1.什么是索引
索引是一种特殊的文件(InnoDB数据表上的索引是表空间的一个组成部分)，它们包含着对数据表里所有记录的引用指针。
更通俗的说，数据库索引好比是一本书前面的目录，能加快数据库的查询速度。
2.索引的类型
1、普通索引 2、唯一性索引 3、主键索引（主索引） 4、复合索引
2.1. 普通索引
最基本的索引，不具备唯一性，就是加快查询速度
2.1.1 创建普通索引
方法一：创建表时添加索引
语法：
create table 表名（ 列定义 index 索引名称 （字段） index 索引名称 （字段） ）
注：可以使用key关键字，也可以使用index关键字。索引名称，可以加也可以不加，不加使用字段名作为索引名。
MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+8 rows in set (0.00 sec)
MariaDB [book]> create table demo(id int(4),name varchar(20),pwd varchar(20),index(pwd));
Query OK, 0 rows affected (0.08 sec)
MariaDB [book]> create table demo1(id int(4),name varchar(20),pwd varchar(20),key(pwd));
Query OK, 0 rows affected (0.01 sec)
MariaDB [book]> create table demo2(id int(4),name varchar(20),pwd varchar(20),key index_pwd(pwd)); # 为索引加上名称Query OK, 0 rows affected (0.02 sec)注意
index和 key作用是相同的
方法二：当表创建完成后，使用alter为表添加索引：
语法：
alter table 表名 add index 索引名称 （字段1，字段2.....）;
例子：
MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+8 rows in set (0.00 sec)
MariaDB [book]> alter table books add key (bName,bTypeId);
Query OK, 44 rows affected (0.01 sec)
Records: 44 Duplicates: 0 Warnings: 0MariaDB [book]> desc books; # 可以看到在bName字段上添加的索引+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | MUL | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+8 rows in set (0.00 sec)2.1.2 查看索引
语法：
desc 索引名(或索引字段)
例子：
MariaDB [book]> desc demo;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(4) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
| pwd | varchar(20) | YES | MUL | NULL | |
+-------+-------------+------+-----+---------+-------+3 rows in set (0.00 sec)
MariaDB [book]> desc demo1;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(4) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
| pwd | varchar(20) | YES | MUL | NULL | |
+-------+-------------+------+-----+---------+-------+3 rows in set (0.00 sec)
MariaDB [book]> desc demo2;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(4) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
| pwd | varchar(20) | YES | MUL | NULL | |
+-------+-------------+------+-----+---------+-------+3 rows in set (0.00 sec)
MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | MUL | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+8 rows in set (0.00 sec)注意：如果Key是MUL, 那么该列的值可以重复。该列是一个非唯一索引的前导列(第一列)或者是一个唯一性索引的组成部分但是可以含有空值NULL。
2.1.3 删除索引
语法：
alter table 表名 drop key 索引名(或字段名);
例子：
MariaDB [book]> alter table demo drop key pwd; Query OK, 0 rows affected (0.02 sec)Records: 0 Duplicates: 0 Warnings: 0MariaDB [book]> alter table demo1 drop key pwd; Query OK, 0 rows affected (0.01 sec)Records: 0 Duplicates: 0 Warnings: 0MariaDB [book]> alter table demo2 drop key index_pwd; Query OK, 0 rows affected (0.01 sec)Records: 0 Duplicates: 0 Warnings: 0MariaDB [book]> alter table books add key (bName,bTypeId); Query OK, 44 rows affected (0.01 sec) Records: 44 Duplicates: 0 Warnings: 0
2.2 唯一索引
与普通索引基本相同，但有一个区别：
索引列的所有值都只能出现一次，即必须唯一，用来约束内容。 字段值只能出现一次时，应该加唯一索引。 唯一性允许有NULL值<允许为空>。
2.2.1 创建唯一索引
方法一：创建表时加唯一索引
语法：
create table 表名( 列定义： unique key 索引名 （字段）; )
注意：常用在值不能重复的字段上，比如说用户名，电话号码，身份证号。
例子：
MariaDB [book]> create table demo3(id int(4) auto_increment primary key,uName varchar(20),uPwd varchar(20),unique index(uName)); Query OK, 0 rows affected (0.07 sec) MariaDB [book]> desc demo3; +-------+-------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+----------------+ | id | int(4) | NO | PRI | NULL | auto_increment | | uName | varchar(20) | YES | UNI | NULL | | | uPwd | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+----------------+3 rows in set (0.00 sec)
方法二：修改表时加唯一索引
语法：
alter table 表名 add unique 索引名 (字段);
例子：
MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+8 rows in set (0.01 sec)
MariaDB [book]> alter table books add unique index_bName (bName);
Query OK, 43 rows affected (0.01 sec)
Records: 43 Duplicates: 0 Warnings: 0MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | UNI | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+8 rows in set (0.02 sec)2.2.2 删除唯一索引
alter table 表名 drop key 索引名(或字段名);
例子：
MariaDB [book]> alter table books drop key index_bName;
