慢速 SQL 查询：在两个不同的连接中使用同一个表会导致查询速度变慢 10 倍！

真的希望某种性能专家可以向我解释为什么单个连接会导致查询速度慢 10 倍。 （另外，请不要嘲笑这个查询的大小！我想取出数据库中的整个目录以通过一个查询输出。我不确定将其分解为较小的查询是否会更快，但是似乎不对。）

SELECT `c`.`categoryID`,
       `cl`.`name` AS `category_name`,
       `v`.*,
       TRUE AS `categoried`,
       GROUP_CONCAT(DISTINCT t_v.iso_3166_1_alpha_2) AS `video_territories`,
       GROUP_CONCAT(DISTINCT t_c.iso_3166_1_alpha_2) AS `category_territories`,
       `vl`.*,
       GROUP_CONCAT(DISTINCT kl.name) AS `keywords`
FROM `tblCategories` AS `c`
INNER JOIN `tblCategoryLocalisedData` AS `cl` ON c.categoryID = cl.categoryID
LEFT JOIN `tblCategoryDurations` AS `cd` ON c.categoryID = cd.categoryID
LEFT JOIN `tblCategoryRules` AS `cr` ON c.categoryID = cr.categoryID
LEFT JOIN `tblCategoryVideos` AS `cv` ON c.categoryID = cv.categoryID
LEFT JOIN `tblVideos` AS `v` ON cv.videoID = v.videoID
LEFT JOIN `tblVideoTerritories` AS `vt` ON vt.videoID = v.videoID
LEFT JOIN `tblCategoryTerritories` AS `ct` ON ct.categoryID = c.categoryID
INNER JOIN `tblTerritories` AS `t_v` ON t_v.territoryID = vt.territoryID
INNER JOIN `tblTerritories` AS `t_c` ON t_c.territoryID = ct.territoryID
INNER JOIN `tblVideoLocalisedData` AS `vl` ON vl.videoID = v.videoID
LEFT JOIN `tblVideoKeywords` AS `vk` ON v.videoID = vk.videoID
LEFT JOIN `tblKeywords` AS `k` ON vk.keywordID = k.keywordID
LEFT JOIN `tblKeywordLocalisedData` AS `kl` ON kl.keywordID = k.keywordID
INNER JOIN `tblLanguages` AS `l`
WHERE (cv.disabled IS NULL)
  AND (cd.start_date < NOW() OR cd.start_date IS NULL)
  AND (cd.end_date > NOW() OR cd.end_date IS NULL)
  AND (cr.name IS NULL)
  AND (l.languageID = cl.languageID OR cl.languageID IS NULL)
  AND (l.languageID = kl.languageID OR kl.languageID IS NULL)
  AND (l.languageID = vl.languageID OR vl.languageID IS NULL)
  AND (l.iso_639_1 = 'en')
GROUP BY `v`.`videoID`, `c`.`categoryID`
ORDER BY `c`.`categoryID` ASC

