Add options to control whether VACUUM runs vac_update_datfrozenxid.

VACUUM normally ends by running vac_update_datfrozenxid(), which
requires a scan of pg_class.  Therefore, if one attempts to vacuum a
database one table at a time --- as vacuumdb has done since v12 ---
we will spend O(N^2) time in vac_update_datfrozenxid().  That causes
serious performance problems in databases with tens of thousands of
tables, and indeed the effect is measurable with only a few hundred.
To add insult to injury, only one process can run
vac_update_datfrozenxid at the same time per DB, so this behavior
largely defeats vacuumdb's -j option.

Hence, invent options SKIP_DATABASE_STATS and ONLY_DATABASE_STATS
to allow applications to postpone vac_update_datfrozenxid() until the
end of a series of VACUUM requests, and teach vacuumdb to use them.

Per bug #17717 from Gunnar L.  Sadly, this answer doesn't seem
like something we'd consider back-patching, so the performance
problem will remain in v12-v15.

Tom Lane and Nathan Bossart

Discussion: https://postgr.es/m/17717-6c50eb1c7d23a886@postgresql.org

1 files changed, 7 insertions, 2 deletions

diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index e40bd39b3f3..f5ea381c53e 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -2854,8 +2854,13 @@ table_recheck_autovac(Oid relid, HTAB *table_toast_map,
 		tab->at_relid = relid;
 		tab->at_sharedrel = classForm->relisshared;
 
-		/* Note that this skips toast relations */
-		tab->at_params.options = (dovacuum ? VACOPT_VACUUM : 0) |
+		/*
+		 * Select VACUUM options.  Note we don't say VACOPT_PROCESS_TOAST, so
+		 * that vacuum() skips toast relations.  Also note we tell vacuum() to
+		 * skip vac_update_datfrozenxid(); we'll do that separately.
+		 */
+		tab->at_params.options =
+			(dovacuum ? (VACOPT_VACUUM | VACOPT_SKIP_DATABASE_STATS) : 0) |
 			(doanalyze ? VACOPT_ANALYZE : 0) |
 			(!wraparound ? VACOPT_SKIP_LOCKED : 0);