PH45533: INCORRECT VALUE OF CARDF IN SYSIBM.SYSCOLDIST WHEN RUNSTATS COLLECTS KEYCARD STATISTICS ON DPSI INDEX

APAR status

• Closed as program error.

Error description

• The user has a partitioned table with at least a Data Secondary
  Partitioned index (DPSI).
  The first two columns of the partitioned index have the
  following cardinality:
  COL1=1
  COL2=5
  
  So the combination of the distinct values of thetwo columns
  should be 5.
  
  However after a RUNSTATS , SYSIBM.SYSCOLDIST contains the
  following:
  !  COLGROUPCOLNO   !CARDF! NUMCOLUMNS
  ------------------------------------------------
  ! 00010002         !  8  !   2
  ! 000100020003     !  3  !   3
  ! 0001000200030004 !  5  !   4
  
  CARDF of Colgroup 1,2 is 8 and obviously wrong.
  Effectively the CARDF of Colgroup 1,2 seems to be the sum of the
  CARDF of the other colgroups.
  

Local fix

• BYPASS/CIRCUMVENTION:
  Define an NPI as the first index on the table and define the
  DPSI index after that. In this scenario the cardinality stats is
  reported correctly for the DPSI index.
  

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * All Db2 12 for z/OS utility users of                         *
  * RUNSTATS or inline STATISTICS                                *
  * gathering statistics on a multi-column                       *
  * partitioned index that is not a DPSI.                        *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * INCORROUT MSGDSNU616I intermediate                           *
  * keycard statistics during                                    *
  * RUNSTATS INDEX on multi-column                               *
  * partitioned indexes resulting in                             *
  * incorrect CARDF in SYSIBM.SYSCOLDIST                         *
  ****************************************************************
  * RECOMMENDATION:                                              *
  * Apply corrective PTF when available                          *
  ****************************************************************
  When the user executed RUNSTATS INDEX on a partitioned index
  with more than two key columns, the statistics reported
  incorrect intermediate correlation values in DSNU616I and the
  SYSIBM.SYSCOLDIST catalog table was updated with those values.
  The leading key columns had the same column value appearing
  across adjacent partitions. This resulted in wrong
  aggregation of the intermediate key cardinalities across
  partitions.
  This problem can also occur during REORG, LOAD, REBUILD
  INDEX utilities with inline STATISTICS index processing.
  

Problem conclusion

• The code was modified to take into consideration the same
  intermediate key values spanning across partitions and arrive
  at the correct cardinality values for each set of columns.
  With this APAR, the intermediate key cardinalities are now
  calculated using a Hash algorithm. The intermediate key
  cardinalities will be accurate when the number of
  distinct values are lower than 100. For indexes with high
  number of distinct values at the intermediate level, the
  derived cardinalities will be only an estimate.
  With this APAR, since hash structures are now used for
  arriving at the intermediate cardinality, the user may
  receive MSGDSNU623I during aggregation, when executing the
  utility at the partition level.
  DSNU623I can be avoided by specifying FORCEROLLUP keyword
  when executing the utility at the partition level.
  

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:

  UI80643

Modules/Macros

• DSNUSIDX DSNUSIIX DSNUSEOF
  

Fix information

• Fixed component name

  DB2 OS/390 & Z/

• Fixed component ID

  5740XYR00

Applicable component levels