81. The machine-readable storage medium of claim 80 , wherein the instructions that cause determining the set of candidate queries and costs further comprise instructions which, when executed by the one or more processors, cause the one or more processors to perform the steps of: determining a partial cost for a first portion of the certain query; and if the partial cost satisfies the first particular mathematical relationship with the current-best cost, determining a second cost based on the entire particular semantically equivalent query.
80. The machine-readable storage medium of claim 77 , wherein the instructions that cause determining the set of candidate queries and costs comprise instructions which, when executed by the one or more processors, cause the one or more processors to perform the steps of: initially establishing the query as a current-best query; determining a current-best cost based on the current-best query; for each certain query of the set of candidate queries: determining a current cost based on the certain query; if the current cost satisfies a first particular mathematical relationship with the current-best cost, then: establishing the certain query as the current-best query; and establishing the current cost as the current-best cost.