Horton

Re: Conflict detection for update_deleted in logical replication

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: shveta malik <shveta.malik@gmail.com>
Cc: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, vignesh C <vignesh21@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2025-05-16T10:13:33Z
Lists: pgsql-hackers 
On Fri, May 16, 2025 at 2:40 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Fri, May 16, 2025 at 12:01 PM shveta malik <shveta.malik@gmail.com> wrote:
> >
> > On Fri, May 16, 2025 at 11:15 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > >
> > >
> > > BTW, another related point is that when we decide to stop retaining
> > > dead tuples (via should_stop_conflict_info_retention), should we also
> > > consider the case that the apply worker didn't even try to get the
> > > publisher status because previously it decided that
> > > oldest_nonremovable_xid cannot be advanced due to its
> > > OldestActiveTransactionId?
> > >
> >
> > Do you mean avoid  stop-conflict-retention in such a case as apply
> > worker itself did not request status from the publisher? If I
> > understood your point correctly, then we can do that by advancing the
> > timer to a new value even if we did not update candidate-xid and did
> > not ask the status from the publisher.
> >
>
> But candidate_xid_time is also used in wait_for_local_flush() to check
> clock_skew between publisher and subscriber, so for that purpose, it
> is better to set it along with candidate_xid. However, can't we rely
> on the valid value of candidate_xid to ensure that apply worker didn't
> send any request? Note that we always reset candidate_xid once we have
> updated oldest_nonremovable_xid.
>

I think this is automatically taken care of because we call
should_stop_conflict_info_retention() only during 'wait' phase, which
should be done after candidate_xid is set. Having said that, we should
have assert for candidate_xid in should_stop_conflict_info_retention()
and also add in comments that it should be called only during the
'wait' phase. Additionally, we can also have an assert that
should_stop_conflict_info_retention() is called only during the 'wait'
phase.

-- 
With Regards,
Amit Kapila.

Commits

  1. Fix intermittent BF failures in 035_conflicts.

  2. Resume conflict-relevant data retention automatically.

  3. Fix intermittent test failure introduced in 6456c6e2c4.

  4. Fix Coverity issue reported in commit a850be2fe.

  5. Add test to prevent premature removal of conflict-relevant data.

  6. Post-commit review fixes for 228c370868.

  7. Add max_retention_duration option to subscriptions.

  8. Detect and report update_deleted conflicts.

  9. Preserve conflict-relevant data during logical replication.

  10. Integrate FullTransactionIds deeper into two-phase code

  11. Improve checks for GUC recovery_target_timeline

  12. Prevent excessive delays before launching new logrep workers.

  13. Keep WAL segments by slot's last saved restart LSN

  14. Rework some code handling pg_subscription data in psql and pg_dump

  15. Use generateClonedIndexStmt to propagate CREATE INDEX to partitions.