我在执行简单的 LEFT JOIN 时遇到问题。
当我运行 EXPLAIN SELECT * FROM Feature f LEFT JOIN feature_translations t ON f.id = t.object_id 时，我得到了以下结果:
+----+-------------+-------+------+---------------------------+------+---------+------+--------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------------------+------+---------+------+--------+----------------------------------------------------+
| 1 | SIMPLE | f | ALL | NULL | NULL | NULL | NULL | 249200 | NULL |
| 1 | SIMPLE | t | ALL | IDX_7ED26C14232D562B,olol | NULL | NULL | NULL | 1 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------+---------------------------+------+---------+------+--------+----------------------------------------------------+
在这里我可以看到Join没有使用现有索引!如果我删除几乎所有行，则使用索引。这就是为什么我认为只是一个配置变量但我不知道是哪一个!
遵循我的表格结构。
CREATE TABLE IF NOT EXISTS `Feature` (
`id` int(11) NOT NULL,
`site_id` int(11) NOT NULL,
`name` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`clientId` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
`signature` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL
) ENGINE=InnoDB AUTO_INCREMENT=1866705 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
ALTER TABLE `Feature`
ADD PRIMARY KEY (`id`), ADD UNIQUE KEY `UNIQ_D06A4CFAF6BD1646AE880141` (`site_id`,`signature`), ADD KEY `IDX_D06A4CFAF6BD1646` (`site_id`), ADD KEY `IDX_D06A4CFAAE880141F6BD1646` (`signature`,`site_id`), ADD KEY `IDX_D06A4CFAEA1CE9BEF6BD1646` (`clientId`,`site_id`);
CREATE TABLE IF NOT EXISTS `feature_translations` (
`id` int(11) NOT NULL,
`object_id` int(11) DEFAULT NULL,
`locale` varchar(8) COLLATE utf8_unicode_ci NOT NULL,
`field` varchar(32) COLLATE utf8_unicode_ci NOT NULL,
`content` longtext COLLATE utf8_unicode_ci
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
ALTER TABLE `feature_translations`
ADD PRIMARY KEY (`id`), ADD KEY `IDX_7ED26C14232D562B` (`object_id`), ADD KEY `feature_translation_idx` (`locale`,`field`,`object_id`);
ALTER TABLE `feature_translations`
ADD CONSTRAINT `FK_7ED26C14232D562B` FOREIGN KEY (`object_id`) REFERENCES `Feature` (`id`) ON DELETE CASCADE;
谢谢!
编辑
感谢您的回复!
我们认为这个请求来自一个更大的请求。我正在使用 Doctrine，并尝试在我用过的表上使用大连接。这个工作正常，但是有 3 个表大大降低了我的性能。在这种情况下使用联接的性能低于多个请求。
我想要运行的请求是
SELECT * FROM Product p1_ INNER JOIN Poi p0_ ON p1_.id = p0_.id INNER JOIN Site s42_ ON p0_.site_id = s42_.id LEFT JOIN PoiLogoLink p2_ ON p0_.id = p2_.poi_id LEFT JOIN File f3_ ON p2_.file_id = f3_.id LEFT JOIN PoiPictureLink p4_ ON p0_.id = p4_.poi_id LEFT JOIN File f5_ ON p4_.file_id = f5_.id LEFT JOIN poi_translations p6_ ON p0_.id = p6_.object_id LEFT JOIN CustomObject c7_ ON p0_.id = c7_.poi_id LEFT JOIN Picto p8_ ON c7_.id = p8_.id LEFT JOIN Label l9_ ON c7_.id = l9_.id LEFT JOIN custom_objects_translations c10_ ON c7_.id = c10_.object_id LEFT JOIN category_poi c43_ ON p0_.id = c43_.poi_id LEFT JOIN Category c11_ ON c11_.id = c43_.category_id LEFT JOIN category_translations c12_ ON c11_.id = c12_.object_id LEFT JOIN place_poi p44_ ON p0_.id = p44_.poi_id LEFT JOIN Place p13_ ON p13_.id = p44_.place_id LEFT JOIN PoiLink p14_ ON p0_.id = p14_.childId LEFT JOIN Poi p15_ ON p14_.parentId = p15_.id LEFT JOIN Person p16_ ON p15_.id = p16_.id LEFT JOIN