Sql-server – repeatable read and nolock

nolocksql server

Does anyone know what the net effect of having a block of t-sql with a transaction isolation level of repeatble read and then inside having a select with nolock.

Does the nolock become invalid because the isolation level has already been set?

Best Answer

The table hint in the query takes precedence, as I would have expected.

Here's my test script:

CREATE TABLE t1 (a int);

INSERT INTO t1(a) SELECT TOP 1000 message_id FROM sys.messages;


SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;

BEGIN TRANSACTION;

    SELECT * FROM t1 WITH(NOLOCK); /* Try it with and without the hint */

    EXEC sp_lock;

ROLLBACK;

With the NOLOCK hint included, no locks are taken by the SELECT statement.

In other words, setting the isolation level on the transaction makes that isolation level the default within the transaction, but that default can still be overridden by table hints, as I demonstrated.

Related Solutions

SQL Server NOLOCK – Use NOLOCK Hint When Calling Table Valued Function

You cannot use a WITH (NOLOCK) on a Table-Valued Function, unless you put it on every single table in the code inside the function.

Your best bet would be, like you said, to SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED.

To change this back to the default, you need to find out what isolation level is currently set (before changing it above).

This can be done by running DBCC USEROPTIONS and checking the value for isolation level.

The default tends to be READ COMMITTED, so to change it to that, you'll want to write a statement like the following:

SET TRANSACTION ISOLATION LEVEL READ COMMITTED

Sql-server – Dirty Read on SQL Server

From this documentation - SET TRANSACTION ISOLATION LEVEL - under REMARKS

Only one of the isolation level options can be set at a time, and it remains set for that connection until it is explicitly changed. All read operations performed within the transaction operate under the rules for the specified isolation level unless a table hint in the FROM clause of a statement specifies different locking or versioning behavior for a table.

You can use this query to determine the isolation level you are currently at:

SELECT CASE transaction_isolation_level 
WHEN 0 THEN 'Unspecified' 
WHEN 1 THEN 'ReadUncommitted' 
WHEN 2 THEN 'ReadCommitted' 
WHEN 3 THEN 'Repeatable' 
WHEN 4 THEN 'Serializable' 
WHEN 5 THEN 'Snapshot' END AS TRANSACTION_ISOLATION_LEVEL 
FROM sys.dm_exec_sessions 
where session_id = @@SPID

Best Answer

Related Solutions

SQL Server NOLOCK – Use NOLOCK Hint When Calling Table Valued Function

Sql-server – Dirty Read on SQL Server

Related Question