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Oracle's Query Execution Plan (QEP)

Whenever an SQL statement is executed, Oracle (that is the RBO or CBO) designs an execution plan for it. This execution plan is basically a step by step instruction for how the statement must be executed. That is, the order in which tables are read, if indexes are used, which join methods are used to join tables and so on.
The execution plan for an SQL statement can be viewed with the explain plan statement.
The query execution plan is elaborated during an SQL statement's parse phase.

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Thanks to Muhammed Imran for correcting a typo on this page.