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

The ratio (ST04 statement cache “rows examined”/QDBSEL) is the number of
rows examined for each row that satisfied the predicates. If this is high, it can be
a sign of access path problems caused by predicate columns without indexes, or
that DB2 is not able to use a filtering predicate early in the query to reduce the
number of candidate rows.
When a query is run with indexes that filter the result early and effectively, this
ratio may be as low as 5 or 10. By itself, this ratio is not a reliable indicator of
performance problems. For example, if a query contains a small table without a
good index, the table may be repeatedly scanned and this ratio will be high, but
the query may still run very quickly, since scanning a few dozen rows of a table in
memory is fast. If you find a query that is slow, and where this ratio is high (for
example, 100–500, or more), then use the ST04 Explain tool.
8.4 DB2 Visual Explain
There is an alternative way to interpret the plan table output manually. You may
want to consider using DB2 Visual Explain. It is a software tool that lets you view
and analyze DB2 access paths graphically to facilitate your analysis and tuning
of SQL queries or statements.
The tool offers suggestions for improving the performance of your SQL queries
or statements. You can change an SQL statement and dynamically explain it to
see whether the access path is improved by the change. You can also use this
tool to browse the current values of the subsystem parameters.
DB2 Visual Explain makes Distributed Relational Database Architecture
(DRDA) queries through a DB2 client on the (for example, Microsoft® Windows)
workstation to get the information it needs. The subsystem parameter values are
retrieved by calling a stored procedure on the host, which makes an IFI call to the
appropriate DB2 trace record and returns them to the workstation.
Basically, DB2 Visual Explain helps database administrators and application
developers do the following:
► See the access path for a given SQL statement.
► Tune SQL statements for better performance.
► View the current values for subsystem parameters.
174 Best Practices for SAP BI using DB2 9 for z/OS