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

partitioning and table-controlled partitioning, see DB2 9 for z/OS documentation
or the DB2 Version 9.1 for z/OS Administration Guide, SES1-2935-01.
Partitioning provides many benefits in the areas of data management, data
availability, and query performance, especially for large tables like DataStore
objects. The support from DB2 allows addition and rotation of partitions and
provides more options for managing data. DB2 utilities can operate on a subset
of the partitions, thus allowing the rest of the data to be available. Partitioned
tables may be able to take advantage of parallelism for online queries, batch
processing, and utilities. Query performance can improve if partitions can be
eliminated from the search. The DB2 feature of data partitioned secondary index
(DPSI) promotes partition-independence, which can reduce lock contention and
improve index availability.
Clustering is the physical sequence in which records are stored in a table. The
clustering sequence is defined by the clustering index for the table. DB2 tries to
insert rows into the table to maintain the order of the clustering index. If you do
not specify one of the table’s indexes as clustering, then, by default, the
clustering index is the first index that DB2 creates for the table.
You use the CLUSTER keyword on the CREATE INDEX statement to explicitly
define the clustering index. When SAP creates tables, it arbitrarily chooses the
primary index (index 0) as the clustering index. However, this may not
necessarily be the best choice. The desired clustering sequence depends on
how the data is accessed and used, which is mostly customer specific.
In DB2, the clustering index can be altered without the requirement to drop and
recreate the affected indexes. In addition, with table-controlled partitioning, you
have more flexibility over the choice of a clustering index, since it does not need
to be the partitioning index.
When a range of rows is retrieved, the query performance can be improved if
these rows are clustered and accessed by a clustering index. The rows can be
read with fewer I/O operations, since the pages read are likely to contain more
rows that need to be retrieved and DB2 can take advantage of its sequential
prefetch feature.
Partitioning provides forced clustering because data within a partition cannot
exceed the boundaries of that partition. Clustering is only a guide that DB2 tries
to achieve. So, partitioning is more precise than clustering.
Also, partitioning can be used instead of indexing in some situations. Lower
cardinality columns that are chosen for partitioning may then be removed from
the index chosen as the clustering index.
Chapter 7. Best practices for DataStore 109