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On-Line Analytical Processing (OLAP) systems are database-like systems optimised for querying in aggregate. Normally OLAP systems use specialised query languages and APIs, different from relational database systems.

Several different architectures are used in OLAP systems:

  • MOLAP (Multi-dimensional OLAP) systems persist the data in a specialised data store, and calculate and persist aggregate rollups of the data that can be used for faster querying. Examples of MOLAP systems include Microsoft SSAS and Hyperion Essbase.

  • ROLAP (Relational OLAP) systems front a standard database system and issue queries to the underlying database. Examples of ROLAP systems include Business Objects and Microsoft Report Builder.

  • HOLAP (Hybrid OLAP) systems query data from an underlying database store but calculate and cache aggregates in memory for performance. Examples of HOLAP systems include Mondrian and Microsoft SSAS.

  • In-memory OLAP systems read data from sources and maintain an in-memory data structure for fast queries. Examples of in-memory OLAP systems include QlikView and Microsoft PowerPivot.

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