Posted by: Fatherjack | 17/Aug/09

Unblocking your SQL Server

Finding which process is locking up your SQL Server can cause some headaches to people new to DBA activities. Digging deaper into the information that is available and refining the data to inform how you proceed is something that can still cause a well established DBA.

sp_who is a stored procedure that ships with SQL Server. If you run EXEC sp_who from a Management Studio query window then you have a grid of data returned that details the connection details of all activity on your server. This is useful as it shows who is connected to which database and what they are up to. Now this can be a small amount of data on your test server but on a production server could return hundreds of rows. This is also instantaneous, in so far as when you run it next it could all have changed.

I find it useful to get these results into a temporary table so that I can shake it up a little and refer to the same set of results more than once while I am trying to resolve block or some such. To do this you need to get the output of sp_who into the aforementioned temporary table with a section of code like this:

IF OBJECT_ID(‘tempdb.dbo.#who’) > 0
[Status] NVARCHAR(255) ,
[HostName] NVARCHAR(255) ,
[Blk] NVARCHAR(255) ,
[DBName] NVARCHAR(255) ,
[Cmd] NVARCHAR(255) ,
INSERT INTO #who EXEC sp_who ;

You can then recall this information at any time by running
SELECT * FROM [#who]

If your headache is blocking flavoured then running this will show you which SPID is the cause:

CAST([w].[Status] AS NVARCHAR(30)) AS [status],
[w].[SPID] ,
[w].[ECID] ,
[w].[HostName] ,
[w].[Cmd] ,
[w].[Blk] AS [Blocked by],
[#who] AS w
ORDER BY [DBName], [blocked BY]

The Blocked by column will have a number in it if that process is being blocked. The value in the column is the SPID of the problem process or, if that process itself has a number in its Blocked By column then it too is being blocked. Keep following the trail until you reach a process that has a zero in its Blocked by column but is being shown as blocking a different process.

Using a temporary table means that the information gathered is only available in the single process and will be dropped when you close the query.

I have this script set up as a SQL Snippet in SQL Prompt so that it is readily available in 3 keystrokes. SQL Prompt is an application that provides shortcuts to SQL Editors, making SQL development simpler and faster from Red Gate. Other editors are available and it is possible to create a script template in SSMS if you are not fortunate enough to have any third party tools working for you.

sp_who is documented in Books Online here so please read more about it there. There is also an undocumented version called sp_who2 that is very similar but has even more information regarding each process that is connecting to your databases.


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