12 Configuring Options for Optimizer Statistics Gathering

This chapter explains what optimizer statistics collection is and how to set statistics preferences.

This chapter contains the following topics:

12.1 About Optimizer Statistics Collection

In Oracle Database, optimizer statistics collection is the gathering of optimizer statistics for database objects, including fixed objects.

The database can collect optimizer statistics automatically. You can also collect them manually using the DBMS_STATS package.

This section contains the following topics:

12.1.1 Purpose of Optimizer Statistics Collection

The contents of tables and associated indexes change frequently, which can lead the optimizer to choose suboptimal execution plan for queries. To avoid potential performance issues, statistics must be kept current.

To minimize DBA involvement, Oracle Database automatically gathers optimizer statistics at various times. Some automatic options are configurable, such enabling AutoTask to run DBMS_STATS.

12.1.2 User Interfaces for Optimizer Statistics Management

You can manage optimizer statistics either through Oracle Enterprise Manager Cloud Control (Cloud Control) or using PL/SQL on the command line.

This section contains the following topics:

12.1.2.1 Graphical Interface for Optimizer Statistics Management

The Manage Optimizer Statistics page in Cloud Control is a GUI that enables you to manage optimizer statistics.

This section contains the following topics:

12.1.2.1.1 Accessing the Database Home Page in Cloud Control

Oracle Enterprise Manager Cloud Control enables you to manage multiple databases within a single GUI-based framework.

To access a database home page using Cloud Control:

  1. Log in to Cloud Control with the appropriate credentials.

  2. Under the Targets menu, select Databases.

  3. In the list of database targets, select the target for the Oracle Database instance that you want to administer.

  4. If prompted for database credentials, then enter the minimum credentials necessary for the tasks you intend to perform.

See Also:

Cloud Control online help

12.1.2.1.2 Accessing the Optimizer Statistics Console

You can perform most necessary tasks relating to optimizer statistics through pages linked to by the Optimizer Statistics Console page.

To manage optimizer statistics using Cloud Control:

  1. In Cloud Control, access the Database Home page.

  2. From the Performance menu, select SQL, then Optimizer Statistics.

    The Optimizer Statistics Console appears.

See Also:

Online Help for Oracle Enterprise Manager Cloud Control

12.1.2.2 Command-Line Interface for Optimizer Statistics Management

The DBMS_STATS package performs most optimizer statistics tasks.

To enable and disable automatic statistics gathering, use the DBMS_AUTO_TASK_ADMIN PL/SQL package.

See Also:

Oracle Database PL/SQL Packages and Types Reference to learn how to use DBMS_STATS and DBMS_AUTO_TASK_ADMIN

12.2 Setting Optimizer Statistics Preferences

This topic explains how to set optimizer statistics defaults using DBMS_STATS.SET_*_PREFS procedures.

This section contains the following topics:

12.2.1 About Optimizer Statistics Preferences

The optimizer statistics preferences set the default values of the parameters used by automatic statistics collection and the DBMS_STATS statistics gathering procedures.

You can set optimizer statistics preferences at the table, schema, database (all tables), and global level. A global preference refers to tables with no preferences and any tables created in the future. The procedure names follow the form SET_*_PREFS.

This section contains the following topics:

12.2.1.1 Purpose of Optimizer Statistics Preferences

Preferences enable you to maintain optimizer statistics automatically when some objects require settings that differ from the default. In this way, you have more control over how Oracle Database gathers statistics.

Preferences that you can set include, but are not limited to, the following:

  • ESTIMATE_PERCENT

    This preference determines the percentage of rows to estimate.

  • CONCURRENT

    This preference determines whether the database gathers statistics concurrently on multiple objects, or serially, one object at a time.

  • STALE_PERCENT

    This preference determines the percentage of rows in a table that must change before the database deems the statistics stale and in need of regathering.

