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

Several things worth noting are:
► There may be skipped values between adjacent quantiles. If there are, then it
means that those skipped values are of zero appearance in the table.
► Each quantile has approximately the same number (or percentage) of rows.
The highly frequent single value may alone by itself occupy a quantile. The
single-value will not be broken into more than one quantiles. Therefore, the
number of quantiles maximum is equivalent to the number of distinct values in
that column. However, DB2 has a maximum limit of 100 quantiles for storage
purposes.
► One particular value may be included into a quantile just because it is closer
to that quantile. The purpose is to avoid the skipped range as much as
possible. For example, value 30 from above has frequency=1%, including it in
the 6th quantile would make the fifth and sixth quantile with frequency=15%
even. But doing that would introduce a large skipped range [28, 34] to the
sixth quantile and we try to avoid that.
► A special case of (3) is that HIGHVALUE or LOWVALUE, when it is too far
away from the rest of the value range, the single value of HIGHVALUE or
LOWVALUE will occupy a quantile by itself.
► The NULL value will occupy a quantile by itself.
► Due to aggregation, quantiles may not always have closed range, that is, the
high value or low value can be inclusive or exclusive. But overall, the
quantiles represent the non-overlapped value range.
You will find more details about histogram statistics in the description of the
corresponding system catalog tables (SYSCOLDIST, SYSCOLDISTSTATS,
SYSKEYTGTDIST, and so on) and in the documentation for RUNSTATS.
4.3 Parallelism enhancements
This section discusses parallelism enhancements such as global query
optimization, which enables DB2 to optimize a query as a whole rather than as
an independent part and cross query block optimization, which is optimization
across, rather than within, query blocks.
4.3.1 Global query optimization
With DB2 V8 the DB2 optimizer chooses the lowest cost sequential plan and
then determines how to parallelize the access path. With Version 9 the way of
using parallelism has been extended. The V9 optimizer considers multiple
sequential plans for parallelism and then executes the one with the lowest costs.
48 Best Practices for SAP BI using DB2 9 for z/OS