当我运行上述查询时，需要 1 秒才能完成。我尝试对其运行 EXPLAIN，它给了我这个：

+----+-------------+-------+--------+--------------------------------------------------------------------------------------+-----------------------------------------+---------+------------------------+------+----------------------------------------------+
| id | select_type | table | type   | possible_keys                                                                        | key                                     | key_len | ref                    | rows | Extra                                        |
+----+-------------+-------+--------+--------------------------------------------------------------------------------------+-----------------------------------------+---------+------------------------+------+----------------------------------------------+
|  1 | SIMPLE      | cv    | ALL    | fk_tblCategoryVideos_tblCategories1,fk_tblCategoryVideos_tblVideos1                  | NULL                                    | NULL    | NULL                   |    2 | Using where; Using temporary; Using filesort |
|  1 | SIMPLE      | c     | eq_ref | PRIMARY                                                                              | PRIMARY                                 | 4       | db.cv.categoryID  |    1 | Using index                                  |
|  1 | SIMPLE      | cd    | ref    | fk_tblCategoryDurations_tblCategories                                                | fk_tblCategoryDurations_tblCategories   | 4       | db.cv.categoryID  |    1 | Using where                                  |
|  1 | SIMPLE      | cr    | ref    | fk_tblCategoryRules_tblCategories1                                                   | fk_tblCategoryRules_tblCategories1      | 4       | db.cv.categoryID  |    1 | Using where; Not exists                      |
|  1 | SIMPLE      | vt    | ref    | fk_tblVideoTerritories_tblVideos1,fk_tblVideoTerritories_tblTerritories1             | fk_tblVideoTerritories_tblVideos1       | 4       | db.cv.videoID     |    1 | Using where                                  |
|  1 | SIMPLE      | t_v   | eq_ref | PRIMARY                                                                              | PRIMARY                                 | 4       | db.vt.territoryID |    1 |                                              |
|  1 | SIMPLE      | v     | eq_ref | PRIMARY                                                                              | PRIMARY                                 | 4       | db.vt.videoID     |    1 | Using where                                  |
|  1 | SIMPLE      | vk    | ref    | fk_tblVideoKeywords_tblVideos1                                                       | fk_tblVideoKeywords_tblVideos1          | 4       | db.cv.videoID     |    6 |                                              |
|  1 | SIMPLE      | k     | eq_ref | PRIMARY                                                                              | PRIMARY                                 | 4       | db.vk.keywordID   |    1 | Using index                                  |
|  1 | SIMPLE      | kl    | ref    | fk_tblKeywordLocalisedData_tblKeywords1                                              | fk_tblKeywordLocalisedData_tblKeywords1 | 4       | db.k.keywordID    |    1 |                                              |
|  1 | SIMPLE      | cl    | ALL    | fk_tblCategoryLocalisedData_tblCategories1,fk_tblCategoryLocalisedData_tblLanguages1 | NULL                                    | NULL    | NULL                   |    5 | Using where; Using join buffer               |
|  1 | SIMPLE      | l     | eq_ref | PRIMARY                                                                              | PRIMARY                                 | 4       | db.cl.languageID  |    1 | Using where                                  |
|  1 | SIMPLE      | ct    | ALL    | fk_tblCategoryTerritories_tblCategories1,fk_tblCategoryTerritories_tblTerritories1   | NULL                                    | NULL    | NULL                   |    2 | Using where; Using join buffer               |
|  1 | SIMPLE      | vl    | ALL    | fk_tblVideoLocalisedData_tblLanguages1,fk_tblVideoLocalisedData_tblVideos1           | NULL                                    | NULL    | NULL                   |    9 | Using where; Using join buffer               |
|  1 | SIMPLE      | t_c   | eq_ref | PRIMARY                                                                              | PRIMARY                                 | 4       | db.ct.territoryID |    1 |                                              |
+----+-------------+-------+--------+--------------------------------------------------------------------------------------+-----------------------------------------+---------+------------------------+------+----------------------------------------------+

但我不知道这意味着什么。我该如何解决这个问题？值得庆幸的是，我确实知道查询的哪些部分会导致速度大幅下降。如果我删除从 tblVideoTerritories (vt) 到 tblTerritories (t_v) 或 tblCategoryTerritories (ct) 到 tblTerritories (t_c) 的连接，那么一切都会大大加快。我认为一开始可能是因为 GROUP_CONCAT 或 DISTINCT，但我尝试删除这些，但几乎没有任何改变。看起来性能问题是由于两次加入同一个表“tblTerritories”引起的。如果我只有其中一个连接，则查询只需要 0.1 秒或 0.2 秒即可运行——这仍然是一个很长的时间，但这是一个更好的开始！

