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=0.084..17555.904 rows=1000000 loops=1)
   Filter: ((id % 20) = 6)
   Rows Removed by Filter: 19000000
   Foreign File: /tmp/testdata.csv
   Foreign File Size: 1504892535
 Planning time: 0.122 ms
 Execution time: 17584.319 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=1.435..18406.866 rows=2000000 loops=1)
   Number of Workers: 1
   ->  Parallel Foreign Scan on test_csv  (cost=0.00..183703.00 rows=94056 width=52) (actual time=0.117..18110.968 rows=1000000 loops=2)
         Filter: ((id % 20) = 6)
         Rows Removed by Filter: 19000000
         Foreign File: /tmp/testdata.csv
         Foreign File Size: 1504892535
 Planning time: 0.121 ms
 Execution time: 18464.476 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=0.352..18983.054 rows=2000000 loops=1)
   Number of Workers: 2
   ->  Parallel Foreign Scan on test_csv  (cost=0.00..183703.00 rows=94056 width=52) (actual time=0.114..14580.335 rows=666667 loops=3)
         Filter: ((id % 20) = 6)
         Rows Removed by Filter: 12666667
         Foreign File: /tmp/testdata.csv
         Foreign File Size: 1504892535
 Planning time: 0.091 ms
 Execution time: 19042.755 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=0.502..19257.240 rows=2000000 loops=1)
   Number of Workers: 3
   ->  Parallel Foreign Scan on test_csv  (cost=0.00..183703.00 rows=94056 width=52) (actual time=0.225..12668.912 rows=500000 loops=4)
         Filter: ((id % 20) = 6)
         Rows Removed by Filter: 9500000
         Foreign File: /tmp/testdata.csv
         Foreign File Size: 1504892535
 Planning time: 0.132 ms
 Execution time: 19318.254 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=0.624..21662.065 rows=2000000 loops=1)
   Number of Workers: 4
   ->  Parallel Foreign Scan on test_csv  (cost=0.00..183703.00 rows=94056 width=52) (actual time=5.084..13596.788 rows=400000 loops=5)
         Filter: ((id % 20) = 6)
         Rows Removed by Filter: 7600000
         Foreign File: /tmp/testdata.csv
         Foreign File Size: 1504892535
 Planning time: 0.130 ms
 Execution time: 21732.910 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=0.561..23396.344 rows=2000000 loops=1)
   Number of Workers: 5
   ->  Parallel Foreign Scan on test_csv  (cost=0.00..183703.00 rows=94056 width=52) (actual time=1.726..14533.409 rows=333333 loops=6)
         Filter: ((id % 20) = 6)
         Rows Removed by Filter: 6333333
         Foreign File: /tmp/testdata.csv
         Foreign File Size: 1504892535
 Planning time: 0.118 ms
 Execution time: 23486.846 ms
(9 rows)