Many developers and DBA's believe that the defined constraints on tables will slow down the execution of DML SQL statements and therefore should be avoided. If this belief is partly true for INSERTS and UPDATES is far from being true for SELECT statements. In reality the constraint definition is beside the object statistics the main source of information for Cost Based Optimizer (CBO) when preparing optimal execution plans. The presentation will cover some of the most important cases where constraints are crucial for preparation of optimal execution plans.