Michaël Thomazo

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We establish complexities of the conjunctive query entailment problem for classes of existential rules (i.e. Tuple-Generating Dependencies or Datalog+/rules). Our contribution is twofold. First, we introduce the class of greedy bounded treewidth sets (gbts), which covers guarded rules, and their known generalizations, namely (weakly) frontier-guarded rules.(More)
Querying large databases while taking ontologies into account is currently a very active domain research. In this paper, we consider ontologies described by existential rules (also known as Datalog+/-), a framework that generalizes lightweight description logics. A common approach is to rewrite a conjunctive query w.r.t an ontology into a union of(More)
We address the issue of Ontology-Based Data Access which consists of exploiting the semantics expressed in ontologies while querying data. Ontologies are represented in the framework of existential rules, also known as Datalog+/-. We focus on the backward chaining paradigm, which involves rewriting the query (assumed to be a conjunctive query, CQ) into a(More)
The need for an ontological layer on top of data, associated with advanced reasoning mechanisms able to exploit ontological knowledge, has been acknowledged in the database, knowledge representation and Semantic Web communities. We focus here on the ontology-based data querying problem, which consists in querying data while taking ontological knowledge into(More)
We address the issue of Ontology-Based Data Access, with ontologies represented in the framework of existential rules, also known as Datalog+/-. A wellknown approach involves rewriting the query using ontological knowledge. We focus here on the basic rewriting technique which consists of rewriting the initial query into a union of conjunctive queries.(More)
We consider existential rules (also called Tuple-Generating Dependencies or Datalog+/rules). These rules are particularly well-suited to the timely ontological query answering problem, which consists of querying data while taking terminological knowledge into account. Since this problem is not decidable in general, various conditions ensuring decidability(More)
Answering queries in information systems that allow for expressive inferencing is currently a field of intense research. This problem is often referred to as ontology-based data access (OBDA). We focus on conjunctive query entailment under logical rules known as tuple-generating dependencies, existential rules or Datalog+/-. One of the most expressive(More)
We address the issue of Ontology-Based Data Access, with ontologies represented in the framework of existential rules, also known as Datalog+/-. A well-known approach involves rewriting the query using ontological knowledge. We focus here on the basic rewriting technique which consists of rewriting a conjunctive query (CQ) into a union of CQs. We assume(More)
Two languages are separable by a piecewise testable language if and only if there exists no infinite tower between them. An infinite tower is an infinite sequence of strings alternating between the two languages such that every string is a subsequence (scattered substring) of all the strings that follow. For regular languages represented by nondeterministic(More)
For a non-negative integer k, a language is k-piecewise testable (k-PT) if it is a finite boolean combination of languages of the form Σa1Σ ∗ · · ·ΣanΣ for ai ∈ Σ and 0 ≤ n ≤ k. We study the following problem: Given a DFA recognizing a piecewise testable language, decide whether the language is k-PT. We provide a complexity bound on this problem and a(More)