IT29099: DB2 MAY RETURN INCORRECT RESULT WHEN EXECUTING QUERY WITH UNION VIEW INCLUDE MULTIPLE TABLES

Db2 Version 11.1 Mod 4 Fix Pack 5 for Linux, UNIX, and Windows
Db2 Version 11.1 Mod 4 Fix Pack 6 for Linux, UNIX, and Windows
Db2 Version 11.1 Mod 4 Fix Pack 6 for Linux, UNIX, and Windows

APAR status

• Closed as program error.

Error description

• Db2 may return incorrect result when executing query with union
  view include multiple tables.
  
  For example, the query reference a view V1.
  
  SELECT C1,
         C2,
         ...
  FROM   V1
  WHERE  C1 = constant
  AND    C2 = constant
  ;
  
  V1 looks like following, with Q1 including multiple subqueries
  and UNIONs which does OUTER JOIN with other tables including T3
  on column C1.
  
  CREATE VIEW V1
  AS
      SELECT
              Q1.C1,
              Q1.C2
              ...
      FROM (
              SELECT
                      T1.C3,
                      T1.C4,
                      ...
                      T2.C1,
                      T2.C2
                      ...
              FROM    T1
                      LEFT OUTER JOIN T2 ON (T1.C5 = T2.T5)
              WHERE
                      ...
              UNION ALL
              SELECT
                      T1.C3,
                      T1.C4,
                      ...
                      T2.C1,
                      T2.C2
                      ...
              FROM    T1
                      LEFT OUTER JOIN T2 ON (T1.C5 = T2.T5)
              WHERE
                      ...
  
              UNION ALL
              SELECT
                      T1.C3,
                      T1.C4,
                      ...
                      T2.C1,
                      T2.C2
                      ...
              FROM    T1
                      LEFT OUTER JOIN T2 ON (T1.C5 = T2.T5)
              WHERE
                      ...
          ) AS Q1
      LEFT OUTER JOIN T3
          ON (Q1.C1 = T3.C1 AND ...)
      ...
      LEFT OUTER JOIN T4
      ...
  
  Because there's an equal predicate on column C1, Db2 Rewrite
  pushes down a local predicate 'Q1.C1 = constant' under T3,
  which makes 'T3.C1 = constant'. During pushing down this
  predicate with other join predicates under the UNION,
  the predicate 'T3.C1 = constant' is removed in error and causing
  a wrong result.
  
  The workaround is to explicitly reference T3.C1 instead of Q1.C1
  as below:
  
  CREATE VIEW V1
  AS
      SELECT
              T3.C1,
              Q1.C2
  ...
  
  with this rewrite the predicate will not be removed.
  

Local fix

• Rewrite the Query as per example in the Error Description.
  

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 is sysrouted FROM one or more of the following:

  IT28945

• 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