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一天，小明很著急地在通訊工具上說：這邊線上出現(xiàn)了個奇怪的問題，麻煩 DBA 大大鑒定下，執(zhí)行語句 select xx from table_name wheere xxx order by 字段A limit offset;，
表數(shù)據(jù)總共 48 條，分頁數(shù)量正常，但出現(xiàn)了結(jié)果混雜的情況，第一頁的數(shù)據(jù)出現(xiàn)在了第二頁上；如果 order by 字段B 就不會出現(xiàn)這個現(xiàn)象，怎么會這樣呢！
其實，這個問題很簡單，如果你有仔細(xì)閱讀官檔的話。~^_^~
我們先來看看官檔是怎么說的：
If multiple rows have identical values in the ORDER BY columns, the server is free to return those rows in any order, and may do so differently depending on the overall execution plan. In other words, the sort order of those rows is nondeterministic with respect to the nonordered columns.
One factor that affects the execution plan is LIMIT, so an ORDER BY query with and without LIMIT may return rows in different orders.
本次實驗使用社區(qū)版 MySQL 5.6.26（因為小明出現(xiàn)問題的環(huán)境就是這個版本O(∩_∩)O~），下面先創(chuàng)建實驗環(huán)境和初始化測試數(shù)據(jù)：
root@localhost [(none)]>select @@version; +------------+ | @@version | +------------+ | 5.6.26-log | +------------+1 row in set (0.00 sec) root@localhost [(none)]>show variables like "sql_mode"; +---------------+------------------------+ | Variable_name | Value | +---------------+------------------------+ | sql_mode | NO_ENGINE_SUBSTITUTION | +---------------+------------------------+1 row in set (0.00 sec) root@localhost [(none)]>create database glon_ho; Query OK, 1 row affected (0.04 sec) root@localhost [(none)]>use glon_ho Database changed root@localhost [glon_ho]>create table glon( -> id int not null auto_increment primary key, -> name varchar(20) not null, -> create_time datetime not null, -> age tinyint unsigned default 18 -> ); Query OK, 0 rows affected (0.01 sec) root@localhost [glon_ho]>INSERT INTO `glon` VALUES (1, 'Eason Chan', '2017-05-02 08:10:10', 19),(2, 'Glon Ho', '2017-05-03 12:10:10', 18),(3, '趙敏', '2017-05-03 14:10:10', 17),(4, 'Jacky Cheung', '2017-05-02 14:00:00', 22),(5, '周芷若', '2017-05-02 14:00:00', 16),(6, 'Andy Lau', '2017-05-02 14:00:00', 50),(7, '至尊寶', '2017-05-02 14:00:00', 20),(8, '劉三姐', '2017-05-02 14:00:00', 19); Query OK, 8 rows affected (0.01 sec) Records: 8 Duplicates: 0 Warnings: 0root@localhost [glon_ho]>select * from glon; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | +----+--------------+---------------------+------+8 rows in set (0.00 sec)
這里創(chuàng)建了一個 glon 表，字段有自增 id, 姓名 name, 年齡 age, 及用戶注冊時間 create_time。
接著來復(fù)現(xiàn)問題：
根據(jù)用戶注冊時間 create_time 來排序：
root@localhost [glon_ho]>select * from glon ORDER BY create_time limit 0, 4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | +----+--------------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY create_time limit 4, 4; +----+-----------+---------------------+------+ | id | name | create_time | age | +----+-----------+---------------------+------+ | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | +----+-----------+---------------------+------+4 rows in set (0.00 sec)
可以看到兩次查詢結(jié)果中都出現(xiàn)了 id 為 8 的劉三姐，從上面初始化數(shù)據(jù)來看，總共有 8 條數(shù)據(jù)，現(xiàn)在不但分頁出現(xiàn)重復(fù)數(shù)據(jù)，還丟了一條！
問題確實重現(xiàn)了，不過先不急，我們再來試多幾組其他的排序方式。
create_time 和 age 組合排序
root@localhost [glon_ho]>select * from glon ORDER BY create_time,age limit 0, 4; +----+------------+---------------------+------+ | id | name | create_time | age | +----+------------+---------------------+------+ | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | +----+------------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY create_time,age limit 4, 4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | +----+--------------+---------------------+------+4 rows in set (0.00 sec)
create_time 和 id 組合排序
root@localhost [glon_ho]>select * from glon ORDER BY create_time,id limit 0, 4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | +----+--------------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY create_time,id limit 4, 4; +----+-----------+---------------------+------+ | id | name | create_time | age | +----+-----------+---------------------+------+ | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | +----+-----------+---------------------+------+4 rows in set (0.00 sec)
主鍵 id 排序
root@localhost [glon_ho]>select * from glon ORDER BY id limit 0, 4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | +----+--------------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY id limit 4, 4; +----+-----------+---------------------+------+ | id | name | create_time | age | +----+-----------+---------------------+------+ | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | +----+-----------+---------------------+------+4 rows in set (0.00 sec)
