Is there an actual reason you're rolling your own code when will_paginate handles this prettttty well already?
And of course you need the right indexes in place, but Customer.find (:all) kindof defeats the whole purpose of pagination. A non worst- possible solution for pagination involves using LIMIT and OFFSET in your query. But I'm really curious as to why you're insistent on re- inventing the wheel (honestly, fairly poorly) here.
