If the input is ordered by all the columns, then OrderedDistinct can be used. This algorithm reads each row and compares it to the previous row. If it is the same, it is ignored; otherwise, it is output. OrderedDistinct is effective if rows are already ordered (perhaps because of an index or a merge join); if the input is not ordered, the optimizer inserts a sort. No work table is used by the OrderedDistinct itself, but one is used by any inserted sort.