Cluster tables are special types of tables present in the SAP data dictionary. They are logical tables maintained as records of the normal SAP tables, which are commonly known as transparent tables. A key advantage of using cluster tables is that data is stored in a compressed format, reducing memory space and the landscape network load for retrieving information from these tables.
Techopedia explains Cluster Table
With cluster tables, functionally dependent data is distributed over many tables and grouped under one cluster table. The key fields in these tables are stored as key fields of cluster tables.
Prominent features of cluster tables are as follows:
Native SQL cannot be used on these tables.
They cannot be directly managed by database system tools.
Administration of cluster tables is easier because the tables are not created separately, like transparent tables in a database.
Functionally dependent data stored in cluster tables may be read together, thus reducing access to the database.
Like pool tables, indexes for non-key fields cannot be created. However, primary or other indexes can be created for a set of fields that are to be key fields.
Database views, join statements or append statements cannot be used on these tables.