 typedef struct PgStat_StatDBEntry
 {
 	PgStat_Counter xact_commit;
@@ -500,6 +482,25 @@ typedef struct PgStat_WalStats
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
+typedef struct PgStat_Backend
+{
+	TimestampTz stat_reset_timestamp;
+	PgStat_BktypeIO io_stats;
+	PgStat_WalCounters wal_counters;
+} PgStat_Backend;
+
+/* ---------
+ * PgStat_BackendPending	Non-flushed backend stats.
+ * ---------
+ */
+typedef struct PgStat_BackendPending
+{
+	/*
+	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
+	 */
+	PgStat_PendingIO pending_io;
+} PgStat_BackendPending;
+
 /*
  * Functions in pgstat.c
  */
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 36d228e3558b..d5557e6e998c 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -622,7 +622,8 @@ extern void pgstat_archiver_snapshot_cb(void);
 
 /* flags for pgstat_flush_backend() */
 #define PGSTAT_BACKEND_FLUSH_IO		(1 << 0)	/* Flush I/O statistics */
-#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO)
+#define PGSTAT_BACKEND_FLUSH_WAL   (1 << 1) /* Flush WAL statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL   (PGSTAT_BACKEND_FLUSH_IO | PGSTAT_BACKEND_FLUSH_WAL)
 
 extern bool pgstat_flush_backend(bool nowait, bits32 flags);
 extern bool pgstat_backend_flush_cb(bool nowait);
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 3c9ebbcd69c0..641ba27c95b4 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -38,6 +38,14 @@
  */
 static PgStat_BackendPending PendingBackendStats;
 
+/*
+ * WAL usage counters saved from pgWalUsage at the previous call to
+ * pgstat_report_wal(). This is used to calculate how much WAL usage
+ * happens between pgstat_report_wal() calls, by subtracting
+ * the previous counters from the current ones.
+ */
+static WalUsage prevBackendWalUsage;
+
 /*
  * Utility routines to report I/O stats for backends, kept here to avoid
  * exposing PendingBackendStats to the outside world.
@@ -185,6 +193,57 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 	MemSet(&PendingBackendStats.pending_io, 0, sizeof(PgStat_PendingIO));
 }
 
+/*
+ * To determine whether WAL usage happened.
+ */
+static bool
+pgstat_backend_wal_have_pending(void)
+{
+	return pgWalUsage.wal_records != prevBackendWalUsage.wal_records;
+}
+
+/*
+ * Flush out locally pending backend WAL statistics.  Locking is managed
+ * by the caller.
+ */
+static void
+pgstat_flush_backend_entry_wal(PgStat_EntryRef *entry_ref)
+{
+	PgStatShared_Backend *shbackendent;
+	PgStat_WalCounters *bktype_shstats;
+	WalUsage	wal_usage_diff = {0};
+
+	/*
+	 * This function can be called even if nothing at all has happened. Avoid
+	 * taking lock for nothing in that case.
+	 */
+	if (!pgstat_backend_wal_have_pending())
+		return;
+
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	bktype_shstats = &shbackendent->stats.wal_counters;
+
+	/*
+	 * We don't update the WAL usage portion of the local WalStats elsewhere.
+	 * Calculate how much WAL usage counters were increased by subtracting the
+	 * previous counters from the current ones.
+	 */
+	WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
+
+#define WALSTAT_ACC(fld, var_to_add) \
+	(bktype_shstats->fld += var_to_add.fld)
+	WALSTAT_ACC(wal_buffers_full, wal_usage_diff);
+	WALSTAT_ACC(wal_records, wal_usage_diff);
+	WALSTAT_ACC(wal_fpi, wal_usage_diff);
+	WALSTAT_ACC(wal_bytes, wal_usage_diff);
+#undef WALSTAT_ACC
+
+	/*
+	 * Save the current counters for the subsequent calculation of WAL usage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
+}
+
 /*
  * Flush out locally pending backend statistics
  *
@@ -212,6 +271,9 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 	if (flags & PGSTAT_BACKEND_FLUSH_IO)
 		pgstat_flush_backend_entry_io(entry_ref);
 
+	if (flags & PGSTAT_BACKEND_FLUSH_WAL)
+		pgstat_flush_backend_entry_wal(entry_ref);
+
 	pgstat_unlock_entry(entry_ref);
 
 	return false;
@@ -259,6 +321,8 @@ pgstat_create_backend(ProcNumber procnum)
 	pgstat_unlock_entry(entry_ref);
 
 	MemSet(&PendingBackendStats, 0, sizeof(PgStat_BackendPending));
+
+	prevBackendWalUsage = pgWalUsage;
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 943be0cbeefd..c1c2e6dc3868 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -52,6 +52,7 @@ pgstat_report_wal(bool force)
 
 	/* flush wal stats */
 	(void) pgstat_wal_flush_cb(nowait);
+	pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_WAL);
 
 	/* flush IO stats */
 	pgstat_flush_io(nowait);
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 68830db8633d..85061e29bd1b 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1606,8 +1606,8 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 /*
  * pg_stat_wal_build_tuple
  *
- * Helper routine for pg_stat_get_wal() returning one tuple based on the
- * contents of wal_counters.
+ * Helper routine for pg_stat_get_wal() and pg_stat_get_backend_wal()
+ * returning one tuple based on the contents of wal_counters.
  */
 static Datum
 pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
@@ -1656,6 +1656,28 @@ pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns WAL statistics for a backend with given PID.
+ */
+Datum
+pg_stat_get_backend_wal(PG_FUNCTION_ARGS)
+{
+	int			pid;
+	PgStat_Backend *backend_stats;
+	PgStat_WalCounters bktype_stats;
+
+	pid = PG_GETARG_INT32(0);
+	backend_stats = pgstat_fetch_stat_backend_by_pid(pid, NULL);
+
+	if (!backend_stats)
+		PG_RETURN_NULL();
+
+	bktype_stats = backend_stats->wal_counters;
+
+	/* save tuples with data from this PgStat_WalCounters */
+	return (pg_stat_wal_build_tuple(bktype_stats, backend_stats->stat_reset_timestamp));
+}
+
 /*
  * Returns statistics of WAL activity
  */
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index 093e6368dbbe..b3c303c98cb5 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -832,6 +832,8 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -851,6 +853,18 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
  t
 (1 row)
 
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+ ?column? 
+----------
+ t
+(1 row)
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 0a44e14d9f4a..ad3f7b7e66ae 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -423,6 +423,9 @@ SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
 
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 
@@ -435,6 +438,9 @@ CHECKPOINT;
 SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;
 SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
 
+SELECT pg_stat_force_next_flush();
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 9178f1d34efd..f4c37c811ba0 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -4860,6 +4860,25 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para></entry>
       </row>
 
+      <row>
+       <entry id="pg-stat-get-backend-wal" role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_stat_get_backend_wal</primary>
+        </indexterm>
+        <function>pg_stat_get_backend_wal</function> ( <type>integer</type> )
+        <returnvalue>record</returnvalue>
+       </para>
+       <para>
+        Returns WAL statistics about the backend with the specified
+        process ID. The output fields are exactly the same as the ones in the
+        <structname>pg_stat_wal</structname> view.
+       </para>
+       <para>
+        The function does not return WAL statistics for the checkpointer,
+        the background writer, the startup process and the autovacuum launcher.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
-- 
2.47.2

From 38ca4b60869a6c96c22e6bdfd33cac07827cef88 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 10:00:00 +0000
Subject: [PATCH v12 1/2] per backend WAL statistics

Now that commit 9aea73fc61 added backend-level statistics to pgstats (and
per backend IO statistics) we can more easily add per backend statistics.

This commit adds per backend WAL statistics using the same layer as pg_stat_wal,
except that it is now possible to know how much WAL activity is happening in each
backend rather than an overall aggregate of all the activity.  A function called
pg_stat_get_backend_wal() is added to access this data depending on the
PID of a backend.

The same limitation as in 9aea73fc61 persists, meaning that Auxiliary processes
are not included in this set of statistics.

XXX: bump catalog version
XXX: bump of stats file format not required, as backend stats do not
persist on disk.
---
 doc/src/sgml/monitoring.sgml                | 19 ++++++
 src/backend/utils/activity/pgstat_backend.c | 64 +++++++++++++++++++++
 src/backend/utils/activity/pgstat_wal.c     |  1 +
 src/backend/utils/adt/pgstatfuncs.c         | 26 ++++++++-
 src/include/catalog/pg_proc.dat             |  7 +++
 src/include/pgstat.h                        | 37 ++++++------
 src/include/utils/pgstat_internal.h         |  3 +-
 src/test/regress/expected/stats.out         | 14 +++++
 src/test/regress/sql/stats.sql              |  6 ++
 9 files changed, 156 insertions(+), 21 deletions(-)
  15.9% doc/src/sgml/
  39.3% src/backend/utils/activity/
  15.6% src/backend/utils/adt/
   8.8% src/include/catalog/
   4.5% src/include/utils/
   8.4% src/test/regress/expected/
   6.4% src/test/regress/sql/

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 9178f1d34ef..f4c37c811ba 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -4860,6 +4860,25 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para></entry>
       </row>
 
+      <row>
+       <entry id="pg-stat-get-backend-wal" role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_stat_get_backend_wal</primary>
+        </indexterm>
+        <function>pg_stat_get_backend_wal</function> ( <type>integer</type> )
+        <returnvalue>record</returnvalue>
+       </para>
+       <para>
+        Returns WAL statistics about the backend with the specified
+        process ID. The output fields are exactly the same as the ones in the
+        <structname>pg_stat_wal</structname> view.
+       </para>
+       <para>
+        The function does not return WAL statistics for the checkpointer,
+        the background writer, the startup process and the autovacuum launcher.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 3c9ebbcd69c..641ba27c95b 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -38,6 +38,14 @@
  */
 static PgStat_BackendPending PendingBackendStats;
 
+/*
+ * WAL usage counters saved from pgWalUsage at the previous call to
+ * pgstat_report_wal(). This is used to calculate how much WAL usage
+ * happens between pgstat_report_wal() calls, by subtracting
+ * the previous counters from the current ones.
+ */
+static WalUsage prevBackendWalUsage;
+
 /*
  * Utility routines to report I/O stats for backends, kept here to avoid
  * exposing PendingBackendStats to the outside world.
@@ -185,6 +193,57 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 	MemSet(&PendingBackendStats.pending_io, 0, sizeof(PgStat_PendingIO));
 }
 
+/*
+ * To determine whether WAL usage happened.
+ */
+static bool
+pgstat_backend_wal_have_pending(void)
+{
+	return pgWalUsage.wal_records != prevBackendWalUsage.wal_records;
+}
+
+/*
+ * Flush out locally pending backend WAL statistics.  Locking is managed
+ * by the caller.
+ */
+static void
+pgstat_flush_backend_entry_wal(PgStat_EntryRef *entry_ref)
+{
+	PgStatShared_Backend *shbackendent;
+	PgStat_WalCounters *bktype_shstats;
+	WalUsage	wal_usage_diff = {0};
+
+	/*
+	 * This function can be called even if nothing at all has happened. Avoid
+	 * taking lock for nothing in that case.
+	 */
+	if (!pgstat_backend_wal_have_pending())
+		return;
+
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	bktype_shstats = &shbackendent->stats.wal_counters;
+
+	/*
+	 * We don't update the WAL usage portion of the local WalStats elsewhere.
+	 * Calculate how much WAL usage counters were increased by subtracting the
+	 * previous counters from the current ones.
+	 */
+	WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
+
+#define WALSTAT_ACC(fld, var_to_add) \
+	(bktype_shstats->fld += var_to_add.fld)
+	WALSTAT_ACC(wal_buffers_full, wal_usage_diff);
+	WALSTAT_ACC(wal_records, wal_usage_diff);
+	WALSTAT_ACC(wal_fpi, wal_usage_diff);
+	WALSTAT_ACC(wal_bytes, wal_usage_diff);
+#undef WALSTAT_ACC
+
+	/*
+	 * Save the current counters for the subsequent calculation of WAL usage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
+}
+
 /*
  * Flush out locally pending backend statistics
  *
@@ -212,6 +271,9 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 	if (flags & PGSTAT_BACKEND_FLUSH_IO)
 		pgstat_flush_backend_entry_io(entry_ref);
 
+	if (flags & PGSTAT_BACKEND_FLUSH_WAL)
+		pgstat_flush_backend_entry_wal(entry_ref);
+
 	pgstat_unlock_entry(entry_ref);
 
 	return false;
@@ -259,6 +321,8 @@ pgstat_create_backend(ProcNumber procnum)
 	pgstat_unlock_entry(entry_ref);
 
 	MemSet(&PendingBackendStats, 0, sizeof(PgStat_BackendPending));
+
+	prevBackendWalUsage = pgWalUsage;
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 943be0cbeef..c1c2e6dc386 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -52,6 +52,7 @@ pgstat_report_wal(bool force)
 
