7 research outputs found
On decidability and tractability of querying in temporal EL
We study access to temporal data with TEL, a temporal extension of the tractable description logic EL. Our aim is to establish a clear computational complexity landscape for the atomic query answering problem, in terms of both data and combined complexity. Atomic queries in full TEL turn out to be undecidable even in data complexity. Motivated by the negative result, we identify well-behaved yet expressive fragments of TEL. Our main contributions are a semantic and sufficient syntactic conditions for decidability and three orthogonal tractable fragments, which are based on restricted use of rigid roles, temporal operators, and novel acyclicity conditions on the ontologies
Computing FO-Rewritings in EL in Practice: from Atomic to Conjunctive Queries
A prominent approach to implementing ontology-mediated queries (OMQs) is to
rewrite into a first-order query, which is then executed using a conventional
SQL database system. We consider the case where the ontology is formulated in
the description logic EL and the actual query is a conjunctive query and show
that rewritings of such OMQs can be efficiently computed in practice, in a
sound and complete way. Our approach combines a reduction with a decomposed
backwards chaining algorithm for OMQs that are based on the simpler atomic
queries, also illuminating the relationship between first-order rewritings of
OMQs based on conjunctive and on atomic queries. Experiments with real-world
ontologies show promising results
On the containment of SPARQL queries under entailment regimes
Most description logics (DL) query languages allow instance retrieval from an ABox. However, SPARQL is a schema query language allowing access to the TBox (in addition to the ABox). Moreover, its entailment regimes enable to take into account knowledge inferred from knowledge bases in the query answering process. This provides a new perspective for the containment problem. In this paper, we study the containment of SPARQL queries over OWL EL axioms under entailment. OWL EL is the language used by many large scale
ontologies and is based on EL++. The main contribution is a novel approach to rewriting queries using SPARQL property paths and the
μ-calculus in order to reduce containment test
under entailment into validity check in the
μ-calculus
On decidability and tractability of querying in temporal EL
We study access to temporal data with TEL, a temporal extension of
the tractable description logic EL. Our aim is to establish a clear computational
complexity landscape for the atomic query answering problem, in terms of both
data and combined complexity. Atomic queries in full TEL turn out to be undecidable
even in data complexity. Motivated by the negative result, we identify
well-behaved yet expressive fragments of TEL. Our main contributions are a semantic
and sufficient syntactic conditions for decidability and three orthogonal
tractable fragments, which are based on restricted use of rigid roles, temporal
operators, and novel acyclicity conditions on the ontologies
Deciding FO-Rewritability in EL
Abstract. We consider the problem of deciding, given an instance query A(x), an EL-TBox T, and possibly an ABox signature Σ, whether A(x) is FO-rewritable relative to T and Σ-ABoxes. Our main results are PSPACE-completeness for the case where Σ comprises all symbols and EXPTIME-completeness for the general case. We also show that the problem is in PTIME for classical TBoxes and that every instance query is FO-rewritable into a polynomial-size FO query relative to every (semi)-acyclic TBox (under some mild assumptions on the data).