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Mr SQL's Blog

Musings on Microsoft SQL Server

Monthly Archives: January 2011

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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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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UK Postcodes can be difficult to handle in SQL, especially when trying to group results based on area code. Postcodes in the UK come in a variety of formats, the most common being AA9 9AA, A9 9AA, A99 9AA and AA99 9AA. Problems arise due to the variation in the total number of characters in a code, the different combinations of numbers and letters, and also the location of the space.

If you need to write a query to select the characters up to but excluding the space (e.g. AA9, A9, A99 or AA99), then the following approach can be used. It uses the LEFT command in conjunction with CHARINDEX to select the left side of the full code.

SELECT CompanyName, LEFT(Postcode, CHARINDEX(' ', Postcode) -1) AS 'Postcode'
FROM CompanyAddresses

However, if you need to select only the leading letters and not numbers (A or AA), then you can use a CASE statement:-

SELECT CompanyName, CASE
WHEN ISNUMERIC(RIGHT(LEFT(Postcode, 2), 1)) = '0' THEN LEFT(Postcode, 2)
ELSE LEFT(Postcode, 1)
END AS 'Postcode'
FROM CompanyAddresses

Using either of the above two methods, you can successfully group results into Postcode areas. Use either method to group and select on Postcode.

SELECT CASE
WHEN ISNUMERIC(RIGHT(LEFT(Postcode, 2), 1)) = '0' THEN LEFT(Postcode, 2)
ELSE LEFT(Postcode, 1)
END AS 'Postcode', COUNT(CompanyName) AS 'Companies in Postal Area'
FROM CompanyAddresses
GROUP BY CASE
WHEN ISNUMERIC(RIGHT(LEFT(Postcode, 2), 1)) = '0' THEN LEFT(Postcode, 2)
ELSE LEFT(Postcode, 1)
END

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The following script defragments all indexes in a database that are above a given fragmentation threshold. I can’t claim to have written this – this script can be found on the MSDN Website. I would suggest creating a stored procedure in your database and calling it from a scheduled SQL Agent job every evening. Be careful of running this on a database with many indexes or with large tables as it may take some time to complete.

/*Perform a 'USE <database name>' to select the database in which to run the script.*/
-- Declare variables
SET NOCOUNT ON;
DECLARE @tablename varchar(255);
DECLARE @execstr   varchar(400);
DECLARE @objectid  int;
DECLARE @indexid   int;
DECLARE @frag      decimal;
DECLARE @maxfrag   decimal;

-- Decide on the maximum fragmentation to allow for.
SELECT @maxfrag = 30.0;

-- Declare a cursor.
DECLARE tables CURSOR FOR
   SELECT TABLE_SCHEMA + '.' + TABLE_NAME
   FROM INFORMATION_SCHEMA.TABLES
   WHERE TABLE_TYPE = 'BASE TABLE';

-- Create the table.
CREATE TABLE #fraglist (
   ObjectName char(255),
   ObjectId int,
   IndexName char(255),
   IndexId int,
   Lvl int,
   CountPages int,
   CountRows int,
   MinRecSize int,
   MaxRecSize int,
   AvgRecSize int,
   ForRecCount int,
   Extents int,
   ExtentSwitches int,
   AvgFreeBytes int,
   AvgPageDensity int,
   ScanDensity decimal,
   BestCount int,
   ActualCount int,
   LogicalFrag decimal,
   ExtentFrag decimal);

-- Open the cursor.
OPEN tables;

-- Loop through all the tables in the database.
FETCH NEXT
   FROM tables
   INTO @tablename;

WHILE @@FETCH_STATUS = 0
BEGIN
-- Do the showcontig of all indexes of the table
   INSERT INTO #fraglist 
   EXEC ('DBCC SHOWCONTIG (''' + @tablename + ''') 
      WITH FAST, TABLERESULTS, ALL_INDEXES, NO_INFOMSGS');
   FETCH NEXT
      FROM tables
      INTO @tablename;
END;

-- Close and deallocate the cursor.
CLOSE tables;
DEALLOCATE tables;

-- Declare the cursor for the list of indexes to be defragged.
DECLARE indexes CURSOR FOR
   SELECT ObjectName, ObjectId, IndexId, LogicalFrag
   FROM #fraglist
   WHERE LogicalFrag >= @maxfrag
      AND INDEXPROPERTY (ObjectId, IndexName, 'IndexDepth') > 0;

-- Open the cursor.
OPEN indexes;

-- Loop through the indexes.
FETCH NEXT
   FROM indexes
   INTO @tablename, @objectid, @indexid, @frag;

WHILE @@FETCH_STATUS = 0
BEGIN
   PRINT 'Executing DBCC INDEXDEFRAG (0, ' + RTRIM(@tablename) + ',
      ' + RTRIM(@indexid) + ') - fragmentation currently '
       + RTRIM(CONVERT(varchar(15),@frag)) + '%';
   SELECT @execstr = 'DBCC INDEXDEFRAG (0, ' + RTRIM(@objectid) + ',
       ' + RTRIM(@indexid) + ')';
   EXEC (@execstr);

   FETCH NEXT
      FROM indexes
      INTO @tablename, @objectid, @indexid, @frag;
END;

-- Close and deallocate the cursor.
CLOSE indexes;
DEALLOCATE indexes;

-- Delete the temporary table.
DROP TABLE #fraglist;
GO

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