Exclusives Locks With Transaction Isolation Level Readuncommitted

Mar 12, 2008

Hello,

I have some locks issues on production database (win 2k3 SP1, sql server 2k5).
In fact, I have an asynchronous process that makes SELECT TOP 1 in a table and UPDATE the selected row. The transaction isolation level for doing this action is READUNCOMMITTED.
The isolation level readuncommitted is ignored for the update if I'm not wrong.
On the other hand, I have some transactional activities with the isolation level read uncommitted too.

But when I control the database activity, I find very often locks between the asynchronous part and the transactional part. This is the transactional activity that is locking the asynchronous activity.
The transactional activity is a simple SELECT and this type of query, in spite of the isolation level readuncommitted, makes exclusives locks when the asynchronous makes LCK_M_U.

I tried to modify the strored procedure for the SELECT/UPDATE of the asynchronous process with a "UPDATE my_table ... FROM my_table" query in order to reduce the transaction time. But the problem is always present.

Can someone help me to understand how a select query with the isolation level readuncommtted can make exclusives locks ?

Thanks in advance.

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