Question

在这里，我将添加部分查询，因为它是敏感信息，我将不使用真实姓名：

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，除了优化器不能使用索引的情况。我应该怎么做才能强制该索引并且不对表进行全面扫描？我试图删除entity_videos_entity_id_index我也试图将uploads表信息添加到where子句，但是对我来说没有任何作用。有任何想法吗？非常感谢你的帮助

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)。和查询玩了一段时间我发现在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

让我们开始将讨厌的OR变成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)

（我认为entity有PRIMARY KEY(id)？请提供SHOW CREATE TABLE所以我不必猜测。）如果这么长的索引有问题，请告诉我;我可以提供一种解决方法。

通过将OR转换为UNION，可以为每个部分使用不同的索引。使用OR时，优化程序通常会进行填充并执行效率低下的操作。

请验证上面的查询运行速度快并产生合理的输出。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

除了每张桌子上的PRIMARY KEY(id)，您还需要

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

请勿使用FORCE INDEX。

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

即使使用索引或强制索引，MySQL也会忽略索引

1 个答案: