Re: Logical Replication of sequences

On Fri, 5 Sept 2025 at 04:01, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Wed, Aug 20, 2025 at 4:57 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Mon, Aug 18, 2025 at 3:36 PM vignesh C <vignesh21(at)gmail(dot)com> wrote:
> > >
> > > Thanks for the comments, the updated version has the changes for the same.
> > >
> >
> > I wanted to first discuss a few design points. The patch implements
> > "ALTER SUBSCRIPTION ... REFRESH PUBLICATION SEQUENCES" such that it
> > copies the existing sequences values and also adds/removes any missing
> > sequences. For the second part (add/remove sequences), we already have
> > a separate command "ALTER SUBSCRIPTION ... REFRESH PUBLICATION". So, I
> > feel the new command should only copy the sequence values, as that
> > will keep the interface easy to define and understand. Additionally,
> > it will help to simplify the code in the patch, especially in the
> > function AlterSubscription_refresh.
>
> While I agree that the new command just copies the sequence values,
> I'm not sure the command should be implemented as an extension of
> ALTER SUBSCRIPTION ... REFRESH PUBLICATION command. Probably what the
> new command does is quite different from what REFRESH PUBLICATION
> command does?

Alternatively, the syntax options could be:
ALTER SUBSCRIPTION subname RESYNC PUBLICATION SEQUENCES;
or
ALTER SUBSCRIPTION subname RESYNC SEQUENCES;

Do you have a preference between the two?

> >
> > We previously discussed *not* to launch an apply worker if the
> > corresponding publication(s) only publish sequences. See [1]. We
> > should consider it again to see if that is a good idea. It will have
> > some drawbacks as compared to the current approach of doing sync via
> > sync worker. The command could take time for a large number of
> > sequences, and on failure, retry won't happen which can happen with
> > background workers. Additionally, when the connect option is false for
> > a subscription during creation, the user needs to later call REFRESH
> > to sync the sequences after enabling the subscription. OTOH, doing the
> > sync during the command will bring more predictability and simplify
> > the patch. What do others think?
>
> It seems okay to me that we launch an apply worker for a subscription
> corresponding to sequence-only publications. I think the situation
> seems somewhat similar to the case where we launch an apply worker
> even for a subscription corresponding to empty publications. It would
> be quite a rare case in practice where publications have only
> sequences.

I agree with this.

> I guess that it would rather simplify the patch if we can
> cut the part of doing the sync during the command (i.e., not
> distinguish between table-and-sequence publications and sequence-only
> publications), no?

Currently, we don’t make a distinction between the two. Just to
clarify, could you point me to the specific part of the patch you’re
referring to?

Regards,
Vignesh