MIME-Version: 1.0
References: 
 <CACJufxGqm7cYQ5C65Eoh1z-f+aMdhv9_7V=NoLH_p6uuyesi6A@mail.gmail.com>
 <4c358bf8-87e8-4760-83d5-a6f9fcd88907@postgresfriends.org>
 <CACJufxF4OW=x2rCwa+ZmcgopDwGKDXha09qTfTpCj3QSTG6Y9Q@mail.gmail.com>
 <1b702de4-86b0-4144-a2d0-38266296894f@postgresfriends.org>
 <CACJufxEH-8UPdbPoUoqNRaiOePw+s2W2DG4OpXtoSYDaW30oAg@mail.gmail.com>
 <CAKFQuwZXYxPw4wofq+zZ5D7nH0sHDBwr--qku9jWYv7dS+P8RQ@mail.gmail.com>
In-Reply-To: 
 <CAKFQuwZXYxPw4wofq+zZ5D7nH0sHDBwr--qku9jWYv7dS+P8RQ@mail.gmail.com>
From: jian he <jian.universality@gmail.com>
Date: Fri, 1 Aug 2025 16:22:52 +0800
Message-ID: 
 <CACJufxGTeSnHn7w8r-qoQrqwCFQSC+CbiEthzpHKUd+bPr_0mA@mail.gmail.com>
Subject: Re: implement CAST(expr AS type FORMAT 'template')
To: "David G. Johnston" <david.g.johnston@gmail.com>
Cc: Vik Fearing <vik@postgresfriends.org>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Archived-At: 
 <https://www.postgresql.org/message-id/CACJufxGTeSnHn7w8r-qoQrqwCFQSC%2BCbiEthzpHKUd%2BbPr_0mA%40mail.gmail.com>
Precedence: bulk

On Tue, Jul 29, 2025 at 11:54=E2=80=AFAM David G. Johnston
<david.g.johnston@gmail.com> wrote:
>
> The slight variation to this would be to specify these 2/4 and 1-arg func=
tions as optional =E2=80=9Cformat_in=E2=80=9D and =E2=80=9Cformat_out=E2=80=
=9D optional properties (like typmod_in).  The format-aware code can look f=
or these which will end up having the full implementation while the current=
 IO functions would simply stub out calls, passing null as the format. (Or =
maybe some variation that looks similar to typmod handling=E2=80=A6which I =
haven=E2=80=99t looked at.)
>

This may also work.
typmod_in, typmod_out, which is associated with typmod, which is used
in many places.
The only use case for (typformatin, typformatout) is CAST expression.
so we also need to consider the overhead of adding
two oid columns (typformatin, typformatout) to pg_type.

another question is:
should we first implement CAST(expr AS type FORMAT 'template') for limited =
types
(to_date, to_char, to_number, to_timestamptz)
or first try to make it more generic?