Rule-based optimizer. Chooses an execution plan for SQL statements based on the access paths available and the ranks of these access paths. If there is more than one way, then the RBO uses the operation with the lowest rank. Note: This feature has been desupported.
Rule-based optimizer. Chooses an execution plan for SQL statements based on the access paths available and the ranks of these access paths (if there is more than one way, then the RBO uses the operation with the lowest rank). The RBO is used if no statistics are available, otherwise the CBO is used.