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

Figure 3-29 Access path graph in Visual Explain
If we click the rectangle indicating (4)SYSTABLESPACE in the graph, it shows that
SYSTABLESPACE has 828 rows and it is estimated that approximately 794 rows of the table
qualify in the table space scan.
On the left side of the graph there is one tree structure that shows information about the
columns, indexes and table space related to the selected table SYSTABLESPACE. If there is
a frequency distribution on one column of the table, under the respective column there is a
Coldist folder. If you select i, you can see the values of the column and the filtering factor for
the value.
For each qualified row in the outer table, the subquery is executed using a matching index
scan using predicate TB.DBNAME=TS.DBNAME with filtering factor of 0.0098 returning 18
RIDs.
The report function generates information about the cardinalities used. If there are no
statistics available, default cardinality is used by the optimizer.
Example 3-7 shows the RUNSTATS statement given by the Statistics Advisor of Visual
Explain for the query in Example 3-6 after the execution of the analyze function. Visual
Explain enhancements are reported in 3.15, “Visual Explain” on page 110.
Example 3-7 RUNSTATS statement provided by the Statistics Advisor
RUNSTATS TABLESPACE DSNDB06.SYSDBASE
TABLE(SYSIBM.SYSTABLESPACE)
COLUMN(NAME)
TABLE(SYSIBM.SYSTABLES)
COLUMN(TYPE,TSNAME)
COLGROUP(TYPE) FREQVAL COUNT 10
Chapter 3. SQL performance 73