I've upped max_worker_processes to 16, but I still can't get it to launch workers unless I use EXPLAIN ANALYZE. I've also found that this simplified setup exhibits the same behavior:
create table big as ( SELECT generate_series(1,30000000) AS id ); explain analyze SELECT avg(id) from big where id % 17 = 0; With explain analyze, the plan includes: Workers Planned: 4 Workers Launched: 4 But without the explain analyze, it never launches workers: Workers Planned: 4 Workers Launched: 0 I've tried this on a CentOS VM (VirtualBox on Windows) that I gave 2 cores, and it worked as expected (it launched workers with and without explain analyze), so I've only been able to reproduce this on Windows. Thanks, Jay K On Thu, Sep 29, 2016 at 9:07 PM David Rowley <david.row...@2ndquadrant.com> wrote: > On 30 September 2016 at 10:47, Jay Knight <j...@jayknight.com> wrote: > >>What's max_worker_processes set to? > > > > 8 > > > >>One theory would be that, the worker might not have been available > >>when you performed the query execution, but it just happened to be > >>when you did the EXPLAIN ANALYZE > > > > This happens consistently this way. Every time I run it with explain > > analyze it uses parallel workers, and every time I run it without it > > doesn't. I just enabled auto_explain, and see that it is "planning" to > use > > a worker, but doesn't launch it. Is there a way to know why a planned > > worker doesn't get launched? > > It seems not, at least not unless you're willing to attach a debugger. > > > 2016-09-29 16:45:44 CDT LOG: duration: 50703.595 ms plan: > > Query Text: select count(*) from t1 > > join t2 on t1.item = t2.item and t1.low <= t2.high and t1.high >= > t2.low > > Finalize Aggregate (cost=14609058.99..14609059.00 rows=1 width=8) > > (actual time=50703.584..50703.584 rows=1 loops=1) > > -> Gather (cost=14609058.88..14609058.99 rows=1 width=8) (actual > > time=50703.581..50703.581 rows=1 loops=1) > > Workers Planned: 1 > > Workers Launched: 0 > > -> Partial Aggregate (cost=14608958.88..14608958.89 rows=1 > > width=8) (actual time=50703.579..50703.579 rows=1 loops=1) > > -> Nested Loop (cost=0.42..13608937.28 rows=400008641 > > width=0) (actual time=0.534..50577.673 rows=3669891 loops=1) > > -> Parallel Seq Scan on t1 (cost=0.00..3386.71 > > rows=176471 width=12) (actual time=0.041..18.351 rows=300000 loops=1) > > -> Index Only Scan using t2_item_low_high_idx > on t2 > > (cost=0.42..63.77 rows=1333 width=12) (actual time=0.167..0.168 rows=12 > > loops=300000) > > Index Cond: ((item = t1.item) AND (low <= > > t1.high) AND (high >= t1.low)) > > Heap Fetches: 0 > > That's interesting. > > How about trying to increase max_worker_processes to say, 16. I know > you've said you've tried multiple times and it seems consistent, but > increasing this seems to be worth a try, if anything, to rule that > out. > > -- > David Rowley http://www.2ndQuadrant.com/ > PostgreSQL Development, 24x7 Support, Training & Services >
