A Relaxed But Not Necessarily Constrained Way from the Top to the Sky

As P2P systems are a very popular approach to connect a possibly large number of peers, efficient query processing plays an important role. Appropriate strategies have to take the characteristics of these systems into account. Due to the possibly large number of peers, extensive flooding is not possible. The application of routing indexes is a commonly used technique to avoid flooding. Promising techniques to further reduce execution costs are query operators such as top-N and skyline, constraints, and the relaxation of exactness and/or completeness. In this paper, we propose strategies that take all these aspects into account. The choice is left to the user if and to what extent he is willing to relax exactness or apply constraints. We provide a thorough evaluation that uses two types of distributed data summaries as examples for routing indexes.