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Re: advancing snapshot's xmin

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
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 14:59:59
Message-ID: 3803.1206716399@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
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.

			regards, tom lane

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