Peter M.G. Apers

Dataflow query execution in a parallel main-memory environment

In this paper, the performance and characteristics of the execution of various join-trees on a parallel DBMS are studied. The results of this study are a step into the direction of the design of a query optimization strategy that is fit for parallel execution of complex queries.Among others, synchronization issues are identified to limit the performance gain from parallelism. A new hash-join algorithm is introduced that has fewer synchronization constraints than the known hash-join algorithms. Also, the behavior of individual join operations in a join-tree is studied in a simulation experiment. The results show that the introduced Pipelining hash-join algorithm yields a better performance for multi-join queries. The format of the optimal join-tree appears to depend on the size of the operands of the join: A multi-join between small operands performs best with a bushy schedule; larger operands are better off with a linear schedule. The results from the simulation study are confirmed with an analytic model for dataflow query execution.

Integrity control in relational database systems: an overview

This paper gives an overview of research regarding integrity control or integrity constraint handling in relational database management systems. The topic of constraint handling is discussed from two points of view. First, constraint handling is discussed by identifying a number of important research issues, and by treating each issue in detail. Second, a number of projects is described that have resulted in the realization of database management systems supporting integrity constraints; the various projects are compared with respect to a number of system characteristics. Together, both approaches give a broad overview of the state of the art in the field at this moment.

Towards Context-Aware Data Management for Ambient Intelligence

Ambient Intelligence (AmI) is a vision of future Information Society, where people are surrounded by an electronic environment which is sensitive to their needs, personalized to their requirements, anticipatory of their behavior, and responsive to their presence. It emphasizes on greater user-friendliness, user-empowerment, and more effective service support, with an aim to make peoplersquos daily activities more convenient, thus improving the quality of human life. To make AmI real, effective data management support is indispensable. High-quality information must be available to any user, anytime, anywhere, and on any lightweight device. Beyond that, AmI also raises many new challenges related to context-awareness and natural user interaction, entailing us to re-think current database techniques. The aim of this paper is to address the impact of AmI, particularly its user-centric context-awareness requirement on data management strategies and solutions. We first provide a multidimensional view of database access context. Taking diverse contextual information into account, we then present five context-aware data management strategies, using the most fundamental database operation-context-aware query request as a case in point. We execute the proposed strategies via a two-layered infrastructure, consisting of public data manager(s) and a private data manager. Detailed steps of processing a context-aware query are also described in the paper.

Global transaction support for workflow management systems: from formal specification to practical implementation

Abstract. In this paper, we present an approach to global transaction management in workflow environments. The transaction mechanism is based on the well-known notion of compensation, but extended to deal with both arbitrary process structures to allow cycles in processes and safepoints to allow partial compensation of processes. We present a formal specification of the transaction model and transaction management algorithms in set and graph theory, providing clear, unambiguous transaction semantics. The specification is straightforwardly mapped to a modular architecture, the implementation of which is first applied in a testing environment, then in the prototype of a commercial workflow management system. The modular nature of the resulting system allows easy distribution using middleware technology. The path from abstract semantics specification to concrete, real-world implementation of a workflow transaction mechanism is thus covered in a complete and coherent fashion. As such, this paper provides a complete framework for the application of well-founded transactional workflows.

Optimization of nested queries in a complex object model

Transformation of nested SQL queries into join queries is advantageous because a nested SQL query can be looked upon as a nested-loop join, which is just one of the several join implementations that may be available in a relational DBMS. In join queries, dangling (unmatched) operand tuples are lost, which causes a problem in transforming nested queries having the aggregate function COUNT between query blocks-a problem that has become well-known as the COUNT bug. In the relational context, the outerjoin has been employed to solve the COUNT bug. In complex object models supporting an SQL-like query language, transformation of nested queries into join queries is an important optimization issue as well. The COUNT bug turns out to be a special case of a general problem being revealed in a complex object model. To solve the more general problem, we introduce the nest join operator, which is a generalization of the outerjoin for complex objects.

