Modify the Find Best Plan(S) function to create a function Find Best Plan(S, O), where O is the desired sort order for S, and which considers interesting sort orders. A null order indicates that the order is not relevant. An algorithm A may give the desired order O; if not a sort operation may need to be added to get the desired order. If A is a merge-join, Find Best Plan must be invoked on the two inputs with the desired orders for the inputs.