BEGIN;
SET max_parallel_workers_per_gather=4;
SET force_parallel_mode=on;
CREATE TABLE parallel_test(i int, data ajbool) WITH (parallel_workers = 4);
INSERT INTO parallel_test
SELECT i, CASE i%3 WHEN 0 THEN 't' WHEN 1 THEN 'f' ELSE 'u' END::ajbool as data
FROM generate_series(1,1e6) i;
EXPLAIN (costs off,verbose)
SELECT COUNT(*) FROM parallel_test WHERE data;
                         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.data
(9 rows)

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

ROLLBACK;