Marc H. Scholl

The relational model with relation-valued attributes

Abstract Motivated by new applications of database systems we introduce relations which may have relation-valued attributes and propose a related algebra. Formal definitions for this extended relational model can be given by applying usual notions recursively. The main problem considered in this paper is the formal definition of an appropriate relational algebra for these non-first-normal-form relations. We allow the application of the basic relational operators to any relation-valued attribute within a relation. This leads to a (hierarchically) nested relational algebra.

Updatable Views in Object-Oriented Databases

Object-oriented database systems (ooDBMSs) are supposed to offer at least the functionality available in commercial relational DBMSs of today. One important consequence of this is that they have to provide a separation of the global (conceptual) database schema from the external schema (“subschema”) of a particular task. Views are a mechanism to realize this data independence. In addition, they also support multiple levels of detail, security and authorization, and interoperability in a heterogeneous environment. In a relational DBMS, views are defined by queries. However, they can not be freely updated. We describe concepts of an object model and query language that are necessary for object view definitions. We show that updating object views is much more feasible than in the case of relational views. The key property of a query language leading to this result is object preserving operator semantics. That is, in contrast to many previous object algebras, query results are sets of existing objects instead of data tuples or new objects. Consequently, we have to solve the classification problem: where to include the view in the type and class lattices.

Advances in Database Technology — EDBT 2000

While we can take as a fact “the Web changes everything”, we argue that “XML is the means” for such a change to make a significant step forward. We therefore regard XML-related research as the most promising and challenging direction for the community of database researchers. In this paper, we approach XML-related research by taking three progressive perspectives. We first consider XML as a data representation standard (in the small), then as a data interchange standard (in the large), and finally as a basis for building a new repository technology. After a broad and necessarily coarse-grain analysis, we turn our focus to three specific research projects which are currently ongoing at the Politecnico di Milano, concerned with XML query languages, with active document management, and with XML-based specifications of Web sites.

The DASDBS project: objectives, experiences, and future prospects

A retrospective of the Darmstadt database system project, also known as DASDBS, is presented. The project is aimed at providing data management support for advanced applications, such as geo-scientific information systems and office automation. Similar to the dichotomy of RSS and RDS in System R, a layered architectural approach was pursued: a storage management kernel serves as the lowest common denominator of the requirements of the various applications classes, and a family of application-oriented front-ends provides semantically richer functions on top of the kernel. The lessons that were learned from building the DASDBS system are discussed. Particular emphasis is placed on the following issues: the role of nested relations, the experiences with using object buffers for coupling the system with the programming-language environment and the learning process in implementing multilevel transactions. >

Architecture and implementation of the Darmstadt database kernel system

The multi-layered architecture of the DArmStadt Data Base System (DASDBS) for advanced applications is introduced DASDBS is conceived as a family of application-specific database systems on top of a common database kernel system. The main design problem considered here is, What features are common enough to be integrated into the kernel and what features are rather application-specific? Kernel features must be simple enough to be efficiently implemented and to serve a broad class of clients, yet powerful enough to form a convenient basis for application-oriented layers. Our kernel provides mechanisms to efficiently store hierarchically structured complex objects, and offers operations which are set-oriented and can be processed in a single scan through the objects. To achieve high concurrency in a layered system, a multi-level transaction methodology is applied. First experiences with our current implementation and some lessons we have learned from it are reported.

A relational object model

The relational model and its extensions are often considered incompatible with object-orientation. However, on the one hand nested relations provide the complex object features demanded by object models. Particularly, powerful query languages exploit the complex data structure while keeping the advantages of the declarative, set-oriented paradigm. On the other hand, object models provide semantically rich constructs for advanced modeling, and abstractions of operations as well as data. In this paper, we show an evolutionary path from relational, essentially nested relational, to object-oriented data models and query languages. Basically, allowing nested relation schemes to be recursively defined yields the necessary w.r.t. structure. The query language, i.e., nested relational algebra, carries over to this “network” model. As a first step towards the object-oriented integration of cooperative systems, different views onto the objects have to be supported. We present a powerful view definition facility that basically allows object views as well as relational views to be defined in our object algebra.

Schema transformation without database reorganization

We argue for avoiding database reorganizations due to schema modification in object-oriented systems, since these are expensive operations and they conflict with reusing existing software components. We show that data independence, which is a neglected concept in object databases, helps to avoid reorganizations in case of capacity preserving and reducing schema transformations. We informally present a couple of examples to illustrate the idea of a schema transformation methodology that avoids database reorganization.

Theoretical foundation of algebraic optimization utilizing unnormalized relations

Unnormalized (NF2) relations, not conforming to the first normal form condition (1NF) of the relational model have been proposed recently for a variety of new applications. In this paper we extend NF2 relational theory in a way that it becomes possible to use NF2 relations as storage structures for conventional 1NF relational database interfaces. Physical clustering of precomputed joins can be achieved this way without introducing redundancy. However, applying transformation rules to internal relations straightforwardly, will still yield unnecessary join operations. With the equivalence rules introduced here we prove that efficient algebraic optimization can be performed using standard (1NF) techniques. Particularly, all redundant joins can be properly removed.

Meta Object Management and its Application to Database Evolution

In this paper, we address the problem of supporting more flexibility on the schema of object-oriented databases. We describe a general framework based on an object-oriented data model, where three levels of objects are distinguished: data objects, schema objects, and metaschema objects. We discuss the prerequisites for applying the query and update operations of an object algebra uniformly on all three levels. As a sample application of the framework, we focus on database evolution, that is, realizing incremental changes to the database schema and their propagation to data instances. We show, how each schema update of a given taxonomy is realized by direct updating of schema objects, and how this approach can be used to build a complete tool for database evolution.

The COCOON object model

The COCOON model was intended to extend the concepts of relational database management systems DBMSs beyond nested relational to object oriented ones Key characteristics of COCOON and its database language COOL are generic set oriented query and update operators similar to relational algebra and SQL updates respectively object preserving seman tics of query operators which allows for the de nition of updatable views update operations that keep model inherent integrity constraints consis tent a separation of the two aspects of programming language classes type vs collection predicative description of collections similar to de ned concepts in KL One like knowledge representation languages automatic classi cation of objects and views positioning in the class hierarchy This report gives a comprehensive introduction to the COCOON model and its language COOL as well as a formal de nition Our formalization uses de notational semantics a popular technique in programming languages We found that standard set theoretic formalizations of data and object models were not equally well suited to express update semantics This however is essential for object oriented as opposed to value based languages This can also be taken as an indication for the convergence of database languages to generell programming languages Along these same lines we emphasized static type checking of COOL Second revised edition January