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

3.4.1 New indexable predicates in V7
PTF UQ61237 for APAR PQ54042 resolves a case of incorrect output resulting from SQL
statements that contain a BETWEEN predicate. The type of BETWEEN predicate that may
have this problem is:
col BETWEEN col-expression AND col-expression
Where “col” is a simple column and “col-expression” is any expression involving a column.
In addition, BETWEEN predicates of the form:
T1.COL BETWEEN T2.COL1 AND T2.COL2
can now be indexable when accessing the table on the left hand side of the BETWEEN
keyword if the tables on the right hand side of the BETWEEN keyword have already been
accessed.
3.4.2 More stage 1 and indexable predicates in V8
In V8, more predicates with mismatched data types and mismatched lengths can be applied
by DB2. This allows some stage 2 predicates to become stage 1 predicates, and stage 1
predicates allow more index usage, and it applies to both join and local predicates.
The potential for performance improvement with no application change is very significant.
Note that mismatched string data types in join became stage 1 in V6.
Mismatched numeric types
Figure 3-25 shows an example of a predicate using mismatched numeric types that is stage 2
in V7 and becomes stage 1 and indexable in V8 NFM.
In the example, the selection of rows from employee table has to scan the table space in V7
because the predicate indicates salary defined as decimal (12,2) is compared to a host
variable defined as floating point that is stage 2.
The same predicate is stage 1 in V8 and it allows matching index scan to find the rows for the
execution of the SQL.
Example of mismatched numeric type
employee ( name character (20),
salary decimal (12,2),
deptid character (3) );
SELECT * FROM employee
WHERE salary > :hv_float ;
in V7
Stage-2 predicate
Table space scan
Figure 3-25 Example of mismatched numeric types
V8
Stage-1 predicate
salary indexable
Chapter 3. SQL performance 67
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