I've come in to lots of situations to tune a system, only to find that the database design needs work.
Of course, by the time the system goes live, it's difficult if not impossible to change the basic design. So it's vital that you check your design nine ways to Sunday to make sure you've covered everything. Remember, you're not hte final authority on the design - document it and pop it up there for everyone to see and comment on. If you've done you're job, you'll be able to defend most everything and be big enough to change what needs changing.
On this very topic, I've been reading a good book on the subject. Check out http://www.informit.com/articles/article.aspx?p=1225693.
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