mysql 表字段太多 varchar类型保存不了 mysql存储大段文字

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attitude 2023-08-30 22:47:06

文章标签 大数据数据结构与算法 python sed 数据 文章分类 MySQL 数据库

最近，在工作中遇到了MySQL中如何存储长度较长的字段类型问题，于是花了一周多的时间抽空学习了一下，并且记录下来。

MySQL大致的逻辑存储结构在这篇文章中有介绍，做为基本概念：InnoDB 逻辑存储结构

注：文中所指的大数据指的是长度较长的数据字段，包括varchar/varbinay/text/blob。

Compact行格式

我们首先来看一下行格式为Compact是如何存储大数据的：

mysql> select version();
+-----------+
| version() |
+-----------+
| 5.1.73    |
+-----------+
1 row in set (0.01 sec)

mysql> show table status like 'row'\G;
*************************** 1. row ***************************
           Name: row
         Engine: InnoDB
        Version: 10
     Row_format: Compact
           Rows: 1
 Avg_row_length: 81920
    Data_length: 81920
Max_data_length: 0
   Index_length: 0
      Data_free: 0
 Auto_increment: NULL
    Create_time: 2017-01-04 21:46:02
    Update_time: NULL
     Check_time: NULL
      Collation: latin1_swedish_ci
       Checksum: NULL
 Create_options: 
        Comment: 
1 row in set (0.00 sec)
mysql> select version();
+-----------+
| version() |
+-----------+
| 5.1.73    |
+-----------+
1 row in set (0.01 sec)

mysql> show table status like 'row'\G;
*************************** 1. row ***************************
           Name: row
         Engine: InnoDB
        Version: 10
     Row_format: Compact
           Rows: 1
 Avg_row_length: 81920
    Data_length: 81920
Max_data_length: 0
   Index_length: 0
      Data_free: 0
 Auto_increment: NULL
    Create_time: 2017-01-04 21:46:02
    Update_time: NULL
     Check_time: NULL
      Collation: latin1_swedish_ci
       Checksum: NULL
 Create_options: 
        Comment: 
1 row in set (0.00 sec)

我们建立一张测试表，插入数据：

CREATE TABLE `row` (
  `content` varchar(65532) NOT NULL DEFAULT ''
) ENGINE=InnoDB DEFAULT CHARSET=latin1

mysql> insert into row(content) select repeat('a',65532);
Query OK, 1 row affected (0.03 sec)
Records: 1  Duplicates: 0  Warnings: 0
CREATE TABLE `row` (
  `content` varchar(65532) NOT NULL DEFAULT ''
) ENGINE=InnoDB DEFAULT CHARSET=latin1

mysql> insert into row(content) select repeat('a',65532);
Query OK, 1 row affected (0.03 sec)
Records: 1  Duplicates: 0  Warnings: 0

我们使用py_innodb_page_info.py工具来查看表中的页分布：

[root@localhost mysql]# python py_innodb_page_info.py -v com/row.ibd 
page offset 00000000, page type <File Space Header>
page offset 00000001, page type <Insert Buffer Bitmap>
page offset 00000002, page type <File Segment inode>
page offset 00000003, page type <B-tree Node>, page level <0000>
page offset 00000004, page type <Uncompressed BLOB Page>
page offset 00000005, page type <Uncompressed BLOB Page>
page offset 00000006, page type <Uncompressed BLOB Page>
page offset 00000007, page type <Uncompressed BLOB Page>
Total number of page: 8:
Insert Buffer Bitmap: 1
Uncompressed BLOB Page: 4
File Space Header: 1
B-tree Node: 1
File Segment inode: 1
[root@localhost mysql]# python py_innodb_page_info.py -v com/row.ibd 
page offset 00000000, page type <File Space Header>
page offset 00000001, page type <Insert Buffer Bitmap>
page offset 00000002, page type <File Segment inode>
page offset 00000003, page type <B-tree Node>, page level <0000>
page offset 00000004, page type <Uncompressed BLOB Page>
page offset 00000005, page type <Uncompressed BLOB Page>
page offset 00000006, page type <Uncompressed BLOB Page>
page offset 00000007, page type <Uncompressed BLOB Page>
Total number of page: 8:
Insert Buffer Bitmap: 1
Uncompressed BLOB Page: 4
File Space Header: 1
B-tree Node: 1
File Segment inode: 1

