PM38935: PARALLELISM ADDS LIST PREFETCH TO AN ACCESS PATH EVEN THOUGH TOO MANY RIDS QUALIFY RESULTING IN RIDPOOL OVERFLOW AND POOR PERFM

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APAR status

  • Closed as program error.

Error description

  • If too many RIDS qualify the index filtering (IMFFADJ), then
    parallelism should not change the access type to LIST PREFETCH.
    Parellelism may add LIST PREFETCH to an access path even though
    too many rids qualify possibly resulting in a ridpool overflow,
    and a fallback to rscan.
    

Local fix

  • n/a
    

Problem summary

  • ****************************************************************
    * USERS AFFECTED: Users running statements with equal          *
    *                 matching index scan with list prefetch       *
    *                 on the leading table when parallelism        *
    *                 is enabled.                                  *
    ****************************************************************
    * PROBLEM DESCRIPTION: If a statement uses equal matching      *
    *                      index scan on the leading table,        *
    *                      an inefficient LIST PREFETCH might be   *
    *                      chosen with parallelism enabled         *
    *                      even when the RID pool size is not big  *
    *                      enough for processing the estimated     *
    *                      RIDs.                                   *
    ****************************************************************
    * RECOMMENDATION:                                              *
    ****************************************************************
    If parallelism is enabled for statements using equal matching
    index scan on the leading table, DB2 could possibly choose
    an inefficient LIST PREFETCH plan for the index scan even when
    the RID pool size is not big enough for processing the
    estimated RIDs.
    

Problem conclusion

  • If parallelism is enabled for statements using equal matching
    index scan on the leading table, DB2 now will not choose LIST
    PREFETCH for the index scan when the RID pool size is not big
    enough for processing the estimated RIDs.
    
    
    ADDITIONAL KEYWORDS:
    PARALLELISM SQLPARALLELISM SQLCPUP SQLACCESSPATH
    

Temporary fix

Comments

APAR Information

  • APAR number

    PM38935

  • Reported component name

    DB2 OS/390 & Z/

  • Reported component ID

    5740XYR00

  • Reported release

    910

  • Status

    CLOSED PER

  • PE

    NoPE

  • HIPER

    NoHIPER

  • Special Attention

    NoSpecatt

  • Submitted date

    2011-05-11

  • Closed date

    2011-07-15

  • Last modified date

    2011-08-01

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

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

    UK69792 UK69793

Modules/Macros

  •    DSNXOGP2
    

Fix information

  • Fixed component name

    DB2 OS/390 & Z/

  • Fixed component ID

    5740XYR00

Applicable component levels

  • RA10 PSY UK69792

       UP11/07/30 P F107

  • R910 PSY UK69793

       UP11/07/30 P F107

Fix is available

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Document information


More support for:

DB2 for z/OS

Software version:

910

Reference #:

PM38935

Modified date:

2011-08-01

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