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

On Thu, Jul 2, 2026 at 11:13 PM Peter Eisentraut <peter(at)eisentraut(dot)org> wrote:
>
> On 18.06.26 16:51, Robert Haas wrote:
> > The reason I somewhat hesitate to endorse that specific proposal is
> > that I'm not convinced that we should actually treat this as a form of
> > casting. Casts can be set to IMPLICIT or ASSIGNMENT or EXPLICIT, and
> > they can be WITHOUT FUNCTION or WITH INOUT, and none of that can be
> > relevant here. A CAST with FORMAT always needs to be implemented by a
> > function, is always explicit from a syntax point of view, and the code
> > to implement probably looks pretty different from the code needed for
> > a non-FORMAT cast. I am somewhat inclined to think we want something
> > that's like a cast function but actually a wholly separate mechanism,
> > e.g. a new pg_formatter catalog. I note that Jian He proposed putting
> > something in pg_type but I don't see how that can work, since there
> > are two types involved.
> >
> > I don't accept the argument that we should start with this and extend
> > it later. The patch as proposed is just syntactic sugar. Said another
> > way, there is existing syntax that already delivers the functionality.
> > So I don't see why we would rush out support for a bit of new syntax;
> > anyone who wants to use this functionality can already do so. Getting
> > the infrastructure right is, IMHO, the interesting part of the
> > project, and I think that work needs to be done first.
>
> I don't really understand the purpose of this feature to begin with.
> You can already call the formatting functions directly. Does this cast
> syntax offer anything on top of that?
>

https://www.postgresql.org/message-id/762ae707-7fdc-43d8-a77a-3a10d12ce21d%40postgresfriends.org

After this implemented, we can begin fully supporting:

<cast specification> ::=
CAST <left paren>
<cast operand> AS <cast target>
[ FORMAT <cast template> ]
[ <cast error behavior> ON CONVERSION ERROR ]
<right paren>