《一起学mysql》4

索引的使用

索引太少返回结果很慢，但是索引太多，又会占用空间。每次插入新记录时，索引都会针对变化重新排序

什么时候使用索引

1.where 从句中用到的字段

select * from tb where f1 = 'xx' ,如果f1 进行了索引，那么这条sql 语句的销毁就会提高

2 max(),min()

select max(id) from tb

对id 索引非常有用，因为索引是按照顺序排序的，所以直接返回最后一个值即可

3.当返回的内容是索引的一部分时

这种情况不必扫描全表，只需查看索引即可

这种情况是不行的：f1 char(20),index f1(10),select f1 from tb ,因为索引只有10个字符，返回的却是20个

字符，必须扫描全表，扫描索引是不行的

4.order by

select * from tb order by f1,若 f1 是被索引的，则按照索引排序的顺序返回即可

5.在连接条件处使用

select t1.c1,t1.c2,t2.c3

from t1 join t2

on t1.c4=t2.c5

where t1.c6=xx;

如果 c4 和 c5 加了索引，也可以被使用到

6.不以% 开头的like 语句vs

select * from tb where name like 'jas%'

name 列加了索引也会被使用到

但是这种情况是不行的：select * from tb where name like '%jas%'

因为索引是按照第一个字符的字母顺序排序的

创建索引的一些建议：

1.where 条件中的字段创建索引

2.被索引的字段内容重复率越低越好，主键是最好的索引，每个id 只返回一条记录，枚举就比较差劲，比方

说性别，查询性别‘男’，可能会返回近一半的记录

3.使用前缀索引

4.不要创建太多的索引，太多的索引会拖慢插入和更新速度

最左原则

这个问题单纯的口头描述很难说清楚，直接看伪代码

create table tb (f1 char(20), f2 char(20), f3 char(20), f4 char(20), index(f1,f2,f3))

对于上面的表我们来进行下面几个sql 查询

1. select * from tb where f1='xx1' and f2 ='xx2' and f3='xx3'

2. select * from tb where f1='xx1' and f2 ='xx2'

3. select * from tb where f1='xx1'

4. select * from tb where f2 ='xx2' and f3='xx3'

5. select * from tb where f1='xx1' and f3='xx3'

上面的sql 中 1，2，3 的索引全部其作用

4中索引不会起作用

5中只有索引f1 起作用

也就是说复合索引使用的时候，必须是从最左侧开始，且是连续的，这样所有的索引才会被使用到

explain

下面来说一个很有用的东西，explain

先来建两张表，存储学生和老师信息，

create table student

(

stu_id int(3) zerofill,

name char(10),

majar char(10),

teacher_id int

);

create table teacher

(

teacher_id int,

name char(10)

);

insert into student values

(1,'丽丽','chemistry',1),

(2,'丽娟','english',2),

(3,'芳丽','儒学',3),

(4,'光头强','佛学',1),

(5,'熊大','math',2),

(6,'熊二','地理',3),

(7,'陶峰','高分子',4),

(8,'波波','机电',1),

(9,'米老鼠','土木',2);

insert into teacher values

(1,'刘老师'),

(2,'王老师'),

(3,'李老师'),

(4,'柴老师');

赶紧来试试explain

MariaDB [jason]> explain select * from student where stu_id=1;

+------+-------------+---------+------+---------------+------+---------+------+------+-------------+

+------+-------------+---------+------+---------------+------+---------+------+------+-------------+

+------+-------------+---------+------+---------------+------+---------+------+------+-------------+

给stu_id 加上索引再来试一下

MariaDB [jason]> alter table student add index(stu_id);

Query OK, 0 rows affected (0.02 sec)

Records: 0 Duplicates: 0 Warnings: 0

MariaDB [jason]> explain select * from student where stu_id=1;

+------+-------------+---------+------+---------------+--------+---------+-------+------+-------+

+------+-------------+---------+------+---------------+--------+---------+-------+------+-------+

| 1 | SIMPLE | student | ref | stu_id | stu_id | 5 | const | 1 | |

+------+-------------+---------+------+---------------+--------+---------+-------+------+-------+

有木有发现不一样？下面我们来解释一下explain 各个字段含义

说实话，我对参考资料中的的解释有些困惑，我想先自己造几个例子看看

MariaDB [jason]> show indexes from student ;

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

| student | 1 | stu_id | 1 | stu_id | A | 9 | NULL | NULL | YES | BTREE | | |

| student | 1 | teacher_id | 1 | teacher_id | A | 9 | NULL | NULL | YES | BTREE | | |

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

MariaDB [jason]> drop index stu_id on student;

MariaDB [jason]> alter table student add primary key(stu_id);

MariaDB [jason]> show indexes from teacher;

