 Index to Exclude Uninteresting Values</strong></p><div class="example-contents"><p>
    If you have a table that contains both billed and unbilled orders,
    where the unbilled orders take up a small fraction of the total
    table and yet those are the most-accessed rows, you can improve
    performance by creating an index on just the unbilled rows.  The
    command to create the index would look like this:
</p><pre class="programlisting">
CREATE INDEX orders_unbilled_index ON orders (order_nr)
    WHER