OPTIMIZER_MODE

Parameter type	String
Syntax	OPTIMIZER_MODE = {first_rows_[1 | 10 | 100 | 1000] | first_rows | all_rows | choose | rule}
Default value	choose
Parameter class	Dynamic: ALTER SESSION

OPTIMIZER_MODE establishes the default behavior for choosing an optimization approach for the instance.

Values:

• rule

  The optimizer chooses a rule-based approach for all SQL statements regardless of the presence of statistics.

• choose

  The optimizer chooses between a cost-based approach and a rule-based approach based on whether statistics are available.

  If the data dictionary contains statistics for at least one of the accessed tables, then the optimizer uses a cost-based approach and optimizes with a goal of best throughput. If the data dictionary contains only some statistics, then the cost-based approach is used, and the optimizer must guess the statistics for the subjects without any statistics. This can result in sub-optimal execution plans. If the data dictionary contains no statistics for any of the accessed tables, then the optimizer uses a rule-based approach.

• first_rows_n

  The optimizer uses a cost-based approach, regardless of the presence of statistics, and optimizes with a goal of best response time to return the first n rows (where n = 1, 10, 100, 1000).

• first_rows

  The optimizer uses a mix of costs and heuristics to find a best plan for fast delivery of the first few rows.

• all_rows

  The optimizer uses a cost-based approach for all SQL statements in the session regardless of the presence of statistics and optimizes with a goal of best throughput (minimum resource use to complete the entire statement).

  See Also: Oracle9i Database Performance Guide and Reference for more information on setting this parameter Oracle9i Database Concepts and Oracle9i Database Performance Guide and Reference for more information about the optimizer