From: Tom Lane <tgl@sss.pgh.pa.us>
To: "Kevin Viraud" <kevin.viraud@rocket-internet.de>
cc: pgsql-performance@postgresql.org
Subject: Re: Weird CASE WHEN behaviour causing query to be suddenly very slow
In-reply-to: <00a001d06b90$2d1bbe80$87533b80$@rocket-internet.de>
References: <00a001d06b90$2d1bbe80$87533b80$@rocket-internet.de>
Comments: In-reply-to "Kevin Viraud" <kevin.viraud@rocket-internet.de>
 message dated "Tue, 31 Mar 2015 10:53:35 +0200"
Date: Tue, 31 Mar 2015 09:58:30 -0400
Message-ID: <16234.1427810310@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-performance-owner@postgresql.org

"Kevin Viraud" <kevin.viraud@rocket-internet.de> writes:
> I have an issue with a rather large CASE WHEN and I cannot figure out why
> it is so slow...

Do all the arms of the CASE usually fail, leaving you at the ELSE?

I suspect what's happening is that you're running into the MAX_CACHED_RES
limit in src/backend/utils/adt/regexp.c, so that instead of just compiling
each regexp once and then re-using 'em, the regexps are constantly falling
out of cache and then having to be recompiled.  They'd have to be used in
a nearly perfect round robin in order for the behavior to have such a big
cliff as you describe, though.  In this CASE structure, that suggests that
you're nearly always testing every regexp because they're all failing.

I have to think there's probably a better way to do whatever you're trying
to do, but there's not enough info here about your underlying goal to
suggest a better approach.  At the very least, if you need a many-armed
CASE, it behooves you to make sure the common cases appear early.

			regards, tom lane


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