  • AUTO_STAT_EXTENSIONS

    When set to the non-default value of ON, this preference enables a SQL plan directive to trigger the creation of column group statistics based on usage of columns in the predicates in the workload.

  • INCREMENTAL

    This preference determines whether the database maintains the global statistics of a partitioned table without performing a full table scan. Possible values are TRUE and FALSE.

    For example, by the default setting for INCREMENTAL is FALSE. You can set INCREMENTAL to TRUE for a range-partitioned table when the last few partitions are updated. Also, when performing a partition exchange operation on a nonpartitioned table, Oracle recommends that you set INCREMENTAL to TRUE and INCREMENTAL_LEVEL to TABLE. With these settings, DBMS_STATS gathers table-level synopses on this table.

  • INCREMENTAL_LEVEL

    This preference controls what synopses to collect when INCREMENTAL preference is set to TRUE. It takes two values: TABLE or PARTITION.

  • APPROXIMATE_NDV_ALGORITHM

    This preference controls which algorithm to use when calculating the number of distinct values for partitioned tables using incremental statistics.

12.2.1.2 DBMS_STATS Procedures for Setting Statistics Preferences

The DBMS_STATS.SET_*_PREFS procedures change the defaults of parameters used by the DBMS_STATS.GATHER_*_STATS procedures. To query the current preferences, use the DBMS_STATS.GET_PREFS function.

When setting statistics preferences, the order of precedence is:

  1. Table preference (set for a specific table, all tables in a schema, or all tables in the database)

  2. Global preference

  3. Default preference

The following table summarizes the relevant DBMS_STATS procedures.

Table 12-1 DBMS_STATS Procedures for Setting Optimizer Statistics Preferences

Procedure Scope

SET_TABLE_PREFS

Specified table only.

SET_SCHEMA_PREFS

All existing tables in the specified schema.

This procedure calls SET_TABLE_PREFS for each table in the specified schema. Calling SET_SCHEMA_PREFS does not affect any new tables created after it has been run. New tables use the GLOBAL_PREF values for all parameters.

SET_DATABASE_PREFS

All user-defined schemas in the database. You can include system-owned schemas such as SYS and SYSTEM by setting the ADD_SYS parameter to true.

This procedure calls SET_TABLE_PREFS for each table in the specified schema. Calling SET_DATABASE_PREFS does not affect any new objects created after it has been run. New objects use the GLOBAL_PREF values for all parameters.

SET_GLOBAL_PREFS

Any table that does not have an existing table preference.

All parameters default to the global setting unless a table preference is set or the parameter is explicitly set in the DBMS_STATS.GATHER_*_STATS statement. Changes made by SET_GLOBAL_PREFS affect any new objects created after it runs. New objects use the SET_GLOBAL_PREFS values for all parameters.

With SET_GLOBAL_PREFS, you can set a default value for the parameter AUTOSTATS_TARGET. This additional parameter controls which objects the automatic statistic gathering job running in the nightly maintenance window affects. Possible values for AUTOSTATS_TARGET are ALL, ORACLE, and AUTO (default).

You can only set the CONCURRENT preference at the global level. You cannot set the preference INCREMENTAL_LEVEL using SET_GLOBAL_PREFS.

See Also:

12.2.1.3 Statistics Preference Overrides

The preference_overrides_parameter statistics preference determines whether, when gathering optimizer statistics, to override the input value of a parameter with the statistics preference. In this way, you control when the database honors a parameter value passed to the statistics gathering procedures.

When preference_overrides_parameter is set to FALSE (default), the input values for statistics gathering procedures are honored. When set to TRUE, the input values are ignored.

Set the preference_overrides_parameter preference using the SET_TABLE_PREFS, SET_SCHEMA_PREFS, or SET_GLOBAL_PREFS procedures in DBMS_STATS. Regardless of whether preference_overrides_parameter is set, the database uses the same order of precedence for setting statistics:

  1. Table preference (set for a specific table, all tables in a schema, or all tables in the database)

  2. Global preference

  3. Default preference

Example 12-1 Overriding Statistics Preferences at the Table Level

In this example, legacy scripts set estimate_percent explicitly rather than using the recommended AUTO_SAMPLE_SIZE. Your goal is to prevent users from using these scripts to set preferences on the sh.costs table.

