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help / color / mirror / Atom feedFrom: David G. Johnston <[email protected]>
To: Robert Haas <[email protected]>
Cc: Haibo Yan <[email protected]>
Cc: jian he <[email protected]>
Cc: Corey Huinker <[email protected]>
Cc: Zsolt Parragi <[email protected]>
Cc: Vik Fearing <[email protected]>
Cc: PostgreSQL-development <[email protected]>
Subject: Re: implement CAST(expr AS type FORMAT 'template')
Date: Thu, 2 Jul 2026 14:52:02 -0700
Message-ID: <CAKFQuwam6CKm+MJbNwFgoMJYJFuzWQ68+WqMf39ku8Q6kqtXYg@mail.gmail.com> (raw)
In-Reply-To: <CA+TgmobibgO2a3UDr7DrbKVnDG9ZViYTn6+nKR3Kq88Aii7-fw@mail.gmail.com>
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On Thursday, July 2, 2026, Robert Haas <[email protected]> wrote:
> On Thu, Jul 2, 2026 at 4:51 PM David G. Johnston
> <[email protected]> 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
There is no wanting the formatless-variant to fail. There is recognition
that sometime the formatless variant might fail with a syntax error and we
have a way to pass a format to make the cast work.
If there is no cast from thistype to thattype both will fail with cast not
found errors. Once a cast exists the first one could pass or fail
depending on the content being casted. If the content has an internal
structure/syntax the failure mode would then be a syntax error. Overcoming
a syntax error is done by specifying a format. But a format does you no
good if there isn’t some cast pathway already available to use it. IOW a
format should never be required - some default exists, like for dates
today, that allows the non-format cast to work. We just need some way to
pass in a format to that cast function if one is specified.
For me the presence of the word cast in the syntax drives this way of
thinking about the problem/design. I’d rather just stick with our
polymorphic to_char functions if we want some way of outputting text with a
format at without having to call doing so a cast.
David J.
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Subject: Re: implement CAST(expr AS type FORMAT 'template')
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