Re: Firing an event trigger for ADD COLUMN?

On Mon, May 11, 2020 at 5:23 PM Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
wrote:

> On 2020-May-11, Michel Pelletier wrote:
>
> > Is this asymmetry a bug? I realize these event trigger functions are
> > typically written in C, but I'd like to keep this idea contained to a
> > plpgsql function and cover as many reversible changes as I can.
>
> Sadly, you cannot obtain all the necessary details using only plpgsql.
> The only way is to compile a C extension. There's a very rough,
> incomplete skeleton to show how to do this in
> src/test/modules/test_ddl_deparse. A much more complete approach, which
> was not completed, was used to produce JSON output from the C
> structures -- see
> https://www.postgresql.org/message-id/20150409161419.GC4369@alvh.no-ip.org

Ah thank you, test_ddl_deparse gives me a good idea what's going on now.
Very interesting thread, thanks for your work on this.

Keep in mind ALTER TABLE is a very sophisticated; you can do things like
> ALTER TABLE one ADD COLUMN two INT, DROP COLUMN three, ADD COLUMN four
> text;
> (i.e. do several things in one command) so if you were to use a
> simplistic approach, you might end up missing things.
>

This is a support feature for some users so they are encouraged to write
down scripts. The idea being the tool would spit one out and they'll be
advised to use it as a starting point for the actual post-hoc down script,
so catching CREATE TABLE/INDEX is a good start. As this ddl deparsing
feature evolves in the future, I can add more complex cases.

Thanks again!

-Michel

> --
> Álvaro Herrera https://www.2ndQuadrant.com/
> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
>