Published on

January 15, 2013

Understanding SQL Server Query Hints: NOLOCK vs NOWAIT

As a SQL Server user, you may have come across situations where you need to retrieve data from a table that is locked by another transaction. In such cases, you might have wondered whether to use the NOLOCK or NOWAIT query hint. In this blog post, we will explore the differences between these two query hints and discuss their behavior.

First, let’s clarify the purpose of these query hints. The NOLOCK hint allows you to read data from a table even if it is locked by another transaction. On the other hand, the NOWAIT hint specifies that the query should not wait for the lock to be released and should return an error immediately if the table is locked.

It is important to note that neither of these query hints guarantees the accuracy of the data retrieved. When using the NOLOCK hint, you may get inconsistent or incorrect data if the table is being modified by another transaction. Similarly, when using the NOWAIT hint, you may receive an error if the table is locked, but there is no guarantee that the data returned will be accurate.

Let’s consider an example to illustrate the differences between these query hints. We will create a table with a single row and open a transaction to delete that row. We will then attempt to read the data using both the NOLOCK and NOWAIT hints. After the test, we will rollback the transaction to ensure there is no change in the result set.

USE tempdb
GO
CREATE TABLE First (ID INT, Col1 VARCHAR(10))
GO
INSERT INTO First (ID, Col1)
VALUES (1, 'First')
GO

-- First Connection
BEGIN TRAN
DELETE FROM First
WHERE ID = 1

-- Second Connection
SELECT ID, Col1
FROM First WITH (NOWAIT)
WHERE ID = 1

-- Third Connection
SELECT ID, Col1
FROM First WITH (NOLOCK)
WHERE ID = 1

As you can see from the example, when using the NOLOCK hint, the query returns the data even though the table is locked. However, there is no guarantee that the data is accurate. On the other hand, when using the NOWAIT hint, an error is returned because the table is locked.

To ensure that you retrieve the most accurate and committed data, it is recommended to wait until the transaction lock on the original table is released before reading the data. This will ensure that you get the appropriate results without any inconsistencies.

In conclusion, the behavior of the NOLOCK and NOWAIT query hints in SQL Server is opposite to each other. While NOLOCK allows you to read data regardless of locks, it may result in inconsistent or incorrect data. NOWAIT, on the other hand, returns an error if the table is locked but does not guarantee accurate data. It is important to carefully consider the implications of using these query hints and choose the approach that best suits your specific requirements.

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