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MySQL與MariaDB功能對比 ---子查詢中order by的處理
MariaDB 聲稱是 MySQL的drop-in replacement，也就是說可以直接替換程序就可以使用。那麼，這兩者的功能是不是100%完全相同呢 ？ 很多用戶都非常關心這個問題。其實還是有蠻多細微的差別的，後面我會發文若干篇講一講這些差別。
本文講一下兩者在子查詢中order by的處理方面的差別。
MySQL與MariaDB對嵌套的查詢語句當中的order by的處理方法不同。MySQL會忠實執行內層查詢的排序子句，但是MariaDB會將這個order by去掉，理論依據就是關係理論 --- 一個表是行的集合，因此沒有順序要求。由於嵌套的查詢語句是外層查詢的數據表，因此可以以任何順序提供給外層查詢。如果在內層查詢語句中不僅有order by,還有limit子句，那麼這時這個order by是不會被MariaDB忽略的，因爲此時的行的順序會決定哪些行會返回給外層查詢。
MySQL的查詢執行過程
mysql> create table t1 (a int, b int);
Query OK, 0 rows affected (0.02 sec)
mysql> insert into t1 values(1, 3),(3, 2),(2, 4);
Query OK, 3 rows affected (0.01 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select*from (select * from t1 order by a) as t2;
+------+------+
| a | b |
+------+------+
| 1 | 3 |
| 2 | 4 |
| 3 | 2 |
+------+------+
3 rows in set (0.00 sec)
mysql> select*from (select * from t1 order by b) as t2;
+------+------+
| a | b |
+------+------+
| 3 | 2 |
| 1 | 3 |
| 2 | 4 |
+------+------+
3 rows in set (0.00 sec)
MariaDB的查詢執行過程
MariaDB [test]> create table t1 (a int, b int);
Query OK, 0 rows affected (0.06 sec)
MariaDB [test]> insert into t1 values(1, 3),(3, 2),(2, 4);
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0
MariaDB [test]> select*from (select * from t1 order by a) as t2;
+------+------+
| a | b |
+------+------+
| 1 | 3 |
| 3 | 2 |
| 2 | 4 |
+------+------+
3 rows in set (0.00 sec)
MariaDB [test]> select*from (select * from t1 order by b) as t2;
+------+------+
| a | b |
+------+------+
| 1 | 3 |
| 3 | 2 |
| 2 | 4 |
+------+------+
3 rows in set (0.00 sec)
MariaDB [test]> select*from (select * from t1 order by a limit 10000000) as t2;
+------+------+
| a | b |
+------+------+
| 1 | 3 |
| 2 | 4 |
| 3 | 2 |
+------+------+
3 rows in set (0.00 sec)
MariaDB [test]> select*from (select * from t1 order by b limit 10000000) as t2;
+------+------+
| a | b |
+------+------+
| 3 | 2 |
| 1 | 3 |
| 2 | 4 |
+------+------+
3 rows in set (0.00 sec)
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