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Re: advancing snapshot's xmin

From: Simon Riggs <simon(at)2ndquadrant(dot)com>
To: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, 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 14:47:40
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Fri, 2008-03-28 at 11:26 -0300, Alvaro Herrera wrote:
> Simon Riggs wrote:
> > On Fri, 2008-03-28 at 10:35 -0300, Alvaro Herrera wrote:
> > 
> > > The problem is that we always consider every transaction's PGPROC->xid
> > > in calculating MyProc->xmin.  So if you have a long running
> > > transaction, it doesn't matter how far beyond the snapshots are -- the
> > > value returned by GetOldestXmin will always be at most the old
> > > transaction's Xid.  Even if that transaction cannot see the old rows
> > > because all of its snapshots are way in the future.
> > 
> > It may not have a TransactionId yet.
> How is this a problen?  If it ever gets one, it will be in the future.

Yeh, that was my point. So the problem you mention mostly goes away.

  Simon Riggs

  PostgreSQL UK 2008 Conference:

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