On 16/03/2016 18:42, Robert Haas wrote: > On Wed, Mar 16, 2016 at 1:23 PM, Julien Rouhaud > <julien.rouh...@dalibo.com> wrote: >> On 16/03/2016 17:55, Robert Haas wrote: >>> On Wed, Mar 16, 2016 at 12:47 PM, Julien Rouhaud >>> <julien.rouh...@dalibo.com> wrote: >>>> Something like a "min_parallel_degree" then ? >>> >>> Why not just parallel_degree without any prefix? As in, when scanning >>> this table in parallel, the reloption suggests using N workers. >>> >> >> Agreed. >> >> PFA v2 that implements that. > > I think create_parallel_paths shouldn't actually run the loop if the > reloption is specified; it should just adopt the specified value (or > max_parallel_degree, whichever is less). Right now, you have it doing > the work to compute the default value but then overriding it. >
After a second look at this part: /* * Limit the degree of parallelism logarithmically based on the size of the * relation. This probably needs to be a good deal more sophisticated, but we * need something here for now. */ while (rel->pages > parallel_threshold * 3 && parallel_degree < max_parallel_degree) { parallel_degree++; parallel_threshold *= 3; if (parallel_threshold >= PG_INT32_MAX / 3) break; } Shouldn't we also check "parallel_degree < max_worker_process" ? There's no need to compute any further than that. I think the best fix would be to add a CheckHook or AssignHook on max_parallel_degree GUC to make sure it's not more than max_worker_process. -- Julien Rouhaud http://dalibo.com - http://dalibo.org -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers