BW中聚集是指根据CUBE中几个或者一个维度信息对象创建的小的CUBE，其实是新生成一张事实表，外加一些对应的维度表，可以提高数据的访问效率。
An aggregate is a roll up of fact data where a total value is sufficient and no detailed information is needed.
Technically speaking, an aggregate is a separate InfoCube with its own fact table
and dimension tables. When an aggregate is created ,it is given a six-digit number
<1NNNNN> that starts with a ì1?. The table name for an aggregate is derive
in the same way from this number as InfoCube table names are derived from
InfoCube names.
For example, if an aggregate has the technical name 100001, its fact tables are
called /BIC/E100001 and /BIC/F100001. Its dimensions have the table names
/BIC/D100001P, /BIC/D100001T, and so on.
Dimension tables can be shared between an InfoCube and an aggregate. In this
example, dimension 2 (the country dimension) is shared between the InfoCube
and the aggregate. It is not necessary to create a new dimension table. A link to
this dimension table is created in the aggregate fact table.
在BW中，创建聚集的大致步骤如下：
创建聚集的方法如下，选择CUBE，点击“管理”，选择“滚动（rollup）”,点击 “Aggregates”，进入聚集功能界面，可以手动生成，也可以由系统提议，其中聚集维度可以分为“层次结构的聚集”（H）、“固定值的聚集维度”及“一般聚集维度”；其主要用法根据实际使用来确定，例如某张报表只会访问到某个层次结构的N层次，再往下就不看了，则可以给CUBE生成一个对应的聚集，该聚集维度基于该层的“层级结构维度”。
其中对于聚集中维度数据和事实表数据的更新，如果是导航属性生成的维度表，则通过信息对象的change run可以同步聚集维度表数据，而事实表数据通过ROLL UP可以更新。