Finding and troubleshooting SQL Server deadlocks

Problem
One thing that will you most certainly face at some time as a DBA is dealing with deadlocks. A deadlock occurs when two processes are trying to update the same record or set of records, but the processing is done in a different order and therefore SQL Server selects one of the processes as a deadlock victim and rolls back the statements.

For example you have two sessions that are updating the same data, session 1 starts a transaction updates table A and then session 2 starts a transaction and updates table B and then updates the same records in table A. Session 1 then tries to update the same records in table B. At this point it is impossible for the transactions to be committed, because the data was updated in a different order and SQL Server selects one of the processes as a deadlock viticm.

To further illustrate how deadlocks work you can run the following code in the Northwind database.

To create a deadlock you can issue commands similar to the commands below.
Step	Commands
1	--open a query window (1) and run these commands begin tran update products set supplierid = 2
2	-- open another query window (2) and run these commands begin tran update employees set firstname = 'Bob' update products set supplierid = 1
3	-- go back to query window (1) and run these commands update employees set firstname = 'Greg' At this point SQL Server will select one of the process as a deadlock victim and roll back the statement
4	--issue this command in query window (1) to undo all of the changes rollback
5	--go back to query window (2) and run these commands to undo changes rollback

Solution
The only solution for handling deadlocks is to find the problem in your code and then modify your processing to avoid deadlock situations. The first thing you need to do is find the deadlock situations and then investigate the problem. There are a couple of ways of doing this.

The first approach is to turn on the trace flag to find the deadlocks. This can be done with the following statement run in Query Analyzer.

From this output we can see that SPID 53: was updating object 1977058079 and SPID 52: was updating object 117575457. But what do these numbers mean. These numbers are the objectIDs. To determine what table is affected you will need to query the sysobjects table in the Northwind database or whatever user database the deadlock occurred in using the following command and the find the ID that matches the ID from the deadlock information.

With this information it is possible to see what tables were part of the deadlock process, but trying to figure out what statements caused the problem is much more difficult. To provide further information about the deadlock process you will need to run a Trace to capture all of the information and then try to decipher what is going on. This can be done by either using Profiler or by using a Server Side Trace. With the trace there are a couple of additional items that need to be captured to help figure out what is going on and with what objects.

To do this using SQL Profiler, you will need to capture the Lock Events Lock:Deadlock and Lock:Deadlock Chain.

For a Server Side Trace the following additional information will need to collected to capture the deadlock information.

In addition, you will also need to capture this additional column to see what objects are part of the deadlock chain.

ColumnNumber	Column	Description
22	ObjectID	System-assigned ID of the object.

The output from our trace would show the following information:

From here we can see what was occurring at the time of the deadlock. This is a very simple example, but you can see how this additional information from a trace can help solve the problem.

When you have a lot of information to go through it is easier to load the data into a SQL Server table and then query the data for the particular timeframe and SPIDs in question. This was covered in a Server Side Trace tip. Here is a sample query that can help you narrow down the timeframe when the deadlock occurred. By changing the date values and the SPIDs to look at you can narrow down what was occurring at the time or right around the time that the deadlock occurred.

DECLARE @lowDate AS datetime, @highDate AS datetime 

SET @lowDate = '2006-08-01 13:47:17.000' 
SET @highDate = '2006-08-01 13:47:18.999' 


SELECT  
       TextData, 
       StartTime,  
       EndTime, 
       SPID,  
       Duration,  
       Reads,  
       Writes,  
       EventClass       
FROM  
       TraceFile 
WHERE  
       SPID IN (52,53,4) AND 
       (StartTime BETWEEN @lowDate AND @highDate  
        OR EndTime BETWEEN @lowDate AND @highDate  
        OR StartTime < @lowDate AND EndTime > @lowDate) 
ORDER BY  
       StartTime