 	/* flush wal stats */
 	(void) pgstat_wal_flush_cb(nowait);
+	pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_WAL);
 
 	/* flush IO stats */
 	pgstat_flush_io(nowait);
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 68830db8633..85061e29bd1 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1606,8 +1606,8 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 /*
  * pg_stat_wal_build_tuple
  *
- * Helper routine for pg_stat_get_wal() returning one tuple based on the
- * contents of wal_counters.
+ * Helper routine for pg_stat_get_wal() and pg_stat_get_backend_wal()
+ * returning one tuple based on the contents of wal_counters.
  */
 static Datum
 pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
@@ -1656,6 +1656,28 @@ pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns WAL statistics for a backend with given PID.
+ */
+Datum
+pg_stat_get_backend_wal(PG_FUNCTION_ARGS)
+{
+	int			pid;
+	PgStat_Backend *backend_stats;
+	PgStat_WalCounters bktype_stats;
+
+	pid = PG_GETARG_INT32(0);
+	backend_stats = pgstat_fetch_stat_backend_by_pid(pid, NULL);
+
+	if (!backend_stats)
+		PG_RETURN_NULL();
+
+	bktype_stats = backend_stats->wal_counters;
+
+	/* save tuples with data from this PgStat_WalCounters */
+	return (pg_stat_wal_build_tuple(bktype_stats, backend_stats->stat_reset_timestamp));
+}
+
 /*
  * Returns statistics of WAL activity
  */
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index cd9422d0bac..3e35f8b8e99 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5954,6 +5954,13 @@
   proargmodes => '{o,o,o,o,o}',
   proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
   prosrc => 'pg_stat_get_wal' },
+{ oid => '8037', descr => 'statistics: backend WAL activity',
+  proname => 'pg_stat_get_backend_wal', provolatile => 'v',
+  proparallel => 'r', prorettype => 'record', proargtypes => 'int4',
+  proallargtypes => '{int4,int8,int8,numeric,int8,timestamptz}',
+  proargmodes => '{i,o,o,o,o,o}',
+  proargnames => '{backend_pid,wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
+  prosrc => 'pg_stat_get_backend_wal' },
 { oid => '6248', descr => 'statistics: information about WAL prefetching',
   proname => 'pg_stat_get_recovery_prefetch', prorows => '1', proretset => 't',
   provolatile => 'v', prorettype => 'record', proargtypes => '',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 4aad10b0b6d..06359b9157d 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -340,24 +340,6 @@ typedef struct PgStat_IO
 	PgStat_BktypeIO stats[BACKEND_NUM_TYPES];
 } PgStat_IO;
 
-typedef struct PgStat_Backend
-{
-	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO io_stats;
-} PgStat_Backend;
-
-/* ---------
- * PgStat_BackendPending	Non-flushed backend stats.
- * ---------
- */
-typedef struct PgStat_BackendPending
-{
-	/*
-	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
-	 */
-	PgStat_PendingIO pending_io;
-} PgStat_BackendPending;
-
 typedef struct PgStat_StatDBEntry
 {
 	PgStat_Counter xact_commit;
@@ -500,6 +482,25 @@ typedef struct PgStat_WalStats
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
+typedef struct PgStat_Backend
+{
+	TimestampTz stat_reset_timestamp;
+	PgStat_BktypeIO io_stats;
+	PgStat_WalCounters wal_counters;
+} PgStat_Backend;
+
+/* ---------
+ * PgStat_BackendPending	Non-flushed backend stats.
+ * ---------
+ */
+typedef struct PgStat_BackendPending
+{
+	/*
+	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
+	 */
+	PgStat_PendingIO pending_io;
+} PgStat_BackendPending;
+
 /*
  * Functions in pgstat.c
  */
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 36d228e3558..d5557e6e998 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -622,7 +622,8 @@ extern void pgstat_archiver_snapshot_cb(void);
 
 /* flags for pgstat_flush_backend() */
 #define PGSTAT_BACKEND_FLUSH_IO		(1 << 0)	/* Flush I/O statistics */
-#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO)
+#define PGSTAT_BACKEND_FLUSH_WAL   (1 << 1) /* Flush WAL statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL   (PGSTAT_BACKEND_FLUSH_IO | PGSTAT_BACKEND_FLUSH_WAL)
 
 extern bool pgstat_flush_backend(bool nowait, bits32 flags);
 extern bool pgstat_backend_flush_cb(bool nowait);
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index 093e6368dbb..b3c303c98cb 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -832,6 +832,8 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -851,6 +853,18 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
  t
 (1 row)
 
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+ ?column? 
+----------
+ t
+(1 row)
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 0a44e14d9f4..ad3f7b7e66a 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -423,6 +423,9 @@ SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
 
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 
@@ -435,6 +438,9 @@ CHECKPOINT;
 SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;
 SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
 
+SELECT pg_stat_force_next_flush();
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
-- 
2.34.1

From 146d2ed10e43053174e0688d80481a2beca5379b Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Fri, 28 Feb 2025 06:17:59 +0000
Subject: [PATCH v12 2/2] Add backend type check in
 pgstat_fetch_stat_backend_by_pid() and pg_stat_reset_backend_stats()

This is relevant specially since a051e71e28a where pgstat_tracks_backend_bktype()
has been modified for B_WAL_RECEIVER, B_WAL_SUMMARIZER and B_WAL_WRITER.
---
 src/backend/utils/activity/pgstat_backend.c | 19 ++++++++-----------
 src/backend/utils/activity/pgstat_wal.c     |  1 +
 src/backend/utils/adt/pgstatfuncs.c         | 21 +++++++++++++++------
 3 files changed, 24 insertions(+), 17 deletions(-)
  46.3% src/backend/utils/activity/
  53.6% src/backend/utils/adt/

diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 641ba27c95b..13c4f3f29d6 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -111,18 +111,19 @@ pgstat_fetch_stat_backend_by_pid(int pid, BackendType *bktype)
 	if (bktype)
 		*bktype = B_INVALID;
 
-	/*
-	 * This could be an auxiliary process but these do not report backend
-	 * statistics due to pgstat_tracks_backend_bktype(), so there is no need
-	 * for an extra call to AuxiliaryPidGetProc().
-	 */
+	/* This could be an auxiliary process */
 	if (!proc)
-		return NULL;
+	{
+		proc = AuxiliaryPidGetProc(pid);
+		if (!proc)
+			return NULL;
+	}
 
 	procNumber = GetNumberFromPGProc(proc);
 
 	beentry = pgstat_get_beentry_by_proc_number(procNumber);
-	if (!beentry)
+	/* Check if the backend type tracks statistics */
+	if (!beentry || !pgstat_tracks_backend_bktype(beentry->st_backendType))
 		return NULL;
 
 	backend_stats = pgstat_fetch_stat_backend(procNumber);
@@ -133,10 +134,6 @@ pgstat_fetch_stat_backend_by_pid(int pid, BackendType *bktype)
 	if (beentry->st_procpid != pid)
 		return NULL;
 
-	/* backend may be gone, so recheck in case */
-	if (beentry->st_backendType == B_INVALID)
-		return NULL;
-
 	if (bktype)
 		*bktype = beentry->st_backendType;
 
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index c1c2e6dc386..16a1ecb4d90 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -56,6 +56,7 @@ pgstat_report_wal(bool force)
 
 	/* flush IO stats */
 	pgstat_flush_io(nowait);
+	(void) pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_IO);
 }
 
 /*
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 85061e29bd1..9ab0429e749 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1941,19 +1941,28 @@ Datum
 pg_stat_reset_backend_stats(PG_FUNCTION_ARGS)
 {
 	PGPROC	   *proc;
+	PgBackendStatus *beentry;
+	ProcNumber	procNumber;
 	int			backend_pid = PG_GETARG_INT32(0);
 
 	proc = BackendPidGetProc(backend_pid);
 
-	/*
-	 * This could be an auxiliary process but these do not report backend
-	 * statistics due to pgstat_tracks_backend_bktype(), so there is no need
-	 * for an extra call to AuxiliaryPidGetProc().
-	 */
+	/* This could be an auxiliary process */
 	if (!proc)
+	{
+		proc = AuxiliaryPidGetProc(backend_pid);
+		if (!proc)
+			PG_RETURN_VOID();
+	}
+
+	procNumber = GetNumberFromPGProc(proc);
+
+	beentry = pgstat_get_beentry_by_proc_number(procNumber);
+	/* Check if the backend type tracks statistics */
+	if (!beentry || !pgstat_tracks_backend_bktype(beentry->st_backendType))
 		PG_RETURN_VOID();
 
-	pgstat_reset(PGSTAT_KIND_BACKEND, InvalidOid, GetNumberFromPGProc(proc));
+	pgstat_reset(PGSTAT_KIND_BACKEND, InvalidOid, procNumber);
 
 	PG_RETURN_VOID();
 }
-- 
2.34.1

From 40100d26d7b6d69367de8670f5df75abbfcb0d57 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 10:00:00 +0000
Subject: [PATCH v13] per backend WAL statistics

Now that commit 9aea73fc61 added backend-level statistics to pgstats (and
per backend IO statistics) we can more easily add per backend statistics.

This commit adds per backend WAL statistics using the same layer as pg_stat_wal,
except that it is now possible to know how much WAL activity is happening in each
backend rather than an overall aggregate of all the activity.  A function called
pg_stat_get_backend_wal() is added to access this data depending on the
PID of a backend.

The same limitation as in 9aea73fc61 persists, meaning that Auxiliary processes
are not included in this set of statistics.

XXX: bump catalog version
XXX: bump of stats file format not required, as backend stats do not
persist on disk.
---
 doc/src/sgml/monitoring.sgml                | 19 ++++++
 src/backend/utils/activity/pgstat_backend.c | 67 ++++++++++++++++++++-
 src/backend/utils/activity/pgstat_wal.c     |  1 +
 src/backend/utils/adt/pgstatfuncs.c         | 26 +++++++-
 src/include/catalog/pg_proc.dat             |  7 +++
 src/include/pgstat.h                        | 37 ++++++------
 src/include/utils/pgstat_internal.h         |  3 +-
 src/test/regress/expected/stats.out         | 14 +++++
 src/test/regress/sql/stats.sql              |  6 ++
 9 files changed, 158 insertions(+), 22 deletions(-)
  15.5% doc/src/sgml/
  41.0% src/backend/utils/activity/
  15.2% src/backend/utils/adt/
   8.5% src/include/catalog/
   4.4% src/include/utils/
   8.2% src/test/regress/expected/
   6.2% src/test/regress/sql/

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 9178f1d34ef..f4c37c811ba 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -4860,6 +4860,25 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para></entry>
       </row>
 
+      <row>
+       <entry id="pg-stat-get-backend-wal" role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_stat_get_backend_wal</primary>
+        </indexterm>
+        <function>pg_stat_get_backend_wal</function> ( <type>integer</type> )
+        <returnvalue>record</returnvalue>
+       </para>
+       <para>
+        Returns WAL statistics about the backend with the specified
+        process ID. The output fields are exactly the same as the ones in the
+        <structname>pg_stat_wal</structname> view.
+       </para>
+       <para>
+        The function does not return WAL statistics for the checkpointer,
+        the background writer, the startup process and the autovacuum launcher.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index a9343b7b59e..abacdaed330 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -38,6 +38,14 @@
  */
 static PgStat_BackendPending PendingBackendStats;
 
+/*
+ * WAL usage counters saved from pgWalUsage at the previous call to
+ * pgstat_report_wal(). This is used to calculate how much WAL usage
+ * happens between pgstat_report_wal() calls, by subtracting
+ * the previous counters from the current ones.
+ */
+static WalUsage prevBackendWalUsage;
+
 /*
  * Utility routines to report I/O stats for backends, kept here to avoid
  * exposing PendingBackendStats to the outside world.
@@ -184,6 +192,57 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 	MemSet(&PendingBackendStats.pending_io, 0, sizeof(PgStat_PendingIO));
 }
 
