To: "Kevin Grittner" <Kevin.Grittner@wicourts.gov>
cc: "Nikolas Everett" <nik9000@gmail.com>, pgsql-performance@postgresql.org
Subject: Re: Why does this query write to the disk? 
In-reply-to: <48D24E6C.EE98.0025.0@wicourts.gov> 
References: <d4e11e980809181030t3f09db94vd5b69d7868271906@mail.gmail.com>
	<48D24E6C.EE98.0025.0@wicourts.gov>
Comments: In-reply-to "Kevin Grittner" <Kevin.Grittner@wicourts.gov>
	message dated "Thu, 18 Sep 2008 12:49:48 -0500"
Date: Thu, 18 Sep 2008 14:13:20 -0400
Message-ID: <23587.1221761600@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

"Kevin Grittner" <Kevin.Grittner@wicourts.gov> writes:
> "Nikolas Everett" <nik9000@gmail.com> wrote: 
>> I'm a bit confused as to why this query writes to the disk:
 
> It's probably writing hint bits to improve performance of subsequent
> access to the table.  The issue is discussed here:
> http://wiki.postgresql.org/wiki/Hint_Bits

Hint-bit updates wouldn't be WAL-logged.  If the table has been around a
long time, it might be freezing old tuples, which *would* be WAL-logged
(since 8.2 or so) --- but that would be a one-time, non-repeatable
behavior.  How sure are you that there was WAL output?

What I was thinking was more likely was that the hash table for the hash
join was spilling out to temp files.  That wouldn't be WAL-logged
either, but depending on your tablespace setup it might result in I/O on
some other disk than the table proper.

			regards, tom lane