Skip redundant anti-wraparound vacuums

An anti-wraparound vacuum has to be by definition aggressive as it needs
to work on all the pages of a relation.  However it can happen that due
to some concurrent activity an anti-wraparound vacuum is marked as
non-aggressive, which makes it redundant with a previous run, and
it is actually useless as an anti-wraparound vacuum should process all
the pages of a relation.  This commit makes such vacuums to be skipped.

An anti-wraparound vacuum not aggressive can be found easily by mixing
low values of autovacuum_freeze_max_age (to control anti-wraparound) and
autovacuum_freeze_table_age (to control the aggressiveness).

28a8fa9 has added some extra logging printing all the possible
combinations of anti-wraparound and aggressive vacuums, which now gets
simplified as an anti-wraparound vacuum also non-aggressive gets
skipped.

Per discussion mainly between Andrew Dunstan, Robert Haas, Álvaro
Herrera, Kyotaro Horiguchi, Masahiko Sawada, and myself.

Author: Kyotaro Horiguchi, Michael Paquier
Reviewed-by: Andrew Dunstan, Álvaro Herrera
Discussion: https://postgr.es/m/20180914153554.562muwr3uwujno75@alvherre.pgsql

diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
index 5c554f94659b60ff658e1156c20ba97d664c9ee4..b5b464e4a9d744c089d2aeec83425ffe54f1cfa4 100644 (file)
--- a/src/backend/access/heap/vacuumlazy.c
+++ b/src/backend/access/heap/vacuumlazy.c
@@ -248,6 +248,23 @@ heap_vacuum_rel(Relation onerel, VacuumParams *params,
        if (params->options & VACOPT_DISABLE_PAGE_SKIPPING)
                aggressive = true;
 
+       /*
+        * Normally the relfrozenxid for an anti-wraparound vacuum will be old
+        * enough to force an aggressive vacuum.  However, a concurrent vacuum
+        * might have already done this work that the relfrozenxid in relcache has
+        * been updated.  If that happens this vacuum is redundant, so skip it.
+        */
+       if (params->is_wraparound && !aggressive)
+       {
+               ereport(DEBUG1,
+                               (errmsg("skipping redundant vacuum to prevent wraparound of table \"%s.%s.%s\"",
+                                               get_database_name(MyDatabaseId),
+                                               get_namespace_name(RelationGetNamespace(onerel)),
+                                               RelationGetRelationName(onerel))));
+               pgstat_progress_end_command();
+               return;
+       }
+
        vacrelstats = (LVRelStats *) palloc0(sizeof(LVRelStats));
 
        vacrelstats->old_rel_pages = onerel->rd_rel->relpages;
@@ -375,10 +392,9 @@ heap_vacuum_rel(Relation onerel, VacuumParams *params,
                        initStringInfo(&buf);
                        if (params->is_wraparound)
                        {
-                               if (aggressive)
-                                       msgfmt = _("automatic aggressive vacuum to prevent wraparound of table \"%s.%s.%s\": index scans: %d\n");
-                               else
-                                       msgfmt = _("automatic vacuum to prevent wraparound of table \"%s.%s.%s\": index scans: %d\n");
+                               /* an anti-wraparound vacuum has to be aggressive */
+                               Assert(aggressive);
+                               msgfmt = _("automatic aggressive vacuum to prevent wraparound of table \"%s.%s.%s\": index scans: %d\n");
                        }
                        else
                        {