Distributed Aggregation Strategies for Preference Queries

Networks of cooperating peers are a new exciting paradigm for evaluating queries in a distributed environment. In this scenario, a query originated at a peer propagates through the network, and the overall result is obtained by aggregating those returned by the peers involved in the evaluation. In this paper we consider the relevant case of preference queries, in which the user is interested in obtaining all and only the “best” results. We highlight the fundamental difference between queries in which preferences define a weak order (wo) over objects and the more general ones for which a strict partial order (spo) has to be considered. While for wo queries a simple algorithm that minimizes the overall number of objects to be transmitted across the network can be easily derived, we show that this is not the case for spo queries. Then, we detail a set of basic issues whose solution is a key to the derivation of an efficient distributed algorithm.