ode class="structfield">amount</code> column would be, since the system has to
    scan the entire index.  Yet, if there are relatively few unbilled
    orders, using this partial index just to find the unbilled orders
    could be a win.
   </p><p>
    Note that this query cannot use this index:
</p><pre class="programlisting">
SELECT * FROM orders WHERE order_nr = 3501;
</pre><p>
    The order 3501 might be among the billed or unbilled
    orders.
   </p></div></div><br class="example-break" /><p>
   <a c