Myisam的碎片空间

mysql> create database bbs_user2;
Query OK, 1 row affected (0.00 sec)

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| bbs_user           |
| bbs_user2          |
| mysql              |
| performance_schema |
| test               |
+--------------------+
6 rows in set (0.00 sec)

mysql> use bbs_user2;
Database changed
mysql> create table one(
    -> id int unsigned not null auto_increment primary key,
    -> username varchar(32)
    -> );
Query OK, 0 rows affected (0.01 sec)

mysql> insert into one(username) values('zhangyi');
Query OK, 1 row affected (0.00 sec)

mysql> insert into one(username) values('zhanger');
Query OK, 1 row affected (0.00 sec)

mysql> insert into one(username) values('zhangsan');
Query OK, 1 row affected (0.00 sec)

mysql> insert into one(username) values('zhangsi');
Query OK, 1 row affected (0.00 sec)

mysql> select * from one;
+----+----------+
| id | username |
+----+----------+
|  1 | zhangyi  |
|  2 | zhanger  |
|  3 | zhangsan |
|  4 | zhangsi  |
+----+----------+
4 rows in set (0.00 sec)

mysql> delete from one where id=3;
Query OK, 1 row affected (0.00 sec)

mysql> select * from one;
+----+----------+
| id | username |
+----+----------+
|  1 | zhangyi  |
|  2 | zhanger  |
|  4 | zhangsi  |                //当我删除id 3的时候 没有清理空间 
+----+----------+
3 rows in set (0.00 sec)

mysql> insert into one (username) values('zhangwu');
Query OK, 1 row affected (0.00 sec)

mysql> select * from one;
+----+----------+
| id | username |
+----+----------+
|  1 | zhangyi  |                //重新插入id 5的数据 跑到id 3的地方出现 
|  2 | zhanger  |
|  5 | zhangwu  |
|  4 | zhangsi  |
+----+----------+
4 rows in set (0.00 sec)

mysql>



                    <------optimize修改遗留数据------> 


mysql> create database bbs_user;
Query OK, 1 row affected (0.00 sec)

mysql> use bbs_user;
Database changed
mysql> create table one (
    -> id int unsigned not null auto_increment primary key,
    -> username varchar(32)
    -> );
Query OK, 0 rows affected (0.01 sec)

mysql> insert into one (username) values('zhang1');
Query OK, 1 row affected (0.01 sec)

mysql> insert into one (username) values('zhang2');
Query OK, 1 row affected (0.00 sec)

mysql> insert into one (username)  values('zhang3');
Query OK, 1 row affected (0.00 sec)

mysql> insert  into one (username) values('zhang4');
Query OK, 1 row affected (0.00 sec)                            //插入数据 

mysql> select * from one ;
+----+----------+
| id | username |
+----+----------+
|  1 | zhang1   |
|  2 | zhang2   |
|  3 | zhang3   |
|  4 | zhang4   |
+----+----------+
4 rows in set (0.00 sec)

mysql> delete from one where id=3;
Query OK, 1 row affected (0.00 sec)

mysql> optimize table one;      //这里用optimize清理删除id 3后的遗留空间 
+--------------+----------+----------+----------+
| Table        | Op       | Msg_type | Msg_text |
+--------------+----------+----------+----------+
| bbs_user.one | optimize | status   | OK       |
+--------------+----------+----------+----------+
1 row in set (0.02 sec)

mysql> insert into one (username) values('zhang5');
Query OK, 1 row affected (0.00 sec)

mysql> select * from one;
+----+----------+
| id | username |              //再次添加zhangwu数据时 id 5数据下面出现
+----+----------+
|  1 | zhang1   |
|  2 | zhang2   |
|  4 | zhang4   |
|  5 | zhang5   |
+----+----------+
4 rows in set (0.00 sec)