1. A system for optimizing query processing in a federated information system, said system comprising: a processor; memory elements comprising a computer readable medium operatively connected to said processor; a calibrator, operating through said processor and memory elements, in communication with said federated information system and adapted to intercept queries to said federated information system; a simulator, operating through said processor and memory elements, in communication with said calibrator and adapted to derive alternative query plans for each of said queries, wherein each of said query plans comprises a plurality of query fragment plans associated with at least one data source, wherein said simulator is further adapted to determine first query fragment cost estimates for each of said query fragment plans, wherein said calibrator is further adapted to calibrate source-specific cost factors for each data source in said federated information system based on workload associated with said corresponding data source, and wherein said simulator is further adapted to calculate second query fragment cost estimates by multiplying each of said first query fragment cost estimates times a corresponding one of said source-specific cost factors.