MySQL:为什么使用索引时仍然是 'using filesort'？

Question

mysql-5.6.24-win32.1432006610

我有两个用户消息表。

TMessageBody (id, body) 存储消息正文。

TMessage (id, uid, folderId, msgBodyId, subject)将用户的消息存储在收件箱、发件箱等文件夹中。

---

创建表 SQL:

create table TMessageBody (
    id          int unsigned    not null    primary key auto_increment,
    body        text            not null
);


create table TMessage (
    id          int unsigned    not null    primary key auto_increment,
    uid         int unsigned    not null,
    folderId    int unsigned    not null,
    msgBodyId   int unsigned    not null,
    subject     varchar(256)    not null
);

---

部分测试数据:

insert into TMessageBody 
    (body) values 
    ('Here is body 1')
    ,('Here is body 2')
    ,('Here is body 3')
    ,('Here is body 4')
    ,('Here is body 5')
    ,('Here is body 6')
    ,('Here is body 7')
    ,('Here is body 8')
    ,('Here is body 9')
    ;

insert into TMessage
    (uid, folderId, msgBodyId, subject) values
    (1, 999, 1, 'Hello jack')
    , (1, 999, 2, 'Jack, how are you')
    , (1, 888, 3, 'Good morning jack')
    , (2, 888, 4, 'I love you, rose')
    , (2, 999, 5, 'I love you, rose')
    , (3, 888, 6, 'Peter, please call back')
    , (3, 999, 7, 'What are you doing, Peter')
    , (3, 999, 8, 'Happy birthday, perter')
    , (4, 999, 9, 'Let me know if you are ready')
    ;

---

索引:

create index Idx_MsgBodyId      on TMessage(msgBodyId);
create index Idx_Uid_FolderId   on TMessage(uid, folderId);

---

1.FileSort 显示 folderId 不在 WHERE 子句中

以下查询获取给定用户 ID 的所有消息，包括消息正文:

SET @uid=3;
SET @folderId=999;

EXPLAIN
SELECT *
FROM        TMessage
INNER JOIN  TMessageBody 
ON          TMessage.msgBodyId=TMessageBody.id
WHERE       TMessage.uid=@uid
                #AND TMessage.folderId=@folderId
ORDER BY    TMessage.id DESC
;

解释结果是:

mysql> EXPLAIN
-> SELECT *
->  FROM            TMessage
->  INNER JOIN      TMessageBody
->  ON              TMessage.msgBodyId=TMessageBody.id
->  WHERE           TMessage.uid=@uid
->                      #AND TMessage.folderId=@folderId
->  ORDER BY        TMessage.id DESC
->  ;
+----+-------------+--------------+--------+--------------------------------+------------------+---------+-------------------------+------+-----------------------------+
| id | select_type | table        | type   | possible_keys                  | key              | key_len | ref                     | rows | Extra                       |
+----+-------------+--------------+--------+--------------------------------+------------------+---------+-------------------------+------+-----------------------------+
|  1 | SIMPLE      | TMessage     | ref    | Idx_MsgBodyId,Idx_Uid_FolderId | Idx_Uid_FolderId | 4       | const                   |    3 | Using where; Using filesort |
|  1 | SIMPLE      | TMessageBody | eq_ref | PRIMARY                        | PRIMARY          | 4       | test.TMessage.msgBodyId |    1 | NULL                        |
+----+-------------+--------------+--------+--------------------------------+------------------+---------+-------------------------+------+-----------------------------+
2 rows in set (0.00 sec)

2.当folderId在WHERE子句中时FileSort消失

除了 WHERE 子句外，查询与上面的查询相同:

SET @uid=3;
SET @folderId=999;

EXPLAIN
SELECT *
FROM        TMessage
INNER JOIN  TMessageBody 
ON          TMessage.msgBodyId=TMessageBody.id
WHERE       TMessage.uid=@uid
            　　AND TMessage.folderId=@folderId
ORDER BY    TMessage.id DESC
;

解释结果是:

mysql> EXPLAIN
-> SELECT *
->  FROM            TMessage
->  INNER JOIN      TMessageBody
->  ON              TMessage.msgBodyId=TMessageBody.id
->  WHERE           TMessage.uid=@uid
->                            AND TMessage.folderId=@folderId
->  ORDER BY        TMessage.id DESC
->  ;
+----+-------------+--------------+--------+--------------------------------+------------------+---------+-------------------------+------+-------------+
| id | select_type | table        | type   | possible_keys                  | key              | key_len | ref                     | rows | Extra       |
+----+-------------+--------------+--------+--------------------------------+------------------+---------+-------------------------+------+-------------+
|  1 | SIMPLE      | TMessage     | ref    | Idx_MsgBodyId,Idx_Uid_FolderId | Idx_Uid_FolderId | 8       | const,const             |    2 | Using where |
|  1 | SIMPLE      | TMessageBody | eq_ref | PRIMARY                        | PRIMARY          | 4       | test.TMessage.msgBodyId |    1 | NULL        |
+----+-------------+--------------+--------+--------------------------------+------------------+---------+-------------------------+------+-------------+
2 rows in set (0.00 sec)

---

问题:

这两个查询的区别在于 folderId 列是否在 WHERE 子句中。根据 EXPLAIN 结果，两个查询都使用了 Idx_Uid_FolderId 索引。我想知道为什么一个显示 FileSort 而另一个不显示。

---

更新

尝试在第一个查询中使用 ORDER BY TMessage.folderId, TMessage.id DESC。但是 Using filesort 仍然存在于 EXPLAIN 结果中。

mysql> EXPLAIN
-> SELECT *
->  FROM        TMessage
->  INNER JOIN  TMessageBody
->  ON          TMessage.msgBodyId=TMessageBody.id
->  WHERE       TMessage.uid=@uid
->                   #AND TMessage.folderId=@folderId
->  ORDER BY    TMessage.folderId, TMessage.id DESC
->  ;
+----+-------------+--------------+--------+--------------------------------+------------------+---------+-------------------------+------+-----------------------------+
| id | select_type | table        | type   | possible_keys                  | key              | key_len | ref                     | rows | Extra                       |
+----+-------------+--------------+--------+--------------------------------+------------------+---------+-------------------------+------+-----------------------------+
|  1 | SIMPLE      | TMessage     | ref    | Idx_MsgBodyId,Idx_Uid_FolderId | Idx_Uid_FolderId | 4       | const                   |    3 | Using where; Using filesort |
|  1 | SIMPLE      | TMessageBody | eq_ref | PRIMARY                        | PRIMARY          | 4       | test.TMessage.msgBodyId |    1 | NULL                        |
+----+-------------+--------------+--------+--------------------------------+------------------+---------+-------------------------+------+-----------------------------+
2 rows in set (0.06 sec)

Answer 1

将索引视为连接值。

在这种情况下，你的索引是

uid | folderId | id

id 排在最后，因为它是二级索引中对主键的引用。

在第一种情况下，您按 uid 过滤，然后按 id 排序。问题是 MySQL 不能假设 id 是按时排序的，因为索引实际上是在按 id 过滤之前按 folderId 排序的。

例如:

uid | folderId | id
  1 |        1 |  1
  1 |        2 |  2
  2 |        1 |  3

因此该索引不能用于排序，因为索引的排序不同于 order by 子句。

现在，我的问题是:您为什么要避免文件排序？除非您遇到性能问题，否则使用文件排序非常好。尽管有名称，排序是在内存中完成的，除非另有说明(使用临时)。