code: function(executionStep) {
      // In order to use and index for implementing a join, the entire relation
      // must be fed to the JoinStep, otherwise the index can't be used.
      if (!(executionStep instanceof lf.proc.TableAccessFullStep)) {
        return null;
      }
      var candidateColumn = getColumnForTable(executionStep.table);
      return goog.isNull(candidateColumn.getIndex()) ? null : candidateColumn;
    }
docs: Extracts the candidate indexed column for the given execution step node.
  @param {!lf.proc.PhysicalQueryPlanNode} executionStep
  @return {?lf.schema.Column} The candidate column or null if no such column
  exists.