我想知道如何解决这个性能问题？为什么两次加入同一个表会导致查询时间延长 10 倍？！

谢谢你的帮助！

edit:tblVideoTerritories 上的 SHOW CREATE TABLE 给了我这个：

CREATE TABLE `tblVideoTerritories` (
  `videoTerritoryID` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `videoID` int(10) unsigned NOT NULL,
  `territoryID` int(10) unsigned NOT NULL,
  PRIMARY KEY (`videoTerritoryID`),
  KEY `fk_tblVideoTerritories_tblVideos1` (`videoID`),
  KEY `fk_tblVideoTerritories_tblTerritories1` (`territoryID`),
  CONSTRAINT `fk_tblVideoTerritories_tblTerritories1` FOREIGN KEY (`territoryID`) REFERENCES `tblTerritories` (`territoryID`) ON DELETE NO ACTION ON UPDATE NO ACTION,
  CONSTRAINT `fk_tblVideoTerritories_tblVideos1` FOREIGN KEY (`videoID`) REFERENCES `tblVideos` (`videoID`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

tblCategoryTerritories 上的 SHOW CREATE TABLE 给了我这个：

CREATE TABLE `tblCategoryTerritories` (
  `categoryTerritoryID` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `categoryID` int(10) unsigned NOT NULL,
  `territoryID` int(10) unsigned NOT NULL,
  PRIMARY KEY (`categoryTerritoryID`),
  KEY `fk_tblCategoryTerritories_tblCategories1` (`categoryID`),
  KEY `fk_tblCategoryTerritories_tblTerritories1` (`territoryID`),
  CONSTRAINT `fk_tblCategoryTerritories_tblCategories1` FOREIGN KEY (`categoryID`) REFERENCES `tblCategories` (`categoryID`) ON DELETE NO ACTION ON UPDATE NO ACTION,
  CONSTRAINT `fk_tblCategoryTerritories_tblTerritories1` FOREIGN KEY (`territoryID`) REFERENCES `tblTerritories` (`territoryID`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

tblTerritories 上的 SHOW CREATE TABLE 给了我这个：

CREATE TABLE `tblTerritories` (
  `territoryID` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `iso_3166_1_alpha_2` char(2) COLLATE utf8_unicode_ci DEFAULT NULL,
  `iso_3166_1_alpha_3` char(3) COLLATE utf8_unicode_ci DEFAULT NULL,
  `defaultLanguageID` int(10) unsigned DEFAULT NULL,
  PRIMARY KEY (`territoryID`),
  KEY `fk_tblTerritories_tblLanguages1` (`defaultLanguageID`),
  KEY `iso_3166_1_alpha_2` (`iso_3166_1_alpha_2`),
  CONSTRAINT `fk_tblTerritories_tblLanguages1` FOREIGN KEY (`defaultLanguageID`) REFERENCES `tblLanguages` (`languageID`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

edit2:两次加入同一区域的原因是我需要使用查询顶部的 GROUP_CONCAT 生成两个单独的区域列表。我需要一个用于视频，一个用于其所属类别。

edit3:有趣的是，如果我将查询精简到最简单的形式，那么即使两次加入同一个表，它也会非常快（0.00 秒）：

    SELECT `c`.`categoryID`,
           `v`.`videoID`,
           GROUP_CONCAT(DISTINCT t_v.iso_3166_1_alpha_2) AS `video_territories`,
           GROUP_CONCAT(DISTINCT t_c.iso_3166_1_alpha_2) AS `category_territories`
FROM `tblCategories` AS `c`
LEFT JOIN `tblCategoryVideos` AS `cv` ON c.categoryID = cv.categoryID
LEFT JOIN `tblVideos` AS `v` ON cv.videoID = v.videoID
LEFT JOIN `tblVideoTerritories` AS `vt` ON vt.videoID = v.videoID
LEFT JOIN `tblCategoryTerritories` AS `ct` ON ct.categoryID = c.categoryID
INNER JOIN `tblTerritories` AS `t_v` ON t_v.territoryID = vt.territoryID
INNER JOIN `tblTerritories` AS `t_c` ON t_c.territoryID = ct.territoryID
GROUP BY `v`.`videoID`, `c`.`categoryID`

