Rafi Ahmed

The Pegasus heterogeneous multidatabase system

Pegasus, a heterogeneous multidatabase management system that responds to the need for effective access and management of shared data across in a wide range of applications, is described. Pegasus provides facilities for multidatabase applications to access and manipulate multipole autonomous heterogeneous distributed object-oriented relational, and other information systems through a uniform interface. It is a complete data management system that integrates various native and local databases. Pegasus takes advantage of object-oriented data modeling and programming capabilities. It uses both type and function abstractions to deal with mapping and integration problems. Function implementation can be defined in an underlying database language or a programming language. Data abstraction and encapsulation facilities in the Pegasus object model provide an extensible framework for dealing with various kinds of heterogeneities in the traditional database systems and nontraditional data sources.<<ETX>>

Version management of composite objects in CAD databases

Anti-jack-knifing apparatus for articulated vehicles, wherein the anti-jack-knifing force is exerted independently of the operation of the vehicle braking system by the provision of a hydraulic pressure source including two working chambers communicating via a throttling bore in a piston, the moving part of the pressure source being connected to the trailer while the stationary part is fixedly connected to the tractor, and the working spaces of the pressure source are connected with the brake cylinder of the anti-jack-knifing brake construction to actuate the latter, via throttle valve units including by-pass ducts and non-return valves.

An overview of Pegasus

Pegasus is a multidatabase management system. It provides access to multiple, autonomous, heterogeneous, distributed database management systems through a uniform interface by integrating native and various external databases. The external databases can have different data models, query languages, and world-views. The authors present an overview of the Pegasus system and highlight its main features.<<ETX>>

Enhanced subquery optimizations in Oracle

This paper describes enhanced subquery optimizations in Oracle relational database system. It discusses several techniques -- subquery coalescing, subquery removal using window functions, and view elimination for group-by queries. These techniques recognize and remove redundancies in query structures and convert queries into potentially more optimal forms. The paper also discusses novel parallel execution techniques, which have general applicability and are used to improve the scalability of queries that have undergone some of these transformations. It describes a new variant of antijoin for optimizing subqueries involved in the universal quantifier with columns that may have nulls. It then presents performance results of these optimizations, which show significant execution time improvements.

Automatic importation of relational schemas in Pegasus

The paper describes a technique for importing automatically an external relational schema into Pegasus, a heterogeneous, multidatabase system. The Pegasus data model is a functional object model which supports specialization, object-identity, and nested aggregation. The import algorithm generates the definition of a Pegasus schema which captures the semantics of a relational schema containing information about primary keys and referential integrity constraints. The algorithm described can be adapted easily for other semantic and object-oriented data models.<<ETX>>

Object Identification in Multidatabase Systems

Abstract In the Pegasus prototype multidatabase system, the key to the problem of object identity is a differentiation between the view of a data administrator, capturing all the underlying discrepancies and their solutions, and the view of an end user or application, in which only a consistent result is visible. New language constructs with which the administrator can describe solutions to identity problems include underlying and unifying types, the Image function, and producer types.

Multidatabase management in Pegasus

Pegasus is an object-oriented multidatabase system being developed at Hewlett-Packard Laboratories. The goal of the system is to provide facilities for multidatabase applications for accessing and manipulating multiple autonomous heterogeneous object-oriented, relational and other databases. This paper gives an overview of the architecture, describes the common object model and gives the justification for adapting the model for a multidatabase system. It describes the database environment of the system, discusses how to manage schemas consisting of types and functions from multiple databases. It briefly describes the features of the data language and gives an example. It discusses a method for dealing with object identification issues in a multidatabase system, reviews schema integration issues and briefly mentions schema mapping to relational models.<<ETX>>

Pegasus: a system for seamless integration of heterogeneous information sources

Pegasus, a heterogeneous multidatabase management system that is under development, is described. The goal of the system is to provide facilities for applications to access and manipulate multiple autonomous heterogeneous object-oriented, relational, and other information systems. Pegasus defines a common object model for unifying the data models of the underlying systems. The data language of Pegasus allows transparent as well as explicit access to multiple information systems in a single declarative statement. The Pegasus architecture is described, and attention is given to data integration and query processing in Pegasus.<<ETX>>

Of snowstorms and bushy trees

Many workloads for analytical processing in commercial RDBMSs are dominated by snowstorm queries, which are characterized by references to multiple large fact tables and their associated smaller dimension tables. This paper describes a technique for bushy join tree optimization for snowstorm queries in Oracle database system. This technique generates bushy join trees containing subtrees that produce substantially reduced sets of rows and, therefore, their joins with other subtrees are generally much more efficient than joins in the left-deep trees. n nThe generation of bushy join trees within an existing commercial physical optimizer requires extensive changes to the optimizer. Further, the optimizer will have to consider a large join permutation search space to generate efficient bushy join trees. The novelty of the approach is that bushy join trees can be generated outside the physical optimizer using logical query transformation that explores a considerably pruned search space. The paper describes an algorithm for generating optimal bushy join trees for snowstorm queries using an existing query transformation framework. It also presents performance results for this optimization, which show significant execution time improvements.

Integration strategies in Pegasus object oriented multidatabase system

The paper identifies various problems in integrating schema, data and methods in heterogeneous systems and describes the strategies used in Pegasus to deal with them. Pegasus is an object-oriented multidatabase system that is being developed at Hewlett-Packard Laboratories. The goal of the first running prototype is to provide facilities for multidatabase applications to access and manipulate multiple autonomous heterogeneous object-oriented and relational databases. Pegasus provides both type and procedural abstractions to support integration. Type abstraction is used to organize objects from multiple databases in categories that are related by having some common properties. The common properties of objects in different databases may be in incompatible forms or in inconsistent states. The system provides procedural abstraction that enables the schema integrator to develop arbitrary mappings between data from different sources and to write necessary transformations to convert object properties to a common form. Because of the importance of existing relational databases, some important integration techniques are demonstrated through the object-oriented views over SQL databases.<<ETX>>