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

Figure 3-12 Query rewrite for join
Query rewrite for join
CREATE TABLE MQT201 (CRE,INT,COUNT) AS(
SELECT CRE,INT,COUNT(*)
FROM CLA,COI,CIA
WHERE COI.POL=CIA.POL AND
COI.CVI=CIA.CVI AND
COI.CLI=CIA.CLI AND
CIA.CLI=CLA.CLI AND
CIA.CAI=CLA.CAI
GROUP BY CRE,INT)
DATA INITIALLY DEFERRED
REFRESH DEFERRED
MAINTAINED BY USER
ENABLE QUERY OPTIMIZATION;
V7 PLAN_TABLE
TABLE
SIZE
TNAME TABLE_TYPE METHOD
633 K
1.5 M
1.3 M
COI
CIA
CLA
V8 PLAN_TABLE
T
T
T
0
1
1
TNAME TABLE_TYPE METHOD
MQT201 M 0
171 rows in MQT
Elapsed sec CPU sec
GROUP BY and predicate matching
The MQT remains a candidate for query rewrite if the query GROUP BY clause results in a
subset of the rows that are in the MQT
The predicate in the query subsumes the predicate in the MQT. Example: C1>8 subsumes
C1>0.
The predicates in the query should be coded as those in the MQT subselect. Otherwise,
matching may fail on some complex predicates.
Figure 3-13 shows the impact of regrouping the MQT data when DB2 obtains the final answer
set in the query.
30
25
20
15
10
5
0
V7 DEGREE(1)
V7 DEGREE(ANY)
V8 using MQT
31X
236
X
Chapter 3. SQL performance 49