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Date: Thu, 2 Jul 2026 17:13:50 +0200
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Subject: Re: implement CAST(expr AS type FORMAT 'template')
To: Robert Haas <robertmhaas@gmail.com>,
 Corey Huinker <corey.huinker@gmail.com>
Cc: jian he <jian.universality@gmail.com>,
 Zsolt Parragi <zsolt.parragi@percona.com>,
 Vik Fearing <vik@postgresfriends.org>,
 "David G. Johnston" <david.g.johnston@gmail.com>,
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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?