When hte SQL statement is executed Oracle 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.