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Don Burleson Blog 







Oracle SQL Profiles

Expert Oracle Database Tips by Donald BurlesonMarch 28, 2015


Automatic SQL Tuning

The Oracle SQL optimizer has always been a key component of execution (explain) plan management.  The optimizer was charged with the responsibility of making decisions about execution plans.  However, the optimizer has not always been given sufficient time to gather the information necessary for the best possible decision.  Automatic SQL tuning and SQL Profiles have made this process easier for the optimizer.

The evolution of the internal aids to the optimizer, including SQL Profiles, is as follows:

  • Stored Outlines:  Also called Optimizer Plan Stability, stored outlines were cumbersome to manage.  Additionally it was difficult to swap out execution plans when using plan stability.
  • SQL Profiles - A SQL Profile is a bundle of improved optimizer statistics that is stored in the data dictionary.  SQL Profiles are create by running Automatic SQL Tuning.  The SQL Profile approach first appeared in Oracle 10g.  Allowed to run in tuning mode, the optimizer can gather additional information for making tuning recommendations on specific SQL statements.  Tuning mode also allows the optimizer to recommend the gathering of statistics on objects with stale or missing statistics.  These additional statistics are stored in an SQL Profile.  A SQL tuning Set (STS) could be tested as a workload, and Oracle would allow the DBA to implement changes to execution plans. 
  • SQL Plan management - Starting in 11g, there is an even easier to use approach to locking-down SQL execution plans.  The 10g SQL Profile approach is deprecated, and uses only two parameters, optimizer_capture_sql_plan_baselines and optimizer_use_sql_plan_baselines.  Also see dba_sql_plan_baselines tips.

SQL Profiles

Alluded to above, the automatic tuning advisor allowed the implementation of tuning suggestions in the form of SQL profiles (enhanced CBO statistics, stored in the data dictionary) that will improve performance.  SQL profiles can be used the same way as their historic counterparts, stored outlines.  However, the SQL Profile is easier to use and manage with the help of the automatic tuning advisor. 

Simply put, the SQL Profile is a collection of historical information from prior runs of the SQL statement.  The SQL Profile can contain comparison details of the actual and estimated cardinality and predicate selectivity, etc. A SQL Profile is stored persistently in the data dictionary, so it does not require any application code changes.

In comparison to normal optimization, a SQL profile helps generate a better execution plan  because it is tested against a real-world workload in the SQL Tuning Set (STS).  The SQL statement is thoroughly profiled with the aid of additional tasks like checking for advanced predicate selectivity, correlation between columns, join skews, and complex predicates such as functions. Once a SQL statement is profiled and stored, differing execution plans can be invoked at will.

The optimizer may also be able to improve performance by altering session specific parameters such as the optimizer_mode.  The SQL Profile stores the information on possible improvements.  The information in the SQL Profile, if accepted, is available to the optimizer when running in normal rather than tuning mode.  Where stored outlines fixed the execution plan, a SQL Profile can continue to be beneficial as the contents of the table change.  Thus, it is not necessary or preferred to update SQL Profiles as long as they are beneficial.

Swapping SQL Profiles

While it may not be necessary to swap SQL Profiles as often as it was necessary to swap stored outlines, there are times when it becomes desirable.  The concept of  swapping SQL profiles  is fairly simple.

A SQL profile that specifies the SQL statement is defined for the statement to be tuned.  The new SQL Profile includes an alternative execution plan, in the form of hints.  When this SQL is executed and hits the library cache, Oracle detects that a SQL profile exists for this statement, and automatically applies the hints to change the execution plan.

This approach allows SQL statements to be tuned without ever having to touch the SQL statement itself.  To do this, we use the DBMS_SQLTUNE package which has an import_sql_profile procedure which allows you to swap hints from one SQL profile into another SQL profile.

   sql_text => 'select * from emp',
   profile => sqlprof_attr('ALL_ROWS','IGNORE_OPTIM_EMBEDDED_HINTS')
   category => 'DEFAULT',
   name => 'change_emp',
   force_match => &&6

To see how you can swap-out SQL profiles, examine the dbms_sqltune package and the import_sql_profile procedure.

Although SQL Profiles have been an improvement over stored outlines, execution plan management in 11g goes further to help lock down critical SQL execution plans, and tests execution timings before implementing changes.

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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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