|
Problem IC62996
|
Status: Closed |
STATISTICAL VIEW MIGHT NOT HELP TO FIX CARDINALITY UNDERESTIMATION UNDER CERTAIN CONDITIONS |
| product: |
DB2 FOR LUW / DB2FORLUW / 950 - DB2 |
| Problem description: |
The DB2 Query Optimizer may misestimate the cardinality for
queries that contain effectively cartesian predicates and use
statistical views that don't reference these predicates. For
example, consider the following query:
select * from T1, T2
where T1.A=T2.A and T1.B=1 and T2.B=1
The two predicates T1.B=1 and T2.B=1 imply a third predicate
T1.B=T2.B. Thus, at the join operation, the join on T1.B=T2.B is
effectively a cartesian product of two result sets that have all
1s in the join columns. If there is an applicable statistical
view containing the predicate T1.A=T2.A and not T1.B=T2.B, then
the optimizer may misestimate the cardinality of the join of T1
and T2, which may result in a sub-optimal query execution plan. |
| Problem Summary: |
USERS AFFECTED: All
PROBLEM DESCRIPTION:
see ERROR DESCRIPTION
PROBLEM SUMMARY:
see ERROR DESCRIPTION |
| Local Fix: |
Try to use SELECTIVITY clause to adjust cardinality. |
| available fix packs: |
DB2 Version 9.5 Fix Pack 6a for Linux, UNIX, and Windows
DB2 Version 9.5 Fix Pack 7 for Linux, UNIX, and Windows
DB2 Version 9.5 Fix Pack 8 for Linux, UNIX, and Windows
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 |
The complete fix for this problem first appears in DB2 UDB
Version 9.5 FixPak 6. |
| Workaround |
not known / see Local fix |
| BUG-Tracking |
forerunner : APAR is sysrouted TO one or more of the following: IC68432
follow-up : |
| Timestamps |
Date - problem reported :
Date - problem closed :
Date - last modified :
| 07.09.2009
14.05.2010
14.05.2010 |
| Problem solved at the following versions (IBM BugInfos) |
9.5.
|
| Problem solved according to the fixlist(s) of the following version(s) |
