Roel Vercammen’s research while affiliated with University of Antwerp and other places

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Publications (17)


Figure 2.1: The data model for an example XML document.
Figure 3.1: Example XML tree without the document node. 
Figure 4.4: Document scan by followingSibling algorithm. 
Supporting Positional Predicates in Efficient XPath Axis Evaluation for DOM Data Structures
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  • Full-text available

August 2013

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68 Reads

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Abstract In this technical report we propose algorithms for implementing the axes for element nodes in XPath given a DOM-like representation of the document. First, we construct algorithms for evaluating simple step expressions, withoout any (positional) predicates. The time complexity of these algorithms is at most O(l + m) where l is the size of the input list and m,the size of the output list. This improves upon results in [6] where also algorithms with linear time complexity are presented, but these are linear in the size of the entire document whereas our algorithms are linear in the size of the intermediate results which are often much smaller. In a second phase we give a description of how the support for positional predicates can be added to the algorithms with a focus on maintaining the eciency,of evaluation. Each algorithm assumes an input list that is sorted in document order and duplicate-free and returns a sorted and duplicate-free list of the result of following a certain axis from the nodes in the input list. Contents

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On the Expressibility of Functions in XQuery Fragments

June 2008

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25 Reads

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2 Citations

Information Systems

XQuery is a powerful XML query language with many features and syntactic con- structs. For many common queries we do not need all the expressive power of XQuery. We investigate the e!ect of omitting certain features of XQuery on the expressive power of the language. We start from a simple base fragment which can be extended by several optional features being aggregation functions such as count and sum, sequence generation, node construction, position information in for loops, and recursion. In this way we obtain 64 di!erent XQuery fragments which can be divided into 17 di!erent equivalence classes such that two fragments can express the same functions i! they are in the same equivalence class. Moreover, we investigate the relationships between these equivalence classes and derive some properties of the fragments within these equivalence classes.


Figure 2: Query Q1a as a tree pattern and its textual representation. 
How to Recognise Different Kinds of Tree Patterns From Quite a Long Way Away.

January 2007

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59 Reads

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13 Citations

Tree patterns are one of the main abstractions used to ac- cess XML data. Tree patterns are used, for instance, to define XML indexes, and support a number of ecient eval- uation algorithms. Unfortunately deciding whether a par- ticular query, or query fragment, is a tree pattern is unde- cidable for most XML Query languages. In this paper, we identify a subset of XQuery for which the problem is decid- able. We then develop a sound and complete algorithm to recognize the corresponding tree patterns for that XQuery subset. The algorithm relies on a normal form along with a set of rewriting rules that we show to be strongly nor- malizing. The rules have been implemented and result in a normal form which is suitable for compiling tree patterns into an appropriate XML algebra.


On the Expressive Power of XQuery-Based Update Languages

September 2006

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21 Reads

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12 Citations

Lecture Notes in Computer Science

XQuery 1.0, the XML query language which is about to become a W3C Recommendation, lacks the ability to make persistent changes to instances of its data model. A number of proposals to ex- tend XQuery with update facilities have been made lately, including a W3C Working Draft. In order to investigate some of the dierent con- structs that are introduced in these proposals, we define an XQuery- based update language that combines them. By doing so, we show that it is possible to give a concise, complete and formal definition of such a language. We define subsets of this language to examine the relative expressive power of the dierent constructs, and we establish the rela- tionships between these subsets in terms of queries and updates that can be expressed. Finally, we discuss the relationships between these subsets and existing XQuery-based update languages.


Query Translation for XPath-Based Security Views

March 2006

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86 Reads

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10 Citations

Lecture Notes in Computer Science

Since XML is used as a storage format in an increasing num- ber of applications, security has become an important issue in XML databases. One aspect of security is restricting access to data by cer- tain users. This can, for example, be achieved by means of access rules or XML security views, which define projections over XML documents. The usage of security views avoids information leakage that may occur when we use certain access rules. XML views can be implemented by ma- terialized views, but materialization and maintenance of views may cause considerable overhead. Therefore, we study translations from queries on views to equivalent queries on the original XML documents, assuming both the security views and the queries are specified by XPath expres- sions. Especially, we investigate which XPath fragments are closed under the composition of a view and a query.


Figure 1.2: Result node types  
Figure 2.1: NCE and determinism  
Figure 2.4: Subdivision of deep equal result nodes
Expressive power of XQuery node construction

January 2006

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33 Reads

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5 Citations

In the relational model it has been shown that the flat relational algebra has the same expressive power as the nested relational algebra, as far as queries over flat relations and with flat results are concerned [11]. Hence, for each query that uses the nested relational model and that, with a flat table as input always has a flat table as output, there exists an equivalent flat query that only uses the flat relational model. In [12] a very direct proof is given of this fact using a simulation technique. In analogy, we study a related flat-flat problem for XQuery. We show that for each expression that only has original, copied or equal nodes in its result sequence there exists an expression without node construction yielding the same or a deep-equal result. In this work we will show how to generate automatically equivalent constructor-free expressions for node-conservative expressions. This result gives an indication of the expressive power of the node construction.


LiXQuery

December 2005

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6 Reads

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6 Citations

ACM SIGMOD Record

XQuery is considered to become the standard query language for XML documents. However, the complete XQuery syntax and semantics seem too complicated for research and educational purposes. By defining a concise backwards compatible subset of XQuery with a complete formal description, we provide a practical foundation for XQuery research. We pay special attention to usability by supporting the most typical XQuery expressions.


