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

Figure 4-7 explains how to influence the DB2 optimizer to use the appropriate
index by explicitly faking the DB2 statistics. You will find the details in SAP note
688133. Make sure that you test your RSADMIN entries by running function
module RSDU_ANALYZE_TABLE_DB2 for the given table and check the output
list where the generated and executed INSERT or UPDATE statements are
shown. Figure 4-7 further highlights SAP note 688133.
How to influence DB2 statistics (note 688133)
� specific RSADMIN parameters for influencing the DB2
statistics
� Use this possibility only if no other solution helps
� Collect additional frequencies for an given index
– DB2_""= FREQVAL NUMCOLS n COUNT c
Example: DB2_"/BIC/DMYDIMT~0" = FREQVAL NUMCOLS 5 COUNT 30
� Change the clusterratio of an index
-> to force the optimizer to use a known index set the
clusterratio to 100
– DB2_"" = CLUSTERRATIO x
Example: DB2_"/BIC/FMYCUBE~030" = CLUSTERRATIO 100
– DB2 catalog is updated accordingly after the RUNSTATS
run
Figure 4-7 Faking the DB2 statistics with RSADMIN parameters
4.2.2 New to DB2 9: histogram statistics
In this section we discuss histogram statistics.
Overview and advantages
Data distribution statistics are very important for query optimization. DB2
chooses the best access path based on costing, and the very basic foundation of
costing is predicate selectivity estimation, which heavily relies on data
distribution statistics.
Up to DB2 v8, the only distribution statistics supported in DB2 for z/OS were
frequency statistics collected on single values, which could be for either
single-column or multi-column. For multi-column, it is a concatenated
multi-column value. Such frequency statistics are most commonly collected on
the most biased values (that is, the most frequent or the least frequent).
Chapter 4. Benefits of DB2 9 for SAP BI 45