Re: Transaction Snapshot Cloning

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@2ndquadrant.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? :-)

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