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The EXPLAIN PLAN results let you determine whether the optimizer selects a particular execution plan, such as, nested loops join.

Parallel execution, such as the distribution method of join inputs.

Partitioning, such as the set of accessed partitions.

Optimization, such as the cost and cardinality of each operation.

In addition to the row source tree, the plan table contains information about the following: Data operations like filter, sort, or aggregation.A join method for tables affected by join operations in the statement.An access method for each table mentioned in the statement.

An ordering of the tables referenced by the statement.The row source tree is the core of the execution plan. A statement's execution plan is the sequence of operations Oracle performs to run the statement. The EXPLAIN PLAN statement displays execution plans chosen by the Oracle optimizer for SELECT, UPDATE, INSERT, and DELETE statements. Oracle9i SQL Reference for the syntax of EXPLAIN PLAN