From: | Greg Smith <gsmith(at)gregsmith(dot)com> |
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To: | Gunther Mayer <gunther(dot)mayer(at)googlemail(dot)com> |
Cc: | pgsql-performance(at)postgresql(dot)org |
Subject: | Re: VERY slow queries at random |
Date: | 2007-06-07 19:42:41 |
Message-ID: | Pine.GSO.4.64.0706071529380.2676@westnet.com |
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Lists: | pgsql-performance |
On Thu, 7 Jun 2007, Gunther Mayer wrote:
> wal checkpoint config is on pg defaults everywhere, all relevant config
> options are commented out. I'm no expert in wal stuff but I don't see how
> that could cause the problem?
Checkpoints are very resource intensive and can cause other processes
(including your selects) to hang for a considerable period of time while
they are processing. With the default parameters, they can happen very
frequently. Normally checkpoint_segments and checkpoint_timeout are
increased in order to keep this from happening.
This would normally be an issue only if you're writing a substantial
amount of data to your tables. If there are a lot of writes going on, you
might get some improvement by adjusting those parameters upward; the
defaults are pretty low. Make sure you read
http://www.postgresql.org/docs/8.2/static/wal-configuration.html first so
you know what you're playing with, there are some recovery implications
invoved.
--
* Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD
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