DB2 FOR LUW / DB2FORLUW / B10 - DB2

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  -n  -s  , 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.

****************************************************************
* 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                                                          *
****************************************************************

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

see Local Fix

forerunner  : 
follow-up : IT29961 

Date  - problem reported    :
Date  - problem closed      :
Date  - last modified       :

08.08.2019
20.01.2020
20.01.2020