SELECT xx.x1,yy.y1,xx.x2,yy.y2

FROM

(
	SELECT tx1.x2 as 'x1',tx2.x1 as 'x2'
	FROM xy tx1, xy tx2
	WHERE tx1.x2 < tx2.x1
)xx,
(
	SELECT ty1.y2 as 'y1',ty2.y1 as 'y2'
	FROM xy ty1, xy ty2
	WHERE ty1.y2 < ty2.y1
)yy

WHERE 
	NOT EXISTS
	(
		SELECT 1 
		FROM xy 
		WHERE
			(	(xy.x1 >= xx.x1 AND xy.x1 < xx.x2) OR (xy.x2 > xx.x1 AND xy.x2 <= xx.x2)	) AND
			(	(xy.y1 >= yy.y1 AND xy.y1 < yy.y2) OR (xy.y2 > yy.y1 AND xy.y2 <= yy.y2)	)
	)	 

ORDER BY xx.x1,yy.y1,xx.x2,yy.y2

CREATE PROCEDURE `pFree`(IN `dx` INT, IN `dy` INT)
	LANGUAGE SQL
	NOT DETERMINISTIC
	READS SQL DATA
	SQL SECURITY DEFINER
	COMMENT ''
BEGIN
SELECT 
	xx.x1,
	yy.y1,
	LEAST(xx.x2, xx.x1+dx) as 'x2',
	LEAST(yy.y2, yy.y1+dy) as 'y2', 
	xx.x2 - xx.x1 - dx + yy.y2 - yy.y1 - dy as 'delta'

FROM

(
	SELECT DISTINCT tx1.x2 as 'x1',tx2.x1 as 'x2'
	FROM xy tx1, xy tx2
	WHERE tx1.x2 < tx2.x1
)xx,
(
	SELECT DISTINCT ty1.y2 as 'y1',ty2.y1 as 'y2'
	FROM xy ty1, xy ty2
	WHERE ty1.y2 < ty2.y1
)yy

WHERE 
	xx.x2 - xx.x1 >= dx AND
	yy.y2 - yy.y1 >= dy AND
	NOT EXISTS
	(
		SELECT 1 
		FROM xy 
		WHERE
			(	(xy.x1 >= xx.x1 AND xy.x1 < xx.x2) OR 
				(xy.x2 > xx.x1 AND xy.x2 <= xx.x2) OR
				(xy.x1 <= xx.x1 AND xy.x2 >= xx.x2)
			) AND
			(	(xy.y1 >= yy.y1 AND xy.y1 < yy.y2) OR 
				(xy.y2 > yy.y1 AND xy.y2 <= yy.y2) OR
				(xy.y1 <=yy.y1 AND xy.y2 >= yy.y2)	
			)
	)	 

ORDER BY xx.x1,yy.y1,xx.x2,yy.y2
LIMIT 1;
END