|
Problem IC98771
|
Status: Closed |
DB2 OPTIMIZER MAY USE FREQUENCY VALCOUNT IN SYSSTAT.COLDIST INCORRECTLY AND GIVE SUB-OPTIMIAL ACCESS PLAN. |
| product: |
DB2 FOR LUW / DB2FORLUW / A50 - DB2 |
| Problem description: |
Under rare circumstances, DB2 optimizer may use frequency
VALCOUNT of a COLVALUE that doesn't match with the value in a
query. The problem happens on CHAR or VARCHAR data type columns
only.
For example,
1. In TAB1, value "0A00000005" appears in 1000 rows, and others
values appear in 1 row only.
db2 "select count(*),c1 from tab1 group by c1"
1 C1
----------- ----------
1 0000000009
1 0A00000000
1000 0A00000005
1 0A00000019
1 0A00000123
1 0A99900999
1 0B99990099
2. SYSSTAT.COLDIST has valid data, and frequency VALCOUNT of
"0A00000005" is 1000.
select substr(colvalue, 1, 30) colvalue, valcount
from sysstat.coldist
where colname = 'C1' and tabname ='TAB1' and
tabschema = '<SCHEMA>' and TYPE = 'F'
COLVALUE VALCOUNT
------------------------------ --------------------
'0A00000005' 1000
- -1
- -1
- -1
- -1
3. For the following query, optimizer incorrectly uses frequency
VALCOUNT of "0A00000005" for "c1='0A00000019'" predicate and
estimate 1000 returned rows.
select * from tab1 where c1='0A00000019';
Rows
RETURN
( 1)
Cost
I/O
|
1000
DTQ
( 2)
0.850451
0
|
1000
IXSCAN
( 3)
0.502298
0
|
1006
INDEX: SHULIN
TAB1_IDX1
Q1 |
| Problem Summary: |
****************************************************************
* USERS AFFECTED: *
* All Users *
****************************************************************
* PROBLEM DESCRIPTION: *
* See Error Description *
****************************************************************
* RECOMMENDATION: *
* Upgrade to DB2 V105FP4 or higher version. *
**************************************************************** |
| Local Fix: |
statistical views can be used to work around the issue. |
| available fix packs: |
DB2 Cancun Release 10.5.0.4 (also known as Fix Pack 4) for Linux, UNIX, and Windows
DB2 Version 10.5 Fix Pack 9 for Linux, UNIX, and Windows
|
| Solution |
Fixed on DB2 V105FP4 |
| Workaround |
statistical views can be used to work around the issue. |
| Timestamps |
Date - problem reported :
Date - problem closed :
Date - last modified :
| 16.01.2014
15.09.2014
15.09.2014 |
| Problem solved at the following versions (IBM BugInfos) |
|
| Problem solved according to the fixlist(s) of the following version(s) |
| 10.5.0.4
|
 |
