code: func (b *builtinFieldIntSig) evalInt(row chunk.Row) (int64, bool, error) {
  	str, isNull, err := b.args[0].EvalInt(b.ctx, row)
  	if isNull || err != nil {
  		return 0, err != nil, err
  	}
  	for i, length := 1, len(b.args); i < length; i++ {
  		stri, isNull, err := b.args[i].EvalInt(b.ctx, row)
  		if err != nil {
  			return 0, true, err
  		}
  		if !isNull && str == stri {
  			return int64(i), false, nil
  		}
  	}
  	return 0, false, nil
  }
docs: // evalInt evals FIELD(str,str1,str2,str3,...).
  // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_field