From: Tom Lane <tgl@sss.pgh.pa.us>
To: Scott Marlowe <scott.marlowe@gmail.com>
cc: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: Query plan good in 8.4, bad in 9.2 and better in 9.3
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Comments: In-reply-to Scott Marlowe <scott.marlowe@gmail.com>
 message dated "Thu, 15 May 2014 10:35:11 -0600"
Date: Thu, 15 May 2014 12:52:48 -0400
Message-ID: <28358.1400172768@sss.pgh.pa.us>
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Scott Marlowe <scott.marlowe@gmail.com> writes:
> OK so we have a query that does OK in 8.4, goes to absolute crap in
> 9.2 and then works great in 9.3. Thing is we've spent several months
> regression testing 9.2 and no time testing 9.3, so we can't just "go
> to 9.3" in an afternoon. But we might have to. 9.2 seems hopelessly
> broken here.

> The query looks something like this:

> SELECT COUNT(*) FROM u, ug
> WHERE u.ugid = ug.id
> AND NOT u.d
> AND ug.somefield IN  (SELECT somefunction(12345));

You really should show us somefunction's definition if you want
useful comments.  I gather however that it returns a set.  8.4
seems to be planning on the assumption that the set contains
only one row, which is completely unjustified in general though
it happens to be true in your example.  9.2 is assuming 1000 rows
in the set, and getting a sucky plan because that's wrong.  9.3
is still assuming that; and I rather doubt that you are really
testing 9.3 on the same data, because 9.2 is finding millions of
rows in a seqscan of u while 9.3 is finding none in the exact
same seqscan.

I'd suggest affixing a ROWS estimate to somefunction, or better
declaring it to return singleton not set if that's actually
always the case.

			regards, tom lane


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