ecxt_scantuple has wrong TupleDesc

Hi all,

I wonder why my ecxt_scantuple has a TupleDesc matching the subplan's tlist, not my plan's tlist. Basically, my
questionis what is x in
 

econtext->ecxt_scantuple = x ?

I have written a new executor node Foo, with corresponding ExecFoo and make_foo functions. I have also written a new
Exprtype called Bar, along with a ExecEvalBar. 
 

In make_foo I append some Bar columns. When I try to evaluate the Bar columns in ExecFoo via ExecProject, I need the
Barcolumns to access each other. 
 

But sadly, the ecxt_scantuple doesn't have any Bar columns because it has a TupleDesc from the subplan.

What are the steps I have to take to make sure I can access my Bar columns (which only exist in Foo) in ExecEvalBar?
Wheredo I get a TupleTableSlot with the desired TupleDesc from?
 

I'll be happy to provide more information, if needed.

Regards
Peter