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

► A single value is never broken into more than one interval, meaning that the
maximum number of intervals is equal to the number of distinct values on the
column. The maximum number of intervals cannot exceed 100, which is the
maximum number that DB2 supports.
► Adjacent intervals sometimes skip values that do not appear in the table,
especially when doing so avoids a large range of skipped values within an
interval. For example, if the value 30 above has 1% frequency, placing it in
the seventh interval would balance the percentage of rows in the sixth and
seventh intervals. However, doing so would introduce a large skipped range
to the seventh interval.
► HIGHVALUE and LOWVALUE can be inclusive or exclusive, but an interval
generally represents a non-overlapped value range.
► NULL values, if any exist, occupy a single interval.
► Because DB2 cannot break any single value into two different intervals, the
maximum number of intervals is limited to the number of distinct values in the
column, and cannot exceed the DB2 maximum of 100 intervals.
Example of equal-depth histogram statistics
Equal-depth Histogram cuts the whole value range so that each quantile has
about the same number of rows. The important parameters to describe
equal-depth histogram statistics includes the total number of quantiles (N) and
for each quantile, the pair of LOWVALUE/HIGHVALUE, the number of distinctive
values (CARD) and the frequency (or the number of rows). To give an example,
on a table IBM_EMPLOYEE column YRS_OF_EXPERIENCE, it may have (N=7)
quantiles, as shown in Table 4-1.
Table 4-1 Histogram statistics example
SEQ.NR LOWVALUE HIGHVALUE CARD FREQUANCY
1 0 3 4 14%
2 4 15 8 14%
3 18 24 7 12%
4 25 25 7 12%
5 26 26 1 15%
6 27 30 4 16%
7 35 40 6 14%
Chapter 4. Benefits of DB2 9 for SAP BI 47