tests_success_constraint ON tests (subject, target)
    WHERE success;
</pre><p>
    This is a particularly efficient approach when there are few
    successful tests and many unsuccessful ones.  It is also possible to
    allow only one null in a column by creating a unique partial index
    with an <code class="literal">IS NULL</code> restriction.
   </p></div></div><br class="example-break" /><p>
   Finally, a partial index can also be used to override the system's
   query plan choices.  Also, data sets