code: func (b *builtinInstrSig) evalInt(row chunk.Row) (int64, bool, error) {
  	str, IsNull, err := b.args[0].EvalString(b.ctx, row)
  	if IsNull || err != nil {
  		return 0, true, err
  	}
  	str = strings.ToLower(str)
  
  	substr, IsNull, err := b.args[1].EvalString(b.ctx, row)
  	if IsNull || err != nil {
  		return 0, true, err
  	}
  	substr = strings.ToLower(substr)
  
  	idx := strings.Index(str, substr)
  	if idx == -1 {
  		return 0, false, nil
  	}
  	return int64(utf8.RuneCountInString(str[:idx]) + 1), false, nil
  }
docs: // evalInt evals INSTR(str,substr), case insensitive
  // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_instr