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IT20463: INCORRECT RESULTS ARE POSSIBLE WHEN CONCURRENT QUERIES ACCESS COLUMNAR ORGANIZED TABLES AND USE CS ISOLATION

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APAR status

  • Closed as program error.

Error description

  • Concurrency issues can arise if updates to entries are committed
    before other queries fetch data from CDE; this results in rows
    that originally matched the query being returned with updated
    values when they should not have been selected.
    
    Example:
    
    1)    Session 1 executes an update to an entry but does not
    commit the work: UPDATE t1 SET COL1 = 9 WHERE COL1 = 8
    
    2)    Session 2 executes a select query:  SELECT * FROM t1 WHERE
    col1=8 with CS
    DB2 returns an FETCH-ISCAN access plan.  The runtime has not
    started the ISCAN operation yet.
    
    3)    Session 1 commits the work for the UPDATE, which means
    that we should not have any entries where COL1 = 8
    
    4)    Session 2 completes its execution of the FAST FETCH-ISCAN
    plan and returns rows where COL1 = 9 when they were previously
    COL1 = 8, which is not expected.
    
    The ISCAN operator returns pseudo deleted keys, which have rows
    that have COL1=8 (because the UPDATE has not been committed
    yet).  When DB2 fetches columns from CDE tables with TSN
    corresponding to tuples with the old COL1 = 8, the original
    index predicate (WHERE COL1 = 8) was not being reapplied.  As a
    result, these tuples were incorrectly returned when in fact
    their updated values are now 9.
    

Local fix

Problem summary

  • ****************************************************************
    * USERS AFFECTED:                                              *
    * CDE environments                                             *
    ****************************************************************
    * PROBLEM DESCRIPTION:                                         *
    * See Error Description                                        *
    ****************************************************************
    * RECOMMENDATION:                                              *
    * Upgrade to the latest fix pack.                              *
    ****************************************************************
    

Problem conclusion

  • Problem was first fixed in DB2 UDB Version 11.1 fix pack 2.
    

Temporary fix

Comments

APAR Information

  • APAR number

    IT20463

  • Reported component name

    DB2 FOR LUW

  • Reported component ID

    DB2FORLUW

  • Reported release

    B10

  • Status

    CLOSED PER

  • PE

    NoPE

  • HIPER

    YesHIPER

  • Special Attention

    NoSpecatt / Xsystem

  • Submitted date

    2017-05-04

  • Closed date

    2017-06-23

  • Last modified date

    2017-06-23

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

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

    IT20475

Fix information

  • Fixed component name

    DB2 FOR LUW

  • Fixed component ID

    DB2FORLUW

Applicable component levels

[{"Business Unit":{"code":"BU053","label":"Cloud & Data Platform"},"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:
29 June 2020