Re: Conflict detection for update_deleted in logical replication

On Thu, May 15, 2025 at 6:02 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, Apr 25, 2025 at 4:06 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> >
> > >
> > > Here is V30 patch set includes the following changes:
> > >
> >
> > Thank You for the patch, please find few concerns:
> >
> > 1)
> > By looking at code of ApplyLauncherMain(), it appears that we stopped
> > advancing shared-slot's xmin if any of the subscriptions with
> > retain_conflict_info is disabled. If a subscription is not being used
> > and is disabled forever (or for quite long), that means xmin will
> > never be advanced and we will keep accumulating dead-rows even if
> > other subscribers with retain_conflict_info are actively setting their
> > oldest_xmin. This could be problematic. Here too, there should be some
> > way to set stop-conflict-rettention for such a subscription like we do
> > for 'subscriber not able to catch-up case'.
> > But I understand it can be complex to implement as we do not know for
> > how long a subscription is disabled. If we do not find a simpler way
> > to implement it, then at least we can document such cases and the
> > risks associated with disabled subscription which has
> > 'retain_conflict_info' enabled. Thoughts?
> >
>
> Yeah, I agree that this is the case to be worried about but OTOH, in
> such cases, even the corresponding logical_slot on the primary won't
> be advanced and lead to bloat on the primary as well. So, we can
> probably give WARNING when user tries to disable a subscription or
> create a disabled subscription with retention flag true. We may need
> to write a LOG or raise a WARNING when while exiting apply worker
> disables the subscription due disable_on_error option. In addition to
> that, we can even document this case. Will that address your concern?
>

Yes, it should solve the purpose. Thanks.

thanks
Shveta