Gabriel M. Kuper

Structural properties of XPath fragments

We study structural properties of each of the main sublanguages of navigational XPath (W3c Recommendation) commonly used in practice. First, we characterize the expressive power of these language fragments in terms of both logics and tree patterns. Second, we investigate closure properties, focusing on the ability to perform basic Boolean operations while remaining within the fragment. We give a complete picture of the closure properties of these fragments, treating XPath expressions both as functions of arbitrary nodes in a document tree, and as functions that are applied only at the root of the tree. Finally, we provide sound and complete axiom systems and normal forms for several of these fragments. These results are useful for simplification of XPath expressions and optimization of XML queries.

Structural Properties of XPath Fragments

We study structural properties of each of the main sublanguages of XPath [8] commonly used in practice. First, we characterize the expressive power of these language fragments in terms of both logics and tree patterns. Second, we investigate closure properties, focusing on the ability to perform basic Boolean operations while remaining within the fragment. We give a complete picture of the closure properties of these fragments, treating XPath expressions both as functions of arbitrary nodes in a document tree, and as functions that are applied only at the root of the tree. Finally, we provide sound and complete axiom systems and normal forms for several of these fragments. These results are useful for simplification of XPath expressions and optimization of XML queries.

A Robust Logical and Computational Characterisation of Peer-to-Peer Database Systems

In this paper we give a robust logical and computational characterisation of peer-to-peer (p2p) database systems. We first define a precise model-theoretic semantics of a p2p system, which allows for local inconsistency handling. We then characterise the general computational properties for the problem of answering queries to such a p2p system. Finally, we devise tight complexity bounds and distributed procedures for the problem of answering queries in few relevant special cases.

Queries and updates in the coDB peer to peer database system

In this short paper we present the coDB P2P DB system. A network of databases, possibly with different schemas, are interconnected by means of GLAV coordination rules, which are inclusions of conjunctive queries, with possibly existential variables in the head; coordination rules may be cyclic. Each node can be queried in its schema for data, which the node can fetch from its neighbours, if a coordination rule is involved.

A unified constraint model for XML

Abstract Integrity constraints are an essential part of modern schema definition languages. They are useful for semantic specification, update consistency control, query optimization, etc. In this paper, we propose UCM, a model of integrity constraints for XML that is both simple and expressive. Because it relies on a single notion of keys and foreign keys, the UCM model is easy to use and makes formal reasoning possible. Because it relies on a powerful type system, the UCM model is expressive, capturing in a single framework the constraints found in relational databases, object-oriented schemas and XML document type definitions. We study the problem of consistency of UCM constraints, the interaction between constraints and subtyping, and algorithms for implementing these constraints.

A distributed algorithm for robust data sharing and updates in p2p database networks

In this paper we thoroughly analyze a distributed procedure for the problem of local database update in a network of database peers, useful for data exchange scenarios The algorithm supports dynamic networks: even if nodes and coordination rules appear or disappear during the computation, the proposed algorithm will eventually terminate with a sound and complete result.

A constrant-based spatial extension to SQL

The con~traint database model provides a uniform framework for handling spatial and non-spatial data. We describe our proposal for a query language for GISS based on this model. Our language is called CSQL, as it is based on SQL. CSQL provides the full query capabilities of SQL on the non-spatial part of the database while being tightly integrated with the spatial part. This allows us to optimize C!SQL queries in a unified fashion, considering both the spatial amdnon-spatial attributes simultaneously. TtTeare in the process of implementing a constraint database system using CSQL as a query language, via a relational algebra CALG, and an e~tensible database system.

Linear vs Polynomial Constraints in Database Query Languages

We prove positive and negative results on the expressive power of the relational calculus augmented with linear constraints. We show the non-expressibility of some properties expressed by polynomial constraints. We consider in particular some queries involving the existence of lines. We study mainly constraints over real numbers, but we also present some results in the case of the natural numbers.

Generalized XML security views

We investigate a generalization of the notion of XML security view introduced by Stoica and Farkas (Proceedings of the 16th International Conference on Data and Applications Security (IFIP’02). IFIP Conference Proceedings, vol. 256, pp. 133–146. Kluwer, Dordrecht, 2002) and later refined by Fan et al. (Proceedings of the ACM SIG- MOD International Conference on Management of Data (SIGMOD’04), pp. 587–598. ACM Press, New York, 2004). The model consists of access control policies specified over DTDs with XPath expressions for data-dependent access control. We provide the notion of security views characterizing information accessible to authorized users. This is a trans- formed DTD schema that can be used by users for query formulation. We develop an algorithm to materialize an authorized version of the document from the view and an algorithm to construct the view from an access control specification. We show that our view construction combined with materialization produces the same result as the direct application of the DTD access specification on the document. We also propose a number of generalizations of possible security policies and show how they affect view construction algorithm. Finally, we provide an evaluation of our system.

Tractable recursion over geometric data (Extended Abstract)

We study the issue of adding a recursion operator to constraint query languages for linear spatial databases. We introduce a language with a bounded inflationary fixpoint operator which is closed and captures the set of polynomial time computable queries over linear constraint databases. This is the first logical characterization of the class of PTIME queries in this context. To prove the result, we develop original techniques to perform arithmetical and geometric operations with constraints. Work supported in part by TMR Project Chorochronos. Work supported by Inria, FNRS and ULB.