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IZ50527: OPTIMIZER MAY STILL GENERATE A SORT FOR JOINS WHEN SETTING EITHE R NO_SORT_NLJOIN OR NO_SORT_MGJOIN FOR DB2_REDUCED_OPTIMIZATION

Fixes are available

DB2 Version 9.1 Fix Pack 8  for Linux, UNIX and Windows
DB2 Version 9.1 Fix Pack 9  for Linux, UNIX and Windows
DB2 Version 9.1 Fix Pack 10  for Linux, UNIX and Windows
DB2 Version 9.1 Fix Pack 11  for Linux, UNIX and Windows
DB2 Version 9.1 Fix Pack 12  for Linux, UNIX and Windows

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APAR status

  • Closed as program error.

Error description

  • When the DB2_REDUCED_OPTIMIZATION registry variable is set to
NO_SORT_NLJOIN, the optimizer may still generate query plans
that force sorts for nested loop joins (NLJN).  Likewise, when
the DB2_REDUCED_OPTIMIZATION registry variable is set to
NO_SORT_MGJOIN, the optimizer may still generate query plans
that force sorts for merge scan joins (MSJN).
.
The DB2 Information Center contains futher information about the
DB2_REDUCED_OPTIMIZATION registry variable:
http://publib.boulder.ibm.com/infocenter/db2luw/v9r5/index.jsp?t
opic=/com.ibm.db2.luw.admin.regvars.doc/doc/r0005664.html
.
This problem does not occur if both NO_SORT_NLJOIN and
NO_SORT_MGJOIN are set.
.

Local fix

  • In order to workaround the problem, you can set the registry
setting to both, eg:
.
db2set DB2_REDUCED_OPTIMIZATION=NO_SORT_NLJOIN,NO_SORT_MSJOIN
.
These types of sorts can be useful for improving performance.
Care must be take when considering the use of the NO_SORT_NLJOIN
and NO_SORT_MGJOIN options, as performance can be severely
impacted.

Problem summary

  • When the DB2_REDUCED_OPTIMIZATION registry variable is set to
NO_SORT_NLJOIN, the optimizer may still generate query plans
that force sorts for nested loop joins (NLJN).  Likewise, when
the DB2_REDUCED_OPTIMIZATION registry variable is set to
NO_SORT_MGJOIN, the optimizer may still generate query plans
that force sorts for merge scan joins (MSJN).
The DB2 Information Center contains futher information about the
DB2_REDUCED_OPTIMIZATION registry variable:
http://publib.boulder.ibm.com/infocenter/db2luw/v9r5/index.jsp?t
opic=/com.ibm.db2.luw.admin.regvars.doc/doc/r0005664.html
.
This problem does not occur if both NO_SORT_NLJOIN and
NO_SORT_MGJOIN are set.

Problem conclusion

  • apar: iz50527
module: engn_sqno
fixed >= v91 fpk8

Temporary fix

  • In order to workaround the problem, you can set the registry
setting to both, eg:
.
db2set DB2_REDUCED_OPTIMIZATION=NO_SORT_NLJOIN,NO_SORT_MSJOIN
.
These types of sorts can be useful for improving performance.
Care must be take when considering the use of the NO_SORT_NLJOIN
and NO_SORT_MGJOIN options, as performance can be severely
impacted.

Comments

  • 



APAR Information




    

  • APAR number
    IZ50527
    • 

  • Reported component name
    DB2 UDB ESE SOL
    • 

  • Reported component ID
    5765F4102
    • 

  • Reported release
    910
    • 

  • Status
    CLOSED  PER
    • 

  • PE
    NoPE
    • 

  • HIPER
    NoHIPER
    • 

  • Special Attention
    NoSpecatt
    • 

  • Submitted date
    2009-05-01
    • 

  • Closed date
    2009-12-02
    • 

  • Last modified date
    2009-12-02
    • 





    

  • APAR is sysrouted FROM one or more of the following:
    

    • 

  • APAR is sysrouted TO one or more of the following:
    

    
IZ50528 IC64042
    

    • 



Modules/Macros


    

  • ENG_SQNO

    



Fix information


    

  • Fixed component name
    DB2 UDB ESE SOL
    • 

  • Fixed component ID
    5765F4102
    • 



Applicable component levels


    

  • R910  PSN
       UP
    • 








      
              
                            

              

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Document Information


            

            


                        

            Modified date:
            

            02 December 2009
            

            
                      

        


        

        


      

    

  



    

  

  
    
            

          
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