Automatic SQL Tuning in Oracle Database 11g

posted Aug 3, 2011, 5:13 AM by Sachchida Ojha
As part of Automatic SQL Tuning, Oracle 11g automatically runs the SQL Tuning Advisor against high impact SQL statements during maintenance windows. This process involves the following steps:
  • AWR statistics are used to compile an ordered list of the SQL statements with the greatest performance impact on the system, where the impact is the sum of the CPU and I/O times for the statement during the past week. The list excludes statements that are inherently less tunable, such as recently (within a month) tuned recursive statements, parallel queries, DML, DDL and SQL statements whose performance problems are caused by concurrency issues.
  • The SQL tuning advisor is run against each statement in turn. The outcome may include both SQL profiles and other recommendations.
  • Suggested SQL profiles are performance tested, and those that result in at least a threefold improvement are accepted if the ACCEPT_SQL_PROFILES parameter is set to TRUE, or reported if it is set to FALSE.
  • The accepted SQL profiles are optionally implemented . Several factors many prevent SQL profiles from being implemented automatically, including stale optimizer statistics of dependent objects. The TYPE column of the DBA_SQL_PROFILES view indicates if SQL profiles are created manually (MANUAL) or automatically (AUTO-TUNE).
The ENABLE and DISABLE procedures of the DBMS_AUTO_TASK_ADMIN package control whether automatic SQL tuning is included in the automated maintenance tasks.
-- Enable
    client_name => 'sql tuning advisor',
    operation   => NULL, 
    window_name => NULL);

-- Disable
    client_name => 'sql tuning advisor',
    operation   => NULL, 
    window_name => NULL);
It is also indirectly disabled by setting the STATISTICS_LEVEL parameter to BASIC, as this stops automatic statistics gathering by the AWR.

The SET_TUNING_TASK_PARAMETER procedure of the DBMS_SQLTUNE package controls the behavior of the SQL tuning advisor. The parameters specifically for the automatic runs include:
  • ACCEPT_SQL_PROFILES - Automatically accept SQL profiles (default FALSE).
  • MAX_SQL_PROFILES_PER_EXEC - The maximum number of SQL profiles automatically implemented per run (default 20).
  • MAX_AUTO_SQL_PROFILES - The maximum number of automatic SQL profiles allowed on the system (default 10000).
The current parameter values are displayed using the %_ADVISOR_PARAMETERS views.
COLUMN parameter_value FORMAT A30

SELECT parameter_name, parameter_value
FROM   dba_advisor_parameters
AND    parameter_name IN ('ACCEPT_SQL_PROFILES',

------------------------------ ------------------------------
MAX_AUTO_SQL_PROFILES          10000

3 rows selected.

The following code shows how the SET_TUNING_TASK_PARAMETER procedure is used to turn on acceptance of automatically generated SQL profiles.
    task_name => 'SYS_AUTO_SQL_TUNING_TASK',
    parameter => 'ACCEPT_SQL_PROFILES',
    value     => 'TRUE');
The REPORT_AUTO_TUNING_TASK function of the DBMS_SQLTUNE package returns a CLOB containing a report from the specified automatic tuning task. Setting the BEGIN_EXEC and END_EXEC parameters to NULL produces a report from the most recent execution.
  :l_report := DBMS_SQLTUNE.report_auto_tuning_task(
    begin_exec   => NULL,
    end_exec     => NULL,
    type         => DBMS_SQLTUNE.type_text,     -- 'TEXT'
    level        => DBMS_SQLTUNE.level_typical, -- 'TYPICAL'
    section      => DBMS_SQLTUNE.section_all,   -- 'ALL'
    object_id    => NULL,
    result_limit => NULL);

SET LONG 1000000
PRINT :l_report

Tuning Task Name                        : SYS_AUTO_SQL_TUNING_TASK
Tuning Task Owner                       : SYS
Workload Type                           : Automatic High-Load SQL Workload
Execution Count                         : 31
Current Execution                       : EXEC_1_25
Execution Type                          : TUNE SQL
Scope                                   : COMPREHENSIVE
Global Time Limit(seconds)              : 3600
Per-SQL Time Limit(seconds)             : 1200
Completion Status                       : COMPLETED
Started at                              : 01/16/2008 22:00:06
Completed at                            : 01/16/2008 22:00:46
Number of Candidate SQLs                : 0
Cumulative Elapsed Time of SQL (s)      : 0


As you can see from the above report, this was run against a very quiet system, so there were no candidate SQL statements to process. If this were run against a more active system, you might expect the report to contain the following sections:
  • General information - High-level information about the SQL tuning task.
  • Summary - A summary of the SQL statements tuned during the task, including the estimated benefit associated with the tuning operation.
  • Tuning finding - Information about findings, acceptance of the profile, implementation of the profile, and detailed execution statistics for each analyzed statement.
  • Explain plans - The old and new execution plans for each analyzed statement.
  • Errors - Any errors encountered during the task.