Expressive Logical Combinators for Free

International audienceA popular technique for the analysis of web query languages relies on the translation of queries into logical formulas. These formulas are then solved for satisfiability using an off-the-shelf satisfiabil-ity solver. A critical aspect in this approach is the size of the obtained logical formula, since it constitutes a factor that affects the combined complexity of the global approach. We present logical combi-nators whose benefit is to provide an exponential gain in succinctness in terms of the size of the logical representation. This opens the way for solving a wide range of problems such as satisfiability and containment for expressive query languages in exponential-time, even though their direct formulation into the underlying logic results in an exponential blowup of the formula size, yielding an incorrectly presumed two-exponential time complexity. We illustrate this from a practical point of view on a few examples such as numerical occurrence constraints and tree frontier properties which are concrete problems found with semi-structured data

A Logical Approach To Deciding Semantic Subtyping

International audienceWe consider a type algebra equipped with recursive, product, function, intersection, union, and complement types together with type variables and implicit universal quantification over them. We consider the subtyping relation recently defined by Castagna and Xu over such type expressions and show how this relation can be decided in EXPTIME, answering an open question. The novelty, originality and strength of our solution reside in introducing a logical modeling for the semantic subtyping framework. We model semantic subtyping in a tree logic and use a satisfiability-testing algorithm in order to decide subtyping. We report on practical experiments made with a full implementation of the system. This provides a powerful polymorphic type system aiming at maintaining full static type-safety of functional programs that manipulate trees, even with higher-order functions, which is particularly useful in the context of XML

SPARQLGX in Action: Efficient Distributed Evaluation of SPARQL with Apache Spark

International audienceWe demonstrate SPARQLGX: our implementation of a distributed sparql evaluator. We show that sparqlgx makes it possible to evaluate SPARQL queries on billions of triples distributed across multiple nodes, while providing attractive performance figures

Mobile Augmented Reality Applications for Smart Cities

International audienceToday, one of the main challenges in mobile augmented reality applications design is understanding how our perception of reality can be profitably augmented and how this augmentation can be made to fit seamlessly with the user’s interaction with the world. The European VENTURI project which began in 2011 is aiming to develop the first generation of ubiquitous augmented reality (AR) tools that meet real user needs and fit within the context they operate in

Mobile Augmented Reality Applications for Smart Cities

International audienceToday, one of the main challenges in mobile augmented reality applications design is understanding how our perception of reality can be profitably augmented and how this augmentation can be made to fit seamlessly with the user’s interaction with the world. The European VENTURI project which began in 2011 is aiming to develop the first generation of ubiquitous augmented reality (AR) tools that meet real user needs and fit within the context they operate in

Compiling XPath into a State-less Forward-only Subset

International audienceWe show how the context state of XPath, accessed through the position() and last() pseudo-functions, can be eliminated in most cases by translating references to the context state with an equivalent contextfree expression, and how this enables the use of context state in combination with a subsequent forward-only transformation, allowing for execution of (almost) full XPath on any of the emerging streaming subsets. Specifically we show how the normalization into a core language as proposed in the current W3C Last Call draft of the XPath/XQuery Formal Semantics can be extended such that the context state and reverse axes can be eliminated from the core XPath (and potentially XQuery) language

Mu-Calculus Based Resolution of XPath Decision Problems

XPath is the standard declarative notation for navigating XML data and returning a set of matching nodes. In the context of XSLT/XQuery analysis, query optimization, and XML type checking, XPath decision problems arise naturally. They notably include XPath containment (whether or not for any tree the result of a particular query is included in the result of a second one), and XPath satisfiability (whether or not an expression yields a non-empty result), in the presence (or the absence) of XML DTDs. In this paper, we propose a unifying logic for XML, namely the alternation-free modal mu-calculus with converse. We show how to translate major XML concepts such as XPath and DTDs into this logic. Based on these embeddings, we show how XPath decision problems can be solved using a state-of-the-art EXPTIME decision procedure for mu-calculus satisfiability. We provide preliminary experiments which shed light, for the first time, on the cost of solving XPath decision problems in practice

Evaluating and benchmarking SPARQL query containment solvers

International audienceQuery containment is the problem of deciding if the answers to a query are included in those of another query for any queried database. This problem is very important for query optimization purposes. In the SPARQL context, it can be equally useful. This problem has recently been investigated theoretically and some query containment solvers are available. Yet, there were no benchmarks to compare theses systems and foster their improvement. In order to experimentally assess implementation strengths and limitations, we provide a first SPARQL containment test benchmark. It has been designed with respect to both the capabilities of existing solvers and the study of typical queries. Some solvers support optional constructs and cycles, while other solvers support projection, union of conjunctive queries and RDF Schemas. No solver currently supports all these features or OWL entailment regimes. The study of query demographics on DBPedia logs shows that the vast majority of queries are acyclic and a significant part of them uses UNION or projection. We thus test available solvers on their domain of applicability on three different benchmark suites. These experiments show that (i) tested solutions are overall functionally correct, (ii) in spite of its complexity, SPARQL query containment is practicable for acyclic queries, (iii) state-of-the-art solvers are at an early stage both in ter

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

XQTC: A Static Type-Checker for XQuery Using Backward Type Inference

We present a novel technique and a tool for static type-checking of XQuery programs. The tool looks for errors in the program by jointly analyzing the source code of the program, input and output schemas that respectively describe the sets of documents admissible as input and as output of the program. The crux and the novelty of our results reside in the joint use of backward type inference and a two-way logic to represent inferred tree type portions. This allowed us to design and implement a type-checker for XQuery which is more precise and supports a larger XQuery fragment compared to the approaches previously proposed in the literature; in particular compared to the only few actually implemented static type-checkers such as the one in Galax. The whole system uses compilers and a satisfiability solver for deciding containment for two-way regular tree expressions. Our tool takes an XQuery program and two schemas Sin and Sout as input. If the program is found incorrect, then it automatically generates a counter-example valid w.r.t. Sin and such that the program produces an invalid output w.r.t Sout. This counter-example can be used by the programmer to fix the program.Nous présentons une technique nouvelle et un outil pour le contrôle de type statique des programmes XQuery. L'outil recherche les erreurs dans le programme en analysant à la fois le code source du programme et les schémas d'entrée et de sortie qui décrivent respectivement les ensembles de documents admissibles en entrée et en sortie. L'originalité de nos résultats réside dans l'utilisation conjointe de l'inférence de type arrière et d'une logique avec programmes inverses pour représenter des fragments de types d'arbre. Cela nous a permis de concevoir et de réaliser un contrôleur de type pour XQuery qui est plus précis et supporte un fragment de XQuery plus large que les approches proposées précédemment dans la littérature, en particulier si on se réfère aux quelques contrôleurs de type statiques effectivement réalisés, tel que celui de Galax. L'ensemble du système utilise des compilateurs et un solveur pour décider de l'inclusion des expressions d'arbres régulières bi-directionnelles. Notre outil prend en entrée un programme XQuery et deux schémas Sin et Sout. Si le programme est reconnu incorrect, l'outil engendre automatiquement un contre-exemple valide vis-à-vis de Sin et tel que le programme produise un résultat invalide vis-à-vis de Sout. Ce contre-exemple peut alors être utilisé par le programmeur pour corriger son programme