Automatic SQL Tuning
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.
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:
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.
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
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.
It is important to note that the SQL Profile does not freeze the execution plan of a SQL statement, as done by stored outlines. As tables grow or indexes are created or dropped, the execution plan can change with the same SQL Profile. The information stored in it continues to be relevant, even as the data distribution or access path of the corresponding statement change. In general, it is not necessary to refresh the SQL profiles. However, over a long period of time, its content can become outdated and may need to be regenerated. This can be done by running the SQL Tuning Advisor again on the same statement to regenerate the SQL Profile.
SQL Profiles apply to the following statement types:
INSERTstatements (only with a
TABLEstatements (only with the
MERGEstatements (the update or insert operations)
A complete set of functions are provided for management of SQL Profiles. See "SQL Profiles".
Indexes can tremendously enhance performance of a SQL statement by reducing the need for full table scans on large tables. Effective indexing is a common tuning technique. The Automatic Tuning Optimizer also explores whether a new index can significantly enhance the performance of a query. If such an index is identified, it recommends its creation.
Because the Automatic Tuning Optimizer does not analyze how its index recommendation can affect the entire SQL workload, it also recommends running the SQL Access Advisor utility on the SQL statement along with a representative SQL workload. The SQL Access Advisor looks at the impact of creating an index on the entire SQL workload before making any recommendations. See "Automatic SQL Tuning Features".
The Automatic Tuning Optimizer identifies common problems with structure of SQL statements that can lead to poor performance. These could be syntactic, semantic, or design problems with the statement. In each of these cases the Automatic Tuning Optimizer makes relevant suggestions to restructure the SQL statements. The alternative suggested is similar, but not equivalent, to the original statement.
For example, the optimizer may suggest to replace
UNION operator with
ALL or to replace
EXISTS. An application developer can then determine if the advice is applicable to their situation or not. For instance, if the schema design is such that there is no possibility of producing duplicates, then the
ALL operator is much more efficient than the
UNION operator. These changes require a good understanding of the data properties and should be implemented only after careful consideration.
The SQL Tuning Advisor takes one or more SQL statements as an input and invokes the Automatic Tuning Optimizer to perform SQL tuning on the statements. The output of the SQL Tuning Advisor is in the form of an advice or recommendations, along with a rationale for each recommendation and its expected benefit. The recommendation relates to collection of statistics on objects, creation of new indexes, restructuring of the SQL statement, or creation of a SQL profile. You can choose to accept the recommendation to complete the tuning of the SQL statements.
Oracle Database can automatically tune SQL statements by identifying problematic SQL statements and implementing tuning recommendations using the SQL Tuning Advisor during system maintenance windows. You can also run the SQL Tuning Advisor selectively on a single or a set of SQL statements that have been identified as problematic.
Oracle Database automatically runs the SQL Tuning Advisor on selected high-load SQL statements from the Automatic Workload Repository (AWR) that qualify as tuning candidates. This task, called Automatic SQL Tuning, runs in the default maintenance windows on a nightly basis. You can customize attributes of the maintenance windows, including start and end time, frequency, and days of the week.
Once automatic SQL tuning begins, which by default runs for at most one hour during a maintenance window, the following steps are performed:
Identify SQL candidates in the AWR for tuning.
Oracle Database analyzes statistics in the AWR and generates a list of potential SQL statements that are eligible for tuning. These statements include repeating high-load statements that have a significant impact on the system. Only SQL statements that have an execution plan with a high potential for improvement will be tuned. Recursive SQL and statements that have been tuned recently (in the last month) are ignored, as are parallel queries, DMLs, DDLs, and SQL statements with performance problems that are caused by concurrency issues. The SQL statements that are selected as candidates are then ordered based on their performance impact. The performance impact of a SQL statement is calculated by summing the CPU time and the I/O times captured in the AWR for that SQL statement in the past week.
Tune each SQL statement individually by calling the SQL Tuning Advisor.
During the tuning process, all recommendation types are considered and reported, but only SQL profiles can be implemented automatically.
Test SQL profiles by executing the SQL statement.
If a SQL profile is recommended, test the new SQL profile by executing the SQL statement both with and without the SQL profile. If the performance improvement improves at least threefold, the SQL profile will be accepted (when the
ACCEPT_SQL_PROFILEStask parameter is set to
TRUE). Otherwise, only the recommendation to create a SQL profile will be reported in the automatic SQL tuning reports.
