A Study on Query Optimization for Federated Database Systems

In this paper, we explore the design space for a query optimizer in this environment and demonstrate the need for decoupling various aspects of the optimization process. We present minimum-communication decoupled variants of various query optimization techniques, and discuss trade-offs in their performance in this scenario. We have implemented these techniques in the Cohera federated database system and our experimental results, somewhat surprisingly, indicate that a simple two-phase optimization scheme performs fairly well as long as the physical database design is known to the optimizer , though more aggressive algorithms are required otherwise