Get parent page on the root level

We have next structure: Page=Subpage=Subsubpage=Subsubsubpage. We are on "Subsubsubpage" and we want to get parent page on the root level of current page.
Add this code to functions.php file:

<?php
function get_root_parent_id($page_id) {
	global $wpdb;
	$parent = $wpdb->get_var("SELECT post_parent FROM $wpdb->posts WHERE post_type='page' AND post_status='publish' AND ID = '$page_id'");
	if ($parent == 0) return $page_id;
	else return get_root_parent_id($parent);
}
?>

Add this code to template file:

<?php
global $post;
$page_id = $post->ID;
//Here is the Root Parent page
$root_parent_id = get_root_parent_id($page_id);
$root_parent_page = get_post($root_parent_id);
//Here is the page title
$root_parent_title = $root_parent_page->post_title;
//Here is the page link
$root_parent_link = get_permalink($root_parent_id);
?>
<h3><a href="<?php echo $root_parent_link;?>" title="<?php echo $root_parent_title;?>"><?php echo $root_parent_title;?></a></h3>
<ul class="sidebar_menu">
	<?php wp_list_pages("title_li=&child_of=$root_parent_id"); ?>
</ul>