wp_query (pre_get_posts) issue

Hi, I got such issue – wp_query editing doesn`t work for search queries. The code:

function my_modify_main_query($query)
{
	if(is_category() || is_search())
	{
		$sorting = $_GET['sorting'];

		session_start();

		if($sorting != '')
		{
			$_SESSION['sorting'] = $sorting;
		}
		else
		{
			$sorting = $_SESSION['sorting'];
		}

		if($sorting == 'latest')
		{
			$query->set('orderby', 'date');
			$query->set('order', 'DESC');
		}

		if($sorting == 'price-l-h')
		{
			$query->set('meta_key', 'amzn_FlatPrice');
			$query->set('orderby', 'meta_value');
			$query->set('order', 'ASC');
		}

		if($sorting == 'price-h-l')
		{
			$query->set('meta_key', 'amzn_FlatPrice');
			$query->set('orderby', 'meta_value');
			$query->set('order', 'DESC');
		}
	}
}

add_action( 'pre_get_posts', 'my_modify_main_query' );

All works fine for categories but doesnt work for search. I made echo and I saw that my code is processed for search queries too, but for some reason ORDER options doesnt apply..