From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Stephen Frost <sfrost(at)snowman(dot)net> |
Cc: | David Rowley <dgrowleyml(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Dilip kumar <dilip(dot)kumar(at)huawei(dot)com> |
Subject: | Re: Allowing join removals for more join types |
Date: | 2014-06-02 15:42:19 |
Message-ID: | 13298.1401723739@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Stephen Frost <sfrost(at)snowman(dot)net> writes:
> * Tom Lane (tgl(at)sss(dot)pgh(dot)pa(dot)us) wrote:
>> TBH I think that trying to do anything at all for inner joins is probably
>> a bad idea. The cases where the optimization could succeed are so narrow
>> that it's unlikely to be worth adding cycles to every query to check.
> I agree that we don't want to add too many cycles to trivial queries but
> I don't think it's at all fair to say that FK-check joins are a narrow
> use-case and avoiding that join could be a very nice win.
[ thinks for a bit... ] OK, I'd been thinking that to avoid a join the
otherwise-unreferenced table would have to have a join column that is both
unique and the referencing side of an FK to the other table's join column.
But after consuming more caffeine I see I got that backwards and it would
need to be the *referenced* side of the FK, which is indeed a whole lot
more plausible case.
regards, tom lane
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