code: func (b *builtinIntervalIntSig) evalInt(row chunk.Row) (int64, bool, error) {
  	args0, isNull, err := b.args[0].EvalInt(b.ctx, row)
  	if err != nil {
  		return 0, true, err
  	}
  	if isNull {
  		return -1, false, nil
  	}
  	idx, err := b.binSearch(args0, mysql.HasUnsignedFlag(b.args[0].GetType().Flag), b.args[1:], row)
  	return int64(idx), err != nil, err
  }
docs: // evalInt evals a builtinIntervalIntSig.
  // See http://dev.mysql.com/doc/refman/5.7/en/comparison-operators.html#function_interval