Table 12-2 Overriding Statistics Preferences at the Table Level

Action Description
SQL> SELECT DBMS_STATS.GET_PREFS 
('estimate_percent', 'sh','costs') 
AS "STAT_PREFS" FROM DUAL;
 
STAT_PREFS
----------
DBMS_STATS.AUTO_SAMPLE_SIZE

No preference for estimate_percent is set for sh.costs or at the global level, so the preference defaults to AUTO_SAMPLE_SIZE.

SQL> EXEC DBMS_STATS.SET_TABLE_PREFS 
('sh', 'costs', 
'preference_overrides_parameter', 'true');
 
PL/SQL procedure successfully completed.

By default, Oracle Database accepts preferences that are passed to the GATHER_*_STATS procedures. To override these parameters, you use SET_TABLE_PREFS to set the preference_overrides_parameter preference to true for the costs table only.

SQL> EXEC DBMS_STATS.GATHER_TABLE_STATS
('sh', 'costs', estimate_percent=>100);

PL/SQL procedure successfully completed.

You attempt to set estimate_percent to 100 when gathering statistics for sh.costs. However, because preference_overrides_parameter is true for this table, Oracle Database does not honor the estimate_percent=>100setting. Instead, the database gathers statistics using AUTO_SAMPLE_SIZE, which is the default.

Example 12-2 Overriding Statistics Preferences at the Global Level

In this example, you set estimate_percent to 5 at the global level, which means that this preference applies to every table in the database that does not have a table preference set. You then set an override on the sh.sales table, which does not have a table-level preference set, to prevent users from overriding the global setting in their scripts.

Table 12-3 Overriding Statistics Preferences at the Global Level

Action Description
SQL> SELECT DBMS_STATS.GET_PREFS 
('estimate_percent', 'sh','sales') 
AS "STAT_PREFS" FROM DUAL;
 
STAT_PREFS
----------
DBMS_STATS.AUTO_SAMPLE_SIZE

No preference for estimate_percent is set for sh.sales or at the global level, so the preference defaults to AUTO_SAMPLE_SIZE.

SQL> EXEC DBMS_STATS.SET_GLOBAL_PREFS 
('estimate_percent', '5');

PL/SQL procedure successfully completed.

You use the SET_GLOBAL_PREFS procedure to set the estimate_percent preference to 5 for every table in the database that does not have a table preference set.

SQL> SELECT DBMS_STATS.GET_PREFS 
('estimate_percent', 'sh','sales') 
AS "STAT_PREFS" FROM DUAL;
 
STAT_PREFS
----------
5

Because sh.sales does not have a preference set, the global setting applies to this table. A query of the preferences for sh.sales now shows that the estimate_percent setting is 5, which is the global setting.

SQL> EXEC DBMS_STATS.SET_TABLE_PREFS 
('sh', 'sales', 
'preference_overrides_parameter', 'true');
 
PL/SQL procedure successfully completed.

You use SET_TABLE_PREFS to set the preference_overrides_parameter preference to true for the sh.sales table only.

SQL> EXEC DBMS_STATS.GATHER_TABLE_STATS
('sh', 'sales', estimate_percent=>10);

PL/SQL procedure successfully completed.

You attempt to set estimate_percent to 10 when gathering statistics for sh.sales. However, because preference_overrides_parameter is true for the sales table, and because a global preference is defined, Oracle Database actually gathers statistics using the global setting of 5.

See Also:

Oracle Database PL/SQL Packages and Types Reference to learn about the DBMS_STATS procedures for setting optimizer statistics

12.2.1.4 Setting Statistics Preferences: Example

This example illustrates the relationship between SET_TABLE_PREFS, SET_SCHEMA_STATS, and SET_DATABASE_PREFS.