可以看出，第4页的<B-tree Node>, page level <0000>格式为数据页，存放着MySQL的行数据。<Uncompressed BLOB Page>可以理解为MySQL存放大数据的地方，暂且叫作外部存储页。Compact格式没有将大数据全部放在数据页中，而是将一部分数据放在了外部存储页中。那么，是全部数据在外部存储页中，还是一部分数据。假如是一部分数据，这一部分是多少呢？

我们使用hexdump -Cv row.ibd查看一下数据页<B-tree Node>, page level <0000>，也就是第4页：

3073 0000c000  8c 25 17 57 00 00 00 03  ff ff ff ff ff ff ff ff  |.%.W....????????|
3074 0000c010  00 00 00 00 00 07 3a b8  45 bf 00 00 00 00 00 00  |......:?E?......|
3075 0000c020  00 00 00 00 00 02 00 02  03 a6 80 03 00 00 00 00  |.........?......|
3076 0000c030  00 7f 00 05 00 00 00 01  00 00 00 00 00 00 00 00  |................|
3077 0000c040  00 00 00 00 00 00 00 00  00 13 00 00 00 02 00 00  |................|
3078 0000c050  00 02 00 f2 00 00 00 02  00 00 00 02 00 32 01 00  |...?.........2..|
3079 0000c060  02 00 1c 69 6e 66 69 6d  75 6d 00 02 00 0b 00 00  |...infimum......|
3080 0000c070  73 75 70 72 65 6d 75 6d  14 c3 00 00 10 ff f1 00  |supremum.?...??.|
3081 0000c080  00 00 00 04 03 00 00 00  00 13 12 80 00 00 00 2d  |...............-|
3082 0000c090  01 10 61 61 61 61 61 61  61 61 61 61 61 61 61 61  |..aaaaaaaaaaaaaa|
3083 0000c0a0  61 61 61 61 61 61 61 61  61 61 61 61 61 61 61 61  |aaaaaaaaaaaaaaaa|
3084 0000c0b0  61 61 61 61 61 61 61 61  61 61 61 61 61 61 61 61  |aaaaaaaaaaaaaaaa|
3085 0000c0c0  61 61 61 61 61 61 61 61  61 61 61 61 61 61 61 61  |aaaaaaaaaaaaaaaa|
....
....
3128 0000c370  61 61 61 61 61 61 61 61  61 61 61 61 61 61 61 61  |aaaaaaaaaaaaaaaa|
3129 0000c380  61 61 61 61 61 61 61 61  61 61 61 61 61 61 61 61  |aaaaaaaaaaaaaaaa|
3130 0000c390  61 61 00 00 00 02 00 00  00 04 00 00 00 26 00 00  |aa...........&..|
3131 0000c3a0  00 00 00 00 fc fc 00 00  00 00 00 00 00 00 00 00  |....??..........|
3132 0000c3b0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
3133 0000c3c0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
3134 0000c3d0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
...
...
4093 0000ffc0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
4094 0000ffd0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
4095 0000ffe0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
4096 0000fff0  00 00 00 00 00 70 00 63  01 a1 6c 2b 00 07 3a b8  |.....p.c.?l+..:?|

我们可以看出，数据页中存储了一部分数据，算下来一共是768字节，然后剩余部分存储在外部存储页中。那么数据页与外部存储页、外部存储页与外部存储页是如何连接在一起的呢？

我们观察这一行：

3130 0000c390  61 61 00 00 00 02 00 00  00 04 00 00 00 26 00 00  |aa...........&..|
3131 0000c3a0  00 00 00 00 fc fc 00 00  00 00 00 00 00 00 00 00  |................|

这一行是前缀768字节的结尾。注意最后的20个字节：

00 00 00 02：4字节，代表外部存储页所在的space id
00 00 00 04：4字节，代表第一个外部页的Page no
00 00 00 26：4字节，值为38，指向blob页的header
00 00 00 00 00 00 fc fc：8字节，代表该列存在外部存储页的总长度。此处的值为64764，加上前缀768正好是65532。（注意一点，虽然表示BLOB长度的是8字节，实际只有4个字节能使用，所有对于BLOB字段，存储数据的最大长度为4GB。）

验证下第一个外部存储页的头部信息：

4097 00010000  cd c3 b6 8e 00 00 00 04  00 00 00 00 00 00 00 00  |?ö.............|
4098 00010010  00 00 00 00 00 06 b8 a2  00 0a 00 00 00 00 00 00  |......??........|
4099 00010020  00 00 00 00 00 02 00 00  3f ca 00 00 00 05 61 61  |........??....aa|
4100 00010030  61 61 61 61 61 61 61 61  61 61 61 61 61 61 61 61  |aaaaaaaaaaaaaaaa|
...
...

