aligputf8=# explain select count(1) from pg_class group by oid;
                              QUERY PLAN                              
----------------------------------------------------------------------
 HashAggregate  (cost=1721.40..2020.28 rows=23910 width=4)
   Group By: oid
   ->  Seq Scan on pg_class  (cost=0.00..1004.10 rows=143460 width=4)
 Settings:  enable_seqscan=on
(4 rows)