OPTIMIZER_MODE

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).