Team mostrare

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Section: New Results

Modeling XML document transformations

XML database queries, logic and automata

Participants : Olivier Gauwin, Joachim Niehren, Sophie Tison, Sławek Staworko, Grégoire Laurence, Aurélien Lemay, Anne-Cécile Caron, Yves Roos, Benoît Groz.

Query answering on XML streams. Gauwin [11] introduces in his PhD thesis a hierarchy of streamability notions for query languages that restrict both space and time. He shows that query languages with hard satisfiability problems are not streamable even for the weakest notion of streamability. This result contradicts prominent streaming algorithms for small syntactic fragments of XPath in the database literature.

With his advisors Niehren and Tison [16] , Gauwin shows that earliest query answering is feasible for queries defined by deterministic streaming tree automata. This permits to establish positive streamability results for n -ary queries defined by deterministic nested word automata, under the condition that the concurrency of queries and the depth of trees are bounded. It equally implies streamability results for fragments of XPath with schema restrictions. Gauwin, Niehren and Tison [15] show that bounded concurrency is decidable in polynomial time for queries defined by deterministic tree automata. The proof is by reduction to proving bounded valuedness of recognizable relations between ranked trees, which in turn can be reduced to bounded valuedness of bottom-up tree transducers.

Bagan started his Postdoc studies on efficient answer enumeration for queries in Conditional XPath with variables, in cooperation with Niehren. The topic was prepared by an internship of A. Venant from ENS Cachan-Bretagne supervised by Niehren.

Tree Transformations. The objective of PhD project of Laurence directed by Staworko, Lemay, and Niehren is to learn tree transformations. The novelty of their approach is to base such transformation tasks on deterministic nested word to word transducers [22] . Such transducers are useful for defining XML transformations such as used in XML style sheets. The paper shows that equivalence testing is in polynomial time for deterministic nested word to word transducers. This first result is obtained by reduction to Plandowski's result on polynomial time equivalence checking for morphism on context tree languages. The class of deterministic nested word to word transducers seems promising as starting point for automatic style sheet induction from examples.

Groz, Staworko, Caron, Roos and Tison study the view-based security framework for XML for an expressive class of security access specifications, the expressive RegXPath (regular XPath) query language, and without any restrictions on the DTD [17] . They are interested in query rewriting, allowing to answer user queries on a security view without materialization, and devise a quadratic algorithm for query rewriting. As second contribution they propose three approximations of the user DTD, each of which is indistinguishable from the real schema by a particular class of queries. Finally, they propose two methods for comparing security policies.

Programming languages

Participant : Joachim Niehren.

Niehren continues co-steering the newly created BioComputing project of the LIFL with C. Lhoussaine, who is the official leader. Jointly with L. Uhrmacher from the University of Rostock, they supervised the PhD project of M. John. In [13] they present the attribute pi-calculus with priorities, with generalizes on various pi-calculi used for stochastic simulation in systems biology. In [18] , they enrich the attributed pi-calculus further, by an imperative store for global control. This allows to model dynamic compartments with mutable configurations, which are of interest for modeling biochemistry in live cells. An implementation of the imperative pi-calculus demonstrates high accuracy for modeling operations of compartment dissolution and merging, while keeping good efficiency.

Niehren continues his collaboration with M. Schmidt-Schauß and D. Sabel from Frankfurt and J. Schwinghammer from Saarbrücken. In [21] , they show the equivalence of two synchronization primitives in Alice ML, an concurrent extension of Standard ML with handled futures and concurrent buffers. The proof uses a semantic-preserving translation, thus illustrating that observational semantics proof techniques allow to provide comprehensive proofs of the equivalence of concurrency primitives in realistic concurrent programming languages.


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