Why query cache do not work?

Did you try giving it a username, password and database name?

Obviously, the data have been removed ahah

You can't use the database driver for sessions on a database connection which has the cache enabled. So, you would either need to configure a second database connection with cache enabled to be used by the rest of your application, or selectively enable query caching when you want to use it, using $this->db->cache_on() / $this->db->cache_off().

Thanks for the reply   I'll try to use cache_on and cache_off