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.com PostgreSQL Training, Services and Support -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
