This paper investigates cyclic query processing issues in objectoriented databases. We first define a data and cyclic query model for an object-oriented database system using a graph model. We then discuss the efficient processing of a general object-oriented cyclic query. For efficient processing, we first define a general cyclic query, the access plans generated for a given query, then develop a cost model to determine the cost for each access plan generated. We also investigate the retrieval algorithms used for actual data retrieval. [i] INTRODUCTIONObject-oriented databases have recently become a very active research area. Several projects on object-oriented databases are mentioned in [DITT86]. Object-oriented database systems research are attempts to overcome the rather limited modelling capabilities that are provided by traditional database systems. These traditional systems are not well-suited to handling advanced applications like CAD/CAM design, office automation, VLSI design and AI applications among others. Object-oriented database systems have the capabilities to handle the complex applications mentioned above by supporting complex data types, generalization/specialization abstraction and aggregation abstraction among others.This paper investigates query processing issues in object-oriented databases. Specifically we are interested in efficient processing of cyclic queries. Query processing issues in an object-oriented database bring out many of the same issues as in conventional databases. One issue in objectoriented query processing is the order in which classes involved in a query must be evaluated. This is similar to the order in which the relations involved in a join must be evaluated in relational databases and the order in which record @pes are visited in evaluating a query in network databases.Query processing involves the following three elements.Identify the class of cyclic queries to consider. Identify all access plans for a given query. Estimate the cost of each access plan and select reasonable access plans for a given query.(1) (2) (3)Most research in cyclic query processing has focused on recursive query processing and optimization in relational and logic databases. In general, the solution to a recursive query cannot be expressed as a finite relational algebraic expression and therefore cannot be evaluated directly by a conventional relational database system. One approach to solve this problem is to provide an appropriate interface between PROLOG and a relational database system [JARK84]; while others provide some means of recursion to overcome the limitations of relationally complete query languages [KERS84, JARK85.LINN861. The optimization techniques for this kind of recursion can be found in [BANC86, ULLM85, GARD861. The above solutions are for conventional systems using flat tables. LJNN871shows that this is cumbersome for the user as well as for the optimization of recursive queries and proposes a better solution combining the non-first normal form relation model with a recurs...

This paper investigates parallel query processing issues in object-oriented databases. We first introduce the query model to be used. We then identify three areas where parallelism can be exploited in the query model. The perf o m n c e of each of the three areas is measured by mapping the query model to a parallel architecture and using analytic comparisons to the case when query processing is performed on a single processor. Our results show that there is always some speed-up in query processing time for parallel processing of a query over single processor processing. [0] INTRODUCTIONWe discussed query processing issues in object-oriented databases for different classes of queries in [KIM% KLM891. Our assumption was that query processing is performed on a single processor. In this paper, we focus attention to parallel query processing issues in object-oriented databases by exploiting parallelism during query processing and using a parallel architecture to process the query. We then compare the query processing time when using a parallel processor versus when using a single processor. Our motivation is mainly due to our emphasis to increase the performance of object-oriented query processing.It is not our intention to discuss in detail the parallel methods for object-oriented databases. We are merely interested in identifying the situations in the query model in which parallelism can be exploited during query evaluation and observe if performance can be enhanced through parallel techniques. Our analytical results show that there is some speedup in query evaluation when parallel techniques are used. However, the speedup is marginal at best. This may be due to the Amdahl's Law [AMDA67], which states that for a program with serial fraction s, the maximum parallel speedup obtainable is bounded by l/s. This has led to the assertation that the serial fraction will dominate execution time for any large parallel ensemble of processors, limiting the advantages of the parallel approach. We will show in the query model that serial processing is a major component of parallel query processing.[AGRA84] demonstrated that the main limitation to parallelism in database machines is the available I/O bandwidth provided by the underlying disk subsystem containing the database. Unless this problem is investigated and tackled, the use of a large number of processing elements does not seem justified. PIC851 showed that one approach to increase the potential YO bandwidth is to replace a single large disk with several smaller units. They also introduced a data model that would permit the exploitation of a significant number of parallel disk units. Similary, [KHOS88] states that there are two solutions for solving the VO bottleneck. One is to increase the U 0 bandwidth through U 0 parallelism and the other is to minimize the number of VOs through a clustering method.An object-oriented query model can exploit parallelism following the strategy of [BICSS]. The strategy is to increase U 0 bandwidth through VO parallelism using multiple disk...