ALTER TABLE NOCHECK CONSTRAINT timeout randomly

I have an C# application that performance ETL process. For self referencing table, the application will run "ALTER TABLE [tableName] NOCHECK CONSTRAINT [constraintName]" which turns off any FK constraint(s ) check of this table. Once all the data is loaded, the constraint(s) are enable again.

The database time out is set to 3 minutes, however, the above SQL command would fail because of database would timeout in 30 sec.

What could be the cause of this timeout?

Are there database system tables I should check for abnormality?

Other information: I checked the app, it only has one active thread doing the ETL, so I don't think the application locks any database resource. In addition, the database runs on the same machine as the application.

Event the application closes all its database connections, it would timeout again if it runs ETL process the next time. If I run the sql man开发者_如何学Goually using SQL Manager Studio, it has no problem at all.

Thanks

UPDATE - The application is turning off a number of constraints. It turns out the time out only happens to 1 particular constraint. This constraint is referencing to the Date Dimension table.

UPDATE - It looks like there is some weird abnormality for the testing database that I was working. I tried the same ETL process with other data warehouse and it has no problem so far. Other developers in the team also haven't encounter this issue. This application runs on every midnight. I will keep it running overnight and hopefully I can reproduce the same issue on other databases. So far no luck on figuring out what is going on.

Altering a table requires an exclusive lock for the table. If there is another process reading/writing to the table in question, the schema change can't take place until that process releases its lock.

When you experience a long run time for the table, run sp_who2 in a different connection and see if any connections are blocking your ETL connection. You can then look at the command buffer for that connection to determine what its doing.