edit4:如果我不再使用 WHERE 作为临时 ON，那么我仍然有一个需要 0.98 秒的查询：

SELECT `c`.`categoryID`,
       `cl`.`name` AS `category_name`,
       `v`.*,
       TRUE AS `categoried`,
       GROUP_CONCAT(DISTINCT t_v.iso_3166_1_alpha_2) AS `video_territories`,
       GROUP_CONCAT(DISTINCT t_c.iso_3166_1_alpha_2) AS `category_territories`,
       `vl`.*,
       GROUP_CONCAT(DISTINCT kl.name) AS `keywords`
FROM `tblCategories` AS `c`
INNER JOIN `tblCategoryLocalisedData` AS `cl` ON c.categoryID = cl.categoryID
LEFT JOIN `tblCategoryDurations` AS `cd` ON c.categoryID = cd.categoryID
LEFT JOIN `tblCategoryRules` AS `cr` ON c.categoryID = cr.categoryID
LEFT JOIN `tblCategoryVideos` AS `cv` ON c.categoryID = cv.categoryID
LEFT JOIN `tblVideos` AS `v` ON cv.videoID = v.videoID
LEFT JOIN `tblVideoTerritories` AS `vt` ON vt.videoID = v.videoID
LEFT JOIN `tblCategoryTerritories` AS `ct` ON ct.categoryID = c.categoryID
INNER JOIN `tblTerritories` AS `t_v` ON t_v.territoryID = vt.territoryID
INNER JOIN `tblTerritories` AS `t_c` ON t_c.territoryID = ct.territoryID
INNER JOIN `tblVideoLocalisedData` AS `vl` ON vl.videoID = v.videoID
LEFT JOIN `tblVideoKeywords` AS `vk` ON v.videoID = vk.videoID
LEFT JOIN `tblKeywords` AS `k` ON vk.keywordID = k.keywordID
LEFT JOIN `tblKeywordLocalisedData` AS `kl` ON kl.keywordID = k.keywordID
INNER JOIN `tblLanguages` AS `l` ON (l.languageID = cl.languageID OR cl.languageID IS NULL) AND (l.languageID = kl.languageID OR kl.languageID IS NULL) AND (l.languageID = vl.languageID OR vl.languageID IS NULL)
WHERE (cv.disabled IS NULL)
  AND (cd.start_date < NOW() OR cd.start_date IS NULL)
  AND (cd.end_date > NOW() OR cd.end_date IS NULL)
  AND (cr.name IS NULL) AND (l.iso_639_1 = 'en')
GROUP BY `v`.`videoID`, `c`.`categoryID`
ORDER BY `c`.`categoryID` ASC

edit5:如果我删除与关键字相关的连接，查询将在 0.09 秒内发生...删除 tblKeyword 和 tblKeywordLocalizedData 但保留 tblVideoKeywords 会给我 0.80 秒。删除 tblVideoKeywords 只需要 0.09 秒。

但它似乎有索引，所以我再次不明白：

CREATE TABLE `tblVideoKeywords` (
  `videoKeywordID` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `videoID` int(10) unsigned NOT NULL,
  `keywordID` int(10) unsigned NOT NULL,
  PRIMARY KEY (`videoKeywordID`),
  KEY `fk_tblVideoKeywords_tblVideos1` (`videoID`),
  KEY `fk_tblVideoKeywords_tblKeywords1` (`keywordID`),
  CONSTRAINT `fk_tblVideoKeywords_tblKeywords1` FOREIGN KEY (`keywordID`) REFERENCES `tblKeywords` (`keywordID`) ON DELETE NO ACTION ON UPDATE NO ACTION,
  CONSTRAINT `fk_tblVideoKeywords_tblVideos1` FOREIGN KEY (`videoID`) REFERENCES `tblVideos` (`videoID`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=18 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

