最近，我们将数据库从 MariaDB 10.2 切换到 Percona Server (Mysql 5.7)，并且我们有一个查询需要大约 15 秒(之前约为 0.5 秒)，因为查询优化器未在主表上使用任何索引。因为应用逻辑，我们无法改变查询格式，我们需要让DB使用索引。
查询结构很简单:
EXPLAIN SELECT `clients`.`id` AS `t0_c0`,
`client`.`name1` AS `t0_c1`,
`client`.`name2` AS `t0_c2`,
`users`.`id` AS `t1_c0`,
`users`.`suffix` AS `t1_c1`,
`users`.`position` AS `t1_c2`,
`users`.`first_name` AS `t1_c3`,
`users`.`last_name` AS `t1_c4`,
`privateData`.`id` AS `t2_c0`,
`privateData`.`first_name` AS `t2_c1`,
`privateData`.`last_name` AS `t2_c2`,
`tariff`.`id` AS `t3_c0`,
`tariff`.`provider_id` AS `t3_c1`,
`tariff`.`tariff_type` AS `t3_c2`,
`tariff`.`name` AS `t3_c3`,
`providers`.`id` AS `t4_c0`,
`providers`.`name1` AS `t4_c1`,
`providers`.`name2` AS `t4_c2`,
`addresses`.`id` AS `t5_c0`,
`addresses`.`zipcode` AS `t5_c1`,
`addresses`.`country` AS `t5_c2`,
`addresses`.`city` AS `t5_c3`,
`private`.`id` AS `t6_c0`,
`private`.`first_name` AS `t6_c1`,
`private`.`last_name` AS `t6_c2`,
`commercial`.`id` AS `t7_c0`,
`commercial`.`name1` AS `t7_c1`,
`commercial`.`name2` AS `t7_c2`,
`commercial`.`name_on_invoice` AS `t7_c3`,
`commercial`.`organization_type` AS `t7_c4`,
`organizations`.`id` AS `t8_c0`,
`organizations`.`person_id` AS `t8_c1`,
`organizations`.`address_id` AS `t8_c2`,
`organizations`.`status` AS `t8_c3`,
`shaddresses`.`id` AS `t9_c0`,
`shaddresses`.`zipcode` AS `t9_c1`,
`shaddresses`.`country` AS `t9_c2`,
`shaddresses`.`city` AS `t9_c3`,
`shprivate`.`id` AS `t10_c0`,
`shprivate`.`first_name` AS `t10_c1`,
`shprivate`.`last_name` AS `t10_c2`,
`coraddresses`.`id` AS `t11_c0`,
`coraddresses`.`zipcode` AS `t11_c1`,
`coraddresses`.`country` AS `t11_c2`,
`corprivate`.`id` AS `t12_c0`,
`corprivate`.`first_name` AS `t12_c1`,
`corprivate`.`last_name` AS `t12_c2`,
FROM `client` `client`
LEFT OUTER JOIN `users` `users` ON (`client`.`user_id`=`users`.`id`)
AND (users.status!=5)
LEFT OUTER JOIN `private` `privateData` ON (`users`.`person_id`=`privateData`.`id`)
LEFT OUTER JOIN `tariff` `tariff` ON (`client`.`rate_id`=`tariff`.`id`)
LEFT OUTER JOIN `providers` `providers` ON (`client`.`provider_id`=`providers`.`id`)
LEFT OUTER JOIN `addresses` `addresses` ON (`client`.`main_address_id`=`addresses`.`id`)
LEFT OUTER JOIN `private` `private` ON (`client`.`main_person_id`=`private`.`id`)
LEFT OUTER JOIN `commercial` `commercial` ON (`client`.`main_organization_id`=`commercial`.`id`)
LEFT OUTER JOIN `organizations` `organizations` ON (`client`.`id_organization`=`organizations`.`id`)
AND (organizations.status!=5)
LEFT OUTER JOIN `addresses` `shaddresses` ON (`client`.`shipping_address_id`=`shaddresses`.`id`)
LEFT OUTER JOIN `private` `shprivate` ON (`client`.`shipping_person_id`=`shprivate`.`id`)
LEFT OUTER JOIN `addresses` `coraddresses` ON (`client`.`correspondense_address_id`=`coraddresses`.`id`)
LEFT OUTER JOIN `private` `corprivate` ON (`client`.`correspondense_person_id`=`corprivate`.`id`)
WHERE (client.status!=5)
ORDER BY client.id DESC
LIMIT 10
OFFSET 10
我可以更改任何索引，但是，我无法更改查询。 在旧主机上，它的运行时间为 0.2 秒，但是，它的优化器使用客户端表中的索引。使用 Percona Server (mysql 5.7) 需要 15 秒。优化器不使用客户端表中的任何索引。使用来自客户端表的 FORCE INCEX() 的时间将低于 1 秒(复合索引的时间约为 0.2 秒)。 表“providers”只有 1 行。 我已经在“客户”表上设置了索引，但是，在解释中，它们没有显示为可能的键。
我尝试将 MySql 变量“max_seeks_for_key”设置为 1，但是它仍然没有使用索引。
我认为我错过了一些基本的东西，但我不知道是什么。
此查询的解释是:
ORDER BY 正在生成临时表，并且正在使用所有资源(即使没有 INDEX，没有 order by 也会在一秒内运行)。
任何想法都会受到赞赏。
最佳答案
这应该比使用 FORCE 更有效:
SELECT ...
FROM ( SELECT id
FROM client
WHERE status != 5
ORDER BY id DESC
LIMIT 10 OFFSET 10
) AS ids
JOIN client USING(id)
LEFT OUTER JOIN `users` `users` ON (`client`.`user_id`=`users`.`id`)
AND (users.status!=5)
LEFT OUTER JOIN `private` `privateData` ON (`users`.`person_id`=`privateData`.`id`)
LEFT OUTER JOIN `tariff` `tariff` ON (`client`.`rate_id`=`tariff`.`id`)
LEFT OUTER JOIN `providers` `providers` ON (`client`.`provider_id`=`providers`.`id`)
LEFT OUTER JOIN `addresses` `addresses` ON (`client`.`main_address_id`=`addresses`.`id`)
LEFT OUTER JOIN `private` `private` ON (`client`.`main_person_id`=`private`.`id`)
LEFT OUTER JOIN `commercial` `commercial` ON (`client`.`main_organization_id`=`commercial`.`id`)
LEFT OUTER JOIN `organizations` `organizations` ON (`client`.`id_organization`=`organizations`.`id`)
AND (organizations.status!=5)
LEFT OUTER JOIN `addresses` `shaddresses` ON (`client`.`shipping_address_id`=`shaddresses`.`id`)
LEFT OUTER JOIN `private` `shprivate` ON (`client`.`shipping_person_id`=`shprivate`.`id`)
LEFT OUTER JOIN `addresses` `coraddresses` ON (`client`.`correspondense_address_id`=`coraddresses`.`id`)
LEFT OUTER JOIN `private` `corprivate` ON (`client`.`correspondense_person_id`=`corprivate`.`id`)
ORDER BY client.id DESC -- Yes, repeated here
关于MySql 查询不使用索引集，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/50319510/