ASA SQL User's Guide
Query Optimization and Execution
Query execution algorithms
Duplicate elimination
If the input is ordered by all the columns, then an ordered distinct 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. The ordered distinct 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 ordered distinct itself, but one is used by any inserted sort.