58 research outputs found
Decidability of Querying First-Order Theories via Countermodels of Finite Width
We propose a generic framework for establishing the decidability of a wide
range of logical entailment problems (briefly called querying), based on the
existence of countermodels that are structurally simple, gauged by certain
types of width measures (with treewidth and cliquewidth as popular examples).
As an important special case of our framework, we identify logics exhibiting
width-finite finitely universal model sets, warranting decidable entailment for
a wide range of homomorphism-closed queries, subsuming a diverse set of
practically relevant query languages. As a particularly powerful width measure,
we propose Blumensath's partitionwidth, which subsumes various other commonly
considered width measures and exhibits highly favorable computational and
structural properties. Focusing on the formalism of existential rules as a
popular showcase, we explain how finite partitionwidth sets of rules subsume
other known abstract decidable classes but -- leveraging existing notions of
stratification -- also cover a wide range of new rulesets. We expose natural
limitations for fitting the class of finite unification sets into our picture
and provide several options for remedy
A Decision Procedure for XPath Containment
XPath is the standard language for addressing parts of an XML document. We present a sound and complete decision procedure for containment of XPath queries. The considered XPath fragment covers most of the language features used in practice. Specifically, we show how XPath queries can be translated into equivalent formulas in monadic second-order logic. Using this translation, we construct an optimized logical formulation of the containment problem, which is decided using tree automata. When the containment relation does not hold between two XPath expressions, a counter-example XML tree is generated. We provide a complexity analysis together with practical experiments that illustrate the efficiency of the decision procedure for realistic scenarios
The Impact of Disjunction on Query Answering Under Guarded-Based Existential Rules
Abstract. We give the complete picture of the complexity of conjunctive query answering under (weakly-)(frontier-)guarded disjunctive existential rules, i.e., existential rules extended with disjunction, and their main subclasses, linear rules and inclusion dependencies.
Views and Queries: Determinacy and Rewriting
International audienceWe investigate the question of whether a query Q can be answered using a set V of views. We first define the problem in information-theoretic terms: we say that V determines Q if V provides enough information to uniquely determine the answer to Q . Next, we look at the problem of rewriting Q in terms of V using a specific language. Given a view language V and query language Q , we say that a rewriting language R is complete for V -to- Q rewritings if every Q ∈ Q can be rewritten in terms of V ∈ V using a query in R , whenever V determines Q . While query rewriting using views has been extensively investigated for some specific languages, the connection to the information-theoretic notion of determinacy, and the question of completeness of a rewriting language have received little attention. In this article we investigate systematically the notion of determinacy and its connection to rewriting. The results concern decidability of determinacy for various view and query languages, as well as the power required of complete rewriting languages. We consider languages ranging from first-order to conjunctive queries
Programming Languages and Systems
This open access book constitutes the proceedings of the 28th European Symposium on Programming, ESOP 2019, which took place in Prague, Czech Republic, in April 2019, held as Part of the European Joint Conferences on Theory and Practice of Software, ETAPS 2019
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