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Oct 7, 2012

How to avoid deadlocking on your SQL Server


Deadlocking occurs when two user processes have locks on separate objects and each process is trying to acquire a lock on the object that the other process has. When this happens, SQL Server ends the deadlock by automatically choosing one and aborting the process, allowing the other process to continue. The aborted transaction is rolled back and an error message is sent to the user of the aborted process. Generally, the transaction that requires the least amount of overhead to rollback is the transaction that is aborted.

Some tips for reducing the deadlock:
  • Ensure the database design is properly normalized.
  • Have the application access server objects in the same order each time.
  • During transactions, don't allow any user input. Collect it before the transaction begins.
  • Avoid cursors.
  • Keep transactions as short as possible. One way to help accomplish this is to reduce the number of round trips between your application and SQL Server by using stored procedures or keeping transactions with a single batch.
  • Another way of reducing the time a transaction takes to complete is to make sure you are not performing the same reads over and over again. If you do need to read the same data more than once, cache it by storing it in a variable or an array, and then re-reading it from there.
  • Reduce lock time. Try to develop your application so that it grabs locks at the latest possible time, and then releases them at the very earliest time.
  • If appropriate, reduce lock escalation by using the ROWLOCK or PAGLOCK.
  • Consider using the NOLOCK hint to prevent locking if the data being locked is not modified often.
  • If appropriate, use as low of an isolation level as possible for the user connection running the transaction. Consider using bound connections.

You are a database developer for a clothing retailer. The company has a database named Sales. This
database contains a table named Inventory. The Inventory table contains the list of items for sale and the
quantity available for each of those items. When sales information is inserted into the database, this table
is updated. The stored procedure that updates the Inventory table is shown in the exhibit.
CREATE PROCEDURE UpdateInventory @IntID int
DECLARE @Count int
SELECT @Count = Available
WHERE InventoryID = @IntID
IF (@Count > 0)
UPDATE Inventory SET Available = @Count – 1
WHERE InventoryID = @IntID

When this procedure executes, the database server occasionally returns the following error message:
Transaction (Process ID 53) was deadlocked on {lock} resources with another
process and has been chosen as the deadlock victim. Rerun the transaction.

You need to prevent the error message from occurring while maintaining data integrity.
Change the table hint to UPDLOCK.
Explanation: This is a deadlock problem. We must resolve this problem. The SQL batch of this scenario
basically consists of an SELECT statement and an UPDATE statement. The SELECT statement includes a table
This table hint is very restrictive. The rows are completely locked. We need to remove this restrictive table hint
and replace it with the table hint UPDLOCK, which is less restrictive than HOLDLOCK. It allows reads of the
rows, but no updates.

Note: table hint

A table hint specifies that a table scan, or one or more indexes, must be used by the query optimizer, or a
locking method must be used by the query optimizer with this table and for this SELECT. The query optimizer
can usually pick the best optimization method without hints being specified, therefore it is recommended that
hints only be used as a last resort by experienced developers and database administrators.

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