postgres=# explain analyze select * from test_csv where id % 100 = 100;
                                                       QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
 Foreign Scan on test_csv  (cost=0.00..2158874.49 rows=94056 width=52) (actual time=17550.811..17550.811 rows=0 loops=1)
   Filter: ((id % 100) = 100)
   Rows Removed by Filter: 20000000
   Foreign File: /tmp/testdata.csv
   Foreign File Size: 1504892535
 Planning time: 1.175 ms
 Execution time: 17551.011 ms
(7 rows)

postgres=# SET max_parallel_degree = 1;
SET
postgres=# explain analyze select * from test_csv where id % 100 = 100;
                                                              QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=1000.00..194108.60 rows=94056 width=52) (actual time=18054.651..18054.651 rows=0 loops=1)
   Number of Workers: 1
   ->  Parallel Foreign Scan on test_csv  (cost=0.00..183703.00 rows=94056 width=52) (actual time=18048.975..18048.975 rows=0 loops=2)
         Filter: ((id % 100) = 100)
         Rows Removed by Filter: 20000000
         Foreign File: /tmp/testdata.csv
         Foreign File Size: 1504892535
 Planning time: 0.461 ms
 Execution time: 18055.185 ms
(9 rows)

postgres=# SET max_parallel_degree = 2;
SET
postgres=# explain analyze select * from test_csv where id % 100 = 100;
                                                              QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=1000.00..194108.60 rows=94056 width=52) (actual time=18567.518..18567.518 rows=0 loops=1)
   Number of Workers: 2
   ->  Parallel Foreign Scan on test_csv  (cost=0.00..183703.00 rows=94056 width=52) (actual time=14593.974..14593.974 rows=0 loops=3)
         Filter: ((id % 100) = 100)
         Rows Removed by Filter: 13333333
         Foreign File: /tmp/testdata.csv
         Foreign File Size: 1504892535
 Planning time: 0.133 ms
 Execution time: 18567.660 ms
(9 rows)

postgres=# SET max_parallel_degree = 3;
SET
postgres=# explain analyze select * from test_csv where id % 100 = 100;
                                                              QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=1000.00..194108.60 rows=94056 width=52) (actual time=18649.676..18649.676 rows=0 loops=1)
   Number of Workers: 3
   ->  Parallel Foreign Scan on test_csv  (cost=0.00..183703.00 rows=94056 width=52) (actual time=12671.429..12671.429 rows=0 loops=4)
         Filter: ((id % 100) = 100)
         Rows Removed by Filter: 10000000
         Foreign File: /tmp/testdata.csv
         Foreign File Size: 1504892535
 Planning time: 0.132 ms
 Execution time: 18649.819 ms
(9 rows)

postgres=# SET max_parallel_degree = 4;
SET
postgres=# explain analyze select * from test_csv where id % 100 = 100;
                                                              QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=1000.00..194108.60 rows=94056 width=52) (actual time=20619.039..20619.039 rows=0 loops=1)
   Number of Workers: 4
   ->  Parallel Foreign Scan on test_csv  (cost=0.00..183703.00 rows=94056 width=52) (actual time=13606.715..13606.715 rows=0 loops=5)
         Filter: ((id % 100) = 100)
         Rows Removed by Filter: 8000000
         Foreign File: /tmp/testdata.csv
         Foreign File Size: 1504892535
 Planning time: 0.133 ms
 Execution time: 20619.184 ms
(9 rows)

postgres=# SET max_parallel_degree = 5;
SET
postgres=# explain analyze select * from test_csv where id % 100 = 100;
                                                              QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=1000.00..194108.60 rows=94056 width=52) (actual time=22557.451..22557.451 rows=0 loops=1)
   Number of Workers: 5
   ->  Parallel Foreign Scan on test_csv  (cost=0.00..183703.00 rows=94056 width=52) (actual time=14594.420..14594.420 rows=0 loops=6)
         Filter: ((id % 100) = 100)
         Rows Removed by Filter: 6666667
         Foreign File: /tmp/testdata.csv
         Foreign File Size: 1504892535
 Planning time: 0.125 ms
 Execution time: 22557.575 ms
(9 rows)