IT29948: TRAP / SIGSEGV OCCURS WHEN DROPPING A TABLE (OR INDEX OF A TABLE) AND A LARGE STATISTICS PROFILE EXISTS

APAR status

• Closed as program error.

Error description

• The dropping of a table with a large statistics profile (or
  dropping of an index on such a table) may result in a large
  memory overwrite, causing DB2 to trap/crash with a wide variety
  of symptoms.  Those symptoms may not include a trap/stack of the
  drop table/index activity.
  
  A large statistics profile is most likely to exist on a wide
  table (> 500 columns) with AUTO_CG_STATS set to ON.  This
  setting is only available in version 11.5 and is OFF by default.
  The "ON" setting may result in collecting statistics on a large
  number of column groups.
  
  The problem stack (not necessarily seen without analyzing a
  coredump) shows a stack similar to the following (it is the same
  for drop table or drop index)
  memmove
  sqlrlDropIndex
  ...
  
  Vulnerable tables have a statistics profile >= 65536 bytes.
  This can be checked by
  1. finding tables > 500 columns :
  select tabname, tabschema from syscat.tables where TYPE='T' and
  colcount>500
  
  2. running db2cat on those tables :
  db2cat -d <database> -n <tabname> -s <tabschema> , check if the
  pd length - stats profile offset is >= 65536
  for example :
         PACKED DESCRIPTOR HEADER
  ..
  Length                    : 357256
  Stats Profile Offset      : 260696
  is clearly vulnerable as the difference is 97256 bytes.  Note if
  the statistics profile is "0", no profile exists and the table
  is not vulnerable to the problem.
  
  We can also retrieve and modify the profile :
  SELECT STATISTICS_PROFILE FROM SYSCAT.TABLES WHERE TABNAME='..'
  AND TABSCHEMA='..'
  Alter the RUNSTATS statement to remove most or all of the column
  group statistics, then run the statement adding "SET PROFILE
  ONLY" at the end.  After this, Auto CGS will no longer perform
  discovery i.e. it won't lengthen the stats profile.
  

Local fix

• db2 runstats on table <table> unset profile
  more permanently, set AUTO_CG_STATS to OFF until a fix for the
  APAR is applied
  

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * database with wide tables, especially those using automatic  *
  * column group stats (AUTO_CG_STATS = ON, v11.5 only)          *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * See Error Description                                        *
  ****************************************************************
  * RECOMMENDATION:                                              *
  * TBD                                                          *
  ****************************************************************
  

Problem conclusion

• TBD
  

Temporary fix

• see Local Fix
  

Comments

APAR Information

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

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

  IT29961

Fix information

• Fixed component name

  DB2 FOR LUW

• Fixed component ID

  DB2FORLUW

Applicable component levels

• RB10 PSN

     UP