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Re: The vacuum-ignore-vacuum patch

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgresql(dot)org, Hannu Krosing <hannu(at)skype(dot)net>
Subject: Re: The vacuum-ignore-vacuum patch
Date: 2006-07-29 02:16:39
Message-ID: 20060729021639.GA6616@surnet.cl (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
Tom Lane wrote:
> Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> > Tom Lane wrote:
> >> But the patch changes things so that *everyone* excludes the vacuum from
> >> their xmin.  Or at least I thought that was the plan.
> 
> > We shouldn't do that, because that Xmin is also used to truncate
> > SUBTRANS.
> 
> Yeah, but you were going to change that, no?  Truncating SUBTRANS will
> need to include the vacuum xact's xmin, but we don't need it for any
> other purpose.

That's correct.

> > but it means
> > lazy vacuum will never be able to use subtransactions.
> 
> This patch already depends on the assumption that lazy vacuum will never
> do any transactional updates, so I don't see what it would need
> subtransactions for.

Here is a patch pursuant to there ideas.  The main change is that in
GetSnapshotData, a backend is skipped entirely if inVacuum is found to
be true.

I've been trying to update my SSH CVS several times today but I can't
reach the server.  Maybe it's the DoS attach that it's been under, I
don't know.

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

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