OLAP (Online Analytical Processing) – a class of IT systems to suport tactical and strategic organization activities, in opposite to transactional systems which are supporting current operating activity. There are different types of OLAP in use. The most popular implementation of this type of system is data warehouse, often called relational OLAP – ROLAP. Data warehouse is based on the same solution as transactional database of more denormalize structure – like e.g. a star or a snowflake. Intentional denormalization allows to efficiently query big groups of data, e.g. to make reports. Multidimensional cube is also the OLAP type of solution. Mostly it is another layer of abstraction set on data warehouse, where the data is storaged in files of special multidimensional structures. MOLAP has its own engine of processing, which is an for intuitive and fast way of quering data in mentioned files. Earlier calculated aggregations are also storaged in multidimensional structures. Thanks to the database engine does not have to process the data on high level of details. In fact OLAP is often identified with multidimensional analytic cube. A lot of tools like SQL Server let us create a hybrid version between relational and multidimensional OLAP.