Simon Riggs wrote:
On Fri, 2009-01-16 at 22:09 +0200, Heikki Linnakangas wrote:
RecentGlobalXmin is just a hint, it lags behind the real oldest xmin
that GetOldestXmin() would return. If another backend has a more recent
RecentGlobalXmin value, and has killed more recent tuples on the page,
the latestRemovedXid written here is too old.
What do you think we should do instead?
Dunno. Maybe call GetOldestXmin().
We are discussing btree deletes, not btree vacuums.
Pardon my ignorance, but what's the difference?
If we are doing
btree delete then we have an unreleased snapshot therefore we also have
a non-zero xmin. How can another backend have a later RecentGlobalXmin
or result from GetOldestXmin() than we do?
Sure it can, for example:
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 4 kills tuple, using its RecentGlobalxmin of 1
9. Transaciont 3 splits the page, emits a delete xlog record, setting
latestRemovedXid to its RecentGlobalXmin of 2
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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