edit6:使用 DRapp 提供的查询使一切变得更快。他的查询的解释现在给了我：

+----+-------------+---------+--------+--------------------------------------------------------------------------------------+-----------------------------------------+---------+------------------------+------+----------------------------------------------+
| id | select_type | table   | type   | possible_keys                                                                        | key                                     | key_len | ref                    | rows | Extra                                        |
+----+-------------+---------+--------+--------------------------------------------------------------------------------------+-----------------------------------------+---------+------------------------+------+----------------------------------------------+
|  1 | SIMPLE      | c       | index  | PRIMARY                                                                              | PRIMARY                                 | 4       | NULL                   |    3 | Using index; Using temporary; Using filesort |
|  1 | SIMPLE      | cl      | ALL    | fk_tblCategoryLocalisedData_tblCategories1,fk_tblCategoryLocalisedData_tblLanguages1 | NULL                                    | NULL    | NULL                   |    5 | Using where; Using join buffer               |
|  1 | SIMPLE      | lang_cl | ALL    | PRIMARY                                                                              | NULL                                    | NULL    | NULL                   |    2 | Using where; Using join buffer               |
|  1 | SIMPLE      | cd      | ref    | fk_tblCategoryDurations_tblCategories                                                | fk_tblCategoryDurations_tblCategories   | 4       | db.c.categoryID   |    1 |                                              |
|  1 | SIMPLE      | cr      | ref    | fk_tblCategoryRules_tblCategories1                                                   | fk_tblCategoryRules_tblCategories1      | 4       | db.c.categoryID   |    1 | Using where; Not exists                      |
|  1 | SIMPLE      | cv      | ALL    | fk_tblCategoryVideos_tblCategories1,fk_tblCategoryVideos_tblVideos1                  | NULL                                    | NULL    | NULL                   |    2 | Using where; Using join buffer               |
|  1 | SIMPLE      | ct      | ALL    | fk_tblCategoryTerritories_tblCategories1,fk_tblCategoryTerritories_tblTerritories1   | NULL                                    | NULL    | NULL                   |    2 | Using where; Using join buffer               |
|  1 | SIMPLE      | t_c     | eq_ref | PRIMARY                                                                              | PRIMARY                                 | 4       | db.ct.territoryID |    1 |                                              |
|  1 | SIMPLE      | v       | eq_ref | PRIMARY                                                                              | PRIMARY                                 | 4       | db.cv.videoID     |    1 | Using where                                  |
|  1 | SIMPLE      | vt      | ref    | fk_tblVideoTerritories_tblVideos1,fk_tblVideoTerritories_tblTerritories1             | fk_tblVideoTerritories_tblVideos1       | 4       | db.v.videoID      |    1 | Using where                                  |
|  1 | SIMPLE      | t_v     | eq_ref | PRIMARY                                                                              | PRIMARY                                 | 4       | db.vt.territoryID |    1 |                                              |
|  1 | SIMPLE      | vl      | ALL    | fk_tblVideoLocalisedData_tblLanguages1,fk_tblVideoLocalisedData_tblVideos1           | NULL                                    | NULL    | NULL                   |    9 | Using where; Using join buffer               |
|  1 | SIMPLE      | lang_vl | eq_ref | PRIMARY                                                                              | PRIMARY                                 | 4       | db.vl.languageID  |    1 | Using where                                  |
|  1 | SIMPLE      | vk      | ALL    | fk_tblVideoKeywords_tblVideos1,fk_tblVideoKeywords_tblKeywords1                      | NULL                                    | NULL    | NULL                   |   15 | Using where; Using join buffer               |
|  1 | SIMPLE      | k       | eq_ref | PRIMARY                                                                              | PRIMARY                                 | 4       | db.vk.keywordID   |    1 | Using where; Using index                     |
|  1 | SIMPLE      | kl      | ref    | fk_tblKeywordLocalisedData_tblKeywords1,fk_tblKeywordLocalisedData_tblLanguages1     | fk_tblKeywordLocalisedData_tblKeywords1 | 4       | db.k.keywordID    |    1 | Using where                                  |
|  1 | SIMPLE      | lang_kl | eq_ref | PRIMARY                                                                              | PRIMARY                                 | 4       | db.kl.languageID  |    1 | Using where                                  |
+----+-------------+---------+--------+--------------------------------------------------------------------------------------+-----------------------------------------+---------+------------------------+------+----------------------------------------------+
17 rows in set (0.01 sec)

