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

Global query optimization addresses query performance problems caused when
DB2 V8 breaks a query into multiple parts and optimizes each of those parts
independently. While each of the individual parts may be optimized to run
efficiently, when these parts are combined the overall result may be inefficient.
For example, consider the following query:
SELECT * FROM T1
WHERE EXISTS (SELECT 1 FROM T2, T3 WHERE T2.C2 = T3.C2
AND T2.C1 = T1.C1);
DB2 V8 will break this query into two parts (the correlated subquery and the
outer query), and each of these parts will be optimized independently. The
access path for the subquery does not take into account the different ways in
which the table in the outer query may be accessed and vice versa.
DB2 V8 may choose to do a table scan of T1, resulting in much random I/O when
accessing T2, while a nonmatching index scan of T1 would avoid the random I/O
on T2. In addition, DB2 will not consider reordering these two parts. The
correlated subquery will always be performed after accessing T1 to get the
correlation value. If T1 is a large table, and T2 is a small table, it may be much
more efficient to access T2 first and then T1 (especially if there is no index on
T2.C1 but here is an index on T1.C1).
In summary, Global Query Optimization will allow DB2 to optimize a query as a
whole rather than as independent parts. This is accomplished by allowing DB2
to:
► Consider the effect of one queryblock on another.
► Consider reordering queryblocks.
4.3.2 Cross query block optimization
This new feature addresses query performance problems caused when DB2
breaks a query into multiple parts and optimizes each of those parts
independently. The V9 optimizer treats subqueries as virtual tables, which allows
it to do a cross query block optimization.
4.4 Extended sparse index and in-memory workfile
DB2 9 introduces the new installation parameter MXDTCACH to specify the
maximum memory allocated for data caching. The recommended minimal setting
of MXDTCACH for SAP BI systems is 128 MB. The recommended settings of
new V9 parameters after migration from a V8 SAP system are documented in
SAP note 1032273.
Chapter 4. Benefits of DB2 9 for SAP BI 49