postgres=# explain analyze select * from test_csv where id % 20 = 6;
                                                        QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------
 Foreign Scan on test_csv  (cost=0.00..2158874.49 rows=94056 width=52) (actual time=0.079..17564.457 rows=1000000 loops=1)
   Filter: ((id % 20) = 6)
   Rows Removed by Filter: 19000000
   Foreign File: /tmp/testdata.csv
   Foreign File Size: 1504892535
 Planning time: 0.598 ms
 Execution time: 17592.498 ms
(7 rows)

postgres=# SET max_parallel_degree = 1;
SET
postgres=# explain analyze select * from test_csv where id % 20 = 6;
                                                               QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=1000.00..194108.60 rows=94056 width=52) (actual time=0.237..12123.367 rows=1000000 loops=1)
   Number of Workers: 1
   ->  Parallel Foreign Scan on test_csv  (cost=0.00..183703.00 rows=94056 width=52) (actual time=0.469..11983.485 rows=500000 loops=2)
         Filter: ((id % 20) = 6)
         Rows Removed by Filter: 9500000
         Foreign File: /tmp/testdata.csv
         Foreign File Size: 1504892535
 Planning time: 0.053 ms
 Execution time: 12152.998 ms
(9 rows)

postgres=# SET max_parallel_degree = 2;
SET
postgres=# explain analyze select * from test_csv where id % 20 = 6;
                                                               QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=1000.00..194108.60 rows=94056 width=52) (actual time=0.548..10616.097 rows=1000000 loops=1)
   Number of Workers: 2
   ->  Parallel Foreign Scan on test_csv  (cost=0.00..183703.00 rows=94056 width=52) (actual time=1.803..10502.100 rows=333333 loops=3)
         Filter: ((id % 20) = 6)
         Rows Removed by Filter: 6333333
         Foreign File: /tmp/testdata.csv
         Foreign File Size: 1504892535
 Planning time: 0.120 ms
 Execution time: 10647.858 ms
(9 rows)

postgres=# SET max_parallel_degree = 3;
SET
postgres=# explain analyze select * from test_csv where id % 20 = 6;
                                                              QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=1000.00..194108.60 rows=94056 width=52) (actual time=0.184..9603.485 rows=1000000 loops=1)
   Number of Workers: 3
   ->  Parallel Foreign Scan on test_csv  (cost=0.00..183703.00 rows=94056 width=52) (actual time=0.731..9509.899 rows=250000 loops=4)
         Filter: ((id % 20) = 6)
         Rows Removed by Filter: 4750000
         Foreign File: /tmp/testdata.csv
         Foreign File Size: 1504892535
 Planning time: 0.049 ms
 Execution time: 9635.445 ms
(9 rows)

postgres=# SET max_parallel_degree = 4;
SET
postgres=# explain analyze select * from test_csv where id % 20 = 6;
                                                               QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=1000.00..194108.60 rows=94056 width=52) (actual time=0.619..11143.383 rows=1000000 loops=1)
   Number of Workers: 4
   ->  Parallel Foreign Scan on test_csv  (cost=0.00..183703.00 rows=94056 width=52) (actual time=15.336..10863.293 rows=200000 loops=5)
         Filter: ((id % 20) = 6)
         Rows Removed by Filter: 3800000
         Foreign File: /tmp/testdata.csv
         Foreign File Size: 1504892535
 Planning time: 0.136 ms
 Execution time: 11175.456 ms
(9 rows)

postgres=# SET max_parallel_degree = 5;
SET
postgres=# explain analyze select * from test_csv where id % 20 = 6;
                                                               QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=1000.00..194108.60 rows=94056 width=52) (actual time=0.674..12544.940 rows=1000000 loops=1)
   Number of Workers: 5
   ->  Parallel Foreign Scan on test_csv  (cost=0.00..183703.00 rows=94056 width=52) (actual time=23.652..12279.323 rows=166667 loops=6)
         Filter: ((id % 20) = 6)
         Rows Removed by Filter: 3166667
         Foreign File: /tmp/testdata.csv
         Foreign File Size: 1504892535
 Planning time: 0.139 ms
 Execution time: 12586.457 ms
(9 rows)