On Sat, Aug 17, 2019 at 10:41 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
Try this: alter system set max_parallel_workers = 20; and restart the system.
max_parallel_workers is still 8, according to both SHOW and pg_controldata, nor can you launch more than 8 workers.
Even odder, if you just do
regression=# set max_parallel_workers = 200; SET regression=# show max_parallel_workers; max_parallel_workers ---------------------- 200 (1 row)
which should certainly not happen for a PGC_POSTMASTER parameter.
We seem to have an awful lot of mechanism that's concerned with adjustments of max_parallel_workers, for something that apparently might as well be a compile-time #define ... so I assume it's supposed to be changeable at restart and somebody broke it. But it's not working as I'd expect in any branch from 10 onwards.
regards, tom lane
If I understand that correctly it works for me.
postgres=# alter system set max_parallel_workers = 1; $ pg_ctl restart -l log $ psql postgres psql (13devel) postgres=# explain analyze select * from test where b > 1;
QUERY PLAN ----------------------------------------------------------------------------------------------------------------------- Gather (cost=1000.00..98294.84 rows=1 width=8) (actual time=1635.959..1636.028 rows=0 loops=1) Workers Planned: 2 Workers Launched: 1 -> Parallel Seq Scan on test (cost=0.00..97294.74 rows=1 width=8) (actual time=1632.239..1632.239 rows=0 loops=2) Filter: (b > 1) Rows Removed by Filter: 5050000
Planning Time: 0.533 ms Execution Time: 1636.080 ms (8 rows)
postgres=# alter system set max_parallel_workers = 2; ALTER SYSTEM postgres=# \q vagrant@vagrant:~/percona/postgresql$ pg_ctl restart -l log vagrant@vagrant:~/percona/postgresql$ psql postgres psql (13devel) Type "help" for help. postgres=# explain analyze select * from test where b > 1; QUERY PLAN ----------------------------------------------------------------------------------------------------------------------- Gather (cost=1000.00..98294.84 rows=1 width=8) (actual time=1622.210..1622.274 rows=0 loops=1) Workers Planned: 2 Workers Launched: 2 -> Parallel Seq Scan on test (cost=0.00..97294.74 rows=1 width=8) (actual time=1616.000..1616.000 rows=0 loops=3) Filter: (b > 1) Rows Removed by Filter: 3366667 Planning Time: 0.699 ms Execution Time: 1622.325 ms (8 rows)