Mysql常見索引介紹

　　索引是一種特殊的文件，包含了對數據表中所有記錄的引用指針。InnoDB引擎的數據庫，其上的索引是表空間的一個組成部分。

(1).索引的優缺點

　　優點：加快搜索速度，減少查詢時間

　　缺點：索引是以文件的形式存儲，如果索引過多，會占用磁盤較大的空間。而且影響insert、update、delete的執行時間。

　　　　　索引中的數據必須與數據表中的人數據同步，如果索引過多，當表中數據更新，索引也要同步更新，這就降低了效率。

(2).普通索引

　　最基本的索引，不具備唯一性，僅加快查詢速度。

1）創建普通索引

　　create table [表名] ([字段名] [字段類型] [字段約束],...,[ index | key ] [索引名稱]([字段名稱]));

mysql> create table com_index1(id int,name varchar(20),key (name));
Query OK, 0 rows affected (0.02 sec)

mysql> desc com_index1;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  | MUL | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> create table com_index2(id int,name varchar(20),index com_name(name));    
Query OK, 0 rows affected (0.11 sec)

mysql> desc com_index2;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  | MUL | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.03 sec)

mysql> create table com_index(id int,name varchar(20),index (name));         
Query OK, 0 rows affected (0.02 sec)

mysql> desc com_index;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  | MUL | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

　　注意1：索引文件是一表一個，所以在沒有外鍵約束的情況下，只要表內沒有重名索引即可。

　　注意2：在Key列值為MUL的一般是普通索引，可以使用show index from [表名]來確認索引類型，里面有索引類型列如果為BTREE那么就是普通索引。

　　注意3：如果沒有指定索引名，那么默認索引名與字段名一致。

2）添加普通索引

　　alter table [表名] add [ index | key ] [索引名稱]([字段名]);

mysql> alter table com_index1 add key (id);    
Query OK, 0 rows affected (0.11 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc com_index1;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  | MUL | NULL    |       |
| name  | varchar(20) | YES  | MUL | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> alter table com_index2 add index com_id(id); 
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc com_index2;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  | MUL | NULL    |       |
| name  | varchar(20) | YES  | MUL | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> alter table com_index add index (id);       
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc com_index;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  | MUL | NULL    |       |
| name  | varchar(20) | YES  | MUL | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

3）刪除普通索引

　　alter table [表名] drop [ key | index ] [索引名];

mysql> alter table com_index1 drop key name;
Query OK, 0 rows affected (0.20 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc com_index1;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  | MUL | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> alter table com_index1 drop index id;
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> desc com_index1;             
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.01 sec)

(3).唯一索引

　　設置唯一索引的列中所有值都只能出現一次，允許NULL。唯一索引一般只在創建數據表以及數據表沒有數據之前創建或添加，如果已經存在數據，必須保證添加的列所有值具有唯一性。

1）創建唯一索引

　　create table [表名] ([字段名] [字段類型] [字段約束],...,unique [索引名稱]([字段名稱]));

mysql> create table uni_index(id int,name varchar(20),unique (id));
Query OK, 0 rows affected (0.12 sec)

mysql> desc uni_index;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  | UNI | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

　　注意：在Key列值為UNI的即是唯一索引。

2）添加唯一索引（盡量不要用，用也盡量在存放數據之前）

　　alter table [表名] add unique [索引名]([字段名]);

mysql> alter table uni_index add unique (name);
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc uni_index;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  | UNI | NULL    |       |
| name  | varchar(20) | YES  | UNI | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

3）刪除唯一索引

　　alter table [表名] drop key [索引名];

mysql> alter table uni_index drop key name;      
Query OK, 0 rows affected (0.13 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc uni_index;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  | UNI | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

(4).主鍵索引

　　查詢數據庫按主鍵查詢是最快的，每個數據表只允許存在一個主鍵，主鍵必須唯一，不允許為NULL。主鍵和唯一索引一樣，一般只在創建數據表以及數據表沒有數據之前創建或添加，如果已經存在數據，必須保證添加的列所有值具有唯一性。

1）創建主鍵

　　create table [表名] ([字段名] [字段類型] [字段約束],...,primary key [索引名稱]([字段名稱]));

　　create table [表名] ([字段名] [字段類型] [字段約束] [ primary key | key ],[字段名] [字段類型] [字段約束]...);

mysql> create table pri_index1 (id int,primary key (id));
Query OK, 0 rows affected (0.01 sec)

mysql> desc pri_index1;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   | PRI | NULL    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.01 sec)

mysql> create table pri_index2 (id int key);         
Query OK, 0 rows affected (0.01 sec)

mysql> desc pri_index2;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   | PRI | NULL    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)

mysql> create table pri_index3 (id int primary key);
Query OK, 0 rows affected (0.02 sec)

mysql> desc pri_index3;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   | PRI | NULL    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)

