Re: [HACKERS] Partition-wise aggregation/grouping

Hi Jeevan,

I have started testing partition-wise-aggregate and got one observation, please take a look.
with the v2 patch, here if I change target list order, query is not picking full partition-wise-aggregate.

SET enable_partition_wise_agg TO true;
SET partition_wise_agg_cost_factor TO 0.5;
SET enable_partition_wise_join TO true;
SET max_parallel_workers_per_gather TO 0;

CREATE TABLE pagg_tab (a int, b int, c int) PARTITION BY RANGE(a);
CREATE TABLE pagg_tab_p1 PARTITION OF pagg_tab FOR VALUES FROM (0) TO (10);
CREATE TABLE pagg_tab_p2 PARTITION OF pagg_tab FOR VALUES FROM (10) TO (20);
CREATE TABLE pagg_tab_p3 PARTITION OF pagg_tab FOR VALUES FROM (20) TO (30);
INSERT INTO pagg_tab SELECT i % 30, i % 30, i % 50 FROM generate_series(0, 299) i;
ANALYZE pagg_tab;

postgres=# explain (verbose, costs off) select a,b,count(*) from pagg_tab group by a,b order by 1,2;
                          QUERY PLAN                         
--------------------------------------------------------------
 Sort
   Output: pagg_tab_p1.a, pagg_tab_p1.b, (count(*))
   Sort Key: pagg_tab_p1.a, pagg_tab_p1.b
   ->  Append
         ->  HashAggregate
               Output: pagg_tab_p1.a, pagg_tab_p1.b, count(*)
               Group Key: pagg_tab_p1.a, pagg_tab_p1.b
               ->  Seq Scan on public.pagg_tab_p1
                     Output: pagg_tab_p1.a, pagg_tab_p1.b
         ->  HashAggregate
               Output: pagg_tab_p2.a, pagg_tab_p2.b, count(*)
               Group Key: pagg_tab_p2.a, pagg_tab_p2.b
               ->  Seq Scan on public.pagg_tab_p2
                     Output: pagg_tab_p2.a, pagg_tab_p2.b
         ->  HashAggregate
               Output: pagg_tab_p3.a, pagg_tab_p3.b, count(*)
               Group Key: pagg_tab_p3.a, pagg_tab_p3.b
               ->  Seq Scan on public.pagg_tab_p3
                     Output: pagg_tab_p3.a, pagg_tab_p3.b
(19 rows)

-- changing target list order
-- picking partial partition-wise aggregation path
postgres=# explain (verbose, costs off) select b,a,count(*) from pagg_tab group by a,b order by 1,2;
                                 QUERY PLAN                                
----------------------------------------------------------------------------
 Finalize GroupAggregate
   Output: pagg_tab_p1.b, pagg_tab_p1.a, count(*)
   Group Key: pagg_tab_p1.b, pagg_tab_p1.a
   ->  Sort
         Output: pagg_tab_p1.b, pagg_tab_p1.a, (PARTIAL count(*))
         Sort Key: pagg_tab_p1.b, pagg_tab_p1.a
         ->  Append
               ->  Partial HashAggregate
                     Output: pagg_tab_p1.b, pagg_tab_p1.a, PARTIAL count(*)
                     Group Key: pagg_tab_p1.b, pagg_tab_p1.a
                     ->  Seq Scan on public.pagg_tab_p1
                           Output: pagg_tab_p1.b, pagg_tab_p1.a
               ->  Partial HashAggregate
                     Output: pagg_tab_p2.b, pagg_tab_p2.a, PARTIAL count(*)
                     Group Key: pagg_tab_p2.b, pagg_tab_p2.a
                     ->  Seq Scan on public.pagg_tab_p2
                           Output: pagg_tab_p2.b, pagg_tab_p2.a
               ->  Partial HashAggregate
                     Output: pagg_tab_p3.b, pagg_tab_p3.a, PARTIAL count(*)
                     Group Key: pagg_tab_p3.b, pagg_tab_p3.a
                     ->  Seq Scan on public.pagg_tab_p3
                           Output: pagg_tab_p3.b, pagg_tab_p3.a
(22 rows)

Thanks & Regards,