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

| teacher | 1 | teacher_id | 1 | teacher_id | A | 1 | NULL | NULL | YES | BTREE | | |

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

1 row in set (0.00 sec)

MariaDB [jason]> drop index teacher_id on teacher;

Query OK, 0 rows affected (0.00 sec)

Records: 0 Duplicates: 0 Warnings: 0

MariaDB [jason]> show indexes from teacher;

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

| teacher | 1 | teacher_id | 1 | teacher_id | A | 1 | NULL | NULL | YES | BTREE | | |

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

1 row in set (0.00 sec)

MariaDB [jason]> drop index teacher_id on teacher;

Query OK, 0 rows affected (0.00 sec)

Records: 0 Duplicates: 0 Warnings: 0

MariaDB [jason]> alter talbe teacher add primary key(teacher_id);

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'talbe teacher add primary key(teacher_id)' at line 1

MariaDB [jason]> alter table teacher add primary key(teacher_id);

我们先把student表中的stu_id 和tercher表中的teacher_id 变成了主键

example1：

MariaDB [jason]> explain select * from student where stu_id = 1;

+------+-------------+---------+-------+---------------+---------+---------+-------+------+-------+

+------+-------------+---------+-------+---------------+---------+---------+-------+------+-------+

+------+-------------+---------+-------+---------------+---------+---------+-------+------+-------+

example2：

MariaDB [jason]> explain select * from student where stu_id > 2;

+------+-------------+---------+-------+---------------+---------+---------+------+------+-------------+

+------+-------------+---------+-------+---------------+---------+---------+------+------+-------------+

+------+-------------+---------+-------+---------------+---------+---------+------+------+-------------+

example3：

MariaDB [jason]> explain select * from student where teacher_id = 1;

+------+-------------+---------+------+---------------+------------+---------+-------+------+-------+

+------+-------------+---------+------+---------------+------------+---------+-------+------+-------+

| 1 | SIMPLE | student | ref | teacher_id | teacher_id | 5 | const | 3 | |

+------+-------------+---------+------+---------------+------------+---------+-------+------+-------+

example4：

MariaDB [jason]> explain select * from student where teacher_id>2;

+------+-------------+---------+-------+---------------+------------+---------+------+------+-----------------------+

+------+-------------+---------+-------+---------------+------------+---------+------+------+-----------------------+

+------+-------------+---------+-------+---------------+------------+---------+------+------+-----------------------+

example5：

ariaDB [jason]> explain select s.stu_id,s.name,s.majar,s.teacher_id,t.name

-> from student s join teacher t

-> on s.teacher_id = t.teacher_id;

+------+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+

+------+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+

| 1 | SIMPLE | t | ALL | PRIMARY | NULL | NULL | NULL | 4 | |

+------+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+

example6：

MariaDB [jason]> explain select s.stu_id,s.name,s.majar,s.teacher_id,t.name

-> from student s left join teacher t

-> on s.teacher_id = t.teacher_id;

+------+-------------+-------+--------+---------------+---------+---------+--------------------+------+-------+

+------+-------------+-------+--------+---------------+---------+---------+--------------------+------+-------+

| 1 | SIMPLE | s | ALL | NULL | NULL | NULL | NULL | 9 | |

| 1 | SIMPLE | t | eq_ref | PRIMARY | PRIMARY | 4 | jason.s.teacher_id | 1 | |

+------+-------------+-------+--------+---------------+---------+---------+--------------------+------+-------+

example7：

MariaDB [jason]> explain select s.stu_id,s.name,s.majar,s.teacher_id,t.name

-> from student s left join teacher t

-> on s.teacher_id = t.teacher_id

-> where t.name is not null;

+------+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+

+------+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+

+------+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+

example8：

MariaDB [jason]> explain select s.stu_id,s.name,s.majar,s.teacher_id,t.name

-> from student s right join teacher t

-> on s.teacher_id = t.teacher_id;

+------+-------------+-------+------+---------------+------------+---------+--------------------+------+-------+

+------+-------------+-------+------+---------------+------------+---------+--------------------+------+-------+

| 1 | SIMPLE | t | ALL | NULL | NULL | NULL | NULL | 4 | |

| 1 | SIMPLE | s | ref | teacher_id | teacher_id | 4 | jason.t.teacher_id | 1 | |

+------+-------------+-------+------+---------------+------------+---------+--------------------+------+-------+

我尝试过使用full outer join ，但是mysql 不支持外连接

example9：

MariaDB [jason]> explain select stu_id,teacher_id from student;

+------+-------------+---------+-------+---------------+------------+---------+------+------+-------------+

+------+-------------+---------+-------+---------------+------------+---------+------+------+-------------+

