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Don Burleson Blog 







Oracle SQL Plan Regression

Oracle Database Tips by Donald BurlesonJanuary 20,  2015


In traditional regression analysis, past points are used in a predictive model to forecast future performance.  In Oracle11g, SQL plan regression refers to the process of tracking changed to execution plans and ensuring that the SQL does not "regress" into a sub-optimal execution plan when changes are made to optimizer parameters of CBO statistics.

One perplexing issue with Oracle is the dynamic nature of Oracle SQL, and the optimizer's propensity to change execution plans whenever CBO statistics change.  This has been dubbed "Monday Morning Mayhem" because some SQL explain plans will change for the worst.

Inside Oracle SQL plan regression

The CBO group notes that SQL plan regression of the processes of "reverting" to a sub-optimal execution plan.

"this best-cost plan will not be used but instead added to the statement's plan history for later verification.

In other words, the optimizer will use a known plan from the SQL plan baseline instead of a new and hitherto unknown plan."

The SQL plan regression feature operates on the processes of historical testing of SQL execution plans.  For details see my notes on SQL history analysis in 11g.

When invoking dbms_xplan.display_cursor, you can tell when SQL plan regression is being used by this message:

- SQL plan baseline SYS_SQL_PLAN_fcc170b0a62d0f4d used for this statement

If you purchased the extra-cost AWR, you can use the dbms_xplan.display_awr procedure allows you to input only a SQL_ID, and Oracle will show you the of the explain plans for that SQL_ID. This simple query will show changes to SQL explain plan history, if you have purchased the extra-cost licenses for AWR.

  select * from TABLE(DBMS_XPLAN.DISPLAY_AWR('&sqlid'));

In sum, Oracle SQL plan regression is a method that uses real-world empirical data to ensure that the optimal SQL execution plan is always used.

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