Umeå universitets logga

We study the relative expressiveness of ontology-mediated queries (OMQs) formulated in the expressive Description Logic ALCHOIQ extended with closed predicates. In particular, we present a polynomial time translation from OMQs into Datalog with negation under the stable model semantics, the formalism that underlies Answer Set Programming. This is a novel and non-trivial result: the considered OMQs are not only non-monotonic, but also feature a tricky combination of nominals, inverse roles, and counting. We start with atomic queries and then lift our approach to a large class of first-order queries where quantification is “guarded” by closed predicates. Our translation is based on a characterization of the query answering problem via integer programming, and a specially crafted program in Datalog with negation that finds solutions to dynamically generated systems of integer inequalities. As an important by-product of our translation we get that the query answering problem is co-NP-complete in data complexity for the considered class of OMQs. Thus, answering these OMQs in the presence of closed predicates is not harder than answering them in the standard setting. This is not obvious as closed predicates are known to increase data complexity for some existing ontology languages.

The Shapes Constraint Language (SHACL) was recommended by the W3C in 2017 for describing constraints on web data (specifically, on the so-called RDF graphs) and validating them. At first glance, it may not seem to be a topic for logicians, but as it turns out, SHACL can be approached as a formal logic, and actually quite an interesting one. In this paper, we give a brief introduction to SHACL tailored towards logicians and frame key uses of SHACL as familiar logic reasoning tasks. We discuss how SHACL relates to description logics, which are the basis of the OWL Web Ontology Languages, a related yet orthogonal standard for web data. Finally, we summarize some of our recent work in the SHACL world, hoping that this may shed light on how ideas, results, and techniques from well-established areas of logic can advance the state of the art in this emerging field.

Circumscription is a prominent approach to bring non-monotonicity to Description Logics (DLs), but unfortunately, it usually displays very high computational complexity of reasoning. Many works have studied circumscribed DLs, but most of them focus on expressive DLs containing ALC, and the results for low-complexity DLs are limited. This paper summarises some recent progress in characterizing the computational complexity of reasoning in circumscribed DL-Lite. We perform a two-dimensional analysis, considering different languages of the DL-Lite family, and varying how concepts and roles are treated. In addition to classical circumscription, we consider the recently studied pointwise circumscription, which shows better complexity, in some cases, and remains decidable in the presence of minimized roles.

Circumscription is one of the most powerful ways to extend Description Logics (DLs) with non-monotonic reasoning features, albeit with huge computational costs and undecidability in many cases. In this paper, we introduce pointwise circumscription for DLs, which is not only intuitive in terms of knowledge representation, but also provides a sound approximation of classic circumscription and has reduced computational complexity. Our main idea is to replace the second-order quantification step of classic circumscription with a series of (pointwise) local checks on all domain elements and their immediate neighbourhood. Our main positive results are for ontologies in DLs ALCIO and ALCI: we prove that for TBoxes of modal depth 1 (i.e. without nesting of existential or universal quantifiers) standard reasoning problems under pointwise circumscription are (co)NEXPTIME-complete and EXPTIMEcomplete, respectively. The restriction of modal depth still yields a large class of ontologies useful in practice, and it is further justified by a strong undecidability result for pointwise circumscription with general TBoxes in ALCIO.

The complexity and relative expressiveness of Ontology-mediated Queries (OMQs) is quite well understood by now. In this paper, we study the expressive power of OMQs from a descriptive complexity perspective, where the central question is to understand whether a given OMQ language is powerful enough to express all queries that can be computed within some bound on time or space. We show that the OMQ language that pairs instance queries with ontologies in the very expressive DL ALCHOI with closed predicates cannot express all coNP-computable Boolean queries, despite being coNP-complete in data complexity. We, then, propose an extension of this OMQ language that is expressive enough to precisely capture the class of all Boolean queries computable in coNP. This involves adding functionality as well as path expressions and nominal schemata, which are restricted in a way that allows us to carefully incorporate them into the existing mosaic technique for the DL ALCHOIF with closed predicates without affecting the coNP upper bound in data complexity.

OWL and SHACL are two prominent W3C standards for managing RDF graphs, the data model of the Web. They are used for different purposes and make different assumptions about the completeness of data: SHACL is used for expressing integrity constraints on complete data, while OWL allows inferring implicit facts from incomplete data; SHACL reasoners perform validation, while OWL reasoners do logical inference. Integrating these two tasks into one uniform approach is a relevant but challenging problem. The SHACL standard envisions graph validation in combination with OWL entailment, but it does not provide technical guidance on how to realize this. To address this problem, we propose a new intuitive semantics for validating SHACL constraints with OWL 2 QL ontologies based on a suitable notion of the chase. We propose an algorithm that rewrites a set of recursive SHACL constraints (with stratified negation) and an OWL 2 QL ontology into a stand-alone set of SHACL constraints that preserves validation for every input graph, which can in turn be evaluated using an off-the-shelf SHACL validator. We show that validation in this setting is EXPTIME-complete in combined complexity, but only PTIME-complete in data complexity, i.e., if the constraints and the ontology are fixed.

This extended abstract summarizes our recent work [1] on SHACL validation in the presence of OWL 2 QL ontologies. To overcome the challenge posed by the non-monotonic behavior of SHACL constraints, we propose a new intuitive validation semantics and a rewriting algorithm that embeds the effects of the ontological axioms into the SHACL constraints. We analyze the complexity of validation in this setting.

The ontology-based data access (OBDA) paradigm has successfully grown over the last decade as a powerful means to access data from possibly diverse and incomplete sources, using a domain ontology as a mediator. The ability to query generic graph-structured data is often highlighted as an advantage of OBDA, but in practice, existing solutions do not allow to access data in popular graph database management systems (DBMS) (e.g., Neo4j) that adopt the so-called 'property graph' data model and support dedicated query languages such as Cypher. Towards overcoming this major limitation, we propose a technique for ontology-mediated querying (OMQ) of property graphs. We tailor a suitable query language that supports path navigation in a form that can be naturally expressed in Cypher and other important graph query languages. It keeps the data complexity of query evaluation tractable even under trail semantics and is sufficient for our motivating use case in the autonomous driving domain. We address the semantic gap between the traditional path semantics adopted by most works on graph databases, and the trail semantics used in Cypher, and identify cases where both semantics coincide. To our knowledge, OMQs with trail semantics had not been addressed before. We develop a rewriting algorithm for queries mediated by DL-Lite ontologies that enables query answering using plain Cypher. The experimental evaluation of our proof-of-concept prototype on a sample set of use case queries reveals that the approach is promising, and can be a stepping stone to making OBDA applicable to data stored in graph DBMS.