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IT28286: POSSIBLE PERFORMACE REGRESSION FOR SELECT DISTINCT SUB-QUERY WHEN MIGRATING TO DB2 V11.1.3

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APAR status

  • Closed as program error.

Error description

  • If a query contains a sub-query with SELECT DISTINCT together
    with other complex operations such as OUTER JOIN and UNION ALL,
    the query performance might be regressed.
    
    Example query:
    
    SELECT t3.c4
    FROM (SELECT t1.c2 FROM t1 LEFT OUTER JOIN t2 ON t1.c1 = t2.c1)
    AS v1(c2), t3
    WHERE v1.c2 = t3.c1 AND t3.c3 IN (SELECT DISTINCT c3 FROM t3
    WHERE c2 = 100)
    
    In v11.1.3, the SELECT DISTINCT sub-query is merged into the
    outer query block and DISTINCT is executed as part of the outer
    SELECT clause.
    
    Prior to V11.1.3, the sub-query is not merged and DISTINCT is
    executed as part of the sub-query.
    

Local fix

  • Workaround: Apply the following registry setting using the
    optimization guideline to the affected queries:
    
    /* <OPTGUIDELINES>
       <REGISTRY>
            <OPTION NAME='DB2COMPOPT' VALUE='NO_EXTDEEPKEYPU'/>
       </REGISTRY>
    </OPTGUIDELINES> */
    

Problem summary

  • ****************************************************************
    * USERS AFFECTED:                                              *
    * ALL                                                          *
    ****************************************************************
    * PROBLEM DESCRIPTION:                                         *
    * See Error Description                                        *
    ****************************************************************
    * RECOMMENDATION:                                              *
    * Upgrade to Db2 11.1 Mod 4 Fixpack 5 or higher                *
    ****************************************************************
    

Problem conclusion

  • First fixed in Db2 11.1 Mod 4 Fixpack 5
    

Temporary fix

Comments

APAR Information

  • APAR number

    IT28286

  • Reported component name

    DB2 FOR LUW

  • Reported component ID

    DB2FORLUW

  • Reported release

    B10

  • Status

    CLOSED PER

  • PE

    NoPE

  • HIPER

    NoHIPER

  • Special Attention

    NoSpecatt / Xsystem

  • Submitted date

    2019-03-04

  • Closed date

    2020-01-16

  • Last modified date

    2020-01-16

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

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

Fix information

  • Fixed component name

    DB2 FOR LUW

  • Fixed component ID

    DB2FORLUW

Applicable component levels

  • RB10 PSN

       UP

[{"Business Unit":{"code":"BU058","label":"IBM Infrastructure w\/TPS"},"Product":{"code":"SSEPGG","label":"DB2 for Linux- UNIX and Windows"},"Platform":[{"code":"PF025","label":"Platform Independent"}],"Version":"11.1","Line of Business":{"code":"LOB10","label":"Data and AI"}}]

Document Information

Modified date:
04 May 2022