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Problem IC62996
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Status: Geschlossen |
STATISTICAL VIEW MIGHT NOT HELP TO FIX CARDINALITY UNDERESTIMATION UNDER CERTAIN CONDITIONS |
| Produkt: |
DB2 FOR LUW / DB2FORLUW / 950 - DB2 |
| Problembeschreibung: |
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-Zusammenfassung: |
USERS AFFECTED: All
PROBLEM DESCRIPTION:
see ERROR DESCRIPTION
PROBLEM SUMMARY:
see ERROR DESCRIPTION |
| Local-Fix: |
Try to use SELECTIVITY clause to adjust cardinality. |
| verfügbare FixPacks: |
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
|
| Lösung |
The complete fix for this problem first appears in DB2 UDB
Version 9.5 FixPak 6. |
| Workaround |
keiner bekannt / siehe Local-Fix |
| Bug-Verfolgung |
Vorgänger : APAR is sysrouted TO one or more of the following: IC68432
Nachfolger : |
| Weitere Daten |
Datum - Problem gemeldet :
Datum - Problem geschlossen :
Datum - der letzten Änderung:
| 07.09.2009
14.05.2010
14.05.2010 |
| Problem behoben ab folgender Versionen (IBM BugInfos) |
9.5.
|
| Problem behoben lt. FixList in der Version |
