索引

資料量很大時，查詢慢的問題；

資料量巨大時，索引無效

索引定義：是一個排好序的，便於快速查詢的，資料結構。

主鍵是一種特殊的索引，

index

pid : parent id ->pid

parent : 父母，雙親

可以在任何欄位上建立索引，但不是每個欄位都適合做索引；

檢視索引

show index from 表名；

建立索引的命令

create index idx_索引名  on 表名（欄位名（索引長度））

索引型別

主鍵、唯一索引、普通索引、聯合（複合索引）、全文索引

新增唯一索引命令

alter table 表名 add unique index idx_索引名(要索引的欄位（長度）)

如果添加了 唯一索引 新增重複的資料，則會報類似錯誤：

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

刪除索引

drop index 索引名稱 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：索引名稱

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)

建立全文索引

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)

事務

事務：一系列的mysql操作，它的特性具有acid特性。

acid:原子性，一致性，隔離性，永續性。

start transaction;/begin;

commit;

提交事務的案例

1. 開啟 視窗1

2. 開啟視窗2

3. 兩個視窗都使用同一資料庫

4. 視窗一 start transaction;或者begin開啟事務

5. 視窗一 執行插入或修改的操作，

6. 視窗一檢視，這時可以看到操作的結果

7. 視窗二檢視，這時看不到操作的結果

8. 視窗一commit

9. 視窗一檢視，可以看到操作結果，視窗二檢視，可以看到操作結果，完成事務操作。