DB2 UDB for z/OS Version 8 Performance Topics - IBM Redbooks

Chapter 3. SQL performance
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In this chapter we discuss the major performance enhancements that affect SQL. The chapter
contains the following sections:
► Multi-row FETCH, INSERT, cursor UPDATE and DELETE
Multi-row operations can be executed in a single SQL statement explicitly to process a
rowset. It is used implicitly by default in the distributed environment as described in
Chapter 7, “Networking and e-business” on page 281. The DSNTEP4 sample is similar to
DSNTEP2 and exploits multi-row fetch as described in Chapter 9, “Installation and
migration” on page 341.
► Materialized query table
Materialized query tables (MQT) can be created in NFM as a pre-computed join, sort or
aggregation and re-used by DB2 query rewrite in dynamic SQL to improve the execution
of long running queries in the Data Warehouse type environment.
► Star join processing enhancements
The execution of star join is improved by: Access path enhancements, better estimates of
the filtering effect of dimensions, the use of sparse index in cache, and the use of
in-memory work file.
► Stage 1 and indexable predicates
The predicates comparing mismatched string data types or comparing mismatched
numeric data types that are Stage 2 in DB2 V7 can become Stage 1 and indexable in DB2
Version 8. This benefit can be obtained by rebinding the plan or package.
► Multi-predicate access path enhancement
Queries which contain two or more predicates referencing a table may get a better access
path with statistics for a group of columns (non-indexed) collected by RUNSTATS.
► Multi-column sort merge join
Access path selection allows multi-column predicates for sort merge join (SMJ) with
mismatched string data types; decides to sort new table based on cost and enables query
parallelism for multi-column sort merge join (MCSMJ).
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