Re: SSI predicate locking on heap -- tuple or row?

"Kevin Grittner" wrote:
> Dan Ports wrote:

>> Does that make sense to you?
>
> Makes sense to me. Like the proof I offered, you have shown that
> there is no cycle which can develop with the locks copied which
> isn't there anyway if we don't copy the locks.

I woke up with the nagging thought that while the above is completely
accurate, it deserves some slight elaboration. These proofs show that
there is no legitimate cycle which could cause an anomaly which the
move from row-based to tuple-based logic will miss. They don't prove
that the change will generate all the same serialization failures;
and in fact, some false positives are eliminated by the change.
That's a good thing. In addition to the benefits mentioned in prior
posts, there will be a reduction in the rate of rollbacks (in
particular corner cases) from what people see in beta1 without a loss
of correctness.

-Kevin

On Tue, May 24, 2011 at 04:18:37AM -0500, Kevin Grittner wrote:
> These proofs show that
> there is no legitimate cycle which could cause an anomaly which the
> move from row-based to tuple-based logic will miss. They don't prove
> that the change will generate all the same serialization failures;
> and in fact, some false positives are eliminated by the change.

Yes, that's correct. That's related to the part in the proof where I
claimed T3 couldn't have a conflict out *to some transaction T0 that
precedes T1*.

I originally tried to show that T3 couldn't have any conflicts out that
T2 didn't have, which would mean we got the same set of serialization
failures, but that's not true. In fact, it's not too hard to come up
with an example where there would be a serialization failure with the
row version links, but not without. However, because the rw-conflict
can't be pointing to a transaction that precedes T1 in the serial
order, it won't create a cycle. In other words, there are serialization
failures that won't happen anymore, but they were false positives.

Dan

--
Dan R. K. Ports MIT CSAIL http://drkp.net/