Loredana Afanasiev

PDL for Ordered Trees

This paper is about a special version of PDL, proposed by Marcus Kracht, for reasoning about sibling ordered trees. It has four basic programs corresponding to the child, parent, left- and right-sibling relations in such trees. The original motivation for this language is rooted in the field of model-theoretic syntax. Motivated by recent developments in the area of semi-structured data, and, especially, in the field of query languages for XML (eXtensible Markup Language) documents, we revisit the language. This renewed interest comes with a special focus on complexity and expressivity aspects of the language, aspects that have so far largely been ignored. We survey and derive complexity results, and spend most of the paper on the most important open question concerning the language: what is its expressive power? We approach this question from two angles: Which first-order properties can be expressed? And which second-order properties? While we are still some way from definitive answers to these questions, we discuss two first-order fragments of the PDL language for ordered trees, and show how the language can be used to express some typical (second-order) problems, like the boolean circuit and the frontier problem.

MemBeR: a micro-benchmark repository for XQuery

XQuery is a feature-rich language with complex semantics. This makes it hard to come up with a benchmark suite which covers all performance-critical features of the language, and at the same time allows one to individually validate XQuery evaluation techniques. This paper presents MemBeR, a micro-benchmark repository, allowing the evaluation of an XQuery implementation with respect to precise evaluation techniques. We take the view that a fixed set of queries is probably insufficient to allow testing for various performance aspects, thus, the users of the repository must be able to add new data sets and/or queries for specific performance assessment tasks. We present our methodology for constructing the micro-benchmark repository, and illustrate with some sample micro-benchmarks.

CTL model checking for processing simple XPath queries

The eXtensible Markup Language (XML) was designed to describe the content of a document and its hierarchical structure, and the XML Path language (XPath) is a language for selecting elements from XML documents. There is a close connection between the query processing problem for XPath and the model checking problem for temporal logics. Both boil down to checking which nodes of a graph satisfy a property. We investigate the potential of a technique based on computation tree logic (CTL) model checking for evaluating queries expressed in (a subset of) XPath. To this aim, we isolate a simple fragment of XPath that is naturally embeddable into CTL. We report on experiments based on the model checker NuSMV, and compare our results with alternative academic XPath processors. We comment on the advantages and drawbacks of the application of our model checking-based approach to XPath processing.

An analysis of XQuery benchmarks

This paper presents a survey and an analysis of the XQuery benchmark publicly available in 2006-XMach-1, XMark, X007, the Michigan benchmark, and XBench-from different perspectives. We address three simple questions about these benchmarks: How are they used? What do they measure? What can one learn from using them? One focus of our analysis is to determine whether the benchmarks can be used for micro-benchmarking. Our conclusions are based on an usage analysis, on an in-depth analysis of the benchmark queries, and on experiments run on four XQuery engines: Galax, SaxonB, Qizx/Open, and MonetDB/XQuery.

An Inflationary Fixed Point Operator in XQuery

We introduce a controlled form of recursion in XQuery, an inflationary fixed point operator, familiar from the context of relational databases. This operator imposes restrictions on the expressible types of recursion, but we show that it is sufficiently versatile to capture a wide range of interesting use cases, including Regular XPath and its core transitive closure operator. While the optimization of general user-defined recursive functions in XQuery appears elusive, we describe how inflationary fixed points can be efficiently evaluated, provided that the recursive XQuery expressions are distributive. We test distributivity syntactically and algebraically, and provide experimental evidence that XQuery processors can benefit substantially from this mode of evaluation.