Score region algebra: building a transparent XML-R database

A unified database framework that will enable better comprehension of ranked XML retrieval is still a challenge in the XML database field. We propose a logical algebra, named score region algebra, that enables transparent specification of information retrieval (IR) models for XML databases. The transparency is achieved by a possibility to instantiate various retrieval models, using abstract score functions within algebra operators, while logical query plan and operator definitions remain unchanged. Our algebra operators model three important aspects of XML retrieval: element relevance score computation, element score propagation, and element score combination. To illustrate the usefulness of our algebra we instantiate four different, well known IR scoring models, and combine them with different score propagation and combination functions. We implemented the algebra operators in a prototype system on top of a low-level database kernel. The evaluation of the system is performed on a collection of IEEE articles in XML format provided by INEX. We argue that state of the art XML IR models can be transparently implemented using our score region algebra framework on top of any low-level physical database engine or existing RDBMS, allowing a more systematic investigation of retrieval model behavior.

Semantics and architecture of global transaction support in workflow environments

We present an approach to global transaction management in workflow environments. The transaction mechanism is based on the well-known notion of sagas, but extended to deal with arbitrary process structures including cycles and savepoints that allow partial compensation. We present a formal specification of the transaction model and transaction management mechanisms in set and graph theory, providing clear, unambiguous transaction semantics. The specification is straightforwardly mapped to a modular architecture, the implementation of which is applied in the prototype of a commercial workflow management system. The loosely-coupled nature of the resulting system allows easy distribution using middleware technology.

On the Applicability of Schema Integration Techniques to Database Interoperation

We discuss the applicability of schema integration techniques developed for tightly-coupled database interoperation to interoperation of databases stemming from different modelling contexts. We illustrate that in such an environment, it is typically quite difficult to infer the real-world semantics of remote classes from their definition in remote databases. However, defining relationships between the real-world semantics of schema elements is essential in existing schema integration techniques. We propose to base database interoperation in such environments on instance-level semantic relationships, to be defined using what we call object comparison rules. Both the local and the remote classifications of the appropriately merged instances are maintained, allowing for the derivation of a global class hierarchy if desired.

Facilitating fine grained data provenance using temporal data model

E-science applications use fine grained data provenance to maintain the reproducibility of scientific results, i.e., for each processed data tuple, the source data used to process the tuple as well as the used approach is documented. Since most of the e-science applications perform on-line processing of sensor data using overlapping time windows, the overhead of maintaining fine grained data provenance is huge especially in longer data processing chains. This is because data items are used by many time windows. In this paper, we propose an approach to reduce storage costs for achieving fine grained data provenance by maintaining data provenance on the relation level instead on the tuple level and make the content of the used database reproducible. The approach has prototypically been implemented for streaming and manually sampled data.

The webspace method: on the integration of database technology with multimedia retrieval

Large collections of documents containing various types of multimedia, are made available to theWWW. Unfortunately, due to the un-structuredness of Internet environments it is hard to find specific information when one is looking for it. Search engines available can only rely their results on information retrieval techniques and most of the time they lack the desired power in query formulation. Modelling data on the web, as if it was designed for use within databases, should provide us with the necessary basis for enhancing this query formulation. This of course requires special care for dealing with the included multimedia data and the semi-structured aspects of data on the web.Modelling the entire web would be too ambitious, therefore we focus on a more feasible environment, like the intranet, where one can find large collections of related data. With the webspace method we have already shown how to deal with the various aspects of semi-structured data in large collections of related documents. In this paper we focus on the integration of our webspace method for concept-based search with content -based multimedia information retrieval (IR). A webspace consists of two levels. At the document level, a webspace is considered to be a collection of related documents. At the semantical level, concepts are defined to be used in the documents at the document level. By modelling these concepts using a webspace schema a semantical level of abstraction is gained. This supplies the necessary platform for querying data available within a specific webspace. For the integration with content-based information retrieval an existing IR model is adopted. We will discuss how this is used in the context of Mirror, a Multimedia DBMS, and how this framework is used for the integration with the webspace method for concept-based search.