Table 12-4 Changing Preferences for Statistics Gathering Procedures

Action Description
SQL> SELECT DBMS_STATS.GET_PREFS 
('incremental', 'sh','costs') 
AS "STAT_PREFS" FROM DUAL;
 
STAT_PREFS
----------
TRUE

You query the INCREMENTAL preference for costs and determine that it is set to true.

SQL> EXEC DBMS_STATS.SET_TABLE_PREFS 
('sh', 'costs', 'incremental', 'false');
 
PL/SQL procedure successfully completed.

You use SET_TABLE_PREFS to set the INCREMENTAL preference to false for the costs table only.

SQL> SELECT DBMS_STATS.GET_PREFS 
('incremental', 'sh', 'costs') 
AS "STAT_PREFS" FROM DUAL;

STAT_PREFS 
----------
FALSE

You query the INCREMENTAL preference for costs and confirm that it is set to false.

SQL> EXEC DBMS_STATS.SET_SCHEMA_PREFS 
('sh', 'incremental', 'true');

PL/SQL procedure successfully completed.

You use SET_SCHEMA_PREFS to set the INCREMENTAL preference to true for every table in the sh schema, including costs.

SQL> SELECT DBMS_STATS.GET_PREFS 
('incremental', 'sh', 'costs') 
AS "STAT_PREFS" FROM DUAL;
 
STAT_PREFS
----------
TRUE

You query the INCREMENTAL preference for costs and confirm that it is set to true.

SQL> EXEC DBMS_STATS.SET_DATABASE_PREFS 
('incremental', 'false');

PL/SQL procedure successfully completed.

You use SET_DATABASE_PREFS to set the INCREMENTAL preference for all tables in all user-defined schemas to false.

SQL> SELECT DBMS_STATS.GET_PREFS 
('incremental', 'sh', 'costs') 
AS "STAT_PREFS" FROM DUAL;

STAT_PREFS
----------
FALSE

You query the INCREMENTAL preference for costs and confirm that it is set to false.

12.2.2 Setting Global Optimizer Statistics Preferences Using Cloud Control

A global preference applies to any object in the database that does not have an existing table preference. You can set optimizer statistics preferences at the global level using Cloud Control.

To set global optimizer statistics preferences using Cloud Control:

  1. In Cloud Control, access the Database Home page.

  2. From the Performance menu, select SQL, then Optimizer Statistics.

    The Optimizer Statistics Console appears.

  3. Click Global Statistics Gathering Options.

    The Global Statistics Gathering Options page appears.

  4. Make your desired changes, and click Apply.

See Also:

Online Help for Oracle Enterprise Manager Cloud Control

12.2.3 Setting Object-Level Optimizer Statistics Preferences Using Cloud Control

You can set optimizer statistics preferences at the database, schema, and table level using Cloud Control.

To set object-level optimizer statistics preferences using Cloud Control:

  1. In Cloud Control, access the Database Home page.

  2. From the Performance menu, select SQL, then Optimizer Statistics.

    The Optimizer Statistics Console appears.

  3. Click Object Level Statistics Gathering Preferences.

    The Object Level Statistics Gathering Preferences page appears.

  4. To modify table preferences for a table that has preferences set at the table level, do the following (otherwise, skip to the next step):

    1. Enter values in Schema and Table Name. Leave Table Name blank to see all tables in the schema.

      The page refreshes with the table names.

    2. Select the desired tables and click Edit Preferences.

      The General subpage of the Edit Preferences page appears.

    3. Change preferences as needed and click Apply.

  5. To set preferences for a table that does not have preferences set at the table level, do the following (otherwise, skip to the next step):

    1. Click Add Table Preferences.

      The General subpage of the Add Table Preferences page appears.

    2. In Table Name, enter the schema and table name.

    3. Change preferences as needed and click OK.

  6. To set preferences for a schema, do the following:

    1. Click Set Schema Tables Preferences.

      The General subpage of the Edit Schema Preferences page appears.