Optionally implement the SQL profiles provided they meet the criteria of threefold performance improvement.
Note that other factors are considered when deciding whether or not to implement the SQL profile. For example, a SQL profile is not implemented if the objects referenced in the SQL statement have stale optimizer statistics. You can identify which SQL profiles have been implemented automatically as their type will be set to
If SQL plan management is used and there is already an existing plan baseline for the SQL statement, a new plan baseline will be added when a SQL profile is created. As a result, the new and improved SQL execution plan will be used by the optimizer immediately after the SQL profile is created. For information about SQL plan management, see Chapter 15, "Using SQL Plan Management".
At any time during or after the automatic SQL tuning process, you can view the results using the automatic SQL tuning report. This report describes in detail all the SQL statements that were analyzed, the recommendations generated, and the SQL profiles that were automatically implemented.
Figure 17-1 illustrates the steps performed by Oracle Database during the automatic SQL tuning process.
This section contains the following topics:
Automatic SQL tuning runs as part of the automated maintenance tasks infrastructure.
To enable automatic SQL tuning, use the
ENABLE procedure in the
BEGIN DBMS_AUTO_TASK_ADMIN.ENABLE( client_name => 'sql tuning advisor', operation => NULL, window_name => NULL); END; /
To disable automatic SQL tuning, use the DISABLE procedure in the DBMS_AUTO_TASK_ADMIN package:
BEGIN DBMS_AUTO_TASK_ADMIN.DISABLE( client_name => 'sql tuning advisor', operation => NULL, window_name => NULL); END; /
You can pass a specific window name using the
window_name parameter to enable or disable the task in certain maintenance windows only.
STATISTICS_LEVEL parameter to
BASIC will disable automatic statistics gathering by the AWR and, as a result, also disable automatic SQL tuning.
The behavior of the automatic SQL tuning task can be configured using the
DBMS_SQLTUNE package. To use the APIs, the user needs at least the
In addition to configuring the standard behavior of the SQL Tuning Advisor, the
DBMS_SQLTUNE package enables you to configure automatic SQL tuning by specifying the task parameters using the
SET_TUNING_TASK_PARAMETER procedure. Because the automatic tuning task is owned by SYS, only the SYS user can set the task parameters.
Table 17-2 lists the parameters that are specific to automatic SQL tuning which can be configured.
Table 17-1 SET_TUNING_TASK_PARAMETER Automatic SQL Tuning Parameters
Specifies whether to accept SQL profiles automatically.
Specifies the limit of SQL profiles that are accepted for each automatic SQL tuning task. Consider setting the limit of SQL profiles that are accepted for each automatic SQL tuning task based on the acceptable level of changes that can be made to the system on a daily basis.
Specifies the limit of SQL profiles that are accepted in total.
Specifies the number of days for which to save the task history in the advisor framework schema. By default, the task history is saved for 30 days before it expires.
To configure automatic SQL tuning, run the
SET_TUNING_TASK_PARAMETER procedure in the
BEGIN DBMS_SQLTUNE.SET_TUNING_TASK_PARAMETER( task_name => 'SYS_AUTO_SQL_TUNING_TASK', parameter => 'ACCEPT_SQL_PROFILES', value => 'TRUE'); END; /
In this example, the automatic SQL tuning task is configured to automatically accept SQL profiles recommended by the SQL Tuning Advisor.
The automatic SQL tuning report is generated using the
REPORT_AUTO_TUNING_TASK function and contains information about all executions of the automatic SQL tuning task. To run this report, you need the
ADVISOR privilege and SELECT privileges on the
DBA_ADVISOR views. Unlike the standard SQL tuning report generated using the
REPORT_TUNING_TASK function, which only contains information about a single task execution of the SQL Tuning Advisor, the automatic SQL tuning report contains information about multiple executions of the automatic SQL tuning task.
To view the automatic SQL tuning report, run the
REPORT_AUTO_TUNING_TASK function in the
variable my_rept CLOB; BEGIN :my_rept :=DBMS_SQLTUNE.REPORT_AUTO_TUNING_TASK( begin_exec => NULL, end_exec => NULL, type => 'TEXT', level => 'TYPICAL', section => 'ALL', object_id => NULL, result_limit => NULL); END; / print :my_rept
In this example, a text report is generated to display all SQL statements that were analyzed in the most recent execution, including recommendations that were not implemented, and all sections of the report are included.
