86. The machine-readable storage medium of claim 77 , wherein the query has one or more query blocks and 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 step of determining a particular semantically equivalent query based on the query and an alternative query block, wherein the alternative query block is determined based on a first query block of the one or more query blocks.
86. The machine-readable storage medium of claim 77 , wherein the query has one or more query blocks and 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 step of determining a particular semantically equivalent query based on the query and an alternative query block, wherein the alternative query block is determined based on a first query block of the one or more query blocks. 87. The machine-readable storage medium of claim 86 , wherein the one or more sequences of instructions further comprise instructions which, when executed by the one or more processors, cause the one or more processors to perform the step of determining the alternative query block based on the first query block.