Date: Wed, 14 May 2008 21:39:59 -0400 (EDT)
From: Greg Smith <gsmith@gregsmith.com>
To: pgsql-performance@postgresql.org
Subject: Re: I/O on select count(*)
In-Reply-To: <482B1D5F.EE98.0025.0@wicourts.gov>
Message-ID: <Pine.GSO.4.64.0805142133250.27290@westnet.com>
References: <920841.71690.qm@web44808.mail.sp1.yahoo.com>
	<482B1D5F.EE98.0025.0@wicourts.gov>
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On Wed, 14 May 2008, Kevin Grittner wrote:

> If this is the first time that the rows are being read since they were
> inserted (or since the database was loaded, including from backup), it
> may be rewriting the rows to set hint bits, which can make subsequent
> access faster.

This is the second time this has come up recently, and I know it used to 
puzzle me too.  This is a particularly relevant area to document better 
for people doing benchmarking.  As close I've found to a useful commentary 
on this subject is the thread at 
http://archives.postgresql.org/pgsql-patches/2005-07/msg00390.php

I still don't completely understand this myself though, if I did I'd add a 
FAQ on it.  Anyone want to lecture for a minute on the birth and care of 
hint bits?  I'll make sure any comments here get onto the wiki.

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD