Tengo dos tablas: una con puntos, la otra con polys.

CREATE TABLE `points` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `point` point NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM;

CREATE TABLE `ranges` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `poly` polygon NOT NULL,
  PRIMARY KEY (`id`),
  SPATIAL KEY `poly` (`poly`)
) ENGINE=MyISAM;

Quiero unir rangos a puntos en puntos dentro de polys. Las consultas se ven simples:

SELECT * 
  FROM points 
    LEFT JOIN ranges 
      ON MBRCONTAINS(poly, point) 
  WHERE points.id = 2;

Esta consulta funciona rápido y utiliza índices, parte de la explicación:

table  | type  | possible_keys | key  | key_len
ranges | range | poly          | poly | 34

Pero, cuando trato de unirme con varias filas de la tablapoints:

SELECT * 
  FROM points 
   LEFT JOIN ranges 
    ON MBRCONTAINS(poly, point) 
  WHERE points.id IN (1,2,3);

todo se descompone:

+----+-------------+------------+-------+---------------+---------+---------+------+--------+-------------+
| id | select_type | table      | type  | possible_keys | key     | key_len | ref  | rows   | Extra       |
+----+-------------+------------+-------+---------------+---------+---------+------+--------+-------------+
|  1 | SIMPLE      | points     | range | PRIMARY       | PRIMARY | 4       | NULL |      3 | Using where |
|  1 | SIMPLE      | ranges     | ALL   | poly          | NULL    | NULL    | NULL | 155183 |             |
+----+-------------+------------+-------+---------------+---------+---------+------+--------+-------------+

AgregandoFORCE INDEX (poly) no ayuda

Muestra de datos para probar consultas (lo siento, solo la versión php, no soy común con los procedimientos SQL):

//points
for($i=0;$i<=500;$i++) {
    $point = mt_rand();
    mysql_query('INSERT INTO points (point) VALUES (POINTFROMWKB(POINT('.$point.', 0)))');
}

$qty = 20000;
$max = mt_getrandmax();
$add = $max / $qty
$end = 0;

//polys
while($end < $max) {
    $start = $end;
    $end = mt_rand($start, $start + $add);
    mysql_query('INSERT INTO ranges (poly) VALUES (
        GEOMFROMWKB(POLYGON(LINESTRING(
            POINT('.$start.', -1),
            POINT('.$end.',   -1),
            POINT('.$end.',    1),
            POINT('.$start.',  1),
            POINT('.$start.', -1)
          )))
    )');
}