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

1 files changed, 24 insertions, 2 deletions

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,
@@ -1660,6 +1660,28 @@ pg_stat_wal_build_tuple(PgStat_WalCounters wal_counters,
 }
 
 /*
+ * 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
  */
 Datum