Optimizing Sorting and Duplicate Elimination in XQuery Path Expressions

September 2005

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86 Reads

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20 Citations

Lecture Notes in Computer Science

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.


On the Expressive Power of XQuery Fragments

August 2005

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25 Reads

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4 Citations

Lecture Notes in Computer Science

XQuery is known to be a powerful XML query language with many bells and whistles. For many common queries we do not need all the expressive power of XQuery. We investigate the eect of omitting certain features of XQuery on the expressive power of the language. We start from a simple base fragment which can be extended by several optional features being aggregation functions such as count and sum, se- quence generation, node construction, position information in for loops, and recursion. In this way we obtain 64 dierent XQuery fragments which can be divided into 17 dierent equivalence classes such that two frag- ments can express the same functions i they are in the same equivalence class. Moreover, we investigate the relationships between these equiva- lence classes.


Instance-Independent View Serializability for Semistructured Databases

June 2005

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103 Reads

Semistructured databases require tailor-made concurrency control mechanisms since traditional solutions for the relational model have been shown to be inadequate. Such mechanisms need to take full advantage of the hierarchical structure of semistructured data, for instance allowing concurrent updates of subtrees of, or even individual elements in, XML documents. We present an approach for concurrency control which is document-independent in the sense that two schedules of semistructured transactions are considered equivalent if they are equivalent on all possible documents. We prove that it is decidable in polynomial time whether two given schedules in this framework are equivalent. This also solves the view serializability for semistructured schedules polynomially in the size of the schedule and exponentially in the number of transactions.


Citations (13)


... For example, Benedikt, Fan, and Kuper studied structural properties of XPath fragments [1], the computational complexity of query evaluation for a number of XPath fragments was investigated by Gottlob, Koch, and Pichler in [5], and Marx [12] increased the expressive power of XPath by extending it in order to be first order complete. It was not until recently that similar efforts were made for XQuery: Koch studies the computational complexity of query evaluation for nonrecursive XQuery fragments without aggregation functions, position information in for loops and sequence generation [8], Koch and Benedikt study the relationship between similar fragments and fragments of first-order logic with counters [2], Vansummeren looks into the well-definedness problem for XQuery fragments [14], and the expressive power of the node construction in XQuery is studied in [9]. In this article we investigate the expressive power of XQuery fragments in a similar fashion as was done for the relational algebra [13] and SQL [11] . ...

Reference:

On the Expressibility of Functions in XQuery Fragments
Expressive power of XQuery node construction

... In the context of these implementations that literally implement the formal semantics, some partial techniques have been developed to tackle the problem of order and duplicates. For instance, in [11,4] it is statically decided which sorting and duplicate-elimination operations are necessary to guarantee correctness. This approach, however, does not solve the problem of unnecessarily accessing nodes multiple times, and in many cases some sorting and duplicate-elimination operations still occur in the evaluation of a path expression. ...

Automata for Avoiding Unnecessary Ordering Operations in XPath Evaluation Plans

... These properties are ensured by using the AS- SIGN operator with the sort-distinct-nodes-asc-or-atomics expression. In VXQuery, properties of the data are tracked through the query plan using knowledge of each operator and expression (similar to [19] ...

Optimizing Sorting and Duplicate Elimination in XQuery Path Expressions
  • Citing Conference Paper
  • September 2005

Lecture Notes in Computer Science

... • Support the entire XQuery language: The possibility to handle multiple join graphs contained in one execution plan allows ROX to support the entire XQuery language while focusing on the optimization of the crucial order of relational joins and step operators. We also stress that the fragment of the XQuery language that can be mapped to an execution query with a single join graph is more expressive than the twig queries widely considered in other previous work [26,35,129,33,65]. • Seamless handling of step and relational joins: The fact that XPath steps and relational joins co-exist together in the same join graph gives ROX the possibility to optimize seamlessly the execution order of these two different types of operators. ...

How to Recognise Different Kinds of Tree Patterns From Quite a Long Way Away.

... That is one reason why we decided to investigate the active, rule-based part of Demaq rather than reiterating results already established for XPath and XQuery. Attempts to isolate a core sublanguage have also been made for XQuery and XQuery Update by Hidders et al. [10, 11]. The focus, however, was on suitability for educational and research purposes rather than on combination of decidability and practical usefulness. ...

On the Expressive Power of XQuery-Based Update Languages
  • Citing Conference Paper
  • September 2006

Lecture Notes in Computer Science

... Much of this work is geared towards static analysis, so aims to capture computations of limited expressiveness for which questions such as emptiness remain decidable [9,18,19]. There is little work on highly expressive languages on trees with data, and it usually adopts a model of ordered unranked trees (siblings are ordered) [8,11,12,14]. In contrast, we consider a model of unordered trees. ...

A Light but Formal Introduction to XQuery

Lecture Notes in Computer Science

... Much of this work is geared towards static analysis, so aims to capture computations of limited expressiveness for which questions such as emptiness remain decidable [9,18,19]. There is little work on highly expressive languages on trees with data, and it usually adopts a model of ordered unranked trees (siblings are ordered) [8,11,12,14]. In contrast, we consider a model of unordered trees. ...

On the Expressive Power of XQuery Fragments
  • Citing Conference Paper
  • August 2005

Lecture Notes in Computer Science