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

For instance, in a data warehouse environment, data is usually extracted from other sources,
transformed, and loaded into data warehouse tables. In this case the referential integrity
constraints can be maintained and enforced by means other than the database manager to
avoid the serialization and overhead of enforcing them by DB2.
ALTER a base table to MQT
We use the extended ALTER TABLE SQL statement to change an existing base table into an
MQT. With this statement we:
► Specify a fullselect associated with a table
► Specify one of the mechanisms to refresh the table
► Enable/disable an MQT for automatic query rewrite
► Switch between system-maintained and user-maintained types
See Example 3-4.
Example 3-4 Alter a table to MQT
ALTER TABLE TRANSCNT ADD MATERIALIZED QUERY AS (
SELECT ACCTID, LOCID, YEAR, COUNT(*) AS CNT
FROM TRANS
GROUP BY ACCTID, LOCID, YEAR )
DATA INITIALLY DEFERRED
REFRESH DEFERRED
MAINTAINED BY USER;
3.2.2 Populating MQTs
We have mentioned that an MQT can be maintained by system or user.
► System-maintained MQTs (default) are populated with the new REFRESH TABLE option.
This option:
– Deletes all the rows in the MQT
– Executes the fullselect associated with the MQT to recalculate the data
– Inserts the calculated result into the MQT
– Updates the catalog for the refresh timestamp and cardinality of the MQT
– Cannot be updated by load, insert, update and delete
You need to refresh materialized query tables periodically to maintain data currency with
base tables. However, realize that refreshing materialized query tables can be an
expensive process.
► User-maintained MQTs can be populated by:
– The REFRESH TABLE option
– Load, insert, update and delete
3.2.3 Controlling query rewrite
Query rewrite is the process that can use an MQT if the query being optimized by DB2 has
common source tables and all the predicates of the fullselect that make up the MQT.
DB2 uses two special registers for dynamic SQL to control query rewrite:
► CURRENT REFRESH AGE
► CURRENT MAINTAINED TABLE TYPES
Chapter 3. SQL performance 41