Re: [RFC] GSoC Work on readonly queries done so far

From: Jeff Davis <pgsql(at)j-davis(dot)com>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: "Florian G(dot) Pflug" <fgp(at)phlo(dot)org>, Postgresql-Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [RFC] GSoC Work on readonly queries done so far
Date: 2007-06-07 00:48:28
Message-ID: 1181177308.7660.176.camel@dogma.v10.wvs
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On Wed, 2007-06-06 at 22:36 +0100, Simon Riggs wrote:
> > .) Transactions are assigned a dummy xid ReadOnlyTransactionId, that
> > is considered to be "later" than any other xid.
>
> So you are bumping FirstNormalTransactionId up by one for this?
>
> You're assuming then that we will "freeze" replay while we run a query?
> Otherwise doing this will mean the snapshot changes as a query executes.

Is it possible to put a normal xmax for the snapshot?

It wouldn't be a real transaction on the slave, and also the master will
use that ID for a real transaction itself. However, I don't see a real
problem on the slave because it would only be used for the purpose of
the snapshot we need at that moment.

Regards,
Jeff Davis

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