There are already quite a few integration tests using window functions, but the unit tests coverage for window funtions is not ideal.
We'd like to test the following aspects:
- different partition clauses (none, one, multiple)
- different order clauses (none, one, multiple, asc/desc, nulls first/last)
- Frames and their combinations
- Aggregate function types
- Cover the conditions that WindowExec should spill at least once