Horton

Re: Conflict detection for update_deleted in logical replication

Masahiko Sawada <sawada.mshk@gmail.com>

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From: Masahiko Sawada <sawada.mshk@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Nisha Moond <nisha.moond412@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, shveta malik <shveta.malik@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2025-01-31T21:23:30Z
Lists: pgsql-hackers 
On Thu, Jan 30, 2025 at 10:39 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Fri, Jan 31, 2025 at 4:10 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > I have one question about the 0004 patch; it implemented
> > max_conflict_retntion_duration as a subscription parameter. But the
> > launcher invalidates the pg_conflict_detection slot only if all
> > subscriptions with retain_conflict_info stopped retaining dead tuples
> > due to the max_conflict_retention_duration parameter. Therefore, even
> > if users set the parameter to a low value to avoid table bloats, it
> > would not make sense if other subscriptions set it to a larger value.
> > Is my understanding correct?
> >
>
> Yes, your understanding is correct. I think this could be helpful
> during resolution because the worker for which the duration has
> exceeded cannot detect conflicts reliably but others can. So, this
> info can be useful while performing resolutions. Do you have an
> opinion/suggestion on this matter?

I imagined a scenario like where two apply workers are running and
have different max_conflict_retention_duration values (say '5 min' and
'15 min'). Suppose both workers are roughly the same behind the
publisher(s), when both workers cannot advance the workers' xmin
values for 5 min or longer, one worker stops retaining dead tuples.
However, the pg_conflict_detection slot is not invalidated yet since
another worker is still using it, so both workers would continue to be
getting slower. The subscriber would end up retaining dead tuples
until both workers are behind for 15 min or longer, before
invalidating the slot. In this case, stopping dead tuple retention on
the first worker would help neither advance the slot's xmin nor
improve another worker's performance. I was not sure of the point of
making the max_conflict_retention_duration a per-subscription
parameter.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com

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.