On Fri, Mar 11, 2011 at 10:11 AM, Rainer Kwesi Schweigkoffer
<kwesi_at_sap.com> wrote:
> Matthew Adams, am 10 Mar 2011 hast Du um 17:12 zum Thema "[jsr338-experts] Re: query improvements: downcast" geschrieben :
>
>> If so, then I would restate **for the deferred,
>> not-currently-specified projection use of TREAT AS** that if either
>> Book is not assignment-compatible with Product or the filter results
>> in type-incompatible instances with the projection expression, an
>> exception is thrown.
>
> Do you see a specific use case that would require to admit TREAT AS for
> the select clause as well ?
>
I suppose not.
Theoretically, using TREAT AS in the projection (the select clause) is
not required. If the user wants to exclude certain types from the
query, the mechanism by which I'd recommend that they do it would be
to use FROM ... TREAT ... AS. To use SELECT ... TREAT ... AS requires
that the filter only return objects of the expected type; if they want
objects of that type, then why not just use FROM ... TREAT ... AS? It
seems like they're just asking for an exception to be thrown if they
use SELECT ... TREAT ... AS. The only behavior that they'd get with
SELECT ... TREAT ... AS is either an exception if their query was
wrong or the same behavior as FROM ... TREAT ... AS. Seems
unnecessary to me.
Let's see if anyone comes up with a use case for SELECT ... TREAT ...
AS that can't be solved with a FROM ... TREAT ... AS. I'd be
surprised if there were.
-matthew