BW InfoCube and Extended Star Schema

下面的内容是我从BW310里面截取的一段，个人觉得非常重要，它简单的讲解了InfoCube与Extended Star Schema，非常容易理解。

SAP NetWeaver BW InfoCube (InfoCube)

• InfoCubes are the central objects of the multidimensional model in SAP NetWeaver BW. Most BEx reports and analyses are based on these. From a reporting perspective, an InfoCube describes a self-contained data set within a business area, for which you can define queries.

• An InfoCube consists of a quantity of relational tables arranged multidimensionally, meaning that it consists of a central fact table surrounded by several dimension tables. SID tables link these dimension tables to their respective master data tables.

Hint: There are various types of InfoCubes in SAP NetWeaver BW. The InfoCube with type InfoCube is the InfoCube most relevant for modeling discussions, since physical database objects (objects that contain data) are the core of your SAP NetWeaver BW project.

Figure 161: A Bigger Example of an Extended Star Schema

The figure above shows more dimensions and more master data. Since we discussed in the first section of this lesson, the facts in the fact table are referred to as key figures and the dimension attributes are known as characteristics. The dimension tables are linked the central fact table by way of key relationships. In contrast to the classic star schema, characteristics are not components of the dimension tables; in other words, the characteristic values are not stored in the dimension tables. A numerical SID key is generated for each characteristic. This “alias” key replaces the characteristic as the component of the dimension table. Here, SID stands for Master Data ID or Surrogate ID (replacement key). In the Master-Data-Bearing Characteristic InfoObjects figure, this these keys are given the prefix SID_. For example, SID_Cost_Center is the SID key for the characteristic Cost Center . The dimension table has a generated numerical primary key, called the dimension key. Dimension tables are denoted with the prefix DIM_ID_. Here, DIM_ID_CostCenter is the dimension key for the Cost Center dimension. As in the classic star schema, the primary key of the fact table is made up of dimension keys.

The customer (you) can define up to 13 dimension tables for an InfoCube, and must define at least one. SAP provides three dimension tables for a total maximum of 16. The Package (DIM_ID_DATAPAKET), Time (DIM_ID_TIME), and Unit (DIM_ID_UNITS) are the three supplied by SAP. The time of a transaction is necessary to add meaning, so the Time dimension is a required. The Unit dimension stores the unit of measure or currency of the key figures, again a critical piece of information. Finally, the Package dimension is used as is a technical load identifier.

Figure 162: A Complete SAP NetWeaver BW InfoCube: Extended Star Schema

One special aspect of the master data is that is shared (linked) with all InfoCubes that have the associated characteristic InfoObject as part of a dimension. This was made possible by removal of master data from the dimension tables using SID technology to create the links. The result is that you to use the master data with different SAP NetWeaver BW InfoCubes. In other words, the master data is InfoCube independent, and can be used by several queries off several different InfoCubes at the same time. This concept is shown in the figure below.

Figure 163: Shared Master Data Across Basic InfoCubes