How the query optimizer makes your queries more efficient

The optimizer is an important part of DB2 UDB for AS/400 because the optimizer:

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

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

• Cost estimation for queries
• Access plan validation
• Join optimization
• Grouping optimization