Depending on the sections that were included in the report, you can view information about the automatic SQL tuning task in the following sections of the report:
The general information section provides a high-level description of the automatic SQL tuning task, including information about the inputs given for the report, the number of SQL statements tuned during the maintenance, and the number of SQL profiles that were created
The summary section lists the SQL statements (by their SQL identifiers) that were tuned during the maintenance window and the estimated benefit of each SQL profile, or their actual execution statistics after test executing the SQL statement with the SQL profile
This section contains the following information about each SQL statement analyzed by the SQL Tuning Advisor:
All findings associated with each SQL statement
Whether the profile was accepted on the system, and why
Whether the SQL profile is currently enabled on the system
Detailed execution statistics captured when testing the SQL profile
This section shows the old and new explain plans used by each SQL statement analyzed by the SQL Tuning Advisor.
This section lists all errors encountered by the automatic SQL tuning task.
The SQL Tuning Advisor can be invoked manually for on-demand tuning of one or more SQL statements. To tune multiple statements, you need to create a SQL tuning set (STS). A SQL tuning set is a database object that stores SQL statements along with their execution context. You can create a SQL tuning set using command line APIs or Oracle Enterprise Manager. See "SQL Tuning Sets".
This section contains the following topics:
Running the SQL Tuning Advisor
The input for the SQL Tuning Advisor can come from several sources. These input sources include:
Automatic Database Diagnostic Monitor
The primary input source is the Automatic Database Diagnostic Monitor (ADDM). By default, ADDM runs proactively once every hour and analyzes key statistics gathered by the Automatic Workload Repository (AWR) over the last hour to identify any performance problems including high-load SQL statements. If a high-load SQL is identified, ADDM recommends running SQL Tuning Advisor on the SQL. See "Overview of the Automatic Database Diagnostic Monitor".
Automatic Workload Repository
The second most important input source is the Automatic Workload Repository (AWR). The AWR takes regular snapshots of the system activity, including high-load SQL statements ranked by relevant statistics, such as CPU consumption and wait time.
You can view the AWR and manually identify high-load SQL statements and run the SQL Tuning Advisor on them, though this is done automatically by Oracle Database as part of the automatic SQL tuning process. By default, the AWR retains data for the last eight days. Any high-load SQL that ran within the retention period of the AWR can be located and tuned using this method. See "Overview of the Automatic Workload Repository".
The third likely source of input is the cursor cache. This source is used for tuning recent SQL statements that are yet to be captured in the AWR. The cursor cache and AWR together provide the capability to identify and tune high-load SQL statements from the current time going as far back as the AWR retention allows, which by default is at least 8 days.
SQL Tuning Set
Another possible input source for the SQL Tuning Advisor is the SQL Tuning Set. A SQL Tuning Set (STS) is a database object that stores SQL statements along with their execution context. An STS can include SQL statements that are yet to be deployed, with the goal of measuring their individual performance, or identifying the ones whose performance falls short of expectation. When a set of SQL statements are used as input, a SQL Tuning Set (STS) has to be first constructed and stored. See "SQL Tuning Sets".
The SQL Tuning Advisor provides options to manage the scope and duration of a tuning task. The scope of a tuning task can be set to limited or comprehensive.
If the limited option
is chosen, the SQL Tuning Advisor produces recommendations based on statistics checks, access path analysis, and SQL structure analysis. SQL Profile recommendations are not generated.
If the comprehensive option is selected, the SQL Tuning Advisor carries out all the analysis it performs under limited scope plus SQL Profiling. With the comprehensive option you can also specify a time limit for the tuning task, which by default is 30 minutes.
After analyzing the SQL statements, the SQL Tuning Advisor provides advice on optimizing the execution plan, the rationale for the proposed optimization, the estimated performance benefit, and the command to implement the advice. You simply have to choose whether or not to accept the recommendations to optimize the SQL statements.
The recommended interface for running the SQL Tuning Advisor is the Oracle Enterprise Manager. Whenever possible, you should run the SQL Tuning Advisor using Oracle Enterprise Manager, as described in theOracle Database 2 Day + Performance Tuning Guide . If Oracle Enterprise Manager is unavailable, you can run the SQL Tuning Advisor using procedures in the
DBMS_SQLTUNE package. To use the APIs, the user must be granted specific privileges.
