MySQL--06(索引)

索引

数据量很大时，查询慢的问题；

数据量巨大时，索引无效

索引定义：是一个排好序的，便于快速查找的，数据结构。

主键是一种特殊的索引，

index

pid : parent id ->pid

parent : 父母，双亲

可以在任何字段上创建索引，但不是每个字段都适合做索引；

查看索引

show index from 表名；

创建索引的命令

create index idx_索引名  on 表名（字段名（索引长度））

索引类型 

主键、唯一索引、普通索引、联合（复合索引）、全文索引

添加唯一索引命令

alter table 表名 add unique index idx_索引名(要索引的字段（长度）)

如果添加了 唯一索引 添加重复的数据，则会报类似错误：

ERROR 1062 (23000):** Duplicate** entry '湖南省永州市' for key 'idx_address'

删除索引

drop index 索引名称 on 表名;

MariaDB [books]> drop index idx_address on student;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

MariaDB [books]> show index from student;
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table   | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| student |          0 | PRIMARY  |            1 | id          | A         |           6 |     NULL | NULL   |      | BTREE      |         |               |
| student |          1 | idx_name |            1 | name        | A         |           6 |       10 | NULL   |      | BTREE      |         |               |
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)

Key_name：索引名称

MariaDB [books]> show index from student;
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table   | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| student |          0 | PRIMARY  |            1 | id          | A         |           6 |     NULL | NULL   |      | BTREE      |         |               |
| student |          1 | idx_name |            1 | name        | A         |           6 |       10 | NULL   |      | BTREE      |         |               |
| student |          1 | idx_fuhe |            1 | name        | A         |           6 |     NULL | NULL   |      | BTREE      |         |               |
| student |          1 | idx_fuhe |            2 | address     | A         |           6 |     NULL | NULL   | YES  | BTREE      |         |               |
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
4 rows in set (0.00 sec)

创建全文索引

MariaDB [books]> alter table student add  FULLTEXT idx_full(address);
Query OK, 0 rows affected, 1 warning (0.18 sec)
Records: 0  Duplicates: 0  Warnings: 1

MariaDB [books]> create table idx(
    ->     id int primary key auto_increment,
    ->     name varchar(20),
    ->     email varchar(60),
    ->     unique idx_email (email(20))
    -> )engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.02 sec)

MariaDB [books]> show index from idx;
+-------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name  | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| idx   |          0 | PRIMARY   |            1 | id          | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
| idx   |          0 | idx_email |            1 | email       | A         |           0 |       20 | NULL   | YES  | BTREE      |         |               |
+-------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)

事务

事务：一系列的mysql操作，它的特性具有acid特性。

acid:原子性，一致性，隔离性，持久性。

start transaction;/begin;

commit;

提交事务的案例

1. 开启 窗口1

2. 开启窗口2

3. 两个窗口都使用同一数据库

4. 窗口一 start transaction;或者begin开启事务

5. 窗口一 执行插入或修改的操作，

6. 窗口一查看，这时可以看到操作的结果

7. 窗口二查看，这时看不到操作的结果

8. 窗口一commit

9. 窗口一查看，可以看到操作结果，窗口二查看，可以看到操作结果，完成事务操作。