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Re: Transaction Snapshot Cloning

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Chris Browne <cbbrowne(at)acm(dot)org>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Transaction Snapshot Cloning
Date: 2008-03-28 02:36:34
Message-ID: 200803280236.m2S2aY816209@momjian.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Added to TODO:

* Allow one transaction to see tuples using the snapshot of another
  transaction

  This would assist multiple backends in working together.
  http://archives.postgresql.org/pgsql-hackers/2008-01/msg00400.php


---------------------------------------------------------------------------

Chris Browne wrote:
> simon(at)2ndquadrant(dot)com (Simon Riggs) writes:
> > On Fri, 2008-01-11 at 20:39 +0000, Simon Riggs wrote:
> >> On Fri, 2008-01-11 at 15:05 -0500, Tom Lane wrote:
> >> Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> >> > > If we had a function 
> >> > > 	replace_serializable_snapshot(master_xid, txid_snapshot)
> >> > > this would allow us to use the txid_snapshot values to replace our
> >> > > transaction's serializable snapshot.
> >> > 
> >> > ... whereupon we'd get wrong answers.  Certainly you could not allow
> >> > transaction xmin to go backwards, and I'm not sure what other
> >> > restrictions there would be, but the whole thing gives me the willies.
> >
> > Sorry, forgot to add
> > - global xmin isn't going backwards
> > - neither is latest completed xid
> >
> > The xmin of the transaction will go backwards, but as long as we don't
> > do anything prior to the setting of the cloned snapshot, what can go
> > wrong? :-)
> 
> Note that we required that the "provider transaction" have the
> attributes IsXactIsoLevelSerializable and XactReadOnly both being
> true, so we have the mandates that the resultant backend process:
> 
> a) Is in read only mode, and
> b) Is in serializable mode.
> 
> That's a pair of (possibly stretching-wide!) suspenders worth of
> support from Evil...
> -- 
> (format nil "~S(at)~S" "cbbrowne" "cbbrowne.com")
> http://linuxfinances.info/info/internet.html
> Trying  to be happy  is like trying to build   a machine for which the
> only specification is that it should run noiselessly.
> 
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-- 
  Bruce Momjian  <bruce(at)momjian(dot)us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +

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