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Don Burleson Blog 







Historical analysis of SQL execution plans

Oracle Database Tips by Donald BurlesonJanuary 9,  2015


SQL Plan history Analysis is not an easy chore.  As the world's most robust and complex database, Oracle offers something for everyone, and it can be confusing to optimize Oracle because of the plethora of features.

One wonderful feature of Oracle is the ability to have SQL execution plans change when the characteristics of the underlying table changes.   For shops where a table is huge one day and small the next, of shops where column distribution vary wildly, dynamic SQL is a Godsend.

However, dynamic SQL changes are not so great for stable shops where there exists one, and only one optimal execution plan for any given SQL statement.

To learn more, see my notes on identifying your SQL philosophy.

Because SQL plans will change whenever we make changes to the instance, any SQL statement will have a history of execution plans.  Here are some common acts that will change execution plans:

  • Enabling dynamic sampling

  • Table in index DDL (e.g. dropping an index)

  • Re-analyze the schema statistics (done automatically, starting in 10g)

  • Changing an optimizer parameter (optimizer_mode, optimizer_index_cost_adj)

  • Enabling parallelism

This execution plan history can be seen by running scripts against the STATSPACK (stats$sql_plan and stats$sql_plan_usage) or the AWR (dba_hist_sql_plan) tables. 

Once you have acquired the SQL_ID for a statement, you can use the built-in dbms_xplan.display_awr procedure to see all the different execution plans for the query.

The dbms_xplan.display_awr allows you to input only a SQL_ID, and Oracle will show you the of the explain plans for that SQL_ID, as recorded in the AWR. 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'));

For more detailed scripts, and free STATSPACK script to monitor SQL execution patterns with details on historical SQL plan analysis, see my book "Oracle Tuning: The Definitive Reference".

Oracle 11g has introduced a wealth of new tools to help us "freeze" optimal SQL execution plan, so that they do not change when the data changes, tools such as the 11g SQL Performance Analyzer which uses a scientific approach to run real-world workloads and only change the SQL execution plans when they are at least 3x faster.  For complete details, see the 11g New Features book.

Also, see these notes from the Oracle CBO group on SQL plan history analysis., where they coined the term "plan regression" for changes that result in a sub-optimal execution plan.  While Oracle 11g has tools for tracking explain plans, you can still choose manual methods, gathering changes to execution plans from STATSPACK or the extra-cost AWR tables.

The following script will identify those SQL statement that have more than one execution plan in the library cache.  Note that this SQL also looks into historical SQL plans (using dba_hist_sqlstat).  This is an important report for showing the effect of DDL and statistics changes on the execution plans of SQL in a controlled, production environment.

   v$session vs,
   v$sql     sq
   sq.plan_hash_value not in
      dba_hist_sqlstat ss
      ss.sql_id=sq.sql_id) and
   0 <
           dba_hist_sqlstat ss

Get the Complete
Oracle SQL Tuning Information 

The landmark book "Advanced Oracle SQL Tuning  The Definitive Reference"  is filled with valuable information on Oracle SQL Tuning. This book includes scripts and tools to hypercharge Oracle 11g performance and you can buy it for 30% off directly from the publisher.



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