This chapter discusses the automatic SQL tuning features of Oracle Database. Automatic SQL tuning automates the manual process, which is complex, repetitive, and time-consuming.

This chapter contains the following sections:

See Also:
Oracle Database 2 Day + Performance Tuning Guide for information about using the automatic SQL tuning features of Oracle Database with Oracle Enterprise Manager

Apr 03, 2016 These messages indicate that an auto kill of a 'hung'/long running tuning task has taken place. This is a protective measure purely to avoid the task from over-running its time limit because of a single task and protects a the system from harm caused by such over-running. End automatic SQL Tuning Advisor run for special tuning task 'SYSAUTOSQLTUNINGTASK'. The database was in this case moved to a different system, i.e. From a solaris global zone to another global zone. Sign In: To view full details, sign in with your My Oracle Support account. Oracle 12c offers a useful utility called the SQL Tuning Advisor. You can use this built-in tool to provide suggestions or recommendations about certain SQL statements. Although it may not always give perfect advice, just like anything else, having it in your toolbox of tuning techniques is beneficial. Use PL/SQL and the internal package DBMSSQLTUNE. Oracle Toad expert blog for developers, admins and data analysts. With solutions for Toad for Oracle, Toad for MySQL, Toad for SQL Server, DB2, SAP and more. Apr 03, 2016 These messages indicate that an auto kill of a 'hung'/long running tuning task has taken place. This is a protective measure purely to avoid the task from over-running its time limit because of a single task and protects a the system from harm caused by such over-running.

17.1 Automatic Tuning Optimizer

When SQL statements are executed by the Oracle database, the query optimizer is used to generate the execution plans of the SQL statements. The query optimizer operates in two modes: a normal mode and a tuning mode.

In normal mode, the optimizer compiles the SQL and generates an execution plan. The normal mode of the optimizer generates a reasonable execution plan for the vast majority of SQL statements. Under normal mode, the optimizer operates with very strict time constraints, usually a fraction of a second, during which it must find a good execution plan.

In tuning mode, the optimizer performs additional analysis to check whether the execution plan produced under normal mode can be improved further. The output of the query optimizer is not an execution plan, but a series of actions, along with their rationale and expected benefit for producing a significantly superior plan. When running in the tuning mode, the optimizer is referred to as the Automatic Tuning Optimizer.

Under tuning mode, the optimizer can take several minutes to tune a single statement. It is both time and resource intensive to invoke the Automatic Tuning Optimizer every time a query has to be hard-parsed. The Automatic Tuning Optimizer is meant to be used for complex and high-load SQL statements that have non-trivial impact on the entire system. The Automatic Database Diagnostic Monitor (ADDM) proactively identifies high-load SQL statements which are good candidates for SQL tuning. See Chapter 6, 'Automatic Performance Diagnostics'. The automatic SQL tuning feature of Oracle Database also automatically identifies problematic SQL statements and implements tuning recommendations during system maintenance windows as an automated maintenance task.

The Automatic Tuning Optimizer performs four types of tuning analysis:

17.1.1 Statistics Analysis

The query optimizer relies on object statistics to generate execution plans. If these statistics are stale or missing, the optimizer does not have the necessary information it needs and can generate poor execution plans. The Automatic Tuning Optimizer checks each query object for missing or stale statistics, and produces two types of output:

Recommendations to gather relevant statistics for objects with stale or no statistics.
Because optimizer statistics are automatically collected and refreshed, this problem may be encountered only when automatic optimizer statistics collection has been turned off. See 'Automatic Optimizer Statistics Collection'.
Auxiliary information in the form of statistics for objects with no statistics, and statistic adjustment factor for objects with stale statistics.

This auxiliary information is stored in an object called a SQL Profile.

17.1.2 SQL Profiling

The query optimizer can sometimes produce inaccurate estimates about an attribute of a statement due to lack of information, leading to poor execution plans. Traditionally, users have corrected this problem by manually adding hints to the application code to guide the optimizer into making correct decisions. For packaged applications, changing application code is not an option and the only alternative available is to log a bug with the application vendor and wait for a fix.

Automatic SQL tuning deals with this problem with its SQL profiling capability. The Automatic Tuning Optimizer creates a profile of the SQL statement called a SQL Profile, consisting of auxiliary statistics specific to that statement. The query optimizer under normal mode makes estimates about cardinality, selectivity, and cost that can sometimes be off by a significant amount resulting in poor execution plans. SQL Profile addresses this problem by collecting additional information using sampling and partial execution techniques to verify and, if necessary, adjust these estimates.

During SQL Profiling, the Automatic Tuning Optimizer also uses execution history information of the SQL statement to appropriately set optimizer parameter settings, such as changing the OPTIMIZER_MODE initialization parameter setting from ALL_ROWS to FIRST_ROWS for that SQL statement.

The output of this type of analysis is a recommendation to accept the SQL Profile. A SQL Profile, once accepted, is stored persistently in the data dictionary. Note that the SQL Profile is specific to a particular query. If accepted, the optimizer under normal mode uses the information in the SQL Profile in conjunction with regular database statistics when generating an execution plan. The availability of the additional information makes it possible to produce well-tuned plans for corresponding SQL statement without requiring any change to the application code.

The scope of a SQL Profile can be controlled by the CATEGORY profile attribute. This attribute determines which user sessions can apply the profile. You can view the CATEGORY attribute for a SQL Profile in CATEGORY column of the DBA_SQL_PROFILES view. By default, all profiles are created in the DEFAULT category. This means that all user sessions where the SQLTUNE_CATEGORY initialization parameter is set to DEFAULT can use the profile.

By altering the category of a SQL profile, you can determine which sessions are affected by the creation of a profile. For example, by setting the category of a SQL Profile to DEV, only those users sessions where the SQLTUNE_CATEGORY initialization parameter is set to DEV can use the profile. All other sessions do not have access to the SQL Profile and execution plans for SQL statements are not impacted by the SQL profile. This technique enables you to test a SQL Profile in a restricted environment before making it available to other user sessions.

Sys_auto_sql_tuning_task