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From: Robert Haas <robertmhaas@gmail.com>
Date: Thu, 2 Jul 2026 16:37:08 -0400
Message-ID: 
 <CA+TgmoaWpZyokE-Dj=qRyPRMLRv73C4eVvFm31wq6f-ifW_c-A@mail.gmail.com>
Subject: Re: implement CAST(expr AS type FORMAT 'template')
To: "David G. Johnston" <david.g.johnston@gmail.com>
Cc: Haibo Yan <tristan.yim@gmail.com>, jian he <jian.universality@gmail.com>,
	Corey Huinker <corey.huinker@gmail.com>,
 Zsolt Parragi <zsolt.parragi@percona.com>,
	Vik Fearing <vik@postgresfriends.org>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>
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On Wed, Jul 1, 2026 at 4:59=E2=80=AFPM David G. Johnston
<david.g.johnston@gmail.com> wrote:
> I think the fact the standard put this inside the 'cast(...)' means it's =
quite reasonable to consider the aspect part of a cast definition as oppose=
d to something wholly different.
>
> When we issue "create table" both pg_class and pg_attribute are modified.=
  It seems quite reasonable that executing "create cast" causes both pg_cas=
t and pg_cast_format to be populated.

I don't see how this would work. The existence of a regular cast
doesn't mean there has to be a format cast, and the existence of a
format cast doesn't mean there has to be a regular cast. Expressions
that use a format cast need to depend on the pg_format_cast entry, and
expressions that use a regular cast need to depend on the pg_cast
entry.

I mean, I'm not saying something else is totally impossible. You could
for example create the pg_cast entry always and make it some kind of
dummy entry when there's only a format cast. Then all casts could
depend on the pg_cast entry. But then we'd still have to have ALTER
CAST commands that modify the format-cast part of the object and the
non-format cast part of the object separately. That seems extremely
unpleasant to sort out, especially since right now a cast can't be
modified after it's created, and changing that might have security
implications. But even apart from that it doesn't seems like it will
make for a very awkward user experience.

I agree with Peter that I'm not really sure why we want to implement
this in the first place; it doesn't seem to add any value over just
having functions. The above is just a discussion about what we should
do (or what I think we should do) if we do want to have it.

--=20
Robert Haas
EDB: http://www.enterprisedb.com