One suggestion from a MSDN blog is: "[SCD]: for large number of rows that will not exist in the dimension table, consider using a lookup before the SCD. " http://blogs.msdn.com/b/ashvinis/archive/2005/09/27/474563.aspx But I don't konw the reason. Why we need to add another Lookup? What's the internal action in SCD? For example, SCDII, it will check if the record exists in destination, and then update or insert. But how doesn't SCD check the record? Please let me konw your idea.