Re: Using xmin to identify last modified rows

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Richard Broersma <richard(dot)broersma(at)gmail(dot)com>
Cc: Gregory Stark <stark(at)enterprisedb(dot)com>, Stéphane A(dot) Schildknecht <stephane(dot)schildknecht(at)postgresqlfr(dot)org>, Postgres General <pgsql-general(at)postgresql(dot)org>
Subject: Re: Using xmin to identify last modified rows
Date: 2009-02-25 16:26:33
Message-ID: 26861.1235579193@sss.pgh.pa.us
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Richard Broersma <richard(dot)broersma(at)gmail(dot)com> writes:
> On Wed, Feb 25, 2009 at 5:21 AM, Gregory Stark <stark(at)enterprisedb(dot)com> wrote:
>> I'm not sure using xmin is such a great idea really. It's handy for ad-hoc
>> queries but there are all kinds of cases where it might not give you the
>> results you expect.

> Its been a while since the following emails were written. Has the
> treatment of xmin changed since then, or is using a timestamp a better
> practice?

> http://archives.postgresql.org/pgsql-novice/2007-02/msg00079.php
> http://archives.postgresql.org/pgsql-hackers/2004-02/msg00654.php

Those statements are all still true, but notice that nowhere do they
suggest doing anything except simple equality comparisons on XIDs.
The OP was looking for ordering, which is a lot trickier, especially
if you might be dealing with old (frozen) tuples.

regards, tom lane

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