Question

We are using ADO.NET to connect to a SQL 2005 server, and doing a number of inserts/updates and selects in it. We changed one of the updates to be inside a transaction however it appears to (b)lock the entire table when we do it, regardless of the IsolationLevel we set on the transaction.

The behavior that I seem to see is that:

1. If you have no transactions then it’s an all out fight (losers getting dead locked)
2. If you have a few transactions then they win all the time and block all others out unless
3. If you have a few transactions and you set something like nolock on the rest then you get transactions and nothing blocked. This is because every statement (select/insert/delete/update) has an isolationlevel regardless of transactions.

Is this correct?

Answer 1

The answer to your question is: It depends.

If you are updating a table, SQL Server uses several strategies to decide how many rows to lock, row level locks, page locks or full table locks.

If you are updating more than a certain percentage of the table (configurable as I remember), then SQL Server gives you a table level lock, which may block selects.

The best reference is:

• Understanding Locking in SQL Server: http://msdn.microsoft.com/en-us/library/aa213039(SQL.80).aspx (for SQL Server 2000)
• Introduction to Locking in SQL Server: http://www.sqlteam.com/article/introduction-to-locking-in-sql-server
• Isolation Levels in the Database Engine: http://msdn.microsoft.com/en-us/library/ms189122.aspx (for SQL server 2008, but 2005 version is available).

Good luck.