Andrew Dunstan <and...@dunslane.net> writes: > On 08/06/2015 03:36 PM, Tom Lane wrote: >> Further fixes for degenerate outer join clauses.
> Looks like this might have upset brolga on 9.0 and 9.1 - it's coming up > with a different plan from what's expected. I looked into this, and while I can't be certain of the diagnosis without looking at the assembly code brolga generates (no, I don't want to), I'm pretty sure I know what's going on here. The two plans in question have exactly the same estimated costs on my machine, which results in add_path discarding the second one to arrive. Evidently, on brolga the second path gets some fractionally cheaper estimate due to some compiler-specific variation in the way the arithmetic is done, so it gets kept rather than the first one. In 9.2 and up we got rid of that class of problems with this patch: Author: Tom Lane <t...@sss.pgh.pa.us> Branch: master Release: REL9_2_BR [33e99153e] 2012-04-21 00:51:14 -0400 Use fuzzy not exact cost comparison for the final tie-breaker in add_path. Instead of an exact cost comparison, use a fuzzy comparison with 1e-10 delta after all other path metrics have proved equal. This is to avoid having platform-specific roundoff behaviors determine the choice when two paths are really the same to our cost estimators. Adjust the recently-added test case that made it obvious we had a problem here. but 9.0 and 9.1 still have platform-dependent treatment of paths with essentially equal cost estimates. I'm not entirely sure what to do about this. We could back-patch that patch into 9.0 and 9.1, but it's conceivable somebody would squawk about planner behavioral changes. The only other idea that seems practical is to remove regression test cases that have platform-specific results in those branches. Probably that wouldn't result in a real reduction in the quality of the test coverage for those branches (we could still execute the query, just not EXPLAIN it). But it seems like a pretty ad-hoc answer, and the next case might be one that hurts more not to test. Thoughts? regards, tom lane -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers