（八）MySQL索引操作

（1）准备环境

mysql> create table t1(id int,name varchar(50));
mysql> d $$
mysql> create procedure autoinsert_t1()
    begin
    declare i int default 1;
    while(i<200000)do
    insert into school.t1 values(i,'ccc');
    set i=i+1
    ;
    end while;
    END $$
mysql> d ;
mysql> show create procedure autoinsert_t1G
mysql> call autoinsert_t1();

（2）创建索引

1）创建表时创建索引

创建普通索引：mysql> create table departname(id int,name varchar(50),comment varchar(50),index(name));
创建唯一索引：mysql> create table department2(id int,name varchar(50),comment varchar(50),unique index(name));
创建全文索引：mysql> create table department3(id int,name varchar(50),log text,fulltext index(log)); 针对文章给文章内容起索引
创建多列索引：mysql> create table department5(id int,name varchar(50),comment varchar(50),index index_name_comment(name,comment));
创建多列索引建议给索引起个名字

2）在已存在表上创建索引：create

语法：create [unique|fulltext|spatial] index 索引名 on 表名(字段[(长度)] [ASC|DESC])
创建普通索引：mysql> create index index_name on department(name); 索引关键字MUL
创建唯一索引：mysql> create unique index index_id on department(id);
创建全文索引：mysql> create fulltext index index_name on department(name);
创建多列索引：mysql> create index index_name_comment on department(name,comment);

3）在已存在表上创建索引：alter table

语法：alter table 表名 add [unique|fulltext|spatial] index 索引名(字段[(长度)] [ASC|DESC])
创建普通索引：alter table department add index index_name(name);
创建唯一索引：alter table department add unique index index_name(name);
创建全文索引：alter table department add fulltext index index_name(name);
创建多列索引：alter table department add index index_name_comment(name,comment);

（3）管理索引

1）查看索引

语法：show create table 表名G

2）测试索引

语法：explain select * from 表名 where 条件

3）删除索引

语法：drop index 索引名 on 表名;
mysql> drop index index_name on department6;
注意：先使用show create table 表名G 查看索引名字，例如 KEY index_name (name) index_name 就是索引名字

（4）测试索引：

未使用索引之前：注意key和rows

mysql> explain  select * from t1 where id=190000;
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
|  1 | SIMPLE      | t1    | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 200186 |    10.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+

创建索引之后：

mysql> explain  select * from t1 where id=190000;
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key      | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | t1    | NULL       | ref  | index_id      | index_id | 5       | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

使用索引：查询语句使用where 条件，条件使用的列必须是索引