Section: New Results

Querying Heterogeneous Linked Data

Angela Bonifati, Gianvito Summa, Esther Pacitt (U Montpellier 2)i and Fady Draidi (U Montpellier 2) [5] consider peer-to-peer data management systems (PDMS), where each peer maintains mappings between its schema and some acquaintances, along with social links with peer friends. In this context, the goal is reformulating conjunctive queries from a peer’s schema into other peer’s schemas. Precisely, queries against a peer node are rewritten into queries against other nodes using schema mappings thus obtaining query rewritings. They propose a new notion of ‘relevance’ of a query with respect to a mapping that encompasses both a local relevance (the relevance of the query w.r.t. the mapping) and a global relevance (the relevance of the query w.r.t. the entire network). Based on this notion, they conceived a new query reformulation approach for social PDMS which achieves great accuracy and flexibility. This has been implemented and experimented in a prototype.

Pierre Bourhis, Andreas Morak and Andreas Pieris [14] investigated classes of queries for which the problem of open query answering of disjunctive guarded TGDs a decent complexity (e, g exptime). The complete picture of the complexity of answering (unions of) conjunctive queries under the main guarded-based classes of disjunctive existential rules has been recently settled. It has been shown that the problem is very hard, namely 2ExpTime-complete, even for fixed sets of rules expressed in lightweight formalisms.The central objective of the present paper is to understand whether simpler query languages (bounded tree width and acyclic queries) have a positive impact on the complexity of query answering under the main guarded-based classes of disjunctive existential rules.

In [3] , a new formalism for schema for unordered trees have been developed. It is based on a notion of regular expressions of multisets of labels. Different problems of static analysis like emptiness and containment are studied and their complexity. Different simpler schema are studied leading to interesting complexity for the different studied problems. Finally, they study the expressive power of the proposed schema languages and compare them with yardstick languages of unordered trees (FO, MSO, and Presburger constraints) and DTDs under commutative closure. The results show that the proposed schema languages are capable of expressing many practical languages of unordered trees and enjoy desirable computational properties.

In [7] , Adrian Boiret, Vincent Hugot and Joachim Niehren and Ralf Treinen (University Paris 7) proposes a notion deterministic tree automata for unordered trees. While the existing notions are well-investigated concerning expressiveness, they all lack a proper notion of determinism, which makes it difficult to distinguish subclasses of automata for which problems such as inclusion, equivalence, and minimization can be solved efficiently. In this paper, the authors propose and investigate different notions of ”horizontal determinism”, starting from automata for unranked trees in which the horizontal evaluation is performed by finite state automata.