Query OK, 43 rows affected (0.01 sec)
Records: 43 Duplicates: 0 Warnings: 0MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+8 rows in set (0.01 sec)
MariaDB [book]> desc demo3;
+-------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id | int(4) | NO | PRI | NULL | auto_increment |
| uName | varchar(20) | YES | UNI | NULL | |
| uPwd | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+----------------+3 rows in set (0.01 sec)
MariaDB [book]> alter table demo3 drop key uName;
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0MariaDB [book]> desc demo3;
+-------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id | int(4) | NO | PRI | NULL | auto_increment |
| uName | varchar(20) | YES | | NULL | |
| uPwd | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+----------------+3 rows in set (0.00 sec)2.3 主键索引
查询数据库，按主键查询是最快的，每个表只能有一个主键列，可以有多个普通索引列。
主键列要求列的所有内容必须唯一，而索引列不要求内容必须唯一，但不允许为空
2.3.1 创建主键索引
方法一：创建表创建主键索引
MariaDB [book]> create table demo4 (id int(4) not null auto_increment,name varchar(4) default null,primary key(id)); Query OK, 0 rows affected (0.02 sec) MariaDB [book]> desc demo4; +-------+------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------+------------+------+-----+---------+----------------+ | id | int(4) | NO | PRI | NULL | auto_increment | | name | varchar(4) | YES | | NULL | | +-------+------------+------+-----+---------+----------------+2 rows in set (0.01 sec) MariaDB [book]> show index from demo4; +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | demo4 | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | | +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+1 row in set (0.00 sec)
方法二：创建表后添加<不推荐>
MariaDB [book]> drop table demo4; Query OK, 0 rows affected (0.01 sec) MariaDB [book]> create table demo4 (id int(4) not null auto_increment primary key,name varchar(4) not null); Query OK, 0 rows affected (0.02 sec) MariaDB [book]> alter table demo4 change id id int(4) not null; Query OK, 0 rows affected (0.03 sec) Records: 0 Duplicates: 0 Warnings: 0MariaDB [book]> desc demo4; +-------+------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+------------+------+-----+---------+-------+ | id | int(4) | NO | PRI | NULL | | | name | varchar(4) | NO | | NULL | | +-------+------------+------+-----+---------+-------+2 rows in set (0.00 sec) MariaDB [book]> show index from demo4; +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | demo4 | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | | +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+1 row in set (0.00 sec)
2.3.2 删除主键索引
MariaDB [book]> alter table demo4 drop primary key; Query OK, 0 rows affected (0.03 sec) Records: 0 Duplicates: 0 Warnings: 0MariaDB [book]> desc demo4; +-------+------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+------------+------+-----+---------+-------+ | id | int(4) | NO | | NULL | | | name | varchar(4) | NO | | NULL | | +-------+------------+------+-----+---------+-------+2 rows in set (0.00 sec) MariaDB [book]> show index from demo4; Empty set (0.00 sec)
主键索引，唯一性索引区别：主键索引不能有NULL，唯一性索引可以有空值
2.4 复合索引
索引可以包含一个、两个或更多个列。两个或更多个列上的索引被称作复合索引
例： 创建一个表存放服务器允许或拒绝的IP和port，但记录中IP和port要唯一。
MariaDB [book]> create table firewall(host varchar(15) not null,port smallint(4) not null,access enum("deny","allow") not null,primary key(host,port)); # 用host和port列作复合索引Query OK, 0 rows affected (0.11 sec)
MariaDB [book]> insert into firewall values("192.168.81.100",22,"deny");
Query OK, 1 row affected (0.02 sec)
MariaDB [book]> select * from firewall;
+---------------+------+--------+
| host | port | access |
+---------------+------+--------+
| 192.168.81.100 | 22 | deny |
+---------------+------+--------+1 row in set (0.00 sec)
MariaDB [book]> insert into firewall values("192.168.81.101",22,"allow");
Query OK, 1 row affected (0.04 sec)
MariaDB [book]> insert into firewall values("192.168.81.100",22,"deny"); # 插入数据时，IP和port要唯一，否则会报错ERROR 1062 (23000): Duplicate entry '192.168.81.100-22' for key 'PRIMARY'MariaDB [book]> select * from firewall;
+----------------+------+--------+
| host | port | access |
+----------------+------+--------+
