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

On Thu, Jul 2, 2026 at 8:01 AM Haibo Yan <tristan(dot)yim(at)gmail(dot)com> wrote:
>
> CAST('{2025T01-01,2026T02-02}' AS date[] FORMAT 'YYYY"T"MM-DD')
>
> could reasonably mean: parse the array structure as usual, and apply the
> formatted text -> date conversion to each element.
>
> The current patch does not implement that. It does an exact source/target
> lookup in pg_format_cast, so an array target would currently require a format
> cast for the array type itself. But I agree that this may not be the behavior
> users would expect, especially given the existing array coercion behavior.
>
> If we decide that element-wise array formatting is required, one possible
> implementation would be to keep the scalar format-cast function signature as
>
> function(source_element_type, text) returns target_element_type
>
> and add a format-aware path in the array coercion logic. The array expression
> would iterate over elements and pass the same FORMAT expression to the scalar
> format cast. In that design, the array container itself would not need a
> separate format-cast entry.
>

In some cases, ArrayCoerceExpr might help, but not here.

CAST('{2025T01-01,2026T02-02}'::TEXT AS date[] FORMAT 'YYYY"T"MM-DD')

parse the array structure as usual, which array_in do all the heavy work.
arrray_in(cstring, oid, integer).
But once array_in is finished, we already get a date[] datum, then
FORMAT is no longer needed.
I also mentioned [1] that add a text argument to array_in is not possible.

I guess not supporting arrays in the initial patch should be fine,
since not all CASTs support CAST FORMAT.
That raises another question: for user-defined CREATE FORMAT CAST,
should we ban anyarray as the first argument type?
If we don't, will it conflict with array support once we implement it later?

[1]: https://www.postgresql.org/message-id/CACJufxGVuCM4XFGqaqiV-VOEiqMtCZ3%2BT-%2BSrG-y6kqdLo1ZqA%40mail.gmail.com