CURSOR_SHARING explained

In Oracle8i, release 2 (version 8.1.6), Oracle introduced a new feature called CURSOR_SHARING. Cursor sharing is an 'auto binder' of sorts. It causes the database to rewrite your query using bind variables before parsing it. This feature will take a query such as:

select * from emp where ename = 'KING'; 

and will automatically rewrite it as: 

select * from emp where ename = :SYS_B_0

CURSOR_SHARING was introduced to help relieve pressure put on the shared pool, specifically the cursor cache, from applications that use literal values rather than bind variables in their SQL statements. It achieves this by replacing the literal values with system generated bind variables thus reducing the number of (parent) cursors in the cursor cache.

In this great article by Maria Colgan the possible values of CURSOR_SHARING are explained.

Also check out this great collection of blog posts related to cursor sharing from the Oracle Optimizer blog.

Enjoy!
Oracle Learner.

Adaptive Cursor Sharing

Adaptive Cursor Sharing is a new feature of Oracle database 11g Release 2 that tries to cure the problem of when you have a cursor with bind variables and some columns have data skew, then a suboptimal plan might be chosen by the optimizer. Adaptive cursors have the ability to be "bind-aware" and choose a different plan for a different bind value.

Check out this great article by Maria Colgan to learn what it is and how it works. Also check out the relevant Oracle Performance Tuning Guide paragraph.

If you want to remember what is cursor sharing and the meaning of the values of the CURSOR_SHARING parameter, then check out our relevant post. Also to understand CURSOR_SHARING combined with Adaptive Cursor Sharing, then read this article.

Enjoy!
Oracle Learner.