Parameter type:	String
Syntax:	OPTIMIZER_MODE = {RULE | CHOOSE | FIRST_ROWS | ALL_ROWS}
Parameter class:	Dynamic. Scope=ALTER SESSION.
Default value:	CHOOSE
OPTIMIZER_MODE specifies the goal of optimization of SQL statements.
RULE specifies that rule-based optimization is to be used unless hints are specified in the query. CHOOSE specifies cost-based optimization for a SQL statement if the data dictionary contains statistics for at least one table accessed in the statement. If no such statistics are present, the optimizer uses the rule-based approach. FIRST_ROWS causes the optimizer to use cost-based optimization to choose execution plans that minimize response time. This setting overrides a nonzero setting for "OPTIMIZER_PERCENT_PARALLEL". ALL_ROWS causes the optimizer to use cost-based optimization to choose execution plans that minimize total execution time.
See Also: Oracle8i Designing and Tuning for Performance for more information on setting this parameter. Oracle8i Concepts and Oracle8i Designing and Tuning for Performance for more information about the optimizer.