+/*
+ * To determine whether WAL usage happened.
+ */
+static bool
+pgstat_backend_wal_have_pending(void)
+{
+	return pgWalUsage.wal_records != prevBackendWalUsage.wal_records;
+}
+
+/*
+ * Flush out locally pending backend WAL statistics.  Locking is managed
+ * by the caller.
+ */
+static void
+pgstat_flush_backend_entry_wal(PgStat_EntryRef *entry_ref)
+{
+	PgStatShared_Backend *shbackendent;
+	PgStat_WalCounters *bktype_shstats;
+	WalUsage	wal_usage_diff = {0};
+
+	/*
+	 * This function can be called even if nothing at all has happened. Avoid
+	 * taking lock for nothing in that case.
+	 */
+	if (!pgstat_backend_wal_have_pending())
+		return;
+
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	bktype_shstats = &shbackendent->stats.wal_counters;
+
+	/*
+	 * We don't update the WAL usage portion of the local WalStats elsewhere.
+	 * Calculate how much WAL usage counters were increased by subtracting the
+	 * previous counters from the current ones.
+	 */
+	WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
+
+#define WALSTAT_ACC(fld, var_to_add) \
+	(bktype_shstats->fld += var_to_add.fld)
+	WALSTAT_ACC(wal_buffers_full, wal_usage_diff);
+	WALSTAT_ACC(wal_records, wal_usage_diff);
+	WALSTAT_ACC(wal_fpi, wal_usage_diff);
+	WALSTAT_ACC(wal_bytes, wal_usage_diff);
+#undef WALSTAT_ACC
+
+	/*
+	 * Save the current counters for the subsequent calculation of WAL usage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
+}
+
 /*
  * Flush out locally pending backend statistics
  *
@@ -199,7 +258,8 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 		return false;
 
 	if (pg_memory_is_all_zeros(&PendingBackendStats,
-							   sizeof(struct PgStat_BackendPending)))
+							   sizeof(struct PgStat_BackendPending))
+		&& !pgstat_backend_wal_have_pending())
 		return false;
 
 	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_BACKEND, InvalidOid,
@@ -211,6 +271,9 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 	if (flags & PGSTAT_BACKEND_FLUSH_IO)
 		pgstat_flush_backend_entry_io(entry_ref);
 
+	if (flags & PGSTAT_BACKEND_FLUSH_WAL)
+		pgstat_flush_backend_entry_wal(entry_ref);
+
 	pgstat_unlock_entry(entry_ref);
 
 	return false;
@@ -258,6 +321,8 @@ pgstat_create_backend(ProcNumber procnum)
 	pgstat_unlock_entry(entry_ref);
 
 	MemSet(&PendingBackendStats, 0, sizeof(PgStat_BackendPending));
+
+	prevBackendWalUsage = pgWalUsage;
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 5d3da4b674e..16a1ecb4d90 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -52,6 +52,7 @@ pgstat_report_wal(bool force)
 
 	/* flush wal stats */
 	(void) pgstat_wal_flush_cb(nowait);
+	pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_WAL);
 
 	/* flush IO stats */
 	pgstat_flush_io(nowait);
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 0212d8d5906..f24dac483c7 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1606,8 +1606,8 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 /*
  * pg_stat_wal_build_tuple
  *
- * Helper routine for pg_stat_get_wal() returning one tuple based on the
- * contents of wal_counters.
+ * Helper routine for pg_stat_get_wal() and pg_stat_get_backend_wal()
+ * returning one tuple based on the contents of wal_counters.
  */
 static Datum
 pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
@@ -1656,6 +1656,28 @@ pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns WAL statistics for a backend with given PID.
+ */
+Datum
+pg_stat_get_backend_wal(PG_FUNCTION_ARGS)
+{
+	int			pid;
+	PgStat_Backend *backend_stats;
+	PgStat_WalCounters bktype_stats;
+
+	pid = PG_GETARG_INT32(0);
+	backend_stats = pgstat_fetch_stat_backend_by_pid(pid, NULL);
+
+	if (!backend_stats)
+		PG_RETURN_NULL();
+
+	bktype_stats = backend_stats->wal_counters;
+
+	/* save tuples with data from this PgStat_WalCounters */
+	return (pg_stat_wal_build_tuple(bktype_stats, backend_stats->stat_reset_timestamp));
+}
+
 /*
  * Returns statistics of WAL activity
  */
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index cd9422d0bac..3e35f8b8e99 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5954,6 +5954,13 @@
   proargmodes => '{o,o,o,o,o}',
   proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
   prosrc => 'pg_stat_get_wal' },
+{ oid => '8037', descr => 'statistics: backend WAL activity',
+  proname => 'pg_stat_get_backend_wal', provolatile => 'v',
+  proparallel => 'r', prorettype => 'record', proargtypes => 'int4',
+  proallargtypes => '{int4,int8,int8,numeric,int8,timestamptz}',
+  proargmodes => '{i,o,o,o,o,o}',
+  proargnames => '{backend_pid,wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
+  prosrc => 'pg_stat_get_backend_wal' },
 { oid => '6248', descr => 'statistics: information about WAL prefetching',
   proname => 'pg_stat_get_recovery_prefetch', prorows => '1', proretset => 't',
   provolatile => 'v', prorettype => 'record', proargtypes => '',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 4aad10b0b6d..06359b9157d 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -340,24 +340,6 @@ typedef struct PgStat_IO
 	PgStat_BktypeIO stats[BACKEND_NUM_TYPES];
 } PgStat_IO;
 
-typedef struct PgStat_Backend
-{
-	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO io_stats;
-} PgStat_Backend;
-
-/* ---------
- * PgStat_BackendPending	Non-flushed backend stats.
- * ---------
- */
-typedef struct PgStat_BackendPending
-{
-	/*
-	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
-	 */
-	PgStat_PendingIO pending_io;
-} PgStat_BackendPending;
-
 typedef struct PgStat_StatDBEntry
 {
 	PgStat_Counter xact_commit;
@@ -500,6 +482,25 @@ typedef struct PgStat_WalStats
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
+typedef struct PgStat_Backend
+{
+	TimestampTz stat_reset_timestamp;
+	PgStat_BktypeIO io_stats;
+	PgStat_WalCounters wal_counters;
+} PgStat_Backend;
+
+/* ---------
+ * PgStat_BackendPending	Non-flushed backend stats.
+ * ---------
+ */
+typedef struct PgStat_BackendPending
+{
+	/*
+	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
+	 */
+	PgStat_PendingIO pending_io;
+} PgStat_BackendPending;
+
 /*
  * Functions in pgstat.c
  */
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 36d228e3558..d5557e6e998 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -622,7 +622,8 @@ extern void pgstat_archiver_snapshot_cb(void);
 
 /* flags for pgstat_flush_backend() */
 #define PGSTAT_BACKEND_FLUSH_IO		(1 << 0)	/* Flush I/O statistics */
-#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO)
+#define PGSTAT_BACKEND_FLUSH_WAL   (1 << 1) /* Flush WAL statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL   (PGSTAT_BACKEND_FLUSH_IO | PGSTAT_BACKEND_FLUSH_WAL)
 
 extern bool pgstat_flush_backend(bool nowait, bits32 flags);
 extern bool pgstat_backend_flush_cb(bool nowait);
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index 093e6368dbb..b3c303c98cb 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -832,6 +832,8 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -851,6 +853,18 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
  t
 (1 row)
 
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+ ?column? 
+----------
+ t
+(1 row)
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 0a44e14d9f4..ad3f7b7e66a 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -423,6 +423,9 @@ SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
 
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 
@@ -435,6 +438,9 @@ CHECKPOINT;
 SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;
 SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
 
+SELECT pg_stat_force_next_flush();
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
-- 
2.34.1

From c1ca96900c8e719820f37408bfff83af027401b1 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Tue, 4 Mar 2025 09:15:08 +0900
Subject: [PATCH] Fix some gaps with pg_stat_wal and WAL-related processes

The WAL receiver and WAL summarizer processes gain each one a call to
pgstat_report_wal(), to make sure that they report their WAL statistics.

pg_stat_io is adjusted so as these processes do not report any rows when
the IOObject is not WAL, making the view easier to use.
---
 src/backend/postmaster/walsummarizer.c |  4 ++++
 src/backend/replication/walreceiver.c  | 10 ++++++++++
 src/backend/utils/activity/pgstat_io.c | 12 +++++++++++-
 src/bin/pg_walsummary/t/002_blocks.pl  |  7 +++++++
 src/test/recovery/t/001_stream_rep.pl  |  7 +++++++
 5 files changed, 39 insertions(+), 1 deletion(-)

diff --git a/src/backend/postmaster/walsummarizer.c b/src/backend/postmaster/walsummarizer.c
index f4d61c1f3bb8..17cb6984a01d 100644
--- a/src/backend/postmaster/walsummarizer.c
+++ b/src/backend/postmaster/walsummarizer.c
@@ -33,6 +33,7 @@
 #include "common/blkreftable.h"
 #include "libpq/pqsignal.h"
 #include "miscadmin.h"
+#include "pgstat.h"
 #include "postmaster/auxprocess.h"
 #include "postmaster/interrupt.h"
 #include "postmaster/walsummarizer.h"
@@ -1543,6 +1544,9 @@ summarizer_read_local_xlog_page(XLogReaderState *state,
 				HandleWalSummarizerInterrupts();
 				summarizer_wait_for_wal();
 
+				/* report pending statistics to the cumulative stats system */
+				pgstat_report_wal(false);
+
 				/* Recheck end-of-WAL. */
 				latest_lsn = GetLatestLSN(&latest_tli);
 				if (private_data->tli == latest_tli)
diff --git a/src/backend/replication/walreceiver.c b/src/backend/replication/walreceiver.c
index 82f7302ff9fd..6cb22ec2bfd7 100644
--- a/src/backend/replication/walreceiver.c
+++ b/src/backend/replication/walreceiver.c
@@ -583,6 +583,16 @@ WalReceiverMain(const void *startup_data, size_t startup_data_len)
 					 */
 					bool		requestReply = false;
 
+					/*
+					 * Report pending statistics to the cumulative stats
+					 * system. This location is useful for the report as it is
+					 * not within a tight loop in the WAL receiver, which
+					 * would bloat requests to pgstats, while also making sure
+					 * that the reports happen at least each time a status
+					 * update is sent.
+					 */
+					pgstat_report_wal(false);
+
 					/*
 					 * Check if time since last receive from primary has
 					 * reached the configured limit.
diff --git a/src/backend/utils/activity/pgstat_io.c b/src/backend/utils/activity/pgstat_io.c
index ba11545a17f3..eb5750255961 100644
--- a/src/backend/utils/activity/pgstat_io.c
+++ b/src/backend/utils/activity/pgstat_io.c
@@ -435,12 +435,22 @@ pgstat_tracks_io_object(BackendType bktype, IOObject io_object,
 	 */
 	no_temp_rel = bktype == B_AUTOVAC_LAUNCHER || bktype == B_BG_WRITER ||
 		bktype == B_CHECKPOINTER || bktype == B_AUTOVAC_WORKER ||
-		bktype == B_STANDALONE_BACKEND || bktype == B_STARTUP;
+		bktype == B_STANDALONE_BACKEND || bktype == B_STARTUP ||
+		bktype == B_WAL_SUMMARIZER || bktype == B_WAL_WRITER ||
+		bktype == B_WAL_RECEIVER;
 
 	if (no_temp_rel && io_context == IOCONTEXT_NORMAL &&
 		io_object == IOOBJECT_TEMP_RELATION)
 		return false;
 
+	/*
+	 * Some BackendTypes only perform IO under IOOBJECT_WAL, hence exclude all
+	 * rows for all the other objects for these.
+	 */
+	if ((bktype == B_WAL_SUMMARIZER || bktype == B_WAL_RECEIVER ||
+		 bktype == B_WAL_WRITER) && io_object != IOOBJECT_WAL)
+		return false;
+
 	/*
 	 * Some BackendTypes do not currently perform any IO in certain
 	 * IOContexts, and, while it may not be inherently incorrect for them to
diff --git a/src/bin/pg_walsummary/t/002_blocks.pl b/src/bin/pg_walsummary/t/002_blocks.pl
index 27f29a3b0c68..270332780a45 100644
--- a/src/bin/pg_walsummary/t/002_blocks.pl
+++ b/src/bin/pg_walsummary/t/002_blocks.pl
@@ -46,6 +46,13 @@ SELECT EXISTS (
 EOM
 ok($result, "WAL summarization caught up after insert");
 
+# The WAL summarizer should have generated some IO statistics.
+my $stats_reads = $node1->safe_psql(
+	'postgres',
+	qq{SELECT sum(reads) > 0 FROM pg_stat_io
+   WHERE backend_type = 'walsummarizer' AND object = 'wal'});
+is($stats_reads, 't', "WAL summarizer generates statistics for WAL reads");
+
 # Find the highest LSN that is summarized on disk.
 my $summarized_lsn = $node1->safe_psql('postgres', <<EOM);
 SELECT MAX(end_lsn) AS summarized_lsn FROM pg_available_wal_summaries()
diff --git a/src/test/recovery/t/001_stream_rep.pl b/src/test/recovery/t/001_stream_rep.pl
index ee57d234c861..3945f00ab884 100644
--- a/src/test/recovery/t/001_stream_rep.pl
+++ b/src/test/recovery/t/001_stream_rep.pl
@@ -506,6 +506,13 @@ $node_standby_2->append_conf('postgresql.conf', "primary_slot_name = ''");
 $node_standby_2->enable_streaming($node_primary);
 $node_standby_2->reload;
 
+# The WAL receiver should have generated some IO statistics.
+my $stats_reads = $node_standby_1->safe_psql(
+	'postgres',
+	qq{SELECT sum(writes) > 0 FROM pg_stat_io
+   WHERE backend_type = 'walreceiver' AND object = 'wal'});
+is($stats_reads, 't', "WAL receiver generates statistics for WAL writes");
+
 # be sure do not streaming from cascade
 $node_standby_1->stop;
 
-- 
2.47.2

From c9500ba1ed93f97aee2f9f4a97bcbfaaff998cad Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 10:00:00 +0000
Subject: [PATCH v14] per backend WAL statistics

Now that commit 9aea73fc61 added backend-level statistics to pgstats (and
per backend IO statistics) we can more easily add per backend statistics.

