MySQL5.7.20-通过合并列按顺序左连接-非常奇怪的行为

我面临一个非常奇怪的问题，希望你能解释给我听。 我试图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

我有两张桌子：

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;

当一个用户（假设ID=1）正在浏览该网站时，我想向他展示其他用户的最爱在底部。 因此，我从这个查询开始：

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，也是我所期望的：

+----+-------+------------------------+
| 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，所以我添加了一个包装器查询，但结果仍然是一样的

为什么ORDER BY\u favorited\u Coalized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

而不是别名，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)

编辑2 我发现了另一种奇怪的行为。如果我尝试按ID列排序，我会得到以下结果：

--------------
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 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

正如我在评论中所建议的：

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)

此外：

编辑4:

我跑过：

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)

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)

如评论中所述

---

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);

这就是bug（或者至少与之密切相关）

它（以一种非常类似的形式）仍然存在于MySQL 8.0.12中（例如，请参见dbfiddle，尽管它希望在修复后不会显示不正确的行为）：虽然它现在实际订购正确（可能是因为您对其进行了计算），但它仍然返回了不正确的

is\u favorited

：

select user.*, favorites.is_favorited, 
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 desc

+----+-------+--------------+------------------------+
| id | name  | is_favorited | is_favorited_coalesced |
+----+-------+--------------+------------------------+
|  1 | user1 |              |                      1 |
|  2 | user2 |              |                      1 |
|  3 | user3 |              |                      0 |
|  4 | user4 |              |                      0 |
+----+-------+--------------+------------------------+

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

限制

）来解决大多数错误：

---

正如@RaymondNijland提到的，还有其他解决方法，例如在运行该查询之前禁用派生表与的合并。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一个已损坏的查询，只需为已验证不受影响的查询启用该功能（这样它们就可以再次从优化中获益）。

对我有效：@juergend它对SqlFIDLE有效，所以我不知道为什么它不能在我的机器上工作。我还发现了另一个奇怪的行为，我现在更新我的问题。谢谢你，你能执行一个

解释吗；显示警告像这样（）并以这种方式将结果发布到这里，我们可以了解MySQL优化器是如何重写或处理查询的。。您还可以看到优化器正在使用
coalesce（1,0）优化
coalesce（favorites.is\u favorited，0）
，因为
is\u favorited\u coalesced`还可以将SQL模式发布到MySQL服务器<代码>选择@@sql\U模式
我可以在运行5.7.12的Rextester上复制。在我看来，这是一个错误。谢谢你，我会在这个问题上摔碎我的头好长时间的。标记为已接受。@ruddle我觉得这就是问题所在，为什么我要求使用optimizer\u开关配置。唯一的问题是
limit1000000
将始终使用临时触发
；使用filesort远远不是最佳的，更好的方法是使用
设置会话优化器\u switch='derived'u merge=off'见演示。@RaymondNijland虽然有几种方法可以解决这个问题，但开关和
限制
的行为应该完全相同（我认为它们不应该引发比
限制
更多的文件排序，当然在派生表中没有
order by
）。使用
limit
的优点是，它可以在需要时通过手术使用（不是每个派生表，甚至可能不是在同一查询中都需要修复），并且不需要第二次查询，而开关可以全局使用，无需检查每个查询是否已损坏。我将在答案中添加该选项。“虽然有几种方法可以解决此问题，但开关和限制都应与sam完全相同”，在这种情况下，选择“是”。。因为别名列上的查询顺序（当然会使用“Using filesort”，当优化器认为需要对大量记录进行排序时，也可以使用“Using temporary”）更好的方法是使用
EXPLAIN
检查这两种变通方法，以确保哪种变通方法生成最佳查询计划。。“我认为它们不应该引发超过限制的更多文件排序，在派生表中没有顺序当然”我更担心组合“使用文件排序；使用temporary“在将临时表写入磁盘时，它可能会变成大量的随机磁盘I/O…”和
LIMIT
，其中包含大量始终使用的“Using filesort”；在解释中使用“临时”。。。
select user.*, favorites.is_favorited, 
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' limit 1000000
) favorites on favorites.favorited_user_id = user.id
order by is_favorited_coalesced desc

+----+-------+--------------+------------------------+
| id | name  | is_favorited | is_favorited_coalesced |
+----+-------+--------------+------------------------+
|  1 | user1 |            1 |                      1 |
|  2 | user2 |            1 |                      1 |
|  3 | user3 |              |                      0 |
|  4 | user4 |              |                      0 |
+----+-------+--------------+------------------------+