Re: Support logical replication of DDLs

On Fri, Apr 8, 2022 at 4:30 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Tue, Mar 29, 2022 at 9:47 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > >
> > > The idea is to force skipping any direct data population (which can
> > > potentially cause data inconsistency on the subscriber)
> > > in CREATE AS and SELECT INTO command on the subscriber by forcing the
> > > skipData flag in the intoClause of the parsetree after
> > > the logical replication worker parses the command. The data sync will
> > > be taken care of by the DML replication after the DDL replication
> > > finishes.
> >
> > Okay, something like that should work, I am not sure it is the best
> > design though.
> >
>
> Even if this works, how will we make Alter Table statement work where
> it needs to rewrite the table? There also I think we can face a
> similar problem if we directly send the statement, once the table will
> be updated due to the DDL statement and then again due to table
> rewrite as that will have a separate WAL.

I agree. But here the bigger question is what is the correct behavior
in case of the Alter Table? I mean for example in the publisher the
table gets rewritten due to the Access Method change then what should
be the behavior of the subscriber. One expected behavior is that on
subscriber also the access method gets changed and the data remains
the same as on the subscriber table(table can be locally rewritten
based on new AM). Which seems quite sensible behavior to me. But if
we want this behavior then we can not replay the logical messages
generated by DML WAL because of table rewrite, otherwise we will get
duplicate data, unless we plan to get rid of the current data and just
get all new data from the publisher. And if we do that then the data
will be as per the latest data in the table based on the publisher, so
I think first we need to define the correct behavior and then we can
design it accordingly.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com