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

Looking at the explanation, we can see that column SEX has COLCARDF of 2. This is a low
COLCARDF. Columns with low cardinality (relative to table cardinality) have significant
opportunity for skew, for that reason frequencies are desirable. The purpose of this example
is to illustrate it may take two runs of SA to get more robust suggestions if defaults are initially
provided. We obtain the statistics shown in Figure 3-54.
Figure 3-54 Statistical output
To illustrate conflict statistic checking, FULLKEYCARDF has been set to 100,000. The table
cardinality is much lower. Rerunning SA returns a complete recollection. When statistics are
conflicting, the issue is often due to an inconsistent statistics collection approach.
We have observed some customers collecting statistics very aggressively on the indexes, and
not on the tables, or vice versa. When index and table statistics are inconsistent, it can result
in inaccurate estimations of filtering for an index, or join size. This can lead to inefficient
access paths. SA can solve this problem quickly by identifying and suggesting recollection of
inconsistent statistics, as shown in Figure 3-55.
120 DB2 UDB for z/OS Version 8 Performance Topics