Re: Logical Replication of sequences

On Fri, Oct 17, 2025 at 1:35 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>
> On Fri, Oct 17, 2025 at 10:01 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Thu, Oct 16, 2025 at 4:53 PM Zhijie Hou (Fujitsu)
> > <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> > >
> > > Regarding whether we can avoid creating slot/origin for seq-only publication.
> > > I think the main challenge lies in ensuring the apply worker operates smoothly
> > > without a replication slot. Currently, the apply worker uses the
> > > START_REPLICATION command with a replication slot to acquire the slot on the
> > > publisher. To bypass this, it's essential to skip starting the replication and
> > > specifically, avoid entering the LogicalRepApplyLoop().
> > >
> > > To address this, I thought to implement a separate loop dedicated to
> > > sequence-only subscriptions. Within this loop, the apply worker would only call
> > > functions like ProcessSyncingSequencesForApply() to manage sequence
> > > synchronization while periodically checking for any new tables added to the
> > > subscription. If new tables are detected, the apply worker would exit this loop
> > > and enter the LogicalRepApplyLoop().
> > >
> > > I chose not to consider allowing the START_REPLICATION command to operate
> > > without a logical slot, as it seems like an unconventional approach requiring
> > > modifications in walsender and to skip logical decoding and related processes.
> > >
> > > Another consideration is whether to address scenarios where tables are
> > > subsequently removed from the subscription, given that slots and origins would
> > > already have been created in such cases.
> > >
> > > Since it might introduce addition complexity to the patches, and considering
> > > that we already allow slot/origin to be created for empty subscription, it might
> > > also be acceptable to allow it to be created for sequence-only subscription. So,
> > > I chose to add some comments to explain the reason for it in latest version.
> > >
> > > Origin case might be slightly easier to handle, but it could also require some
> > > amount of implementations. Since origin is less harmful than a replication slot
> > > and maintaining it does not have noticeable overhead, it seems OK to me to
> > > retain the current behaviour and add some comments in the patch to clarify the
> > > same.
> > >
> >
> > I agree that avoiding to create a slot/origin for sequence-only
> > subscription is not worth the additional complexity at other places,
> > especially when we do create them for empty subscriptions.
>
> +1.
>
> While testeing 001 patch alone, I found that for sequence-only
> subscription, we get error in tablesync worker :
> ERROR: relation "public.seq1" type mismatch: source "table", target "sequence"
>
> This error comes because during copy_table(),
> logicalrep_relmap_update() does not update relkind and thus later
> CheckSubscriptionRelkind() ends up giving the above error.

I faced the same error while reviewing the 0001 patch. I think if
we're going to push these patches separately the 0001 patch should
have at least minimal regression tests. Otherwise, I'm concerned that
buildfarm animals won't complain but we could end up blocking other
logical replication developments.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com