DB2 UDB for z/OS Version 8 Performance Topics - IBM Redbooks

► Parallel sort enhancement
Parallel sort is enabled for multiple-table sort (not restricted for single-table sort) for long
running queries resulting in reduction of elapsed time. Parallel sort may need bigger work
file buffer pool size, more work file or use Parallel Access Volume.
► Table UDF improvement
DB2 allows you to specify the cardinality option when you reference a user-defined table
function in an SQL statement. With this option, users have the capability to better tune the
performance of queries that contain user-defined table functions.
► ORDER BY in SELECT INTO with FETCH FIRST N ROWS
It enables SELECT INTO to get the top rows based on a user-specified ordering, thereby
reducing the need for a number of FETCHes.
► Trigger enhancements
This enhancement saves work file allocation if WHEN condition evaluates to false or
transition file fits into working storage. This avoids work file creation at each trigger
invocation.
► REXX support improvements
The performance of REXX programs accessing DB2 tables is improved in DB2 V8 if the
program issues large numbers of SQL statements by avoiding loading DSNREXX at each
REXX API invocation and optimizing REXX API initialization.
► Dynamic scrollable cursors
Dynamic scrollable cursor is a new option which allows the application to see the
updated/inserted rows and goes against the base table. It does not create the temporary
table needed for static scrollable cursors.
► Backward index scan
Potential performance enhancements through better access paths and potentially fewer
required indexes.
► Multiple distinct
With DB2 V7 only one DISTINCT keyword per SELECT/HAVING clause is allowed. If we
need to aggregate (COUNT, SUM) by distinct values in more than one column, we must
submit multiple queries. With DB2 V8, a query with multiple DISTINCTs reduces CPU and
elapsed time when compared to the time necessary in DB2 V7 to execute multiple
queries.
► Visual Explain
We provide an overview of the new Visual Explain and show examples of using the new
Statistic Advisor function. Visual Explain was rewritten to provide better, faster analysis of
problem queries. It provides much more detailed information about the access path. It
formats the information into an XML document which can be sent to the lab for analysis.
The EXPLAIN tables have been enhanced for DB2 V8, and a summary of these changes
is provided in Appendix D, “EXPLAIN and its tables” on page 407.
30 DB2 UDB for z/OS Version 8 Performance Topics