This commit adds per backend WAL statistics using the same layer as pg_stat_wal,
except that it is now possible to know how much WAL activity is happening in each
backend rather than an overall aggregate of all the activity.  A function called
pg_stat_get_backend_wal() is added to access this data depending on the
PID of a backend.

The same limitation as in 9aea73fc61 persists, meaning that Auxiliary processes
are not included in this set of statistics.

XXX: bump catalog version
XXX: bump of stats file format not required, as backend stats do not
persist on disk.
---
 doc/src/sgml/monitoring.sgml                | 19 ++++++
 src/backend/utils/activity/pgstat_backend.c | 70 ++++++++++++++++++++-
 src/backend/utils/activity/pgstat_wal.c     |  1 +
 src/backend/utils/adt/pgstatfuncs.c         | 26 +++++++-
 src/include/catalog/pg_proc.dat             |  7 +++
 src/include/pgstat.h                        | 37 +++++------
 src/include/utils/pgstat_internal.h         |  3 +-
 src/test/regress/expected/stats.out         | 14 +++++
 src/test/regress/sql/stats.sql              |  6 ++
 9 files changed, 160 insertions(+), 23 deletions(-)
  15.1% doc/src/sgml/
  42.6% src/backend/utils/activity/
  14.7% src/backend/utils/adt/
   8.3% src/include/catalog/
   4.2% src/include/utils/
   8.0% src/test/regress/expected/
   6.1% src/test/regress/sql/

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 16646f560e8..b1710680705 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -4866,6 +4866,25 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para></entry>
       </row>
 
+      <row>
+       <entry id="pg-stat-get-backend-wal" role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_stat_get_backend_wal</primary>
+        </indexterm>
+        <function>pg_stat_get_backend_wal</function> ( <type>integer</type> )
+        <returnvalue>record</returnvalue>
+       </para>
+       <para>
+        Returns WAL statistics about the backend with the specified
+        process ID. The output fields are exactly the same as the ones in the
+        <structname>pg_stat_wal</structname> view.
+       </para>
+       <para>
+        The function does not return WAL statistics for the checkpointer,
+        the background writer, the startup process and the autovacuum launcher.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index a9343b7b59e..01e072a9bfd 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -38,6 +38,14 @@
  */
 static PgStat_BackendPending PendingBackendStats;
 
+/*
+ * WAL usage counters saved from pgWalUsage at the previous call to
+ * pgstat_report_wal(). This is used to calculate how much WAL usage
+ * happens between pgstat_report_wal() calls, by subtracting
+ * the previous counters from the current ones.
+ */
+static WalUsage prevBackendWalUsage;
+
 /*
  * Utility routines to report I/O stats for backends, kept here to avoid
  * exposing PendingBackendStats to the outside world.
@@ -184,6 +192,57 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 	MemSet(&PendingBackendStats.pending_io, 0, sizeof(PgStat_PendingIO));
 }
 
+/*
+ * To determine whether WAL usage happened.
+ */
+static bool
+pgstat_backend_wal_have_pending(void)
+{
+	return pgWalUsage.wal_records != prevBackendWalUsage.wal_records;
+}
+
+/*
+ * Flush out locally pending backend WAL statistics.  Locking is managed
+ * by the caller.
+ */
+static void
+pgstat_flush_backend_entry_wal(PgStat_EntryRef *entry_ref)
+{
+	PgStatShared_Backend *shbackendent;
+	PgStat_WalCounters *bktype_shstats;
+	WalUsage	wal_usage_diff = {0};
+
+	/*
+	 * This function can be called even if nothing at all has happened. Avoid
+	 * taking lock for nothing in that case.
+	 */
+	if (!pgstat_backend_wal_have_pending())
+		return;
+
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	bktype_shstats = &shbackendent->stats.wal_counters;
+
+	/*
+	 * We don't update the WAL usage portion of the local WalStats elsewhere.
+	 * Calculate how much WAL usage counters were increased by subtracting the
+	 * previous counters from the current ones.
+	 */
+	WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
+
+#define WALSTAT_ACC(fld, var_to_add) \
+	(bktype_shstats->fld += var_to_add.fld)
+	WALSTAT_ACC(wal_buffers_full, wal_usage_diff);
+	WALSTAT_ACC(wal_records, wal_usage_diff);
+	WALSTAT_ACC(wal_fpi, wal_usage_diff);
+	WALSTAT_ACC(wal_bytes, wal_usage_diff);
+#undef WALSTAT_ACC
+
+	/*
+	 * Save the current counters for the subsequent calculation of WAL usage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
+}
+
 /*
  * Flush out locally pending backend statistics
  *
@@ -199,7 +258,8 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 		return false;
 
 	if (pg_memory_is_all_zeros(&PendingBackendStats,
-							   sizeof(struct PgStat_BackendPending)))
+							   sizeof(struct PgStat_BackendPending))
+		&& !pgstat_backend_wal_have_pending())
 		return false;
 
 	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_BACKEND, InvalidOid,
@@ -211,6 +271,9 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 	if (flags & PGSTAT_BACKEND_FLUSH_IO)
 		pgstat_flush_backend_entry_io(entry_ref);
 
+	if (flags & PGSTAT_BACKEND_FLUSH_WAL)
+		pgstat_flush_backend_entry_wal(entry_ref);
+
 	pgstat_unlock_entry(entry_ref);
 
 	return false;
@@ -223,7 +286,8 @@ bool
 pgstat_backend_have_pending_cb(void)
 {
 	return (!pg_memory_is_all_zeros(&PendingBackendStats,
-									sizeof(struct PgStat_BackendPending)));
+									sizeof(struct PgStat_BackendPending))
+			|| pgstat_backend_wal_have_pending());
 }
 
 /*
@@ -258,6 +322,8 @@ pgstat_create_backend(ProcNumber procnum)
 	pgstat_unlock_entry(entry_ref);
 
 	MemSet(&PendingBackendStats, 0, sizeof(PgStat_BackendPending));
+
+	prevBackendWalUsage = pgWalUsage;
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 5d3da4b674e..16a1ecb4d90 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -52,6 +52,7 @@ pgstat_report_wal(bool force)
 
 	/* flush wal stats */
 	(void) pgstat_wal_flush_cb(nowait);
+	pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_WAL);
 
 	/* flush IO stats */
 	pgstat_flush_io(nowait);
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 9172e1cb9d2..662ce46cbc2 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1609,8 +1609,8 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 /*
  * pg_stat_wal_build_tuple
  *
- * Helper routine for pg_stat_get_wal() returning one tuple based on the
- * contents of wal_counters.
+ * Helper routine for pg_stat_get_wal() and pg_stat_get_backend_wal()
+ * returning one tuple based on the contents of wal_counters.
  */
 static Datum
 pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
@@ -1659,6 +1659,28 @@ pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns WAL statistics for a backend with given PID.
+ */
+Datum
+pg_stat_get_backend_wal(PG_FUNCTION_ARGS)
+{
+	int			pid;
+	PgStat_Backend *backend_stats;
+	PgStat_WalCounters bktype_stats;
+
+	pid = PG_GETARG_INT32(0);
+	backend_stats = pgstat_fetch_stat_backend_by_pid(pid, NULL);
+
+	if (!backend_stats)
+		PG_RETURN_NULL();
+
+	bktype_stats = backend_stats->wal_counters;
+
+	/* save tuples with data from this PgStat_WalCounters */
+	return (pg_stat_wal_build_tuple(bktype_stats, backend_stats->stat_reset_timestamp));
+}
+
 /*
  * Returns statistics of WAL activity
  */
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index cd9422d0bac..3e35f8b8e99 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5954,6 +5954,13 @@
   proargmodes => '{o,o,o,o,o}',
   proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
   prosrc => 'pg_stat_get_wal' },
+{ oid => '8037', descr => 'statistics: backend WAL activity',
+  proname => 'pg_stat_get_backend_wal', provolatile => 'v',
+  proparallel => 'r', prorettype => 'record', proargtypes => 'int4',
+  proallargtypes => '{int4,int8,int8,numeric,int8,timestamptz}',
+  proargmodes => '{i,o,o,o,o,o}',
+  proargnames => '{backend_pid,wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
+  prosrc => 'pg_stat_get_backend_wal' },
 { oid => '6248', descr => 'statistics: information about WAL prefetching',
   proname => 'pg_stat_get_recovery_prefetch', prorows => '1', proretset => 't',
   provolatile => 'v', prorettype => 'record', proargtypes => '',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 4aad10b0b6d..06359b9157d 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -340,24 +340,6 @@ typedef struct PgStat_IO
 	PgStat_BktypeIO stats[BACKEND_NUM_TYPES];
 } PgStat_IO;
 
-typedef struct PgStat_Backend
-{
-	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO io_stats;
-} PgStat_Backend;
-
-/* ---------
- * PgStat_BackendPending	Non-flushed backend stats.
- * ---------
- */
-typedef struct PgStat_BackendPending
-{
-	/*
-	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
-	 */
-	PgStat_PendingIO pending_io;
-} PgStat_BackendPending;
-
 typedef struct PgStat_StatDBEntry
 {
 	PgStat_Counter xact_commit;
@@ -500,6 +482,25 @@ typedef struct PgStat_WalStats
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
+typedef struct PgStat_Backend
+{
+	TimestampTz stat_reset_timestamp;
+	PgStat_BktypeIO io_stats;
+	PgStat_WalCounters wal_counters;
+} PgStat_Backend;
+
+/* ---------
+ * PgStat_BackendPending	Non-flushed backend stats.
+ * ---------
+ */
+typedef struct PgStat_BackendPending
+{
+	/*
+	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
+	 */
+	PgStat_PendingIO pending_io;
+} PgStat_BackendPending;
+
 /*
  * Functions in pgstat.c
  */
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 36d228e3558..d5557e6e998 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -622,7 +622,8 @@ extern void pgstat_archiver_snapshot_cb(void);
 
 /* flags for pgstat_flush_backend() */
 #define PGSTAT_BACKEND_FLUSH_IO		(1 << 0)	/* Flush I/O statistics */
-#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO)
+#define PGSTAT_BACKEND_FLUSH_WAL   (1 << 1) /* Flush WAL statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL   (PGSTAT_BACKEND_FLUSH_IO | PGSTAT_BACKEND_FLUSH_WAL)
 
 extern bool pgstat_flush_backend(bool nowait, bits32 flags);
 extern bool pgstat_backend_flush_cb(bool nowait);
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index 093e6368dbb..b3c303c98cb 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -832,6 +832,8 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -851,6 +853,18 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
  t
 (1 row)
 
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+ ?column? 
+----------
+ t
+(1 row)
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 0a44e14d9f4..ad3f7b7e66a 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -423,6 +423,9 @@ SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
 
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 
@@ -435,6 +438,9 @@ CHECKPOINT;
 SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;
 SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
 
+SELECT pg_stat_force_next_flush();
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
-- 
2.34.1

From db6b4601108c4ec4a636d5085c30baad758b6f5e Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 10:00:00 +0000
Subject: [PATCH v15] per backend WAL statistics

Now that commit 9aea73fc61 added backend-level statistics to pgstats (and
per backend IO statistics) we can more easily add per backend statistics.

This commit adds per backend WAL statistics using the same layer as pg_stat_wal,
except that it is now possible to know how much WAL activity is happening in each
backend rather than an overall aggregate of all the activity.  A function called
pg_stat_get_backend_wal() is added to access this data depending on the
PID of a backend.

The same limitation as in 9aea73fc61 persists, meaning that Auxiliary processes
are not included in this set of statistics.

