I wrote:
> Bottom line seems to be that we should run through the in_info_list and
> force Vars mentioned therein to be propagated up at least to the
> "righthand" join level, ensuring they're available if we decide to
> unique-ify above that point.
I've committed a patch along these lines --- if you need it right now
see
http://archives.postgresql.org/pgsql-committers/2007-10/msg00093.php
regards, tom lane