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
> But I can't see how there can be one: Two concurrent vacuums can have
> different OldestXmin values, but two concurrent transactions cannot.
Of course they can.
-- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com
