On Tue, Mar 15, 2016 at 8:45 PM, David Rowley <david.row...@2ndquadrant.com> wrote: > On 16 March 2016 at 13:26, Julien Rouhaud <julien.rouh...@dalibo.com> wrote: >> On 15/03/2016 21:12, Robert Haas wrote: >>> I thought about this a bit more. There are a couple of easy things we >>> could do here. >>> >>> The 1000-page threshold could be made into a GUC. >>> >>> We could add a per-table reloption for parallel-degree that would >>> override the calculation. >>> >>> Neither of those things is very smart, but they'd probably both help >>> some people. If someone is able to produce a patch for either or both >>> of these things *quickly*, we could possibly try to squeeze it into >>> 9.6 as a cleanup of work already done. >>> >> >> I'm not too familiar with parallel planning, but I tried to implement >> both in attached patch. I didn't put much effort into the >> parallel_threshold GUC documentation, because I didn't really see a good >> way to explain it. I'd e happy to improve it if needed. Also, to make >> this parameter easier to tune for users, perhaps we could divide the >> default value by 3 and use it as is in the first iteration in >> create_parallel_path() ? >> >> Also, global max_parallel_degree still needs to be at least 1 for the >> per table value to be considered. > > Thanks for working on this. I've only skimmed the patch so far, but > will try to look more closely later. > > This did get me wondering why we have the parallel_threshold at all, > and not just allow the parallel_setup_cost to make parallel plans look > less favourable for smaller relations. I assume that this is so that > we don't burden the planner with the overhead of generating parallel > paths for smaller relations?
Right. And, also, we need some heuristic for judging how many workers to deploy. parallel_setup_cost is of no use in making that decision. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers