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Old 05-05-06, 11:06
chavadb2 chavadb2 is offline
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Posts: 138
question on prefetching

UDB 8.1 fp6a, AIX 5.2 ML4

One of our queries is doing list prefetch and it's just hanging there at the fetch operation holding onto locks..The table always goes out of clustering because of the deletes, updates happening on that..I scheduled weekly reorgs and still I face the problem of fetch operation hanging couple of days after the reorg. .the table has only 1000 rows and I placed it in seperate sms tablespace with extent size 32, prefetch size 32..

This is an oltp env..

what can I do differently to avoid this issue?
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Old 05-05-06, 11:30
chavadb2 chavadb2 is offline
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there is an apar on diabling list prefetch..but this is for siebel pipelined plans
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IY72977: INTRODUCE OPTION OF DISABLING LIST PREFETCH FOR PIPELINED PLANS

APAR status
Closed as program error.

Error description
When pipelined plans is enabled for Siebel environments, we
introduce the option of disabling list prefetch accesses for
performance reasons.
A list prefetch access can be seen in the access plan of an SQL
as (FETCH->RIDSCAN->SORT->IXSCAN) or if you gather a db2exfmt
output for a query, you will see:
PREFETCH: (Type of Prefetch)
LIST
Note, list prefetch is a good access in general. But in
specific cases of pipelined plans, avoiding this access can
result in better perforance. To enable this fix, please contact
Siebel or IBM to set the registry to a specific value.
Local fix
Problem summary
USERS AFFECTED:
Siebel specific environments or instances where the pipelined
plans registry is set.

PROBLEM DESCRIPTION:
Provide additional option to registry variable to avoid list
prefetch accesses in certain cases.

PROBLEM SUMMARY:
For environments where the pipelined plans registry is set,
avoiding list prefetch through a special setting of the registry
can help performance.
Problem conclusion
Problem was first fixed in DB2 UDB Version 8, FixPak 11
Temporary fix
Comments
APAR information
APAR number IY72977
Reported component name DB2 UDB ESE AIX
Reported component ID 5765F4100
Reported release 810
Status CLOSED PER
PE NoPE
HIPER NoHIPER
Submitted date 2005-06-17
Closed date 2006-02-06
Last modified date 2006-02-06

APAR is sysrouted FROM one or more of the following:

APAR is sysrouted TO one or more of the following:

Modules/Macros
ENG_SQNO


Fix information
Fixed component name DB2 UDB ESE AIX
Fixed component ID 5765F4100

Applicable component levels
R810 PSN UP
R820 PSN UP

Last edited by chavadb2; 05-05-06 at 11:37.
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