Of course you are right and I'm sure you understand these concepts inside and out. But the OP sounded to me like a complete beginner and your rigorous analysis may be quite beyond him in this early stage of his explorations. I just suggested one quick way he might get up and running.
Knowing how much help to offer is always a sticky question, isnt' it?
BTW, at no time did I tell say that people should always use identity fields as 'default' keys. False notion or not, I didn't say that.
I thought what you posted was perfectly reasonable. I just had a little fun with the PKs. I think you're right on track for helping out the OP.
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Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning