On 2017-04-04 08:01:32 -0400, Robert Haas wrote: > On Tue, Apr 4, 2017 at 12:47 AM, Andres Freund <and...@anarazel.de> wrote: > > I don't think the parallel seqscan is comparable in complexity with the > > parallel append case. Each worker there does the same kind of work, and > > if one of them is behind, it'll just do less. But correct sizing will > > be more important with parallel-append, because with non-partial > > subplans the work is absolutely *not* uniform. > > Sure, that's a problem, but I think it's still absolutely necessary to > ramp up the maximum "effort" (in terms of number of workers) > logarithmically. If you just do it by costing, the winning number of > workers will always be the largest number that we think we'll be able > to put to use - e.g. with 100 branches of relatively equal cost we'll > pick 100 workers. That's not remotely sane.
I'm quite unconvinced that just throwing a log() in there is the best way to combat that. Modeling the issue of starting more workers through tuple transfer, locking, startup overhead costing seems a better to me. If the goal is to compute the results of the query as fast as possible, and to not use more than max_parallel_per_XXX, and it's actually beneficial to use more workers, then we should. Because otherwise you really can't use the resources available. - Andres -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers