Invalidate relcache for pg_appendonly change. (#7437)

In Greenplum, data structure RelationData includes the related pg_appendonly
row for an ao table, when pg_appendonly changes for that ao table, we should
invalidate the relcache for it also to prevent possible inconsistency.

I can not find a real case to hit this after some tries, but this is an issue
we could hit in theory.

Reviewed-by: Hubert Zhang
Reviewed-by: Asim R P
Reviewed-by: Jimmy Yih

src/backend/utils/cache/inval.c

@@ -1196,6 +1196,13 @@ CacheInvalidateHeapTuple(Relation relation,
 		relationId = gptup->localoid;
 		databaseId = MyDatabaseId;
 	}
+	else if (tupleRelId == AppendOnlyRelationId)
+	{
+		FormData_pg_appendonly *aotup = (FormData_pg_appendonly *) GETSTRUCT(tuple);
+
+		relationId = aotup->relid;
+		databaseId = MyDatabaseId;
+	}
 	else if (tupleRelId == IndexRelationId)
 	{
 		Form_pg_index indextup = (Form_pg_index) GETSTRUCT(tuple);