Jan Hidders

Satisfiability of XPath Expressions

In this paper, we investigate the complexity of deciding the satisfiability of XPath 2.0 expressions, i.e., whether there is an XML document for which their result is nonempty. Several fragments that allow certain types of expressions are classified as either in PTIME or NP-hard to see which type of expression make this a hard problem. Finally, we establish a link between XPath expressions and partial tree descriptions which are studied in computational linguistics.

Path locks for XML document collaboration

The hierarchical and semistructured nature of XML data can cause complicated update-behavior. The updates are not limited to entire document trees, but can involve subtrees and even individual elements. These document parts correspond to, e.g. sections in text documents or sub-diagrams in vector graphics files. Providing suitable locking mechanisms for semi-structured data can significantly improve collaboration systems that store their data as XML documents. We show that concurrency control mechanisms in CVS, relational, and object oriented database systems are inadequate for collaboration systems based on semistructured data. We therefore propose a new locking scheme of fine granularity based on path locks. We also show that our proposed mechanism avoids conflicts by ensuring serializability, supports both top-down and bottom-up query evaluation, and is relatively efficient.

A formal semantics for the Taverna 2 workflow model

This paper presents a formal semantics for the Taverna 2 scientific workflow system. Taverna 2 is a successor to Taverna, an open-source workflow system broadly adopted within the e-science community worldwide. The new version improves upon the existing model in two main ways: (i) by adding support for data pipelining, which in turns enables input streams of indefinite length to be processed efficiently; and (ii) by providing new extensibility points that make it possible to add new operators to the workflow model. Consistent with previous work by some of the authors, we use trace semantics to describe the effect of workflow computations, and we show how they can be used to describe the new features in the Taverna 2 model.

A Light but Formal Introduction to XQuery

We give a light-weight but formal introduction to XQuery by defining a sublanguage of XQuery. We ignore typing, and don’t consider namespaces, comments, programming instructions, and entities. To avoid confusion we call our version LiXQuery (Light XQuery). LiXQuery is fully downwards compatible with XQuery. Its syntax and its semantics are far less complex than that of XQuery, but the typical expressions of XQuery are included in LiXQuery. We claim that LiXQuery is an elegant and simple sublanguage of XQuery that can be used for educational and research purposes. We give the complete syntax and the formal semantics of LiXQuery.

A Transaction Model for XML Databases

The hierarchical and semistructured nature of XML data may cause complicated update behavior. Updates should not be limited to entire document trees, but should ideally involve subtrees and even individual elements. Providing a suitable scheduling algorithm for semistructured data can significantly improve collaboration systems that store their data—e.g., word processing documents or vector graphics—as XML documents. In this paper we show that concurrency control mechanisms in CVS, relational, and object-oriented database systems are inadequate for collaborative systems based on semistructured data. We therefore propose two new locking schemes based on path locks which are tightly coupled to the document instance. We also introduce two scheduling algorithms that can both be used with any of the two proposed path lock schemes. We prove that both schedulers guarantee serializability, and show that the conflict rules are necessary.

Typing Graph-Manipulation Operations

We present a graph-based data model called GDM where database instances and database schemas are described by certain types of labeled graphs called instance graphs and schema graphs. For this data model we introduce two graph-manipulation operations, an addition and a deletion, that are based on pattern matching and can be represented in a graphical way. For these operations it is investigated if they can be typed such that it is guaranteed for well-typed operations that the result belongs to a certain database schema graph, and what the complexity of deciding this well-typedness is.

Optimizing sorting and duplicate elimination in XQuery path expressions

XQuery expressions can manipulate two kinds of order: document order and sequence order. While the user can impose or observe the order of items within a sequence, the results of path expressions must always be returned in document order. Correctness can be obtained by inserting explicit (and expensive) operations to sort and remove duplicates after each XPath step. However, many such operations are redundant. In this paper, we present a systematic approach to remove unnecessary sorting and duplicate elimination operations in path expressions in XQuery 1.0. The technique uses an automaton-based algorithm which we have applied successfully to path expressions within a complete XQuery implementation. Experimental results show that the algorithm detects and eliminates most redundant sorting and duplicate elimination operators and is very effective on common XQuery path expressions.

Avoiding Unnecessary Ordering Operations in XPath

We present a sound and complete rule set for determining whether sorting by document order and duplicate removal operations in the query plan of XPath expressions are unnecessary. Additionally we define a deterministic automaton that illustrates how these rules can be translated into an efficient algorithm. This work is an important first step in the understanding and tackling of XPath/XQuery optimization problems that are related to ordering and duplicate removal.

A structural approach to indexing triples

As an essential part of the W3Cs semantic web stack and linked data initiative, RDF data management systems (also known as triplestores) have drawn a lot of research attention. The majority of these systems use value-based indexes (e.g., B+-trees) for physical storage, and ignore many of the structural aspects present in RDF graphs. Structural indexes, on the other hand, have been successfully applied in XML and semi-structured data management to exploit structural graph information in query processing. In those settings, a structural index groups nodes in a graph based on some equivalence criterion, for example, indistinguishability with respect to some query workload (usually XPath). Motivated by this body of work, we have started the SAINT-DB project to study and develop a native RDF management system based on structural indexes. In this paper we present a principled framework for designing and using RDF structural indexes for practical fragments of SPARQL, based on recent formal structural characterizations of these fragments. We then explain how structural indexes can be incorporated in a typical query processing workflow; and discuss the design, implementation, and initial empirical evaluation of our approach.

Storing and Indexing Massive RDF Datasets

The resource description framework (RDF for short) provides a flexible method for modeling information on the Web [34, 40]. All data items in RDF are uniformly represented as triples of the form (subject, predicate, object), sometimes also referred to as (subject, property, value)triples.