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

If your query has long run times that are unacceptable to the user, and if the
query summarizes many rows, then using aggregates is often the most effective
way to speed up the query. To determine whether your query would benefit from
aggregates, you can look at the detailed breakdown of the response time on
ST03.
QDBSEL/QDBTRANS
If the QDBTIME component of the response time is very high, examine the fields
QDBSEL/QDBTRANS to determine whether the query is reading too many
records from the cube in relation to the amount of rows that are transferred back
to the OLAP engine.
The field QDBSEL is the number of rows that satisfied the predicates. The field
QDBTRANS tells you how many records were transferred from the database to
the application server after the GROUP BY statement in the SQL.
As a rule of thumb (RoT), if the ratio between QDBSEL/QDBTRANS is > 10, then
you may be able to get larger improvements in performance though the use of
aggregates than through other database or SQL tuning. However, this does not
mean that you should ignore other methods for further improving performance by
analyzing the SQL used by the query.
QDBSEL/QDBTRANS is the number of rows selected for each row returned to
the query user. When this is high, it means that DB2 must summarize many
selected rows into each result row, so an SAP aggregate table would probably
help performance, since the aggregate table would contain pre-summarized
data. After the E- fact table is summarized into an aggregate table, the query will
retrieve fewer rows to create the result.
The SAP rule of thumb is that when this ratio is greater than 10, an aggregate
may be appropriate. Use the frequency of query execution, the importance of
performance for the query, and the time available for aggregate roll-ups in
conjunction with this rule of thumb when evaluating the need for new aggregate
tables.
For instance, if an aggregate is not frequently used, and if the query is very slow
but time and CPU are available in the aggregate roll-up window, then creating an
aggregate is a way to move part of the report generation time to the nighttime.
If the ratio QDBSEL/QDBTRANS is low (which indicates that aggregates may not
help) and the QDBTIME is still very high, then you should look at the relationship
of the fields QDBSEL/QTIMEDB. This is a measure of how fast (rows per
second) the rows are retrieved from the database. As a rule of thumb, if the
figure is 300–400 or lower, then you should further analyze the database to
evaluate the way the SQL is executing. Continue down the database leg of our
162 Best Practices for SAP BI using DB2 9 for z/OS