Re: Skipping logical replication transactions on subscriber side

On Wed, Dec 8, 2021 at 12:36 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Wed, Dec 8, 2021 at 3:50 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Wed, Dec 8, 2021 at 11:48 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > >
> > > >
> > > > Can't we think of just allowing prepare in this case and updating the
> > > > skip_xid only at commit time? I see that in this case, we would be
> > > > doing prepare for a transaction that has no changes but as such cases
> > > > won't be common, isn't that acceptable?
> > >
> > > In this case, we will end up committing both the prepared (empty)
> > > transaction and the transaction that updates the catalog, right?
> > >
> >
> > Can't we do this catalog update before committing the prepared
> > transaction? If so, both in prepared and non-prepared cases, our
> > implementation could be the same and we have a reason to accomplish
> > the catalog update in the same transaction for which we skipped the
> > changes.
>
> But in case of a crash between these two transactions, given that
> skip_xid is already cleared how do we know the prepared transaction
> that was supposed to be skipped?
>

I was thinking of doing it as one transaction at the time of
commit_prepare. Say, in function apply_handle_commit_prepared(), if we
check whether the skip_xid is the same as prepare_data.xid then update
the catalog and set origin_lsn/timestamp in the same transaction. Why
do we need two transactions for it?

--
With Regards,
Amit Kapila.