Update relation's stats in pg_class during vacuum full.

Hash index depends on estimation of numbers of tuples and pages of relations,
incorrect value could be a reason of significantly growing of index. Vacuum
full recreates heap and reindex all indexes before renewal stats. The patch
fixes that, so indexes will see correct values.

Backpatch to v10 only because earlier versions haven't usable hash index and
growing of hash index is a single user-visible symptom.

Author: Amit Kapila
Reviewed-by: Ashutosh Sharma, me
Discussion: https://www.postgresql.org/message-id/flat/20171115232922.5tomkxnw3iq6jsg7@inml.weebeastie.net

src/backend/commands/cluster.c

@@ -738,6 +738,9 @@ copy_heap_data(Oid OIDNewHeap, Oid OIDOldHeap, Oid OIDOldIndex, bool verbose,
 	Relation	NewHeap,
 				OldHeap,
 				OldIndex;
+	Relation	relRelation;
+	HeapTuple	reltup;
+	Form_pg_class relform;
 	TupleDesc	oldTupDesc;
 	TupleDesc	newTupDesc;
 	int			natts;
@@ -756,6 +759,7 @@ copy_heap_data(Oid OIDNewHeap, Oid OIDOldHeap, Oid OIDOldIndex, bool verbose,
 	double		num_tuples = 0,
 				tups_vacuumed = 0,
 				tups_recently_dead = 0;
+	BlockNumber num_pages;
 	int			elevel = verbose ? INFO : DEBUG2;
 	PGRUsage	ru0;
 
@@ -1079,6 +1083,8 @@ copy_heap_data(Oid OIDNewHeap, Oid OIDOldHeap, Oid OIDOldIndex, bool verbose,
 	/* Reset rd_toastoid just to be tidy --- it shouldn't be looked at again */
 	NewHeap->rd_toastoid = InvalidOid;
 
+	num_pages = RelationGetNumberOfBlocks(NewHeap);
+
 	/* Log what we did */
 	ereport(elevel,
 			(errmsg("\"%s\": found %.0f removable, %.0f nonremovable row versions in %u pages",
@@ -1098,6 +1104,30 @@ copy_heap_data(Oid OIDNewHeap, Oid OIDOldHeap, Oid OIDOldIndex, bool verbose,
 		index_close(OldIndex, NoLock);
 	heap_close(OldHeap, NoLock);
 	heap_close(NewHeap, NoLock);
+
+	/* Update pg_class to reflect the correct values of pages and tuples. */
+	relRelation = heap_open(RelationRelationId, RowExclusiveLock);
+
+	reltup = SearchSysCacheCopy1(RELOID, ObjectIdGetDatum(OIDNewHeap));
+	if (!HeapTupleIsValid(reltup))
+		elog(ERROR, "cache lookup failed for relation %u", OIDNewHeap);
+	relform = (Form_pg_class) GETSTRUCT(reltup);
+
+	relform->relpages = num_pages;
+	relform->reltuples = num_tuples;
+
+	/* Don't update the stats for pg_class.  See swap_relation_files. */
+	if (OIDOldHeap != RelationRelationId)
+		CatalogTupleUpdate(relRelation, &reltup->t_self, reltup);
+	else
+		CacheInvalidateRelcacheByTuple(reltup);
+
+	/* Clean up. */
+	heap_freetuple(reltup);
+	heap_close(relRelation, RowExclusiveLock);
+
+	/* Make the update visible */
+	CommandCounterIncrement();
 }
 
 /*