title: Efficient to do math in a MySql "order by" clause?
body: <p>I've been told on several occasions that it is quite efficient to SELECT using math and that it is NOT very efficient to use math in a WHERE clause.  Are these sentiments correct?  And how does this apply to ORDER BY clauses?</p>
  
  <p>Thanks!!</p>
  
  <p>Example: </p>
  
  <pre><code>SELECT a.* FROM a ORDER BY (a.field_1*a.field_2)
  </code></pre>