Best Practices for SAP BI using DB2 9 for z/OS - IBM Redbooks

7.4.2 Choosing and changing the clustering index
The clustering index determines the order in which rows are stored in a table. It
is defined with the CLUSTER keyword on the CREATE INDEX statement. The
clustering index is also the clustering key. There can only be one clustering index
on a table.
By default, in SAP, the primary index is always the clustering index. However,
depending on specific customer situations, this is not always the best choice for
the clustering index. This is especially true for DataStore objects.
To optimize the query performance on DataStore objects, you should consider
explicitly defining a clustering index. Choosing the clustering index is similar to
choosing the partitioning key — you need to understand your data and know how
it is used. You should focus on how the data is commonly retrieved.
If there are queries that access a table index sequentially through an index that is
not defined as clustering, and if this is a predominant way that the table is
accessed, then the performance can be improved by specifying this index as
clustering. Index-sequential access occurs in most cases for the range
predicates (BETWEEN, >,