Cache query?

[eluser]Hamed[/eluser]
I enable query cache in this way:
1-config/database.php
TRUE for cache query and set cache folder.
2-Change cache folder permission to 777.

Now when I want to cache a query I use $this->db->cache_on(); before query and after that I use $this->db->cache_off();

Now I have one question:
All queries cached or just query with cache_on cached?
Just I need to cache queries with cache_on.