postgres=# explain analyze select * from test_csv where id % 100 = 56;
                                                        QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------
 Foreign Scan on test_csv  (cost=0.00..2158874.49 rows=94056 width=52) (actual time=0.353..17566.875 rows=200000 loops=1)
   Filter: ((id % 100) = 56)
   Rows Removed by Filter: 19800000
   Foreign File: /tmp/testdata.csv
   Foreign File Size: 1504892535
 Planning time: 1.202 ms
 Execution time: 17573.572 ms
(7 rows)

postgres=# SET max_parallel_degree = 1;
SET
postgres=# explain analyze select * from test_csv where id % 100 = 56;
                                                               QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=1000.00..194108.60 rows=94056 width=52) (actual time=1.756..18085.642 rows=400000 loops=1)
   Number of Workers: 1
   ->  Parallel Foreign Scan on test_csv  (cost=0.00..183703.00 rows=94056 width=52) (actual time=0.300..18020.790 rows=200000 loops=2)
         Filter: ((id % 100) = 56)
         Rows Removed by Filter: 19800000
         Foreign File: /tmp/testdata.csv
         Foreign File Size: 1504892535
 Planning time: 0.143 ms
 Execution time: 18098.070 ms
(9 rows)

postgres=# SET max_parallel_degree = 2;
SET
postgres=# explain analyze select * from test_csv where id % 100 = 56;
                                                               QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=1000.00..194108.60 rows=94056 width=52) (actual time=0.835..18663.106 rows=400000 loops=1)
   Number of Workers: 2
   ->  Parallel Foreign Scan on test_csv  (cost=0.00..183703.00 rows=94056 width=52) (actual time=0.513..14600.749 rows=133333 loops=3)
         Filter: ((id % 100) = 56)
         Rows Removed by Filter: 13200000
         Foreign File: /tmp/testdata.csv
         Foreign File Size: 1504892535
 Planning time: 0.134 ms
 Execution time: 18676.078 ms
(9 rows)

postgres=# SET max_parallel_degree = 3;
SET
postgres=# explain analyze select * from test_csv where id % 100 = 56;
                                                               QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=1000.00..194108.60 rows=94056 width=52) (actual time=0.824..18817.112 rows=400000 loops=1)
   Number of Workers: 3
   ->  Parallel Foreign Scan on test_csv  (cost=0.00..183703.00 rows=94056 width=52) (actual time=0.511..12731.459 rows=100000 loops=4)
         Filter: ((id % 100) = 56)
         Rows Removed by Filter: 9900000
         Foreign File: /tmp/testdata.csv
         Foreign File Size: 1504892535
 Planning time: 0.120 ms
 Execution time: 18830.597 ms
(9 rows)

postgres=# SET max_parallel_degree = 4;
SET
postgres=# explain analyze select * from test_csv where id % 100 = 56;
                                                              QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=1000.00..194108.60 rows=94056 width=52) (actual time=0.851..21002.776 rows=400000 loops=1)
   Number of Workers: 4
   ->  Parallel Foreign Scan on test_csv  (cost=0.00..183703.00 rows=94056 width=52) (actual time=4.608..13590.657 rows=80000 loops=5)
         Filter: ((id % 100) = 56)
         Rows Removed by Filter: 7920000
         Foreign File: /tmp/testdata.csv
         Foreign File Size: 1504892535
 Planning time: 0.133 ms
 Execution time: 21015.842 ms
(9 rows)

postgres=# SET max_parallel_degree = 5;
SET
postgres=# explain analyze select * from test_csv where id % 100 = 56;
                                                               QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=1000.00..194108.60 rows=94056 width=52) (actual time=0.878..22852.304 rows=400000 loops=1)
   Number of Workers: 5
   ->  Parallel Foreign Scan on test_csv  (cost=0.00..183703.00 rows=94056 width=52) (actual time=11.043..14634.342 rows=66667 loops=6)
         Filter: ((id % 100) = 56)
         Rows Removed by Filter: 6600000
         Foreign File: /tmp/testdata.csv
         Foreign File Size: 1504892535
 Planning time: 0.122 ms
 Execution time: 22865.496 ms
(9 rows)