E billed is not true;
</pre><p>
   </p><p>
    A possible query to use this index would be:
</p><pre class="programlisting">
SELECT * FROM orders WHERE billed is not true AND order_nr &lt; 10000;
</pre><p>
    However, the index can also be used in queries that do not involve
    <code class="structfield">order_nr</code> at all, e.g.:
</p><pre class="programlisting">
SELECT * FROM orders WHERE billed is not true AND amount &gt; 5000.00;
</pre><p>
    This is not as efficient as a partial index on the
    <c