我在尝试加速查询时遇到了问题，该查询仅需要大约 11 秒就处理 200 万行。 Here is a link to my sqlfiddle .这是我要运行的语句和我的 EXPLAIN 语句。

查询:

SELECT crawl.pk Pk,domains.domain Domain, 
CONCAT(schemes.scheme, "://", domains.domain, remainders.remainder) Uri, 
crawl.redirect Redirect FROM crawl 
LEFT JOIN dates ON crawl.date_crawled=dates.pk     
LEFT JOIN schemes ON crawl.scheme=schemes.pk 
LEFT JOIN domains ON crawl.domain=domains.pk 
LEFT JOIN remainders ON crawl.remainder=remainders.pk 
WHERE (dates.date < CURDATE() - INTERVAL 30 DAY) 
AND crawl.redirect=0 
GROUP BY crawl.domain 
ORDER BY crawl.date_crawled ASC 
LIMIT 50

解释:

+----+-------------+------------+--------+-----------------------+-----------------------+---------+----------------------------+--------+----------------------------------------------+
| id | select_type | table      | type   | possible_keys         | key                   | key_len | ref                        | rows   | Extra                                        |
+----+-------------+------------+--------+-----------------------+-----------------------+---------+----------------------------+--------+----------------------------------------------+
|  1 | SIMPLE      | dates      | ALL    | PRIMARY,date          | NULL                  | NULL    | NULL                       |      7 | Using where; Using temporary; Using filesort |
|  1 | SIMPLE      | crawl      | ref    | date_crawled_redirect | date_crawled_redirect | 8       | mytable.dates.pk,const     | 408644 |                                              |
|  1 | SIMPLE      | schemes    | eq_ref | PRIMARY               | PRIMARY               | 4       | mytable.crawl.scheme       |      1 |                                              |
|  1 | SIMPLE      | domains    | eq_ref | PRIMARY               | PRIMARY               | 4       | mytable.crawl.domain       |      1 |                                              |
|  1 | SIMPLE      | remainders | eq_ref | PRIMARY               | PRIMARY               | 4       | mytable.crawl.remainder    |      1 |                                              |
+----+-------------+------------+--------+-----------------------+-----------------------+---------+----------------------------+--------+----------------------------------------------+
5 rows in set (2.26 sec)

编辑#1: 根据评论，我已经用 Joins 替换了 Left Joins 并通过 join 移动了日期过滤器。遗憾的是，这并没有减少查询时间。

SELECT crawl.pk Pk,domains.domain Domain, CONCAT(schemes.scheme, "://", domains.domain, remainders.remainder) Uri, crawl.redirect Redirect
FROM crawl
JOIN schemes ON crawl.scheme=schemes.pk
JOIN domains ON crawl.domain=domains.pk
JOIN remainders ON crawl.remainder=remainders.pk
JOIN dates ON crawl.date_crawled=dates.pk AND dates.date < CURDATE() - INTERVAL 30 DAY
WHERE crawl.redirect=0
GROUP BY crawl.domain
ORDER BY crawl.date_crawled ASC
LIMIT 50

编辑#2: 我更新的解释:

+----+-------------+------------+--------+---------------------------------------------------------+-----------------------+---------+----------------------------+--------+-----------------------------------------------------------+
| id | select_type | table      | type   | possible_keys                                           | key                   | key_len | ref                        | rows   | Extra                                                     |
+----+-------------+------------+--------+---------------------------------------------------------+-----------------------+---------+----------------------------+--------+-----------------------------------------------------------+
|  1 | SIMPLE      | dates      | range  | PRIMARY,date,date_pk,dateBtreeIdx,pk                     | date_pk                | 3       | NULL                       |      4 | Using where; Using index; Using temporary; Using filesort |
|  1 | SIMPLE      | crawl      | ref    | domain_remainder,remainder,scheme,date_crawled_redirect | date_crawled_redirect | 8       | mytable.dates.pk,const     | 408644 |                                                           |
|  1 | SIMPLE      | schemes    | ALL    | PRIMARY                                                 | NULL                  | NULL    | NULL                       |      2 | Using where; Using join buffer                            |
|  1 | SIMPLE      | domains    | eq_ref | PRIMARY                                                 | PRIMARY               | 4       | mytable.crawl.domain       |      1 |                                                           |
|  1 | SIMPLE      | remainders | eq_ref | PRIMARY                                                 | PRIMARY               | 4       | mytable.crawl.remainder    |      1 |                                                           |
+----+-------------+------------+--------+---------------------------------------------------------+-----------------------+---------+----------------------------+--------+-----------------------------------------------------------+

编辑 #3

+----+--------------------+------------+-----------------+------------------------------------------+---------+---------+----------------------------+---------+---------------------------------+
| id | select_type        | table      | type            | possible_keys                            | key     | key_len | ref                        | rows    | Extra                           |
+----+--------------------+------------+-----------------+------------------------------------------+---------+---------+----------------------------+---------+---------------------------------+
|  1 | PRIMARY            | schemes    | ALL             | PRIMARY                                  | NULL    | NULL    | NULL                       |       2 | Using temporary; Using filesort |
|  1 | PRIMARY            | crawl      | ref             | domain_remainder,remainder,scheme,domain | scheme  | 4       | mytable.schemes.pk         | 1448223 | Using where                     |
|  1 | PRIMARY            | domains    | eq_ref          | PRIMARY                                  | PRIMARY | 4       | mytable.crawl.domain       |       1 |                                 |
|  1 | PRIMARY            | remainders | eq_ref          | PRIMARY                                  | PRIMARY | 4       | mytable.crawl.remainder    |       1 |                                 |
|  2 | DEPENDENT SUBQUERY | dates      | unique_subquery | PRIMARY,date,date_pk,dateBtreeIdx,pk     | PRIMARY | 4       | func                       |       1 | Using where                     |
+----+--------------------+------------+-----------------+------------------------------------------+---------+---------+----------------------------+---------+---------------------------------+
5 rows in set (0.04 sec)

