Re: Support logical replication of DDLs, take2

Hi,

On 2026-04-29 10:07:04 +0200, Hannu Krosing wrote:
> On Wed, Apr 29, 2026 at 5:39 AM Dilip Kumar <dilipbalaut(at)gmail(dot)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