Troubleshooting Timeout SqlExceptions

Question

I have some curious behavior that I'm having trouble figuring out why is occurring. I'm seeing intermittent timeout exceptions. I'm pretty sure it's related to volume because it's not reproducible in our development environment. As a bandaid solution, I tried upping the sql command timeout to sixty seconds, but as I've found, this doesn't seem to help. Here's the strange part, when I check my logs on the process that is failing, here are the start and end times:

• 09/16/2008 16:21:49
• 09/16/2008 16:22:19

So how could it be that it's timing out in thirty seconds when I've set the command timeout to sixty??

Just for reference, here's the exception being thrown:

System.Data.SqlClient.SqlException: Timeout expired.  The timeout period elapsed prior to completion of the operation or the server is not responding.
   at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection)
   at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj)
   at System.Data.SqlClient.TdsParser.Run(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj)
   at System.Data.SqlClient.SqlDataReader.ConsumeMetaData()
   at System.Data.SqlClient.SqlDataReader.get_MetaData()
   at System.Data.SqlClient.SqlCommand.FinishExecuteReader(SqlDataReader ds, RunBehavior runBehavior, String resetOptionsString)
   at System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean async)
   at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, DbAsyncResult result)
   at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method)
   at System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior, String method)
   at System.Data.SqlClient.SqlCommand.ExecuteReader()
   at SetClear.DataAccess.SqlHelper.ExecuteReader(CommandType commandType, String commandText, SqlParameter[] commandArgs)

Answer 1

SQL commands time out because the query you're using takes longer than that to execute. Execute it in Query Analyzer or Management Studio, with a representative amount of data in the database, and look at the execution plan to find out what's slow.

If something is taking a large percentage of the time and is described as a 'table scan' or 'clustered index scan', look at whether you can create an index that would turn that operation into a key lookup (an index seek or clustered index seek).