Store s17_ ON p15_.id = s17_.id LEFT JOIN Product p18_ ON p15_.id = p18_.id LEFT JOIN Exhibitor e19_ ON p15_.id = e19_.id LEFT JOIN Room r20_ ON p15_.id = r20_.id LEFT JOIN Service s21_ ON p15_.id = s21_.id LEFT JOIN PoiLink p22_ ON p0_.id = p22_.parentId LEFT JOIN Poi p23_ ON p22_.childId = p23_.id LEFT JOIN Person p24_ ON p23_.id = p24_.id LEFT JOIN Store s25_ ON p23_.id = s25_.id LEFT JOIN Product p26_ ON p23_.id = p26_.id LEFT JOIN Exhibitor e27_ ON p23_.id = e27_.id LEFT JOIN Room r28_ ON p23_.id = r28_.id LEFT JOIN Service s29_ ON p23_.id = s29_.id LEFT JOIN poi_translations p30_ ON p23_.id = p30_.object_id LEFT JOIN PoiMediaLink p31_ ON p0_.id = p31_.poi_id LEFT JOIN Media m32_ ON p31_.media_id = m32_.id LEFT JOIN MediaText m33_ ON m32_.id = m33_.id LEFT JOIN MediaImage m34_ ON m32_.id = m34_.id LEFT JOIN MediaVideo m35_ ON m32_.id = m35_.id LEFT JOIN MediaPdf m36_ ON m32_.id = m36_.id LEFT JOIN Movie m37_ ON m32_.id = m37_.id LEFT JOIN MediaUrl m38_ ON m32_.id = m38_.id LEFT JOIN media_translations m39_ ON m32_.id = m39_.object_id LEFT JOIN product_feature p45_ ON p1_.id = p45_.product_id LEFT JOIN Feature f40_ ON f40_.id = p45_.feature_id LEFT JOIN feature_translations f41_ ON f40_.id = f41_.object_id WHERE s42_.id IN (15) ORDER BY p0_.id ASC LIMIT 100 OFFSET 0;
100 rows in set (15,43 sec)
+----+-------------+-------+--------+------------------------------------------------------------+----------------------+---------+------------------------+------ +----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+------------------------------------------------------------+----------------------+---------+------------------------+------+----------------------------------------------------+
| 1 | SIMPLE | s42_ | const | PRIMARY | PRIMARY | 4 | const | 1 | Using temporary; Using filesort |
| 1 | SIMPLE | p0_ | ref | PRIMARY,UNIQ_45F65936F6BD1646AE880141,IDX_45F65936F6BD1646 | IDX_45F65936F6BD1646 | 4 | const | 8875 | NULL |
| 1 | SIMPLE | p1_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p0_.id | 1 | NULL |
| 1 | SIMPLE | p2_ | ref | IDX_7990D9C97EACE855 | IDX_7990D9C97EACE855 | 4 | adsum.p0_.id | 1 | NULL |
| 1 | SIMPLE | f3_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p2_.file_id | 1 | NULL |
| 1 | SIMPLE | p4_ | ref | IDX_6CC279957EACE855 | IDX_6CC279957EACE855 | 4 | adsum.p0_.id | 1 | NULL |
| 1 | SIMPLE | f5_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p4_.file_id | 1 | NULL |
| 1 | SIMPLE | p6_ | ref | IDX_9478F0F7232D562B | IDX_9478F0F7232D562B | 5 | adsum.p0_.id | 1 | NULL |
| 1 | SIMPLE | c7_ | ref | IDX_FE0561D77EACE855 | IDX_FE0561D77EACE855 | 5 | adsum.p0_.id | 1 | NULL |
| 1 | SIMPLE | p8_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.c7_.id | 1 | NULL |
| 1 | SIMPLE | l9_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.c7_.id | 1 | NULL |
| 1 | SIMPLE | c10_ | ref | IDX_A653A52A232D562B | IDX_A653A52A232D562B | 5 | adsum.c7_.id | 1 | NULL |
| 1 | SIMPLE | c43_ | ref | IDX_5879B28C7EACE855 | IDX_5879B28C7EACE855 | 4 | adsum.p0_.id | 1 | Using index |
| 1 | SIMPLE | c11_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.c43_.category_id | 1 | NULL |
