What is an Aggregate?
Multidimensional databases generally have hierarchies or formula-based relationships of data within each dimension. Aggregation or consolidation involves computing all of these data relationships for one or more dimensions, for example, adding up all Departments to get Total Department data.
It has been claimed that for complex queries OLAP Cubes can produce an answer in about 0.1% of the time for the same query on OLTP relational data. The single most important mechanism in OLAP, which allows it to achieve such performance, is the use of aggregations. Aggregations are also built from the fact table by changing the granularity on specific dimensions and aggregating data up along these dimensions during the ETL process.