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JR34918: OPTIMIZER UNEXPECTEDLY FAVOURS THE REVERSE ISCAN

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APAR status

  • Closed as program error.

Error description

  • Performance degradation due to reverse iscan over index with 3
    or more levels (nlevels >= 3).
    

Local fix

  • db2set DB2_EXTENDED_OPTIMIZATION=NO_REVSCAN_JOIN
    

Problem summary

  • ****************************************************************
    * USERS AFFECTED:                                              *
    * ALL                                                          *
    ****************************************************************
    * PROBLEM DESCRIPTION:                                         *
    * Performance degradation due to reverse iscan on an index     *
    * with NLEVEL > 2.                                             *
    *                                                              *
    * When the number of levels in an index is more then 2, the    *
    * optimizer may pick an Index Scan in the REVERSE direction.   *
    * The reverse index scan can lead to problems with prefetching *
    * of table pages into the buffer pool, if there is an          *
    * associated with FETCH with the IXSCAN, and/or it can lead to *
    * problems with locking. The use of a reverse IXSCAN can be    *
    * confirmed by examining the Query Access Plan through one of  *
    * the explain facilities.                                      *
    ****************************************************************
    * RECOMMENDATION:                                              *
    * Upgrade to Version 9.5 Fix Pack 5.                           *
    ****************************************************************
    

Problem conclusion

  • First fixed in Version 9.5 Fix Pack 5. At a minimum, this fix
    should be applied on the server.
    

Temporary fix

  • db2set DB2_EXTENDED_OPTIMIZATION=NO_REVSCAN_JOIN
    

Comments

APAR Information

  • APAR number

    JR34918

  • Reported component name

    DB2 UDB ESE WIN

  • Reported component ID

    5765F4101

  • Reported release

    950

  • Status

    CLOSED PER

  • PE

    NoPE

  • HIPER

    NoHIPER

  • Special Attention

    NoSpecatt

  • Submitted date

    2009-11-19

  • Closed date

    2010-02-24

  • Last modified date

    2010-02-24

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

    IZ25720

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

Fix information

  • Fixed component name

    DB2 UDB ESE WIN

  • Fixed component ID

    5765F4101

Applicable component levels

  • R950 PSN

       UP

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Document Information

Modified date:
24 February 2010