| 1 | SIMPLE | c12_ | ref | IDX_1C60F915232D562B | IDX_1C60F915232D562B | 5 | adsum.c11_.id | 1 | NULL |
| 1 | SIMPLE | p44_ | ref | IDX_720979277EACE855 | IDX_720979277EACE855 | 4 | adsum.p0_.id | 1 | Using index |
| 1 | SIMPLE | p13_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p44_.place_id | 1 | NULL |
| 1 | SIMPLE | p14_ | ref | IDX_64E377A92FD6B47 | IDX_64E377A92FD6B47 | 5 | adsum.p0_.id | 4 | NULL |
| 1 | SIMPLE | p15_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p14_.parentId | 1 | NULL |
| 1 | SIMPLE | p16_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p15_.id | 1 | NULL |
| 1 | SIMPLE | s17_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p15_.id | 1 | NULL |
| 1 | SIMPLE | p18_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p15_.id | 1 | NULL |
| 1 | SIMPLE | e19_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p15_.id | 1 | NULL |
| 1 | SIMPLE | r20_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p15_.id | 1 | NULL |
| 1 | SIMPLE | s21_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p15_.id | 1 | NULL |
| 1 | SIMPLE | p22_ | ref | IDX_64E377A910EE4CEE | IDX_64E377A910EE4CEE | 5 | adsum.p0_.id | 2 | NULL |
| 1 | SIMPLE | p23_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p22_.childId | 1 | NULL |
| 1 | SIMPLE | p24_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p23_.id | 1 | NULL |
| 1 | SIMPLE | s25_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p23_.id | 1 | NULL |
| 1 | SIMPLE | p26_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p23_.id | 1 | NULL |
| 1 | SIMPLE | e27_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p23_.id | 1 | NULL |
| 1 | SIMPLE | r28_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p23_.id | 1 | NULL |
| 1 | SIMPLE | s29_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p23_.id | 1 | NULL |
| 1 | SIMPLE | p30_ | ref | IDX_9478F0F7232D562B | IDX_9478F0F7232D562B | 5 | adsum.p23_.id | 1 | NULL |
| 1 | SIMPLE | p31_ | ref | IDX_5D2C1AE27EACE855 | IDX_5D2C1AE27EACE855 | 4 | adsum.p0_.id | 1 | NULL |
| 1 | SIMPLE | m32_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p31_.media_id | 1 | NULL |
| 1 | SIMPLE | m33_ | ALL | PRIMARY | NULL | NULL | NULL | 1 | Using where; Using join buffer (Block Nested Loop) |
| 1 | SIMPLE | m34_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.m32_.id | 1 | NULL |
| 1 | SIMPLE | m35_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.m32_.id | 1 | NULL |
| 1 | SIMPLE | m36_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.m32_.id | 1 | NULL |
| 1 | SIMPLE | m37_ | ALL | PRIMARY | NULL | NULL | NULL | 1 | Using where; Using join buffer (Block Nested Loop) |
| 1 | SIMPLE | m38_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.m32_.id | 1 | NULL |
| 1 | SIMPLE | m39_ | ref | IDX_AF46700B232D562B | IDX_AF46700B232D562B | 5 | adsum.m32_.id | 1 | NULL |
| 1 | SIMPLE | p45_ | ref | PRIMARY,IDX_CE0E6ED64584665A | IDX_CE0E6ED64584665A | 4 | adsum.p0_.id | 2 | Using index |
| 1 | SIMPLE | f40_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p45_.feature_id | 1 | NULL |
