MySQL-慢速查询优化

我在优化一个非常慢的prestashop SQL查询时遇到了一个问题（mysql大约需要3.5秒才能得到结果）

查询：

SELECT
    SQL_CALC_FOUND_ROWS a.*, a.id_order AS id_pdf, 
    CONCAT(LEFT(c.`firstname`, 1), '. ', c.`lastname`) AS `customer`, 
    osl.`name` AS `osname`, 
    os.`color`, 
    IF((SELECT COUNT(so.id_order) FROM `ps_orders` so WHERE so.id_customer = a.id_customer) > 1, 0, 1) as new, 
    (SELECT COUNT(od.`id_order`) FROM `ps_order_detail` od WHERE od.`id_order` = a.`id_order` GROUP BY `id_order`) AS product_number 
FROM `ps_orders` a 
LEFT JOIN `ps_customer` c 
    ON (c.`id_customer` = a.`id_customer`) 
LEFT JOIN `ps_order_history` oh 
    ON (oh.`id_order` = a.`id_order`) 
LEFT JOIN `ps_order_state` os 
    ON (os.`id_order_state` = oh.`id_order_state`) 
LEFT JOIN `ps_order_state_lang` osl 
    ON (os.`id_order_state` = osl.`id_order_state` AND osl.`id_lang` = 6) 
WHERE 1 
    AND oh.`id_order_history` = (SELECT MAX(`id_order_history`) FROM `ps_order_history` moh WHERE moh.`id_order` = a.`id_order` GROUP BY moh.`id_order`) 
ORDER BY `date_add` DESC
LIMIT 0,50

解释结果：

SELECT
    SQL_CALC_FOUND_ROWS a.*, a.id_order AS id_pdf, 
    CONCAT(LEFT(c.`firstname`, 1), '. ', c.`lastname`) AS `customer`, 
    osl.`name` AS `osname`, 
    os.`color`, 
    IF((SELECT COUNT(so.id_order) FROM `ps_orders` so WHERE so.id_customer = a.id_customer) > 1, 0, 1) as new, 
    (SELECT COUNT(od.`id_order`) FROM `ps_order_detail` od WHERE od.`id_order` = a.`id_order` GROUP BY `id_order`) AS product_number 
FROM `ps_orders` a 
LEFT JOIN `ps_customer` c 
    ON (c.`id_customer` = a.`id_customer`) 
LEFT JOIN `ps_order_history` oh 
    ON (oh.`id_order` = a.`id_order`) 
LEFT JOIN `ps_order_state` os 
    ON (os.`id_order_state` = oh.`id_order_state`) 
LEFT JOIN `ps_order_state_lang` osl 
    ON (os.`id_order_state` = osl.`id_order_state` AND osl.`id_lang` = 6) 
WHERE 1 
    AND oh.`id_order_history` = (SELECT MAX(`id_order_history`) FROM `ps_order_history` moh WHERE moh.`id_order` = a.`id_order` GROUP BY moh.`id_order`) 
ORDER BY `date_add` DESC
LIMIT 0,50

我该怎么办

---

提前感谢您的回答。

这篇评论有点长

explain

看起来很合理。除了最后的order by需要对204480行进行文件排序，这可能会占用很多时间。不清楚列

date\u add

来自何处。但是，也许你知道，所有的争吵都将发生在上周左右。如果是这样，那么额外的

where

子句可能会有帮助：

where date_added >= now() - interval 7 day and . . .

---

你有索引吗？是的，有创建的索引（prestashop默认）内部查询的时间，选择MAX（

id\u order\u history

）从

ps\u order\u history

moh WHERE moh.

id\u order

=a.

id\u order

按moh分组

id\u order

并且您是否创建了适当的索引，即用于比较的列？上面的查询速度很快（大约0.1005秒），我认为索引是正确的。在删除IF（（选择COUNT（so.id\u order）后也尝试运行从

ps\u订单

so，其中so.id\u客户=a.id\u客户）>1，0，1）作为新的（选择计数（od.

id\u订单

）从

ps\u订单详情

od，其中od.

id\u订单

=a.

id\u订单

分组作为产品编号，这些都会影响性能。我还有一个关于“ORDER BY

date\u add

DESC”的问题，这个日期\u add属于哪个表。当我删除

ORDER BY

子句时，仍然需要大约3秒钟。@user1768501。删除

SQL\u CALC\u FOUND\u行时会发生什么情况？这也明显降低了查询速度。哇，现在大约需要0.0116秒，所以我认为您已经找到了速度缓慢的原因，非常感谢，非常感谢。