我正面临一个非常奇怪的问题，希望你能解释给我听。我要做的是根据子查询中的合并列对结果集进行排序。让我解释清楚。
我有两张table：
CREATE TABLE `user` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
CREATE TABLE `user_favorites_user` (
`source_user_id` int(11) NOT NULL,
`favorited_user_id` int(11) NOT NULL,
KEY `source_user_id` (`source_user_id`),
KEY `favorited_user_id` (`favorited_user_id`),
CONSTRAINT `user_favorites_user_ibfk_1` FOREIGN KEY (`source_user_id`) REFERENCES `user` (`id`),
CONSTRAINT `user_favorites_user_ibfk_2` FOREIGN KEY (`favorited_user_id`) REFERENCES `user` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;由于一个用户（假设id=1）正在浏览该网站，我想向他展示其他用户的最爱与他的底部订购。所以，我从这个问题开始：
select user.*, coalesce(favorites.is_favorited,0) as is_favorited_coalesced from user
left join (
select 1 as is_favorited, favorited_user_id from user_favorites_user
where source_user_id = '1'
) favorites on favorites.favorited_user_id = user.id到目前为止还不错，这是我得到的和我期望的：
+----+-------+------------------------+
| id | name | is_favorited_coalesced |
+----+-------+------------------------+
| 3 | user3 | 1 |
| 4 | user4 | 1 |
| 1 | user1 | 0 |
| 2 | user2 | 0 |
+----+-------+------------------------+
4 rows in set (0.00 sec)现在，我想订购结果集。我认为按条款订货就足够了：
select user.*, coalesce(favorites.is_favorited,0) as is_favorited_coalesced from user
left join (
select 1 as is_favorited, favorited_user_id from user_favorites_user
where source_user_id = '1'
) favorites on favorites.favorited_user_id = user.id
order by is_favorited_coalesced asc在这一点上，我得到了与上面相同的结果：
+----+-------+------------------------+
| id | name | is_favorited_coalesced |
+----+-------+------------------------+
| 3 | user3 | 1 |
| 4 | user4 | 1 |
| 1 | user1 | 0 |
| 2 | user2 | 0 |
+----+-------+------------------------+
4 rows in set (0.00 sec)然后我认为coalesce不适合动态排序，所以我添加了一个 Package 器查询，但结果仍然是一样的。
为什么order by\u favorited\u Coalsed不起作用？我错过了什么？
编辑：我尝试使用：
order by coalesce(favorites.is_favorited,0) asc但我得到了相同的结果：
select user.*, coalesce(favorites.is_favorited,0) as is_favorited_coalesced from user left join ( select 1 as is_favorited, favorited_user_id from user_favorites_user where source_user_id = '1' ) favorites on favorites.favorited_user_id = user.id order by coalesce(favorites.is_favorited,0)
--------------
+----+-------+------------------------+
| id | name | is_favorited_coalesced |
+----+-------+------------------------+
| 3 | user3 | 1 |
| 4 | user4 | 1 |
| 1 | user1 | 0 |
| 2 | user2 | 0 |
+----+-------+------------------------+
4 rows in set (0.00 sec)我发现了另一个奇怪的行为。如果我尝试按id列排序，我得到的结果如下：
--------------
select user.*, coalesce(favorites.is_favorited,0) as is_favorited_coalesced from user left join ( select 1 as is_favorited, favorited_user_id from user_favorites_user where source_user_id = '1' ) favorites on favorites.favorited_user_id = user.id order by id asc
--------------
+----+-------+------------------------+
| id | name | is_favorited_coalesced |
+----+-------+------------------------+
| 1 | user1 | 1 |
| 2 | user2 | 1 |
| 3 | user3 | 1 |
| 4 | user4 | 1 |
+----+-------+------------------------+
4 rows in set (0.00 sec)我不知道为什么会这样。我在使用virtualbox的windows下的虚拟fedora25上使用mysql 5.7.20。
编辑3
正如我在评论中所说：
mysql> explain select user.*, coalesce(favorites.is_favorited,0) as is_favorited_coalesced from user left join ( select 1 as is_favorited, favorited_user_id from user_favorites_user where source_user_id = '1' ) favorites on favorites.favorited_user_id = user.id order by is_favorited_coalesced asc;show warnings;
