On 10.10.2012 23:05, Andres Freund wrote:
> On Wednesday, October 10, 2012 10:43:57 PM Thom Brown wrote:
>> On 10 October 2012 21:21, Tomas Vondra <tv@fuzzy.cz> wrote:
>>> Hi,
>>>
>>> I've just noticed a change of LOCK command behavior between 9.1 and 9.2,
>>> and I'm not sure whether this is expected or not.
>>>
>>> Let's use a very simple table
>>>
>>> CREATE TABLE x (id INT);
>>>
>>> Say there are two sessions - A and B, where A performs some operations
>>> on "x" and needs to protect them with an "ACCESS EXCLUSIVE" lock (e.g.
>>> it might be a pg_bulkload that acquires such locks, and we need to do
>>> that explicitly on one or two places).
>>>
>>> Session B is attempting to read the data, but is blocked and waits. On
>>> 9.1 it sees the commited data (which is what we need) but on 9.2 it sees
>>> only data commited at the time of the lock attemt.
>>>
>>> Example:
>>>
>>> A: BEGIN;
>>> A: LOCK x IN ACCESS EXCLUSIVE MODE;
>>> A: INSERT INTO x VALUES (100);
>>> B: SELECT * FROM x;
>>> A: COMMIT;
>>>
>>> Now on 9.1, B receives the value "100" while on 9.2 it gets no rows.
>>>
>>> Is this expected? I suspect the snapshot is read at different time or
>>> something, but I've checked release notes but I haven't seen anything
>>> relevant.
>>>
>>> Without getting the commited version of data, the locking is somehow
>>> pointless for us (unless using a different lock, not the table itself).
>>
>> I suspect it's this commit: d573e239f03506920938bf0be56c868d9c3416da
>>
>> http://archives.postgresql.org/pgsql-committers/2011-12/msg00167.php
> Very likely, yes. In fact you get the same beaviour in 9.1 if you modify the
> example slightly:
>
> B: PREPARE foo AS SELECT * FROM x;
> A: BEGIN;
> A: LOCK x IN ACCESS EXCLUSIVE MODE;
> A: INSERT INTO x VALUES (100);
> B: EXECUTE foo;
> A: COMMIT;
>
> If you think about it for a second its not that surprising anymore. We start to
> execute a query, acquire a snapshot for that, and then wait for the locks on
> the target relations. We continue executing in the same snapshot for the
> duration of the statement and thus cannot see any of the new rows which
> committed *after* we assembled our snapshot.
Yes, that was my guess too (that the snapshot is acquired before asking
for the lock and not re-acquired after getting the lock).
> The easy workaround is acquiring a AccessShareLock in the B transaction
> separately.
I know - I've mentioned explicit locking as a possible solution in my
first message, although it would make the whole process more complex.
The question is whether that should be necessary or whether the 9.2
should behave the same as 9.1.
Tomas
