From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | "Kevin Viraud" <kevin(dot)viraud(at)rocket-internet(dot)de> |
Cc: | pgsql-performance(at)postgresql(dot)org |
Subject: | Re: Weird CASE WHEN behaviour causing query to be suddenly very slow |
Date: | 2015-03-31 13:58:30 |
Message-ID: | 16234.1427810310@sss.pgh.pa.us |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-performance |
"Kevin Viraud" <kevin(dot)viraud(at)rocket-internet(dot)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
From | Date | Subject | |
---|---|---|---|
Next Message | Kevin Viraud | 2015-03-31 15:58:57 | Re: Weird CASE WHEN behaviour causing query to be suddenly very slow |
Previous Message | Pavel Stehule | 2015-03-31 09:41:48 | Re: Weird CASE WHEN behaviour causing query to be suddenly very slow |