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
Date: Tue, 31 Mar 2015 09:58:30 -0400
Message-ID: <16234.1427810310@sss.pgh.pa.us> (raw)
In-Reply-To: <00a001d06b90$2d1bbe80$87533b80$@rocket-internet.de>
References: <00a001d06b90$2d1bbe80$87533b80$@rocket-internet.de>
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"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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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, kevin.viraud@rocket-internet.de
Subject: Re: Weird CASE WHEN behaviour causing query to be suddenly very slow
In-Reply-To: <16234.1427810310@sss.pgh.pa.us>
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