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

05 SERIAL-NUMBER PIC S9(9) COMP-4 OCCURS 10 TIMES.
Tip: Verify that the PTFs for APARs PQ91898, PQ93620, PQ89181, and PQ87969 are
applied:
► PTF UQ92546 for performance APAR PQ91898 reduces excessive lock and getpage
requests in multi-row insert.
► PTF UQ96567 for performance APAR PQ93620 reduces CPU overhead during
multi-row cursor delete.
► PTFs UQ90464 and UQ92692, respectively for APAR PQ89181 and PQ87969, reduce
CPU usage by providing improved lock management.
PTF UK06848 for APAR PQ99482 provides QMF for TSO/CICS 8.1 with the optional
support of multi-row fetch and insert.
3.2 Materialized query table
For many years, the optimization for decision-support queries has been a difficult task,
because such queries typically operate over huge amounts of data (1 to 10 terabytes),
performing multiple joins and complex aggregation.
When working with the performance of a long running query executed via dynamic SQL,
especially in Data Warehousing, involving joins of many tables, DBAs often created summary
tables manually in order to:
► Improve the response time
► Avoid the redundant work of scanning (sometimes in the DB2 work file), aggregation and
joins of the detailed base tables (history)
► Simplify SQL to be coded
However, you need to be aware of the existence of the DBA’s summary tables and make a
decision whether to use them or the base tables depending on the query. Another issue is
setting up the synchronization of these summary tables with base tables.
A materialized query table (MQT) is a table containing materialized data derived from one or
more source tables specified by a fullselect to satisfy long running queries requiring good
response time (minutes or even seconds).
The characteristics of MQTs are:
► Source tables for MQTs can be base tables, views, table expressions or user-defined table
expressions.
► MQTs can be chosen by the optimizer to satisfy dynamic query (through automatic query
rewrite) when a base table or view is referenced. No query rewrite is done for static SQL,
you need to address the MQT directly.
► MQTs are used to precompute once expensive joins, sorts or aggregations. They can be
reused many times to improve query performance.
► MQTs can be accessed directly by static and dynamic SQL.
Materialized query tables are also known as automatic summary tables in other platforms.
Chapter 3. SQL performance 39