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: Dilip Kumar <dilipbalaut@gmail.com>
Cc: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, shveta malik <shveta.malik@gmail.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: 2025-07-01T08:54:24Z
Lists: pgsql-hackers 
On Tue, Jul 1, 2025 at 10:53 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Tue, Jul 1, 2025 at 10:31 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> >
> > On Mon, Jun 30, 2025 at 6:59 PM Zhijie Hou (Fujitsu)
> > <houzj.fnst@fujitsu.com> wrote:
> > >
> > > On Mon, Jun 30, 2025 at 7:22 PM Amit Kapila wrote:
> > > >
> >
> > I was looking at 0001, it mostly looks fine to me except this one
> > case.  So here we need to ensure that commits must be acquired after
> > marking the flag, don't you think we need to ensure strict statement
> > ordering using memory barrier, or we think it's not required and if so
> > why?
> >

Good point. I also think we need a barrier here, but a write barrier
should be sufficient as we want ordering of two store operations.

> > RecordTransactionCommitPrepared()
> > {
> > ..
> > + MyProc->delayChkptFlags |= DELAY_CHKPT_IN_COMMIT;
> > +
> > + /*
> > + * Note it is important to set committs value after marking ourselves as
> > + * in the commit critical section (DELAY_CHKPT_IN_COMMIT). This is because
> > + * we want to ensure all transactions that have acquired commit timestamp
> > + * are finished before we allow the logical replication client to advance
> > + * its xid which is used to hold back dead rows for conflict detection.
> > + * See maybe_advance_nonremovable_xid.
> > + */
> > + committs = GetCurrentTimestamp();
> > }
>
> I'm unsure whether the function call inherently acts as a memory
> barrier, preventing the compiler from reordering these operations.
> This needs to be confirmed.
>

As per my understanding, function calls won't be a memory barrier. In
this regard, we need a similar change in RecordTransactionCommit as
well.

-- 
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.