Re: In-place updates and serializable transactions

On Fri, Nov 16, 2018 at 4:07 AM Kevin Grittner <kgrittn(at)gmail(dot)com> wrote:
>
> On Thu, Nov 15, 2018 at 3:03 AM Kuntal Ghosh <kuntalghosh(dot)2007(at)gmail(dot)com> wrote:
>
> > The test multiple-row-versions is failing because of the
> > above-discussed scenario. I've attached the regression diff file and
> > the result output file for the same. Here is a brief summary of the
> > test w.r.t. heap:
> >
> > Step 1: T1-> BEGIN; Read FROM t where id=1000000;
> > Step 2: T2-> BEGIN; UPDATE t where id=1000000; COMMIT; (creates T1->T2)
> > Step 3: T3-> BEGIN; UPDATE t where id=1000000; Read FROM t where id=500000;
> > Step 4: T4-> BEGIN; UPDATE t where id= 500000; Read FROM t where id=1;
> > COMMIT; (creates T3->T4)
> > Step 5: T3-> COMMIT;
> > Step 6: T1-> UPDATE t where id=1; COMMIT; (creates T4->T1,)
> >
> > At step 6, when the update statement is executed, T1 is rolled back
> > because of T3->T4->T1.
> >
> > But for zheap, step 3 also creates a dependency T1->T3 because of
> > in-place update. When T4 commits in step 4, it marks T3 as doomed
> > because of T1 --> T3 --> T4. Hence, in step 5, T3 is rolled back.
>
> If I understand this, no permutation (order of execution of the
> statements in a set of concurrent transactions vulnerable to
> serialization anomalies) which have succeeded with the old storage
> engine now fail with zheap; what we have with zheap is an earlier
> failure in one case. More importantly, zheap doesn't create any false
> negatives (cases where a serialization anomaly is missed).
>

Your understanding is correct. Thanks for sharing your feedback.

> I would say this should be considered a resounding success. We should
> probably add an alternative result file to cover this case, but
> otherwise I don't see anything which requires action.
>
> Congratulations on making this work so well!
>

Thanks.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com