+------+-------------+---------+-------+---------------+------------+---------+------+------+-------------+

example10：

MariaDB [jason]> explain select max(teacher_id) from student;

+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------+

+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------+

+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------+

example11：

MariaDB [jason]> explain select * from student order by teacher_id desc;

+------+-------------+---------+------+---------------+------+---------+------+------+----------------+

+------+-------------+---------+------+---------------+------+---------+------+------+----------------+

+------+-------------+---------+------+---------------+------+---------+------+------+----------------+

example12：

MariaDB [jason]> explain select * from student force index(teacher_id) order by teacher_id desc;

+------+-------------+---------+-------+---------------+------------+---------+------+------+-------+

+------+-------------+---------+-------+---------------+------------+---------+------+------+-------+

+------+-------------+---------+-------+---------------+------------+---------+------+------+-------+

example13：

MariaDB [jason]> alter table student add index(name);

MariaDB [jason]> explain select * from student where name like '丽%';

+------+-------------+---------+-------+---------------+------+---------+------+------+-----------------------+

+------+-------------+---------+-------+---------------+------+---------+------+------+-----------------------+

+------+-------------+---------+-------+---------------+------+---------+------+------+-----------------------+

example14：

MariaDB [jason]> explain select * from student where teacher_id +5 < 7;

+------+-------------+---------+------+---------------+------+---------+------+------+-------------+

+------+-------------+---------+------+---------------+------+---------+------+------+-------------+

+------+-------------+---------+------+---------------+------+---------+------+------+-------------+

1 row in set (0.01 sec)

MariaDB [jason]> explain select * from student where teacher_id < 2;

+------+-------------+---------+-------+---------------+------------+---------+------+------+-----------------------+

+------+-------------+---------+-------+---------------+------------+---------+------+------+-----------------------+

+------+-------------+---------+-------+---------------+------------+---------+------+------+-----------------------+

1 row in set (0.00 sec)

example 15:

MariaDB [jason]> explain select s.stu_id,s.name,s.majar,s.teacher_id,t.name

-> from student s left join teacher t

-> on s.teacher_id = t.teacher_id

-> where t.teacher_id is null;

+------+-------------+-------+--------+---------------+---------+---------+--------------------+------+-------------------------+

+------+-------------+-------+--------+---------------+---------+---------+--------------------+------+-------------------------+

| 1 | SIMPLE | s | ALL | NULL | NULL | NULL | NULL | 9 | |

+------+-------------+-------+--------+---------------+---------+---------+--------------------+------+-------------------------+

2 rows in set (0.00 sec)

example 16:

MariaDB [jason]> explain select *

-> from t2 left join t1 on t2.f2=t1.f1

-> left join t3 on t3.f3=t2.f2

-> left join t4 on t4.f4=t3.f3

-> where t1.f1=1;

+------+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+

+------+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+

+------+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+

这里没有列出最左原则的例子，大家可以自己个儿试一下，

关于expalin 各列的解释可以看这里

analyze table

mysql 在执行时select 或join 时会使用索引的分布信息来决定表的关联顺序、或使用那个索引，

analyze table tablename 就可以分析并存储索引的分布信息，

MariaDB [jason]> analyze table student;

+---------------+---------+----------+----------+

+---------------+---------+----------+----------+

+---------------+---------+----------+----------+

可以通过 show index from tb_name 来查看索引的情况

MariaDB [jason]> show index from student;

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

| student | 1 | name | 1 | name | A | 10 | NULL | NULL | YES | BTREE | | |

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

各字段的解释看这里

重点解释一下Cardinality

1. 列值代表的是此列中存储的唯一值的个数（如果此列为primary key 则值为记录的行数）

2. 列值只是个估计值，并不准确。

3. 列值不会自动更新，需要通过Analyze table来更新一张表或者mysqlcheck -Aa来进行更新整个数据库。

4. 列值的大小影响Join时是否选用这个Index的判断。

5. 创建Index时，MyISAM的表Cardinality的值为null，InnoDB的表Cardinality的值大概为行数。

6. MyISAM与InnoDB对于Cardinality的计算方式不同。

optimize

如果对表进行了大量的删除和更新操作，那么表中就会留下缝隙，optimize table tb_name 可以将缝隙删除

将分割的记录连接在一起

MariaDB [jason]> optimize table student;

+---------------+----------+----------+-------------------------------------------------------------------+

+---------------+----------+----------+-------------------------------------------------------------------+

+---------------+----------+----------+-------------------------------------------------------------------+

什么鬼？原来是innodb 不支持optimize, 百度找到了方法

ALTER TABLE table.name ENGINE='InnoDB';

This will create a copy of the original table, and drop the original table, and replace to the original place.

Although this is safe, but I suggest you do backup and test first before doing this.

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