tpch10s=# set enable_hashagg=off;
tpch10s=# set optimizer=on;
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=0.00..6012.40 rows=59986052 width=16)
   ->  HashAggregate  (cost=0.00..3351.42 rows=7498257 width=16)
         Group Key: l_partkey, l_suppkey
         ->  Redistribute Motion 8:8  (slice1; segments: 8)  (cost=0.00..1457.66 rows=7498257 width=13)
               Hash Key: l_partkey, l_suppkey
               ->  Seq Scan on lineitem  (cost=0.00..971.25 rows=7498257 width=13)
 Optimizer: Pivotal Optimizer (GPORCA)