MySQL 不使用主索引

Question

我有这个查询:

SELECT SQL_NO_CACHE
    COUNT(*) AS `numrows`
FROM
    (`citations`)
        LEFT JOIN
    `projects` ON `projects`.`project_id` = `citations`.`project_id`
        LEFT JOIN
    `users` ON `users`.`user_id` = `projects`.`user_id`
WHERE
    `users`.`role` = '0'
        AND `citations`.`created` BETWEEN 1360213200 AND 1360299599
        AND `citations`.`in_card` = '0'
        AND `citations`.`citation_id` NOT IN (SELECT 
            user_stats_citations.citation_id
        FROM
            user_stats_citations,
            user_stats FORCE INDEX (user_stats_type_index)
        WHERE
            user_stats_citations.user_stat_id = user_stats.id
                AND user_stats.type IN (69 , 70, 71, 75, 76));

我在用户表上有这些索引:

users            0  PRIMARY                             1  user_id      A                42836    (NULL)  (NULL)          BTREE                               
users            1  users_industry_id_index             1  industry_id  A                  118    (NULL)  (NULL)  YES     BTREE                               
users            1  users_sponsor_index                 1  sponsor      A                   12    (NULL)  (NULL)  YES     BTREE

这是 EXPLAIN EXTENDED 的输出

  id    select_type table   type    possible_keys   key key_len ref rows    filtered    Extra

  1 PRIMARY users   ALL PRIMARY \N  \N  \N  42836   100.00  Using where
  1 PRIMARY projects    ref PRIMARY\,projects_user_id_index projects_user_id_index  4   citelighter.users.user_id   1   100.00  Using where; Using index
  1 PRIMARY citations   ref citations_project_id_index  citations_project_id_index  4   citelighter.projects.project_id 4   100.00  Using index condition; Using where
  2 SUBQUERY    user_stats  range   user_stats_type_index   user_stats_type_index   2   \N  410768  100.00  Using where; Using index
  2 SUBQUERY    user_stats_citations    ref user_stats_citations_index_user_stat_id\,user_stats_citations_index_citation_id user_stats_citations_index_user_stat_id 8   citelighter.user_stats.id   1   100.00  \N

我尝试在用户 LEFT JOIN 上添加 FORCE INDEX，但未使用索引。你能帮我解决这个问题吗，因为这个查询在我的本地大约需要 10 秒，在生产环境上大约需要 1 秒。

Answer 1

我注意到的第一件事是，where 子句中的谓词:WHERE users.role = '0' 将您的 LEFT JOIN 转换为 INNER JOIN ，所以您也可以将它们设为内部连接。

其次，MySQL 在优化相关子查询方面存在问题，并且在派生表方面也表现不佳。例如在 this simple query :

SELECT *
FROM (SELECT * FROM T) T
JOIN (SELECT * FROM T) T2 ON T.ID = T2.ID;

尽管 ID 是 T 上的主键，但主键不用于连接，因为它无法从派生表中级联出来。同样，有时当你写:

SELECT *
FROM T
WHERE Afield NOT IN (SELECT Afield FROM T WHERE AnotherField = 1);

MySQL 不一定具体化子查询并使用它，它通常会将查询重写为:

SELECT *
FROM T
WHERE NOT EXISTS (SELECT 1 
                    FROM T T2 
                    WHERE T.Afield = T2.Afield  
                    AND T2.AnotherField = 1);

子查询是针对外部查询中的每一行执行的，因此，如果外部查询中有大量行，则为每一行执行子查询会变得非常昂贵。解决方案是尽可能避免子查询。在您的情况下，您可以将查询重写为:

SELECT  SQL_NO_CACHE
        COUNT(*) AS `numrows`
FROM    `citations`
        INNER JOIN `projects` 
            ON `projects`.`project_id` = `citations`.`project_id`
        INNER JOIN `users` 
            ON `users`.`user_id` = `projects`.`user_id`
        LEFT JOIN (user_stats_citations
            INNER JOIN user_stats
                ON user_stats_citations.user_stat_id = user_stats.id
                AND user_stats.type IN (69 , 70, 71, 75, 76))
            ON user_stats_citations.citation_id = `citations`.`citation_id`
WHERE   `users`.`role` = '0'
AND     `citations`.`created` BETWEEN 1360213200 AND 1360299599
AND     `citations`.`in_card` = '0'
AND      user_stats_citations.citation_id IS NULL;

如果没有子查询，就没有派生表，也没有子查询的逐行执行。这应该会缩短执行时间。