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Re: VERY slow queries at random

From: Greg Smith <gsmith(at)gregsmith(dot)com>
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 (view raw or flat)
Thread:
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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