Re: Failure to coerce unknown type to specific type

On Wed, 2015-04-29 at 15:37 -0700, Tom Lane wrote:
> We should IMO, but there's been push-back about backwards compatibility
> when this has been proposed in the past.  But I'd rather break backwards
> compatibility to the extent of saying "you can't do that" than to try to
> make unknown a full-fledged type, which is what this patch wants to do.

My intention was to make can_coerce_type and coerce_type consistent --
right now, coerce_type can fail after can_coerce_type returns true.

(I also wanted to improve the composability of subqueries, but I got
enough resistance that I'm setting that argument aside.)

It really has nothing to do with creating real tables with real columns
of type unknown.

   => select u=t from (select 'x' as u, 'y'::text as t) s;
   ERROR:  failed to find conversion function from unknown to text

That's an elog (not an ereport, just plain elog) for what is a
high-level query compilation error of a fairly sane-looking query, which
seems wrong.

The code comment above the error says that the caller blew it, but as
far as I can tell there's nothing the caller can do about it. That seems
wrong, too.

Regards,
    Jeff Davis