Horton

Re: Conflict detection for update_deleted in logical replication

shveta malik <shveta.malik@gmail.com>

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From: shveta malik <shveta.malik@gmail.com>
To: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: Dilip Kumar <dilipbalaut@gmail.com>, Amit Kapila <amit.kapila16@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>, shveta malik <shveta.malik@gmail.com>
Date: 2025-08-08T06:34:00Z
Lists: pgsql-hackers 
On Thu, Aug 7, 2025 at 10:10 AM Zhijie Hou (Fujitsu)
<houzj.fnst@fujitsu.com> wrote:
>
> On Tuesday, August 5, 2025 10:09 AM Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com> wrote:
> > Here is V57 patch set which addressed most of comments.
> >
> > In this version, I also fixed a bug that the apply worker continued to find dead
> > tuples even if it has already stop retaining dead tuples.
>
> Here is a V58 patch set which improved few things by internal review:
>
> 0001:
>

Thank You for the patches, please find  a few comments on 001 alone:

1)
+ /*
+ * Return if the wait time has not exceeded the maximum limit
+ * (max_conflict_retention_duration).
+ */
+ if (!TimestampDifferenceExceeds(rdt_data->candidate_xid_time, now,
+ max_conflict_retention_duration +
+ rdt_data->table_sync_wait_time))

We can add comments here as in why we are adding table-sync time to
max_conflict_retention_duration.

2)
relmutex comment says:

        /* Used for initial table synchronization. */
        Oid                     relid;
        char            relstate;
        XLogRecPtr      relstate_lsn;
        slock_t         relmutex;

We shall update this comment as now we are using it for other
purposes. Also name is specific to relation (due to originally created
for table-sync case). We can rename it to be more general so that it
can be used for oldest-xid access purposes as well.

3)
+ Assert(TransactionIdIsValid(rdt_data->candidate_xid));
+ Assert(rdt_data->phase == RDT_WAIT_FOR_PUBLISHER_STATUS ||
+    rdt_data->phase == RDT_WAIT_FOR_LOCAL_FLUSH);
+
+ if (!max_conflict_retention_duration)
+ return false;

Shall we move 'max_conflict_retention_duration' NULL check as the
first step. Or do you think it will be better to move it to the caller
before should_stop_conflict_info_retention is invoked?

4)
+        The information useful for conflict detection is no longer retained if
+        all apply workers associated with the subscriptions, where
+        <literal>retain_dead_tuples</literal> is enabled, confirm that the
+        retention duration exceeded the
+        <literal>max_conflict_retention_duration</literal>. To re-enable
+        retention, you can disable <literal>retain_dead_tuples</literal> and
+        re-enable it after confirming this replication slot has been dropped.

But the replication slot will not be dropped unless all the
subscriptions have disabled retain_dead_tuples. So shall the above doc
somehow mention this part as well otherwise it could be misleading for
users.


5)
pg_stat_subscription_stats: retain_dead_tuples

Can it cause confusion as both subscription's parameter and
pg_stat_subscription_stats's column have the same name while may have
different values. Shall the stats one be named as
'effective_retain_dead_tuples'?

thanks
Shveta

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.