Optimizer

Optimizer is an important module of the i5/OS™ Query component because it makes the key decisions for good database performance. Its main objective is to find the most efficient access path to the data.

Query optimization is a trade-off between the time spent to select a query implementation and the time spent to run it. Query optimization must handle the following distinct user needs:

Quick interactive response
Efficient use of total-machine resources

In deciding how to access data, Optimizer does the following:

Determines possible implementations
Picks the optimal implementation for the i5/OS Query component to execute

Cost estimation
At runtime, the Optimizer chooses an optimal access method for the query by calculating an implementation cost based on the current state of the database.
Optimizer decision-making rules
In performing its function, Optimizer uses a general set of guidelines to choose the best method for accessing data.

Parent topic: SQL performance