On Wed, Jan 11, 2017 at 4:05 PM, Robert Haas <robertmh...@gmail.com> wrote: >> While investigating why Rushabh Lathia's Gather Merge patch sometimes >> fails to pick a Gather Merge plan even when it really ought to do so, >> I ran smack into this problem. I discovered that this is more than a >> cosmetic issue. The costing itself is actually badly broken. >> >> The reason why this is happening is that final_cost_nestloop(), >> final_cost_hashjoin(), and final_cost_mergejoin() don't care a whit >> about whether the path they are generating is partial. They apply the >> row estimate for the joinrel itself to every such path generated for >> the join, except for parameterized paths which are a special case. I >> think this generally has the effect of discouraging parallel joins, >> because the inflated row count also inflates the join cost. I think >> the right thing to do is probably to scale the row count estimate for >> the joinrel's partial paths by the leader_contribution value computed >> in cost_seqscan. >> >> Despite my general hatred of back-patching things that cause plan >> changes, I'm inclined to think the fix for this should be back-patched >> to 9.6, because this is really a brown-paper-bag bug. If the >> consensus is otherwise I will of course defer to that consensus. > > And here is a patch which seems to fix the problem.
Since nobody seems to have any comment here, I've committed and back-patched this to 9.6. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
