On Sat, 17 Jun 2006, paolo romano wrote:
The original point I was moving is if there were any concrete reason
(which still I can't see) to require Multixacts recoverability (by means
of logging).
Concerning the prepare state of two phase commit, as I was pointing out
in my previous post, shared locks can safely be released once a
transaction gets precommitted, hence they do not have to be made
durable.
No, it's not safe to release them until 2nd phase commit.
Imagine table foo and table bar. Table bar has a foreign key reference to
foo.
1. Transaction A inserts a row to bar, referencing row R in foo. This
acquires a shared lock on R.
2. Transaction A precommits, releasing the lock.
3. Transaction B deletes R. The new row inserted by A is not visible to
B, so the delete succeeds.
4. Transaction A and B commit. Oops, the new row in bar references R that
doesn't exist anymore.
Holding the lock until the true end of transaction, the 2nd phase
of commit, blocks B from deleting R.
- Heikki
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