IT04520: INCORRECT RESULTS MIGHT BE PRODUCED IF HSJOIN HAS 2 NLJNs ON THE PROBE SIDE AND HSJN PROBE PUSH DOWN INTO THE SECOND NLJN DONE

DB2 Version 10.1 Fix Pack 6 for Linux, UNIX, and Windows

APAR status

• Closed as program error.

Error description

• An SQL might produce incorrect results if all the following
  conditions are met:
  
  (a) The statement leverages a hash join (HSJOIN).
  (b) The plan has 2 nested loop joins (NLJOIN); one on the inner
  of another, and the other on the probe side of the HSJOIN.
  (c) An internal optimization on the probe side of the HSJN
  called a "push down" is implemented. The DB2 trace example below
  shows sqlrihsjnProbePD, which identifies this optimization
  taking place.
  
  We can use the EXPLAIN statement to see if the given SQL
  satisfies the conditions (a) and (b).
  
  The plan would have a graph like as below:
  
                    HSJOIN
                   /           \
            NLJOIN (x)
            /         \
                       NLJOIN  (y)
  
  Please note that this issue is likely to occur if NLJN(x) outer
  is not a real table, but a TABFNC: SYSIBM.GENROW
  
  Also, we can take db2trc to see if the HSJOIN is running into
  the condition (c), like as below:
  
  1. Turn on the trace.
  db2trc on -f trc.dmp
  
  2. Run the query.
  
  3. Turn off the trace, then format it.
  db2trc off
  db2trc flw trc.dmp trc.flw
  
  4. Search one sqlrihsjnProbePD that immediately returns with
  rc=1 inside sqlriFetch.
  grep -e sqlrihsjnProbePD -e sqlriFetch trc.flw
  59255   | | | sqlriFetch entry
  59417   | | | | | | | | | | | sqlrihsjnProbePD entry
  59420   | | | | | | | | | | | sqlrihsjnProbePD exit [rc = 1]
  50526   | | | sqlriFetch exit
  
  Note that the result might vary depending on the order and
  volume of the data.
  

Local fix

• Users can disable all the push down by using the following
  setting.
  
  db2set DB2_TCG_DEFAULT_OPTIONS="set disable_pushdown on"
  
  Note that disabling push down can impact on query performance.
  

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * All users                                                    *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * See Error Description                                        *
  ****************************************************************
  * RECOMMENDATION:                                              *
  * Upgrade to DB2 version 10.1.0.5.                             *
  ****************************************************************
  

Problem conclusion

• The problem is first fixed in DB2 version 10.1.0.5.
  

Temporary fix

Comments

APAR Information

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

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

  IT05904

Fix information

• Fixed component name

  DB2 FOR LUW

• Fixed component ID

  DB2FORLUW

Applicable component levels

• RA10 PSN

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