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Mr SQL's Blog

“Musings on Microsoft SQL Server”

Daily Archives: 31/01/2011

31/01/11 Checking for Locking

Locking occurs frequently within a database and is a desired feature for a relational database system. SQL Server places locks to allow the sharing of resources (tables, pages, rows etc) when multiple users attempt to access or alter the same data.

However, locking can become problematic when many users (or processes) are constantly trying to access the same data. This can lead to contention and deadlocking. The following DMV query can assist with the troubleshooting of such issues:-

SELECT  L.request_session_id AS 'SPID', 
        DB_NAME(L.resource_database_id) AS 'Database Name',
        O.Name AS 'LockedObjectName', 
        P.object_id AS 'LockedObjectId', 
        L.resource_type AS 'LockedResource', 
        L.request_mode AS 'LockType',
        ST.text AS 'SqlStatementText',        
        ES.login_name AS 'LoginName',
        ES.host_name AS 'HostName',
        TST.is_user_transaction as 'IsUserTransaction',
        AT.name as 'TransactionName',
        CN.auth_scheme as 'AuthenticationMethod'
FROM    sys.dm_tran_locks L
        JOIN sys.partitions P ON P.hobt_id = L.resource_associated_entity_id
        JOIN sys.objects O ON O.object_id = P.object_id
        JOIN sys.dm_exec_sessions ES ON ES.session_id = L.request_session_id
        JOIN sys.dm_tran_session_transactions TST ON ES.session_id = TST.session_id
        JOIN sys.dm_tran_active_transactions AT ON TST.transaction_id = AT.transaction_id
        JOIN sys.dm_exec_connections CN ON CN.session_id = ES.session_id
        CROSS APPLY sys.dm_exec_sql_text(CN.most_recent_sql_handle) AS ST
WHERE   resource_database_id = db_id()
ORDER BY L.request_session_id;

The query outputs the SPID that obtained the lock, the database affected, the object (table), the resource affected (e.g. PAGE, KEY etc), the SQL statement that was executed, login name of the user, the host name, whether the query is a user statement and the authentication method. By identifying long running queries and data that is being constantly locked, problems can be understood and solutions can be found to improve performance.

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31/01/11 Querying SQL Server Default Trace Files

SQL Server Profiler is a useful tool for troubleshooting and monitoring database and server performance. Usually, a DBA has to manually setup a trace to monitor a specific area of interest (e.g. Deadlocks, Login audit). However, many are unaware that a default Profiler trace is permanently running on the majority of servers. It is possible to query the trace files from within SQL Server. The default trace captures information such as file growth, mirroring status, object creation/deletion/alteration and security auditing, amongst other items. If you decide that this is not valuable information then you can switch off the default trace.

To check to see whether a default trace is indeed currently running, execute the following:-

SELECT * 
FROM sys.configurations 
WHERE configuration_id = 1568

If the value is ‘1’ then the trace is running. Once you have established that the trace is enabled, you need to run a query to determine the most recent trace file. Trace files automatically rollover, so you need to know which one to query. 

SELECT traceid, value 
FROM [fn_trace_getinfo](default) 
WHERE [property] = 2;

The full path and file name will be returned. You can then run the following query and include the correct path to the most recent file. You could of course setup a variable to automatically populate the path.

SELECT *  
FROM [fn_trace_gettable]('C:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL\LOG\log_49.trc', DEFAULT) 
ORDER BY StartTime;

You can disable the trace by issuing the following commands. However, don’t expect a dramatic increase in server performance as the trace uses only minimal resources.

EXEC master.dbo.sp_configure 'allow updates', 1;
GO 
EXEC master.dbo.sp_configure 'show advanced options', 1;
GO 
EXEC master.dbo.sp_configure 'default trace enabled', 0;
GO
RECONFIGURE WITH OVERRIDE;
GO
EXEC master.dbo.sp_configure 'show advanced options', 0;
GO
EXEC master.dbo.sp_configure 'allow updates', 0;
GO

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