    2. In Schema, enter the schema name.

    3. Change preferences as needed and click OK.

See Also:

Online Help for Oracle Enterprise Manager Cloud Control

12.2.4 Setting Optimizer Statistics Preferences from the Command Line

If you do not use Cloud Control to set optimizer statistics preferences, then you can invoke the DBMS_STATS procedures from the command line.

Prerequisites

This task has the following prerequisites:

  • To set the global or database preferences, you must have SYSDBA privileges, or both ANALYZE ANY DICTIONARY and ANALYZE ANY system privileges.

  • To set schema preferences, you must connect as owner, or have SYSDBA privileges, or have the ANALYZE ANY system privilege.

  • To set table preferences, you must connect as owner of the table or have the ANALYZE ANY system privilege.

To set optimizer statistics preferences from the command line:

  1. In SQL*Plus or SQL Developer, log in to the database as a user with the necessary privileges.

  2. Optionally, call the DBMS_STATS.GET_PREFS procedure to see preferences set at the object level, or at the global level if a specific table is not set.

    For example, obtain the STALE_PERCENT parameter setting for the sh.sales table as follows:

    SELECT DBMS_STATS.GET_PREFS('STALE_PERCENT', 'SH', 'SALES') 
    FROM   DUAL;
    
  3. Execute the appropriate procedure from Table 12-1, specifying the following parameters:

    • ownname - Set schema name (SET_TAB_PREFS and SET_SCHEMA_PREFS only)

    • tabname - Set table name (SET_TAB_PREFS only)

    • pname - Set parameter name

    • pvalue - Set parameter value

    • add_sys - Include system tables (optional, SET_DATABASE_PREFS only)

    The following example specifies that 13% of rows in sh.sales must change before the statistics on that table are considered stale:

    EXEC DBMS_STATS.SET_TABLE_PREFS('SH', 'SALES', 'STALE_PERCENT', '13');
    
  4. Optionally, query the *_TAB_STAT_PREFS view to confirm the change.

    For example, query DBA_TAB_STAT_PREFS as follows:

    COL OWNER FORMAT a5
    COL TABLE_NAME FORMAT a15
    COL PREFERENCE_NAME FORMAT a20
    COL PREFERENCE_VALUE FORMAT a30
    SELECT * FROM DBA_TAB_STAT_PREFS;
    

    Sample output appears as follows:

    OWNER TABLE_NAME      PREFERENCE_NAME      PREFERENCE_VALUE
    ----- --------------- -------------------- ------------------------------
    OE    CUSTOMERS       NO_INVALIDATE        DBMS_STATS.AUTO_INVALIDATE
    SH    SALES           STALE_PERCENT        13

See Also:

Oracle Database PL/SQL Packages and Types Reference for descriptions of the parameter names and values for program units

12.3 Configuring Options for Dynamic Statistics

Dynamic statistics are an optimization technique in which the database uses recursive SQL to scan a small random sample of the blocks in a table.

The sample scan estimate predicate selectivities. Using these estimates, the database determines better default statistics for unanalyzed segments, and verifies its estimates. By default, when optimizer statistics are missing, stale, or insufficient, dynamic statistics automatically run recursive SQL during parsing to scan a small random sample of table blocks.

This section contains the following topics:

12.3.1 About Dynamic Statistics Levels

The dynamic statistics level controls both when the database gathers dynamic statistics, and the size of the sample that the optimizer uses to gather the statistics.

Set the dynamic statistics level using either the OPTIMIZER_DYNAMIC_SAMPLING initialization parameter or a statement hint.

Note:

Dynamic statistics were called dynamic sampling in releases earlier than Oracle Database 12c Release 1 (12.1).

The following table describes the levels for dynamic statistics. Note the following:

  • If OPTIMIZER_DYNAMIC_STATISTICS is TRUE, and if dynamic statistics are not disabled, then the database may choose to use dynamic statistics when a SQL statement uses parallel execution.

