Andreas M. Weiner

Using Structural Joins and Holistic Twig Joins for Native XML Query Optimization

One of the most important factors for success of native XML database systems is a powerful query optimizer. Surprisingly, little has been done to develop cost models to enable cost-based optimization in such systems. Since the entire optimization process is so complex, only a stepwise approach will lead to a satisfying (future) solution. In this work, we are paving the way for cost-based XML query optimization by developing cost formulae for two important join operators, which allow to perform join reordering and join fusion in a cost-aware way, and, therefore, make joint application of Structural Joins and Holistic Twig Joins possible.

An integrative approach to query optimization in native XML database management systems

Even though an effective cost-based query optimizer is of utmost importance for the efficient evaluation of XQuery expressions in native XML database systems, such a component is currently out of sight, because former approaches do not pay attention to the latest advances in the area of physical operators (e. g., Holistic Twig Joins and advanced indexes) or just focus only on some of them. To support the development of native XML query optimizers, we introduce an extensible cost-based optimization framework that integrates the cutting-edge XML query evaluation operators into a single system. Using the well-known plan generation techniques from the relational world and a novel set of plan equivalences---which allows for the generation of alternative query plans consisting of Structural Joins, Holistic Twig Joins, and numerous indexes (especially path indexes and content-and-structure indexes)---our optimizer can now benefit from the knowledge on native XML query evaluation to speed-up query execution significantly.

XTCcmp: XQuery compilation on XTC

XTCcmp, the XQuery Compiler of a native XML database system, extends Starbursts well-known Query Graph Model to serve as an internal representation and basis for query restructuring of XQuery expressions. Furthermore, XTCcmp is able to generate execution plans supporting a wide range of both well-known and newly developed variants of core XML processing algorithms and indexes. Our demo visualizes all rule-based transformation stages, i. e., simplification, algebraic rewriting, and plan generation. Furthermore, via an interface to the extensible rule configuration, it allows interaction with the query compiler to vary configuration parameters and to control the compilation outcome.

Rules for query rewrite in native XML databases

In recent years, the database community has seen many sophisticated Structural Join and Holistic Twig Join algorithms as well as several index structures supporting the evaluation of twig query patterns. Even though almost all XML query evaluation proposals in the literature use one of those evaluation methods, we believe that (1) there is no internal representation that enables a smooth transition between the XQuery language level and physical algebra operators, and (2) there is still no approach that considers the combination of content-and-structure indexes, Structural Join (SJ), and Holistic Twig Join (HTJ) algorithms to speed up the evaluation of twig queries. To overcome this deficit, we propose an enhancement to Starbursts Query Graph Model as an internal representation for XML query languages such as XQuery. This representation permits the usage of SJ and HTJ operators as part of the logical algebra. For twig queries, we define a set of rewrite rules which initiate query graph transformations towards improved processability, e.g., to fuse adjacent binary join operators to an n-way join operator. To enhance the evaluation flexibility of twig queries, we come up with further rewrite rules to prepare query graphs---even before query transformation---for making the most of existing joins and indexes.

Visualizing cost-based XQuery optimization

Developing a full-fledged cost-based XQuery optimizer is a fairly complex task. Nowadays, there is little knowledge concerning suitable cost formulae and optimization strategies for exploring and constraining the tremendously large search space. To allow for a fair assessment of different optimization strategies, physical algebra operators, and indexing approaches, we developed an extensible optimization framework. The framework is accompanied by a supportive visual explain tool that enables user interactions to refine the inspection and the comprehension of the query plans proposed. Using this tool, the optimizer can be dynamically reconfigured and the impact of different optimization strategies on the final query execution plan can be immediately visualized.

„Von der Torfabrik zur Denkfabrik“ Bericht zur 14. Fachtagung „Datenbanksysteme für Business, Technologie und Web“

Seit 1985 richtet der GI-Fachbereich Datenbanken und Informationssysteme (DBIS) alle zwei Jahre seine Fachtagung Datenbanksysteme fur Business, Technologie und Web (BTW) aus, die in diesem Jahr vom 28. Februar bis zum 4. Marz in Kaiserslautern durchgefuhrt wurde. Nach 1991, als die 4. Fachtagung dieser Reihe an der TU Kaiserslautern stattfand, durfte die Arbeitsgruppe DBIS des Fachbereichs Informatik der TU damit zum zweiten Mal die groste Wissenschaftsveranstaltung der deutschsprachigen Datenbankgemeinde ausrichten. Sie wurde von Prof. Dr-.Ing. Dr. h.c. Theo Harder (lokale Organisation) und zwei „Ehemaligen“ der TU Kaiserslautern, Prof. Dr.-Ing. habil. Bernhard Mitschang (Wissenschaftliches Programmkomitee, Universitat Stuttgart) und Dr.-Ing. Harald Schoning (IndustrieProgrammkomitee, Software AG, Darmstadt), organisiert. Jurgen Bittner, Vorstandsvorsitzender der SQL Projekt AG, Dresden, und Prof. Dr. Hans-Jurgen Schek, Emeritus der ETH Zurich, waren als Ehrengaste der BTW eingeladen. Beide hatten 1991 zur 4. BTW-Tagung die Hauptvortrage mit den Themen „Die Architekturkonzeption eines DBMS aus pragmatischer Sicht“ und „Erweiterbarkeit, Kooperation, Foderation von Datenbanksystemen“ gehalten. Das Fachprogramm bestand aus mehr als 40 Vortragen im Wissenschaftsund Industrieprogramm und 12 Demonstrationen von neuen Informatik-Anwendungen. Alle Beitrage wurden in einem anonymen Begutachtungsverfahren von uber 50 Fachleuten aus dem DBIS-Bereich ausgewahlt. Die