> On Sun, Dec 19, 2010 at 4:02 AM, Florian Pflug <fgp@phlo.org> wrote:
>> Note that it's sufficient to check if B can see the effects of the
>> *latest* locker of T. If it can see those, it must also see the
>> effects of any previous locker. But because of this, B cannot
>> distinguish different lock strengths on T - even if A locked T
>> in exclusive mode, some transaction A2 may lock T in shared mode
>> after A has committed but before B inspects T.
>
> This seems to point to a rather serious problem, though. If B sees
> that the last locker A1 aborted, correctness requires that it roll
> back B, because there might have been some other transaction A2 which
> locked locked T and committed before A1 touched it. Implementing that
> behavior could lead to a lot of spurious rollbacks, but NOT
> implementing it isn't fully correct.
Certainly seems serios. How on earth did I manage to miss that one, I
wonder :-(
If only shared locks are invovled, the patch probably works correctly,
though, since they don't remove all traces of previous lockers, they
merely add themselves to the multi-xid holding the lock. That'd also
explain why my concurrency test-suite didn't trip over this. So we may
only need to do what you propose for exclusive locks.
I'll look at this later today in more detail - I'm not currently at home,
so I don't have access to the sources...
>> For serializable transactions, everything is as you explained it. Taking
>> a
>> lock amounts to saying "If you can't see what I did, leave this tuple
>> alone".
>> A read-committed transactions, though, sees different things at
>> different
>> times since it takes a new snapshot for every statement. Since we cannot
>> raise a serialization error in a read-committed transaction, an UPDATE
>> or DELETE statement within a read-committed transaction may very well
>> modify a previously locked row, even if *doesn't* see the effects of
>> some
>> concurrent locker. Any snapshot taken after the UDPATE or DELETE hit the
>> locked row, however, *will* see those changes. This includes the
>> snapshot
>> taken within any AFTER trigger fired on updating the locked row. Thus,
>> things for one fine for RI constraints enforced by triggers.
>
> I can't parse the last sentence of this paragraph. I think there is a
> word or two wrong.
Ups, sorry for that. s/for one/are - the sentence should have read "Thus,
things are find for RI constraints enforced by triggers".
best regards,
Florian Pflug