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CHOOSE	The optimizer chooses between a cost-based approach and a rule-based approach, depending on whether statistics are available. This is the default value. 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 still used, but the optimizer must guess the statistics for the subjects without any statistics. This can result in suboptimal execution plans. If the data dictionary contains no statistics for any of the accessed tables, then the optimizer uses a rule-based approach.
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).
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 number of rows; n can equal 1, 10, 100, or 1000.
FIRST_ROWS	The optimizer uses a mix of cost and heuristics to find a best plan for fast delivery of the first few rows. Note: Using heuristics sometimes leads the CBO to generate a plan with a cost that is significantly larger than the cost of a plan without applying the heuristic. FIRST_ROWS is available for backward compatibility and plan stability.
RULE	The optimizer chooses a rule-based approach for all SQL statements regardless of the presence of statistics.