对于我回答过的其他几个问题，只需添加“STRAIGHT_JOIN”并进行轻微的重组即可有所帮助。查询优化器实际上会尝试为您考虑所有表，尝试找到记录较少的表并将其连接到较大的表，从而导致完全混乱。当我对超过 14 万条记录进行政府数据查询并查找超过 15 个子表时，发生了这种情况......与您在这里发生的情况非常相似。它需要在专用的独立服务器上运行 30 多个小时的查询并将其挂起，时间缩短到不到 2 小时...请尝试以下操作：

除了对我习惯的连接进行一些视觉清理/排序之外，我还采用了一些 NOW() 与 NULL 并将它们移到连接中。如果您查询左联接并将日期作为联接限定符的一部分，那么您将排除这些超出范围的记录，从而留下 NULL 结果集或有效条目，无需加倍该限定符。

SELECT STRAIGHT_JOIN
      c.categoryID,
      cl.name AS category_name,
      v.*,
      TRUE AS categoried,
      GROUP_CONCAT(DISTINCT t_v.iso_3166_1_alpha_2) AS video_territories,
      GROUP_CONCAT(DISTINCT t_c.iso_3166_1_alpha_2) AS category_territories,
      vl.*,
      GROUP_CONCAT(DISTINCT kl.name) AS keywords
   FROM 
      tblCategories AS c
         INNER JOIN tblCategoryLocalisedData AS cl
            ON c.categoryID = cl.categoryID 
            INNER JOIN tblLanguages AS lang_cl
               ON l.languageID = lang_cl.languageID
                  AND lang_cl.iso_639_1 = 'en'
         LEFT JOIN tblCategoryDurations AS cd
            ON c.categoryID = cd.categoryID 
              AND cd.start_date < NOW()
              AND cd.end_date > NOW()
         LEFT JOIN tblCategoryRules AS cr
            ON c.categoryID = cr.categoryID 
         LEFT JOIN tblCategoryVideos AS cv
            ON c.categoryID = cv.categoryID 
         LEFT JOIN tblCategoryTerritories AS ct
            ON c.categoryID = ct.categoryID
            INNER JOIN tblTerritories AS t_c 
               ON ct.territoryID = t_c.territoryID
         LEFT JOIN tblVideos AS v
            ON cv.videoID = v.videoID 
            LEFT JOIN tblVideoTerritories AS vt
               ON v.videoID = vt.videoID
               INNER JOIN tblTerritories AS t_v
                  ON vt.territoryID = t_v.territoryID
            INNER JOIN tblVideoLocalisedData AS vl
               ON v.videoID = vl.videoID
               INNER JOIN tblLanguages AS lang_vl
                   ON vl.languageID = lang_vl.languageID
                      AND lang_vl.iso_639_1 = 'en'
            LEFT JOIN tblVideoKeywords AS vk
               ON v.videoID = vk.videoID 
               LEFT JOIN tblKeywords AS k
                  ON vk.keywordID = k.keywordID 
                  LEFT JOIN tblKeywordLocalisedData AS kl
                     ON k.keywordID = kl.keywordID
                     INNER JOIN tblLanguages AS lang_kl
                        ON kl.languageID = lang_kl.languageID
                          AND lang_kl.iso_639_1 = 'en'
   WHERE 
          (  cv.disabled IS NULL)   
      AND (  cr.name IS NULL)   
   GROUP BY 
      v.videoID, 
      c.categoryID
   ORDER BY 
      c.categoryID ASC 

正如我上面所解释的，STRAIGHT_JOIN 基本上告诉优化器，“不要为我思考”......按照我告诉你的顺序执行查询。在本例中，使用“tblCategories”作为主表并链接其他所有内容。即使有解释，优化器也可能会尝试变慢，并在下次运行查询时尝试不同的方法。因此，它可以尝试首先使用语言表，然后向后划过其他表并进行阻塞。另外，通过将“AND”部分（例如日期）直接指向那些左连接，这些连接简化了 WHERE，如您所见...就像您在 NULL 或它存在的位置中所做的那样，仅应用于该特定连接。 .保持地方清洁。

此外，通过保持关系直接并缩进到他们所加入的内容，更容易理解什么与哪里相关......

我还想看看最后的“解释”，看看它会带来什么。