98. The machine-readable storage medium of claim 97 , 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 partial cost does not satisfy the first particular mathematical relationship with the current-best cost or if the second cost satisfies a second particular mathematical relationship with the current-best cost, then choosing as the particular query a candidate query other than the certain query.
97. The machine-readable storage medium of claim 92 , wherein the set of candidate queries comprises a certain 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 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 certain query.