MySQL 忽略索引，即使使用使用索引或强制索引

Question

在这里，我将添加部分查询，因为它是敏感信息，并且我不会使用真实姓名:

SELECT
  `entity`.`id`,
  `entity`.`requirements`,
  `entity`.`description`,
  `entity`.`status`,
  `entity_videos`.`length`,
  `entity_videos`.`quality`,
  `states`.`name`,
  `uploads`.`id`,
  `uploads`.`name`
    FROM  `entity`
    LEFT JOIN  `states`  ON `states`.`id` = `entity`.`state_id`
    INNER JOIN  `uploads` FORCE INDEX (`uploadable_id_index`)
       ON  `uploads`.`uploadable_type` = 'Entity'
      AND  `uploads`.`category` = 'Icon'
      AND  (`uploads`.`uploadable_id` = `entity`.`id`
              OR  `uploads`.`uploadable_id` = `entity`.`parent_entity_for_icon`
           )
    INNER JOIN  `entity_videos` FORCE INDEX (entity_videos_entity_id_index)
       ON `entity_videos`.`entity_id` = `entity`.`id`
    WHERE  `entity`.`status` = 'active' 

问题是 Mysql 优化器不想使用 uploadable_id_index 索引。部分解释:

据我所知，FORCE INDEX是强制优化器使用索引，除非优化器无法使用索引。我应该怎么做才能强制该索引而不对表进行完整扫描？ 我尝试删除 entity_videos_entity_id_index 也尝试将 uploads 表信息添加到 where 子句，但对我没有任何作用。有任何想法吗？非常感谢您的帮助

更新:

在@Barmar和@PaulSpiegel的帮助下，我发现问题出在(uploads.uploadable_id =entity.id OR uploads.uploadable_id=entity.parent_entity_for_icon)中。经过一段时间的查询，我发现我的情况下最好的解决方案是:

 SELECT
  `entity`.`id`,
  `entity`.`requirements`,
  `entity`.`description`,
  `entity`.`status`,
  `entity_videos`.`length`,
  `entity_videos`.`quality`,
  `states`.`name`,
  `icon`.`id`,
  `icon`.`name`,
  `parent_offer_icon`.`id`,
  `parent_offer_icon`.`name`
    FROM  `entity`
    LEFT JOIN  `states`  ON `states`.`id` = `entity`.`state_id`
    LEFT JOIN  `uploads` as `icon` FORCE INDEX (`uploadable_id_index`)
       ON  `icon`.`uploadable_type` = 'Entity'
      AND  `icon`.`category` = 'Icon'
      AND  `icon`.`uploadable_id` = `entity`.`id`
    LEFT JOIN  `uploads` as `parent_offer_icon` FORCE INDEX (`uploadable_id_index`)
       ON  `parent_offer_icon`.`uploadable_type` = 'Entity'
      AND  `parent_offer_icon`.`category` = 'Icon'
      AND  `parent_offer_icon`.`uploadable_id` = `entity`.`parent_entity_for_icon`
    INNER JOIN  `entity_videos` FORCE INDEX (entity_videos_entity_id_index)
       ON  `entity_videos`.`entity_id` = `entity`.`id`
    WHERE  `entity`.`status` = 'active'
      AND  (parent_offer_icon.id IS NOT NULL
              OR  icon.id IS NOT NULL ) 

我仍然愿意接受其他建议:)

Answer 1

让我们首先将讨厌的 OR 转换为 UNION:

( SELECT  e.id AS eid,
          u.id AS uid,
          u.name AS uname
    FROM `uploads` AS u 
    INNER JOIN  `entity` AS e
       ON  u.`uploadable_id` = e.`id`
    WHERE  u.`uploadable_type` = 'Entity'
      AND  u.`category` = 'Icon'
      AND  e.status = 'active'
) UNION DISTINCT
( SELECT  e.id AS eid,
          u.id AS uid,
          u.name AS uname
    FROM `uploads` AS u 
    INNER JOIN   `entity` AS e
       ON  u.`uploadable_id` = e.`parent_entity_for_icon` 
    WHERE  u.`uploadable_type` = 'Entity'
      AND  u.`category` = 'Icon'
      AND  e.status = 'active'
)

这将需要其中一些索引:

uploads:  INDEX(uploadable_type, category, uploadable_id, id, name)
entity:   INDEX(parent_entity_for_icon, status, id)

(我假设entity有PRIMARY KEY(id)？请提供SHOW CREATE TABLE，这样我就不必猜测。)如果这么长的索引有问题，请告诉我；我也许可以提供一个解决方法。

通过将OR转换为UNION，每个部分可以使用不同的索引。使用OR时，优化器通常会放弃并做一些低效的事情。

请验证上面的查询是否运行快速并产生合理的输出。然后...

SELECT  e.`id`, e.`requirements`, e.`description`, e.`status`,
        ev.`length`, ev.`quality`,
        s.`name`,
        uid, uname
    FROM ( the-query-above ) AS i
    JOIN `entity` AS e  ON e.id = i.eid
    LEFT JOIN  `states` AS s  ON s.`id` = e.`state_id`
    INNER JOIN  `entity_videos` AS ev  ON ev.`entity_id` = i.eid

除了每个表上的PRIMARY KEY(id)之外，您还需要

entity_videos:  INDEX(entity_id)  -- good, or
entity_videos:  INDEX(entity_id, length, quality)  -- better ("covering")

不要使用FORCE INDEX。

有关索引创建的更多信息:http://mysql.rjweb.org/doc.php/index_cookbook_mysql