On Sat, Sep 1, 2012 at 8:47 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > [ sorry for slow response, but I'd not gotten time to think about this... ] > > Sam Ross <ellip...@gmail.com> writes: >> I was wondering why it seems that the query planner can't "see", based >> on the histograms, that two join-columns have a very small >> intersection, and adjust its row estimation accordingly. > > The reason why not is that eqjoinsel() doesn't take any such > consideration into account. It's possible that it'd be a good idea > to teach it to do so. I'm not entirely convinced though. It would > add a fair amount of expense to that function, as well as adding > some possibly shaky assumptions, and I'm not sure how often we'd > get a usefully-better estimate in practice. OTOH, there are a lot > of shaky assumptions in eqjoinsel() already, and we did decide this > was worth worrying about in mergejoin cost estimation. > > Do you want to try it and submit a patch for testing? > > regards, tom lane
Thanks for the answer, and sorry for the slow reply - I'm not sure I have the necessary expertise, but I'll be happy to give it a shot. Is there an already-assembled library of queries that is used to test purported improvements to the planner, or is it expected that I come up with a convincing test-set myself? Sam -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
