Aggregator performance improves dramatically if records are sorted before passing to the aggregator and if the ‘sorted input’ option under Aggregator Properties is checked. The record set should be sorted on those columns that are used in the Group By operation. It is often a good idea to sort the record set in the database level, e.g., inside a source qualifier transformation, unless there is a chance that the already sorted records from the source qualifier can again become unsorted before reaching the aggregator.