| 1 | SIMPLE | f41_ | ALL | IDX_7ED26C14232D562B | NULL | NULL | NULL | 1 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+--------+------------------------------------------------------------+----------------------+---------+------------------------+------+----------------------------------------------------+
但是如果我删除不使用键的表
SELECT * FROM Product p1_ INNER JOIN Poi p0_ ON p1_.id = p0_.id INNER JOIN Site s31_ ON p0_.site_id = s31_.id LEFT JOIN PoiLogoLink p2_ ON p0_.id = p2_.poi_id LEFT JOIN File f3_ ON p2_.file_id = f3_.id LEFT JOIN PoiPictureLink p4_ ON p0_.id = p4_.poi_id LEFT JOIN File f5_ ON p4_.file_id = f5_.id LEFT JOIN poi_translations p6_ ON p0_.id = p6_.object_id LEFT JOIN CustomObject c7_ ON p0_.id = c7_.poi_id LEFT JOIN Picto p8_ ON c7_.id = p8_.id LEFT JOIN Label l9_ ON c7_.id = l9_.id LEFT JOIN custom_objects_translations c10_ ON c7_.id = c10_.object_id LEFT JOIN category_poi c32_ ON p0_.id = c32_.poi_id LEFT JOIN Category c11_ ON c11_.id = c32_.category_id LEFT JOIN category_translations c12_ ON c11_.id = c12_.object_id LEFT JOIN place_poi p33_ ON p0_.id = p33_.poi_id LEFT JOIN Place p13_ ON p13_.id = p33_.place_id LEFT JOIN PoiLink p14_ ON p0_.id = p14_.childId LEFT JOIN Poi p15_ ON p14_.parentId = p15_.id LEFT JOIN Person p16_ ON p15_.id = p16_.id LEFT JOIN Store s17_ ON p15_.id = s17_.id LEFT JOIN Product p18_ ON p15_.id = p18_.id LEFT JOIN Exhibitor e19_ ON p15_.id = e19_.id LEFT JOIN Room r20_ ON p15_.id = r20_.id LEFT JOIN Service s21_ ON p15_.id = s21_.id LEFT JOIN PoiLink p22_ ON p0_.id = p22_.parentId LEFT JOIN Poi p23_ ON p22_.childId = p23_.id LEFT JOIN Person p24_ ON p23_.id = p24_.id LEFT JOIN Store s25_ ON p23_.id = s25_.id LEFT JOIN Product p26_ ON p23_.id = p26_.id LEFT JOIN Exhibitor e27_ ON p23_.id = e27_.id LEFT JOIN Room r28_ ON p23_.id = r28_.id LEFT JOIN Service s29_ ON p23_.id = s29_.id LEFT JOIN poi_translations p30_ ON p23_.id = p30_.object_id WHERE s31_.id IN (15) ORDER BY p0_.id ASC LIMIT 100 OFFSET 0;
100 rows in set (0,02 sec)
+----+-------------+-------+--------+------------------------------------------------------------+----------------------+---------+------------------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+------------------------------------------------------------+----------------------+---------+------------------------+------+-------------+
| 1 | SIMPLE | s31_ | const | PRIMARY | PRIMARY | 4 | const | 1 | NULL |
| 1 | SIMPLE | p0_ | ref | PRIMARY,UNIQ_45F65936F6BD1646AE880141,IDX_45F65936F6BD1646 | IDX_45F65936F6BD1646 | 4 | const | 8875 | Using where |
| 1 | SIMPLE | p1_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p0_.id | 1 | NULL |
| 1 | SIMPLE | p2_ | ref | IDX_7990D9C97EACE855 | IDX_7990D9C97EACE855 | 4 | adsum.p0_.id | 1 | NULL |
| 1 | SIMPLE | f3_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p2_.file_id | 1 | NULL |
| 1 | SIMPLE | p4_ | ref | IDX_6CC279957EACE855 | IDX_6CC279957EACE855 | 4 | adsum.p0_.id | 1 | NULL |
| 1 | SIMPLE | f5_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p4_.file_id | 1 | NULL |
| 1 | SIMPLE | p6_ | ref | IDX_9478F0F7232D562B | IDX_9478F0F7232D562B | 5 | adsum.p0_.id | 1 | NULL |
| 1 | SIMPLE | c7_ | ref | IDX_FE0561D77EACE855 | IDX_FE0561D77EACE855 | 5 | adsum.p0_.id | 1 | NULL |
