anchor.diff:        if (oldVersion <= 1) {
           await db.execute('''
           alter table $_tableCacheObject 
  -        add ${CacheObject.columnKey} text,
  -            ${CacheObject.columnLength} integer;
  +        add ${CacheObject.columnKey} text;
   
           update $_tableCacheObject 
             set ${CacheObject.columnKey} = ${CacheObject.columnUrl}
  
positive.code: 
    @override
    Future open() async {
      db = await openDatabase(path, version: 2,
          onCreate: (Database db, int version) async {
        await db.execute('''
        create table $_tableCacheObject ( 
  
negative.code: 
          create unique index $_tableCacheObject${CacheObject.columnKey} 
            on $_tableCacheObject (${CacheObject.columnKey});
          ''');
        }
        if (oldVersion <= 2) {
          await db.execute('''
          alter table $_tableCacheObject 
          add ${CacheObject.columnLength} integer;
          ''');
        }
      });
  