Cost-Based Optimizer. An SQL Query optimizer that uses data statistics to identify the query plan with the lowest execution cost. The cost is based on the number of rows in a table, index efficiency, etc. All applications should be converted to use CBO, as RBO will not be available in Oracle 10 and above.
Cost-based optimizer. Generates a set of potential execution plans for SQL statements, estimates the cost of each plan, calls the plan generator to generate the plan, compares the costs, and chooses the plan with the lowest cost. This approach is used when the data dictionary has statistics for at least one of the tables accessed by the SQL statements. The CBO is made up of the query transformer, the estimator, and the plan generator.