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Message-ID: <4DE4A652.2040301@wpkg.org>
Date: Tue, 31 May 2011 10:26:58 +0200
From: Tomasz Chmielewski <mangoo@wpkg.org>
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To: Tom Lane <tgl@sss.pgh.pa.us>
CC: Scott Marlowe <scott.marlowe@gmail.com>,
	Kevin Grittner <Kevin.Grittner@wicourts.gov>,
	pgsql-admin@postgresql.org, pgsql-hackers@postgresql.org
Subject: Re: 'SGT DETAIL: Could not open file "pg_clog/05DC": No such
	file or directory' - what to do now?
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On 31.05.2011 05:16, Tom Lane wrote:
> Tomasz Chmielewski<mangoo@wpkg.org>  writes:
>> bookstor=# SELECT 1 FROM core_wot_seq FOR UPDATE;
> 
> Um ... why are you doing that on a sequence?
> 
>> ERROR: could not access status of transaction 1573786613
>> DETAIL: Could not open file "pg_clog/05DC": No such file or directory.
> 
> This doesn't surprise me too much, because sequences are not expected
> to contain any live XIDs, so the XID freezing mechanism ignores them.
> So if you did that in the past, this would eventually happen.
> 
> I think the most appropriate solution may be to disallow SELECT FOR
> UPDATE/SHARE on sequences ... so if you have a good reason why we
> shouldn't do so, please explain it.

That's a good question.

I grepped the sources of the application using postgres, and it certainly doesn't do it.


We use pgpool though, and I see:

pool_process_query.c:		snprintf(qbuf, sizeof(qbuf), "SELECT 1 FROM %s FOR UPDATE", seq_rel_name);


So it looks to be coming from pgpool 3.x (it didn't do it in 2.x version).

This is a message explaining why it was introduced to pgpool:

http://comments.gmane.org/gmane.comp.db.postgresql.pgpool.devel/348


This brings two questions:

1) whatever command I send to postgres, should I expect "Could not open file "pg_clog/05DC": No such file or directory"?
If so, it should be documented, and a way to recover from such a situation should be explained.

2) is pgpool behaviour correct?


-- 
Tomasz Chmielewski
http://wpkg.org