Re: Support logical replication of DDLs

On Wed, Apr 26, 2023 at 2:56 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Wed, Apr 26, 2023 at 10:01 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > On Tue, Apr 25, 2023 at 12:58 PM Zhijie Hou (Fujitsu)
> > <houzj.fnst@fujitsu.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.

Regards,

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