Date: Wed, 16 May 2007 08:44:20 -0400
From: Alvaro Herrera <alvherre@commandprompt.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Chris Browne <cbbrowne@acm.org>, pgsql-patches@postgresql.org
Subject: Re: [DOCS] Autovacuum and XID wraparound
Message-ID: <20070516124420.GA4582@alvh.no-ip.org>
References: <20070514011740.GH14860@fetter.org>
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	<60sl9z6tln.fsf@dba2.int.libertyrms.com>
	<20070514221619.GC8916@alvh.no-ip.org>
	<857.1179184222@sss.pgh.pa.us>
	<20070515221347.GT12731@alvh.no-ip.org>
	<23362.1179292856@sss.pgh.pa.us>
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Tom Lane wrote:
> Alvaro Herrera <alvherre@commandprompt.com> writes:
> > Tom Lane wrote:
> >> I had the idea we were doing that already --- at least I'm pretty sure I
> >> remember it being discussed.  But I see it's not being done in HEAD.
> 
> > Patch to do it attached.  I am thinking we can do something similar in
> > CLUSTER as well.
> 
> Umm ... you'd have to be a lot more conservative in CLUSTER now that
> it's MVCC-safe.  I don't say that CLUSTER can't push up relfrozenxid,
> but there's something wrong if CLUSTER and TRUNCATE are trying to
> push it up the same amount.

No, TRUNCATE will use RecentXmin while the CLUSTER patch I posted uses
OldestXmin, which is what the HeapTupleSatisfiesUpdate test was using.
However, given that Heikki just confirmed that CLUSTER does not freeze
tuples, it's not really possible to do this, so I'll drop the CLUSTER
patch for now.

This means that people using CLUSTER to compact tables won't have the
benefit of advancing relfrozenxid, so they will have to run VACUUM on
those tables at some point anyway, even though there will be no dead
tuples :-(

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.