Aggregator transformation performance can be improved by using the sorted input option. When you use sorted input, the PC Server assumes all data is sorted by group. As the PC Server reads rows for a group, it performs aggregate calculations. It performs aggregate calculation for a group as soon as it encounters a row with new group. When necessary, it stores group information in memory. To use the Sorted Input option, you must pass sorted data to the Aggregator transformation. You can gain performance with sorted ports when you configure the session with multiple partitions.

When you do not use sorted input, the PowerCenter Server performs aggregate calculations as it reads but, since data is not sorted, the PowerCenter Server stores data for each group until it reads the entire source to ensure all aggregate calculations are accurate. 

If you use sorted input and do not presort data correctly, you receive unexpected results.

Do not use sorted input if either of the following conditions are true:

• The aggregate expression uses nested aggregate functions.
• The session uses incremental aggregation.

If you use sorted input and do not sort data correctly, the session fails.