Horton

Re: advancing snapshot's xmin

Simon Riggs <simon@2ndquadrant.com>

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From: Simon Riggs <simon@2ndquadrant.com>
To: Alvaro Herrera <alvherre@commandprompt.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Gregory Stark <stark@enterprisedb.com>, Heikki Linnakangas <heikki@enterprisedb.com>, Neil Conway <neilc@samurai.com>, Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2008-03-28T14:07:55Z
Lists: pgsql-hackers 
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.

So we should have the capability to prevent long running read-only
transactions from causing a build up of dead row versions. But long
running write transactions would still be a problem.

-- 
  Simon Riggs
  2ndQuadrant  http://www.2ndQuadrant.com 

  PostgreSQL UK 2008 Conference: http://www.postgresql.org.uk