<code class="function">avg</code>
    aggregate, but the <code class="literal">OVER</code> clause causes it to be
    treated as a window function and computed across the window frame.)
   </p><p>
    A window function call always contains an <code class="literal">OVER</code> clause
    directly following the window function's name and argument(s).  This is what
    syntactically distinguishes it from a normal function or non-window
    aggregate.  The <code class="literal">OVER</code> clause determines exa