Re: Skipping logical replication transactions on subscriber side

On Wed, Dec 8, 2021 at 5:54 PM Amit Kapila  wrote:
>
> On Wed, Dec 8, 2021 at 12:36 PM Masahiko Sawada  wrote:
> >
> > On Wed, Dec 8, 2021 at 3:50 PM Amit Kapila  wrote:
> > >
> > > On Wed, Dec 8, 2021 at 11:48 AM Masahiko Sawada  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?

I meant the two transactions are the prepared transaction and the
transaction that updates the catalog. If I understand your idea
correctly, in apply_handle_commit_prepared(), we update the catalog
and set origin_lsn/timestamp. These are done in the same transaction.
Then, we commit the prepared transaction, right? If the server crashes
between them, skip_xid is already cleared and logical replication
starts from the LSN after COMMIT PREPARED. But the prepared
transaction still exists on the subscriber.

Regards,

-- 
Masahiko Sawada
EDB:  https://www.enterprisedb.com/