4.1 Introduction

Emerging database (DB) applications require the scalable management of a large quantity of complex data together with traditional business data and flexible querying capabilities for business intelligence. Such applications are called universal applications (Stonebraker et al. 1999). Object-relational databases or universal servers are component database management systems (DBMSs) that have been developed to support such applications. They leverage the mature relational database technology, which provides scalability, reliability, and recovery. More important, they enable users to introduce application-specific types and methods into a database. These extensibility features can be seen as infrastructure for providing plug-in components that extend the built-in repertoire of database types and functions with support for additional, nonstandard data types. Tables in a database may now contain such user-defined objects as geographical shapes, images, and semistructured or unstructured text documents.

IBM's DB2 Universal Database product (Chamberlin 1998; Davis 2000) implements important object-relational concepts that have been standardized in SQL:99 (ANSI 1999a), such as structured types with subtyping, inheritance, and value substitutability (Fuh et al. 1999); typed tables, table hierarchies, and object views (Carey et al. 1999); and user-defined functions and methods. With the object-relational...