Re: Properly pathify the union planner

It seems ok that
the ec_indexes are not set for the top-level set RelOptInfo as
get_eclass_for_sort_expr() does not make use of ec_indexes while
searching for an existing EquivalenceClass.  Maybe we should fix this
varno == 0 hack and adjust get_eclass_for_sort_expr() so that it makes
use of the ec_indexes.

It's possible to see this happen with a query such as:

SELECT oid FROM pg_class UNION SELECT oid FROM pg_class ORDER BY oid;

I didn't see that as a reason not to push this patch as this occurs
both with and without this change, so I've now pushed this patch.