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

10.3.1 Index design for post-DSO access
For each table not chosen to provide filtering before the DSO is joined, you must
ensure that these tables have indexes that support the join and local filtering
predicates. This will ensure efficient nested loop join performance for all tables.
Given the following WHERE clause from one of the remaining tables from the
DSO example:
WHERE "/BIC/AZOSASALE00"."PROFIT_CTR" = "/BI0/SPROFIT_CTR"."PROFIT_CTR"
AND "/BI0/SPROFIT_CTR"."SID" = 31
The relevant columns from table /BI0/SPROFIT_CTR are PROFIT_CTR (join
predicate) and SID (local predicate).
For smaller master data tables (less than a few data pages) or master data
tables with local predicates that provide very little filtering, it may be sufficient to
ensure that an index exists on the join columns only. This provides the greatest
degree of flexibility, especially since the local predicates are not going to
significantly limit the number of data pages accessed.
As the master data table becomes larger and the local predicates provide more
filtering, it is important to create indexes that support both the local and join
predicates. Local predicates are preferred as the leading index columns because
they support this table accessed as the leading table accessed or non-leading in
the table join sequence. Additionally, they allow frequency statistics to be
collected on the local predicate column with default RUNSTATS. Both the local
and join predicates should be included in the index because both columns
provide filtering as part of the join.
Therefore, for each smaller table (a few pages) we recommend having an index
that supports the join predicates. This is also true for larger tables with local
predicates that provide minimal filtering. Larger tables with local filtering
predicates should have indexes on local (equal and IN) followed by join
predicates. If local predicates are range predicates, then these should appear
after join columns in the index.
The index recommendation for table /BI0/SPROFIT_CTR is shown in Table 10-4.
Table 10-4 Index recommendation
Column Reason
SID Local (equal) filtering predicate
PROFIT_CTR Filtering join predicate
Chapter 10. Tips for SQL efficiency 223