Robert Haas <robertmhaas@gmail.com> writes:
> On Sun, Oct 18, 2009 at 1:59 PM, Jeff Janes <jeff.janes@gmail.com> wrote:
>> Even if country.id is a primary or unique key?
> Well, we currently don't have any logic for making inferences based on
> unique constraints.
Huh?
http://archives.postgresql.org/pgsql-committers/2009-09/msg00159.php
Admittedly it's just one case and there's lots more to be done, but it's
more than nothing. So this is a *potential* argument for trying to trim
subquery outputs. What I'm not sure about is whether there are common
cases where this would be applicable below a non-flattenable subquery.
regards, tom lane