View Selection in Semantic Web Databases

We consider the setting of a Semantic Web database, containing both explicit data encoded in RDF triples, and implicit data, implied by the RDF semantics. Based on a query workload, we address the problem of selecting a set of views to be materialized in the database, minimizing a combination of query processing, view storage, and view maintenance costs. Starting from an existing relational view selection method, we devise new algorithms for recommending view sets, and show that they scale significantly beyond the existing relational ones when adapted to the RDF context. To account for implicit triples in query answers, we propose a novel RDF query reformulation algorithm and an innovative way of incorporating it into view selection in order to avoid a combinatorial explosion in the complexity of the selection process. The interest of our techniques is demonstrated through a set of experiments.Comment: VLDB201

Tâtonnement expérimental et expériences sur les tâtonnements. Pistes théoriques et méthodologiques sur l'exploration.

La psychologie, bien que se préoccupant de longue date des problèmes d'apprentissage a toujours plus ou moins tenu à l'écart la notiond'exploration, surtout dans l'étude de l'apprentissage. Il n'y a guère que la psychologie du développement et que celle de la perception qui se soientintéressées à deux formes très différentes d'exploration : l'exploration ludique de l'enfant et l'exploration oculaire et haptique.Les raisons de ce désintérêt des psychologues pour l'exploration ou les tâtonnements sont à la fois épistémologiques, théoriques etméthodologiques. Le but de l'exploration et la possibilité même de qualifier un comportement d'exploratoire sont les deux premiers problèmes surlesquels achoppent les approches plus explicatives qu'interprétatives. En effet, qualifier un comportement d'exploratoire est le plus souvent unjugement rétrospectif, qui dépend surtout du but atteint ou de l'objet exploré. Ainsi, les problèmes se corsent dans des situations où le but ou encorel'objet exploré n'est pas facilement définissable. Pourtant Jean-Pierre Mialaret choisit cette situation fort inconfortable pour développer un questionnement psycho-pédagogique sur la musique. Par un jeu de vases communiquant parfois douloureux pour le chercheur, l'intérêt d'unesituation d'observation n'a d'égal que les difficultés théoriques et méthodologiques pour l'étudier

La folie des grands nombres ou la construction sociale des marchés de la recherche

« Ce n’est pas parce que les couteaux coupent qu’il ne faut pas s’en servir », affirmait J.-F. Dhainaut à propos des classements des universités à l’occasion d’un colloque intitulé « Oublier Shanghai ». S’il est raisonnable avec homo sapiens d’accepter la dangerosité des objets tranchants en contrepartie de l’intérêt et de la variété de leurs usages, l’homo academicus peut-il espérer le même profit des classements, comme le suggère cette comparaison ? Les deux principales questions abordées dans ce chapitre sont les suivantes : à quoi servent ces outils et quels sont leurs effets ? Il ne s’agit pas d’une synthèse exhaustive des travaux sur les effets des outils de quantification dans l’évaluation de la recherche, mais d’une proposition, par la transposition au monde de la recherche des principes de l’économie des singularités (Karpik, 2007), d’un cadre théorique permettant d’aborder conjointement ces deux questions

RDFViewS: A Storage Tuning Wizard for RDF Applications

In recent years, the significant growth of RDF data used in numerous applications has made its efficient and scalable manipulation an important issue. In this paper, we present RDFViewS, a system capable of choosing the most suitable views to materialize, in order to minimize the query response time for a specific SPARQL query workload, while taking into account the view maintenance cost and storage space constraints. Our system employs practical algorithms and heuristics to navigate through the search space of potential view configurations, and exploits the possibly available semantic information - expressed via an RDF Schema - to ensure the completeness of the query evaluation

