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The performance of any database application heavily relies on query execution. While the Oracle optimizer is perfectly suited to evaluate the best possible plan without any user intervention, a SQL statement's execution plan can change unexpectedly, for a variety of reasons including: re-gathering optimizer statistics, changing optimizer parameters or schema/metadata definitions. Not being able to guarantee a plan will change always for the better has lead some customers to freeze their execution plans (Stored Outlines) or lock their optimizer statistics. However, doing so prevents such environments from ever taking advantage of new optimizer functionality or access paths, which would improve the SQL statements performance.
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