Since operational databases store huge amounts of data, why not perform online analytical processing directly on such databases instead of spending additional time and resources to construct a separate data warehouse? A major reason for such a separation is to help promote the high performance of both systems. Data warehouse involve the computation of large groups of data at summarize levels, and may require the use of special data organization, access and implementation methods based on multidimensional views. Data warehouses and OLAP tools requires a concise, subject-oriented schema that facilitates online data analysis. The most popular data model for a data warehouse is a multidimensional data model. Such a model can exist in the form of a star schema, a snowflake schema, or a constellation schema