code: func GetDefaultFieldLength(tp byte) int {
  	switch tp {
  	case TypeTiny:
  		return 4
  	case TypeShort:
  		return 6
  	case TypeInt24:
  		return 9
  	case TypeLong:
  		return 11
  	case TypeLonglong:
  		return 21
  	case TypeDecimal, TypeNewDecimal:
  		// See https://dev.mysql.com/doc/refman/5.7/en/fixed-point-types.html
  		return 10
  	case TypeBit, TypeBlob:
  		return -1
  	default:
  		//TODO: Add more types.
  		return -1
  	}
  }
docs: // GetDefaultFieldLength is used for Interger Types, Flen is the display length.
  // Call this when no Flen assigned in ddl.
  // or column value is calculated from an expression.
  // For example: "select count(*) from t;", the column type is int64 and Flen in ResultField will be 21.
  // See https://dev.mysql.com/doc/refman/5.7/en/storage-requirements.html