Re: Failure to coerce unknown type to specific type

On Wed, 2015-04-08 at 21:31 -0400, Tom Lane wrote:
> Jeff Davis <pgsql@j-davis.com> writes:
> > Original report and patch by Karl Schnaitter.
> > create table a(u) as select '1';
>
> We should, in fact, fail that to begin with.  Unknown-type columns are
> a spectactularly horrid idea.

That example was just for illustration. My other example didn't require
creating a table at all:

  SELECT a=b FROM (SELECT ''::text, '  ') x(a,b);

it's fine with me if we want that to fail, but I don't think we're
failing in the right place, or with the right error message.

I'm not clear on what rules we're applying such that the above query
should fail, but:

  SELECT ''::text='  ';

should succeed. Unknown literals are OK, but unknown column references
are not? If that's the rule, can we catch that earlier, and throw an
error like 'column reference "b" has unknown type'?

Regards,
    Jeff Davis