XXX: bump catalog version
XXX: bump of stats file format not required, as backend stats do not
persist on disk.
---
 doc/src/sgml/monitoring.sgml                | 19 ++++++
 src/backend/utils/activity/pgstat_backend.c | 71 ++++++++++++++++++++-
 src/backend/utils/activity/pgstat_wal.c     |  1 +
 src/backend/utils/adt/pgstatfuncs.c         | 26 +++++++-
 src/include/catalog/pg_proc.dat             |  7 ++
 src/include/pgstat.h                        | 37 +++++------
 src/include/utils/pgstat_internal.h         |  3 +-
 src/test/regress/expected/stats.out         | 14 ++++
 src/test/regress/sql/stats.sql              |  6 ++
 9 files changed, 161 insertions(+), 23 deletions(-)
  14.9% doc/src/sgml/
  43.2% src/backend/utils/activity/
  14.6% src/backend/utils/adt/
   8.2% src/include/catalog/
   4.2% src/include/utils/
   7.9% src/test/regress/expected/
   6.0% src/test/regress/sql/

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 16646f560e8..b1710680705 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -4866,6 +4866,25 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para></entry>
       </row>
 
+      <row>
+       <entry id="pg-stat-get-backend-wal" role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_stat_get_backend_wal</primary>
+        </indexterm>
+        <function>pg_stat_get_backend_wal</function> ( <type>integer</type> )
+        <returnvalue>record</returnvalue>
+       </para>
+       <para>
+        Returns WAL statistics about the backend with the specified
+        process ID. The output fields are exactly the same as the ones in the
+        <structname>pg_stat_wal</structname> view.
+       </para>
+       <para>
+        The function does not return WAL statistics for the checkpointer,
+        the background writer, the startup process and the autovacuum launcher.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index a9343b7b59e..7be35e667dd 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -38,6 +38,14 @@
  */
 static PgStat_BackendPending PendingBackendStats;
 
+/*
+ * WAL usage counters saved from pgWalUsage at the previous call to
+ * pgstat_report_wal(). This is used to calculate how much WAL usage
+ * happens between pgstat_report_wal() calls, by subtracting
+ * the previous counters from the current ones.
+ */
+static WalUsage prevBackendWalUsage;
+
 /*
  * Utility routines to report I/O stats for backends, kept here to avoid
  * exposing PendingBackendStats to the outside world.
@@ -184,6 +192,57 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 	MemSet(&PendingBackendStats.pending_io, 0, sizeof(PgStat_PendingIO));
 }
 
+/*
+ * To determine whether WAL usage happened.
+ */
+static bool
+pgstat_backend_wal_have_pending(void)
+{
+	return pgWalUsage.wal_records != prevBackendWalUsage.wal_records;
+}
+
+/*
+ * Flush out locally pending backend WAL statistics.  Locking is managed
+ * by the caller.
+ */
+static void
+pgstat_flush_backend_entry_wal(PgStat_EntryRef *entry_ref)
+{
+	PgStatShared_Backend *shbackendent;
+	PgStat_WalCounters *bktype_shstats;
+	WalUsage	wal_usage_diff = {0};
+
+	/*
+	 * This function can be called even if nothing at all has happened. Avoid
+	 * taking lock for nothing in that case.
+	 */
+	if (!pgstat_backend_wal_have_pending())
+		return;
+
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	bktype_shstats = &shbackendent->stats.wal_counters;
+
+	/*
+	 * We don't update the WAL usage portion of the local WalStats elsewhere.
+	 * Calculate how much WAL usage counters were increased by subtracting the
+	 * previous counters from the current ones.
+	 */
+	WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
+
+#define WALSTAT_ACC(fld, var_to_add) \
+	(bktype_shstats->fld += var_to_add.fld)
+	WALSTAT_ACC(wal_buffers_full, wal_usage_diff);
+	WALSTAT_ACC(wal_records, wal_usage_diff);
+	WALSTAT_ACC(wal_fpi, wal_usage_diff);
+	WALSTAT_ACC(wal_bytes, wal_usage_diff);
+#undef WALSTAT_ACC
+
+	/*
+	 * Save the current counters for the subsequent calculation of WAL usage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
+}
+
 /*
  * Flush out locally pending backend statistics
  *
@@ -199,7 +258,8 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 		return false;
 
 	if (pg_memory_is_all_zeros(&PendingBackendStats,
-							   sizeof(struct PgStat_BackendPending)))
+							   sizeof(struct PgStat_BackendPending))
+		&& !pgstat_backend_wal_have_pending())
 		return false;
 
 	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_BACKEND, InvalidOid,
@@ -211,6 +271,9 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 	if (flags & PGSTAT_BACKEND_FLUSH_IO)
 		pgstat_flush_backend_entry_io(entry_ref);
 
+	if (flags & PGSTAT_BACKEND_FLUSH_WAL)
+		pgstat_flush_backend_entry_wal(entry_ref);
+
 	pgstat_unlock_entry(entry_ref);
 
 	return false;
@@ -223,7 +286,9 @@ bool
 pgstat_backend_have_pending_cb(void)
 {
 	return (!pg_memory_is_all_zeros(&PendingBackendStats,
-									sizeof(struct PgStat_BackendPending)));
+									sizeof(struct PgStat_BackendPending))
+			|| (pgstat_tracks_backend_bktype(MyBackendType) &&
+				pgstat_backend_wal_have_pending()));
 }
 
 /*
@@ -258,6 +323,8 @@ pgstat_create_backend(ProcNumber procnum)
 	pgstat_unlock_entry(entry_ref);
 
 	MemSet(&PendingBackendStats, 0, sizeof(PgStat_BackendPending));
+
+	prevBackendWalUsage = pgWalUsage;
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 5d3da4b674e..16a1ecb4d90 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -52,6 +52,7 @@ pgstat_report_wal(bool force)
 
 	/* flush wal stats */
 	(void) pgstat_wal_flush_cb(nowait);
+	pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_WAL);
 
 	/* flush IO stats */
 	pgstat_flush_io(nowait);
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 9172e1cb9d2..662ce46cbc2 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1609,8 +1609,8 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 /*
  * pg_stat_wal_build_tuple
  *
- * Helper routine for pg_stat_get_wal() returning one tuple based on the
- * contents of wal_counters.
+ * Helper routine for pg_stat_get_wal() and pg_stat_get_backend_wal()
+ * returning one tuple based on the contents of wal_counters.
  */
 static Datum
 pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
@@ -1659,6 +1659,28 @@ pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns WAL statistics for a backend with given PID.
+ */
+Datum
+pg_stat_get_backend_wal(PG_FUNCTION_ARGS)
+{
+	int			pid;
+	PgStat_Backend *backend_stats;
+	PgStat_WalCounters bktype_stats;
+
+	pid = PG_GETARG_INT32(0);
+	backend_stats = pgstat_fetch_stat_backend_by_pid(pid, NULL);
+
+	if (!backend_stats)
+		PG_RETURN_NULL();
+
+	bktype_stats = backend_stats->wal_counters;
+
+	/* save tuples with data from this PgStat_WalCounters */
+	return (pg_stat_wal_build_tuple(bktype_stats, backend_stats->stat_reset_timestamp));
+}
+
 /*
  * Returns statistics of WAL activity
  */
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index cd9422d0bac..3e35f8b8e99 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5954,6 +5954,13 @@
   proargmodes => '{o,o,o,o,o}',
   proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
   prosrc => 'pg_stat_get_wal' },
+{ oid => '8037', descr => 'statistics: backend WAL activity',
+  proname => 'pg_stat_get_backend_wal', provolatile => 'v',
+  proparallel => 'r', prorettype => 'record', proargtypes => 'int4',
+  proallargtypes => '{int4,int8,int8,numeric,int8,timestamptz}',
+  proargmodes => '{i,o,o,o,o,o}',
+  proargnames => '{backend_pid,wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
+  prosrc => 'pg_stat_get_backend_wal' },
 { oid => '6248', descr => 'statistics: information about WAL prefetching',
   proname => 'pg_stat_get_recovery_prefetch', prorows => '1', proretset => 't',
   provolatile => 'v', prorettype => 'record', proargtypes => '',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 4aad10b0b6d..06359b9157d 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -340,24 +340,6 @@ typedef struct PgStat_IO
 	PgStat_BktypeIO stats[BACKEND_NUM_TYPES];
 } PgStat_IO;
 
-typedef struct PgStat_Backend
-{
-	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO io_stats;
-} PgStat_Backend;
-
-/* ---------
- * PgStat_BackendPending	Non-flushed backend stats.
- * ---------
- */
-typedef struct PgStat_BackendPending
-{
-	/*
-	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
-	 */
-	PgStat_PendingIO pending_io;
-} PgStat_BackendPending;
-
 typedef struct PgStat_StatDBEntry
 {
 	PgStat_Counter xact_commit;
@@ -500,6 +482,25 @@ typedef struct PgStat_WalStats
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
+typedef struct PgStat_Backend
+{
+	TimestampTz stat_reset_timestamp;
+	PgStat_BktypeIO io_stats;
+	PgStat_WalCounters wal_counters;
+} PgStat_Backend;
+
+/* ---------
+ * PgStat_BackendPending	Non-flushed backend stats.
+ * ---------
+ */
+typedef struct PgStat_BackendPending
+{
+	/*
+	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
+	 */
+	PgStat_PendingIO pending_io;
+} PgStat_BackendPending;
+
 /*
  * Functions in pgstat.c
  */
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 36d228e3558..d5557e6e998 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -622,7 +622,8 @@ extern void pgstat_archiver_snapshot_cb(void);
 
 /* flags for pgstat_flush_backend() */
 #define PGSTAT_BACKEND_FLUSH_IO		(1 << 0)	/* Flush I/O statistics */
-#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO)
+#define PGSTAT_BACKEND_FLUSH_WAL   (1 << 1) /* Flush WAL statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL   (PGSTAT_BACKEND_FLUSH_IO | PGSTAT_BACKEND_FLUSH_WAL)
 
 extern bool pgstat_flush_backend(bool nowait, bits32 flags);
 extern bool pgstat_backend_flush_cb(bool nowait);
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index 093e6368dbb..b3c303c98cb 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -832,6 +832,8 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -851,6 +853,18 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
  t
 (1 row)
 
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+ ?column? 
+----------
+ t
+(1 row)
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 0a44e14d9f4..ad3f7b7e66a 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -423,6 +423,9 @@ SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
 
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 
@@ -435,6 +438,9 @@ CHECKPOINT;
 SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;
 SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
 
+SELECT pg_stat_force_next_flush();
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
-- 
2.34.1

From 38c718f8cd2c079219e5341e72e99664f150a581 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Thu, 6 Mar 2025 09:49:49 +0000
Subject: [PATCH v16 1/2] Add an extra check in
 pgstat_backend_have_pending_cb()

There is no reason for pgstat_backend_have_pending_cb() to not check
for pgstat_tracks_backend_bktype(). It could wrongly reports true should
PendingBackendStats not be full of zeroes.
---
 src/backend/utils/activity/pgstat_backend.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)
 100.0% src/backend/utils/activity/

diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index a9343b7b59e..1d94d3176b2 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -222,8 +222,11 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 bool
 pgstat_backend_have_pending_cb(void)
 {
-	return (!pg_memory_is_all_zeros(&PendingBackendStats,
-									sizeof(struct PgStat_BackendPending)));
+	if (!pgstat_tracks_backend_bktype(MyBackendType))
+		return false;
+	else
+		return (!pg_memory_is_all_zeros(&PendingBackendStats,
+										sizeof(struct PgStat_BackendPending)));
 }
 
 /*
-- 
2.34.1

From b3edf7c2bd448dc370847ba119ab9007b5b92b8f Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 6 Jan 2025 10:00:00 +0000
Subject: [PATCH v16 2/2] per backend WAL statistics

Now that commit 9aea73fc61 added backend-level statistics to pgstats (and
per backend IO statistics) we can more easily add per backend statistics.

This commit adds per backend WAL statistics using the same layer as pg_stat_wal,
except that it is now possible to know how much WAL activity is happening in each
backend rather than an overall aggregate of all the activity.  A function called
pg_stat_get_backend_wal() is added to access this data depending on the
PID of a backend.

The same limitation as in 9aea73fc61 persists, meaning that Auxiliary processes
are not included in this set of statistics.

