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Oracle 10g release 2 locking changes?

Oracle Database Tips by Donald Burleson

Question:  I just upgraded to 10gr2 (release 10.2) and I now notice lich locking behavior, resulting in high waits on transaction enqueue locks (enq: TX - row lock contention).  I've isolated the update statement, and I still get deadlocks.

Top 5 Timed Events
                                Avg %Total wait Call
Event                           Waits Time (s) (ms) Time
------------------------------- ------------ ----------- ------ ------
enq: TX - row lock contention   5,541        16,593      2995   61.4


Answer:  Oracle has changed their locking mechanism as noted in bug 4,969,880, a workaround for changed locking behavior in 10gr2.  While this applies to explicit locking "select for update", MOSC can give you specific details about the changes.

First, run autotrace from SQL*Plus for both statement and confirm that the execution plans and run statistics are the same on both releases:

set autotrace on;
update xxx . . .

Also, the "enq: TX" wait is often associated with a shortage on the Interested Transaction List (ITL), and increasing INITRANS for the offending table and index may improve concurrency.  It can also be related to referential integrity enforcement and the use of bitmap indexes.

Try try doing a 10046 trace on the update? If you do it on both databases, you will see a difference.

If you like Oracle tuning, see the book "Oracle Tuning: The Definitive Reference", with 950 pages of tuning tips and scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.



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