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

But not all mismatched numeric types become stage 1. The exceptions that are still stage 2
predicates are:
► REAL -> DEC(p,s) where p >15
where REAL is the right hand side predicate -> DEC(p,s) is the left hand side predicate
► FLOAT -> DEC(p,s) where p >15
– REAL or FLOAT column in the outer table of a join
– REAL or FLOAT literal or host variable
DEC_column > REAL_hostvar
REAL and FLOAT values cannot be converted to decimal (the index column) with precision >
15 without possibly changing the collating sequence.
Mismatched string types
Figure 3-26 shows examples of mismatched string types that were stage 2 in V7 and are
stage 1 and indexable in V8 CM.
Example of mismatched string types
SELECT * FROM employee
WHERE deptid = :HV ;
Figure 3-26 Example of mismatched string types
In the example, the selection of rows from the employee table has to scan the table space in
V7 because the column DEPTID is defined as CHAR(3) and is compared to a string of four
characters. This is a stage 2 predicate in DB2 V7.
The same predicate is stage 1 in V8 and it allows a matching index scan.
Not all mismatched string types become stage 1. The exceptions that are still stage 2
predicates are:
► graphic/vargraphic -> char/varchar
where, for example, graphic is the right hand side predicate -> char is the left hand side
predicate
68 DB2 UDB for z/OS Version 8 Performance Topics
SELECT * FROM employee
WHERE deptid = 'M60A' ;
in V7
or
Stage-2 predicate
Tablespace scan
CHAR(3)
V8
:HV > 3 bytes
Stage-1 predicate
Could use index on
deptid column