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>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, pgsql-hackers <pgsql-hackers@postgresql.org>, vignesh C <vignesh21@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>
Date: 2025-08-01T10:50:38Z
Lists: pgsql-hackers 
On Fri, Aug 1, 2025 at 3:58 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> 4.
> +   /*
> +    * Instead of invoking GetOldestNonRemovableTransactionId() for conflict
> +    * detection, we use the conflict detection slot.xmin. This value will be
> +    * greater than or equal to the other threshold and provides a more direct
> +    * and efficient way to identify recently deleted dead tuples relevant to
> +    * the conflict detection. The oldest_nonremovable_xid is not used here,
> +    * as it is maintained only by the leader apply worker and unavailable to
> +    * table sync and parallel apply workers.
> +    */
> +   slot = SearchNamedReplicationSlot(CONFLICT_DETECTION_SLOT, true);
>
> This comment seems a bit confusing to me, Isn't it actually correct to
> just use the "conflict detection slot.xmin" even without any other
> reasoning?
>

But it is *not* wrong to use even GetOldestNonRemovableTransactionId()
because it will anyway consider conflict detection slot's xmin.
However, the value returned by that function could be much older, so
slot's xmin is a better choice. Similarly, it is sufficient to use
oldest_nonremovable_xid value of apply worker and ideally would be
better than slot's xmin because it could give update_deleted in fewer
cases, however, we can't use that because of reasons mentioned in the
comments. Do you think this comment needs improvement for clarity and
if so, do you have any proposal?

-- 
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.