Query answering over disjunctive datalog with existential quantifiers

The paper discusses the impact of adding existential quantifi-
cation in the head of positive disjunctive Datalog rules. After introducing
syntax and semantics of the resulting language, we provide a notion of
instantiation, which has been proven to be adequate for query answering
purposes. Although on the one hand this new formalism is attractive for
knowledge management, especially in domains where ontology-based rea-
soning is needed, on the other hand the presence of existential quantifiers
does not preserve decidability of the typical reasoning tasks even if dis-
junction is forbidden. Therefore, we consider many decidable fragments
of the language that naturally extend analogous classes defined in the
non-disjunctive case, and we give a complete picture of the complexity
of answering conjunctive queries in this setting.