aligputf8=# set enable_hashagg = off;
SET
aligputf8=#  explain select count(1) from pg_class group by oid;
                                 QUERY PLAN                                 
----------------------------------------------------------------------------
 GroupAggregate  (cost=13291.66..14666.48 rows=23910 width=4)
   Group By: oid
   ->  Sort  (cost=13291.66..13650.31 rows=143460 width=4)
         Sort Key: oid
         ->  Seq Scan on pg_class  (cost=0.00..1004.10 rows=143460 width=4)
 Settings:  enable_hashagg=off; enable_seqscan=on
(6 rows)