1. A method of performing a query in a computer system to retrieve data from a database stored on the computer system, the method comprising: (a) generating a plurality of query execution plans (QEPs) for the query; (b) providing an execution model of each of the QEPs; (c) choosing one of the QEPs for execution based on the model associated therewith; and (d) exploiting an empirical measurement from the execution of the chosen QEP to validate the model associated therewith, by determining whether the model is in error, and by computing one or more adjustments to the model to correct the determined error.