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

is run on an empty table (dimension or fact), and statistics have not been
updated since.)
If statistics are current, and the access path does not match with the counts such
that the most filtering dimension/snowflake is accessed before the facttable, then
the most likely scenario is either:
► The dimension/snowflake chosen before the facttable has had its filtering
overestimated (so the filtering looks better than it actually is).
► The good dimension/snowflake (not chosen before the facttable) has had its
filtering underestimated (so the filtering looks worse than it actually is).
For information about analyzing and determining the actual cause, refer to 8.3,
“Analyzing query performance” on page 157.
10.3 Indexing of DSOs
Query performance on DSO objects can be greatly influenced by having
appropriate indexes on the DSO tables. When a DSO object is created within
SAP, only one index is created. This is a primary index, which is usually
comprised of all the key fields of the DSO object.
This index most likely will not provide optimal performance for the queries being
executed against the DSO. Secondary indexes should be created. However,
keep in mind that there is a cost to maintaining indexes. This cost is mostly
incurred during the load DSO phase. Therefore, index design is important and it
depends on the queries that will be executed against the DSO. The point is to
create indexes that are worth the cost of maintenance.
The objective of indexing on the DSO is to minimize the number of DSO table
rows that must be accessed to produce the final result. The closer the number of
rows retrieved is to the number of rows returned by the query, the better the
performance should be. Therefore, indexes should be designed to support the
most filtering of rows. This is more important than the number of matching
columns in an index.
Let us look at the DSO query that is analyzed in 10.1, “SQL efficiency for DSO”
on page 204, and see how additional indexes on the tables accessed can
influence the performance of the query.
The WHERE clause of this query consists of the following predicates, which
proved to provide the greatest degree of filtering:
WHERE "/BIC/AZOSASALE00"."/BIC/ZMATLPLNT" =
"/BIC/XZMATLPLNT"."/BIC/ZMATLPLNT"
Chapter 10. Tips for SQL efficiency 219