看到，后面的幾組排序方式都沒有再出現(xiàn)問題了，結(jié)合官檔，我們知道 order by 排序的時候，如果排序字段中有多行相同的列值，則排序結(jié)果是不確定的。所以后面的幾組組合形式的排序或者是主鍵 id 的排序，因為唯一性高，所以排序是確定的，不會出現(xiàn)結(jié)果混亂的問題。
那是不是可以就此結(jié)束了呢，no way， 我們再來看下面的實驗，繼續(xù)鞏固一下：
根據(jù)年齡 age 來排序：
root@localhost [glon_ho]>select * from glon ORDER BY age limit 0, 4; +----+------------+---------------------+------+ | id | name | create_time | age | +----+------------+---------------------+------+ | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | +----+------------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY age limit 4, 4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | +----+--------------+---------------------+------+4 rows in set (0.00 sec)
咦，這個排序也只是根據(jù)一個字段 age 來排序，怎么就沒有出問題呢？不急，還有招：
root@localhost [glon_ho]>insert into glon values (9,'喬峰','2017-05-03 13:10:10',22),(10,'段譽','2017-05-03 15:10:10',19),(11,'郭靖','2017-05-03 17:10:10',20),(12,'黃蓉','2017-05-03 08:10:10',19); Query OK, 4 rows affected (0.01 sec) Records: 4 Duplicates: 0 Warnings: 0 root@localhost [glon_ho]>select * from glon; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 9 | 喬峰 | 2017-05-03 13:10:10 | 22 | | 10 | 段譽 | 2017-05-03 15:10:10 | 19 | | 11 | 郭靖 | 2017-05-03 17:10:10 | 20 | | 12 | 黃蓉 | 2017-05-03 08:10:10 | 19 | +----+--------------+---------------------+------+12 rows in set (0.00 sec)
我又給 glon 表新增了幾條數(shù)據(jù)，然后再來看看：
root@localhost [glon_ho]>select * from glon ORDER BY create_time limit 0, 4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | +----+--------------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY create_time limit 4, 4; +----+-----------+---------------------+------+ | id | name | create_time | age | +----+-----------+---------------------+------+ | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 12 | 黃蓉 | 2017-05-03 08:10:10 | 19 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | +----+-----------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY create_time limit 8, 4; +----+--------+---------------------+------+ | id | name | create_time | age | +----+--------+---------------------+------+ | 9 | 喬峰 | 2017-05-03 13:10:10 | 22 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | | 10 | 段譽 | 2017-05-03 15:10:10 | 19 | | 11 | 郭靖 | 2017-05-03 17:10:10 | 20 | +----+--------+---------------------+------+4 rows in set (0.00 sec)
根據(jù) create_time 排序，沒有問題了，再來：
root@localhost [glon_ho]>select * from glon ORDER BY age limit 0, 4; +----+------------+---------------------+------+ | id | name | create_time | age | +----+------------+---------------------+------+ | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | +----+------------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY age limit 4, 4; +----+-----------+---------------------+------+ | id | name | create_time | age | +----+-----------+---------------------+------+ | 12 | 黃蓉 | 2017-05-03 08:10:10 | 19 | | 10 | 段譽 | 2017-05-03 15:10:10 | 19 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | +----+-----------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY age limit 8, 4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | | 9 | 喬峰 | 2017-05-03 13:10:10 | 22 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | +----+--------------+---------------------+------+4 rows in set (0.00 sec)
可以看到根據(jù)年齡 age 排序，問題出現(xiàn)了。
然后在看看組合的排序：
root@localhost [glon_ho]>select * from glon ORDER BY create_time,id limit 0, 4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | +----+--------------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY create_time,id limit 4, 4; +----+-----------+---------------------+------+ | id | name | create_time | age | +----+-----------+---------------------+------+ | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 12 | 黃蓉 | 2017-05-03 08:10:10 | 19 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | +----+-----------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY create_time,id limit 8, 4; +----+--------+---------------------+------+ | id | name | create_time | age | +----+--------+---------------------+------+ | 9 | 喬峰 | 2017-05-03 13:10:10 | 22 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | | 10 | 段譽 | 2017-05-03 15:10:10 | 19 | | 11 | 郭靖 | 2017-05-03 17:10:10 | 20 | +----+--------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY create_time,age limit 0, 4; +----+------------+---------------------+------+ | id | name | create_time | age | +----+------------+---------------------+------+ | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | +----+------------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY create_time,age limit 4, 4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | | 12 | 