BEGIN;
SET max_parallel_workers_per_gather=4;
SET force_parallel_mode=on;
CREATE TABLE parallel_test(i int, c language) WITH (parallel_workers = 4);
INSERT INTO parallel_test (i, c)
SELECT i, c.country
FROM generate_series(1,1e6) i,
LATERAL (SELECT CASE WHEN i % 3 = 0 THEN 'en'::language
                     WHEN i % 3 = 1 THEN 'de'::language
                     WHEN i % 3 = 2 THEN 'id'::language
         END as country) c;
EXPLAIN (costs off,verbose)
SELECT COUNT(*) FROM parallel_test WHERE  c = 'id'::language;
                           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.c = 'id'::language)
(9 rows)

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

ROLLBACK;