1. A method, for use in query optimization in a relational database management system, said method comprising the steps of: (a) generating statistical information regarding data which represents the results of an operation involving one or more columns of a database; (b) deriving a statistical soft constraint from said statistical information that reflects a statistical property of said data; and (c) using said statistical soft constraint to estimate a cardinality value for the result of applying one or more query predicates in a query plan.