Re: Support logical replication of DDLs

On Wed, Apr 26, 2023 at 12:01 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Wed, Apr 26, 2023 at 2:56 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Wed, Apr 26, 2023 at 10:01 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > >
> > > On Tue, Apr 25, 2023 at 12:58 PM Zhijie Hou (Fujitsu)
> > > <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> > > >
> > > > Aport from above comments, I splitted the code related to verbose
> > > > mode to a separate patch. And here is the new version patch set.
> > > >
> > >
> > > As for DDL replication, we create event triggers to write deparsed DDL
> > > commands to WAL when creating a publication with the ddl option. The
> > > event triggers are recreated/dropped at ALTER/DROP PUBLICATION. I'm
> > > concerned it's possible that DDLs executed while such a publication
> > > not existing are not replicated. For example, imagine the following
> > > steps,
> > >
> > > 1. CREATE PUBLICATION test_pub ... WITH (ddl = 'table);
> > > 2. CREATE SUBSCRIPTION test_sub ... PUBLICATION test_pub;
> > > 3. ALTER SUBSCRIPTION test_sub DISABLE;
> > > 4. DROP PUBLICATION test_pub;
> > > 5. CREATE PUBLICATION test_pub ... WITH (ddl = 'table);
> > > 6. ALTER SUBSCRIPTION test_sub ENABLE;
> > >
> > > DDLs executed between 4 and 5 won't be replicated.
> > >
> >
> > But we won't even send any DMLs between 4 and 5. In fact, WALSender
> > will give an error for those DMLs that publication doesn't exist as it
> > uses a historic snapshot.
>
> You're right, I missed this point.
>
> > So, why do we expect DDLs between Drop and
> > Create of publication should be replicated?
>
> For example, suppose that a publication is created for a table and
> then a new column is added to the table between 4 and 5, subsequent
> INSERTs could fail due to the missing column. But it's not a problem
> as you pointed out since the user dropped the publication.
>

Right, I think we can add a note about this in the document if you
think so but OTOH it appears quite natural to me.

--
With Regards,
Amit Kapila.