Demystifying aggregates in SAP BI 7.0
Aggregates are subsets of InfoCube data, where the data is pre-aggregated and stored in a InfoCube structure. Aggregates on an Info Cube can be considered similar to that of an Index on a database table. Creating an Aggregate on an InfoCube is one of the few ways to improve performance of SAP BW Query. Subset of an InfoCube data is stored in an Aggregate. As a result, the response time that we get out of reading the data from aggregate will be much faster than reading from an InfoCube.
Read more @ http://www.sdn.sap.com/irj/scn/weblogs?blog=/pub/wlg/26054