  Some window functions act only
    on the rows of the window frame, rather than of the whole partition.
    By default, if <code class="literal">ORDER BY</code> is supplied then the frame consists of
    all rows from the start of the partition up through the current row, plus
    any following rows that are equal to the current row according to the
    <code class="literal">ORDER BY</code> clause.  When <code class="literal">ORDER BY</code> is omitted the
    default frame consists of all rows in the par