For SAP customers Qlikview provides the SAP Connector. This is a valuable component giving access to data structures not reacheble when using direct access to the database.

One of these structures are Cluster Tables.  see details here Pool and Cluster Table Definition . You can read data from these tables, but you need  filters  based on the cluster index.  You also can't use this table in  a join statement. 

The connector allows only reading of data.  

You can't call RFC functions.

To read the SAP tables you need to use SQL statements based on SAP OPEN SQL. see details here Reading with OPEN SQL.

Performance Notes

The performance of an ABAP program is largely determined by the efficiency of its database accesses. It is therefore worth analyzing your SQL statements closely. To help you to do this, you should use the Performance Trace andRuntime Analysis tools (Test menu in the ABAP Workbench). In particular, the SQL Trace in the Performance Traceshows you which parts of Open SQL statements are processed where, and how long they take.  Check detais here.