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Oracle cursor_sharing tips

Oracle Tips by Burleson Consulting
Updated November 17, 2010

Some Oracle databases with high ad-hoc query activity (Crystal Reports, Business Objects) cannot avoid in-line literals inside the SQL, and that's why Oracle introduced the cursor_sharing parameter. This use of cursor_sharing=force has been shown to provide a huge benefit for database plagued with literals (i.e. non-reentrant SQL) in their library cache.

Also, note that in 11g, cursor_sharing=similar has been debugged to the point where it can be used.  See my notes on adaptive cursor sharing and include bind variable peeking.

Note:  The cursor_sharing=similar option has been deprecated in Oracle 11g and will be removed in version 12 per MOSC Note 1169017.1

The dynamic shop often has SQL that is generated by ad-hoc query tools with hard-coded literal values embedded within the SQL. As we know, hard-coded literal values make the SQL statements non-reusable unless cursor_sharing=force is set in the Oracle initialization file.

Shops that are plagued with non-reusable SQL can adopt either the persistent or the dynamic execution plan philosophy. To use optimizer plan stability with non-reusable SQL, the DBA will set cursor_sharing=force and then extract the transformed SQL from the library cache and use optimizer plan stability to make the execution plan persistent.

Note:  For testing purposes, you may want to turn-off optimizer bind variable peeking and this can be done in several ways::

  • Re-set the _optim_peek_user_binds hidden parameter.

  • Re-analyze using dbms_stats with the argument:  method_opt=> 'for all columns size 1';

My other tips for using cursor_sharing include:


Inside Oracle cursor sharing
(by Mark Rittman)

If you're a developer working with relational databases such as the Oracle RDBMS and Microsoft SQL Server, the single most effective thing you can do to ensure your code is efficient and scalable is to use bind variables (and now you can even use them with Oracle 9i OLAP). Bind variables are important because they allow Oracle to reduce the amount of 'hard parsing' (and hence CPU activity) carried out when processing SQL statements, and are pretty much a requirement when building systems with many concurrent users.

Other notes on cursor_sharing:

The dynamic shop often has SQL that is generated by ad-hoc query tools with hard-coded literal values embedded within the SQL. As we know, hard-coded literal values make the SQL statements nonreusable unless cursor_sharing=force is set in the Oracle initialization file. Shops that are plagued with nonreusable SQL can set cursor_sharing=force.

Mike Ault has great scripts for Locating similar SQL in Oracle to see if your SQL is using bind variables.  Systems that need cursor sharing are littered with SQL that contains literal values, like this:

select cust_stuff from customer where cust_name = 'JONES';

These system that benefit from cursor_sharing have low "executions" because the SQL cannot be re-used it is only executed once. (as seen in the v$sql view) and the need for cursor sharing van be seen by Oracle hard parse (sharing criteria) elapsed time.  Other symptoms indicating the need for cursor_sharing include:

  • High CPU attributable to shared pool activity

  • High hard parses

  • Low number of executions for individual SQL statements

You can also tailor cursor_sharing for a specific group of users using Mike Ault's script collection.  Other notes on cursor sharing include:

 

If you like Oracle tuning, you may enjoy my new book "Oracle Tuning: The Definitive Reference", over 900 pages of my favorite tuning tips & scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


 

 

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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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