The ever-changing data requirements of today’s dynamic businesses are not handled well by current OLAP systems. Physical integration
of data into OLAP systems is a time-consuming process, making logical
federations the better choice in many cases. The increasing use of XML suggests that the required data will often be available in XML
format. Thus, federations of OLAP and XML databases will be very attractive in many situations. In an efficient implementation
of OLAP-XML federations, cost-based optimization is a must, creating a need for an effective cost model for OLAP-XML federations.
In this paper we present a cost model for OLAP-XML federations, and outline techniques for estimating the cost model parameters in a federated OLAP-XML environment. The paper also outlines the cost models for the OLAP and XML
components in the federation on which the federation cost model is built. The cost model has been used as the basis for effective
cost-based query optimization in OLAP-XML federations. Experiments show that the cost model is precise enough to make a substantial
difference in the query optimization process.