function wp_admin_meta_dropdown() {
	global $wpdb;
	
	$meta_dropdown = '';
	$meta_keys = $wpdb->get_col("SELECT DISTINCT meta_key FROM $wpdb->postmeta WHERE SUBSTRING(meta_key,1,1) != '_'" );

	if( !empty($meta_keys) ) {
		$sortkey = ( isset($_GET['meta_key']) ) ? attribute_escape($_GET['meta_key']) : '';
		$meta_dropdown .= '<select name="meta_key" id="meta" class="postform">';
		$meta_dropdown .= ($sortkey == ('All' || '')) 
			? '<option selected="selected" value="All">View all meta &nbsp;</option>' 
			: '<option value="'.$metakey.'">View all meta &nbsp;&nbsp;</option>';
		foreach($meta_keys as $key => $metakey) {
			$meta_dropdown .= ($sortkey == $metakey) 
				? '<option selected="selected" value="'.$metakey.'">'.$metakey.'</option>'
				: '<option value="'.$metakey.'">'.$metakey.'</option>';
		}
		$meta_dropdown .= '</select>';
	}
	echo $meta_dropdown;
	return;	
}
function admin_pre_filter($query) {
	global $pagenow;
	if( $query->is_admin && ('edit.php' == $pagenow) ) { 
		$metakey = (isset($_GET['meta_key']) && $_GET['meta_key'] !== 'All') ? attribute_escape($_GET['meta_key']) : '';
		if('' != $metakey) {
			$query->set('orderby','meta_value');
			$query->set('order','ASC');
			$query->set('meta_key',$metakey);
		}
	}
	return $query;
}

add_filter('pre_get_posts', 'admin_pre_filter');
add_action('restrict_manage_posts', 'wp_admin_meta_dropdown',2);