pgstat: scaffolding for transactional stats creation / drop.

One problematic part of the current statistics collector design is that there
is no reliable way of getting rid of statistics entries. Because of that
pgstat_vacuum_stat() (called by [auto-]vacuum) matches all stats for the
current database with the catalog contents and tries to drop now-superfluous
entries. That's quite expensive. What's worse, it doesn't work on physical
replicas, despite physical replicas collection statistics entries.

This commit introduces infrastructure to create / drop statistics entries
transactionally, together with the underlying catalog objects (functions,
relations, subscriptions). pgstat_xact.c maintains a list of stats entries
created / dropped transactionally in the current transaction. To ensure the
removal of statistics entries is durable dropped statistics entries are
included in commit / abort (and prepare) records, which also ensures that
stats entries are dropped on standbys.

Statistics entries created separately from creating the underlying catalog
object (e.g. when stats were previously lost due to an immediate restart)
are *not* WAL logged. However that can only happen outside of the transaction
creating the catalog object, so it does not lead to "leaked" statistics
entries.

For this to work, functions creating / dropping functions / relations /
subscriptions need to call into pgstat. For subscriptions this was already
done when dropping subscriptions, via pgstat_report_subscription_drop() (now
renamed to pgstat_drop_subscription()).

This commit does not actually drop stats yet, it just provides the
infrastructure. It is however a largely independent piece of infrastructure,
so committing it separately makes sense.

Bumps XLOG_PAGE_MAGIC.

Author: Andres Freund <andres@anarazel.de>
Reviewed-By: Thomas Munro <thomas.munro@gmail.com>
Reviewed-By: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/20220303021600.hs34ghqcw6zcokdh@alap3.anarazel.de

1 files changed, 15 insertions, 22 deletions

diff --git a/src/backend/utils/activity/pgstat_relation.c b/src/backend/utils/activity/pgstat_relation.c
index 4f97d2f1d9c..5b9b6dd7c60 100644
--- a/src/backend/utils/activity/pgstat_relation.c
+++ b/src/backend/utils/activity/pgstat_relation.c
@@ -171,33 +171,26 @@ pgstat_relation_init(Relation rel)
 }
 
 /*
- * Tell the collector that we just dropped a relation.
- * (If the message gets lost, we will still clean the dead entry eventually
- * via future invocations of pgstat_vacuum_stat().)
- *
- * Currently not used for lack of any good place to call it; we rely
- * entirely on pgstat_vacuum_stat() to clean out stats for dead rels.
+ * Ensure that stats are dropped if transaction aborts.
  */
-#ifdef NOT_USED
 void
-pgstat_drop_relation(Oid relid)
+pgstat_create_relation(Relation rel)
 {
-	PgStat_MsgTabpurge msg;
-	int			len;
-
-	if (pgStatSock == PGINVALID_SOCKET)
-		return;
-
-	msg.m_tableid[0] = relid;
-	msg.m_nentries = 1;
-
-	len = offsetof(PgStat_MsgTabpurge, m_tableid[0]) + sizeof(Oid);
+	pgstat_create_transactional(PGSTAT_KIND_RELATION,
+								rel->rd_rel->relisshared ? InvalidOid : MyDatabaseId,
+								RelationGetRelid(rel));
+}
 
-	pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_TABPURGE);
-	msg.m_databaseid = MyDatabaseId;
-	pgstat_send(&msg, len);
+/*
+ * Ensure that stats are dropped if transaction commits.
+ */
+void
+pgstat_drop_relation(Relation rel)
+{
+	pgstat_drop_transactional(PGSTAT_KIND_RELATION,
+							  rel->rd_rel->relisshared ? InvalidOid : MyDatabaseId,
+							  RelationGetRelid(rel));
 }
-#endif							/* NOT_USED */
 
 /*
  * Report that the table was just vacuumed.