| 1 | SIMPLE | p8_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.c7_.id | 1 | NULL |
| 1 | SIMPLE | l9_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.c7_.id | 1 | NULL |
| 1 | SIMPLE | c10_ | ref | IDX_A653A52A232D562B | IDX_A653A52A232D562B | 5 | adsum.c7_.id | 1 | NULL |
| 1 | SIMPLE | c32_ | ref | IDX_5879B28C7EACE855 | IDX_5879B28C7EACE855 | 4 | adsum.p0_.id | 1 | Using index |
| 1 | SIMPLE | c11_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.c32_.category_id | 1 | NULL |
| 1 | SIMPLE | c12_ | ref | IDX_1C60F915232D562B | IDX_1C60F915232D562B | 5 | adsum.c11_.id | 1 | NULL |
| 1 | SIMPLE | p33_ | ref | IDX_720979277EACE855 | IDX_720979277EACE855 | 4 | adsum.p0_.id | 1 | Using index |
| 1 | SIMPLE | p13_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p33_.place_id | 1 | NULL |
| 1 | SIMPLE | p14_ | ref | IDX_64E377A92FD6B47 | IDX_64E377A92FD6B47 | 5 | adsum.p0_.id | 4 | NULL |
| 1 | SIMPLE | p15_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p14_.parentId | 1 | NULL |
| 1 | SIMPLE | p16_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p15_.id | 1 | NULL |
| 1 | SIMPLE | s17_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p15_.id | 1 | NULL |
| 1 | SIMPLE | p18_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p15_.id | 1 | NULL |
| 1 | SIMPLE | e19_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p15_.id | 1 | NULL |
| 1 | SIMPLE | r20_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p15_.id | 1 | NULL |
| 1 | SIMPLE | s21_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p15_.id | 1 | NULL |
| 1 | SIMPLE | p22_ | ref | IDX_64E377A910EE4CEE | IDX_64E377A910EE4CEE | 5 | adsum.p0_.id | 2 | NULL |
| 1 | SIMPLE | p23_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p22_.childId | 1 | NULL |
| 1 | SIMPLE | p24_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p23_.id | 1 | NULL |
| 1 | SIMPLE | s25_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p23_.id | 1 | NULL |
| 1 | SIMPLE | p26_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p23_.id | 1 | NULL |
| 1 | SIMPLE | e27_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p23_.id | 1 | NULL |
| 1 | SIMPLE | r28_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p23_.id | 1 | NULL |
| 1 | SIMPLE | s29_ | eq_ref | PRIMARY | PRIMARY | 4 | adsum.p23_.id | 1 | NULL |
| 1 | SIMPLE | p30_ | ref | IDX_9478F0F7232D562B | IDX_9478F0F7232D562B | 5 | adsum.p23_.id | 1 | NULL |
+----+-------------+-------+--------+------------------------------------------------------------+----------------------+---------+------------------------+------+-------------+
在这里，我认为我的第一个请求中有一件非常糟糕的事情
最佳答案
MySQL中LEFT JOIN的执行方式是执行两个嵌套循环:外循环遍历左表，内循环遍历内表。如果内表有很多行，并且在 ON 条件下的列上有索引，它将使用该索引来加速内循环。请注意，外层表上的索引无法帮助加快外层循环的速度，因为无论如何它都需要遍历左表的所有行。
在您提供的解释中，右表只有一行。 MySQL 聪明地得出结论，它不需要任何索引，因为对该表的全表扫描将与使用索引一样快(甚至更快，因为避免了与使用索引相关的开销)，因此它选择在每次迭代时对表执行完整扫描(即对单行进行完整扫描)。
如果您的右表有更多行，那么 MySQL 将选择使用该索引。
最后请注意，如果您的联接是INNER JOIN，而不是LEFT JOIN，MySQL 可能足够聪明，可以颠倒表的顺序，并使用您的联接。 feature_translations 作为外部表，在这种情况下，它将能够利用内部 Feature 表上的索引。
关于Mysql 左连接不使用外键，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/33763883/