Re: Transaction Snapshot Cloning

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

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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

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