I participated in a database server migration from SQL Server 2000 to SQL Server 2005 and noticed some query performance issues that came up during testing. When a database is upgraded from SQL 2000 to SQL Server 2005 using backup\restore or sp_attach, the database retains its current compatibility level. Read my article on the SQL Server World Wide User Group web site as I explain how to set compatibility mode for a single database using T-SQL and how this may affect query performance.