Re: Logical Replication of sequences - Mailing list pgsql-hackers

On Fri, Oct 17, 2025 at 10:01 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Thu, Oct 16, 2025 at 4:53 PM Zhijie Hou (Fujitsu)
> <houzj.fnst@fujitsu.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.

thanks
Shveta