  • If OPTIMIZER_ADAPTIVE_STATISTICS is TRUE, then the optimizer uses dynamic statistics when relevant SQL plan directives exist. The database maintains the resulting statistics in the SQL plan directives store, making them available to other queries.

Table 12-5 Dynamic Statistics Levels

Level When the Optimizer Uses Dynamic Statistics Sample Size (Blocks)

0

Do not use dynamic statistics.

n/a

1

Use dynamic statistics for all tables that do not have statistics, but only if the following criteria are met:

  • At least one nonpartitioned table in the query does not have statistics.

  • This table has no indexes.

  • This table has more blocks than the number of blocks that would be used for dynamic statistics of this table.

32

2

Use dynamic statistics if at least one table in the statement has no statistics. This is the default value.

64

3

Use dynamic statistics if any of the following conditions is true:

  • At least one table in the statement has no statistics.

  • The statement has one or more expressions used in the WHERE clause predicates, for example, WHERE SUBSTR(CUSTLASTNAME,1,3).

64

4

Use dynamic statistics if any of the following conditions is true:

  • At least one table in the statement has no statistics.

  • The statement has one or more expressions used in the WHERE clause predicates, for example, WHERE SUBSTR(CUSTLASTNAME,1,3).

  • The statement uses complex predicates (an OR or AND operator between multiple predicates on the same table).

64

5

The criteria are identical to level 4, but the database uses a different sample size.

128

6

The criteria are identical to level 4, but the database uses a different sample size.

256

7

The criteria are identical to level 4, but the database uses a different sample size.

512

8

The criteria are identical to level 4, but the database uses a different sample size.

1024

9

The criteria are identical to level 4, but the database uses a different sample size.

4086

10

The criteria are identical to level 4, but the database uses a different sample size.

All blocks

11

The database uses adaptive dynamic sampling automatically when the optimizer deems it necessary.

Automatically determined

See Also:

12.3.2 Setting Dynamic Statistics Levels Manually

Determining a database-level setting that would be beneficial to all SQL statements can be difficult.

When setting the level for dynamic statistics, Oracle recommends setting the OPTIMIZER_DYNAMIC_SAMPLING initialization parameter at the session level.

Assumptions

This tutorial assumes the following:

  • You want correct selectivity estimates for the following query, which has WHERE clause predicates on two correlated columns:

      SELECT *
      FROM   sh.customers
      WHERE  cust_city='Los Angeles'
      AND    cust_state_province='CA';
    
  • The preceding query uses serial processing.

  • The sh.customers table contains 932 rows that meet the conditions in the query.

  • You have gathered statistics on the sh.customers table.

  • You created an index on the cust_city and cust_state_province columns.

  • The OPTIMIZER_DYNAMIC_SAMPLING initialization parameter is set to the default level of 2.

To set the dynamic statistics level manually:

  1. In SQL*Plus or SQL Developer, log in to the database as a user with the necessary privileges.

  2. Explain the execution plan as follows:

    EXPLAIN PLAN FOR
      SELECT *
      FROM   sh.customers
      WHERE  cust_city='Los Angeles'
      AND    cust_state_province='CA';
    
  3. Query the plan as follows:

    SET LINESIZE 130
    SET PAGESIZE 0
    SELECT * 
    FROM   TABLE(DBMS_XPLAN.DISPLAY);
    

    The output appears below (the example has been reformatted to fit on the page):

    -------------------------------------------------------------------------------
    |Id| Operation                   | Name             |Rows|Bytes|Cost | Time   |
    -------------------------------------------------------------------------------
    | 0| SELECT STATEMENT            |                   | 53| 9593|53(0)|00:00:01|
    | 1|  TABLE ACCESS BY INDEX ROWID|CUSTOMERS          | 53| 9593|53(0)|00:00:01|
    |*2|   INDEX RANGE SCAN          |CUST_CITY_STATE_IND| 53| 9593| 3(0)|00:00:01|
    -------------------------------------------------------------------------------
     
