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).

28a8fa98 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

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)
+				/* 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
-					msgfmt = _("automatic vacuum to prevent wraparound of table \"%s.%s.%s\": index scans: %d\n");
 			}
 			else
 			{