Parameter Wednesday: DB2 Registry DB2_SKIPDELETED

DB2 Version This Was Written For

9.7

Parameter Name

DB2_SKIPDELETED

Where This Parameter Lives

DB2 Registry (db2set)

How To Check Value

> db2set -all |grep DB2_SKIPDELETED
[i] DB2_SKIPDELETED=YES [DB2_WORKLOAD]

OR

> db2 "select substr(reg_var_name,1,32) name, substr(reg_var_value,1,16) value, level, is_aggregate, substr(aggregate_name,1,32) aggregate_name from SYSIBMADM.reg_variables where reg_var_name='DB2_SKIPDELETED' with ur"

NAME                             VALUE            LEVEL IS_AGGREGATE AGGREGATE_NAME
-------------------------------- ---------------- ----- ------------ --------------------------------
DB2_SKIPDELETED                  YES              I                0 DB2_WORKLOAD

Description

Affects db2’s locking/scanning behavior when using CS or RS isolation levels. Causes DB2 to skip deleted rows and keys. Variable is activated at database start time, and is engaged (or not) at statement compile or bind time.

This variable can still have an effect if Currently Commited behavior is enabled (CUR_COMMIT) – though it only is engaged when currently committed semantics will not help avoid lock contentions.

Impact

Can reduce locking/deadlocking and increase concurrency if applications can tolerate the data integrity changes. Potentially dangerous if the application does not explicitly support this behavior.

Default

NO/OFF

Range/Values

ON, OFF

Recycle Required To Take Effect?

Yes

Can It Be Set To AUTOMATIC?

No, there is no AUTOMATIC option for this parameter.

How To Change It

db2set DB2_SKIPDELETED=YES

Rule of Thumb

If your application does not explicitly support it, do not set this parameter – leave it at the default of NO.

Tuning Considerations

The main consideration here is if your application supports this value. If your application supports this behavior, you should set this parameter.

Along with DB2_SKIPINSERTED and DB2_EVALUNCOMMITTED, you can drastically reduce deadlocking.

Related Error Messages

 

War Stories From The Real World

WebSphere Commerce has supported DB2_SKIPINSERTED, DB2_SKIPDELETED, and DB2_EVALUNCOMMITTED for years. They’re now part of the aggregate DB2 Registry setting DB2_WORKLOAD=WC, but we were setting them independently long before that. I have personally seen them cause a WebSphere Commerce site go from having dozens of deadlocks per day to just one or two. So for WebSphere Commerce databases, they absolutely must be set.

I wonder how having the three related parameters set is different from Currently Committed behavior available in DB2 9.7?

Think long and hard before enabling this, because you are changing the behavior of your isolation level by enabling this, and may allow concurrency phenomena that you did not intend.

Link To Info Center

http://www.ibm.com/support/knowledgecenter/SSEPGG_9.7.0/com.ibm.db2.luw.admin.regvars.doc/doc/r0005665.html

Related Parameters

Note that the links for these are all the same. You have to search that page to find them. I wish IBM would have individual parameter pages for DB2 Registry variables like they do for DB CFG and DBM CFG, but they don’t currently.

Blog Entries Related to this Parameter

What DBAs can do to Reduce Deadlocks

Locking Parameters

Registry Variables and DB2_WORKLOAD=WC

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