Query Optimization in a Mediator for Digital Libraries

Digital libraries are more and more available on the web. Unfortunately, their heterogeneity and distribution make retrieving information from them a difficult process. To overcome these difficulties, several solutions have been proposed. We adopt a virtual data integration approach based on mediation to access various digital libraries. In this context, a query posed to the mediator need to be translated into a sequence of queries to the underlying data sources. In a heterogeneous environment, with sources of diverse and limited query capabilities, not all of the translations are feasible. In this paper, we study the problem of finding feasible and efficient query plans to retrieve data from digital libraries. We present the formalism that we use to describe data sources capabilities. We propose an algorithm for producing equivalent query plans and a cost model for choosing the optimal plan.