IT18021: INCORRECTLY GENERATED DERIVED PREDICATES MIGHT CAUSE INCORRECT QUERY RESULTS DUE TO TRAILING BLANKS

DB2 Version 11.1 Mod 2 Fix Pack 2 for Linux, UNIX, and Windows
Db2 Version 11.1 Mod2 Fix Pack2 iFix001 for Linux, UNIX, and Windows
Db2 Version 11.1 Mod2 Fix Pack2 iFix002 for Linux, UNIX, and Windows
Db2 Version 11.1 Mod 3 Fix Pack 3 for Linux, UNIX, and Windows
Db2 Version 11.1 Mod3 Fix Pack3 iFix001 for Linux, UNIX, and Windows
Db2 Version 11.1 Mod3 Fix Pack3 iFix002 for Linux, UNIX, and Windows
Db2 Version 11.1 Mod4 Fix Pack4 iFix001 for Linux, UNIX, and Windows

APAR status

• Closed as program error.

Error description

• A derived predicate based on a generated column that includes
  the concatenation of a VARCHAR or VARGRAPHIC column to some
  other column or expression, could result in incorrect results if
  some of the data in the VARCHAR or VARGRAPHIC column has
  trailing blanks.
  
  For example:
  
  drop table t1;
  create table t1 (
  c1 varchar(5) not null,
  c2 varchar(5) not null,
  c3 int,
  mc1 varchar(10) generated always as (c1||c2));
  
  create index t1_i1 on t1 (mc1);
  
  insert into t1 (c1,c2,c3) values
  ('AAA','BBB',1),
  ('AAA ','BBB',2);
  
  select * from t1 where c1 = 'AAA' and c2 = 'BBB';
  
  Query results:
  
  db2 => select * from t1 where c1 = 'AAA' and c2 = 'BBB';
  
  C1    C2    C3          MC1
  ----- ----- ----------- ----------
  AAA   BBB             1 AAABBB
  
    1 record(s) selected.
  
  db2 => drop index t1_i1;
  DB20000I  The SQL command completed successfully.
  db2 => select * from t1 where c1 = 'AAA' and c2 = 'BBB';
  
  C1    C2    C3          MC1
  ----- ----- ----------- ----------
  AAA   BBB             1 AAABBB
  AAA   BBB             2 AAA BBB
  
    2 record(s) selected.
  
  The problem is that the derived predicate is: MC1='AAABBB' which
  misses the C1 values with 'AAA' that have trailing blanks.
  
  The problem occurs for an index scan using an index the includes
  the generated column, where the derived predicate is applied as
  a start or stop key. A table scan doesn't have this issue
  because the original predicates are applied instead of the
  derived predicate. However, it is possible for the problem to
  occur for a table scan of a range partition table because
  partitions could be incorrectly eliminated based on the derived
  predicate, which could also lead to incorrectly missing rows.
  

Local fix

• For tables that aren't range partitioned, drop the index that
  includes the generated column. For range partitioned tables,
  change the definition of the generated column to avoid
  concatenating a VARCHAR or VARGRAPHIC column that might contain
  duplicate values that differ only by the number of trailing
  blanks.
  

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
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  * PROBLEM DESCRIPTION:                                         *
  * See Error Description                                        *
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  * RECOMMENDATION:                                              *
  * Upgrade to DB2 11.1 Mod 2 Fix Pack 2 or higher               *
  ****************************************************************
  

Problem conclusion

• First fixed in DB2 11.1 Mod 2 Fix Pack 2
  

Temporary fix

Comments

APAR Information

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

  IT17587

• 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