On 5/11/2010 12:39 PM, Florian Pflug wrote:
On May 11, 2010, at 17:04 , Robert Haas wrote:
2010/5/11 Florian Pflug <f...@phlo.org>:
C1: BEGIN
C1: INSERT INTO child (parent_id) VALUES (0)
C2: BEGIN
C2: SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
C2: SELECT TRUE -- Take snapshot *before* C1 commits
C1: COMMIT
C2: DELETE FROM parent WHERE parent_id = 0 -- Works!
C2: COMMIT
It seems that while SHARE-locking a concurrently deleted row causes a
serialization error, deleting a concurrently SHARE-locked is allowed. I do
wonder if this shouldn't be considered a bug - whether locks conflict or not
does not usually depend on the other in which they are taken.
Wait - I'm confused. The DELETE in your example happens after C1
commits, so C1 can't still be holding any locks (nor does C2 take any
locks prior to the commit of C1).
I used the word "lock" a bit sloppy there.
What I did want to point out is that any UPDATE by a SERIALIZABLE transaction
to a row that has been concurrently updated causes a serialization error. The
same happens when it instead SHARE- or UPDATE-locks the concurrently updated
row. This is also independent from the commit-time of the concurrent
transaction, as long as it is deemed invisible by the UPDATE/LOCK-ing
transaction. In other words, any attempt to UPDATE, SHARE-lock or UPDATE-lock a
row from within a SERIALIZABLE transaction fails if the visible row version
isn't the latest row version. If, however, the order of the events is the other
way around, such that the SHARE-locking or UPDATE-locking happens first, and
the UPDATE afterwards, then no serialization error occurs!
That might seem sensible if you view SHARE-locks and UPDATE-locks as locks, and the "taint" that marks a row
(the existence of a newer row version) after it has been updated by a transaction as "something else". After
all, as you pointed out, the lock is gone as soon as the transaction commits. If, however, you view that
"taint" as a slightly strange kind of lock that a transaction holds on the rows it updated even *after* the
transaction committed, then it stops making sense. You now have a "locking" behavior with order-dependent
conflicts.
Viewing those "taints" as locks is consistent with how that true serializability
algorithm Kevin Grittner is working on deals with those things, I believe - or at least it's
probably that paper in the back of my mind that made me call it "lock" in the first place.
It would be interesting to formalize this in the language of that paper - unfortunately, I probably lack the time to do this in the near future :-(
To avoid more confusion, here are the sequences of commands I have in mind:
No serialization error (and neither with "FOR UPDATE" instead of "FOR SHARE")
C1: BEGIN
C1: SELECT t.* FROM t WHERE id = 1 FOR SHARE
C2: BEGIN
C2: SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
C2: SELECT TRUE --Take snapshot before c1 commits
C1: COMMIT
C2: UPDATE t SET id = 2 WHERE id = 1
C2: COMMIT
Serialization error (and also with "FOR UPDATE" instead of "FOR SHARE")
C1: BEGIN
C1: UPDATE t SET id = 2 WHERE id = 1
C2: BEGIN
C2: SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
C2: SELECT TRUE --Take snapshot before c1 commits
C1: COMMIT
C2: SELECT t.* FROM t WHERE id = 1 FOR SHARE --serialization error
C2: COMMIT
The problem really is that in the case of deleting a PK row while a
concurrent transaction creates such a reference cannot be solved with
user level visibility rules in case of a serializable transacton, unless
you go really expensive routes.
One corner case is that the transaction doing the FK INSERT commits
after the serializable transaction doing the PK DELETE got its snapshot
and also does the PK check before the PK DELETE got the lock on it. No
user level visibility allows it to see that newly created reference. And
unless the FK INSERTer actually UPDATE's the PK row (expensive), the PK
DELETE will not throw anything. It will wait to get the lock and go
ahead with the delete.
The PK DELETE needs to be able to do some sort of dirty scan in order to
see those new references. That is what I think Tom was referring to.
Jan
--
Anyone who trades liberty for security deserves neither
liberty nor security. -- Benjamin Franklin
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