public function removeOphanUserMeta()
    {
        $this->db->query("DELETE FROM " . $this->db->usermeta . "
        WHERE NOT EXISTS (
          SELECT * FROM " . $this->db->users . "
            WHERE " . $this->db->usermeta . ".user_id = " . $this->db->users . ".ID
        )");

        wp_cache_flush();
    }