Disabling cost-based optimizations (CBO) forces the execution plan to be generated from rule-based heuristics. DISABLE_CBO causes the query optimizer to ignore any table boundary statistics or other table statistics that might have been gathered; the query optimizer applies only heuristics-based optimizations to the execution plan.

If DISABLE_CBO is not specified, the query optimizer applies cost-based optimizations in addition to heuristics-based optimizations.

Note: When SELECT options are specified as part of a subquery or subselect, they might not affect the root-level query execution plan.