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Administration of OUTLINE Objects

Oracle Database Tips by Donald Burleson

An outline is a stored query path or set of attributes that will be used by the optimizer to optimize a specific SQL query. Outlines can be enabled or disabled at both the system and session levels by use of the ALTER SESSION and ALTER SYSTEM commands. An outline should not be generated until the statement to be outlined has been tuned and you are absolutely sure that it is performing exactly the way you want it to.

In order to create outlines, users must have the CREATE ANY OUTLINE privilege; to create a clone of an existing outline (in Oracle9i) users must have SELECT_CATALOG_ROLE granted.

Storing plan outlines for SQL statements is known as plan stability; it ensures that changes in the Oracle environment don't affect the way a SQL statement is optimized by the cost-based optimizer.  If you want, Oracle will define plans for all issued SQL statements at the time they are executed, and this stored plan will be reused until altered or dropped. Generally, however, I do not suggest using the automatic outline feature as it can lead to poor plans being reused by the optimizer. It makes more sense to monitor for high-cost statements and tune them as required, storing an outline for them only after they have been properly tuned.

As with the storage of SQL in the shared pool, the storage of outlines depends on the statement being reissued in an identical fashion each time it is used. If even one space is out of place, the stored outline is not reused. Therefore, your queries should be stored as PL/SQL procedures, functions, or packages (or perhaps Java routines), and bind variables should always be used. This allows reuse of the stored image of the SQL as well as reuse of stored outlines.

Remember that to be useful over the life of an application, the outlines will have to be periodically verified by checking SQL statement performance. If performance of SQL statements degrades, the stored outline may have to be dropped and regenerated after the SQL is retuned.

In Oracle9i, the ability to edit a stored outline is provided by means of the DBMS_OTLN_EDIT Oracle-provided package.

You enable or disable the use of stored outlines dynamically for an individual session or for the system:

* Enable the USE_STORED_OUTLINES parameter to use public outlines.

* Enable the USE_PRIVATE_OUTLINES parameter to use private stored outlines.

Creation of an OUTLINE Object

Outlines are created using the CREATE OUTLINE command; the syntax for this command is:

[FROM [PUBLIC|PRIVATE] source_outline_name]
[FOR CATEGORY category_name]
ON statement;


PUBLIC|PRIVATE. By default all outlines are public. Before first creating a private outline, you must run the DBMS_OUTLN_EDIT.CREATE_EDIT_TABLES  procedure to create the required outline tables and indexes in your schema. 

Outline_name. A unique name for the outline.

FROM. Use this clause to create a new outline by copying an existing one. By default, Oracle looks for the source category in the public area. If you specify PRIVATE, Oracle will look for the outline in the current schema. If you specify the FROM clause, you cannot specify the ON clause.

[FOR CATEGORY category_name]. (Optional) Allows more than one outline to be associated with a single query by specifying multiple categories, each named uniquely.

ON statement. Specifies the statement for which the outline is prepared.

An example would be:

COUNT(b.table_name) extents

dba_tables a,
      dba_extents b
      AND a.table_name=b.table_name
      a.owner, a.table_name, a.tablespace_name; 

Assuming the above select is a part of a stored PL/SQL procedure or perhaps part of a view, the stored outline will now be used each time an exactly-matching SQL statement is issued.

Before a public outline can be created, the user must have access to a PLAN_TABLE.

Altering an OUTLINE Object

Outlines are altered using the ALTER OUTLINE or the CREATE OR REPLACE form of the CREATE command. The format of the command is identical whether it is used for the creation or replacement of an existing outline. For example, assume we want to add SUM(b.blocks) to the previous example.

COUNT(b.table_name) extents,
      dba_tables a,
      dba_extents b
      AND a.table_name=b.table_name

      a.owner, a.table_name, a.tablespace_name; 

This example has the effect of altering the stored outline get_tables to include any changes brought about by inclusion of the SUM(b.blocks) in the SELECT list. But what if we want to rename the outline or change a category name? The ALTER OUTLINE command has the format:

ALTER OUTLINE outline_name
[RENAME TO new_outline_name]
[CHANGE CATEGORY TO new_category_name]

The ALTER OUTLINE command allows us to rebuild the outline for an existing outline_name as well as rename the outline or change its category. The benefit of using the ALTER OUTLINE command is that we do not have to respecify the complete SQL statement, as we would have to using the CREATE OR REPLACE command.

Dropping an OUTLINE Object

Outlines are dropped using the DROP OUTLINE command; the syntax for this command is:

DROP OUTLINE outline_name;

This is an excerpt from Mike Ault, bestselling author of "Oracle 10g Grid and Real Application Clusters".


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