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Re: Improvement of procArray.xmin for VACUUM

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Bruce Momjian <bruce(at)momjian(dot)us>
Cc: Simon Riggs <simon(at)2ndquadrant(dot)com>, Heikki Linnakangas <heikki(at)enterprisedb(dot)com>, PostgreSQL-patches <pgsql-patches(at)postgresql(dot)org>
Subject: Re: Improvement of procArray.xmin for VACUUM
Date: 2007-03-27 01:54:01
Message-ID: 9695.1174960441@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-patches
Bruce Momjian <bruce(at)momjian(dot)us> writes:
> Tom Lane wrote:
>> ... but we won't come out ahead unless advancing xmin
>> intra-transaction really helps, and I'm not sure I believe that (except
>> in the special case of VACUUM, and we already have a solution for that,
>> which would be independent of this).

> The improvement is going to be a win for multi-statement transactions
> --- the only question is how often are they long-running.

Uh, no, that's not very clear.  A long-running transaction will be a
VACUUM bottleneck because of its own XID, never mind its xmin.  To make
this helpful, you have to posit a lot of overlapping long-running
transactions (such that the distance back to GlobalXmin might average
about twice the distance back to the oldest live XID).  That's not
impossible but I wonder whether it's not mostly a token of bad
application design.

> It does seem best to put this on the TODO for 8.4, and I will do that
> now.

Agreed.  Quite aside from the time needed for a reasonable
implementation, we'd really need to do more performance-testing than we
have time for now.

			regards, tom lane

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