From: Tom Lane <tgl@sss.pgh.pa.us>
To: =?UTF-8?B?QWxleGlzIEzDqi1RdcO0Yw==?= <alq@datadoghq.com>
cc: pgsql-performance@postgresql.org
Subject: Re: Better performance possible for a pathological query?
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Comments: In-reply-to =?UTF-8?B?QWxleGlzIEzDqi1RdcO0Yw==?= <alq@datadoghq.com>
 message dated "Wed, 07 Aug 2013 11:38:47 -0400"
Date: Wed, 07 Aug 2013 12:07:37 -0400
Message-ID: <6581.1375891657@sss.pgh.pa.us>
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=?UTF-8?B?QWxleGlzIEzDqi1RdcO0Yw==?= <alq@datadoghq.com> writes:
> The query itself is very simple: a primary key lookup on a 1.5x10^7 rows.
> The issue is that we are looking up over 11,000 primary keys at once,
> causing the db to consume a lot of CPU.

It looks like most of the runtime is probably going into checking the
c.key = ANY (ARRAY[...]) construct.  PG isn't especially smart about that
if it fails to optimize the construct into an index operation --- I think
it's just searching the array linearly for each row meeting the other
restrictions on c.

You could try writing the test like this:
    c.key = ANY (VALUES (1), (17), (42), ...)
to see if the sub-select code path gives better results than the array
code path.  In a quick check it looked like this might produce a hash
join, which seemed promising anyway.

			regards, tom lane


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