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

Better filter factor estimation
V7 PLAN_TABLE
IXNAME METHOD MATCHCOL
TSS on PLO
CLAIX
CLBIX
BLAIX
0
1
4
4
Figure 3-30 Better filter factor estimation
0
2
1
1
V8 PLAN_TABLE
IXNAME METHOD MATCHCOL
TSS on PLO
BLAIX
CLBIX
CLAIX
0
1
1
1
0
1
This figure shows a join of four tables and two join predicates between tables PLO and CLA.
The plan table for V7 shows a join of table PLO and CLA using NLJ that is followed by a join
to table CLB and BLA using hybrid join.
In DB2 V8, the same queries allow the DB2 optimizer to find a better access path applying
earlier a better filtering factor. The sequence of join of four tables changes: table BLA, that
was processed last, becomes the second table scanned using NLJ, as the tables that follow.
In DB2 V8, the hybrid join method is not used.
The measurements of this example show:
► Elapsed time = 56.4 seconds in V7 to 19.1 seconds in V8 (66% reduction, a factor of 3)
► CPU time = 39.1 seconds in V7 to 12.4 seconds in V8 (68% reduction, a factor of 3.2)
The improvement ratio can be even more significant in complex queries involving more tables
and more predicates.
Access path improvements - multi-column statistics
Measurements on other queries from the sample workload show improvements in CPU time
resulting from a better access path as shown in Figure 3-31.
60
50
40
30
20
10
V7 DEGREE(1)
V8 DEGREE(1)
3X
1
0
2 Elapsed sec CPU sec
3.2X
Chapter 3. SQL performance 75