title: Mysql: what situation(s) will cause index failure
body: <p>Let's suppose you have a table called <code>user_card_tbl</code> which contains fields <code>userid</code>,<code>cardno</code> etc. And you have created index as seen below:</p>
  
  <pre><code>CREATE INDEX cardno_idx ON user_card_tbl(cardno);
  </code></pre>
  
  <p>I just want to know, what situation(s) will cause <code>cardno_idx</code> failure when you execute query.</p>