Query-Oriented Summarization of RDF Graphs

International audienceThe Resource Description Framework (RDF) is the W3C’s graph data model for Semantic Web applications. We study the problem of RDF graph summarization: given an input RDF graph G, find an RDF graph G' which summarizes G as accurately as possible, while being possibly orders of magnitude smaller than the original graph. Our approach is query-oriented, i.e., querying a summary of a graph should reflect whether the query has some answers against this graph. The summaries are aimed as a help for query formulation and optimization. We introduce two summaries: a baseline which is compact and simple and satisfies certain accuracy and representativeness properties, but may oversimplify the RDF graph, and a refined one which trades some of these properties for more accuracy in representing the structure

Teaching an RDBMS about ontological constraints

International audienceIn the presence of an ontology, query answers must reflect not only data explicitly present in the database, but also implicit data, which holds due to the ontology, even though it is not present in the database. A large and useful set of ontology languages enjoys FOL reducibility of query answering: answering a query can be reduced to evaluating a certain first-order logic (FOL) formula (obtained from the query and ontology) against only the explicit facts. We present a novel query optimization framework for ontology-based data access settings enjoying FOL reducibility. Our framework is based on searching within a set of alternative equivalent FOL queries, i.e., FOL reformulations, one with minimal evaluation cost when evaluated through a relational database system. We apply this framework to the DL-LiteR Description Logic underpinning the W3C's OWL2 QL ontology language, and demonstrate through experiments its performance benefits when two leading SQL systems, one open-source and one commercial, are used for evaluating the FOL query reformulations

Social, Structured and Semantic Search

International audienceSocial content such as blogs, tweets, news etc. is a rich source of interconnected information. We identify a set of requirements for the meaningful exploitation of such rich content, and present a new data model, called S3, which is the first to satisfy them. S3 captures social relationships between users, and between users and content, but also the structure present in rich social content, as well as its semantics. We provide the first top-k keyword search algorithm taking into account the social, structured, and semantic dimensions and formally establish its termination and correctness. Experiments on real social networks demonstrate the efficiency and qualitative advantage of our algorithm through the joint exploitation of the social, structured, and semantic dimensions of S3

SHAMAN : Symbolic and Human-centric view of dAta MANagement

National audienc

Le recours au financement participatif par les médias d’information : levier de communication, travail en soi, idéologie marchande

Le financement participatif (crowdfunding) permet de récolter des fonds via un procédé alternatif qui met en relation des porteurs de projets, des plateformes et des contributeurs. Plusieurs cas de figures sont observables parmi les médias qui choisissent de recourir à ce procédé (création, diversification, coup par coup, sauvetage). Cet article discute du rôle de la communication et de ses effets et implications (un travail en soi), puis de la charge idéologique que véhicule ce système. Ainsi, au-delà de quelques présupposés, notamment l’idée que l’aspect économique est essentiel, nous montrerons que les techniques rationalisées dont use ce type de collecte fraient largement avec des logiques mercatiques.This article aims to focus on the communicational and ideological facets of crowdfunding, which is usually presented as an alternative way of fundraising and as an intermediary between promoters and ordinary people (funders). Several types of projects take advantage of participative-funding in media field: creation, diversification, on a case-by-case basis, rescue. Beyond common assumptions, like the idea that economics is the main motivation, we will show how this phenomenon is largely a marketing issue with specific and rationalized technics. We will also questioned the workload implied by the process and the ideological frame that this one contributes to convey

RDF Analytics: Lenses over Semantic Graphs

International audienceThe development of Semantic Web (RDF) brings new requirements for data analytics tools and methods, going beyond querying to semantics-rich analytics through warehouse-style tools. In this work, we fully redesign, from the bottom up, core data analytics concepts and tools in the context of RDF data, leading to the first complete formal framework for warehouse-style RDF analytics. Notably, we define i) analytical schemas tailored to heterogeneous, semantics-rich RDF graph, ii) analytical queries which (beyond relational cubes) allow flexible querying of the data and the schema as well as powerful aggregation and iii) OLAP-style operations. Experiments on a fully-implemented platform demonstrate the practical interest of our approach