诡异的MySql执行计划的更改

SQL是在从库上执行

正常的执行计划：
explain SELECT h.id hid,r.rate rate FROM ORDER_CUST_INFO o, exchange_rate r, HOTEL_INFO h where o.group_id = h.group_id and o.currency = r.type and r.type != 'CNY' and h.type = 'OWNER' and h.apply_status = 'CHECKED' and h.online = true and h.online_status = true and h.deleted = false ;

+----+-------------+-------+------+---------------+--------------+---------+----------------+-------+--------------------------------+

| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

+----+-------------+-------+------+---------------+--------------+---------+----------------+-------+--------------------------------+

| 1 | SIMPLE | r | ALL | NULL | NULL | NULL | NULL | 2 | Using where |

| 1 | SIMPLE | o | ALL | FK_OCI_GID | NULL | NULL | NULL | 18476 | Using where; Using join buffer |

| 1 | SIMPLE | h | ref | idx_group_id | idx_group_id | 5 | hms.o.GROUP_ID | 1 | Using where |

+----+-------------+-------+------+---------------+--------------+---------+----------------+-------+--------------------------------+

3 rows in set (0.00 sec)

在定时备份时，从库的执行计划变更
mysql> explain SELECT h.id hid,r.rate rate FROM ORDER_CUST_INFO o, exchange_rate r, HOTEL_INFO h where o.group_id = h.group_id and o.currency = r.type and r.type != 'CNY' and h.type = 'OWNER' and h.apply_status = 'CHECKED' and h.online = true and h.online_status = true and h.deleted = false ;

+----+-------------+-------+--------+---------------+------------+---------+----------------+-------+--------------------------------+

| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

+----+-------------+-------+--------+---------------+------------+---------+----------------+-------+--------------------------------+

| 1 | SIMPLE | r | ALL | NULL | NULL | NULL | NULL | 2 | Using where |

| 1 | SIMPLE | h | ALL | idx_group_id | NULL | NULL | NULL | 68935 | Using where; Using join buffer |

| 1 | SIMPLE | o | eq_ref | FK_OCI_GID | FK_OCI_GID | 4 | hms.h.group_id | 1 | Using where |

+----+-------------+-------+--------+---------------+------------+---------+----------------+-------+--------------------------------+

3 rows in set (0.00 sec)

是在想不到好办法，只能强制表的扫描顺序：

SELECT

h.id hid,

r.rate rate

FROM

exchange_rate r
STRAIGHT_JOIN ORDER_CUST_INFO o   ON o.CURRENCY = r.TYPE
STRAIGHT_JOIN HOTEL_INFO h  ON h.group_id = o.GROUP_ID

WHERE o.group_id = h.group_id

AND o.currency = r.TYPE

AND r.TYPE != 'CNY'

AND h.TYPE = 'OWNER'

AND h.apply_status = 'CHECKED'

AND h.online = TRUE

AND h.online_status = TRUE

AND h.deleted = FALSE ;