Support limit per group in Window

We have a lot of queries like the following to retrieve top N tuples per group:

SELECT x, y FROM
     (SELECT x, y, ROW_NUMBER() OVER (PARTITION BY x ORDER BY y) 
     AS rn FROM t1) t2 WHERE rn <= 3;

The performance is not good if each group has a lot more tuples than wanted, because we will retrieve and sort all the tuples, instead of just doing a top-N heap sort.

In order to do optimization for this kind of query, we need to extend window to support limit, if and only if there is only 1 window function, and it is row_number(). We also need a substitute rule to push the limit into window. Of course, we also need to modify executor to support this optimization (can be later).

Filter (rn <= 3)
  +- Window (window#0={Partition by x order by y ROW_NUMBER()})

to

Filter (rn <= 3)
  +- Window (window#0={Partition by x order by y limit 3 ROW_NUMBER()})

Thoughts? Objections?