Estimating Costs of Path Expression Evaluation in Distributed Object Databases

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Efficient evaluation of path expressions in distributed object databases involves choosing among several query processing strategies, due to the rich semantics involved in object-based data models and to the complexity added by the distribution. This work presents a new cost model for object-based query processors and addresses relevant issues, which are ignored or relaxed in other works in the literature, such as the selectivity of the path expression, the sharing degree of the referenced objects, the partial participation of the collections in the relationships, and the distribution of the database objects across the nodes of a network. These issues allowed us to present more realistic estimates for the query optimizer. Our cost model has been validated against experimental results obtained with an object DBMS prototype running in a distributed architecture, using the OO7 benchmark application.