code: func builtinCRC32(args []types.Datum, ctx context.Context) (d types.Datum, err error) {
  	if args[0].IsNull() {
  		return d, nil
  	}
  	x, err := args[0].ToString()
  	if err != nil {
  		return d, errors.Trace(err)
  	}
  	r := crc32.ChecksumIEEE([]byte(x))
  	d.SetUint64(uint64(r))
  	return d, nil
  }
docs: // See http://dev.mysql.com/doc/refman/5.7/en/mathematical-functions.html#function_crc32