XXX: bump catalog version
XXX: bump of stats file format not required, as backend stats do not
persist on disk.
---
 doc/src/sgml/monitoring.sgml                | 19 ++++++
 src/backend/utils/activity/pgstat_backend.c | 70 ++++++++++++++++++++-
 src/backend/utils/activity/pgstat_wal.c     |  1 +
 src/backend/utils/adt/pgstatfuncs.c         | 26 +++++++-
 src/include/catalog/pg_proc.dat             |  7 +++
 src/include/pgstat.h                        | 37 +++++------
 src/include/utils/pgstat_internal.h         |  3 +-
 src/test/regress/expected/stats.out         | 14 +++++
 src/test/regress/sql/stats.sql              |  6 ++
 9 files changed, 160 insertions(+), 23 deletions(-)
  15.1% doc/src/sgml/
  42.6% src/backend/utils/activity/
  14.7% src/backend/utils/adt/
   8.3% src/include/catalog/
   4.2% src/include/utils/
   8.0% src/test/regress/expected/
   6.1% src/test/regress/sql/

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 16646f560e8..b1710680705 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -4866,6 +4866,25 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para></entry>
       </row>
 
+      <row>
+       <entry id="pg-stat-get-backend-wal" role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_stat_get_backend_wal</primary>
+        </indexterm>
+        <function>pg_stat_get_backend_wal</function> ( <type>integer</type> )
+        <returnvalue>record</returnvalue>
+       </para>
+       <para>
+        Returns WAL statistics about the backend with the specified
+        process ID. The output fields are exactly the same as the ones in the
+        <structname>pg_stat_wal</structname> view.
+       </para>
+       <para>
+        The function does not return WAL statistics for the checkpointer,
+        the background writer, the startup process and the autovacuum launcher.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 1d94d3176b2..b2c82f37301 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -38,6 +38,14 @@
  */
 static PgStat_BackendPending PendingBackendStats;
 
+/*
+ * WAL usage counters saved from pgWalUsage at the previous call to
+ * pgstat_report_wal(). This is used to calculate how much WAL usage
+ * happens between pgstat_report_wal() calls, by subtracting
+ * the previous counters from the current ones.
+ */
+static WalUsage prevBackendWalUsage;
+
 /*
  * Utility routines to report I/O stats for backends, kept here to avoid
  * exposing PendingBackendStats to the outside world.
@@ -184,6 +192,57 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 	MemSet(&PendingBackendStats.pending_io, 0, sizeof(PgStat_PendingIO));
 }
 
+/*
+ * To determine whether WAL usage happened.
+ */
+static bool
+pgstat_backend_wal_have_pending(void)
+{
+	return pgWalUsage.wal_records != prevBackendWalUsage.wal_records;
+}
+
+/*
+ * Flush out locally pending backend WAL statistics.  Locking is managed
+ * by the caller.
+ */
+static void
+pgstat_flush_backend_entry_wal(PgStat_EntryRef *entry_ref)
+{
+	PgStatShared_Backend *shbackendent;
+	PgStat_WalCounters *bktype_shstats;
+	WalUsage	wal_usage_diff = {0};
+
+	/*
+	 * This function can be called even if nothing at all has happened. Avoid
+	 * taking lock for nothing in that case.
+	 */
+	if (!pgstat_backend_wal_have_pending())
+		return;
+
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	bktype_shstats = &shbackendent->stats.wal_counters;
+
+	/*
+	 * We don't update the WAL usage portion of the local WalStats elsewhere.
+	 * Calculate how much WAL usage counters were increased by subtracting the
+	 * previous counters from the current ones.
+	 */
+	WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
+
+#define WALSTAT_ACC(fld, var_to_add) \
+	(bktype_shstats->fld += var_to_add.fld)
+	WALSTAT_ACC(wal_buffers_full, wal_usage_diff);
+	WALSTAT_ACC(wal_records, wal_usage_diff);
+	WALSTAT_ACC(wal_fpi, wal_usage_diff);
+	WALSTAT_ACC(wal_bytes, wal_usage_diff);
+#undef WALSTAT_ACC
+
+	/*
+	 * Save the current counters for the subsequent calculation of WAL usage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
+}
+
 /*
  * Flush out locally pending backend statistics
  *
@@ -199,7 +258,8 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 		return false;
 
 	if (pg_memory_is_all_zeros(&PendingBackendStats,
-							   sizeof(struct PgStat_BackendPending)))
+							   sizeof(struct PgStat_BackendPending))
+		&& !pgstat_backend_wal_have_pending())
 		return false;
 
 	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_BACKEND, InvalidOid,
@@ -211,6 +271,9 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 	if (flags & PGSTAT_BACKEND_FLUSH_IO)
 		pgstat_flush_backend_entry_io(entry_ref);
 
+	if (flags & PGSTAT_BACKEND_FLUSH_WAL)
+		pgstat_flush_backend_entry_wal(entry_ref);
+
 	pgstat_unlock_entry(entry_ref);
 
 	return false;
@@ -226,7 +289,8 @@ pgstat_backend_have_pending_cb(void)
 		return false;
 	else
 		return (!pg_memory_is_all_zeros(&PendingBackendStats,
-										sizeof(struct PgStat_BackendPending)));
+										sizeof(struct PgStat_BackendPending))
+				|| pgstat_backend_wal_have_pending());
 }
 
 /*
@@ -261,6 +325,8 @@ pgstat_create_backend(ProcNumber procnum)
 	pgstat_unlock_entry(entry_ref);
 
 	MemSet(&PendingBackendStats, 0, sizeof(PgStat_BackendPending));
+
+	prevBackendWalUsage = pgWalUsage;
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 5d3da4b674e..16a1ecb4d90 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -52,6 +52,7 @@ pgstat_report_wal(bool force)
 
 	/* flush wal stats */
 	(void) pgstat_wal_flush_cb(nowait);
+	pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_WAL);
 
 	/* flush IO stats */
 	pgstat_flush_io(nowait);
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 9172e1cb9d2..662ce46cbc2 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1609,8 +1609,8 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 /*
  * pg_stat_wal_build_tuple
  *
- * Helper routine for pg_stat_get_wal() returning one tuple based on the
- * contents of wal_counters.
+ * Helper routine for pg_stat_get_wal() and pg_stat_get_backend_wal()
+ * returning one tuple based on the contents of wal_counters.
  */
 static Datum
 pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
@@ -1659,6 +1659,28 @@ pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns WAL statistics for a backend with given PID.
+ */
+Datum
+pg_stat_get_backend_wal(PG_FUNCTION_ARGS)
+{
+	int			pid;
+	PgStat_Backend *backend_stats;
+	PgStat_WalCounters bktype_stats;
+
+	pid = PG_GETARG_INT32(0);
+	backend_stats = pgstat_fetch_stat_backend_by_pid(pid, NULL);
+
+	if (!backend_stats)
+		PG_RETURN_NULL();
+
+	bktype_stats = backend_stats->wal_counters;
+
+	/* save tuples with data from this PgStat_WalCounters */
+	return (pg_stat_wal_build_tuple(bktype_stats, backend_stats->stat_reset_timestamp));
+}
+
 /*
  * Returns statistics of WAL activity
  */
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 134b3dd8689..064d47f63a4 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5954,6 +5954,13 @@
   proargmodes => '{o,o,o,o,o}',
   proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
   prosrc => 'pg_stat_get_wal' },
+{ oid => '8037', descr => 'statistics: backend WAL activity',
+  proname => 'pg_stat_get_backend_wal', provolatile => 'v',
+  proparallel => 'r', prorettype => 'record', proargtypes => 'int4',
+  proallargtypes => '{int4,int8,int8,numeric,int8,timestamptz}',
+  proargmodes => '{i,o,o,o,o,o}',
+  proargnames => '{backend_pid,wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
+  prosrc => 'pg_stat_get_backend_wal' },
 { oid => '6248', descr => 'statistics: information about WAL prefetching',
   proname => 'pg_stat_get_recovery_prefetch', prorows => '1', proretset => 't',
   provolatile => 'v', prorettype => 'record', proargtypes => '',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 4aad10b0b6d..06359b9157d 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -340,24 +340,6 @@ typedef struct PgStat_IO
 	PgStat_BktypeIO stats[BACKEND_NUM_TYPES];
 } PgStat_IO;
 
-typedef struct PgStat_Backend
-{
-	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO io_stats;
-} PgStat_Backend;
-
-/* ---------
- * PgStat_BackendPending	Non-flushed backend stats.
- * ---------
- */
-typedef struct PgStat_BackendPending
-{
-	/*
-	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
-	 */
-	PgStat_PendingIO pending_io;
-} PgStat_BackendPending;
-
 typedef struct PgStat_StatDBEntry
 {
 	PgStat_Counter xact_commit;
@@ -500,6 +482,25 @@ typedef struct PgStat_WalStats
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
+typedef struct PgStat_Backend
+{
+	TimestampTz stat_reset_timestamp;
+	PgStat_BktypeIO io_stats;
+	PgStat_WalCounters wal_counters;
+} PgStat_Backend;
+
+/* ---------
+ * PgStat_BackendPending	Non-flushed backend stats.
+ * ---------
+ */
+typedef struct PgStat_BackendPending
+{
+	/*
+	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
+	 */
+	PgStat_PendingIO pending_io;
+} PgStat_BackendPending;
+
 /*
  * Functions in pgstat.c
  */
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 36d228e3558..d5557e6e998 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -622,7 +622,8 @@ extern void pgstat_archiver_snapshot_cb(void);
 
 /* flags for pgstat_flush_backend() */
 #define PGSTAT_BACKEND_FLUSH_IO		(1 << 0)	/* Flush I/O statistics */
-#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO)
+#define PGSTAT_BACKEND_FLUSH_WAL   (1 << 1) /* Flush WAL statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL   (PGSTAT_BACKEND_FLUSH_IO | PGSTAT_BACKEND_FLUSH_WAL)
 
 extern bool pgstat_flush_backend(bool nowait, bits32 flags);
 extern bool pgstat_backend_flush_cb(bool nowait);
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index 093e6368dbb..b3c303c98cb 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -832,6 +832,8 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -851,6 +853,18 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
  t
 (1 row)
 
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+ ?column? 
+----------
+ t
+(1 row)
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 0a44e14d9f4..ad3f7b7e66a 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -423,6 +423,9 @@ SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 -- Test pg_stat_wal (and make a temp table so our temp schema exists)
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
 
+-- Test pg_stat_get_backend_wal (and make a temp table so our temp schema exists)
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 
@@ -435,6 +438,9 @@ CHECKPOINT;
 SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;
 SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
 
+SELECT pg_stat_force_next_flush();
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
-- 
2.34.1

From 353e6c9ff508f52c476ffe4536f1b8033b3ac996 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 10 Mar 2025 16:45:26 +0900
Subject: [PATCH v17] per backend WAL statistics

Now that commit 9aea73fc61 added backend-level statistics to pgstats (and
per backend IO statistics) we can more easily add per backend statistics.

This commit adds per backend WAL statistics using the same layer as pg_stat_wal,
except that it is now possible to know how much WAL activity is happening in each
backend rather than an overall aggregate of all the activity.  A function called
pg_stat_get_backend_wal() is added to access this data depending on the
PID of a backend.

The same limitation as in 9aea73fc61 persists, meaning that Auxiliary processes
are not included in this set of statistics.

XXX: bump catalog version
XXX: bump of stats file format not required, as backend stats do not
persist on disk.
---
 src/include/catalog/pg_proc.dat             |  7 ++
 src/include/pgstat.h                        | 41 +++++-----
 src/include/utils/pgstat_internal.h         |  3 +-
 src/backend/utils/activity/pgstat_backend.c | 87 ++++++++++++++++++++-
 src/backend/utils/activity/pgstat_wal.c     |  1 +
 src/backend/utils/adt/pgstatfuncs.c         | 26 +++++-
 src/test/regress/expected/stats.out         | 17 +++-
 src/test/regress/sql/stats.sql              |  9 ++-
 doc/src/sgml/monitoring.sgml                | 19 +++++
 9 files changed, 184 insertions(+), 26 deletions(-)

diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index cede992b6e22..42e427f8fe87 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5973,6 +5973,13 @@
   proargmodes => '{o,o,o,o,o}',
   proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
   prosrc => 'pg_stat_get_wal' },
+{ oid => '8037', descr => 'statistics: backend WAL activity',
+  proname => 'pg_stat_get_backend_wal', provolatile => 'v', proparallel => 'r',
+  prorettype => 'record', proargtypes => 'int4',
+  proallargtypes => '{int4,int8,int8,numeric,int8,timestamptz}',
+  proargmodes => '{i,o,o,o,o,o}',
+  proargnames => '{backend_pid,wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
+  prosrc => 'pg_stat_get_backend_wal' },
 { oid => '6248', descr => 'statistics: information about WAL prefetching',
   proname => 'pg_stat_get_recovery_prefetch', prorows => '1', proretset => 't',
   provolatile => 'v', prorettype => 'record', proargtypes => '',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 4aad10b0b6d5..def6b370ac11 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -340,24 +340,6 @@ typedef struct PgStat_IO
 	PgStat_BktypeIO stats[BACKEND_NUM_TYPES];
 } PgStat_IO;
 
-typedef struct PgStat_Backend
-{
-	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO io_stats;
-} PgStat_Backend;
-
-/* ---------
- * PgStat_BackendPending	Non-flushed backend stats.
- * ---------
- */
-typedef struct PgStat_BackendPending
-{
-	/*
-	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
-	 */
-	PgStat_PendingIO pending_io;
-} PgStat_BackendPending;
-
 typedef struct PgStat_StatDBEntry
 {
 	PgStat_Counter xact_commit;
@@ -500,6 +482,29 @@ typedef struct PgStat_WalStats
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
+/* -------
+ * PgStat_Backend		Backend statistics
+ * -------
+ */
+typedef struct PgStat_Backend
+{
+	TimestampTz stat_reset_timestamp;
+	PgStat_BktypeIO io_stats;
+	PgStat_WalCounters wal_counters;
+} PgStat_Backend;
+
+/* ---------
+ * PgStat_BackendPending	Non-flushed backend stats.
+ * ---------
+ */
+typedef struct PgStat_BackendPending
+{
+	/*
+	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
+	 */
+	PgStat_PendingIO pending_io;
+} PgStat_BackendPending;
+
 /*
  * Functions in pgstat.c
  */
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 36d228e3558b..d5557e6e998c 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -622,7 +622,8 @@ extern void pgstat_archiver_snapshot_cb(void);
 
