13. The method of claim 4 wherein Total cost is a sum of a first product and a second product, wherein the first product is a product of a factor specific to the database system implementation and the estimated CPU cost; and the second product is a product of a second factor specific to the database system implementation and the estimated I/O cost.
4. The method of claim 1 wherein the cost of computing the query is the weighted sum of an estimated CPU cost and an estimated I/O cost.