How the query optimizer makes your queries more efficient

Data manipulation statements such as SELECT specify only what data the user wants, not how to retrieve that data. This path to the data is chosen by the optimizer and stored in the access plan. You should understand the techniques employed by the query optimizer for performing this task.

The optimizer is an important part of DB2 Universal Database™ for iSeries™ because the optimizer:

Makes the key decisions which affect database performance.
Identifies the techniques which can be used to implement the query.
Selects the most efficient technique.