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DB2 - Problem description

Problem IC78195 Status: Closed

DB2 OPTIMIZER CAN ESTIMATE A TOO SMALL IO COST WHEN RANGE PREDICATES ARE
APPLIED BY AN IXSCAN

product:
DB2 FOR LUW / DB2FORLUW / 950 - DB2
Problem description:
When a range predicate or an equality predicate that is applied 
by an IXSCAN is preceded by another range predicate, DB2 
optimize can estimate a too small IO cost. The problem is more 
likely to occur when the range predicate involves parameter 
marks or host variables. 
 
For example, assume we have the following index 
 
create index idx1 on t1 (c1, c2) 
 
Further assume we have predicates 
 
c1 <= ? and 
c1 >=? and 
c2 = 'a' 
 
If these predicates are applied by an IXSCAN on idx1, then the 
IO cost estimation of the IXSCAN can be too small. This can 
result in sub-optimal query plans. 
 
In db2exfmt output, one can observe the following: 
 
 
    1.93755e+06 
      IXSCAN 
      (   6) 
      19622.5 
         3 
        | 
    1.93755e+08 
  INDEX: SCHM1 
     IDX1 
 
Note that the number of rows out of the IXSCAN is 1.93755e+06, 
but the number of IOs is only 3.
Problem Summary:
**************************************************************** 
* USERS AFFECTED:                                              * 
* All Platforms                                                * 
**************************************************************** 
* PROBLEM DESCRIPTION:                                         * 
* DB2 OPTIMIZER CAN ESTIMATE A TOO SMALL IO COST WHEN RANGE    * 
* PREDICATES ARE APPLIED BY AN IXSCAN                          * 
**************************************************************** 
* RECOMMENDATION:                                              * 
* Upgrade to DB2 v9.5 Fixpack 9 or higher                      * 
****************************************************************
Local Fix:
available fix packs:
DB2 Version 9.5 Fix Pack 9 for Linux, UNIX, and Windows
DB2 Version 9.5 Fix Pack 10 for Linux, UNIX, and Windows

Solution
First Fixed in DB2 v9.5 Fixpack 9
Workaround
not known / see Local fix
Timestamps
Date  - problem reported    :
Date  - problem closed      :
Date  - last modified       :
18.08.2011
07.03.2012
07.03.2012
Problem solved at the following versions (IBM BugInfos)
9.5.FP9
Problem solved according to the fixlist(s) of the following version(s)
9.5.0.9 FixList
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