 /* flags for pgstat_flush_backend() */
 #define PGSTAT_BACKEND_FLUSH_IO		(1 << 0)	/* Flush I/O statistics */
-#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO)
+#define PGSTAT_BACKEND_FLUSH_WAL   (1 << 1) /* Flush WAL statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL   (PGSTAT_BACKEND_FLUSH_IO | PGSTAT_BACKEND_FLUSH_WAL)
 
 extern bool pgstat_flush_backend(bool nowait, bits32 flags);
 extern bool pgstat_backend_flush_cb(bool nowait);
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 6efbb650aa8d..a34a1b40060e 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -38,6 +38,14 @@
  */
 static PgStat_BackendPending PendingBackendStats = {0};
 
+/*
+ * WAL usage counters saved from pgWalUsage at the previous call to
+ * pgstat_report_wal().  This is used to calculate how much WAL usage
+ * happens between pgstat_report_wal() calls, by subtracting the previous
+ * counters from the current ones.
+ */
+static WalUsage prevBackendWalUsage;
+
 /*
  * Utility routines to report I/O stats for backends, kept here to avoid
  * exposing PendingBackendStats to the outside world.
@@ -184,6 +192,57 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 	MemSet(&PendingBackendStats.pending_io, 0, sizeof(PgStat_PendingIO));
 }
 
+/*
+ * To determine whether WAL usage happened.
+ */
+static inline bool
+pgstat_backend_wal_have_pending(void)
+{
+	return pgWalUsage.wal_records != prevBackendWalUsage.wal_records;
+}
+
+/*
+ * Flush out locally pending backend WAL statistics.  Locking is managed
+ * by the caller.
+ */
+static void
+pgstat_flush_backend_entry_wal(PgStat_EntryRef *entry_ref)
+{
+	PgStatShared_Backend *shbackendent;
+	PgStat_WalCounters *bktype_shstats;
+	WalUsage	wal_usage_diff = {0};
+
+	/*
+	 * This function can be called even if nothing at all has happened for WAL
+	 * statistics.  In this case, avoid unnecessarily modifying the stats
+	 * entry.
+	 */
+	if (!pgstat_backend_wal_have_pending())
+		return;
+
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	bktype_shstats = &shbackendent->stats.wal_counters;
+
+	/*
+	 * Calculate how much WAL usage counters were increased by subtracting the
+	 * previous counters from the current ones.
+	 */
+	WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
+
+#define WALSTAT_ACC(fld, var_to_add) \
+	(bktype_shstats->fld += var_to_add.fld)
+	WALSTAT_ACC(wal_buffers_full, wal_usage_diff);
+	WALSTAT_ACC(wal_records, wal_usage_diff);
+	WALSTAT_ACC(wal_fpi, wal_usage_diff);
+	WALSTAT_ACC(wal_bytes, wal_usage_diff);
+#undef WALSTAT_ACC
+
+	/*
+	 * Save the current counters for the subsequent calculation of WAL usage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
+}
+
 /*
  * Flush out locally pending backend statistics
  *
@@ -194,12 +253,23 @@ bool
 pgstat_flush_backend(bool nowait, bits32 flags)
 {
 	PgStat_EntryRef *entry_ref;
+	bool		has_pending_data = false;
 
 	if (!pgstat_tracks_backend_bktype(MyBackendType))
 		return false;
 
-	if (pg_memory_is_all_zeros(&PendingBackendStats,
-							   sizeof(struct PgStat_BackendPending)))
+	/* Some IO data pending? */
+	if ((flags & PGSTAT_BACKEND_FLUSH_IO) &&
+		!pg_memory_is_all_zeros(&PendingBackendStats,
+								sizeof(struct PgStat_BackendPending)))
+		has_pending_data = true;
+
+	/* Some WAL data pending? */
+	if ((flags & PGSTAT_BACKEND_FLUSH_WAL) &&
+		pgstat_backend_wal_have_pending())
+		has_pending_data = true;
+
+	if (!has_pending_data)
 		return false;
 
 	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_BACKEND, InvalidOid,
@@ -211,6 +281,9 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 	if (flags & PGSTAT_BACKEND_FLUSH_IO)
 		pgstat_flush_backend_entry_io(entry_ref);
 
+	if (flags & PGSTAT_BACKEND_FLUSH_WAL)
+		pgstat_flush_backend_entry_wal(entry_ref);
+
 	pgstat_unlock_entry(entry_ref);
 
 	return false;
@@ -226,7 +299,8 @@ pgstat_backend_have_pending_cb(void)
 		return false;
 
 	return (!pg_memory_is_all_zeros(&PendingBackendStats,
-									sizeof(struct PgStat_BackendPending)));
+									sizeof(struct PgStat_BackendPending)) ||
+			pgstat_backend_wal_have_pending());
 }
 
 /*
@@ -261,6 +335,13 @@ pgstat_create_backend(ProcNumber procnum)
 	pgstat_unlock_entry(entry_ref);
 
 	MemSet(&PendingBackendStats, 0, sizeof(PgStat_BackendPending));
+
+	/*
+	 * Initialize prevBackendWalUsage with pgWalUsage so that
+	 * pgstat_backend_flush_cb() can calculate how much pgWalUsage counters
+	 * are increased by subtracting prevBackendWalUsage from pgWalUsage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 5d3da4b674e7..16a1ecb4d90d 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -52,6 +52,7 @@ pgstat_report_wal(bool force)
 
 	/* flush wal stats */
 	(void) pgstat_wal_flush_cb(nowait);
+	pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_WAL);
 
 	/* flush IO stats */
 	pgstat_flush_io(nowait);
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 9172e1cb9d23..662ce46cbc20 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1609,8 +1609,8 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 /*
  * pg_stat_wal_build_tuple
  *
- * Helper routine for pg_stat_get_wal() returning one tuple based on the
- * contents of wal_counters.
+ * Helper routine for pg_stat_get_wal() and pg_stat_get_backend_wal()
+ * returning one tuple based on the contents of wal_counters.
  */
 static Datum
 pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
@@ -1659,6 +1659,28 @@ pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns WAL statistics for a backend with given PID.
+ */
+Datum
+pg_stat_get_backend_wal(PG_FUNCTION_ARGS)
+{
+	int			pid;
+	PgStat_Backend *backend_stats;
+	PgStat_WalCounters bktype_stats;
+
+	pid = PG_GETARG_INT32(0);
+	backend_stats = pgstat_fetch_stat_backend_by_pid(pid, NULL);
+
+	if (!backend_stats)
+		PG_RETURN_NULL();
+
+	bktype_stats = backend_stats->wal_counters;
+
+	/* save tuples with data from this PgStat_WalCounters */
+	return (pg_stat_wal_build_tuple(bktype_stats, backend_stats->stat_reset_timestamp));
+}
+
 /*
  * Returns statistics of WAL activity
  */
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index 30d763c4aee8..f77caacc17dd 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -908,8 +908,11 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 
 -- Test pg_stat_checkpointer checkpointer-related stats, together with pg_stat_wal
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
--- Test pg_stat_wal (and make a temp table so our temp schema exists)
+-- Test pg_stat_wal
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal()
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+-- Make a temp table so our temp schema exists
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -929,6 +932,18 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
  t
 (1 row)
 
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+ ?column? 
+----------
+ t
+(1 row)
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 5e7ef20fef6e..c223800fd19c 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -426,9 +426,13 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 -- Test pg_stat_checkpointer checkpointer-related stats, together with pg_stat_wal
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 
--- Test pg_stat_wal (and make a temp table so our temp schema exists)
+-- Test pg_stat_wal
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
 
+-- Test pg_stat_get_backend_wal()
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+
+-- Make a temp table so our temp schema exists
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 
@@ -441,6 +445,9 @@ CHECKPOINT;
 SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;
 SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
 
+SELECT pg_stat_force_next_flush();
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 16646f560e8d..b1710680705c 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -4866,6 +4866,25 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para></entry>
       </row>
 
+      <row>
+       <entry id="pg-stat-get-backend-wal" role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_stat_get_backend_wal</primary>
+        </indexterm>
+        <function>pg_stat_get_backend_wal</function> ( <type>integer</type> )
+        <returnvalue>record</returnvalue>
+       </para>
+       <para>
+        Returns WAL statistics about the backend with the specified
+        process ID. The output fields are exactly the same as the ones in the
+        <structname>pg_stat_wal</structname> view.
+       </para>
+       <para>
+        The function does not return WAL statistics for the checkpointer,
+        the background writer, the startup process and the autovacuum launcher.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
-- 
2.47.2

From d93548d9b14dfba7c6ef3c8544c441ec29c12361 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Mon, 10 Mar 2025 16:45:26 +0900
Subject: [PATCH v18] per backend WAL statistics

Now that commit 9aea73fc61 added backend-level statistics to pgstats (and
per backend IO statistics) we can more easily add per backend statistics.

This commit adds per backend WAL statistics using the same layer as pg_stat_wal,
except that it is now possible to know how much WAL activity is happening in each
backend rather than an overall aggregate of all the activity.  A function called
pg_stat_get_backend_wal() is added to access this data depending on the
PID of a backend.

The same limitation as in 9aea73fc61 persists, meaning that Auxiliary processes
are not included in this set of statistics.

XXX: bump catalog version
XXX: bump of stats file format not required, as backend stats do not
persist on disk.
---
 doc/src/sgml/monitoring.sgml                | 19 +++++
 src/backend/utils/activity/pgstat_backend.c | 86 ++++++++++++++++++++-
 src/backend/utils/activity/pgstat_wal.c     |  1 +
 src/backend/utils/adt/pgstatfuncs.c         | 26 ++++++-
 src/include/catalog/pg_proc.dat             |  7 ++
 src/include/pgstat.h                        | 41 +++++-----
 src/include/utils/pgstat_internal.h         |  3 +-
 src/test/regress/expected/stats.out         | 17 +++-
 src/test/regress/sql/stats.sql              |  9 ++-
 9 files changed, 183 insertions(+), 26 deletions(-)
  13.0% doc/src/sgml/
  46.2% src/backend/utils/activity/
  12.8% src/backend/utils/adt/
   7.2% src/include/catalog/
   3.7% src/include/utils/
   8.4% src/test/regress/expected/
   6.8% src/test/regress/sql/

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 16646f560e8..b1710680705 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -4866,6 +4866,25 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para></entry>
       </row>
 
+      <row>
+       <entry id="pg-stat-get-backend-wal" role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_stat_get_backend_wal</primary>
+        </indexterm>
+        <function>pg_stat_get_backend_wal</function> ( <type>integer</type> )
+        <returnvalue>record</returnvalue>
+       </para>
+       <para>
+        Returns WAL statistics about the backend with the specified
+        process ID. The output fields are exactly the same as the ones in the
+        <structname>pg_stat_wal</structname> view.
+       </para>
+       <para>
+        The function does not return WAL statistics for the checkpointer,
+        the background writer, the startup process and the autovacuum launcher.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 6efbb650aa8..f414a48c242 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -38,6 +38,14 @@
  */
 static PgStat_BackendPending PendingBackendStats = {0};
 
+/*
+ * WAL usage counters saved from pgWalUsage at the previous call to
+ * pgstat_report_wal().  This is used to calculate how much WAL usage
+ * happens between pgstat_report_wal() calls, by subtracting the previous
+ * counters from the current ones.
+ */
+static WalUsage prevBackendWalUsage;
+
 /*
  * Utility routines to report I/O stats for backends, kept here to avoid
  * exposing PendingBackendStats to the outside world.
@@ -184,6 +192,57 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 	MemSet(&PendingBackendStats.pending_io, 0, sizeof(PgStat_PendingIO));
 }
 
