Mysql e、 您没有示例输入、输出和所需输出。“我只解析了一行”是什么意思？并给出一个完整的例子（代码和数据），展示您的问题。在给出您的规范时，还有哪些其他可声明的惟一列集&FKs列集和非空列？PS“reference”和“searched”列是什么意思？在表中添

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e、 您没有示例输入、输出和所需输出。“我只解析了一行”是什么意思？并给出一个完整的例子（代码和数据），展示您的问题。在给出您的规范时，还有哪些其他可声明的惟一列集&FKs列集和非空列？PS“reference”和“searched”列是什么意思？在表中添加了几十行之后，请再次尝试

EXPLAIN

。您可能会发现它不是以

u

的完整表扫描开始的。但是，

中仍然隐含着一个tmp表。此外，还有一个完整的表扫描（确定，只有2行）来构建索引（

）。@RickJames我用6个用户和50条消息进行了测试，执行计划没有改变。此查询类似于@BillKarwin one，但它只对u执行1次完整表扫描，并且只执行1次嵌套循环（

JOIN

），而不是2和2。从MySQL 5.7开始，为派生表构建合并索引应该具有良好的性能。我的意思是，加入一个联盟应该比两个联盟更好。哇！该问题需要

test_message

和

test_user

中的所有列；此处的查询仅提供来自

test\u user

的列。因此，我认为这不是一个有效的答案。问题是“在这种情况下，如何可能获得‘每个记录的范围检查’”，我的答案给出了问题的描述和避免问题的替代解决方案。如果你不喜欢，别担心。。我会活下来..“为每个记录检查的范围”太神秘了，我不知道它是什么意思。我不能检查，但我相信使用“联合所有”将有助于避免临时表（可能会导致重复，但我认为很少向自己发送消息）@skyboyer:MySQL 5.7.3及更高版本可以在使用

UNION ALL

时消除临时表。请参阅第页的具体说明。MySQL的早期版本没有这种优化。

SELECT *
FROM message AS m
INNER JOIN user AS u
ON u.id = m.sender_id OR u.id = m.receiver_id

+----+-------------+-------+------+---------------+------+---------+------+-------+-----------------------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows  | Extra                                         |
+----+-------------+-------+------+---------------+------+---------+------+-------+-----------------------------------------------+
|  1 | SIMPLE      | u     | ALL  | PRIMARY       | null | null    | null | 75000 | Range checked for each record (index map: 0x1)|
+----+-------------+-------+------+---------------+------+---------+------+-------+-----------------------------------------------+

SELECT *
FROM message AS m
INNER JOIN user AS u
ON u.id IN(m.sender_id, m.receiver_id)

SELECT *
FROM message AS m
INNER JOIN user AS u
ON u.id = m.sender_id

CREATE TABLE test_user (
    id INT NOT NULL AUTO_INCREMENT,
    name VARCHAR(30),
    PRIMARY KEY (id)
);

CREATE TABLE test_message (
    id INT NOT NULL AUTO_INCREMENT, 
    sender_id INT NOT NULL,
    receiver_id INT NOT NULL,
    PRIMARY KEY (id),
    INDEX idx_sender (sender_id),
    INDEX idx_receiver (receiver_id)
);

EXPLAIN SELECT *
FROM test_message AS m
INNER JOIN test_user AS u
    ON u.id = m.sender_id OR u.id = m.receiver_id;

mysql> EXPLAIN 
    SELECT * FROM test_message AS m 
    INNER JOIN test_user AS u ON u.id = m.sender_id 
  UNION
    SELECT * FROM test_message AS m 
    INNER JOIN test_user AS u ON u.id = m.receiver_id;

+----+--------------+------------+--------+---------------+---------+---------+--------------------+------+-----------------+
| id | select_type  | table      | type   | possible_keys | key     | key_len | ref                | rows | Extra           |
+----+--------------+------------+--------+---------------+---------+---------+--------------------+------+-----------------+
|  1 | PRIMARY      | m          | ALL    | idx_sender    | NULL    | NULL    | NULL               |    1 | NULL            |
|  1 | PRIMARY      | u          | eq_ref | PRIMARY       | PRIMARY | 4       | test.m.sender_id   |    1 | NULL            |
|  2 | UNION        | m          | ALL    | idx_receiver  | NULL    | NULL    | NULL               |    1 | NULL            |
|  2 | UNION        | u          | eq_ref | PRIMARY       | PRIMARY | 4       | test.m.receiver_id |    1 | NULL            |
| NULL | UNION RESULT | <union1,2> | ALL    | NULL          | NULL    | NULL    | NULL               | NULL | Using temporary |
+----+--------------+------------+--------+---------------+---------+---------+--------------------+------+-----------------+

EXPLAIN 
SELECT *
FROM test_user AS u
INNER JOIN (
    select id, sender_id as msg_id
    from test_message 
    union all 
    select id, receiver_id 
    from test_message 
    ) AS m 
ON u.id = m.msg_id;

id  select_type table           partitions  type    possible_keys   key             key_len ref         rows    filtered    Extra
1   PRIMARY     u               NULL        ALL     PRIMARY         NULL            NULL    NULL        1       100.00      NULL
1   PRIMARY     <derived2>      NULL        ref     <auto_key0>     <auto_key0>     4       test.u.id   2       100.00      NULL
2   DERIVED     test_message    NULL        index   NULL            idx_sender      4       NULL        1       100.00      "Using index"
3   UNION       test_message    NULL        index   NULL            idx_receiver    4       NULL        1       100.00      "Using index"