title: Index for nullable column
body: <p>I have an index on a nullable column and I want to select all it's values like this:</p>
  
  <pre><code>SELECT e.ename 
  FROM   emp e;
  </code></pre>
  
  <p>In the explain plan I see a <code>FULL TABLE SCAN</code> (even a hint didn't help)  </p>
  
  <pre><code>SELECT e.ename 
  FROM   emp e
  WHERE  e.ename = 'gdoron';
  </code></pre>
  
  <p>Does use the index...</p>
  
  <p>I googled and found out there are no null entries in indexes, thus the first query can't use the index.  </p>
  
  <p>My question is simple: <strong>why there aren't null entries in indexes?</strong></p>