More About Star Schema

SAP BW Star schema is based on the extended of the classic star schema (aka “snowflake” schema). The enhancement comes from the fact that the dimension table does not contain master data information. This master data information is stored in separate tables, called master data tables. In SAP BW Star Schema, the distinction is made between two self-contained areas: infocube and surrogate ID (SID) tables.

Infocube

Infocubes are the central objects on which reports and analysis are based in SAP BW. It describes a self-contained data set within a business area, for which we can define queries.

Infocube consist of a central fact table and several surrounding dimension tables. In SAP BW star schema , the facts in a fact table refers to key figure and the dimension attributes refer to characteristics.

In contrast to classic star schema concepts, characteristics are not component the dimension tables since the characteristic values are not stored in the dimension tables. They are stored in master data tables. There are foreign keys that replace the characteristics as the component of dimension table, i.e.: SID stand for Surrogate ID. In picture above, these keys are given the prefix SID_.

Each Dimension table has a generated primary key, called the dimension key. (In Picture above, the keys are given the prefix DIM_ID_..). As the classic star schema, the primary key of the fact table is made up of dimension keys.

Master data Table or Surrogate ID (SID) Table

In SAP BW, additional information about characteristic is referred to as master data. Master data information is stored in separate tables called master data tables. There are 3 types of master data, i.e.: Attributes, Text and Hierarchies.

Here, SID tables play an important role in linking the data warehouse information structured to the subject-oriented infocubes. The master data tables, text tables, and hierarchy tables are not directly linked to the associated dimension tables. These tables are infocube independent and joined with the dimension tables using SID table. The SID therefore provides the link to the dimension tables.

The connection between Infocube and Master Data Tables

Picture above illustrates the connection between master data table and infocube. Master data tables are connected to an infocube by way of the SID tables. The picture above also explains that in SAP BW star schema, the master data is independent infocube and can be used by several infocubes at the same time.