1. A system for predicting query execution time in a database system, comprising: a cost model determination device for determining a cost model of a database query optimizer for the database system, the cost model for modeling costs of queries applied to the database system; a profiling device for determining a group of profiling queries for profiling input/output cost units and processor cost units relating to the database system, and for profiling the input/output cost units and the processor cost units using one or more of the profiling queries in the group to output profiled input/output cost units and profiled processor cost units; a calibrating device for calibrating cost units in the cost model responsive to the profiled input/output cost units and the profiled processor cost units to output calibrated cost units; a sampling re-estimator for sampling and re-estimating a cardinality estimation of a final query plan to output an updated cardinality estimation; and a predictor for applying the calibrated cost units and the updated cardinality estimation in the cost model to generate a prediction of an execution time of a given query.