Running SQL Tuning Advisor using
DBMS_SQLTUNE package is a multi-step process:
Create a SQL Tuning Set (if tuning multiple SQL statements)
Create a SQL tuning task
Execute a SQL tuning task
Display the results of a SQL tuning task
Implement recommendations as appropriate
A SQL tuning task can be created for a single SQL statement. For tuning multiple statements, a SQL Tuning Set (STS) has to be first created. An STS is a database object that stores SQL statements along with their execution context. An STS can be created manually using command line APIs or automatically using Oracle Enterprise Manager. See "SQL Tuning Sets".
Figure 17-2 shows the steps involved when running the SQL Tuning Advisor using the
This section covers the following topics:
You can create tuning tasks from the text of a single SQL statement, a SQL Tuning Set containing multiple statements, a SQL statement selected by SQL identifier from the cursor cache, or a SQL statement selected by SQL identifier from the Automatic Workload Repository.
For example, to use the SQL Tuning Advisor to optimize a specified SQL statement text, you need to create a tuning task with the SQL statement passed as a CLOB argument. For the following PL/SQL code, the user HR has been granted the
ADVISOR privilege and the function is run as user HR on the
employees table in the HR schema.
DECLARE my_task_name VARCHAR2(30); my_sqltext CLOB; BEGIN my_sqltext := 'SELECT /*+ ORDERED */ * ' || 'FROM employees e, locations l, departments d ' || 'WHERE e.department_id = d.department_id AND ' || 'l.location_id = d.location_id AND ' || 'e.employee_id < :bnd'; my_task_name := DBMS_SQLTUNE.CREATE_TUNING_TASK( sql_text => my_sqltext, bind_list => sql_binds(anydata.ConvertNumber(100)), user_name => 'HR', scope => 'COMPREHENSIVE', time_limit => 60, task_name => 'my_sql_tuning_task', description => 'Task to tune a query on a specified employee'); END; /
In this example, 100 is the value for bind variable
:bnd passed as function argument of type
HR is the user under which the
CREATE_TUNING_TASK function analyzes the SQL statement, the scope is set to
COMPREHENSIVE which means that the advisor also performs SQL Profiling analysis, and 60 is the maximum time in seconds that the function can run. In addition, values for task name and description are provided.
CREATE_TUNING_TASK function returns the task name that you have provided or generates a unique task name. You can use the task name to specify this task when using other APIs. To view the task names associated with a specific owner, you can run the following:
SELECT task_name FROM DBA_ADVISOR_LOG WHERE owner = 'HR';
You can fine tune a SQL tuning task after it has been created by configuring its parameters using the
SET_TUNING_TASK_PARAMETER procedure in the
BEGIN DBMS_SQLTUNE.SET_TUNING_TASK_PARAMETER( task_name => 'my_sql_tuning_task', parameter => 'TIME_LIMIT', value => 300); END; /
In this example, the maximum time that the SQL tuning task can run is changed to 300 seconds.
Table 17-2 lists the parameters that can be configured using the
Table 17-2 SET_TUNING_TASK_PARAMETER Procedure Parameters
Specifies the scope of the tuning task:
Username under which the SQL statement will be parsed
Number of days before the task is deleted
Default execution type if not specified by the
Time limit (in number of seconds) before the task times out
Time limit (in number of seconds) for each SQL statement
Determines if the SQL Tuning Advisor will test execute the SQL statements to verify the recommendation benefit:
Basic filter used for SQL tuning set
Object filter used for SQL tuning set
Plan filter used for SQL tuning set
First ranking measure used for SQL tuning set
Second ranking measure used for SQL tuning set
Third ranking measure used for SQL tuning set
Extra filter used for SQL tuning set (besides
Maximum number of SQL statements to tune
Percentage filter of statements from SQL tuning set
After you have created a tuning task, you need to execute the task and start the tuning process. For example:
BEGIN DBMS_SQLTUNE.EXECUTE_TUNING_TASK( task_name => 'my_sql_tuning_task' ); END; /
Like any other SQL Tuning Advisor task, you can also execute the automatic tuning task
SYS_AUTO_SQL_TUNING_TASK using the
EXECUTE_TUNING_TASK API. The SQL Tuning Advisor will perform the same analysis and actions as it would when run automatically. You can also pass an execution name to the API to name the new execution.
