BEGIN;
create extension country;
SET max_parallel_workers_per_gather=4;
SET force_parallel_mode=on;
CREATE TABLE parallel_test(i int, c country) 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 'US'::country
                     WHEN i % 3 = 1 THEN 'DE'::country
                     WHEN i % 3 = 2 THEN 'CA'::country
         END as country) c;
EXPLAIN (costs off,verbose)
SELECT COUNT(*) FROM parallel_test WHERE  c = 'us';;
                          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
                     Output: i, c
                     Filter: (parallel_test.c = 'us'::country)
(10 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
   ->  Gather Merge
         Output: c, (PARTIAL count(*))
         Workers Planned: 4
         ->  Sort
               Output: c, (PARTIAL count(*))
               Sort Key: parallel_test.c
               ->  Partial HashAggregate
                     Output: c, PARTIAL count(*)
                     Group Key: parallel_test.c
                     ->  Parallel Seq Scan on public.parallel_test
                           Output: i, c
(14 rows)

ROLLBACK;