2009/12/8 黄晓骋 <huangxcl...@gmail.com>: > From the above, I think the tuple lock is unnecessary, because it uses > transaction lock. > > Besides, tuple lock is unlocked after the tuple is updated but not after the > transaction commits. I mean it's not 2PL.
It's a two step process. An update marks the tuple locked. Another transaction which comes along and wants to lock the tuple waits on the transaction marked on the tuple. When the first transaction commits or aborts then the second transaction can proceed and lock the tuple itself. The reason we need both locks is because the first transaction cannot go around the whole database finding every tuple it ever locked to unlock it, firstly that could be a very large list and secondly there would be no way to do that atomically. Tuple locks and all user-visible locks are indeed held until the end of the transaction. -- greg -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