You can check the status of the task by reviewing the information in the USER_ADVISOR_TASKS view or check execution progress of the task in the
V$SESSION_LONGOPS view. For example:
SELECT status FROM USER_ADVISOR_TASKS WHERE task_name = 'my_sql_tuning_task';
You can check the execution progress of the SQL Tuning Advisor in the
V$ADVISOR_PROGRESS view. For example:
SELECT sofar, totalwork FROM V$ADVISOR_PROGRESS WHERE user_name = 'HR' AND task_name = 'my_sql_tuning_task';
After a task has been executed, you display a report of the results with the
REPORT_TUNING_TASK function. For example:
SET LONG 1000 SET LONGCHUNKSIZE 1000 SET LINESIZE 100 SELECT DBMS_SQLTUNE.REPORT_TUNING_TASK( 'my_sql_tuning_task') FROM DUAL;
The report contains all the findings and recommendations of the SQL Tuning Advisor. For each proposed recommendation, the rationale and benefit is provided along with the SQL commands needed to implement the recommendation.
Additional information about tuning tasks and results can be found in DBA views. See "SQL Tuning Information Views".
You can use the following APIs for managing SQL tuning tasks:
INTERRUPT_TUNING_TASKto interrupt a task while executing, causing a normal exit with intermediate results
RESUME_TUNING_TASKto resume a previously interrupted task
CANCEL_TUNING_TASKto cancel a task while executing, removing all results from the task
RESET_TUNING_TASKto reset a task while executing, removing all results from the task and returning the task to its initial state
DROP_TUNING_TASKto drop a task, removing all results associated with the task
A SQL Tuning Set (STS) is a database object that includes one or more SQL statements along with their execution statistics and execution context, and could include a user priority ranking. The SQL statements can be loaded into a SQL Tuning Set from different SQL sources, such as the Automatic Workload Repository, the cursor cache, or custom SQL provided by the user. An STS includes:
A set of SQL statements
Associated execution context, such as user schema, application module name and action, list of bind values, and the cursor compilation environment
Associated basic execution statistics, such as elapsed time, CPU time, buffer gets, disk reads, rows processed, cursor fetches, the number of executions, the number of complete executions, optimizer cost, and the command type
Associated execution plans and row source statistics for each SQL statement (optional)
SQL statements can be filtered using the application module name and action, or any of the execution statistics. In addition, the SQL statements can be ranked based on any combination of execution statistics.
A SQL Tuning Set can be used as input to the SQL Tuning Advisor, which performs automatic tuning of the SQL statements based on other input parameters specified by the user. SQL Tuning Sets are transportable across databases and can be exported from one system to another, allowing for the transfer of SQL workloads between databases for remote performance diagnostics and tuning. When poorly performing SQL statements are encountered on a production system, it may not be desirable for developers to perform their investigation and tuning activities on the production system directly. This feature allows the DBA to transport the problematic SQL statements to a test system where the developers can safely analyze and tune them. To transport SQL Tuning Sets, use the
DBMS_SQLTUNE package procedures.
The recommended interface for managing SQL tuning sets is the Oracle Enterprise Manager. Whenever possible, you should manage SQL tuning sets using Oracle Enterprise Manager, as described in theOracle Database 2 Day + Performance Tuning Guide . If Oracle Enterprise Manager is unavailable, you can manage SQL tuning sets using the
DBMS_SQLTUNE package procedures. Typically you would use the STS operations in the following sequence:
Create a new STS
Load the STS
Select the STS to review the contents
Update the STS if necessary
Create a tuning task with the STS as input
Transporting the STS to another system if necessary
Drop the STS when finished
To use the APIs, you need the
ADMINISTER SQL TUNING SET system privilege to manage SQL Tuning Sets that you own, or the
SET system privilege to manage any SQL Tuning Sets.
Figure 17-3 shows the steps involved when using SQL Tuning Sets APIs.
This section covers the following topics:
CREATE_SQLSET procedure is used to create an empty STS object in the database. For example, the following procedure creates an STS object that could be used to tune I/O intensive SQL statements during a specific period of time:
BEGIN DBMS_SQLTUNE.CREATE_SQLSET( sqlset_name => 'my_sql_tuning_set', description => 'I/O intensive workload'); END; /
my_sql_tuning_set is the name of the STS in the database and
'I/O intensive workload' is the description assigned to the STS.
