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

Figure 3-13 Regrouping on MQT result
In this example the populated MQT661 has 1808 rows as the result of joining two tables and
grouping by BRN,PST. Explain was executed for query number 662 as shown in Example 3-5.
Example 3-5 Explain for Q662
** EXPLAIN ALL SET QUERYNO=662 FOR
SELECT BRN, SUM(OFC),COUNT(DISTINCT PLC.POL),
COUNT(COV.OFC)
FROM COV,PLC
WHERE
COV.POL = PLC.POL AND COV.CVT='B'
GROUP BY BRN
ORDER BY BRN;
The PLAN_TABLE for V8 in Figure 3-13 shows that the DB2 Optimizer has selected the MQT.
The result is a reduction in elapsed time from 45.1 sec. to 0.92 sec. We see a significant
reduction even if we compare it with V7 with parallelism.
The new access path using MQT regroups 1808 rows avoiding the nested loop join of one
table having 1.7 million rows with a table with 624 thousand rows.
Summary of measurements of a query rewrite for simple query:
► V7 DEGREE(1)
45.1 sec. elapsed time and 12.92 sec. CPU time
► V7 DEGREE(ANY)
9.04 sec. elapsed time and 13.87 sec. CPU time
50 DB2 UDB for z/OS Version 8 Performance Topics
Re-grouping on MQT results
CREATE TABLE MQT661 (BRN,PST,COUNT,SUM) AS (
SELECT BRN.PST,COUNT(DISTINCT PLC.POL),
SUM(COV.OFC)
FROM COV, PLC
WHERE COV.POL=PLC.POL
AND COV.CVT= 'B'
GROUP BY BRN,PST)
DATA INITIALLY DEFERRED
REFRESH DEFERRED
MAINTAINED BY USER
ENABLE QUERY OPTIMIZATION;
V7 PLAN_TABLE
TABLE
SIZE
TNAME TABLE_TYPE METHOD
1.7 M
624 K
COV
PLC
T
T
0
1
V8 PLAN_TABLE
TNAME TABLE_TYPE METHOD
MQT661 M 0
1808 rows in MQT
50
40
30
20
10
0
V7 degree=1
V7 degree=any
V8 using MQT
47X
Elapsed sec CPU sec
117
X