getCandidate = 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 candidateColumn.getIndex() === null ? null : candidateColumn;
  }
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.