Pierre Genevès

A system for the static analysis of XPath

XPath is the standard language for navigating XML documents and returning a set of matching nodes. We present a sound and complete decision procedure for containment of XPath queries, as well as other related XPath decision problems such as satisfiability, equivalence, overlap, and coverage. The considered XPath fragment covers most of the language features used in practice. Specifically, we propose a unifying logic for XML, namely, the alternation-free modal μ-calculus with converse. We show how to translate major XML concepts such as XPath and regular XML types (including DTDs) into this logic. Based on these embeddings, we show how XPath decision problems, in the presence or absence of XML types, can be solved using a decision procedure for μ-calculus satisfiability. We provide a complexity analysis of our system together with practical experiments to illustrate the efficiency of the approach for realistic scenarios.

Logic-based XPath optimization

XPath [5] was introduced by the W3C as a standard language for specifying node selection, matching conditions, and for computing values from an XML document. XPath is now used in many XML standards such as XSLT [4] and the forthcoming XQuery [10] database access language. Since efficient XML content querying is crucial for the performance of almost all XML processing architectures, a growing need for studying high performance XPath-based querying has emerged. Our approach aims at optimizing XPath performance through static analysis and syntactic transformation of XPath expressions.

Deciding XPath containment with MSO

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 practical experiments that illustrate the efficiency of the decision procedure for realistic scenarios.

A comparative analysis of attitude estimation for pedestrian navigation with smartphones

We investigate the precision of attitude estimation solutions in the context of Pedestrian Dead-Reckoning (PDR) with commodity smartphones and inertial/magnetic sensors. We propose a concise comparison and analysis of a number of attitude filtering methods in this setting. We conduct an experimental study with a precise ground truth obtained with a motion capture system. We precisely quantify the error in attitude estimation obtained with each filter which combines a 3-axis accelerometer, a 3-axis magnetometer and a 3-axis gyroscope measurements. We discuss the obtained results and analyse advantages and limitations of current technology for further PDR research.

XML query-update independence analysis revisited

XML transformations can be resource-costly in particular when applied to very large XML documents and document sets. Those transformations usually involve lots of XPath queries and may not need to be entirely re-executed following an update of the input document. In this context, a given query is said to be independent of a given update if, for any XML document, the results of the query are not affected by the update. We revisit Benedikt and Cheneys framework for query-update independence analysis and show that performance can be drastically enhanced, contradicting their initial claims. The essence of our approach and results resides in the use of an appropriate logic, to which queries and updates are both succinctly translated. Compared to previous approaches, ours is more expressive from a theoretical point of view, equally accurate, and more efficient in practice. We illustrate this through practical experiments and comparative figures.

Toward automated schema-directed code revision

Updating XQuery programs in accordance with a change of the input XML schema is known to be a time-consuming and error-prone task. We propose an automatic method aimed at helping developers realign the XQuery program with the new schema. First, we introduce a taxonomy of possible problems induced by a schema change. This allows to differentiate problems according to their severity levels, e.g. errors that require code revision, and semantic changes that should be brought to the developers attention. Second, we provide the necessary algorithms to detect such problems using a solver that checks satisfiability of XPath expressions.

Attitude Estimation for Indoor Navigation and Augmented Reality with Smartphones

Abstract We investigate the precision of attitude estimation algorithms in the particular context of pedestrian navigation with commodity smartphones and their inertial/magnetic sensors. We report on an extensive comparison and experimental analysis of existing algorithms. We focus on typical motions of smartphones when carried by pedestrians. We use a precise ground truth obtained from a motion capture system. We test state-of-the-art and built-in attitude estimation techniques with several smartphones, in the presence of magnetic perturbations typically found in buildings. We discuss the obtained results, analyze advantages and limits of current technologies for attitude estimation in this context. Furthermore, we propose a new technique for limiting the impact of magnetic perturbations with any attitude estimation algorithm used in this context. We show how our technique compares and improves over previous works. A particular attention was paid to the study of attitude estimation in the context of augmented reality motions when using smartphones.

On attitude estimation with smartphones

We investigate the precision of attitude estimation algorithms in the particular context of pedestrian navigation with commodity smartphones and their inertial/magnetic sensors. We report on an extensive comparison and experimental analysis of existing algorithms. We focus on typical motions of smartphones when carried by pedestrians. We use a precise ground truth obtained from a motion capture system. We test state-of-the-art attitude estimation techniques with several smartphones, in the presence of magnetic perturbations typically found in buildings. We discuss the obtained results, analyze advantages and limits of current technologies for attitude estimation in this context. Furthermore, we propose a new technique for limiting the impact of magnetic perturbations with any attitude estimation algorithm used in this context. We show how our technique compares and improves over previous works.

Selectivity Estimation for SPARQL Triple Patterns with Shape Expressions

We optimize the evaluation of conjunctive SPARQL queries, on big RDF graphs, by taking advantage of ShEx schema constraints. Our optimization is based on computing ranks for query triple patterns, which indicates their order of execution. We first define a set of well-formed ShEx schemas, that possess interesting characteristics for SPARQL query optimization. We then define our optimization method by exploiting information extracted from a ShEx schema. The experimentations performed shows the advantages of applying our optimization on the top of an existing state-of-the-art query evaluation system.

SPARQL Query Containment under Schema

Query containment is defined as the problem of determining if the result of a query is included in the result of another query for any dataset. It has major applications in query optimization and knowledge base verification. To date, testing query containment has been performed using different techniques: containment mapping, canonical databases, automata theory techniques and through a reduction to the validity problem in logic. Here, we use the latter technique to test containment of SPARQL queries using an expressive modal logic called