Re: Proposal: Conflict log history table for Logical Replication

On Sun, Jun 21, 2026 at 7:53 PM vignesh C <vignesh21@gmail.com> wrote:
>
> While attempting to log a conflict, a concurrent ALTER SUBSCRIPTION
> can change the conflict logging destination from all to log. In this
> scenario, the apply worker may already have cached the conflictlogdest
> information, including the OID of the current conflict log table.
> However, the concurrent ALTER SUBSCRIPTION drops the conflict log
> table as part of the destination change:
> +Relation
> +GetConflictLogDestAndTable(ConflictLogDest *log_dest)
> +{
> +       Oid                     conflictlogrelid;
> +
> +       /*
> +        * Convert the text log destination to the internal enum.
> MySubscription
> +        * already contains the data from pg_subscription.
> +        */
> +       *log_dest = GetConflictLogDest(MySubscription->conflictlogdest);
> +
> +       /* Quick exit if a conflict log table was not requested. */
> +       if (!CONFLICTS_LOGGED_TO_TABLE(*log_dest))
> +               return NULL;
> +
> +       conflictlogrelid = MySubscription->conflictlogrelid;
> +
> +       Assert(OidIsValid(conflictlogrelid));
> +
> +       return table_open(conflictlogrelid, RowExclusiveLock);
> +}
>
> As a result, when the apply worker later attempts to open the cached
> conflict log table, table_open() fails because the relation has
> already been dropped. This causes the error handling path itself to
> fail before the conflict record can be written to either the conflict
> log table or the server log.
>
> In such cases, the conflict record is effectively lost and is not
> logged anywhere. For example:
> 2026-06-21 19:31:13.592 IST [263598] LOG:  logical replication apply
> worker for subscription "sub1" has started
> 2026-06-21 19:32:26.731 IST [263598] ERROR:  could not open relation
> with OID 16405
> 2026-06-21 19:32:26.731 IST [263598] CONTEXT:  processing remote data
> for replication origin "pg_16404" during message type "INSERT" for
> replication target relation "public.t1" in transaction 698, finished
> at 0/017D39A0
> 2026-06-21 19:32:26.735 IST [263471] LOG:  background worker "logical
> replication apply worker" (PID 263598) exited with exit code 1
>
> Ideally, failure to access the conflict log table should not prevent
> the conflict from being reported in the server log. This issue is
> present with the v52 version. I have not yet checked if Amit's recent
> patch posted a few minutes ago at [1] handles this issue.
>

There are two places in the patch from where we LOG/Insert the
conflict data. First is ReportApplyConflict() where we LOG if the
conflict arises from a non-ERROR path (aka conflicts other
INSERT/UPDATE_EXISTS).  In that case, the conflict data will be logged
even when we fail to insert into CLT. Second is the place for
conflicts that arose as ERRORs (aka INSERT/UPDATE_EXISTS), where the
conflict information will be logged along with insert failure as
CONTEXT. Can you please verify your test based on this input and share
your findings and thoughts?

-- 
With Regards,
Amit Kapila.