    Predicate Information (identified by operation id):
    ---------------------------------------------------
     
       2 - access("CUST_CITY"='Los Angeles' AND "CUST_STATE_PROVINCE"='CA')
    

    The columns in the WHERE clause have a real-world correlation, but the optimizer is not aware that Los Angeles is in California and assumes both predicates reduce the number of rows returned. Thus, the table contains 932 rows that meet the conditions, but the optimizer estimates 53, as shown in bold.

    If the database had used dynamic statistics for this plan, then the Note section of the plan output would have indicated this fact. The optimizer did not use dynamic statistics because the statement executed serially, standard statistics exist, and the parameter OPTIMIZER_DYNAMIC_SAMPLING is set to the default of 2.

  4. Set the dynamic statistics level to 4 in the session using the following statement:

    ALTER SESSION SET OPTIMIZER_DYNAMIC_SAMPLING=4;
    
  5. Explain the plan again:

    EXPLAIN PLAN FOR
      SELECT *
      FROM   sh.customers
      WHERE  cust_city='Los Angeles'
      AND    cust_state_province='CA';
    

    The new plan shows a more accurate estimate of the number of rows, as shown by the value 932 in bold:

    PLAN_TABLE_OUTPUT
    ---------------------------------------------------------------------------
    Plan hash value: 2008213504
     
    ---------------------------------------------------------------------------
    | Id  | Operation         | Name      |Rows | Bytes |Cost (%CPU)|Time     |
    ---------------------------------------------------------------------------
    |   0 | SELECT STATEMENT  |           | 932 |   271K|   406 (1)| 00:00:05 |
    |*  1 |  TABLE ACCESS FULL| CUSTOMERS | 932 |   271K|   406 (1)| 00:00:05 |
    ---------------------------------------------------------------------------
     
    Predicate Information (identified by operation id):
    ---------------------------------------------------
     
       1 - filter("CUST_CITY"='Los Angeles' AND "CUST_STATE_PROVINCE"='CA')
     
    Note
    -----
       - dynamic statistics used for this statement (level=4)
    

    The note at the bottom of the plan indicates that the sampling level is 4. The additional dynamic statistics made the optimizer aware of the real-world relationship between the cust_city and cust_state_province columns, thereby enabling it to produce a more accurate estimate for the number of rows: 932 rather than 53.

See Also:

12.3.3 Disabling Dynamic Statistics

In general, the best practice is not to incur the cost of dynamic statistics for queries whose compile times must be as fast as possible, for example, unrepeated OLTP queries.

To disable dynamic statistics at the session level:

  1. Connect SQL*Plus to the database with the appropriate privileges.

  2. Set the dynamic statistics level to 0.

    For example, run the following statement:

    ALTER SESSION SET OPTIMIZER_DYNAMIC_SAMPLING=0;

See Also:

Oracle Database Reference to learn about the OPTIMIZER_DYNAMIC_SAMPLING initialization parameter

12.4 Managing SQL Plan Directives

A SQL plan directive is additional information and instructions that the optimizer can use to generate a more optimal plan.

A directive informs the database that the optimizer is misestimate cardinalities of certain types of predicates, and alerts DBMS_STATS to gather additional statistics in the future. Thus, directives have an effect on statistics gathering.

The database automatically creates and manages SQL plan directives in the SGA, and then periodically writes them to the data dictionary. If the directives are not used within 53 weeks, then the database automatically purges them.

You can use DBMS_SPD procedures and functions to alter, save, drop, and transport directives manually. The following table lists some of the more commonly used procedures and functions.

Table 12-6 DBMS_SPD Procedures

Procedure Description

FLUSH_SQL_PLAN_DIRECTIVE

Forces the database to write directives from memory to persistent storage in the SYSAUX tablespace.

