SQL Tuning - Oracle Cost Based Optimizer (CBO)

Oracle's cost-based SQL optimizer (CBO) is an extremely sophisticated component of Oracle that governs the execution for every Oracle query. The CBO has evolved into one of the world's most sophisticated software components, and it has the challenging job of evaluating any SQL statement and generating the "best" execution plan for the statement.

Refer: http://www.dba-oracle.com/art_otn_cbo.htm