Using Plan Baselines

Once the plan baselines have been created, the Optimizer may be directed to use only applicable plan baselines. The Optimizer will store any newly generated plans for the SQL statement into the plan history. But the plan actually selected for execution will only be the lowest-cost plan currently stored within the plan baseline.

Any newly generated plans, even if they are perceived to have a lower cost, will not be stored in the baseline, but simply be added to the plan history.

optimizer_use_sql_plan_baselines

Using plan baselines may be enabled at either the session or the system level. In this case a session level directive is issued.

Baseline Evolution

Once the Optimizer is instructed to use plan baselines, one will likely want to allow selected baselines to evolve. Once new plans are generated and they are verified by the Optimizer to be acceptable, result in superior performance, and not likely to trigger performance regressions, then these may be added to the baseline.