Date: Mon, 9 May 2022 11:05:12 +0200
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Dilip Kumar <dilipbalaut@gmail.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, Zheng Li <zhengli10@gmail.com>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
	rajesh singarapu <rajesh.rs0541@gmail.com>,
	Ajin Cherian <itsajin@gmail.com>,
	=?utf-8?B?SG91LCBaaGlqaWUv5L6vIOW/l+adsA==?= <houzj.fnst@fujitsu.com>
Subject: Re: Support logical replication of DDLs
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On 2022-May-08, Dilip Kumar wrote:

> On Sat, May 7, 2022 at 9:38 AM Amit Kapila <amit.kapila16@gmail.com> wrote:

> > 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.

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/