The OPTIMIZER_USE_PLAN_BASELINES parameter is set to TRUE. The optimizer
generates a plan for a SQL statement but does not find a matching plan in the SQL plan
baseline.
Which two operations are performed by the optimizer in this scenario? (Choose two.)
A.
The optimizer adds the new plan to the plan history.
B.
The optimizer selects the new plan for the execution of the SQL statement.
C.
The optimizer adds the new plan to the SQL plan baseline as an accepted plan.
D.
The optimizer adds the new plan to the SQL plan baseline but not in the ENABLED state.
E.
The optimizer costs each of the accepted plans in the SQL plan baseline and picks the
one with the lowest cost.
Explanation:
15.2.2 Selecting SQL Plan Baselines
During the SQL plan baseline selection phase, Oracle Database detects plan changes
based on the stored plan history, and selects plans to avoid potential performance
regressions for a set of SQL statements.
Each time the database compiles a SQL statement, the optimizer does the following:
1. Uses a cost-based search method to build a best-cost plan
2. Tries to find a matching plan in the SQL plan baseline
3. Does either of the following depending on whether a match is found:
If found, then the optimizer proceeds using the matched plan
If not found, then the optimizer evaluates the cost of each accepted plan in the SQL plan
baseline and selects the plan with the lowest cost
The best-cost plan found by the optimizer that does not match any plans in the plan history
for the SQL statement represents a new plan. The database adds this plan as a
nonaccepted plan to the plan history. The database does not use the new plan until it is
verified to not cause a performance regression. However, if a change in the system (such as
a dropped index) causes all accepted plans to become non-reproducible, then the optimizer
selects the best-cost plan. Thus, the presence of a SQL plan baseline causes the optimizer
to use conservative plan selection strategy for the SQL statement.
To enable the use of SQL plan baselines, set the
OPTIMIZER_USE_SQL_PLAN_BASELINES initialization parameter to TRUE (default).
A SQL plan baseline contains one or more accepted plans.
The plan history is the set of plans, both accepted and not accepted, that the optimizer
generates for a SQL statement over time, the plans in the baseline form a subset of the plan
history. For example, after the optimizer generates the first acceptable plan for a SQL plan
baseline, subsequent plans are part of the plan history but not part of the plan baseline.
