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Re: I/O on select count(*)

From: Greg Smith <gsmith(at)gregsmith(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: I/O on select count(*)
Date: 2008-05-15 01:39:59
Message-ID: Pine.GSO.4.64.0805142133250.27290@westnet.com (view raw or flat)
Thread:
Lists: pgsql-performance
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(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD

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Subject: Re: I/O on select count(*)
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Subject: poor row estimates with multi-column joins

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