1. A method of processing information relating to the execution of instructions by a database management system, wherein the method is performed by a data processing apparatus and comprises: receiving an instruction; receiving execution information relating to the execution performance of the instruction when executed by the database management system; comparing the instruction to one or more previously received instructions and determining whether, according to a user selectable equivalence criterion, the instruction matches one of the previously received instructions; and if the instruction matches a previously received instruction, combining the execution information with execution information relating to the matching one of the previously received instructions to produce aggregate execution information; wherein the aggregate execution information comprises a plurality of aggregate values each representing a respective performance measure, the aggregate values determined for a plurality of executions of instructions determined as equivalent according to the user selectable equivalence criterion, the plurality of aggregate values including at least one of: a sum value, an average value, a minimum value, and a maximum value; wherein the method further comprises storing a hash table comprising hash values for one or more previously received instructions; and wherein the comparing and determining step comprises: applying a hash function to the received instruction to generate a hash value for the received instruction, and comparing the generated hash value to the hash values in the hash table; responsive to a determination that the generated hash value does not match match one of the hash values in the hash table, placing the received instruction in a standard form to produce a modified instruction, applying a hash function to the modified instruction to generate a hash value for the modified instruction, and comparing the hash value for the modified instruction to the hash values in the hash table; and determining that the received instruction matches a previously received instruction responsive to a determination that either the hash value for the received instruction or the hash value for the modified instruction matches a hash value in the hash table.