Re: Is select_outer_pathkeys_for_merge() too strict now we have Incremental Sorts?

So the idea is if the ECs used by the mergeclauses are prefix of the
query_pathkeys, we use this prefix as pathkeys for the mergejoin. Why
not relax this further that if the ECs in the mergeclauses and the
query_pathkeys have common prefix, we use that prefix as pathkeys? So
that we can have a plan like below:

# explain (costs off) select * from t1 join t2 on t1.c = t2.c and t1.a = t2.a order by t1.a DESC, t1.b;
                      QUERY PLAN
-------------------------------------------------------
 Incremental Sort
   Sort Key: t1.a DESC, t1.b
   Presorted Key: t1.a
   ->  Merge Join
         Merge Cond: ((t1.a = t2.a) AND (t1.c = t2.c))
         ->  Sort
               Sort Key: t1.a DESC, t1.c
               ->  Seq Scan on t1
         ->  Sort
               Sort Key: t2.a DESC, t2.c
               ->  Seq Scan on t2
(11 rows)

Thanks
Richard