Abstract: In a distributed database environment, it is common that queries access data from different sites. In such situations, it is reasonable to attempt to limit the amount of data transfer across sites. Many algorithms have been suggested. In this chapter we present the problems encountered in distributed query processing and some of the common techniques to estimate sizes of intermediate results, to make use of semi-joins to reduce data transfer, to find improved sequences of semi-joins and to handle multiple copies of relations and fragments of relations.

Abstract: This thesis addresses the problem of efficient query evaluation over a deductive database and proposes several methods to optimize the evaluation of a query. The problems addressed in this thesis and the solutions proposed, under the central theme of query optimization can be discussed under- (i) Techniques for interfacing PROLOG with relational databases, (ii) A formalism for semantic query optimization using integrity constraints, and (iii) Multiple query evaluation in deductive databases. We propose several ways in which a PROLOG interpreter can be modified so that it can be interfaced effectively with a database system. Three solutions, namely, a simple modification to the PROLOG query evaluation strategy to accomplish the complied approach, a meta-level interpreter without any modifications to PROLOG and a set evaluation strategy using tables, are proposed in this thesis. A general framework in which domain specific knowledge--in the form of integrity constraints, is used to transform a query, is proposed in this thesis and is termed semantic query optimization. The process of semantic query optimization is carried out in two phases. Initially, the axioms of a database are semantically compiled, wherein, integrity constraints are integrated into the axioms in a suitable manner. Semantic compilation is performed only once prior to the submission of any query. Subsequently, the compiled axioms are utilized for query transformation at the time of query evaluation. The transformed query has restrictions imposed on it by the integrity constraints and hence it may be evaluated more efficiently over the database than the original query. Multiple queries arise in several contexts. In the case of deductive databases, a single query on an intensional predicate may result in several disjunctive queries which may have overlapping computations. We extend the connection graph decomposition algorithm to generate a single plan for a set of disjunctive queries. A multi-query graph is used as a non-procedural representation for a set of queries. The algorithm proposed in this thesis minimizes the number of accesses to the secondary storage where the relations are physically stored as well as the total number of joins.

Abstract: The existing information systems are characterized by sophisticated hardware designs and relatively unforgiving software support. One may expect that future information systems will provide a unified approach to several different types of information processing tasks, as well as more user-friendly processing environments. Some of the requirements of future information systems are described in this note and various advances in retrieval system design are examined, including automatic indexing, automatic query formulation, and extended Boolean query processing.

Abstract: This thesis presents a new approach to distributed query processing in locally distributed database systems, load-balanced query processing (LBQP), which integrates distributed query processing and load balancing. Several observations about previous research work in distributed query processing motivated this study. First, only a few query processing algorithms have been developed specifically for distributed databases based on local networks. Second, the use of multiple copies of data to improve performance in a distributed database system has not been addressed by most existing algorithms. Finally, and perhaps most importantly, existing query optimization algorithms have considered only the static characteristics of a distributed database. The algorithm reported here considers the dynamic load status of the system and the existence of multiple copies of data to provide better performance than is achievable with purely static planning techniques. The dynamic query allocation problem for distributed database systems with fully-replicated data was studied first using simulation. Two new heuristic algorithms were proposed for dynamically choosing a processing site for a newly arrived query in a local network environment. In order to obtain empirical experience regarding distributed query processing in local area networks, a testbed was built to conduct experiments on the performance of distributed join algorithms. A three-phase load-balanced query processing algorithm, Algorithm LBQP, was then developed based on these experimental results and the results of the study of dynamic query allocation. This algorithm first statically generates a logical processing plan for a query. A dynamic query unit allocation algorithm is then applied to the plan to determine the processing sites for each relation. Finally, specific processing methods for the distributed joins in the resulting plan are selected. A model of distributed database systems with partially-replicated data was used to investigate the performance of the dynamic query unit algorithm of LBQP. The results showed significant improvements in both the mean waiting time for queries and the overall system throughput.

Abstract: Of the many problems facing the casual user of a data–base enquiry system probably the most difficult is gaining a competent understanding of the associated query language. Given that he manages to construct a well–formed query expression there is no guarantee that it exactly reflects the original question. The research described here concerns the design of an interpreter from a formal query language to natural language to aid query verification in a relational data–base environment. The system is being developed to work in conjunction with the ICL Natural Language enquiry interface NEL which translates English query expressions into the formal query language QUERYMASTER. The requirements of a natural–language paraphraser are first discussed and the nature of an intermediate representation is defined and motivated with respect to an applied relational calculus. Consideration is then given to choosing a suitable underlying framework with which to underpin the practical work and the choice of Lexic...

Abstract: This paper presents a systematic approach to matching categories of query language interfaces with the requirements of certain user types. The method is based on a trend model of query language development on the dimensions of functional capabilities and usability. From the trend model the following are derived: a classification scheme for query languages, a criterion hierarchy for query language evaluation, a comprehensive classification scheme of query language users and their requirements, and preliminary recommendations for allocating language classes to user types.The method integrates the results of existing human factors studies and provides a structured framework for future research in this area. Current and expected developments are exemplified by the description of "new generation" database query languages. In a practical query language selection problem, the results of this paper can be used for preselecting suitable query language types; the final selection decision will also depend on organization-specific factors, such as the available database management system, hardware and software strategies, and financial system costs.