编辑#4:

+----+-------------+------------+--------+--------------------------------------+-------------------------+---------+----------------------------+---------+-----------------------------------------------------------+
| id | select_type | table      | type   | possible_keys                        | key                     | key_len | ref                        | rows    | Extra                                                     |
+----+-------------+------------+--------+--------------------------------------+-------------------------+---------+----------------------------+---------+-----------------------------------------------------------+
|  1 | SIMPLE      | dates      | range  | PRIMARY,date,date_pk,dateBtreeIdx,pk | date_pk                 | 3       | NULL                       |       4 | Using where; Using index; Using temporary; Using filesort |
|  1 | SIMPLE      | schemes    | ALL    | PRIMARY                              | NULL                    | NULL    | NULL                       |       2 | Using join buffer                                         |
|  1 | SIMPLE      | crawl      | ref    | scheme_domain_remainder              | scheme_domain_remainder | 4       | mytable.schemes.pk         | 1455517 | Using where                                               |
|  1 | SIMPLE      | domains    | eq_ref | PRIMARY                              | PRIMARY                 | 4       | mytable.crawl.domain       |       1 |                                                           |
|  1 | SIMPLE      | remainders | eq_ref | PRIMARY                              | PRIMARY                 | 4       | mytable.crawl.remainder    |       1 |                                                           |
+----+-------------+------------+--------+--------------------------------------+-------------------------+---------+----------------------------+---------+-----------------------------------------------------------+
5 rows in set (0.04 sec)

编辑 #5

SELECT urls.pk PK, domains.domain Domain, CONCAT(schemes.scheme, "://", domains.domain, remainders.remainder) Uri, urls.redirect Redirect, urls.date_crawled DC FROM
(SELECT * FROM (
 SELECT * FROM crawl as urls ORDER BY date_crawled ASC
) AS tmp GROUP BY tmp.domain ) as urls
JOIN schemes ON urls.scheme=schemes.pk
JOIN domains ON urls.domain=domains.pk
JOIN remainders ON urls.remainder=remainders.pk
JOIN dates ON urls.date_crawled=dates.pk AND dates.date < CURDATE() - INTERVAL 30 DAY
WHERE urls.redirect=0
ORDER BY urls.date_crawled ASC
LIMIT 50

最佳答案

您手头有一个近乎最优的查询。唯一的问题是表 dates 中的非最佳索引。正如您在 EXPLAIN 输出中看到的那样，MySQL 无法使用表 dates 中的任何索引，因此它被用作第一个表。这会为您的表 crawl 生成一个半优化的执行计划，其中包含大量要访问的行。

要改善这一点，您应该在 dates.date 列上添加一个 BTREE 索引:

ALTER TABLE dates ADD INDEX dateBtreeIdx USING BTREE (date)

BTREE 索引用于范围条件。在您的情况下，“低于”see here .

基于此，您也可以尝试将连接字段 Dates.pk 添加到索引中。这可能会进一步加快您的查询速度，但取决于您的数据。

编辑

现在 MySQL 可以在 date.dates(type = RANGE 和 rows = 4)上使用索引。您看不到加速，因为现在优化器不会在 schemes 中使用 PRIMARY KEY...

但最大的性能问题在于 crawl。使用 IN 查询尝试不同的方法:

SELECT 
    crawl.pk Pk, domains.domain Domain, 
    CONCAT(schemes.scheme, "://", domains.domain, remainders.remainder) Uri, 
    crawl.redirect Redirect 
FROM 
    crawl, schemes, domains, remainders
WHERE 
    crawl.scheme=schemes.pk
    AND crawl.domain=domains.pk
    AND crawl.remainder=remainders.pk

    AND crawl.date_crawled IN (SELECT pk FROM dates WHERE (dates.date < CURDATE() - INTERVAL 30 DAY))
    AND crawl.redirect=0 

GROUP BY 
    crawl.domain 
ORDER BY 
    crawl.date_crawled ASC 
LIMIT 50

编辑 #2

SELECT 
    urls.pk PK, domains.domain Domain, 
    CONCAT(schemes.scheme, "://", domains.domain, remainders.remainder) Uri, 
    urls.redirect Redirect, 
    urls.date_crawled DC 
FROM 
    (SELECT pk, redirect, date_crawled FROM crawl GROUP BY `domain` ) as urls
JOIN schemes ON urls.scheme=schemes.pk
JOIN domains ON urls.`domain`=domains.pk
JOIN remainders ON urls.remainder=remainders.pk
JOIN dates ON urls.date_crawled=dates.pk AND dates.date < CURDATE() - INTERVAL 30 DAY
WHERE 
    urls.redirect=0
ORDER BY urls.date_crawled ASC
LIMIT 50

关于mysql - 如何加速具有多个连接的 Group By 语句？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/26823804/