What's an explain plan?

An explain plan is a representation of the access path that is taken when a query is executed within Oracle.

The EXPLAIN PLAN statement displays execution plans chosen by the Oracle optimizer for 
SELECT, UPDATE, INSERT, and DELETE statements. 

A statement's execution plan is the sequence of operations Oracle performs to run the statement.

The row source tree is the core of the execution plan. It shows the following information:
	·  An ordering of the tables referenced by the statement
	·  An access method for each table mentioned in the statement
	·  A join method for tables affected by join operations in the statement
	·  Data operations like filter, sort, or aggregation
In addition to the row source tree, the plan table contains information about the following:
	·  Optimization, such as the cost and cardinality of each operation
	·  Partitioning, such as the set of accessed partitions
	·  Parallel execution, such as the distribution method of join inputs
	
The EXPLAIN PLAN results let you determine whether the optimizer selects a particular execution plan, such as, nested loops join. 
It also helps you to understand the optimizer decisions, such as why the optimizer chose a nested loops join instead of a hash join, 
and lets you understand the performance of a query.