title: mysql table: index vs primary
body: <p>i am a newbie and am a bit concerned about the efficiency of mysql table. 
  what are the pros and cons of making a field in mysql table as </p>
  
  <ol>
  <li>index</li>
  <li>primary</li>
  </ol>
  
  <p>what is the followed practise in deciding on the attributes of a field in a mysql table?
  please share your insights, thanks.</p>