Add write_to_file to PgStat_KindInfo for pgstats kinds

This new field controls if entries of a stats kind should be written or
not to the on-disk pgstats file when shutting down an instance.  This
affects both fixed and variable-numbered kinds.

This is useful for custom statistics by itself, and a patch is under
discussion to add a new builtin stats kind where the write of the stats
is not necessary.  All the built-in stats kinds, as well as the two
custom stats kinds in the test module injection_points, set this flag to
"true" for now, so as stats entries are written to the on-disk pgstats
file.

Author: Bertrand Drouvot
Reviewed-by: Nazir Bilal Yavuz
Discussion: https://postgr.es/m/Zz7T47nHwYgeYwOe@ip-10-97-1-34.eu-west-3.compute.internal

src/backend/utils/activity/pgstat.c

@@ -12,7 +12,8 @@
  * Statistics are loaded from the filesystem during startup (by the startup
  * process), unless preceded by a crash, in which case all stats are
  * discarded. They are written out by the checkpointer process just before
- * shutting down, except when shutting down in immediate mode.
+ * shutting down (if the stats kind allows it), except when shutting down in
+ * immediate mode.
  *
  * Fixed-numbered stats are stored in plain (non-dynamic) shared memory.
  *
@@ -281,6 +282,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.name = "database",
 
 		.fixed_amount = false,
+		.write_to_file = true,
 		/* so pg_stat_database entries can be seen in all databases */
 		.accessed_across_databases = true,
 
@@ -297,6 +299,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.name = "relation",
 
 		.fixed_amount = false,
+		.write_to_file = true,
 
 		.shared_size = sizeof(PgStatShared_Relation),
 		.shared_data_off = offsetof(PgStatShared_Relation, stats),
@@ -311,6 +314,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.name = "function",
 
 		.fixed_amount = false,
+		.write_to_file = true,
 
 		.shared_size = sizeof(PgStatShared_Function),
 		.shared_data_off = offsetof(PgStatShared_Function, stats),
@@ -324,6 +328,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.name = "replslot",
 
 		.fixed_amount = false,
+		.write_to_file = true,
 
 		.accessed_across_databases = true,
 
@@ -340,6 +345,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.name = "subscription",
 
 		.fixed_amount = false,
+		.write_to_file = true,
 		/* so pg_stat_subscription_stats entries can be seen in all databases */
 		.accessed_across_databases = true,
 
@@ -359,6 +365,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.name = "archiver",
 
 		.fixed_amount = true,
+		.write_to_file = true,
 
 		.snapshot_ctl_off = offsetof(PgStat_Snapshot, archiver),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, archiver),
@@ -374,6 +381,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.name = "bgwriter",
 
 		.fixed_amount = true,
+		.write_to_file = true,
 
 		.snapshot_ctl_off = offsetof(PgStat_Snapshot, bgwriter),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, bgwriter),
@@ -389,6 +397,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.name = "checkpointer",
 
 		.fixed_amount = true,
+		.write_to_file = true,
 
 		.snapshot_ctl_off = offsetof(PgStat_Snapshot, checkpointer),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, checkpointer),
@@ -404,6 +413,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.name = "io",
 
 		.fixed_amount = true,
+		.write_to_file = true,
 
 		.snapshot_ctl_off = offsetof(PgStat_Snapshot, io),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, io),
@@ -421,6 +431,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.name = "slru",
 
 		.fixed_amount = true,
+		.write_to_file = true,
 
 		.snapshot_ctl_off = offsetof(PgStat_Snapshot, slru),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, slru),
@@ -438,6 +449,7 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.name = "wal",
 
 		.fixed_amount = true,
+		.write_to_file = true,
 
 		.snapshot_ctl_off = offsetof(PgStat_Snapshot, wal),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, wal),
@@ -1617,6 +1629,10 @@ pgstat_write_statsfile(XLogRecPtr redo)
 		if (pgstat_is_kind_builtin(kind))
 			Assert(info->snapshot_ctl_off != 0);
 
+		/* skip if no need to write to file */
+		if (!info->write_to_file)
+			continue;
+
 		pgstat_build_snapshot_fixed(kind);
 		if (pgstat_is_kind_builtin(kind))
 			ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
@@ -1663,6 +1679,10 @@ pgstat_write_statsfile(XLogRecPtr redo)
 		/* if not dropped the valid-entry refcount should exist */
 		Assert(pg_atomic_read_u32(&ps->refcount) > 0);
 
+		/* skip if no need to write to file */
+		if (!kind_info->write_to_file)
+			continue;
+
 		if (!kind_info->to_serialized_name)
 		{
 			/* normal stats entry, identified by PgStat_HashKey */

src/include/utils/pgstat_internal.h

@@ -213,6 +213,9 @@ typedef struct PgStat_KindInfo
 	 */
 	bool		accessed_across_databases:1;
 
+	/* Should stats be written to the on-disk stats file? */
+	bool		write_to_file:1;
+
 	/*
 	 * The size of an entry in the shared stats hash table (pointed to by
 	 * PgStatShared_HashEntry->body).  For fixed-numbered statistics, this is

src/test/modules/injection_points/injection_stats.c

@@ -39,6 +39,7 @@ static bool injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
 static const PgStat_KindInfo injection_stats = {
 	.name = "injection_points",
 	.fixed_amount = false,		/* Bounded by the number of points */
+	.write_to_file = true,
 
 	/* Injection points are system-wide */
 	.accessed_across_databases = true,

src/test/modules/injection_points/injection_stats_fixed.c

@@ -50,6 +50,7 @@ static void injection_stats_fixed_snapshot_cb(void);
 static const PgStat_KindInfo injection_stats_fixed = {
 	.name = "injection_points_fixed",
 	.fixed_amount = true,
+	.write_to_file = true,
 
 	.shared_size = sizeof(PgStat_StatInjFixedEntry),
 	.shared_data_off = offsetof(PgStatShared_InjectionPointFixed, stats),