title: Efficient way to verify that records are unique in Python/PyTables
body: <p>I have a table in PyTables with ~50 million records.  The combination of two fields (specifically userID and date) should be unique (i.e. a user should have at most one record per day), but I need to verify that this is indeed the case.</p>
  
  <p>Illustratively, my table looks like this:</p>
  
  <pre><code>userID |   date
  A      |    1
  A      |    2
  B      |    1
  B      |    2
  B      |    2   &lt;- bad! Problem with the data!
  </code></pre>
  
  <p>Additional details:</p>
  
  <ul>
  <li>The table is currently 'mostly' sorted.</li>
  <li>I can just barely pull one column
  into memory as a numpy array, but I
  can't pull two into memory at the
  same time.</li>
  <li>Both userID and date are integers</li>
  </ul>