Date: Mon, 21 Jul 2014 18:48:57 +0200
From: Andres Freund <andres@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Bruce Momjian <bruce@momjian.us>,
 Alvaro Herrera <alvherre@2ndquadrant.com>,
 PostgreSQL Bugs <pgsql-bugs@postgresql.org>
Subject: Re: pg_upgrade < 9.3 -> >=9.3 misses a step around multixacts
Message-ID: <20140721164857.GO5974@alap3.anarazel.de>
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On 2014-07-21 12:43:24 -0400, Tom Lane wrote:
> Andres Freund <andres@2ndquadrant.com> writes:
> > I'm not wondering so much about vac_update_relstats(). There indeed the
> > calls aren't new and the worst that can happen is a slightly older
> > freeze limit. I'm more wondering about vac_update_datfrozenxid(). Afaics
> > we very well could hit
> > 	newFrozenXid = lastSaneFrozenXid = GetOldestXmin(NULL, true);
> > 	newMinMulti = lastSaneMinMulti = GetOldestMultiXactId();
> > for a relation that has just been vacuumed by another backend.
>
> Hmm ... I see.  The issue is not what the computed minimum datfrozenxid
> etc should be; it's right to err in the backwards direction there.
> It's whether we want to declare that the calculation is bogus and abandon
> truncation if another session manages to sneak in a very-new relfrozenxid.
> Yeah, you're right, we need to be conservative about doing that.  I'd
> wanted to avoid extra calls here but I guess we have to do them after all.
> Will fix.

I wonder if GetTopTransactionId()/MultiXactIdSetOldestMember() and using
lastSane* = ReadNew* isn't sufficient. After the xid assignment
concurrent GetOldest* can't go below the ReadNew* values anymore, right?

Greetings,

Andres Freund

--
 Andres Freund	                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services


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