IT23741: A QUERY OVER NICKNAMES OR COLUMN ORGANIZED TABLES CONTAINING FETCH FIRST N ROWS ONLY CLAUSE MAY RETURN INCORRECT RESULTS

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 Mod 4 Fix Pack 6 for Linux, UNIX, and Windows
Db2 Version 11.1 Mod 4 Fix Pack 5 for Linux, UNIX, and Windows
Db2 Version 11.1 Mod 4 Fix Pack 7 for Linux, UNIX, and Windows

APAR status

• Closed as program error.

Error description

• A query over nicknames containing FETCH FIRST n ROWS ONLY clause
  may return incorrect results when the following conditions are
  met:
  
  1. The query contains a FETCH FIRST n ROWS ONLY clause.
  2. The query contains a join that can be processed in the
  remote data source.
  3. The query join predicate can only be processed locally or
  the query contains other predicates that can only be processed
  locally and in both cases cannot be pushed down into the remote
  data source.
  
  A query containing column organized tables with similar criteria
  may also return incorrect results when the query also contains
  an ORDER BY clause.
  
  Example:
  
  *connect to remote database*
  
  CREATE TABLE remote.t1 (c1 int);
  CREATE TABLE remote.t2 (c1 int, c2 varchar(255));
  
  insert into remote.t1 values (1);
  insert into remote.t2 values (1, 'bar'),(1, 'foo_bar');
  
  *connect to local database*
  CREATE NICKNAME local.t1 FOR myserver.remote.t1;
  CREATE NICKNAME local.t2 FOR myserver.remote.t2;
  
  SET SCHEMA LOCAL;
  
  SELECT t2.c1 FROM t1 JOIN t2 ON t1.c1 = t2.c1 where INSTR(t2.c2,
  'foo_', 1) > 0 FETCH FIRST 1 ROW ONLY;
  
  Expected Results:
  
  C1
  -----------
            1
  
    1 record(s) selected.
  
  Actual Results:
  
  C1
  -----------
  
    0 record(s) selected.
  
  
  An indication of this problem can be found within the DB2 access
  plan
  details.
  
  For example, the access plan for the query above has a FILTER
  operation
  above a SHIP operation, and the SHIP operation is estimated to
  return 1
  row. This may indicate that the remote data source is processing
  the
  FETCH FIRST n ROWS ONLY clause and the local database is
  processing
  the predicate that can only be processed locally:
  
                  Rows
                 RETURN
                 (   1)
                  Cost
                   I/O
                   |
                0.333333
                 FILTER
                 (   2)
                 596.263
                   57
                   |
                    1
                 SHIP
                 (   3)
                 596.228
                   57
            /------+------\
         1000              1000
   NICKNM: LOCAL    NICKNM: LOCAL
          T2                T1
          Q1                Q2
  
  For column organized tables, the access plan above will
  use a CTQ operator instead of a SHIP operator.
  

Local fix

• Either of these workarounds will avoid the incorrect results for
  nicknames:
  
  1. Disable the FETCH FIRST n ROWS ONLY clause from being
  processed by the remote server:
  alter server myserver options(add db2_fetch_n_rows 'N')
  
  2. If the predicate contains a function, create a function
  mapping object so that the predicate can be processed by the
  remote data source. In the example above, a function mapping
  object for the SYSIBM.INSTR function will allow the predicate to
  be processed by the remote data source.
  

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * NICKNAMES OR COLUMN ORGANIZED TABLES                         *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * See Error Description                                        *
  ****************************************************************
  * RECOMMENDATION:                                              *
  * Please, See Error Description and install this fix.          *
  ****************************************************************
  

Problem conclusion

• Please, See Error Description and install this fix.
  

Temporary fix

Comments

APAR Information

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

  IT23267

• 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 PSY

     UP