MySQL 索引未按预期使用

Posted 2023-04-15

【中文标题】MySQL 索引未按预期使用

【英文标题】：MySQL index not being used as expected

【发布时间】：2013-05-03 02:43:46

【问题描述】：

如何让 mysql 使用预期的索引？

我有 4 个表，其中两个包含资源，另一个包含历史变化。

一对正确使用索引，另一对没有，但两者的结构几乎相同。

我尝试更改主键的顺序和其他键的顺序，我尝试更改表结构，以便它们在两个表中使用相同的名称，并且都具有相同的键名，但没有似乎使查询使用了正确的索引。

为简洁起见，已删除不相关的列。

这两个表按预期工作。

CREATE TABLE `players` (
  `player_id` varbinary(36) NOT NULL DEFAULT '',
  `pop_rank_score` double NOT NULL DEFAULT '0',
  PRIMARY KEY (`player_id`),
  KEY `pop_rank_score` (`pop_rank_score`),
  KEY `weblinc_id` (`weblinc_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1

CREATE TABLE `poprankhistory` (
  `day_id` int(11) NOT NULL,
  `player_id` varbinary(36) NOT NULL DEFAULT '',
  `total` double NOT NULL DEFAULT '0',
  `today` double NOT NULL DEFAULT '0',
  PRIMARY KEY (`day_id`,`player_id`),
  KEY `day_id` (`day_id`),
  KEY `player_id` (`player_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1


explain select p.`player_id`, p.pop_rank_score + 0.5 * COALESCE(h1.total,0) as pop_rank_score
from fpme_lua.`Players` p, fpme_lua.PopRankHistory h1
where ( p.`player_id` = h1.`player_id` AND h1.day_id = (SELECT Max(h2.day_id) AS day_id
  FROM   fpme_lua.poprankhistory h2
  WHERE  h2.day_id <= 15786 and h2.player_id = p.`player_id` ));

+----+--------------------+-------+--------+--------------------------+-----------+---------+-----------------------+-------+--------------------------+
| id | select_type        | table | type   | possible_keys            | key       | key_len | ref                   | rows  | Extra                    |
+----+--------------------+-------+--------+--------------------------+-----------+---------+-----------------------+-------+--------------------------+
|  1 | PRIMARY            | h1    | ALL    | PRIMARY,day_id,player_id | NULL      | NULL    | NULL                  | 25391 |                          |
|  1 | PRIMARY            | p     | eq_ref | PRIMARY                  | PRIMARY   | 38      | fpme_lua.h1.player_id |     1 | Using where              |
|  2 | DEPENDENT SUBQUERY | h2    | ref    | PRIMARY,day_id,player_id | player_id | 38      | fpme_lua.p.player_id  |     2 | Using where; Using index |
+----+--------------------+-------+--------+--------------------------+-----------+---------+-----------------------+-------+--------------------------+

这些表未按预期工作（必需）。

CREATE TABLE `pictures` (
  `id` varchar(36) NOT NULL DEFAULT '',
  `pcr_score` double NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`),
  KEY `owner_id` (`owner_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

CREATE TABLE `picpcrhistory` (
  `day_id` int(11) NOT NULL,
  `target_id` varchar(36) NOT NULL DEFAULT '',
  `total` double NOT NULL DEFAULT '0',
  `today` double NOT NULL DEFAULT '0',
  PRIMARY KEY (`day_id`,`target_id`),
  KEY `target_id` (`target_id`),
  KEY `day_id` (`day_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1


explain select p.`id`, p.pcr_score + 0.5 * COALESCE(h1.total,0) as pcr_score
from fpme_lua.`Pictures` p, fpme_lua.PicPcrHistory h1
where ( p.`id` = h1.`target_id` AND h1.day_id = (SELECT Max(h2.day_id) AS day_id
  FROM   fpme_lua.PicPcrHistory h2
  WHERE  h2.day_id <= 15786 and h2.`target_id` = p.`id` ));

+----+--------------------+-------+--------+----------------+---------+---------+------+-------+--------------------------+
| id | select_type        | table | type   | possible_keys  | key     | key_len | ref  | rows  | Extra                    |
+----+--------------------+-------+--------+----------------+---------+---------+------+-------+--------------------------+
|  1 | PRIMARY            | h1    | ALL    | PRIMARY,day_id | NULL    | NULL    | NULL | 65310 |                          |
|  1 | PRIMARY            | p     | eq_ref | PRIMARY        | PRIMARY | 110     | func |     1 | Using where              |
|  2 | DEPENDENT SUBQUERY | h2    | range  | PRIMARY,day_id | day_id  | 4       | NULL | 21824 | Using where; Using index |
+----+--------------------+-------+--------+----------------+---------+---------+------+-------+--------------------------+

【问题讨论】：

第一个查询包括 pk，并且两个索引都是 as possible_keys，但是第二个查询不包括 target_id，这是我想要使用的，即使我已经指定了两个键where 子句。

【参考方案1】：

这些表有不同的字符集，这就是索引不起作用的原因。 我更改了字符集，因此两者都是 utf8 并为三列添加了索引。

ALTER TABLE `fpme_lua`.`colpcrhistory` CHARACTER SET = utf8 ;
ALTER TABLE `fpme_lua`.`picpcrhistory` CHARACTER SET = utf8 ;
ALTER TABLE `fpme_lua`.`picpcrhistory` 
ADD INDEX `indx_tar_day_tot` USING BTREE (`target_id` ASC, `day_id` ASC, `total` ASC) ;

然后我将查询更改为...

SELECT p.id, 
       p.pcr_score + 0.5 * COALESCE(h1.total,0) AS pcr_score
FROM 
    fpme_lua.Pictures AS p
  JOIN 
    fpme_lua.PicPcrHistory AS h1
      ON  h1.target_id = p.id 
  JOIN
    ( SELECT   hh.target_id,
               Max(hh.day_id) AS day_id
      FROM     fpme_lua.PicPcrHistory AS hh
      WHERE    hh.day_id <= 15786 
      GROUP BY hh.target_id
    ) AS h2
      ON  h2.target_id = h1.target_id
      AND h2.day_id = h1.day_id ;

【讨论】：

啊，很好，感谢您记录它。 （好吧，你实际上并没有接受你的答案，所以我们仍然不能 100% 确定。;)

【参考方案2】：

如果要使用特定索引，可以在表名后使用FORCE INDEX(index_name)。 尽管 MySQL 应该足够聪明地选择“最佳”索引。