Stored Outlines and Plan Stability
The cost-based optimizer evaluates SQL statements based on the current cost of operations. The cost calculations are based on the known statistics about the database and its data.
You may decide that the optimizations for your SQL statements are appropriate and that you do not want them to change in the future. If so, you can create a stored outline, which uses hints to direct the optimization of the statement.
With a stored outline, changes in the composition of the database that might result in a different optimal execution path will not affect the optimization of the statement. A stored outline can ensure that all installations of a system use the same optimization. You might also create and use stored outlines before upgrading your Oracle system to ensure that the new version of the optimizer delivered at least equivalent performance.
Oracle refers to the use of stored outlines as plan stability.