From: Tom Lane <tgl@sss.pgh.pa.us>
To: Tomasz Chmielewski <mangoo@wpkg.org>
Cc: Scott Marlowe <scott.marlowe@gmail.com>
Cc: Tatsuo Ishii <t-ishii@sra.co.jp>
Cc: Kevin Grittner <Kevin.Grittner@wicourts.gov>
Cc: pgsql-hackers@postgresql.org
Subject: pgpool versus sequences (was Re: [ADMIN] 'SGT DETAIL: Could not open file "pg_clog/05DC": No such file or directory' - what to do now?)
Date: Tue, 31 May 2011 09:33:34 -0400
Message-ID: <25524.1306848814@sss.pgh.pa.us> (raw)
In-Reply-To: <4DE4A652.2040301@wpkg.org>
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<BANLkTikG3yN0xB_wTy1SVYOrA9E=FNBZLg@mail.gmail.com>
<4DC1B378.6050105@wpkg.org>
<4DC1B6C0.2000507@wpkg.org>
<4DC3B469.80004@wpkg.org>
<4DC5B754.3040506@wpkg.org>
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<16063.1306811813@sss.pgh.pa.us>
<4DE4A652.2040301@wpkg.org>
Tomasz Chmielewski <mangoo@wpkg.org> writes:
> On 31.05.2011 05:16, Tom Lane wrote:
>> 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.
> I grepped the sources of the application using postgres, and it certainly doesn't do it.
> [ but pgpool does, as of a couple months ago ]
> This is a message explaining why it was introduced to pgpool:
> http://comments.gmane.org/gmane.comp.db.postgresql.pgpool.devel/348
Too bad that wasn't mentioned on pgsql-hackers, where someone might have
pointed out the major flaws in the idea.
> 2) is pgpool behaviour correct?
No. Quite aside from the lack-of-XID-maintenance problem, the proposal
seems just plain bizarre to me. SELECT FOR UPDATE wouldn't block
nextval(), so the command doesn't actually guarantee serialization of
sequence value acquisition. Taking a table lock on the sequence could
do so, and wouldn't run into any implementation issues, so I fail to see
why that alternative was rejected. I'm also wondering a bit how one
determines *which* sequence to lock, in a case where the table has
multiple serial columns ...
regards, tom lane
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Cc: tgl@sss.pgh.pa.us, mangoo@wpkg.org, scott.marlowe@gmail.com, t-ishii@sra.co.jp, Kevin.Grittner@wicourts.gov, pgsql-hackers@postgresql.org
Subject: Re: pgpool versus sequences (was Re: [ADMIN] 'SGT DETAIL: Could not open file "pg_clog/05DC": No such file or directory' - what to do now?)
In-Reply-To: <25524.1306848814@sss.pgh.pa.us>
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