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  Oracle Database Tips by Donald Burleson

Plan Stability

This storing of plan outlines for SQL statements is known as plan stability and insures that changes in the Oracle environment don't affect the way a SQL statement is optimized by the cost based optimizer.  If you wish, 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 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 once they have been properly tuned.

As with the storage of SQL in the shared pool, 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. (Note: In Oracle9i excess white space is cleaned from SQL before use, so this limit is only for pre-9i databases.) 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 re-tuned.

The above text is an excerpt from:

Using Oracle SQL Stored Outlines & Optimizer Plan Stability
ISBN 0-9740716-8-4

by Mike Ault

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