MySQL解惑——GROUP BY隱式排序

MySQL中GROUP BY隱式排序是什麼概念呢？ 主要是其它RDBMS沒有這樣的概念，若是沒有認真瞭解過概念，對這個概念會感受有點困惑，咱們先來看看官方文檔的介紹：

 

官方文檔MySQL 5.7 Reference Manual中的「.2.1.14 ORDER BY Optimization」章節有以下介紹：

 

GROUP BY implicitly sorts by default (that is, in the absence of ASC or DESC designators for GROUP BY columns). However, relying on implicit GROUP BY sorting (that is, sorting in the absence of ASC or DESC designators) or explicit sorting for GROUP BY (that is, by using explicit ASC or DESC designators for GROUP BY columns) is deprecated. To produce a given sort order, provide an ORDER BY clause.

 

默認狀況下GROUP BY隱式排序(即，缺乏GROUP BY列的ASC或DESC指示符)。可是，不推薦依賴於隱式GROUP BY排序(即，在沒有ASC或DESC指示符的狀況下排序)或GROUP BY的顯式排序(即，經過對GROUP BY列使用顯式ASC或DESC指示符)。要生成給定的排序 ORDER，請提供ORDER BY子句。

 

從MySQL 8.0開始，GROUP BY字段再也不支持隱式排序. 官方文檔MySQL 8.0 Reference Manual中「8.2.1.16 ORDER BY Optimization」章節有以下介紹：

 

Previously (MySQL 5.7 and lower), GROUP BY sorted implicitly under certain conditions. In MySQL 8.0, that no longer occurs, so specifying ORDER BY NULL at the end to suppress implicit sorting (as was done previously) is no longer necessary. However, query results may differ from previous MySQL versions. To produce a given sort order, provide an ORDER BY clause.

 

那麼來看看MySQL的GROUP BY隱式排序（GROUP BY sorted implicitly）吧。咱們用「Removal of implicit and explicit sorting for GROUP BY」這篇博客中的例子。

 

下面實驗環境爲MySQL 5.6.41（）

 

mysql> select version() from dual;

+------------+

| version()  |

+------------+

| 5.6.41-log |

+------------+

1 row in set (0.00 sec)

mysql> CREATE TABLE t (id INTEGER,  cnt INTEGER);

Query OK, 0 rows affected (0.04 sec)

mysql> INSERT INTO t VALUES (4,1),(3,2),(1,4),(2,2),(1,1),(1,5),(2,6),(2,1),(1,3),(3,4),(4,5),(3,6);

Query OK, 12 rows affected (0.00 sec)

Records: 12  Duplicates: 0  Warnings: 0

 

#MySQL在這裏隱式地對GROUP BY的結果進行排序（即在缺乏GROUP BY列的ASC或DESC指示符的狀況下）。

mysql> SELECT id, SUM(cnt) FROM t GROUP BY id; --GROUP BY隱式排序

+------+----------+

| id   | SUM(cnt) |

+------+----------+

|    1 |       13 |

|    2 |        9 |

|    3 |       12 |

|    4 |        6 |

+------+----------+

4 rows in set (0.00 sec)

 

 

MySQL還支持使用GROUP BY進行顯式排序（即經過對GROUP BY列使用顯式ASC或DESC指示符）

 

mysql> SELECT id, SUM(cnt) FROM t GROUP BY id DESC;  --GROUP BY顯式排序

+------+----------+

| id   | SUM(cnt) |

+------+----------+

|    4 |        6 |

|    3 |       12 |

|    2 |        9 |

|    1 |       13 |

+------+----------+

4 rows in set (0.00 sec)

 

從MySQL8.0開始，MySQL再也不支持GROUP BY的隱式或顯示排序，以下所示：

 

 

#下面實驗環境爲MySQL 8.0.18

mysql> select version();

+-----------+

| version() |

+-----------+

| 8.0.18    |

+-----------+

1 row in set (0.00 sec)

mysql> CREATE TABLE t (id INTEGER,  cnt INTEGER);

Query OK, 0 rows affected (0.39 sec)

mysql> INSERT INTO t VALUES (4,1),(3,2),(1,4),(2,2),(1,1),(1,5),(2,6),(2,1),(1,3),(3,4),(4,5),(3,6);

Query OK, 12 rows affected (0.10 sec)

Records: 12  Duplicates: 0  Warnings: 0

mysql> SELECT id, SUM(cnt) FROM t GROUP BY id;

+------+----------+

| id   | SUM(cnt) |

+------+----------+

|    4 |        6 |

|    3 |       12 |

|    1 |       13 |

|    2 |        9 |

+------+----------+

4 rows in set (0.00 sec)

mysql> SELECT id, SUM(cnt) FROM t GROUP BY id DESC;

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'DESC' at line 1

 

 

如上所示，GROUP BY隱式排序不支持了，在MySQL 8.0中，上面測試例子是無序的。GROUP BY顯示排序則直接報錯。因此若是有數據庫從MySQL 5.7或以前的版本，遷移升級到MySQL 8的話，就須要特別留意這個問題了。正確的作法應該是GROUP BY .. ORDER BY 這種操做。以下所示：

 

mysql>  SELECT id, SUM(cnt) FROM t GROUP BY id

    ->  ORDER BY id;

+------+----------+

| id   | SUM(cnt) |

+------+----------+

|    1 |       13 |

|    2 |        9 |

|    3 |       12 |

|    4 |        6 |

+------+----------+

4 rows in set (0.00 sec)

mysql> 

 

 

MySQL 8.0.13中刪除了GROUP BY的顯式排序。至於爲何MySQL 8.0再也不支持GROUP BY的隱式排序和顯示排序，這篇文章「Removal of implicit and explicit sorting for GROUP BY」已經有詳細的介紹，這裏不多此一舉了。

 

 

 

參考資料：

 

https://mysqlserverteam.com/removal-of-implicit-and-explicit-sorting-for-group-by/