BEGIN;
SET max_parallel_workers_per_gather=4;
SET force_parallel_mode=on;
CREATE TABLE parallel_test(i int, md md5hash, md2 md5hash) WITH (parallel_workers = 4);
INSERT INTO parallel_test (i, md, md2)
SELECT i, md5(i::text), md5((i + 1)::text)
FROM generate_series(1,1e6) i;
EXPLAIN (costs off,verbose)
SELECT COUNT(*) FROM parallel_test WHERE md > md2;
                             QUERY PLAN                             
--------------------------------------------------------------------
 Finalize Aggregate
   Output: count(*)
   ->  Gather
         Output: (PARTIAL count(*))
         Workers Planned: 4
         ->  Partial Aggregate
               Output: PARTIAL count(*)
               ->  Parallel Seq Scan on public.parallel_test
                     Filter: (parallel_test.md > parallel_test.md2)
(9 rows)

EXPLAIN (costs off,verbose)
SELECT md, COUNT(*) FROM parallel_test GROUP BY 1;
                            QUERY PLAN                             
-------------------------------------------------------------------
 Finalize GroupAggregate
   Output: md, count(*)
   Group Key: parallel_test.md
   ->  Sort
         Output: md, (PARTIAL count(*))
         Sort Key: parallel_test.md
         ->  Gather
               Output: md, (PARTIAL count(*))
               Workers Planned: 4
               ->  Partial HashAggregate
                     Output: md, PARTIAL count(*)
                     Group Key: parallel_test.md
                     ->  Parallel Seq Scan on public.parallel_test
                           Output: md
(14 rows)

ROLLBACK;