Efficient way to verify that records are unique in Python/PyTables
<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>
