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

WHERE clause). The performance improvement can be gained only on a table
with triggers defined. Otherwise, it will perform as today. Moreover, it provides a
statement commit option (IMMEDIATE option) for the statement that allows the
truncate operation to become permanent (that is, cannot undo) and immediately
makes deallocated space available for new, subsequent inserts in the same unit
of work.
SAP BI will use the TRUNCATE TABLE statement of DB2 9, but because BI
tables usually have no triggers, the performance is the same as the mass delete
operation.
4.6 FETCH FIRST n ROWS ONLY in subselects
DB2 8 for z/OS disallows the FETCH FIRST n ROWS ONLY clause in a
subselect. That is, one could write:
SELECT * FROM T ORDER BY c1 FETCH FIRST n ROWS ONLY
(specifying the clauses as part of select-statement), but one could not write:
INSERT INTO any_table
(SELECT * FROM T ORDER BY c1 FETCH FIRST n ROWS ONLY)
(specifying the clauses within the subselect).
SAP BI needs this feature to implement a more sophisticated algorithm of the
so-called BI Selective Deletion for InfoCube and DSO data. Here selective
means that the user can choose an arbitrary selection criteria for the data to be
deleted. In this general case the data to be deleted can be distributed over all
table partitions of a partitioned table, so that the deletion of whole partitions
cannot be used.
When deleting a huge amount of data rows from a fact or DSO table, it is better
to separate this deletion in many small portions of delete transactions.
Otherwise, you have one probably very long-running delete transaction, with the
danger of a day-long or even longer rollback in the case the delete transaction
has to be canceled for any purpose. This is a nightmare for every database
administrator.
With the DB2 9 feature of using FETCH FIRST n ROWS ONLY within a
subselect, the portionwise deletion is implemented in the following way.
The large delete operation is split up into multiple smaller units of work. In
Example 4-2, assume that the primary key of table consists of the
columns kf1, kf2, ..., kfn. is a placeholder for the WHERE
Chapter 4. Benefits of DB2 9 for SAP BI 51