What indexes would I add to make this table not use filesort? I've tried all indexes I can think of and it always resorts to filesort.

SELECT CONCAT( REPEAT( ' ', COUNT( parent.title ) -1 ) , node.title ) AS name, GROUP_CONCAT( parent.title
ORDER BY parent.left_
SEPARATOR '~' ) AS 'path', node.left_
FROM category AS node
LEFT JOIN category AS parent ON node.left_
BETWEEN parent.left_
AND parent.right_
GROUP BY node.title
ORDER BY path, node.title;
Explain gives:
Using temporary; Using filesort