Re: advancing snapshot's xmin

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