Re: Invalid query generated by postgres_fdw with UNION ALL and ORDER BY

I've looked at this patch a bit.  I once wondered why we don't check
pathkey->pk_eclass->ec_has_const with EC_MUST_BE_REDUNDANT to see if the
pathkey is not needed.  Then I realized that a child member would not be
marked as constant even if the child expr is a Const, as explained in
add_child_rel_equivalences().

BTW, I wonder if it is possible that we have a pseudoconstant expression
that is not of type Const.  In such cases we would need to check
'bms_is_empty(pull_varnos(em_expr))' instead of 'IsA(em_expr, Const)'.
However, I'm unable to think of such an expression in this context.

The patch looks good to me otherwise.

Thanks
Richard