Query Answering in Probabilistic Datalog+/- Ontologies under Group Preferences

In the recent years, the Web has been changing more and more towards the so-called Social Semantic Web. Rather than being based on the link structure between Web pages, the ranking of search results in the Social Semantic Web needs to be based on something new - we believe that it can be based on user preferences and underlying ontological knowledge. Modeling uncertainty is also playing an increasingly important role in these domains, since uncertainty can arise due to many uncontrollable factors. In this paper, we propose an extension of the Data log+/- ontology language with a model for representing preferences of groups of users and a model for representing the (probabilistic) uncertainty in the domain. Assuming that more probable answers are more preferable, this raises the question of how to rank query results, since the preferences of single users may be in conflict both with the probability-based preferences as well as with each other. To this end, we propose preference merging and aggregation operators, respectively, and study their semantic and computational properties. Based on these operators, we provide algorithms for answering k-rank queries for DAQs (disjunctions of atomic queries), which generalize top-k queries based on the iterative computation of classical skyline answers, and show that, under certain reasonable conditions, they run in polynomial time in the data complexity.