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Re: advancing snapshot's xmin

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Gregory Stark <stark(at)enterprisedb(dot)com>, Heikki Linnakangas <heikki(at)enterprisedb(dot)com>, Neil Conway <neilc(at)samurai(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: advancing snapshot's xmin
Date: 2008-03-28 15:05:09
Message-ID: (view raw or whole thread)
Lists: pgsql-hackers
Tom Lane wrote:
> Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> > As far as I can see, for the purposes of VACUUM we can remove any tuple
> > that was deleted after the old transaction's Xid but before that
> > transaction's Xmin (i.e. all of its live snapshots).  This means we get
> > to ignore Xid in GetOldestXmin and in the TransactionXmin calculations
> > in GetSnapshotData.  It would not surprise me, however, to find out that
> > I am overlooking something and this is incorrect.
> This seems entirely off-base to me.  In particular, if a transaction
> has an XID then its XMIN will never be greater than that, so I don't
> even see how you figure the case will arise.

My point exactly -- can we let the Xmin go past its Xid?  You imply we
can't, but why?

Alvaro Herrera                      
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

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