Re: Support logical replication of DDLs

On Mon, Apr 11, 2022 at 11:01 PM Zheng Li <zhengli10(at)gmail(dot)com> wrote:
>
> >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.
>
> Yes, I think any DDL that can generate DML changes should be listed
> out and handled properly or documented. Here is one extreme example
> involving volatile functions:
> ALTER TABLE nd_ddl ADD COLUMN t timestamp DEFAULT now().
> Again, I think we need to somehow skip the data rewrite on the
> subscriber when replicating such DDL and let DML replication handle
> the rewrite.
>

I am not sure what is the right way to deal with this but another idea
we can investigate is to probably rewrite the DDL such that it doesn't
rewrite the table.

> >Another somewhat unrelated problem I see with this work is how to save
> >recursion of the same command between nodes (when the involved nodes
> >replicate DDLs). For DMLs, we can avoid that via replication origins
> >as is being done in the patch proposed [1] but not sure how will we
> >deal with that here?
>
> I'll need to investigate "recursion of the same command between
> nodes", could you provide an example?
>

See email [1]. I think the same solution should work for your proposal
as well because I see that LogLogicalMessage includes origin but it is
better if you can confirm it.

[1] - https://www.postgresql.org/message-id/flat/CALDaNm0gwjY_4HFxvvty01BOT01q_fJLKQ3pWP9=9orqubhjcQ(at)mail(dot)gmail(dot)com

--
With Regards,
Amit Kapila.