Re: Deriving Recovery Snapshots

On Tue, 2008-10-14 at 18:50 +0100, Simon Riggs wrote:
> I've worked out what I think is a workable, efficient process for
> deriving snapshots during recovery. I will be posting a patch to show
> how this works tomorrow [Wed 15 Oct], just doing cleanup now.

How will this interact with an idea like this?:
http://archives.postgresql.org/pgsql-hackers/2008-01/msg00400.php

> I've had to change the way XidInMVCCSnapshot() works. We search the
> snapshot even if it has overflowed. This is actually a performance win
> in cases where only a few xids have overflowed but most haven't. This is
> essential because if we were forced to check in subtrans *and*
> unobservedxids existed then the snapshot would be invalid. (I could have
> made it this way *just* in recovery, but the change seems better both
> ways).

I don't entirely understand this. Can you explain the situation that
would result in an invalid snapshot?

Regards,Jeff Davis