Codeigniter unset session

嗨，我有一个在CodeIgniter中开发的网站。

在我的一个页面中，我在搜索表单后使用CodeIgniter的分页。

在这种情况下，我将传递的搜索值存储在我的会话中，因为如果我有更多的结果单击下一页，则搜索keppe搜索值。$_POST

但是，当我更改页面时，例如，我想返回到索引，并且在返回到我的搜索表单页面后，会话已经创建，并使用会话的值进行查询。更改页面时，如何销毁或取消设置会话的值？这可能吗？

在我的模型函数中，我检查会话值是否与0不同并且存在，如果为true，则使用会话值进行查询。

这是我的控制器（国家/地区是要存储到会话中的值）

public function region_list(){
        $this->load->model('backend/Nation_model');
        $this->load->library("pagination");

        if($_POST)
        {
            if (isset($_POST['ricerca'])){

                $nation = $this->input->post('nation');
                if(strlen($nation) > 0){
                   $this->session->set_userdata('nation',$nation);
                }

                $config = array();
                $config["base_url"] = base_url() . "index.php/backend/region/region_list";
                $config["total_rows"] = $this->Region_model->countRegionSearch();
                $config["per_page"] = 10;
                $config["uri_segment"] = 4;

                $this->pagination->initialize($config);

                $page = ($this->uri->segment(4)) ? $this->uri->segment(4) : 0;
                $data["regionlist"] = $this->Region_model->regionSearch($config["per_page"], $page);
                $data["links"] = $this->pagination->create_links();
                $data["nationlist"] = $this->Nation_model->nationList();

                $this->load->view('backend/region_list_view',$data);
            }
        }
        else
        {
            $config = array();
            $config["base_url"] = base_url() . "index.php/backend/region/region_list";
            $config["total_rows"] = $this->Region_model->countRegion();
            $config["per_page"] = 10;
            $config["uri_segment"] = 4;

            $this->pagination->initialize($config);

            $page = ($this->uri->segment(4)) ? $this->uri->segment(4) : 0;
            $data["links"] = $this->pagination->create_links();
            $data["regionlist"] = $this->Region_model->regionList($config["per_page"], $page);
            $data["nationlist"] = $this->Nation_model->nationList();

            $this->load->view('backend/region_list_view',$data);
        }
    }

这是我要搜索的模型：

function regionList($limit=null, $start=null) {
        $nation_id = $this->session->userdata('nation');
        if ($this->session->userdata('language')=="it")
            $this->db->select('region.id, region.name_it as name,nation.id as nation_id, nation.name_it as nation_name');
        if ($this->session->userdata('language')=="en")
            $this->db->select('region.id, region.name_en as name,nation.id as nation_id, nation.name_en as nation_name');
        $this->db->from('region');
        $this->db->join('nation', 'region.nation_id = nation.id','left');
        if((isset($nation_id))&&($nation_id!=0))
            $this->db->where('region.nation_id', $nation_id);
        $this->db->order_by("name", "asc");
        $this->db->limit($limit, $start);
        $query = $this->db->get();
        $region = array();
        foreach ($query->result() as $row)
            array_push($region, $row);

        return $region;     
    }

    function countRegion() {
        $nation_id = $this->session->userdata('nation');
        if ($this->session->userdata('language')=="it")
            $this->db->select('region.id, region.name_it as name,nation.id as nation_id, nation.name_it as nation_name');
        if ($this->session->userdata('language')=="en")
            $this->db->select('region.id, region.name_en as name,nation.id as nation_id, nation.name_en as nation_name');
        $this->db->from('region');
        $this->db->join('nation', 'region.nation_id = nation.id','left');
        if((isset($nation_id))&&($nation_id!=0))
            $this->db->where('region.nation_id', $nation_id);
        $this->db->order_by("name", "asc");
        $query = $this->db->get();
        return $query->num_rows();  
    }

    public function regionSearch($limit=null, $start=null){
        $nation_id = $this->session->userdata('nation');
        if ($this->session->userdata('language')=="it"){
            $this->db->select('*,region.id, region.name_it as name,nation.id as nation_id, nation.name_it as nation_name');
            if($this->input->post('name')!="")
                $this->db->where('region.name_it LIKE "%'.$this->input->post('name').'%"');
        }

        if ($this->session->userdata('language')=="en"){
            $this->db->select('*,region.id, region.name_en as name,nation.id as nation_id, nation.name_en as nation_name');
            if($this->input->post('name')!="")
                $this->db->where('region.name_en LIKE "%'.$this->input->post('name').'%"');
        }
        $this->db->from('region');
        $this->db->join('nation', 'region.nation_id = nation.id','left');
        if((isset($nation_id))&&($nation_id!=0))
            $this->db->where('region.nation_id', $nation_id);
        $this->db->order_by("name", "asc");
        $this->db->limit($limit, $start);
        $query = $this->db->get();
        $region = array();
        foreach ($query->result() as $row)
            array_push($region, $row);

        return $region; 
     }

     public function countRegionSearch(){
        $nation_id = $this->session->userdata('nation');
        if ($this->session->userdata('language')=="it"){
            $this->db->select('*,region.id, region.name_it as name,nation.id as nation_id, nation.name_it as nation_name');
            if($this->input->post('name')!="")
                $this->db->where('region.name_it LIKE "%'.$this->input->post('name').'%"');
        }

        if ($this->session->userdata('language')=="en"){
            $this->db->select('*,region.id, region.name_en as name,nation.id as nation_id, nation.name_en as nation_name');
            if($this->input->post('name')!="")
                $this->db->where('region.name_en LIKE "%'.$this->input->post('name').'%"');
        }
        $this->db->from('region');
        $this->db->join('nation', 'region.nation_id = nation.id','left');
        if((isset($nation_id))&&($nation_id!=0))
            $this->db->where('region.nation_id', $nation_id);
        $this->db->order_by("name", "asc");

        $query = $this->db->get();
        return $query->num_rows();      
     }