A Data Warehouse (DW) is a collection of technologies whose purpose is to enable knowledge workers (executive, manager, analyst, etc) to make better and faster decisions. Many researchers and practitioners share the understanding that a data warehouse architecture can be formally understood as layers of materialized views on top of each other. A data warehouse architecture exhibits various layers of data in which data from one layer are derived from data of the lower layer. Data sources, also called operational databases, form the lowest layer. They may consist of structured data stored in open database systems and legacy systems, or unstructured or semi-structured data stored in files.