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.
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. 84. The machine-readable storage medium of claim 81 , 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 certain query.