Mysql常见索引介绍

　　索引是一种特殊的文件，包含了对数据表中所有记录的引用指针。InnoDB引擎的数据库，其上的索引是表空间的一个组成部分。

(1).索引的优缺点

　　优点：加快搜索速度，减少查询时间

　　缺点：索引是以文件的形式存储，如果索引过多，会占用磁盘较大的空间。而且影响insert、update、delete的执行时间。

　　　　　索引中的数据必须与数据表中的人数据同步，如果索引过多，当表中数据更新，索引也要同步更新，这就降低了效率。

(2).普通索引

　　最基本的索引，不具备唯一性，仅加快查询速度。

1）创建普通索引

　　create table [表名] ([字段名] [字段类型] [字段约束],...,[ index | key ] [索引名称]([字段名称]));

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：索引文件是一表一个，所以在没有外键约束的情况下，只要表内没有重名索引即可。

　　注意2：在Key列值为MUL的一般是普通索引，可以使用show index from [表名]来确认索引类型，里面有索引类型列如果为BTREE那么就是普通索引。

　　注意3：如果没有指定索引名，那么默认索引名与字段名一致。

2）添加普通索引

　　alter table [表名] add [ index | key ] [索引名称]([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）删除普通索引

　　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

　　设置唯一索引的列中所有值都只能出现一次，允许NULL。唯一索引一般只在创建数据表以及数据表没有数据之前创建或添加，如果已经存在数据，必须保证添加的列所有值具有唯一性。

1）创建唯一索引

　　create table [表名] ([字段名] [字段类型] [字段约束],...,unique [索引名称]([字段名称]));

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列值为UNI的即是唯一索引。

2）添加唯一索引（尽量不要用，用也尽量在存放数据之前）

　　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）删除唯一索引

　　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).主键索引

　　查询数据库按主键查询是最快的，每个数据表只允许存在一个主键，主键必须唯一，不允许为NULL。主键和唯一索引一样，一般只在创建数据表以及数据表没有数据之前创建或添加，如果已经存在数据，必须保证添加的列所有值具有唯一性。

1）创建主键

　　create table [表名] ([字段名] [字段类型] [字段约束],...,primary key [索引名称]([字段名称]));

　　create table [表名] ([字段名] [字段类型] [字段约束] [ primary key | key ],[字段名] [字段类型] [字段约束]...);

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列值为PRI的即是主键索引。

2）添加主键（尽量不要用，用也尽量在存放数据之前）

　　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）删除主键

　　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).复合索引

　　一个索引可以包含一个或一个以上的列，当索引包含两个及以上的列时，此时的索引被称之为复合索引。基本创建、添加、删除方式没有改变，只是字段名部分可以为一个以上，之间由逗号隔开。

　　创建

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)

　　删除

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

　　全文索引也称全文检索，是目前搜索引擎使用的一种关键技术。它能够利用多种算法智能分析出文本文字中关键词的频率和重要性，然后按照一定的算法规则智能筛选出想要的结果。3.2开始支持全文索引，但无法正确支持中文；5.7.6开始内置ngram全文检索插件，用来支持中文。

　　旧版的全文索引只能用在MyISAM数据库引擎的表上，但5.6.24上InnoDB也加入了全文索引。不够只支持char、varchar和text的字段类型。

1）创建全文索引

　　create table [表名] ([字段名] [字段类型] [字段约束],...,fulltext key [索引名称]([字段名称]));

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：这里Key的值虽然是MUL，但使用show index from [表名]可以看到实际索引类型是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）删除全文索引

　　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).索引创建原则

　　索引并不是越多越好：1.数据量不大时不需要创建索引；

　　　　　　　　　　　　2.列中值变化种类不多不需要创建索引；

　　　　　　　　　　　　3.经常排序（order by）和分组（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键索引，但每个表中只能存在一个主键索引，并且不能为NULL。