|
Problem IC71737
|
Status: Closed |
POSSIBLE NLJN QUERY PERFORMANCE ISSUE FROM UNDER COSTED NLJN DUE TOBAD INNER PAGE ESTIMATE |
| product: |
DB2 FOR LUW / DB2FORLUW / 950 - DB2 |
| Problem description: |
When examining an Query Access Plan with a Nested Loop Join,
usually the cost for the Nested Loop Join will be at least the
sum of the cost of the outer Query Access Plan and the inner
Query Access Plan. You may be experiencing a poorly performing
Query Access Plan that is caused by this APAR if the number of
I/O's associated with the Nested Loop Join is only a few I/Os
more then the number of I/Os from the outer Query Access Plan,
but the inner Query Access Plan has more then a few I/Os
indicated (typically 1 or 2 I/Os, but it could be more).
The db2exfmt plan segment below shows a Nested Loop Join
(NLJOIN) with an outer I/O of 0 I/Os, an inner I/O of 91,252
I/Os, but the Nested Loop Join I/Os of only 3. In this case the
NLJOIN I/Os should have been at least 91,252 I/Os or close to
91,252 I/Os.
715402
NLJOIN
( 23)
7367.04
3.00784 <<< I/Os too small
/-----------+----------\
1 715402
BTQ FETCH
( 24) ( 26)
0.0678342 114167
0 91252
| /-----+-----\
1 1.42166e+06 8.52996e+06
IXSCAN IXSCAN TABLE: DW
( 25) ( 27) FACT
0.0174534 19006.1 Q1
0 2376
| |
6 8.52996e+06
INDEX: DW INDEX: DW
IDX1 XIF1
Q7 Q1 |
| Problem Summary: |
****************************************************************
* USERS AFFECTED: *
* All Users *
****************************************************************
* PROBLEM DESCRIPTION: *
* POSSIBLE NLJN QUERY PERFORMANCE ISSUE FROM UNDER COSTED NLJN *
* DUE TO BAD INNER PAGE ESTIMATE *
****************************************************************
* RECOMMENDATION: *
* Upgrade the server to DB2 for Linux, Unix, Windows, Version *
* 9.5 Fix Pack 7 or later *
**************************************************************** |
| Local Fix: |
One Possible workaround that *may* work is to get detailed stats
for the columns (frequent value & distribution stats). This will
cause the join page selectivity code to use a different
algorithm to compute the filtering. |
| Solution |
The problem is corrected in DB2 for Linux, Unix, Windows,
Version 9.5 Fix Pack 7. |
| Workaround |
not known / see Local fix |
| Timestamps |
Date - problem reported :
Date - problem closed :
Date - last modified :
| 06.10.2010
21.12.2010
21.12.2010 |
| Problem solved at the following versions (IBM BugInfos) |
9.5.FP7
|
| Problem solved according to the fixlist(s) of the following version(s) |
