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