Add WAL data to backend statistics

This commit adds per-backend WAL statistics, providing the same
information 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.  Like pg_stat_wal, the implementation
relies on pgWalUsage, tracking the difference of activity between two
reports to pgstats.

This data can be retrieved with a new system function called
pg_stat_get_backend_wal(), that returns one tuple based on the PID
provided in input.  Like pg_stat_get_backend_io(), this is useful when
joined with pg_stat_activity to get a live picture of the WAL generated
for each running backend, showing how the activity is [un]balanced.

pgstat_flush_backend() gains a new flag value, able to control the flush
of the WAL stats.

This commit relies mostly on the infrastructure provided by
9aea73fc61d4, that has introduced backend statistics.

Bump catalog version.  A bump of PGSTAT_FILE_FORMAT_ID is not required,
as backend stats do not persist on disk.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/Z3zqc4o09dM/Ezyz@ip-10-97-1-34.eu-west-3.compute.internal

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>

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.pending_io,
+								sizeof(struct PgStat_PendingIO)))
+		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;
 }
 
 /*

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);

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
  */

src/include/catalog/catversion.h

@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202503071
+#define CATALOG_VERSION_NO	202503111
 
 #endif

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 => '',

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
  */

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);

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.

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.