+----+-------------+---------------------+------------+-------+----------------------------------+----------------+---------+------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------------------+------------+-------+----------------------------------+----------------+---------+------+------+----------+----------------------------------------------------+
| 1 | SIMPLE | user | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | NULL |
| 1 | SIMPLE | user_favorites_user | NULL | range | source_user_id,favorited_user_id | source_user_id | 4 | NULL | 2 | 100.00 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+---------------------+------------+-------+----------------------------------+----------------+---------+------+------+----------+----------------------------------------------------+
2 rows in set, 1 warning (0.00 sec)
+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note | 1003 | /* select#1 */ select `so_test`.`user`.`id` AS `id`,`so_test`.`user`.`name` AS `name`,coalesce(1,0) AS `is_favorited_coalesced` from `so_test`.`user` left join (`so_test`.`user_favorites_user`) on(((`so_test`.`user_favorites_user`.`favorited_user_id` = `so_test`.`user`.`id`) and (`so_test`.`user_favorites_user`.`source_user_id` = '1'))) where 1 order by `is_favorited_coalesced` |
+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)也：
mysql> SELECT @@sql_mode;
+-------------------------------------------------------------------------------------------------------------------------------------------+
| @@sql_mode |
+-------------------------------------------------------------------------------------------------------------------------------------------+
| ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION |
+-------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)编辑4：
我跑过：
mysql> SELECT @@optimizer_switch;
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| @@optimizer_switch |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| index_merge=on,index_merge_union=on,index_merge_sort_union=on,index_merge_intersection=on,engine_condition_pushdown=on,index_condition_pushdown=on,mrr=on,mrr_cost_based=on,block_nested_loop=on,batched_key_access=off,materialization=on,semijoin=on,loosescan=on,firstmatch=on,duplicateweedout=on,subquery_materialization_cost_based=on,use_index_extensions=on,condition_fanout_filter=on,derived_merge=on |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)如评论所述。
包括用于快速测试的数据集：
SET NAMES utf8;
SET time_zone = '+00:00';
SET foreign_key_checks = 0;
SET sql_mode = 'NO_AUTO_VALUE_ON_ZERO';
CREATE TABLE `user` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
INSERT INTO `user` (`id`, `name`) VALUES
(1, 'user1'),
(2, 'user2'),
(3, 'user3'),
(4, 'user4');
CREATE TABLE `user_favorites_user` (
`source_user_id` int(11) NOT NULL,
`favorited_user_id` int(11) NOT NULL,
KEY `source_user_id` (`source_user_id`),
KEY `favorited_user_id` (`favorited_user_id`),
CONSTRAINT `user_favorites_user_ibfk_1` FOREIGN KEY (`source_user_id`) REFERENCES `user` (`id`),
CONSTRAINT `user_favorites_user_ibfk_2` FOREIGN KEY (`favorited_user_id`) REFERENCES `user` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
INSERT INTO `user_favorites_user` (`source_user_id`, `favorited_user_id`) VALUES
(1, 3),
(1, 4);
1条答案
按热度按时间r7s23pms1#
如果存在orderby（或者至少是密切相关的），那么这个bug查询将返回错误的数据。
它（以非常相似的形式）仍然存在于mysql 8.0.12中（例如，请参阅dbfiddle中的示例，尽管它在修复后不会显示错误的行为）：虽然它现在实际上正确地排序（可能是因为您对它进行了计算），但它仍然返回错误的值
is_favorited:这似乎是一个与（非）物化相关的优化器问题（mysql 5.7有很多这样的问题）。您可以通过强制实现派生表（例如，通过添加
limit):正如@raymondnijland提到的，还有其他解决方法，例如禁用派生表合并
set [GLOBAL|SESSION] optimizer_switch='derived_merge=off'在运行查询之前。您还可以使用此功能全局禁用该功能，直到错误得到修复，这样您就不必检查每个已损坏的查询，只需为已验证不受影响的查询启用它（这样它们就可以再次从优化中获益）。