DROP_SQL_PLAN_DIRECTIVE

Drops a SQL plan directive. If a directive that triggers dynamic sampling is creating unacceptable performance overhead, then you may want to remove it manually.

If a SQL plan directive is dropped manually or automatically, then the database can re-create it. To prevent its re-creation, you can use DBMS_SPM.ALTER_SQL_PLAN_DIRECTIVE to do the following:

  • Disable the directive by setting ENABLED to NO

  • Prevent the directive from being dropped by setting AUTO_DROP to NO

To disable SQL plan directives, set OPTIMIZER_ADAPTIVE_STATISTICS to FALSE.

Prerequisites

You must have the Administer SQL Management Object privilege to execute the DBMS_SPD APIs.

Assumptions

This tutorial assumes that you want to do the following:

  • Write all directives for the sh schema to persistent storage.

  • Delete all directives for the sh schema.

To write and then delete all sh schema plan directives:

  1. In SQL*Plus or SQL Developer, log in to the database as a user with the necessary privileges.

  2. Force the database to write the SQL plan directives to disk.

    For example, execute the following DBMS_SPD program:

    BEGIN 
      DBMS_SPD.FLUSH_SQL_PLAN_DIRECTIVE;
    END;
    /
    
  3. Query the data dictionary for information about existing directives in the sh schema.

    Example 12-3 queries the data dictionary for information about the directive.

  4. Delete the existing SQL plan directive for the sh schema.

    The following PL/SQL program unit deletes the SQL plan directive with the ID 1484026771529551585:

    BEGIN
      DBMS_SPD.DROP_SQL_PLAN_DIRECTIVE ( directive_id => 1484026771529551585 );
    END;
    /

Example 12-3 Display Directives for sh Schema

This example shows SQL plan directives, and the results of SQL plan directive dynamic sampling queries.

SELECT TO_CHAR(d.DIRECTIVE_ID) dir_id, o.OWNER, o.OBJECT_NAME, 
       o.SUBOBJECT_NAME col_name, o.OBJECT_TYPE object, d.TYPE, 
       d.STATE, d.REASON
FROM   DBA_SQL_PLAN_DIRECTIVES d, DBA_SQL_PLAN_DIR_OBJECTS o
WHERE  d.DIRECTIVE_ID=o.DIRECTIVE_ID
AND    o.OWNER IN ('SH')
ORDER BY 1,2,3,4,5;
 
DIR_ID              OWN OBJECT_NA COL_NAME   OBJECT  TYPE     STATE      REASON
------------------- --- --------- ---------- ------- -------- ---------- ------------
1484026771529551585  SH CUSTOMERS COUNTRY_ID  COLUMN DYNAMIC_ SUPERSEDED SINGLE TABLE  
                                                     SAMPLING            CARDINALITY 
                                                                         MISESTIMATE
1484026771529551585  SH CUSTOMERS CUST_STATE_ COLUMN DYNAMIC_ SUPERSEDED SINGLE TABLE
                                  PROVINCE           SAMPLING            CARDINALITY 
                                                                         MISESTIMATE
1484026771529551585  SH CUSTOMERS              TABLE DYNAMIC_ SUPERSEDED SINGLE TABLE 
                                                     SAMPLING            CARDINALITY 
                                                                         MISESTIMATE
9781501826140511330  SH dyg4msnst5           SQL STA DYNAMIC_     USABLE VERIFY
                                             TEMENT  SAMPLING            CARDINALITY 
                                                     _RESULT             ESTIMATE
9872337207064898539  SH TIMES                  TABLE DYNAMIC_     USABLE VERIFY
                                                     SAMPLING            CARDINALITY 
                                                     _RESULT             ESTIMATE
9781501826140511330  SH 2nk1v0fdx0           SQL STA DYNAMIC_     USABLE VERIFY
                                             TEMENT  SAMPLING            CARDINALITY 
                                                     _RESULT             ESTIMATE

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