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DB2 - Problembeschreibung

Problem IC96826 Status: Geschlossen

DB2 OPTIMIZER MIGHT NOT CHOOSE A ONE-FETCH GROUP BY WHEN CARDINALITY IS
UNDERESTIMATED

Produkt:
DB2 FOR LUW / DB2FORLUW / A10 - DB2
Problembeschreibung:
The optimizer will consider an optimization for an SQL statement 
containing a MIN or MAX aggregation without a group by clause, 
referred to as a one-fetch group by operation, if the stream is 
ordered on the aggregation source column. You can identify if 
the group by operation is optimized as a one-fetch by collecting 
an EXPLAIN of the statement and searching for the ONEFETCH 
argument in the group by operator details: 
 
                ONEFETCH: (One Fetch flag) 
                        TRUE 
 
If the input cardinality is significantly underestimated to 
value of 1 or less, then the optimizer might not choose to 
perform this optimization, and the ONEFETCH argument will not 
exist in the formatted EXPLAIN output.
Problem-Zusammenfassung:
**************************************************************** 
* USERS AFFECTED:                                              * 
* ALL                                                          * 
**************************************************************** 
* PROBLEM DESCRIPTION:                                         * 
* DB2 OPTIMIZER MIGHT NOT CHOOSE A ONE-FETCH GROUP BY WHEN     * 
* CARDINALITY IS UNDERESTIMATED                                * 
**************************************************************** 
* RECOMMENDATION:                                              * 
* Upgrade to DB2 V10 fixpack 4.                                * 
****************************************************************
Local-Fix:
In most cases, collecting up-to-date statistics will improve the 
cardinality estimate if the SQL statement contains predicates 
that qualify the latest data in the table and the statistics are 
stale.
verfügbare FixPacks:
DB2 Version 10.1 Fix Pack 4 for Linux, UNIX, and Windows
DB2 Version 10.1 Fix Pack 6 for Linux, UNIX, and Windows

Lösung
First Fixed in DB2 V10 FP4.
Workaround
In most cases, collecting up-to-date statistics will improve the 
cardinality estimate if the SQL statement contains predicates 
that qualify the latest data in the table and the statistics are 
stale.
Weitere Daten
Datum - Problem gemeldet    :
Datum - Problem geschlossen :
Datum - der letzten Änderung:
11.10.2013
10.06.2014
10.06.2014
Problem behoben ab folgender Versionen (IBM BugInfos)
Problem behoben lt. FixList in der Version
10.1.0.4 FixList