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DB2 - Problembeschreibung

Problem IC71737 Status: Geschlossen

POSSIBLE NLJN QUERY PERFORMANCE ISSUE FROM UNDER COSTED NLJN DUE TOBAD
INNER PAGE ESTIMATE

Produkt:
DB2 FOR LUW / DB2FORLUW / 950 - DB2
Problembeschreibung:
When examining an Query Access Plan with a Nested Loop Join, 
usually the cost for the Nested Loop Join will be at least the 
sum of the cost of the outer Query Access Plan and the inner 
Query Access Plan. You may be experiencing a poorly performing 
Query Access Plan that is caused by this APAR if the number of 
I/O's associated with the Nested Loop Join is only a few I/Os 
more then the number of I/Os from the outer Query Access Plan, 
but the inner Query Access Plan has more then a few I/Os 
indicated (typically 1 or 2 I/Os, but it could be more). 
 
The db2exfmt plan segment below shows a Nested Loop Join 
(NLJOIN) with an outer I/O of 0 I/Os, an inner I/O of 91,252 
I/Os, but the Nested Loop Join I/Os of only 3. In this case the 
NLJOIN I/Os should have been at least 91,252 I/Os or close to 
91,252 I/Os. 
 
                     715402 
                     NLJOIN 
                     (  23) 
                     7367.04 
                     3.00784          <<< I/Os too small 
            /-----------+----------\ 
          1                        715402 
       BTQ                         FETCH 
       (  24)                      (  26) 
      0.0678342                    114167 
          0                         91252 
          |                      /-----+-----\ 
          1              1.42166e+06      8.52996e+06 
       IXSCAN          IXSCAN       TABLE: DW 
       (  25)              (  27)            FACT 
      0.0174534        19006.1            Q1 
          0                 2376 
         |                   | 
          6              8.52996e+06 
   INDEX: DW           INDEX: DW 
       IDX1                  XIF1 
         Q7                  Q1
Problem-Zusammenfassung:
**************************************************************** 
* USERS AFFECTED:                                              * 
* All Users                                                    * 
**************************************************************** 
* PROBLEM DESCRIPTION:                                         * 
* POSSIBLE NLJN QUERY PERFORMANCE ISSUE FROM UNDER COSTED NLJN * 
* DUE TO BAD INNER PAGE ESTIMATE                               * 
**************************************************************** 
* RECOMMENDATION:                                              * 
* Upgrade the server to DB2 for Linux, Unix, Windows,  Version * 
* 9.5 Fix Pack 7 or later                                      * 
****************************************************************
Local-Fix:
One Possible workaround that *may* work is to get detailed stats 
for the columns (frequent value & distribution stats). This will 
cause the join page selectivity code to use a different 
algorithm to compute the filtering.
Lösung
The problem is corrected in DB2 for Linux, Unix, Windows, 
 
Version 9.5 Fix Pack 7.
Workaround
keiner bekannt / siehe Local-Fix
Weitere Daten
Datum - Problem gemeldet    :
Datum - Problem geschlossen :
Datum - der letzten Änderung:
06.10.2010
21.12.2010
21.12.2010
Problem behoben ab folgender Versionen (IBM BugInfos)
9.5.FP7
Problem behoben lt. FixList in der Version