plement three methods of IndexReduction:</p>
<ul>
  <li><code>prepareBatch</code> - Passed each dataset before processing. Return value becomes first  argument to <code>reduceIndex</code>.</li>
  <li><code>reduceIndex</code> - Passed batchCtx, valCtx, and rowIdx. Must return an updated or  new valCtx.</li>
  <li><code>finalize</code> - Passed valCtx and must return the final per-row value expected in  result dataset. The default is just to return valCtx.</li>
</ul>
<p>For <code>groupByColumnAgg</code> you d