| 192.168.81.100 | 22 | deny |
| 192.168.81.101 | 22 | allow |
+----------------+------+--------+2 rows in set (0.01 sec)
MariaDB [book]> desc firewall; # 查看索引+--------+----------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+----------------------+------+-----+---------+-------+
| host | varchar(15) | NO | PRI | NULL | |
| port | smallint(4) | NO | PRI | NULL | |
| access | enum('deny','allow') | NO | | NULL | |
+--------+----------------------+------+-----+---------+-------+3 rows in set (0.01 sec)建表的时候如果加各种索引，顺序如下：
create table 表名（字段定义，PRIMARYKEY (`bId`),UNIQUE KEY `bi` (`bImg`),KEY `bn` (`bName`),KEY `ba` (`author`)）
2.5 全文索引 (FULLTEXT INDEX)
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"分词技术"等多种算法智能分析出文本文字中关键字词的频率及重要性，然后按照一定的算法规则智能地筛选出我们想要的搜索结果
mysql 在数据量较大的情况下，高并发连接的情况下:
select 语句 where bName like '%网%'使用% 和 _ 通配符进行查找，因为索引里面保存的是一堆指针，不会保存具体内容，只能进行全表扫描，造成数据库压力大
全文索引只能用在varchar和text字段中
2.5.1 创建全文索引
方法一：创建表时创建
语法：
create table 表名（ 列定义， fulltext key 索引名 （字段）; ）
方法二：修改表时添加
语法：
alter table 表名 add fulltext 索引名 （字段）;
例子：
MariaDB [book]> show create table books;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| books | CREATE TABLE `books` (
`bId` int(4) NOT NULL AUTO_INCREMENT,
`bName` varchar(255) DEFAULT NULL,
`bTypeId` enum('1','2','3','4','5','6','7','8','9','10') DEFAULT NULL,
`publishing` varchar(255) DEFAULT NULL,
`price` int(4) DEFAULT NULL,
`pubDate` date DEFAULT NULL,
`author` varchar(30) DEFAULT NULL,
`ISBN` varchar(255) DEFAULT NULL,
PRIMARY KEY (`bId`)
) ENGINE=MyISAM AUTO_INCREMENT=45 DEFAULT CHARSET=utf8 |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+1 row in set (0.01 sec)
MariaDB [book]> alter table books add fulltext index_bName (`bName`);
Query OK, 43 rows affected (0.04 sec)
Records: 43 Duplicates: 0 Warnings: 0MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | MUL | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+8 rows in set (0.00 sec)
MariaDB [book]> alter table books drop key index_bName;
Query OK, 43 rows affected (0.03 sec)
Records: 43 Duplicates: 0 Warnings: 0MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+8 rows in set (0.00 sec)
MariaDB [book]> alter table books add fulltext index_bName (`bName`,`publishing`);
Query OK, 43 rows affected (0.02 sec)
Records: 43 Duplicates: 0 Warnings: 0MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | MUL | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+8 rows in set (0.01 sec)
MariaDB [book]> alter table books drop key index_bName;
Query OK, 43 rows affected (0.02 sec)
Records: 43 Duplicates: 0 Warnings: 0MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+8 rows in set (0.00 sec)
MariaDB [book]> alter table books add fulltext index_bName (`publishing`);
Query OK, 43 rows affected (0.02 sec)
Records: 43 Duplicates: 0 Warnings: 0MariaDB [book]> desc books;
+------------+------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------------------------------------+------+-----+---------+----------------+
| bId | int(4) | NO | PRI | NULL | auto_increment |
| bName | varchar(255) | YES | | NULL | |
| bTypeId | enum('1','2','3','4','5','6','7','8','9','10') | YES | | NULL | |
| publishing | varchar(255) | YES | MUL | NULL | |
| price | int(4) | YES | | NULL | |
| pubDate | date | YES | | NULL | |
| author | varchar(30) | YES | | NULL | |
| ISBN | varchar(255) | YES | | NULL | |
+------------+------------------------------------------------+------+-----+---------+----------------+8 rows in set (0.01 sec)2.5.2 删除全文索引
MariaDB [book]> alter table books drop key index_bName; Query OK, 39 rows affected (0.02 sec) Records: 39 Duplicates: 0 Warnings: 0
MySQL自带的全文索引只能用于数据库引擎为MyISAM的数据表，如果是其他数据引擎，则全文索引不会生效
MySQL自带的全文索引只能对英文进行全文检索，目前无法对中文进行全文检索。
3.索引设计原则
1、索引并非越多越好2、数据量不大不需要建立索引3、列中的值变化不多不需要建立索引 row id4、经常排序（order by）和分组（group by）的列需要建立索引5、唯一性约束对应使用唯一性索引
4.索引的优点：
为了加快搜索速度，减少查询时间 。
5.索引的缺点
1 索引是以文件存储的。如果索引过多，占磁盘空间较大。而且他影响： insert ,update ,delete 执行时间。2索引中数据必须与数据表数据同步：如果索引过多，当表中数据更新的时候后，索引也要同步更新，这就降低了效率。
原文出处：https://www.cnblogs.com/renpingsheng/p/9574013.html
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