tpch10s=# set enable_hashagg=off;
tpch10s=# set optimizer=off;
tpch10s=# explain select sum(l_quantity), l_partkey, l_suppkey from  lineitem group by l_partkey, l_suppkey;
                                                       QUERY PLAN                                                       
------------------------------------------------------------------------------------------------------------------------
 Gather Motion 8:1  (slice2; segments: 8)  (cost=14249316.35..14444271.04 rows=5998606 width=40)
   ->  GroupAggregate  (cost=14249316.35..14444271.04 rows=749826 width=40)
         Group Key: lineitem.l_partkey, lineitem.l_suppkey
         ->  Sort  (cost=14249316.35..14264312.86 rows=749826 width=40)
               Sort Key: lineitem.l_partkey, lineitem.l_suppkey
               ->  Redistribute Motion 8:8  (slice1; segments: 8)  (cost=12175746.69..12970561.91 rows=749826 width=40)
                     Hash Key: lineitem.l_partkey, lineitem.l_suppkey
                     ->  GroupAggregate  (cost=12175746.69..12850589.79 rows=749826 width=40)
                           Group Key: lineitem.l_partkey, lineitem.l_suppkey
                           ->  Sort  (cost=12175746.69..12325711.82 rows=7498257 width=13)
                                 Sort Key: lineitem.l_partkey, lineitem.l_suppkey
                                 ->  Seq Scan on lineitem  (cost=0.00..654972.52 rows=7498257 width=13)
 Optimizer: Postgres query optimizer