Tractable query answering over ontologies with datalog

Abstract

We present a family of expressive extensions of Datalog, called Datalog, as a new paradigm for query answering over ontologies. The Datalog family admits existentially quantified variables in rule heads, and has suitable
\nrestrictions to ensure highly efficient ontology querying. In particular, we show that query answering under so-called guarded Datalog is PTIME-complete in data complexity, and that query answering under so-called linear Datalog is in
\nAC0 in data complexity. We also show how negative constraints and a general class of key constraints can be added to Datalog while keeping ontology querying tractable. We then show that linear Datalog, enriched with a special class of key constraints, generalizes the well-known DL-Lite family of tractable description logics. Furthermore, the Datalog family is of interest in its own right and can, moreover, be used in various contexts such as data integration and data exchange.