Re: logical decoding and replication of sequences, take 2 - Mailing list pgsql-hackers

On Thu, Dec 14, 2023 at 12:31 PM Ashutosh Bapat
<ashutosh.bapat.oss@gmail.com> wrote:
>
> On Thu, Dec 14, 2023 at 10:53 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> > > >
> > >
> > > It is correct that we can make a wrong decision about whether a change
> > > is transactional or non-transactional when sequence DDL happens before
> > > the SNAPBUILD_FULL_SNAPSHOT state and the sequence operation happens
> > > after that state. However, one thing to note here is that we won't try
> > > to stream such a change because for non-transactional cases we don't
> > > proceed unless the snapshot is in a consistent state. Now, if the
> > > decision had been correct then we would probably have queued the
> > > sequence change and discarded at commit.
> > >
> > > One thing that we deviate here is that for non-sequence transactional
> > > cases (including logical messages), we immediately start queuing the
> > > changes as soon as we reach SNAPBUILD_FULL_SNAPSHOT state (provided
> > > SnapBuildProcessChange() returns true which is quite possible) and
> > > take final decision at commit/prepare/abort time. However, that won't
> > > be the case for sequences because of the dependency of determining
> > > transactional cases on one of the prior records. Now, I am not
> > > completely sure at this stage if such a deviation can cause any
> > > problem and or whether we are okay to have such a deviation for
> > > sequences.
> >
> > Okay, so this particular scenario that I raised is somehow saved, I
> > mean although we are considering transactional sequence operation as
> > non-transactional we also know that if some of the changes for a
> > transaction are skipped because the snapshot was not FULL that means
> > that transaction can not be streamed because that transaction has to
> > be committed before snapshot become CONSISTENT (based on the snapshot
> > state change machinery).  Ideally based on the same logic that the
> > snapshot is not consistent the non-transactional sequence changes are
> > also skipped.  But the only thing that makes me a bit uncomfortable is
> > that even though the result is not wrong we have made some wrong
> > intermediate decisions i.e. considered transactional change as
> > non-transactions.
> >
> > One solution to this issue is that, even if the snapshot state does
> > not reach FULL just add the sequence relids to the hash, I mean that
> > hash is only maintained for deciding whether the sequence is changed
> > in that transaction or not.  So no adding such relids to hash seems
> > like a root cause of the issue.  Honestly, I haven't analyzed this
> > idea in detail about how easy it would be to add only these changes to
> > the hash and what are the other dependencies, but this seems like a
> > worthwhile direction IMHO.
>
> I also thought about the same solution. I tried this solution as the
> attached patch on top of Hayato's diagnostic changes.

I think you forgot to attach the patch.


--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com