Rule Based Optimizer
From Oracle FAQ
The Rule Based Optimizer (RBO) is a SQL Query optimizer that uses heuristic rules to derive optimal query execution plans. RBO is enabled by setting OPTIMIZER_MODE=RULE in the server initialization parameter file, or by altering OPTIMIZER_GOAL for your session.
All applications should be converted to use the Cost Based Optimizer as the Rule Based Optimizer is not supported in Oracle 10g and above releases.
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