An aggregate join index is a join index that specifies SUM, COUNT, MIN, or MAX aggregate operations. No other aggregate functions are permitted in the definition of a join index.
The primary function of an aggregate join index is to provide the Optimizer with a high-performing, cost-effective means for satisfying any query that specifies a frequently made aggregation operation on one or more columns. The aggregate join index permits you to define a summary table without violating schema normalization.