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Ever wanted to generate routing from database with codeigniter? I had to for one of my recent project. Here goes the detailed instruction on doing it.
Extend the Router class. Create new file My_Router.php in your application libraries folder with following content.

class MY_Router extends CI_Router {

    function MY_Router()
    {
        parent::CI_Router();
    }

    function _validate_request($segments)
    {
        // Does the requested controller exist in the root folder?
        if (file_exists(APPPATH.'controllers/'.$segments[0].EXT))
        {
            return $segments;
        }

        // Is the controller in a sub-folder?
        if (is_dir(APPPATH.'controllers/'.$segments[0]))
        {
            // Set the directory and remove it from the segment array
            $this->set_directory($segments[0]);
            $segments = array_slice($segments, 1);

            if (count($segments) > 0)
            {
                // Does the requested controller exist in the sub-folder?
                if ( ! file_exists(APPPATH.'controllers/'.$this->fetch_directory().$segments[0].EXT))
                {
                    show_404($this->fetch_directory().$segments[0]);
                }
            }
            else
            {
                $this->set_class($this->default_controller);
                $this->set_method('index');

                // Does the default controller exist in the sub-folder?
                if ( ! file_exists(APPPATH.'controllers/'.$this->fetch_directory().$this->default_controller.EXT))
                {
                    $this->directory = '';
                    return array();
                }

            }
            return $segments;
        }

	$user_routes = $this->user_routing($segments);
	if($user_routes !== FALSE)
	{
		return $user_routes;
	}

        show_404($segments[0]);
    }

function user_routing($segments)
	{
		if($this->check_username_exist($segments[0]))
		{
			//only profile
			if(count($segments)==1)
			{
				return array('user','profile',$segments[0]);
			}
		}
		return FALSE;
	}

	function check_username_exist($username)
	{
		//connect to database and find the category
		include(APPPATH.'config/database'.EXT);

		$conn = mysql_connect($db['default']['hostname'], $db['default']['username'], $db['default']['password']);
		mysql_select_db($db['default']['database'],$conn);
		$sql = sprintf("SELECT COUNT(id) as count FROM users WHERE username = '%s'", mysql_real_escape_string($username));
		$query = mysql_query($sql);
		$row = mysql_fetch_object($query);
		mysql_close($conn);
		if($row->count)
		{
			return TRUE;
		}
		return FALSE;
	}
}

The above code checks for the username in database table user and routes them to controller user/profile. Offcourse you could use simple routing rules but there can be case when you specifically route for the records in the database only. So this method comes handy in that condition.