　　注意：在Key列值為PRI的即是主鍵索引。

2）添加主鍵（盡量不要用，用也盡量在存放數據之前）

　　alter table [表名] add primary key [索引名]([字段名]);

mysql> create table pri_index4 (id int);
Query OK, 0 rows affected (0.02 sec)

mysql> desc pri_index4;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)

mysql> alter table pri_index4 add primary key (id);
Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc pri_index4;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   | PRI | NULL    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)

3）刪除主鍵

　　alter table [表名] drop primary key;

mysql> alter table pri_index4 drop primary key;
Query OK, 0 rows affected (0.13 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc pri_index4;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.01 sec)

(5).復合索引

　　一個索引可以包含一個或一個以上的列，當索引包含兩個及以上的列時，此時的索引被稱之為復合索引。基本創建、添加、刪除方式沒有改變，只是字段名部分可以為一個以上，之間由逗號隔開。

　　創建

mysql> create table comp_index(id int,name varchar(20), primary key (id,name));
Query OK, 0 rows affected (0.01 sec)

mysql> desc comp_index;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | NO   | PRI | NULL    |       |
| name  | varchar(20) | NO   | PRI | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

　　添加

mysql> create table comp_index1(id int,name varchar(20));
Query OK, 0 rows affected (0.25 sec)

mysql> alter table comp_index1 add primary key (id,name);
Query OK, 0 rows affected (0.17 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc comp_index1;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | NO   | PRI | NULL    |       |
| name  | varchar(20) | NO   | PRI | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

　　刪除

mysql> alter table comp_index drop primary key;
Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc comp_index;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | NO   |     | NULL    |       |
| name  | varchar(20) | NO   |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

(6).全文索引

　　全文索引也稱全文檢索，是目前搜索引擎使用的一種關鍵技術。它能夠利用多種算法智能分析出文本文字中關鍵詞的頻率和重要性，然后按照一定的算法規則智能篩選出想要的結果。3.2開始支持全文索引，但無法正確支持中文；5.7.6開始內置ngram全文檢索插件，用來支持中文。

　　舊版的全文索引只能用在MyISAM數據庫引擎的表上，但5.6.24上InnoDB也加入了全文索引。不夠只支持char、varchar和text的字段類型。

1）創建全文索引

　　create table [表名] ([字段名] [字段類型] [字段約束],...,fulltext key [索引名稱]([字段名稱]));

mysql> create table ful_index(id int,name varchar(20),age varchar(20),fulltext key (name));
Query OK, 0 rows affected (0.25 sec)

mysql> desc ful_index;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  | MUL | NULL    |       |
| age   | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

mysql> show index from ful_index;
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table     | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| ful_index |          1 | name     |            1 | name        | NULL      |           0 |     NULL | NULL   | YES  | FULLTEXT   |         |               |
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)

　　注意：這里Key的值雖然是MUL，但使用show index from [表名]可以看到實際索引類型是FULLTEXT。

2）添加全文索引

　　alter table [表名] add fulltext key [索引名]([字段名]);

mysql> alter table ful_index add fulltext key (age);
Query OK, 0 rows affected (0.26 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show index from ful_index;
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table     | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| ful_index |          1 | name     |            1 | name        | NULL      |           0 |     NULL | NULL   | YES  | FULLTEXT   |         |               |
| ful_index |          1 | age      |            1 | age         | NULL      |           0 |     NULL | NULL   | YES  | FULLTEXT   |         |               |
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.01 sec)

3）刪除全文索引

　　alter table [表名] drop key [索引名];

mysql> alter table ful_index drop key age;
Query OK, 0 rows affected (0.05 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show index from ful_index;
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table     | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| ful_index |          1 | name     |            1 | name        | NULL      |           0 |     NULL | NULL   | YES  | FULLTEXT   |         |               |
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)

(7).索引創建原則

　　索引並不是越多越好：1.數據量不大時不需要創建索引；

　　　　　　　　　　　　2.列中值變化種類不多不需要創建索引；

　　　　　　　　　　　　3.經常排序（order by）和分組（group by）的列需要建立索引；

　　　　　　　　　　　　4.列中值需要具有唯一性，可以使用唯一索引或主鍵索引，但每個表中只能存在一個主鍵索引，並且不能為NULL。