PH09753: MULTI INDEX ACCESS CHOSEN BASED ON A HIGH UNCERTAINTY PREDICATE

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• An inefficient access path utilizing multi-index access can be
  selected based on a range predicate that can be grossly
  underestimated because it is either a host variable or a
  special register such as CURRENT DATE, CURRENT TIMESTAMP, etc.
  
  This can result in RID pool overflows and overall general poor
  performance.
  
  Additional symptoms and keywords.
  SQLPERFORMANCE SQLMIDX
  

Local fix

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * All Db2 12 for z/OS users running                            *
  * queries with predicates referencing                          *
  * parameter markers or special registers                       *
  * and using indexes.                                           *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * An inefficient multi-index access                            *
  * plan can be chosen where the 2nd                             *
  * index plan and beyond don't provide                          *
  * much extra filtering when the                                *
  * following conditions are met.                                *
  *                                                              *
  * (1) Only one extra matching range                            *
  * predicate is provided by the 2nd                             *
  * index plan and beyond.                                       *
  * (2) All other matching predicates                            *
  * from the 2nd index plan and beyond                           *
  * are overlapped with the 1st index                            *
  * plan.                                                        *
  * (3) The filter factor of the range                           *
  * predicate in (1) is underestimated                           *
  * because it references either a host                          *
  * variable or a special register such                          *
  * as CURRENT DATE, CURRENT TIMESTAMP,                          *
  * etc.                                                         *
  *                                                              *
  * The inefficient access plan will                             *
  * overall cause a suboptimal query                             *
  * performance problem.                                         *
  ****************************************************************
  * RECOMMENDATION:                                              *
  * Apply corrective PTF when available                          *
  ****************************************************************
  An inefficient multi-index access plan can be chosen where the
  2nd index plan and beyond don't provide much extra filtering
  when the following conditions are met.
  
  (1) Only one extra matching range predicate is provided by the
  2nd index plan and beyond.
  (2) All other matching predicates from the 2nd index plan and
  beyond are overlapped with the 1st index plan.
  (3) The filter factor of the range predicate in (1) is
  underestimated because it references either a host variable
  or a special register such as CURRENT DATE, CURRENT TIMESTAMP,
  etc.
  
  The inefficient access plan will overall cause a suboptimal
  query performance problem. The reason is that Db2 failed to
  consider the uncertainty in the filter factor of the range
  predicates described in (2).
  

Problem conclusion

• The performance problem described above is fixed by taking
  into account the uncertainty of the predicates that have
  DSN_PREDICATE_TABLE.UNCERTAINTY set.
  
  SQLPERFORMANCE SQLMIDX
  

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:

  UI65971

Modules/Macros

• DSNXOGCM
  

Fix information

• Fixed component name

  DB2 OS/390 & Z/

• Fixed component ID

  5740XYR00

Applicable component levels

• RC10 PSY UI65971

     UP19/10/24 P F910

Fix is available

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