LOAD_SQLSET procedure populates the STS with selected SQL statements. The standard sources for populating an STS are the workload repository, another STS, or the cursor cache. For both the workload repository and STS, predefined table functions can be used to select columns from the source to populate a new STS.
In the following example, procedure calls are used to load
my_sql_tuning_set from an AWR baseline called
baseline. The data has been filtered to select only the top 30 SQL statements ordered by elapsed time. First a ref cursor is opened to select from the specified baseline. Next the statements and their statistics are loaded from the baseline into the STS.
DECLARE baseline_cursor DBMS_SQLTUNE.SQLSET_CURSOR; BEGIN OPEN baseline_cursor FOR SELECT VALUE(p) FROM TABLE (DBMS_SQLTUNE.SELECT_WORKLOAD_REPOSITORY( 'peak baseline', NULL, NULL, 'elapsed_time', NULL, NULL, NULL, 30)) p; DBMS_SQLTUNE.LOAD_SQLSET( sqlset_name => 'my_sql_tuning_set', populate_cursor => baseline_cursor); END; /
SELECT_SQLSET table function reads the contents of the STS. After an STS has been created and populated, you can browse the SQL in the STS using different filtering criteria. The
SELECT_SQLSET procedure is provided for this purpose.
In the following example, the SQL statements in the STS are displayed for statements with a disk-reads to buffer-gets ratio greater than or equal to 75%.
SELECT * FROM TABLE(DBMS_SQLTUNE.SELECT_SQLSET( 'my_sql_tuning_set', '(disk_reads/buffer_gets) >= 0.75'));
Additional details of the SQL Tuning Sets that have been created and loaded can also be displayed with DBA views, such as
SQL statements can be updated and deleted from a SQL Tuning Set based on a search condition. In the following example, the
DELETE_SQLSET procedure deletes SQL statements from
my_sql_tuning_set that have been executed less than fifty times.
BEGIN DBMS_SQLTUNE.DELETE_SQLSET( sqlset_name => 'my_sql_tuning_set', basic_filter => 'executions < 50'); END; /
SQL Tuning Sets can be transported to another system by first exporting the STS from one system to a staging table, then importing the STS from the staging table into another system.
To transport a SQL Tuning Set:
CREATE_STGTAB_SQLSETprocedure to create a staging table where the SQL Tuning Sets will be exported.
The following example shows how to create a staging table named
staging_table. Table names are case-sensitive.
BEGIN DBMS_SQLTUNE.CREATE_STGTAB_SQLSET( table_name => 'staging_table' ); END; /
PACK_STGTAB_SQLSETprocedure to export SQL Tuning Sets into the staging table.
The following example shows how to export a SQL Tuning Set named
my_ststo the staging table.
BEGIN DBMS_SQLTUNE.PACK_STGTAB_SQLSET( sqlset_name => 'my_sts', staging_table_name => 'staging_table'); END; /
Move the staging table to the system where the SQL Tuning Sets will be imported using the mechanism of choice (such as datapump or database link).
On the system where the SQL Tuning Sets will be imported, use the
UNPACK_STGTAB_SQLSETprocedure to import SQL Tuning Sets from the staging table.
The following example shows how to import SQL Tuning Sets contained in the staging table.
BEGIN DBMS_SQLTUNE.UNPACK_STGTAB_SQLSET( sqlset_name => '%', replace => TRUE, staging_table_name => 'staging_table'); END; /
DROP_SQLSET procedure is used to drop an STS that is no longer needed. For example:
BEGIN DBMS_SQLTUNE.DROP_SQLSET( sqlset_name => 'my_sql_tuning_set' ); END; /
You can use the following APIs to manage an STS:
Updating the attributes of SQL statements in an STS
UPDATE_SQLSETprocedure updates the attributes of SQL statements (such as
OTHER) in an existing STS identified by STS name and SQL identifier.
Capturing the full system workload
CAPTURE_CURSOR_CACHE_SQLSETfunction enables the capture of the full system workload by repeatedly polling the cursor cache over a specified interval. This function is a lot more efficient than repeatedly using the
LOAD_SQLSETprocedures to capture the cursor cache over an extended period of time. This function effectively captures the entire workload, as opposed to the AWR—which only captures the workload of high-load SQL statements—or the
LOAD_SQLSETprocedure, which accesses the data source only once.
