On 6 April 2017 at 11:33, Tom Lane <t...@sss.pgh.pa.us> wrote: > David Rowley <david.row...@2ndquadrant.com> writes: >> On 6 April 2017 at 10:48, Tom Lane <t...@sss.pgh.pa.us> wrote: >>> The buildfarm is unhappy about the fact that this changed the API >>> for clauselist_selectivity(). I am not convinced that that change >>> was a good idea, so before telling FDW authors that they need to >>> change their code, I'd like to hear a defense of the API change. > >> Because varReliId is often passed in as 0, and that meant we'd have to >> write some code to check of the clause was made up of RestrictInfos >> from a single relation or not, and look for extended stats on that >> singleton rel. > > Generally, if it's passed as zero, that's a good clue that the clause > *is* a join clause. In any case, this defense fails to address my > other question, which is what's going to happen to this API when you > want to use extended stats in join-clause estimates, which I'd expect > to surely happen before very long. > > Also, I find it hard to believe that a bms_get_singleton_member call is > going to be material in comparison to all the work that will be invoked > indirectly via whatever selectivity estimation function gets called for > each clause. Even a single catcache fetch would swamp that. > > So no, you have not convinced me that this isn't a broken design. > >> FWIW, I found this function being called 72 times in a 5 way join >> search problem. > > And you measured the overhead of doing it the other way to be ... ? > Premature optimization and all that.
I tested with the attached, and it does not seem to hurt planner performance executing: explain select * from ab ab1 inner join ab ab2 on ab1.a = ab2.a and ab1.b = ab2.b inner join ab ab3 on ab1.a = ab3.a and ab1.b = ab3.b inner join ab ab4 on ab1.a = ab4.a and ab1.b = ab4.b inner join ab ab5 on ab1.a = ab5.a and ab1.b = ab5.b inner join ab ab6 on ab1.a = ab6.a and ab1.b = ab6.b inner join ab ab7 on ab1.a = ab7.a and ab1.b = ab7.b inner join ab ab8 on ab1.a = ab8.a and ab1.b = ab8.b; after having executed: create table ab (a int, b int); I get: find_relation_from_clauses tps = 48.992918 (excluding connections establishing) tps = 49.060407 (excluding connections establishing) tps = 49.075815 (excluding connections establishing) Master tps = 48.938027 (excluding connections establishing) tps = 48.066274 (excluding connections establishing) tps = 48.727089 (excluding connections establishing) running pgbench -n -T 60 -f 8wayjoin.sql -- David Rowley http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
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