MyISAM Table Storage Formats




本文主要了解 Static (Fixed-Length) TABLE 和 Dynamic Table 以及对空格的处理。


http://dev.MySQL.com/doc/refman/5.7/en/myisam-table-formats.html
1.创建和更改 MyISAM Table Storage Formats 语法
## table has no BLOB or TEXT ##
Create Table Engine
Create Table table_name ROW_FORMAT=FIXED;
Change Table Engine
Alter Table table_name ROW_FORMAT=DYNAMIC;




2.案例
2.1 创建 FIXED TABLE OF MyISAM Engine 并插入带有空格的字符，观察 FIXED TABLE OF MyISAM Engine 是如何处理空格的。
mysql> create table myisam_char(name char(10)) engine=myisam ROW_FORMAT=FIXED;
Query OK, 0 rows affected (0.04 sec)


mysql> insert into myisam_char values('abcde'),('abcde  '),('  abcde'),('  abcde  ');
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 0


mysql> select name,length(name) from myisam_char;
+---------+--------------+
| name    | length(name) |
+---------+--------------+
| abcde   |            5 |
| abcde   |            5 |
|   abcde |            7 |
|   abcde |            7 |
+---------+--------------+
4 rows in set (0.00 sec)
## FIXED TABLE OF MyISAM Engine 中字段都是非变长字段，数据会按照列的宽度定义补足空格，但是应用访问的时候并不会得到这些空格，这些空格在返回给应用之前已经去掉。
## 所以，开发需求本来就要保存字符后的空格，那就需要注意了。


mysql> show table status fromtest1 like 'myisam_char';
+-------------+--------+---------+------------+------+----------------+-------------+------------------+--------------+-----------+----------------+---------------------+---------------------+------------+-------------------+----------+------------------+---------+
| Name        | Engine | Version | Row_format | Rows | Avg_row_length | Data_length | Max_data_length  | Index_length | Data_free | Auto_increment | Create_time         | Update_time         | Check_time | Collation         | Checksum | Create_options   | Comment |
+-------------+--------+---------+------------+------+----------------+-------------+------------------+--------------+-----------+----------------+---------------------+---------------------+------------+-------------------+----------+------------------+---------+
| myisam_char | MyISAM |      10 | Fixed      |    4 |             11 |          44 | 3096224743817215 |         1024 |         0 |           NULL | 2016-03-22 16:09:26 | 2016-03-22 16:09:35 | NULL       | latin1_swedish_ci |     NULL | row_format=FIXED|         |
+-------------+--------+---------+------------+------+----------------+-------------+------------------+--------------+-----------+----------------+---------------------+---------------------+------------+-------------------+----------+------------------+---------+
1 row in set (0.00 sec)
## SHOW TABLE STATUS 语法直接查看 MyISAM Table Storage Formats


2.2 更改 MyISAM Table Storage Formats
mysql> alter table myisam_char engine=myisam ROW_FORMAT=DYNAMIC;
Query OK, 4 rows affected (0.01 sec)
Records: 4  Duplicates: 0  Warnings: 0


mysql> show table status from test1 like 'myisam_char';
+-------------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+-------------------+----------+--------------------+---------+
| Name        | Engine | Version | Row_format | Rows | Avg_row_length | Data_length | Max_data_length | Index_length | Data_free | Auto_increment | Create_time         | Update_time         | Check_time | Collation         | Checksum | Create_options     | Comment |
+-------------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+-------------------+----------+--------------------+---------+
| myisam_char | MyISAM |      10 | Dynamic    |    4 |             20 |          80 | 281474976710655 |         1024 |         0 |           NULL | 2016-03-22 16:21:44 | 2016-03-22 16:21:44 | NULL       | latin1_swedish_ci |     NULL | row_format=DYNAMIC |         |
+-------------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+-------------------+----------+--------------------+---------+
1 row in set (0.00 sec)


mysql> insert into myisam_char values('ABCDE'),('ABCDE  '),('  ABCDE'),('  ABCDE  ');
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 0


mysql> select name,length(name) from myisam_char;
+---------+--------------+
| name    | length(name) |
+---------+--------------+
| abcde   |            5 |
| abcde   |            5 |
|   abcde |            7 |
|   abcde |            7 |
| ABCDE   |            5 |
| ABCDE   |            5 |
|   ABCDE |            7 |
|   ABCDE |            7 |
+---------+--------------+
8 rows in set (0.00 sec)


3.Static (Fixed-Length) 和 Dynamic Table 特点
3.1 Static (Fixed-Length) 特点
Static-format tables have these characteristics:


CHAR and VARCHAR columns are space-padded to the specified column width, although the column type is not altered. BINARY and VARBINARY columns are padded with 0x00 bytes to the column width.


Very quick.


Easy to cache.


Easy to reconstruct after a crash, because rows are located in fixed positions.


Reorganization is unnecessary unless you delete a huge number of rows and want to return free disk space to the operating system. To do this, use OPTIMIZE TABLE or myisamchk -r.


Usually require more disk space than dynamic-format tables.
## 这种存储方式的优点存储非常迅速，容易缓存，出现故障容易恢复因为行位于固定的位置；缺点是占用的空间比动态表多。


3.2 Dynamic Table 特点


Dynamic-format tables have these characteristics:


All string columns are dynamic except those with a length less than four.


Each row is preceded by a bitmap that indicates which columns contain the empty string (for string columns) or zero (for numeric columns). This does not include columns that contain NULL values. If a string column has a length of zero after trailing space removal, or a numeric column has a value of zero, it is marked in the bitmap and not saved to disk. Nonempty strings are saved as a length byte plus the string contents.


Much less disk space usually is required than for fixed-length tables.


Each row uses only as much space as is required. However, if a row becomes larger, it is split into as many pieces as are required, resulting in row fragmentation. For example, if you update a row with information that extends the row length, the row becomes fragmented. In this case, you may have to run OPTIMIZE TABLE or myisamchk -r from time to time to improve performance. Use myisamchk -ei to obtain table statistics.


More difficult than static-format tables to reconstruct after a crash, because rows may be fragmented into many pieces and links (fragments) may be missing.


The expected row length for dynamic-sized rows is calculated using the following expression:


3
+ (number of columns + 7) / 8
+ (number of char columns)
+ (packed size of numeric columns)
+ (length of strings)
+ (number of NULL columns + 7) / 8
There is a penalty of 6 bytes for each link. A dynamic row is linked whenever an update causes an enlargement of the row. Each new link is at least 20 bytes, so the next enlargement probably goes in the same link. If not, another link is created. You can find the number of links using myisamchk -ed. All links may be removed with OPTIMIZE TABLE or myisamchk -r.
## 这种存储方式比静态表占用空间少；但是，频繁的更新和删除记录会生产锁片。所以，需要定期执行 OPTIMIZE TABLE or myisamchk -r 命令来改善性能；出现故障时恢复比较困难。


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