RE: Conflict detection for update_deleted in logical replication

On Thursday, August 21, 2025 3:47 PM Nisha Moond <nisha(dot)moond412(at)gmail(dot)com> wrote:
>
> On Wed, Aug 20, 2025 at 12:12 PM Zhijie Hou (Fujitsu)
> <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> >
> > I agree. Here is V63 version which implements this approach.
> >
>
> Thank you Hou-san for the patches. Here are couple of comments:
>
> 1) Once retention is stopped for all subscriptions and conflict_slot.xmin is
> reset to NULL, we are no longer retaining dead tuples. In that case, the warning
> shown during subscription disable looks misleading.
>
> For example sub has already stopped the retention and when disabled -
> postgres=# alter subscription sub1 disable;
> WARNING: deleted rows to detect conflicts would not be removed until the
> subscription is enabled
> HINT: Consider setting retain_dead_tuples to false.
> ALTER SUBSCRIPTION
>
> I think we should check if retention is active or not here.
>
> 2) Regarding the logic in the launcher for advancing the slot’s xmin:
> Consider a case where two subscriptions exist, and one of them is disabled
> after it has already stopped retention.
> Example subscriptions in state:
> ...
> Here, sub2 is disabled, and since subretentionactive = 'f', it is not retaining
> dead tuples anymore. But, the current launcher logic still blocks xmin
> advancement as one of the subscriptions with retain_dead_tuples is disabled.
> I think the launcher should consider the subretentionactive value and the xmin
> should be allowed to advance. Thoughts?

I agree that retentionactive needs to be checked in the cases mentioned above.
Here is the V64 patch set addressing this concern. This version also resolves
the bug reported by Shveta[1], where retention could not resume and was stuck
waiting for the publisher status.

In addition, I also improved the comments related to the new phases and
retentionactive flag.

[1] https://www.postgresql.org/message-id/CAJpy0uCP7x_pdVysYohvrjpk0Vtmk36%2BXfnC_DOPiegekxfBLA%40mail.gmail.com

Best Regards,
Hou zj