Mysql左连接不使用外键

当我运行EXPLAIN SELECT * FROM Feature f LEFT JOIN feature_translations t ON f.id = t.object_id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，加入不使用现有的索引！如果我删除几乎所有的行，那么使用索引。这就是为什么我认为只是一个配置变量，但我不知道哪一个！

遵循我的表结构。

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！

认为这个请求来自一个更大的请求。我正在使用Doctrine，并尝试在我使用的表上使用一个大连接。这项工作很好，但有3张桌子大大减少了我的表演。在这种情况下使用连接的性能低于多个请求。

我想跑的请求

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) | 
+----+-------------+-------+--------+------------------------------------------------------------+----------------------+---------+------------------------+------+----------------------------------------------------+

但如果我删除的表不使用钥匙

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，我想我有一个非常糟糕的事情在我的第一个请求

来源

2015-11-17 moroine bentefrit

在MySQL中执行LEFT JOIN的方式是执行两个嵌套循环：在外循环中遍历左表，在内循环中遍历内部表。如果内部表有很多行，并且ON条件的列上有一个索引，它将使用该索引来加速内部循环。请注意，外表上的索引无法帮助加快外循环，因为它无论如何都需要遍历左表的所有行。

在解释你提供的右表只有一行。 MySQL巧妙地得出结论，它不需要任何索引，因为该表上的全表扫描将会像使用索引一样快（或者甚至更快，因为避免了与使用索引相关的开销），所以它选择执行全扫描在每次迭代中表（即，对单个行的完整扫描）。

如果你的表有更多的行，那么MySQL会选择使用该索引。

最后注意，如果您的加盟是一个INNER JOIN，不是LEFT JOIN时，MySQL可能是足够聪明的逆转表的顺序，并使用您的feature_translations作为外部表，在这种情况下，将能够利用内部Feature表上的索引。

来源

2015-11-17 18:20:22 Ishamael

谢谢你的回复！我编辑我的问题，以提供更多的上下文。如果你可以看看这将是非常好的！ –

我能想到的唯一的事实是，第一个解释包含一些块嵌套循环。在执行'SET optimizer_switch ='block_nested_loop = off''之前，尝试禁用它，然后再次运行查询和解释。如果它没有帮助，我建议创建一个新问题，因为在这里只有我看到了这些变化，而新问题对整个社区都是可见的。 – Ishamael

嗨，我试过了，但不幸的是，这并不奏效。 Mysql不使用索引，执行时间增加2秒。我会发布一个新问题，以获得更多的知名度。谢谢 –

Mysql左连接不使用外键

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