Re: Support logical replication of DDLs

> > > I agree that it adds to our maintainability effort, like every time we
> > > enhance any DDL or add a new DDL that needs to be replicated, we
> > > probably need to change the deparsing code. But OTOH this approach
> > > seems to provide better flexibility. So, in the long run, maybe the
> > > effort is worthwhile. I am not completely sure at this stage which
> > > approach is better but I thought it is worth evaluating this approach
> > > as Alvaro and Robert seem to prefer this idea.
> >
> > +1, IMHO with deparsing logic it would be easy to handle the mixed DDL
> > commands like ALTER TABLE REWRITE. But the only thing is that we will
> > have to write the deparsing code for all the utility commands so there
> > will be a huge amount of new code to maintain.
>
> Actually, the largest stumbling block on this, IMO, is having a good
> mechanism to verify that all DDLs are supported. The deparsing code
> itself is typically not very bad, as long as we have a sure way to twist
> every contributor's hand into writing it, which is what an automated
> test mechanism would give us.
>
> The code in test_ddl_deparse is a pretty lame start, not nearly good
> enough by a thousand miles. My real intention was to have a test
> harness that would first run a special SQL script to install DDL
> capture, then run all the regular src/test/regress scripts, and then at
> the end ensure that all the DDL scripts were properly reproduced -- for
> example transmit them to another database, replay them there, and dump
> both databases and compare them. However, there were challenges which I
> no longer remember and we were unable to complete this, and we are where
> we are.
>
> Thanks for rebasing that old code, BTW.

I agree that deparsing could be a very useful utility on its own. Not
only for SQL command replication between PostgreSQL servers, but also
potentially feasible for SQL command replication between PotgreSQL and
other database systems. For example, one could assemble the json
representation of the SQL parse tree back to a SQL command that can be
run in MySQL. But that requires different assembling rules and code
for different database systems. If we're envisioning this kind of
flexibility that the deparsing utility can offer, then I think it's
better to develop the deparsing utility as an extension itself.

Regards,
Zheng