A Cost Model for Path Expressions in Object-Oriented Queries

Query processing remains one of the important challenges of Object-Oriented Database Management Systems. Cost based query optimization involves creating alternative executing plans for a given query and executing the least costly one within a cost model framework. In Object-Oriented Database Management Systems (OODBMSs) objects may store references to other objects (precomputed joins), and path expressions are used in query languages. Although the cost fomulas for explicit joins and the selectivities of attributes and joins are well-known in the relational model, there is no similar work involving path expressions for OODBMSs. However in order to optimize object-oriented queries involving path expressions, a cost model is essential. This information is necessary for deciding whether to use pointer chasing or to convert the path expressions into explicit joins and also for deciding the execution order of path expressions. In this paper, we provide a cost model that includes the formulas for the costs and selectivities of forward and backward path traversals.