li>
  <li><code>finalize</code> - Passed valCtx and must return the final per-row value expected in  result dataset.</li>
</ul>
<p>For <code>groupByColumnAgg</code> you do not need to worry about reduceReductions - there is no merge step.</p></div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>colname</code> - One or more column names. If multiple column names are specified then perElemFn will need to take additional arguments.</dd>
<dd><code>perElemFn</code> - A function that takes th