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Date: Wed, 29 Apr 2026 08:10:17 -0400
From: Andres Freund <andres@anarazel.de>
To: Hannu Krosing <hannuk@google.com>
Cc: Dilip Kumar <dilipbalaut@gmail.com>, 
	Amit Kapila <amit.kapila16@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, 
	Vitaly Davydov <v.davydov@postgrespro.ru>, Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, 
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Support logical replication of DDLs, take2
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Hi,

On 2026-04-29 10:07:04 +0200, Hannu Krosing wrote:
> On Wed, Apr 29, 2026 at 5:39 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> 
> > I am trying to understand your idea. If we are trying to deparse from
> > an actual system table using a snapshot, why don't we just use the
> > WAL? I mean, the WAL should contain the actual catalog modifications
> > it has made.
> 
> We have the full data in the catalog and we would likely need catalog
> queries for any change, even when de-parsing the tree.
> 
> And we should not add the extra load on the original DDL side, just as
> we don't for DML.

That can't be a relevant cost compared to everything else.


> At most we could just serialize the statement tree into the WAL,
> though even that may be an overkill if we can get the change from
> existing records.
> 
> - insert new row in pg_class --> extract the CREATE TABLE (or INDEX, or ...)
> - update row in pg_class or insert, update or delete a row in
> pg_attribute --> extract ALTER TABLE
>   - except when it just updates relfilenod --> extract TRUNCATE
> - delete row in pg_class --> DROP TABLE
> - dml on pg_constraint --> ALTER TABLE
> 
> ... etc

That doesn't work in the general case, think of
ALTER TABLE ... ALTER COLUMN ... TYPE foo USING (...)

There's a big difference between USING(foo::int8) and USING (pg_size_bytes(foo))
but it's nowhere visible in the WAL.

Greetings,

Andres Freund