On Mon, 2009-01-19 at 12:22 +0200, Heikki Linnakangas wrote:
> Simon Riggs wrote:
> > Well, steps 7 and 8 don't make sense.
> >
> > Your earlier comment was that it was possible for a WAL record to be
> > written with a RecentGlobalXmin that was lower than other backends
> > values. In step 9 the RecentGlobalXmin is *not* lower than any other
> > backend, it is the same.
> >
> > So if there is a proof, this isn't it.
>
> Yeah, you're right. I got steps 8 and 9 mixed. Let me try again:
>
> 1. Transaction 1 begins in backend A
> 2. Transaction 2 begins in backend B, xmin = 1
> 3. Transaction 1 ends
> 4. Transaction 3 begins in backend C, xmin = 2
> 5. Backend C gets snapshot, TransactionXmin = 2, RecentGlobalXmin = 1
> 6. Transaction 2 ends.
> 7. Transaction 4 begins in backend A, gets snapshot TransactionXmin = 2,
> RecentGlobalXmin = 2
> 8. Transaction 3 kills tuple, using its RecentGlobalxmin of 2
> 9. Transaction 4 splits the page, emits a delete xlog record, setting
> latestRemovedXid to its RecentGlobalXmin of 1
One of us needs a coffee.
How does Transaction 4 have a RecentGlobalXmin of 2 in step (7), but at
step (9) the value of RecentGlobalXmin has gone backwards?
-- Simon Riggs www.2ndQuadrant.comPostgreSQL Training, Services and Support