+/*
+ * To determine whether WAL usage happened.
+ */
+static inline bool
+pgstat_backend_wal_have_pending(void)
+{
+	return pgWalUsage.wal_records != prevBackendWalUsage.wal_records;
+}
+
+/*
+ * Flush out locally pending backend WAL statistics.  Locking is managed
+ * by the caller.
+ */
+static void
+pgstat_flush_backend_entry_wal(PgStat_EntryRef *entry_ref)
+{
+	PgStatShared_Backend *shbackendent;
+	PgStat_WalCounters *bktype_shstats;
+	WalUsage	wal_usage_diff = {0};
+
+	/*
+	 * This function can be called even if nothing at all has happened for WAL
+	 * statistics.  In this case, avoid unnecessarily modifying the stats
+	 * entry.
+	 */
+	if (!pgstat_backend_wal_have_pending())
+		return;
+
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	bktype_shstats = &shbackendent->stats.wal_counters;
+
+	/*
+	 * Calculate how much WAL usage counters were increased by subtracting the
+	 * previous counters from the current ones.
+	 */
+	WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
+
+#define WALSTAT_ACC(fld, var_to_add) \
+	(bktype_shstats->fld += var_to_add.fld)
+	WALSTAT_ACC(wal_buffers_full, wal_usage_diff);
+	WALSTAT_ACC(wal_records, wal_usage_diff);
+	WALSTAT_ACC(wal_fpi, wal_usage_diff);
+	WALSTAT_ACC(wal_bytes, wal_usage_diff);
+#undef WALSTAT_ACC
+
+	/*
+	 * Save the current counters for the subsequent calculation of WAL usage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
+}
+
 /*
  * Flush out locally pending backend statistics
  *
@@ -194,12 +253,22 @@ bool
 pgstat_flush_backend(bool nowait, bits32 flags)
 {
 	PgStat_EntryRef *entry_ref;
+	bool		has_pending_data = false;
 
 	if (!pgstat_tracks_backend_bktype(MyBackendType))
 		return false;
 
-	if (pg_memory_is_all_zeros(&PendingBackendStats,
-							   sizeof(struct PgStat_BackendPending)))
+	/* Some IO data pending? */
+	if ((flags & PGSTAT_BACKEND_FLUSH_IO) &&
+		!pg_memory_is_all_zeros(&PendingBackendStats.pending_io,
+								sizeof(struct PgStat_PendingIO)))
+		has_pending_data = true;
+	/* Some WAL data pending? */
+	else if ((flags & PGSTAT_BACKEND_FLUSH_WAL) &&
+			 pgstat_backend_wal_have_pending())
+		has_pending_data = true;
+
+	if (!has_pending_data)
 		return false;
 
 	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_BACKEND, InvalidOid,
@@ -211,6 +280,9 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 	if (flags & PGSTAT_BACKEND_FLUSH_IO)
 		pgstat_flush_backend_entry_io(entry_ref);
 
+	if (flags & PGSTAT_BACKEND_FLUSH_WAL)
+		pgstat_flush_backend_entry_wal(entry_ref);
+
 	pgstat_unlock_entry(entry_ref);
 
 	return false;
@@ -226,7 +298,8 @@ pgstat_backend_have_pending_cb(void)
 		return false;
 
 	return (!pg_memory_is_all_zeros(&PendingBackendStats,
-									sizeof(struct PgStat_BackendPending)));
+									sizeof(struct PgStat_BackendPending)) ||
+			pgstat_backend_wal_have_pending());
 }
 
 /*
@@ -261,6 +334,13 @@ pgstat_create_backend(ProcNumber procnum)
 	pgstat_unlock_entry(entry_ref);
 
 	MemSet(&PendingBackendStats, 0, sizeof(PgStat_BackendPending));
+
+	/*
+	 * Initialize prevBackendWalUsage with pgWalUsage so that
+	 * pgstat_backend_flush_cb() can calculate how much pgWalUsage counters
+	 * are increased by subtracting prevBackendWalUsage from pgWalUsage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat_wal.c b/src/backend/utils/activity/pgstat_wal.c
index 5d3da4b674e..16a1ecb4d90 100644
--- a/src/backend/utils/activity/pgstat_wal.c
+++ b/src/backend/utils/activity/pgstat_wal.c
@@ -52,6 +52,7 @@ pgstat_report_wal(bool force)
 
 	/* flush wal stats */
 	(void) pgstat_wal_flush_cb(nowait);
+	pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_WAL);
 
 	/* flush IO stats */
 	pgstat_flush_io(nowait);
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 9172e1cb9d2..662ce46cbc2 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1609,8 +1609,8 @@ pg_stat_get_backend_io(PG_FUNCTION_ARGS)
 /*
  * pg_stat_wal_build_tuple
  *
- * Helper routine for pg_stat_get_wal() returning one tuple based on the
- * contents of wal_counters.
+ * Helper routine for pg_stat_get_wal() and pg_stat_get_backend_wal()
+ * returning one tuple based on the contents of wal_counters.
  */
 static Datum
 pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
@@ -1659,6 +1659,28 @@ pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Returns WAL statistics for a backend with given PID.
+ */
+Datum
+pg_stat_get_backend_wal(PG_FUNCTION_ARGS)
+{
+	int			pid;
+	PgStat_Backend *backend_stats;
+	PgStat_WalCounters bktype_stats;
+
+	pid = PG_GETARG_INT32(0);
+	backend_stats = pgstat_fetch_stat_backend_by_pid(pid, NULL);
+
+	if (!backend_stats)
+		PG_RETURN_NULL();
+
+	bktype_stats = backend_stats->wal_counters;
+
+	/* save tuples with data from this PgStat_WalCounters */
+	return (pg_stat_wal_build_tuple(bktype_stats, backend_stats->stat_reset_timestamp));
+}
+
 /*
  * Returns statistics of WAL activity
  */
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index cede992b6e2..42e427f8fe8 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -5973,6 +5973,13 @@
   proargmodes => '{o,o,o,o,o}',
   proargnames => '{wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
   prosrc => 'pg_stat_get_wal' },
+{ oid => '8037', descr => 'statistics: backend WAL activity',
+  proname => 'pg_stat_get_backend_wal', provolatile => 'v', proparallel => 'r',
+  prorettype => 'record', proargtypes => 'int4',
+  proallargtypes => '{int4,int8,int8,numeric,int8,timestamptz}',
+  proargmodes => '{i,o,o,o,o,o}',
+  proargnames => '{backend_pid,wal_records,wal_fpi,wal_bytes,wal_buffers_full,stats_reset}',
+  prosrc => 'pg_stat_get_backend_wal' },
 { oid => '6248', descr => 'statistics: information about WAL prefetching',
   proname => 'pg_stat_get_recovery_prefetch', prorows => '1', proretset => 't',
   provolatile => 'v', prorettype => 'record', proargtypes => '',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 4aad10b0b6d..def6b370ac1 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -340,24 +340,6 @@ typedef struct PgStat_IO
 	PgStat_BktypeIO stats[BACKEND_NUM_TYPES];
 } PgStat_IO;
 
-typedef struct PgStat_Backend
-{
-	TimestampTz stat_reset_timestamp;
-	PgStat_BktypeIO io_stats;
-} PgStat_Backend;
-
-/* ---------
- * PgStat_BackendPending	Non-flushed backend stats.
- * ---------
- */
-typedef struct PgStat_BackendPending
-{
-	/*
-	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
-	 */
-	PgStat_PendingIO pending_io;
-} PgStat_BackendPending;
-
 typedef struct PgStat_StatDBEntry
 {
 	PgStat_Counter xact_commit;
@@ -500,6 +482,29 @@ typedef struct PgStat_WalStats
 	TimestampTz stat_reset_timestamp;
 } PgStat_WalStats;
 
+/* -------
+ * PgStat_Backend		Backend statistics
+ * -------
+ */
+typedef struct PgStat_Backend
+{
+	TimestampTz stat_reset_timestamp;
+	PgStat_BktypeIO io_stats;
+	PgStat_WalCounters wal_counters;
+} PgStat_Backend;
+
+/* ---------
+ * PgStat_BackendPending	Non-flushed backend stats.
+ * ---------
+ */
+typedef struct PgStat_BackendPending
+{
+	/*
+	 * Backend statistics store the same amount of IO data as PGSTAT_KIND_IO.
+	 */
+	PgStat_PendingIO pending_io;
+} PgStat_BackendPending;
+
 /*
  * Functions in pgstat.c
  */
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 36d228e3558..d5557e6e998 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -622,7 +622,8 @@ extern void pgstat_archiver_snapshot_cb(void);
 
 /* flags for pgstat_flush_backend() */
 #define PGSTAT_BACKEND_FLUSH_IO		(1 << 0)	/* Flush I/O statistics */
-#define PGSTAT_BACKEND_FLUSH_ALL	(PGSTAT_BACKEND_FLUSH_IO)
+#define PGSTAT_BACKEND_FLUSH_WAL   (1 << 1) /* Flush WAL statistics */
+#define PGSTAT_BACKEND_FLUSH_ALL   (PGSTAT_BACKEND_FLUSH_IO | PGSTAT_BACKEND_FLUSH_WAL)
 
 extern bool pgstat_flush_backend(bool nowait, bits32 flags);
 extern bool pgstat_backend_flush_cb(bool nowait);
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index 30d763c4aee..f77caacc17d 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -908,8 +908,11 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 
 -- Test pg_stat_checkpointer checkpointer-related stats, together with pg_stat_wal
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
--- Test pg_stat_wal (and make a temp table so our temp schema exists)
+-- Test pg_stat_wal
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
+-- Test pg_stat_get_backend_wal()
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+-- Make a temp table so our temp schema exists
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
@@ -929,6 +932,18 @@ SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
  t
 (1 row)
 
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+ ?column? 
+----------
+ t
+(1 row)
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 5e7ef20fef6..c223800fd19 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -426,9 +426,13 @@ SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELEC
 -- Test pg_stat_checkpointer checkpointer-related stats, together with pg_stat_wal
 SELECT num_requested AS rqst_ckpts_before FROM pg_stat_checkpointer \gset
 
--- Test pg_stat_wal (and make a temp table so our temp schema exists)
+-- Test pg_stat_wal
 SELECT wal_bytes AS wal_bytes_before FROM pg_stat_wal \gset
 
+-- Test pg_stat_get_backend_wal()
+SELECT wal_bytes AS backend_wal_bytes_before from pg_stat_get_backend_wal(pg_backend_pid()) \gset
+
+-- Make a temp table so our temp schema exists
 CREATE TEMP TABLE test_stats_temp AS SELECT 17;
 DROP TABLE test_stats_temp;
 
@@ -441,6 +445,9 @@ CHECKPOINT;
 SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;
 SELECT wal_bytes > :wal_bytes_before FROM pg_stat_wal;
 
+SELECT pg_stat_force_next_flush();
+SELECT wal_bytes > :backend_wal_bytes_before FROM pg_stat_get_backend_wal(pg_backend_pid());
+
 -- Test pg_stat_get_backend_idset() and some allied functions.
 -- In particular, verify that their notion of backend ID matches
 -- our temp schema index.
-- 
2.34.1

From d1178e65b5070f539677b1134224d6ca81fd0bbf Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Wed, 12 Mar 2025 05:14:21 +0000
Subject: [PATCH v1] Remove the PendingBackendStats initializer

9a8dd2c5a6d added an initializer to PendingBackendStats, but on some GCC versions
-Werror=missing-braces would report a warning. Instead of adding extra braces,
let's remove the initialization that would prevent padding bytes from being
zeroed.

Per buildfarm members ayu, batfish, boa, buri, demoiselle, dhole, rhinoceros,
shelduck and siskin.
---
 src/backend/utils/activity/pgstat_backend.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
 100.0% src/backend/utils/activity/

diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index da4c7edd772..a8cb54a7732 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -36,7 +36,7 @@
  * reported within critical sections so we use static memory in order to avoid
  * memory allocation.
  */
-static PgStat_BackendPending PendingBackendStats = {0};
+static PgStat_BackendPending PendingBackendStats;
 
 /*
  * WAL usage counters saved from pgWalUsage at the previous call to
-- 
2.34.1

From 1605f513ad691b463baacc00e3c305655525ea07 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Date: Mon, 31 Mar 2025 07:02:34 +0000
Subject: [PATCH v1] Fix heap-use-after-free in
 pgstat_fetch_stat_backend_by_pid()

With stats_fetch_consistency set to snapshot the beentry is reset during
the pgstat_fetch_stat_backend() call. So moving this call at the end of
pgstat_fetch_stat_backend_by_pid().

Reported-by: Alexander Lakhin <exclusion@gmail.com>
---
 src/backend/utils/activity/pgstat_backend.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)
 100.0% src/backend/utils/activity/

diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 187c5c76e1e..ec95c302af8 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -133,10 +133,6 @@ pgstat_fetch_stat_backend_by_pid(int pid, BackendType *bktype)
 	if (!pgstat_tracks_backend_bktype(beentry->st_backendType))
 		return NULL;
 
-	backend_stats = pgstat_fetch_stat_backend(procNumber);
-	if (!backend_stats)
-		return NULL;
-
 	/* if PID does not match, leave */
 	if (beentry->st_procpid != pid)
 		return NULL;
@@ -144,6 +140,10 @@ pgstat_fetch_stat_backend_by_pid(int pid, BackendType *bktype)
 	if (bktype)
 		*bktype = beentry->st_backendType;
 
+	backend_stats = pgstat_fetch_stat_backend(procNumber);
+	if (!backend_stats)
+		return NULL;
+
 	return backend_stats;
 }
 
-- 
2.34.1