使用LEFT JOIN + ORDER BY时如何避免FileSort？

Question

mysql-5.6.24-win32.1432006610

我有两个简单的表， TUser(id, name)和TMessage(id, uid, message) 。 TUser持有用户， TMessage持有用户消息。

SQL如下所示，另请参见： http : //sqlfiddle.com/#!9/7f099

CREATE TABLE TUser(
    id     INT UNSIGNED     PRIMARY KEY    NOT NULL    AUTO_INCREMENT,
    name   VARCHAR(128) NOT NULL
);


CREATE TABLE TMessage(
    id      INT UNSIGNED    PRIMARY KEY    NOT NULL    AUTO_INCREMENT,
    uid     INT UNSIGNED    NOT NULL,
    message VARCHAR(256)    NOT NULL
);


CREATE INDEX TMessageIndexUid ON TMessage(uid);

插入一些数据：

INSERT INTO TUser (name) VALUES 
     ('jack')
    ,('rose')
    ,('peter');


INSERT INTO TMessage(uid, message) VALUES
     (1, 'Hello jack')
    , (1, 'Jack, how are you')
    , (1, 'Good morning jack')
    , (2, 'I love you, rose')
    , (3, 'Peter, please call back')
    , (3, 'What are you doing, Peter');

当我运行以下LEFT JOIN + ORDER BY查询时，FileSort显示在EXPLAIN结果中：

EXPLAIN
SELECT *
  FROM        TUser
  LEFT JOIN   TMessage 
  ON          TUser.id=TMessage.uid
  WHERE       TUser.id=3
  ORDER BY    TMessage.id DESC;



id  select_type table    type  possible_keys    key              key_len ref   rows Extra
1   SIMPLE      TUser    const PRIMARY;         PRIMARY          4       const 1    Using temporary; Using filesort
1   SIMPLE      TMessage ref   TMessageIndexUid TMessageIndexUid 4       const 2 \N

有什么不对？

Answer 1

该文件排序由ORDER BY引入：

mysql> EXPLAIN
    -> SELECT *
    ->   FROM        TUser
    ->   LEFT JOIN   TMessage
    ->   ON          TUser.id=TMessage.uid
    ->   WHERE       TUser.id=3
    ->   ORDER BY    TMessage.id DESC;
+----+-------------+----------+------------+-------+------------------+------------------+---------+-------+------+----------+---------------------------------+
| id | select_type | table    | partitions | type  | possible_keys    | key              | key_len | ref   | rows | filtered | Extra                           |
+----+-------------+----------+------------+-------+------------------+------------------+---------+-------+------+----------+---------------------------------+
|  1 | SIMPLE      | TUser    | NULL       | const | PRIMARY          | PRIMARY          | 4       | const |    1 |   100.00 | Using temporary; Using filesort |
|  1 | SIMPLE      | TMessage | NULL       | ref   | TMessageIndexUid | TMessageIndexUid | 4       | const |    4 |   100.00 | NULL                            |
+----+-------------+----------+------------+-------+------------------+------------------+---------+-------+------+----------+---------------------------------+
2 rows in set, 1 warning (0.00 sec)

mysql> EXPLAIN
    -> SELECT *
    ->   FROM        TUser
    ->   LEFT JOIN   TMessage
    ->   ON          TUser.id=TMessage.uid
    ->   WHERE       TUser.id=3;
+----+-------------+----------+------------+-------+------------------+------------------+---------+-------+------+----------+-------+
| id | select_type | table    | partitions | type  | possible_keys    | key              | key_len | ref   | rows | filtered | Extra |
+----+-------------+----------+------------+-------+------------------+------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | TUser    | NULL       | const | PRIMARY          | PRIMARY          | 4       | const |    1 |   100.00 | NULL  |
|  1 | SIMPLE      | TMessage | NULL       | ref   | TMessageIndexUid | TMessageIndexUid | 4       | const |    4 |   100.00 | NULL  |
+----+-------------+----------+------------+-------+------------------+------------------+---------+-------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)

由于order by对LEFT JOIN结果进行操作，因此我不知道如何避免这种文件排序。

如果将LEFT JOIN更改为INNER JOIN是可以接受的，那可能是一种绕过的方法，同时会丢失一些用户信息，而这些信息与TMessages不匹配。

mysql> EXPLAIN
    -> SELECT *
    ->   FROM        TUser
    ->   INNER JOIN   TMessage
    ->   ON          TUser.id=TMessage.uid
    ->   WHERE       TUser.id=3
    ->   ORDER BY    TMessage.id DESC;
+----+-------------+----------+------------+-------+------------------+------------------+---------+-------+------+----------+-------------+
| id | select_type | table    | partitions | type  | possible_keys    | key              | key_len | ref   | rows | filtered | Extra       |
+----+-------------+----------+------------+-------+------------------+------------------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | TUser    | NULL       | const | PRIMARY          | PRIMARY          | 4       | const |    1 |   100.00 | NULL        |
|  1 | SIMPLE      | TMessage | NULL       | ref   | TMessageIndexUid | TMessageIndexUid | 4       | const |    2 |   100.00 | Using where |
+----+-------------+----------+------------+-------+------------------+------------------+---------+-------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

Answer 2

我也不知道'filesort'。 然后我发现类似

"Anytime a sort can’t be performed from an index, it’s a filesort."

以下是引用的链接：

• https://www.percona.com/blog/2009/03/05/what-does-using-filesort-mean-in-mysql/
• http://s.petrunia.net/blog/?p=24
• 避免使用INNER JOIN + ORDER BY进行文件排序