Re: implement CAST(expr AS type FORMAT 'template')

[Vik Fearing]

On 04/08/2025 07:55, David G. Johnston wrote:

On Sunday, August 3, 2025, Corey Huinker <corey.huin...@gmail.com> wrote:

    On Sun, Aug 3, 2025 at 11:36 PM David G. Johnston
    <david.g.johns...@gmail.com> wrote:


        I was under the impression that for format to be allowed in
        the expression one of the two data types involved has to be
        text and the other must not be text.


    I hadn't understood that, but also hadn't thought of a case where
    it might be wanted until just now. What if someone wanted a cast
    from JSONB to their custom type, and the format was a specific
    keypath to extract from the JSONB? It's true that could be
    accomplished by first extracting the keypath and then CASTing that
    expression, but the same is true for text->date, regexing a
    YYYY-MM-DD into the locale default.

Feels like the same basic answer.  Create cast has a single (because it’s one-way) function accepting between 1 and 3 arguments.  Change it to accept between 1 and 4 arguments and the 4th is where the format expression gets passed.  If a format expression is present and the function doesn’t have a 4th argument, error.

This is my position as well.

+1

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Vik Fearing