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Lesson 5The EXPLAIN PLAN utility
ObjectiveUnderstand the Basic Functions of the Oracle EXPLAIN PLAN Utility
View access path to data
More than any other tool, the EXPLAIN PLAN utility is the most indispensable. In fact, all of the other Oracle tools rely on the EXPLAIN PLAN utility to allow the DBA to view the access path to the data. We have no hope of tuning a SQL query until we can see how it is accessing the database, and the EXPLAIN PLAN utility makes the access path visible. The EXPLAIN PLAN utility can be invoked directly on the UNIX server with the EXPLAIN PLAN command syntax, or it can be invoked from Oracle SQL analyze client software.
As we noted in an earlier lesson, it is never clear from looking at an SQL statement how the database will be accessed. To illustrate, consider the following SQL statement presented in the Slide Show.
Oracle Tuning - Definitive ReferenceOracle Tuning: The Definitive Reference

Oracle Explain Plan Utility
We will now take a deeper look into SQL tuning by examining the optimizer modes.