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

3.4.3 Performance
► char/varchar(n1)-> char/varchar(n2)
– where n1>n2 and not equal predicate
► graphic/vargraphic(n1) -> graphic/vargraphic(n2)
– where n1>n2 and not equal predicate
► char/varchar(n1) -> graphic/vargraphic(n2)
– where n1>n2 and not equal predicate
Performance measurements were made to show the type of benefits that can be achieved
using mismatched numeric data types and mismatched string types in predicates that were
stage 2 in V7 and are stage 1 in V8.
Mismatched numeric data types
The measured query is a count of the rows of a result set of a SELECT joining two tables
where the columns in the join predicate are INTEGER and DEC(8).
Figure 3-27 shows the SQL text executed in V7 (with DEGREE(1) and DEGREE(ANY)) and
V8, the PLAN_TABLE indicating the usage of an index to execute the join, and the measured
elapsed and CPU times. The tables used here are the same as in Figure 3-13 on page 50.
Mismatched numeric data types
SELECT COUNT(*) FROM CV, PL
WHERE PL.LP = 19861025
AND CV.CS = PL. LP;
Data type of LP is DEC(8)
Index PLIX on LP column
Data type of CS is INT
Index CVIX on CS column
V7 PLAN_TABLE
IXNAME METHOD MATCHCOL
CVIX
PLIX
Figure 3-27 Mismatched numeric data types
0
1
V8 PLAN_TABLE
IXNAME METHOD MATCHCOL
PLIX
CVIX
0
1
0
1
1
1
Elapsed sec CPU sec
The columns involved in this equi-join have different numeric data types: Column LP is
DEC(8) and column CS is INT.
V7 shows that table CV is joined with table PL using the nested loop join method. PL is
accessed via a one column matching index only access. CV is accessed via a non-matching
index only access that results in reading all leaf pages in the index.
9
8
7
6
5
4
3
2
1
0
V7 DEGREE(1)
V7 DEGREE(ANY)
V8 DEGREE(1)
6X
16
X
Chapter 3. SQL performance 69