Composite-Object Views in Relational DBMS: An Implementation Perspective

We present a novel approach for supporting Composite Objects (CO) as an abstraction over the relational data. This approach brings the advanced CO model to existing relational databases and applications, without requiring an expensive migration to othcr DBMSs which support CO. The concept of views in relational DBMSs (RDBMS) gives the basis for providing the CO abstraction. This model is strictly an extension to the relational model, and it is fully upward compatible with it. We present an overview of the data model. We put emphasis in this paper on showing how we have made the extensions to the architecture and implementation of an RDBMS (Starburst) to support this model. We show that such a major extension to the data model is in fact quite attractive both in terms of implementation cost and query performance. We introduce a CO cache for efficient navigation through components of a CO. Our work on CO enables existing RDBMSs to incorporate efficient CO facilities at a low cost and at a high degree of application reusability and database sharability. 1 Motivation It is widely agreed now that complex applications, such as design applications, multi -media and Al applications, and even advanced business applications can benefit significantly from database interfaces that support composite (or complex) objects (shortly, CO). A generally accepted characterization defines a CO consisting of several components (possibly from different types) with relationships in between [2,3,17,11]. Interestingly, object oriented DBMSs (OODBMSs) have adopted a very similar model [1,21,9,13]. This is particularly true for OODBMSs used in practice. Especially 00 prograrruning environments have made advances in handling of COs. Such environments facilitate the growth of complex applications. As a result, there is considerable pressure on RDBMSs for better support of COs. Torespond to this demand, several systems today are bridging 00 environments with relational. An example of such a system is the Persistence DBMS [12], which builds a layer on top of RDBMSs, providing better support for COs. Thesesystems essentially extract data from a relational database and load it into 00 environments. fu summary, relational systemstobe viable must be able to understand COs (a language extension issue), and must be able to handle them weil (optimization issue). Given the prevalence of SQL as a database interface language both for application and database interoperability, we have proposed an extension to SQL to handle COs. In our approach, called SQL Extended Nonnal Form (for short XNF) and introduced in [18], we derive COs from relational data. That paper covers the language part giving details on syntax and semantics. One major achievement isthat such a powerful extensionwas made with full upward compatibility with SQL, hence opening up a path for the current DBMSs in research 1. The EDBT committee decided to present the full version of the paper in Information Systems, special issue: 'extended database technology', Spring 1994. 2. University ofKaiserslautem, DepL of Computer Science, P.O.Box 3049, 67653 Kaiserslautern, Federal Republic of Gennany, e-mail: [email protected] 3. IBM Heidelberg Seienlifte Center, Vangerowstr. 18,69115 Heidelberg, Federal Republic ofGennany, e-mail: [email protected]