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Pros and Cons of NOLOCK

I have seen some developers use WITH (NOLOCK) when querying in SQL Server and wonder why they use. Now i explored it and found that its useful to improve the performance in executing the query. However there is a disadvantage in using it. The disadvantage is that one may not be sure that they are getting the data which is currently being updated in the Table ie Without lock protection, you cannot be guaranteed that the data isn’t changing during the time that the query is running.

With these "Hints"  you can quickly see where problems can occur:  

SQL Server NOLOCK Hint & other poor ideas.




NOLOCK

This table hint, also known as READUNCOMMITTED, is applicable to SELECT statements only. NOLOCK indicates that no shared locks are issued against the table that would prohibit other transactions from modifying the data in the table.

The benefit of the statement is that it allows you to keep the database engine from issuing locks against the tables in your queries; this increases concurrency and performance because the database engine does not have to maintain the shared locks involved. The downside is that, because the statement does not issue any locks against the tables being read, some "dirty," uncommitted data could potentially be read. A "dirty" read is one in which the data being read is involved in a transaction from another connection. If that transaction rolls back its work, the data read from the connection using NOLOCK will have read uncommitted data. This type of read makes processing inconsistent and can lead to problems. The trick is being able to know when you should use NOLOCK.

As a side note, NOLOCK queries also run the risk of reading "phantom" data, or data rows that are available in one database transaction read but can be rolled back in another. 

The following example shows how NOLOCK works and how dirty reads can occur. In the script below, I begin a transaction and insert a record in the SalesHistory table.

BEGIN TRANSACTION
      INSERT INTO SalesHistory
      (Product, SaleDate, SalePrice)         
      VALUES           
      ('PoolTable', GETDATE(), 500)                  

The transaction is still open, which means that the record that was inserted into the table still has locks issued against it. In a new query window, run the following script, which uses the NOLOCK table hint in returning the number of records in the SalesHistory table.

SELECT COUNT(*) FROM SalesHistory WITH(NOLOCK)

The number of records returned is 301. Since the transaction that entered the record into the SalesHistory table has not been committed, I can undo it. I'll roll back the transaction by issuing the following statement:

ROLLBACK TRANSACTION

This statement removes the record from the SalesHistory table that I previously inserted. Now I run the same SELECT statement that I ran earlier:

SELECT COUNT(*) FROM SalesHistory WITH(NOLOCK)

This time the record count returned is 300. My first query read a record that was not yet committed -- this is a dirty read.



Posted on Thursday, February 2, 2012 11:45 AM | Back to top


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