Adding and removing a reference to an STS
ADD_SQLSET_REFERENCEfunction adds a new reference to an existing STS to indicate its use by a client. The function returns the identifier of the added reference. The
REMOVE_SQLSET_REFERENCEprocedure is used to deactivate an STS to indicate it is no longer used by the client.
While SQL Profiles are usually handled by Oracle Enterprise Manager as part of the Automatic SQL tuning process, SQL Profiles can be managed through the
DBMS_SQLTUNE package. To use the SQL Profiles APIs, you need the
ADMINISTER SQL MANAGEMENT OBJECT privilege.
Figure 17-4 shows the steps involved when using SQL Profiles APIs.
This section covers the following topics:
Accepting a SQL Profile
Altering a SQL Profile
Dropping a SQL Profile
When the SQL Tuning Advisor recommends that a SQL Profile be used, you should accept the SQL Profile that is recommended. In cases where the SQL Tuning Advisor recommends that an index and a SQL Profile be used, both should be used. You can use the
DBMS_SQLTUNE.ACCEPT_SQL_PROFILE procedure to accept a SQL Profile recommended by the SQL Tuning Advisor. This creates and stores a SQL Profile in the database. For example:
DECLARE my_sqlprofile_name VARCHAR2(30); BEGIN my_sqlprofile_name := DBMS_SQLTUNE.ACCEPT_SQL_PROFILE ( task_name => 'my_sql_tuning_task', name => 'my_sql_profile', force_match => TRUE); END;
In this example, my_
sql_tuning_task is the name of the SQL tuning task and
my_sql_profile is the name of the SQL Profile that you want to accept.
Typically, an accepted SQL Profile is associated with the SQL statement through a special SQL signature that is generated using a hash function. This hash function normalizes the SQL statement for case (changes the entire SQL statement to upper case) and white spaces (removes all extra whites spaces) before generating the signature. The same SQL Profile thus will work for all SQL statements that are essentially the same, where the only difference is in case usage and white spaces. However, by setting
force_match to true, the SQL Profile will additionally target all SQL statements that have the same text after normalizing literal values to bind variables. This may be useful for applications that use literal values rather than bind variables, since this will allow SQL with text differing only in its literal values to share a SQL Profile. If both literal values and bind variables are used in the SQL text, or if this parameter is set to false (the default value), literal values will not be normalized.
If SQL plan management is used and there is already an existing plan baseline for the SQL statement, a new plan baseline will be added when a SQL profile is created. If SQL plan management is not used, a new plan baseline will not be added when a SQL profile is created. There is no strict relationship between the SQL profile and the plan baseline. When hard parsing a SQL statement, the optimizer will use the SQL profile to select the best plan baseline from the ones available. In different conditions, the SQL profile may cause the optimizer to select different plan baselines. For information about SQL plan management, see Chapter 15, "Using SQL Plan Management".
You can view information about a SQL Profile in the
You can alter the
CATEGORY attributes of an existing SQL Profile with the
ALTER_SQL_PROFILE procedure. For example:
BEGIN DBMS_SQLTUNE.ALTER_SQL_PROFILE( name => 'my_sql_profile', attribute_name => 'STATUS', value => 'DISABLED'); END; /
In this example,
my_sql_profile is the name of the SQL Profile that you want to alter. The status attribute is changed to disabled, which means the SQL Profile is not used during SQL compilation.
You can drop a SQL Profile with the
DROP_SQL_PROFILE procedure. For example:
BEGIN DBMS_SQLTUNE.DROP_SQL_PROFILE(name => 'my_sql_profile'); END; /
In this example,
my_sql_profile is the name of the SQL Profile you want to drop. You can also specify whether to ignore errors raised if the name does not exist. For this example, the default value of
FALSE is accepted.
This section summarizes the views that you can display to review information that has been gathered for tuning the SQL statements. You need DBA privileges to access these views.
Advisor information views, such as
SQL tuning information views, such as
SQL Tuning Set views, such as
Information on captured execution plans for statements in SQL Tuning Sets are displayed in the
SQL Profile information is displayed in the
TYPEparameter shows if the SQL profile was created manually by the SQL Tuning Advisor (if
MANUAL) or automatically by automatic SQL tuning (if
Advisor execution progress information is displayed in the
Dynamic views containing information relevant to the SQL tuning, such as