Re: Aggregate Push Down - Performing aggregation on foreign server

While checking for shippability, we build the target list which is passed to
the foreign server as fdw_scan_tlist. The target list contains
a. All the GROUP BY expressions
b. Shippable entries from the target list of upper relation
c. Var and Aggref nodes from non-shippable entries from the target list of
upper relation

The code in the patch doesn't seem to add Var nodes explicitly. It assumes that
the Var nodes will be part of GROUP BY clause. The code is correct, I think.