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

V8 shows that PL is joined with CV also using nested loop join method. But the sequence is
different. Both tables are accessed via a one-column matching index only scan, but the
access path in V8 is more efficient.
Other measurement indicators are listed in Table 3-2.
Table 3-2 Measurements for mismatched numeric data types
Measurement V7 V8 Delta
Elapsed time in sec. 7.9 1.4 -82%
CPU time in sec. 5.8 0.3 -95%
Work file getpages 0 0 0
Data getpages 77 43 -44%
Index getpages 2558 8 -99.7%
Matching index 1 2 100%
Mismatched string data types
This measured query is a join of two tables where the local predicates have different lengths
as shown in Figure 3-28.
Figure 3-28 Different lengths for string data type
The predicate comparing strings of different length becomes stage 1, and then the DB2
Optimizer changes the sequence of tables in the join to do an earlier filtering, resulting in
better performance.
70 DB2 UDB for z/OS Version 8 Performance Topics
Different length for string data type
SELECT BRN.REG,PLC.POL,LEN,PLC.BRA,
CMA.SUR,TRC,CST,COV.CVI,LPC,CSC,CVT
FROM BRN,PLC,COV,CIA,CMA,CLB,CCT,CAG
WHERE PLC.BRN = BRN.BRN
AND PLC.POL = COV.POL
AND PLC.POL = CIA.POL
AND PLC.POL = CAG.POL
AND COV.POL = CIA.POL
AND COV.POL = CAG.POL
AND COV.CVI = CIA.CVI
AND CIA.CLI = CLB.CLI
AND CAG.CMA = CMA.CMA
AND COV.CVC = CCT.CVC
AND CST = 'MAILD ' /* CHAR(5) CHAR(6) */
AND COV.CVT IN ('A ','B ','G ','P ')
ORDER BY REG,PLC.POL,PLC.BRN,SUR;
V7 PLAN_TABLE
IXNAME METHOD MATCHCOL
COVIX
PLCIX
0
1
V8 PLAN_TABLE
IXNAME METHOD MATCHCOL
CCTT
COVIM
0
1
0
1
0
1
12
10
8
6
4
2
0
9.6X
V7 DEGREE(1)
V8 DEGREE(1)
40 X
Elapsed sec CPU sec