title: Why manipulate filtered column will affect index efficiency?
body: <p>I'm reading "Tsql Fundamental" by Ben Itzik.</p>
  
  <p>The author briefly mentioned that we shouldn't manipulate the filtered column if we want to use index efficiently. But he didn't really go into detail as to why this is the case.</p>
  
  <p>So could someone please kindly explain the reason behind it?</p>