* Aggregations are precalculated sums of fact table data at some combination of levels from each dimension
* Aggregations are used to answer queries and create additional aggregates
* Storage mode options for facts and aggregates:
- Relational OLAP (ROLAP) – fact data and aggregations are stored in a relational database
- Multidimensional OLAP (MOLAP) – fact data and aggregations are stored on the OLAP server in an optimized multidimensional format
- Hybrid OLAP (HOLAP) – fact data is stored in the relational database, aggregations are stored on the OLAP server in an optimized multidimensional format
* Tradeoffs on precalculating aggregates:
- Precalculate gives fast queries but requires more storage
- Calculate at runtime saves storage but slows down queries