php – 为什么我的MySQL连接速度慢？

SELECT *,A.id AS id,A.username AS username,G.desc AS customer_desc,H.id AS counter,SUM( F.amount_paid ) 
FROM tblcheckin AS A
LEFT OUTER JOIN tblrate AS B ON B.id = A.rate_id
LEFT OUTER JOIN tblrefaccom_type AS C ON C.id = A.id
LEFT OUTER JOIN tblcust_bill AS D ON D.check_in_id = A.id
LEFT OUTER JOIN tblroom AS E ON E.room_no = A.room_id
LEFT OUTER JOIN tblpayment AS F ON F.check_in_id = A.id
LEFT OUTER JOIN tblrefcust_type AS G ON G.id = A.customer_desc
LEFT OUTER JOIN tblindex AS H ON A.id = H.check_in_id
WHERE A.check_out =  "0000-00-00 00:00:00"
GROUP BY F.check_in_id
ORDER BY check_out,check_in

结果：显示0到11行(总共12行,查询占用7.4061秒)

这就是Explain SQL

这是我到目前为止所做的：
我设置optimizer_search_depth = 0,因为我认为它会像Joining many tables in mySQL中提到的那样工作.

我在这做错了什么？我感谢每一个反馈.谢谢.

UPDATE.问题解决了.我从tblpayment,tblcust_bill和tblindex向check_in_id列添加了索引.这是新的Explain SQL：