On Sat, Sep 1, 2012 at 8:47 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> [ sorry for slow response, but I'd not gotten time to think about this... ]
>
> Sam Ross <elliptic(at)gmail(dot)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