Wednesday, December 28, 2011


As a DBA some I feel that Oracle is living thing ! for example query performance. Out of no where a gentle looking SQL statement may pop up your top time consuming query list. A sql query which gives efficient performance in 3 months ago (Or even week ago), now giving horrible performance that threatening you cool database performance :)

Yes, we all DBA know that a gentle statement can screwed up due to
- Re-guthering optimizer statistics
- Change in optimizes parameters
- Change in Schema? Object structure (i.e, metadata definition)

Though Optimizer always try to choose list expensive plan for each SQL statement execution, Some time It (optimizer) choose expensive plans and raise annoying situations for peace loving DBAs :)

Well Oracle 11g come up with a new feature that can handle such weird situation. The simplest solution we can think is a mechanism that only allow trusted SQL plans to be executed and reject untrusted plans. O Yes Oracle 11g gives us a manager named SPM that do the same thing.

SQL PLAN MANAGER (SPM) has three major components

  • SQL Plan Baseline Capture - Create SQL Plan Baseline for trusted (accepted) SQL plans.
  • SQL Plan Baseline Selection - Conform that only accepted SQl plans are used when an SQL statement is executed.
  • SQL Plan Baseline Evolution - Evaluated all SQL Plans (old & new) for each SQL statement. The new/old plan will only be excepted if it gives better or equal performance compared with existing trusted plan otherwise rejected. (If no trusted plan exist then current plan will be accepted)

To enable this cool feature we just do the followings

ALTER SYSTEM set optimizer_capture_sql_plan_baselines= TRUE scope=both; [Default value is false]
ALTER SYSTEM set optimizer_user_sql_plan_baselines= TRUE scope=both; [Default value is true, so just check the parameter value]

You are done ! Keep an eye in DBA_SQL_PLAN_BASELINES view and enjoying SPM's magic
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