An Inflationary Fixed Point Operator in XQuery

We introduce a controlled form of recursion in XQuery, inflationary fixed points, familiar in the context of relational databases. This imposes restrictions on the expressible types of recursion, but we show that inflationary fixed points nevertheless are sufficiently versatile to capture a wide range of interesting use cases, including the semantics of Regular XPath and its core transitive closure construct. While the optimization of general user-defined recursive functions in XQuery appears elusive, we will describe how inflationary fixed points can be efficiently evaluated, provided that the recursive XQuery expressions exhibit a distributivity property. We show how distributivity can be assessed both, syntactically and algebraically, and provide experimental evidence that XQuery processors can substantially benefit during inflationary fixed point evaluation.Comment: 11 pages, 10 figures, 2 table

Harnessing the Deep Web: Present and Future

Over the past few years, we have built a system that has exposed large volumes of Deep-Web content to Google.com users. The content that our system exposes contributes to more than 1000 search queries per-second and spans over 50 languages and hundreds of domains. The Deep Web has long been acknowledged to be a major source of structured data on the web, and hence accessing Deep-Web content has long been a problem of interest in the data management community. In this paper, we report on where we believe the Deep Web provides value and where it does not. We contrast two very different approaches to exposing Deep-Web content -- the surfacing approach that we used, and the virtual integration approach that has often been pursued in the data management literature. We emphasize where the values of each of the two approaches lie and caution against potential pitfalls. We outline important areas of future research and, in particular, emphasize the value that can be derived from analyzing large collections of potentially disparate structured data on the web.Comment: CIDR 200

The Repeatability Experiment of SIGMOD 2008

SIGMOD 2008 was the first database conference that offered to test submitters' programs against their data to verify the experiments published. This paper discusses the rationale for this effort, the community's reaction, our experiences, and advice for future similar efforts

XCheck: a platform for benchmarking XQuery engines

XCheck is a tool for assessing the relative performance of different XQuery engines by means of benchmarks consistingofasetofXQueryqueriesandasetofXMLdocuments. Given a benchmark and a set of engines, XCheck runs the benchmark on these engines and produces highly informative performance output. The current version of XCheck contains all available XQuery benchmarks which are run against four XQuery engines: Galax, Qizx/open, Saxon and MonetDB/XQuery. XCheck’s design makes it easy to include new engines and new benchmarks. 1

On Core XPath with Inflationary Fixed Points

We prove the undecidability of Core XPath 1.0 (CXP) [G. Gottlob and C. Koch, in Proc. of 17th Ann. IEEE Symp. on Logic in Computer Science, LICS ’02 (Copenhagen, July 2002). IEEE CS Press (2002) 189–202.] extended with an Inflationary Fixed Point (IFP) operator. More specifically, we prove that the satisfiability problem of this language is undecidable. In fact, the fragment of CXP+IFP containing only the self and descendant axes is already undecidable

XML Query Evaluation via CTL Model Checking

The Extensible Markup Language (XML) was designed to describe the content of a document and its hierarchical structure. The XML Path language (XPath) is a language for selecting elements from XML documents. We propose and implement a linear embedding of the query evaluation problem for Core XPath, the navigational fragment of XPath, into the model checking problem for Computation Tree Logic (CTL). We extend the embedding to XCPath, an extension of Core XPath that is as expressive as first-order logic on sibling-ordered trees. This allows us to evaluate XCPath queries by exploiting a model checker for CTL. We report on experiments with the state-of-the-art model checker NuSMV, and compare our results with alternative academic XPath processors. Acknowledgements This thesis is the result of work with several people which I will mention below. I was just being in the middle of the process, trying to catch and to learn as much as possible, about myself, about my interests, about my capacities. What I have learned actually is a great deal about the people that I got to know. I learned how people can be generous and patient, professional and loving. I had a school of life more then a school of logic. I consider this thesis to be the work of Massimo Franceschet, Maarten Marx and Balder ten Cate, as much as mine. More of mine was the pleasure of learning from them. Maarten Marx's help cannot be described in words. He has many qualities which I respect. I will mention only one. He was able always to come up with just the right solution for my problems. He o#ered help firmly, quickly, the right one and in the right moment. I wish I could be the same in this respect. Massimo Franceschet was very kind to me and brave, agreeing to be my adviser. He advised me wisely, caring more abou..