Stored Outlines and Plan Stability

A stored outline is a collection of hints associated with a specific SQL statement that allows a standard execution plan to be maintained, regardless of changes in the system environment or associated statistics. Plan stability is based on the preservation of execution plans at a point in time where the performance of a statement is considered acceptable.

The outlines are stored in the OL$, OL$HINTS, and OL$NODES tables, but the[USER|ALL|DBA]_OUTLINES and [USER|ALL|DBA]_OUTLINE_HINTS views should be used to display information about existing outlines.
All of the caveats associated with optimizer hints apply equally to stored outlines. Under normal running the optimizer chooses the most suitable execution plan for the current circumstances. By using a stored outline you may be forcing the optimizer to choose a substandard execution plan, so you should monitor the affects of your stored outlines over time to make sure this isn’t happening. Remember, what works well today may not tomorrow.

• Creating Outlines
• Using Outlines
• Dropping Outlines

For details read article on oracle base