从具有多个联接的表中进行简单选择需要很长时间才能执行

Question

我有一个大约84mb左右的MySQL数据库。 以下查询：

SELECT     
    `reservations`.`id`
FROM       
    `reservations` 
INNER JOIN `reservation_room` 
ON         `reservation_room`.`reservation_id` = `reservations`.`id` 

LEFT JOIN  `users` 
ON         `reservations`.`user_id` = `users`.`id` 

INNER JOIN `rooms` 
ON         `reservation_room`.`room_id` = `rooms`.`id` 

INNER JOIN `stories` 
ON         `rooms`.`story_id` = `stories`.`id`

INNER JOIN `hotels` 
ON         `stories`.`hotel_id` = `hotels`.`id`

INNER JOIN `neighbourhoods` 
ON         `hotels`.`neighbourhood_id` = `neighbourhoods`.`id` 

INNER JOIN `cities` 
ON         `cities`.`id` = `neighbourhoods`.`city_id` 

GROUP BY   `reservations`.`id` 
ORDER BY   `reservations`.`created_at` DESC
LIMIT      30 
OFFSET     0

执行大约需要4到700秒。 reservations表有48k条记录， reservation_room有191k条记录。 该查询经常执行，这会使mysql进程在CPU上变得困难：

 PID USER      PR  NI  VIRT  RES  SHR S  %CPU %MEM    TIME+  COMMAND                                                                                                                                                                                                           
 5021 mysql    20   0  755m 385m 8596 S 748.7  1.2   5353:58 mysqld    

我该如何加快查询速度，或者至少不使整个mysql服务器发疯？

EXPLAIN上面的查询：

98％的执行时间是Copying to temp table 。