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

the materialized query table. If it is, then the materialized query table remains a candidate for
query rewrite.
If the query Q contains predicates that are not identical to predicates in the materialized query
table subselect, they are examined to determine if column references to base table columns
can derive values from materialized query table columns instead. This takes place during the
expression derivation step.
If the contents of a materialized query table overlap with the contents of a query, the query
and the materialized query table are said to match.
If there are multiple materialized query tables that match the query but cannot be used
simultaneously, heuristic rules are used to select one of them. If the query rewrite process is
successful, DB2 determines the cost and access path of the new query. Only if the cost of the
new query is less than the cost of the original query is the new query substituted.
Query rewrite example
A simple example of query rewrite is depicted in Figure 3-8. If MQT1 is selected by automatic
query rewrite for query RefQ, this means that:
► MQT1 is enabled for query optimization by CURRENT REFRESH AGE and CURRENT
MAINTAINED TABLE TYPES
► MQT1 is populated as discussed before
► Query optimization is enabled
► Tables referenced in the query RefQ match (same tables) the source tables in MQT1
► The predicates in MQT1 match (same predicates) with predicates in the query RefQ
► The GROUP BY clause in the query RefQ is a subset (same GROUP BY) of the rows that
are in MQT.
Fact
Store
RefQ
Figure 3-8 Query rewrite example
44 DB2 UDB for z/OS Version 8 Performance Topics
Query rewrite example
prod_id time_id store_id ... ... sale_amt
( reference query)
select S.city_id,
sum(F.sale_amt)
from Fact F, Store S
where F.store_id = S.store_id
group by S.city_id
QRW
(query rewrite)
store_id .... city_id city_name
MQT1
select A.cityid,
A.sm_sale
from MQT1 A
select S.city_id,
sum(F.sale_amt) as sm_sale
from Fact F, Store S
where F.store_id = S.store_id
group by S.city_id