黃蓉 | 2017-05-03 08:10:10 | 19 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | +----+--------------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY create_time,age limit 8, 4; +----+--------+---------------------+------+ | id | name | create_time | age | +----+--------+---------------------+------+ | 9 | 喬峰 | 2017-05-03 13:10:10 | 22 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | | 10 | 段譽 | 2017-05-03 15:10:10 | 19 | | 11 | 郭靖 | 2017-05-03 17:10:10 | 20 | +----+--------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY age,id limit 0, 4; +----+------------+---------------------+------+ | id | name | create_time | age | +----+------------+---------------------+------+ | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | +----+------------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY age,id limit 4, 4; +----+-----------+---------------------+------+ | id | name | create_time | age | +----+-----------+---------------------+------+ | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 10 | 段譽 | 2017-05-03 15:10:10 | 19 | | 12 | 黃蓉 | 2017-05-03 08:10:10 | 19 | | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | +----+-----------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon ORDER BY age,id limit 8, 4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 11 | 郭靖 | 2017-05-03 17:10:10 | 20 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 9 | 喬峰 | 2017-05-03 13:10:10 | 22 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | +----+--------------+---------------------+------+4 rows in set (0.00 sec)
既然排序不定，那么給排序字段加上索引會不會有用呢？
root@localhost [glon_ho]>alter table glon add index ix_age(age); Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0root@localhost [glon_ho]>show create table glon\G *************************** 1. row *************************** Table: glon Create Table: CREATE TABLE `glon` ( `id` int(11) NOT NULL AUTO_INCREMENT, `name` varchar(20) NOT NULL, `create_time` datetime NOT NULL, `age` tinyint(3) unsigned DEFAULT '18', PRIMARY KEY (`id`), KEY `ix_age` (`age`) ) ENGINE=InnoDB AUTO_INCREMENT=13 DEFAULT CHARSET=utf81 row in set (0.00 sec) root@localhost [glon_ho]>select * from glon order by age limit 0,4; +----+------------+---------------------+------+ | id | name | create_time | age | +----+------------+---------------------+------+ | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | +----+------------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon order by age limit 4,4; +----+-----------+---------------------+------+ | id | name | create_time | age | +----+-----------+---------------------+------+ | 12 | 黃蓉 | 2017-05-03 08:10:10 | 19 | | 10 | 段譽 | 2017-05-03 15:10:10 | 19 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | +----+-----------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from glon order by age limit 8,4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | | 9 | 喬峰 | 2017-05-03 13:10:10 | 22 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | +----+--------------+---------------------+------+4 rows in set (0.00 sec)
也可以不在 order by 后面加多一列增加唯一性，可以改寫成下面的形式：
root@localhost [glon_ho]>select * from (select distinct g.* from glon g order by age) t limit 0,4; +----+------------+---------------------+------+ | id | name | create_time | age | +----+------------+---------------------+------+ | 5 | 周芷若 | 2017-05-02 14:00:00 | 16 | | 3 | 趙敏 | 2017-05-03 14:10:10 | 17 | | 2 | Glon Ho | 2017-05-03 12:10:10 | 18 | | 1 | Eason Chan | 2017-05-02 08:10:10 | 19 | +----+------------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from (select distinct g.* from glon g order by age) t limit 4,4; +----+-----------+---------------------+------+ | id | name | create_time | age | +----+-----------+---------------------+------+ | 10 | 段譽 | 2017-05-03 15:10:10 | 19 | | 8 | 劉三姐 | 2017-05-02 14:00:00 | 19 | | 12 | 黃蓉 | 2017-05-03 08:10:10 | 19 | | 11 | 郭靖 | 2017-05-03 17:10:10 | 20 | +----+-----------+---------------------+------+4 rows in set (0.00 sec) root@localhost [glon_ho]>select * from (select distinct g.* from glon g order by age) t limit 8,4; +----+--------------+---------------------+------+ | id | name | create_time | age | +----+--------------+---------------------+------+ | 7 | 至尊寶 | 2017-05-02 14:00:00 | 20 | | 9 | 喬峰 | 2017-05-03 13:10:10 | 22 | | 4 | Jacky Cheung | 2017-05-02 14:00:00 | 22 | | 6 | Andy Lau | 2017-05-02 14:00:00 | 50 | +----+--------------+---------------------+------+4 rows in set (0.00 sec)
總之，如果發(fā)生了，最簡單的方法就是在排序列（如 create time）上加索引，然后在 order by 上明示 primary key，這個問題就非常圓滿的解決了。
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