详解MariaDB数据库的索引
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全部记录的引用指针。mysql
更通俗的说，数据库索引比如是一本书前面的目录，在查找内容以前能够先在目录中查找索引位置，以此快速定位查询数据。算法
对于索引，会保存在额外的文件中。sql
2.索引的类型
一、普通索引 二、惟一性索引 三、主键索引（主索引） 四、复合索引 五、全文索引
2.1. 普通索引
最基本的索引，不具有惟一性，仅加快查询速度数据库
2.1.1 建立普通索引
方法一：建立表时添加索引服务器
语法：并发
create table 表名（
列定义
index 索引名称 （字段）
index 索引名称 （字段）
）
注：可使用key关键字，也可使用index关键字。索引名称，能够加也能够不加，不加使用字段名做为索引名。高并发
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)
注意搜索引擎
index和 key做用是相同的设计
方法二：当表建立完成后，使用alter为表添加索引：
语法：指针
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: 0
MariaDB [book]> desc books; # 能够看到在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：若是Key是MUL, 那么该列的值能够重复。该列是一个非惟一索引的前导列(第一列)或者是一个惟一性索引的组成部分可是能够含有空值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: 0 MariaDB [book]> alter table demo1 drop key pwd; Query OK, 0 rows affected (0.01 sec) Records: 0 Duplicates: 0 Warnings: 0 MariaDB [book]> alter table demo2 drop key index_pwd; Query OK, 0 rows affected (0.01 sec) Records: 0 Duplicates: 0 Warnings: 0 MariaDB [book]> alter table books add key (bName,bTypeId); Query OK, 44 rows affected (0.01 sec) Records: 44 Duplicates: 0 Warnings: 0
2.2 惟一索引
与普通索引基本相同，但有一个区别：
索引列的全部值都只能出现一次，即必须惟一，用来约束内容。 字段值只能出现一次时，应该加惟一索引。 惟一性容许有NULL值<容许为空>。
2.2.1 建立惟一索引
方法一：建立表时加惟一索引
语法：
create table 表名(
列定义：
unique key 索引名 （字段）;
)
注意：经常使用在值不能重复的字段上，好比说用户名，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：修改表时加惟一索引
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: 0
MariaDB [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 删除惟一索引
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: 0
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]> 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: 0
MariaDB [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，每一个表只能有一个主键列，能够有多个普通索引列。
主键列要求列的全部内容必须惟一，而索引列不要求内容必须惟一，但不容许为空
2.3.1 建立主键索引
方法一：建立表建立主键索引
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：建立表后添加 <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: 0 MariaDB [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: 0 MariaDB [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，惟一性索引区别：主键索引不能有NULL，惟一性索引能够有空值
2.4 复合索引
索引能够包含一个、两个或更多个列。两个或更多个列上的索引被称做复合索引
复合索引专门用于组合搜索，其效率大于索引合并
例： 建立一个表存放服务器容许或拒绝的IP和port，但记录中IP和port要惟一。
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列做复合索引
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要惟一，不然会报错
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)
建表的时候若是加各类索引，顺序以下：
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。
它可以利用"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，而后按照必定的算法规则智能地筛选出咱们想要的搜索结果
mysql 在数据量较大的状况下，高并发链接的状况下:
select 语句 where bName like '%网%' 使用% 和 _ 通配符进行查找，由于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，形成数据库压力大
全文索引只能用在varchar和text字段中
2.5.1 建立全文索引
方法一：建立表时建立
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: 0
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)
MariaDB [book]> alter table books drop key index_bName;
Query OK, 43 rows affected (0.03 sec)
Records: 43 Duplicates: 0 Warnings: 0
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 fulltext index_bName (`bName`,`publishing`);
Query OK, 43 rows affected (0.02 sec)
Records: 43 Duplicates: 0 Warnings: 0
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 (0.01 sec)
MariaDB [book]> alter table books drop key index_bName;
Query OK, 43 rows affected (0.02 sec)
Records: 43 Duplicates: 0 Warnings: 0
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 fulltext index_bName (`publishing`);
Query OK, 43 rows affected (0.02 sec)
Records: 43 Duplicates: 0 Warnings: 0
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 | 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，若是是其余数据引擎，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，目前没法对中文进行全文检索。
3.索引设计原则
一、索引并不是越多越好 二、数据量不大不须要创建索引 三、列中的值变化很少不须要创建索引 row id 四、常常排序（order by）和分组（group by）的列须要创建索引 五、惟一性约束对应使用惟一性索引
4.索引的优势：
因为索引是专门用于加速搜索而生，因此加上索引以后，查询效率会很快，查询时间会减小
5.索引的缺点
1.索引是以文件存储的。若是索引过多，占磁盘空间较大。并且他影响： insert ,update ,delete 执行时间。 2.索引中数据必须与数据表数据同步：若是索引过多，当表中数据更新的时候后，索引也要同步更新，这就下降了效率。
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