|
Problem IC79173
|
Status: Closed |
DB2 SQL Optimizer might choose a suboptimal access plan for query with 'OPTIMIZE FOR N ROWS' clause and IS NULL predicate |
| product: |
DB2 FOR LUW / DB2FORLUW / 970 - DB2 |
| Problem description: |
For a query with an 'optimize for N rows' clause and a highly
filtering IS NULL predicate, the DB2 SQL Optimizer might choose
a suboptimal query access plan as in the following example:
select *
from fact left outer join d1 on f1 = id1
left outer join d2 on f2 = id2
left outer join d3 on f3 = id3
left outer join d4 on f4 = id4
where d3.c31 is null;
with access plan graph for this query ( db2exfmt output ):
RETURN
|
>HSJOIN
/--------+---------\
>HSJOIN TBSCAN
/-------+-------\ |
>HSJOIN TBSCAN D4
/---+----\ |
FILTER TBSCAN D2
| |
>HSJOIN D1
/-----+------\
TBSCAN TBSCAN
| |
FACT D3
The IS NULL predicate is applied by the FILTER. The MAXPAGES
argument described in the details of the TBSCAN operator(s)
below the FILTER is a potential indicator of the problem. If
the MAXPAGES argument is small and the IS NULL predicate is
filtering many records, then a large amount of synchronous IO
might be required, resulting in suboptimal performance. |
| Problem Summary: |
****************************************************************
* USERS AFFECTED: *
* ALL *
****************************************************************
* PROBLEM DESCRIPTION: *
* See Error Description *
****************************************************************
* RECOMMENDATION: *
* Upgrade to DB2 version 9.7 Fix Pack 6 *
**************************************************************** |
| Local Fix: |
|
| available fix packs: |
DB2 Version 9.7 Fix Pack 6 for Linux, UNIX, and Windows
DB2 Version 9.7 Fix Pack 7 for Linux, UNIX, and Windows
DB2 Version 9.7 Fix Pack 8 for Linux, UNIX, and Windows
DB2 Version 9.7 Fix Pack 9 for Linux, UNIX, and Windows
DB2 Version 9.7 Fix Pack 9a for Linux, UNIX, and Windows
DB2 Version 9.7 Fix Pack 10 for Linux, UNIX, and Windows
|
| Solution |
Problem was first fixed in DB2 version 9.7 Fix Pack 6 |
| Workaround |
not known / see Local fix |
| BUG-Tracking |
forerunner : APAR is sysrouted TO one or more of the following: IC84597
follow-up : |
| Timestamps |
Date - problem reported :
Date - problem closed :
Date - last modified :
| 11.10.2011
14.06.2012
14.06.2012 |
| Problem solved at the following versions (IBM BugInfos) |
9.7.FP6
|
| Problem solved according to the fixlist(s) of the following version(s) |
| 9.7.0.6
|
 |
