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

3.8.3 Conclusion
The measurements show that SQL statements using user-defined table functions can have
performance improvement if using the cardinality option. Improvements apply to applications
related to Content Management.
3.8.4 Recommendations
Specify cardinality to help the optimizer
Make sure you have sufficient work file buffers and data sets across I/O devices, preferably
with the Parallel Access Volume feature:
► To avoid work file I/O contention, especially with other concurrently running jobs
► For high table UDF cardinality
3.9 ORDER BY in SELECT INTO with FETCH FIRST N ROWS
This function allows you to specify ORDER BY in the SELECT INTO statement. It enables
SELECT INTO to get the top rows based on a user-specified ordering. This means you can
now issue a single SQL statement where in the past you had to issue a number of SQL
statements or even write a program.
An example would be:
SELECT INTO.... ORDER BY ANNUAL_INCOME FETCH FIRST 1 ROW ONLY
In this case up to 30% CPU time reduction can occur when compared to the
Open/Fetch/Close sequence which was required in V7 when there were multiple rows to be
selected, no sort was required. Generally, the entire result set is retrieved and sorted before
the first row is returned.
3.10 Trigger enhancements
Prior to DB2 V8:
► The AFTER trigger causes the creation of a work file for old and new transition variables at
each trigger invocation
► Work files are always created, even when the WHEN condition evaluates to false
With V8:
► Avoid work file allocation if the WHEN condition evaluates to false
► Avoid work file allocation if the transition table fits into 4 KB of working storage.
Use of memory rather than of work files for small transition tables or variable
► Work files are allocated if the transition table exceeds the working storage
Example 3-13 shows the creation of the row trigger used in the measurements.
Example 3-13 Sample row trigger creation
CREATE TRIGGER ADD_AFT
AFTER INSERT ON USRT001.EMPLOYEE
REFERENCING NEW AS NROW
FOR EACH ROW MODE DB2SQL
86 DB2 UDB for z/OS Version 8 Performance Topics