My PostgreSQL Version 13. and below is parallel related parameters.

SELECT name, setting FROM pg_Settings WHERE name LIKE '%parallel%'

name                            |setting|
--------------------------------+-------+
enable_parallel_append          |on     |
enable_parallel_hash            |on     |
force_parallel_mode             |off    |
max_parallel_maintenance_workers|4      |
max_parallel_workers            |96     |
max_parallel_workers_per_gather |2      |
min_parallel_index_scan_size    |64     |
min_parallel_table_scan_size    |1024   |
parallel_leader_participation   |on     |
parallel_setup_cost             |1000   |
parallel_tuple_cost             |0.1    |

When I Run bleow Query, It works good. (only 3 secs)

EXPLAIN (analyze) 
SELECT   t1_code
        ,COUNT(1)  AS cnt
FROM     t1 a
WHERE    1=1
GROUP BY t1_code
      
      
    Finalize GroupAggregate  (cost=620185.13..620185.64 rows=2 width=12) (actual time=2953.797..3186.877 rows=2 loops=1)
      Group Key: t1_code
      ->  Gather Merge  (cost=620185.13..620185.60 rows=4 width=12) (actual time=2953.763..3186.835 rows=6 loops=1)
            Workers Planned: 2
            Workers Launched: 2
            ->  Sort  (cost=619185.11..619185.11 rows=2 width=12) (actual time=2926.805..2926.808 rows=2 loops=3)
                  Sort Key: t1_code
                  Sort Method: quicksort  Memory: 25kB
                  Worker 0:  Sort Method: quicksort  Memory: 25kB
                  Worker 1:  Sort Method: quicksort  Memory: 25kB
                  ->  Partial HashAggregate  (cost=619185.08..619185.10 rows=2 width=12) (actual time=2926.763..2926.768 rows=2 loops=3)
                        Group Key: t1_code
                        Batches: 1  Memory Usage: 24kB
                        Worker 0:  Batches: 1  Memory Usage: 24kB
                        Worker 1:  Batches: 1  Memory Usage: 24kB
                        ->  Parallel Seq Scan on t1 a  (cost=0.00..551015.72 rows=13633872 width=4) (actual time=0.017..1412.845 rows=10907098 loops=3)
    Planning Time: 1.295 ms
    JIT:
      Functions: 21
      Options: Inlining true, Optimization true, Expressions true, Deforming true
      Timing: Generation 2.595 ms, Inlining 156.371 ms, Optimization 112.165 ms, Emission 63.886 ms, Total 335.017 ms
    Execution Time: 3243.358 ms 

But, Without “EXPLAIN Analyze” That Query not use parallel process, when i see the pg_stat_activity. Only 1 process works. so Elapsed time is double. (6 secs)

T1 Table size is 3GB.

Thany for your help.