ue from each column as additional arguments. It must always return the current context.</p>
<p>This is a easy way to instantiate tech.v3.datatype.IndexReduction so if you really need the best possible performance you need to implement 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 valCtx.</