68. The machine-readable storage medium of claim 64 , wherein the instructions that cause determining the set of candidate queries comprise instructions which, when executed by the one or more processors, cause the one or more processors to perform the step of determining a particular semantically equivalent query, and wherein the instructions that cause determining costs comprise instructions which, when executed by the one or more processors, cause the one or more processors to perform the steps of: determining a current-best cost based on a first cost for the query and costs for zero or more semantically equivalent queries; determining a partial cost for a first portion of the particular semantically equivalent query; and if the partial cost satisfies a first particular mathematical relationship with the current-best cost, determining a second cost based on the entire particular semantically equivalent query.
68. The machine-readable storage medium of claim 64 , wherein the instructions that cause determining the set of candidate queries comprise instructions which, when executed by the one or more processors, cause the one or more processors to perform the step of determining a particular semantically equivalent query, and wherein the instructions that cause determining costs comprise instructions which, when executed by the one or more processors, cause the one or more processors to perform the steps of: determining a current-best cost based on a first cost for the query and costs for zero or more semantically equivalent queries; determining a partial cost for a first portion of the particular semantically equivalent query; and if the partial cost satisfies a first particular mathematical relationship with the current-best cost, determining a second cost based on the entire particular semantically equivalent query. 70. The machine-readable storage medium of claim 68 , wherein the instructions that cause choosing the particular query comprise instructions which, when executed by the one or more processors, cause the one or more processors to perform the step of, if the second cost does not satisfy a second particular mathematical relationship with the current-best cost, then choosing as the particular query a candidate query other than the particular semantically equivalent query.