Explanation:
Oracle stores the execution plan of a SQL statement to enable a database administrator (DBA) to analyze the execution plan of a poorly performing SQL statement without having to rerun the query. Oracle9i has introduced the V$SQL_PLAN view to display the cached execution plan, the compilation environment, and the execution statistics of each variable. It also helps the database administrators to determine the actual plan of a query at the time of reported performance problem.
Incorrect Answers:
1: The Cached Execution Plan feature helps to diagnose the execution plan, not to improve SQL statement performance itself.
2: The SQL statement still needs to be cached in the library cache. The Cached Execution Plan does not eliminate this need.
4: The execution plan is available for diagnosis when the SQL statement is in the library cache.
Reference:
OCP Oracle9i Database: New Features for Administrators, Daniel Benjamin, p. 217-218
Chapter 4: Performance and Scalability Enhancements
Oracle 9i New Features, Robert Freeman, p. 185-186
Chapter 6: Oracle9i SQL, PL/SQL New Features

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