Sounds something like ORDERED BY x implying output is "known" ordered by x perhaps enough hint for the planner to make necessary plan choices though I may be wrong.
I may be missing something, but isnt what you mentioned the exact functionality this patch adds?
May be what Amit has in mind is that the planner can choose the most optimal sorting algorithm using the hint that the dataset is probably already sorted. Actually why not just do that? Though AFAIK its against our stated policy of not adding any planner hints.
