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

3.14.2 Conclusion
Percent reduction
Figure 3-46 Multiple DISTINCT performance
Multiple DISTINCT keywords in a single SELECT statement in V8 can largely enhance
application performance, as well as usability of SQL. Specifying four distinct columns in one
statement can make approximately 55% elapsed time improvement and 45% CPU
improvement compared to the total time of the four individual statements. This improvement is
due to reducing the overhead of multiple executions of queries and performing multiple sorts
for multiple distinct columns.
3.14.3 Recommendations
Use multiple DISTINCT keywords in a single SELECT statement in your application as often
as possible, eliminating unnecessary multiple queries. Performance advantages were found
where there more than two DISTINCT clauses in one query.
3.15 Visual Explain
Visual Explain for DB2 V8 provides graphical depictions of the access plans that DB2
chooses for your SQL queries and statements. Such graphs eliminate the need to manually
interpret the PLAN_TABLE output filled by DB2 EXPLAIN. See Appendix D, “EXPLAIN and its
tables” on page 407 for its contents and the contents of the other EXPLAIN tables. The
relationships between database objects, such as tables and indexes, and operations, such as
table space scans and sorts, are clearly illustrated in the graphs. You can use this information
to tune your queries.
Statistics Advisor is a brand new function of Visual Explain, and it is also available from the
Web. Its main function is to help you identify the RUNSTATS executions that help your SQL.
110 DB2 UDB for z/OS Version 8 Performance Topics
Multiple DISTINCT performance
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Time reduction when compared to invidual DISTINCTs
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Number of DISTINCTs in one query
Elapsed Time
CPU Time