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From: Robert Haas <robertmhaas@gmail.com>
Date: Thu, 2 Jul 2026 17:16:52 -0400
Message-ID: 
 <CA+TgmobibgO2a3UDr7DrbKVnDG9ZViYTn6+nKR3Kq88Aii7-fw@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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 <https://www.postgresql.org/message-id/CA%2BTgmobibgO2a3UDr7DrbKVnDG9ZViYTn6%2BnKR3Kq88Aii7-fw%40mail.gmail.com>
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On Thu, Jul 2, 2026 at 4:51=E2=80=AFPM David G. Johnston
<david.g.johnston@gmail.com> wrote:
>> and the existence of a
>> format cast doesn't mean there has to be a regular cast.
>
> Disagree

OK, but on what basis?

If I want 'whatever'::thistype::thattype to fail and CAST(thistype AS
thattype FORMAT 'bumble') to succeed, then I need there to be a format
cast but not a regular cast. Apparently, you'd like to arbitrarily
disallow that case, but I can't see any logical reason for such a
prohibition. Essentially we'd be forcing the user to create a regular
cast that they don't want in order to be able to create a format cast
that they do want. If there were some reason why we had to couple the
two things together that way, that would be one thing, but as far as I
can see, there isn't.

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