指標上主鍵和外鍵

Question

我有一個GUI工具創建一個數據庫，我發現這似乎是不一致的使用KEY（又名INDEX）定義：

CREATE TABLE `foo_bar` (
    `foo_id` int(10) unsigned NOT NULL, 
    `bar_id` int(10) unsigned NOT NULL, 
    PRIMARY KEY (`foo_id`, `bar_id`), 
    KEY `foo_bar_fk2` (`bar_id`), -- <== ??? 
    CONSTRAINT `foo_bar_fk1` FOREIGN KEY (`foo_id`) REFERENCES `foo` (`foo_id`) ON DELETE CASCADE ON UPDATE CASCADE, 
    CONSTRAINT `foo_bar_fk2` FOREIGN KEY (`bar_id`) REFERENCES `bar` (`bar_id`) ON DELETE CASCADE ON UPDATE CASCADE 
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_spanish_ci COMMENT='Links between Foo and Bar'; 

我有以下幾個問題約指標：

1. 是否有必要明確定義索引主鍵和外鍵？
2. 如果不是，你真的會得到兩個指數（和更少的性能）？
3. InnoDB和MyISAM（外鍵除外）有什麼不同？

Answer 1

我一直在做一些實驗上你告訴我，我還以爲我會分享它的答案。

首先，我創建一些測試表：

CREATE TABLE foo (
    foo_id int(10) unsigned NOT NULL, 
    PRIMARY KEY (foo_id) 
) 
ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_spanish_ci; 
CREATE TABLE bar (
    bar_id int(10) unsigned NOT NULL, 
    PRIMARY KEY (bar_id) 
) 
ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_spanish_ci; 


CREATE TABLE foo_bar (
    foo_id int(10) unsigned NOT NULL, 
    bar_id int(10) unsigned NOT NULL 
) 
ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_spanish_ci; 

到目前爲止，還沒有索引存在：

mysql> SHOW INDEXES FROM foo_bar; 
Empty set (0.00 sec) 

添加主鍵，生成索引：

mysql> ALTER TABLE foo_bar 
    -> ADD PRIMARY KEY (`foo_id`, `bar_id`); 
Query OK, 0 rows affected (0.70 sec) 
Records: 0 Duplicates: 0 Warnings: 0 

mysql> SHOW INDEXES FROM foo_bar; 
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ 
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | 
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ 
| foo_bar |   0 | PRIMARY |   1 | foo_id  | A   |   0 |  NULL | NULL |  | BTREE  |   | 
| foo_bar |   0 | PRIMARY |   2 | bar_id  | A   |   0 |  NULL | NULL |  | BTREE  |   | 
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ 
2 rows in set (0.02 sec) 

如果我添加一個外鍵在foo_id它重新使用主鍵索引，因爲該列是索引中的第一個：

mysql> ALTER TABLE foo_bar 
    -> ADD CONSTRAINT `foo_bar_fk1` FOREIGN KEY (`foo_id`) REFERENCES `foo` (`foo_id`) ON DELETE CASCADE ON UPDATE CASCADE; 
Query OK, 0 rows affected (0.27 sec) 
Records: 0 Duplicates: 0 Warnings: 0 

mysql> SHOW INDEXES FROM foo_bar; 
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ 
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | 
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ 
| foo_bar |   0 | PRIMARY |   1 | foo_id  | A   |   0 |  NULL | NULL |  | BTREE  |   | 
| foo_bar |   0 | PRIMARY |   2 | bar_id  | A   |   0 |  NULL | NULL |  | BTREE  |   | 
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ 
2 rows in set (0.00 sec) 

如果我添加上bar_id外鍵，它會創建一個索引，因爲沒有現成的指數可以重複使用：

mysql> ALTER TABLE foo_bar 
    -> ADD CONSTRAINT `foo_bar_fk2` FOREIGN KEY (`bar_id`) REFERENCES `bar` (`bar_id`) ON DELETE CASCADE ON UPDATE CASCADE; 
Query OK, 0 rows affected (0.25 sec) 
Records: 0 Duplicates: 0 Warnings: 0 

mysql> SHOW INDEXES FROM foo_bar; 
+---------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ 
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | 
+---------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ 
| foo_bar |   0 | PRIMARY  |   1 | foo_id  | A   |   0 |  NULL | NULL |  | BTREE  |   | 
| foo_bar |   0 | PRIMARY  |   2 | bar_id  | A   |   0 |  NULL | NULL |  | BTREE  |   | 
| foo_bar |   1 | foo_bar_fk2 |   1 | bar_id  | A   |   0 |  NULL | NULL |  | BTREE  |   | 
+---------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ 
3 rows in set (0.02 sec) 

我們的一個外鍵的使用主鍵索引。這意味着我們不能刪除這樣的索引！

mysql> ALTER TABLE foo_bar 
    -> DROP PRIMARY KEY; 
ERROR 1025 (HY000): Error on rename of '.\test\#sql-568_c7d' to '.\test\foo_bar' (errno: 150) 

除非我們建立外鍵索引，或者我們刪除密鑰本身：

mysql> ALTER TABLE foo_bar 
    -> DROP FOREIGN KEY `foo_bar_fk1`; 
Query OK, 0 rows affected (0.19 sec) 
Records: 0 Duplicates: 0 Warnings: 0 

mysql> ALTER TABLE foo_bar 
    -> DROP PRIMARY KEY; 
Query OK, 0 rows affected (0.23 sec) 
Records: 0 Duplicates: 0 Warnings: 0 

---

的結論是MySQL的功能會在需要時，他們會自動創建索引（但只有在嚴格需要的情況下才可以）。

Answer 2

外鍵索引用於創建referential constraints。

它們允許您定義級聯刪除子句，以便從父表中刪除行將從子表中刪除行。它也可以用來確保如果你試圖在子表中創建一行，它可以鏈接到父母（即Childrow.ParentId有效）

編輯：啊抱歉我誤解了。 InnoDB自動爲外鍵創建索引。請參閱does-mysql-index-foreign-key-columns-automatically

Answer 3

1）主鍵在mysql中自動編入索引。 2）參見上文，不必做 3）MyISAM不支持約束。 （我想你是暗示的，所以這對其他人來說更是如此）。當您嘗試在MyISAM中使用外鍵時，某些圖形工具（如MySQL Workbench）不會引發警報，但顯然不會創建它們。

順便說一句：你用什麼GUI（在哪個操作系統上）？

很好看的：d ata warehousing whitepaper（包含MyISAM和InnoDB表的比較）