Datawarehousing and OLAP techniques

A data warehouse might be used to find the day of the week on which a company sold the most widgets in May 1992, or how employee sick leave the week before the winter break differed between California and New York from 2001–2005.

While operational systems are optimized for simplicity and speed of modification (see OLTP) through heavy use of database normalization and an entity-relationship model, the data warehouse is optimized for reporting and analysis (online analytical processing, or OLAP). Frequently data in data warehouses are heavily denormalised, summarised or stored in a dimension-based model. However, this is not always required to achieve acceptable query response times.