前38个字节为File Header(关于InnoDB数据页的详细结构请参见《MySQL技术内幕 InnoDB存储引擎》4.4)，这个简单提一下：

cd c3 b6 8e：4字节，该页的checksum。
00 00 00 04：4字节，页偏移，此页为表空间中的第5个页。
00 00 00 00：4字节，当前页的上一个页。此页为<Uncompressed BLOB Page>，所以没有上一页。
00 00 00 00：4字节，当前页的下一个页。此页为<Uncompressed BLOB Page>，所以没有下一页。
00 00 00 00 00 06 b8 a2：8字节，该页最后被修改的日志序列位置LSN。
00 0a：2字节，页类型，0x000A代表BLOB页。
00 00 00 00 00 00 00 00：8字节，略过。
00 00 00 02：页属于哪个表空间，此处指表空间的ID为2。

之后是4字节的00 00 3f ca，这里的值为16330，代表此BLOB页的有效数据的字节数。00 00 00 05代表下一个BLOB页的page number。

我们看最后一个<Uncompressed BLOB Page>，第8个页：

7169 0001c000  fa 78 9b 27 00 00 00 07  00 00 00 00 00 00 00 00  |?x.'............|
7170 0001c010  00 00 00 00 00 07 3a b8  00 0a 00 00 00 00 00 00  |......:?........|
7171 0001c020  00 00 00 00 00 02 00 00  3d 9e ff ff ff ff 61 61  |........=.????aa|
7172 0001c030  61 61 61 61 61 61 61 61  61 61 61 61 61 61 61 61  |aaaaaaaaaaaaaaaa|
7173 0001c040  61 61 61 61 61 61 61 61  61 61 61 61 61 61 61 61  |aaaaaaaaaaaaaaaa|
...
...

最后一页的有效数据大小为0x00003d9e=15774，768+16330*3+15774 = 65532字节，符合初始插入数据的大小。
由于这是最后一个<Uncompressed BLOB Page>，所以指向下一个<Uncompressed BLOB Page>的指针为ff ff ff ff。

由此我们可以很清晰的看出数据页与BLOB页的连接关系(引用淘宝数据库月报上的一张图)：

mysql 表字段太多 varchar类型保存不了 mysql存储大段文字_sed

我们来再看一个比较有意思的例子。：

CREATE TABLE `testblob` (
  `blob1` blob NOT NULL,
  `blob2` blob NOT NULL,
  `blob3` blob NOT NULL,
  `blob4` blob NOT NULL,
  `blob5` blob NOT NULL,
  `blob6` blob NOT NULL,
  `blob7` blob NOT NULL,
  `blob8` blob NOT NULL,
  `blob9` blob NOT NULL,
  `blob10` blob NOT NULL,
  `blob11` blob NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

mysql> insert into testblob select repeat('a',1000),repeat('b',1000),repeat('c',1000),repeat('d',1000),repeat('e',1000),repeat('f',1000),repeat('g',1000),repeat('h',1000),repeat('i',1000),repeat('j',1000),repeat('k',1000);
ERROR 1030 (HY000): Got error 139 from storage engine
CREATE TABLE `testblob` (
  `blob1` blob NOT NULL,
  `blob2` blob NOT NULL,
  `blob3` blob NOT NULL,
  `blob4` blob NOT NULL,
  `blob5` blob NOT NULL,
  `blob6` blob NOT NULL,
  `blob7` blob NOT NULL,
  `blob8` blob NOT NULL,
  `blob9` blob NOT NULL,
  `blob10` blob NOT NULL,
  `blob11` blob NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

mysql> insert into testblob select repeat('a',1000),repeat('b',1000),repeat('c',1000),repeat('d',1000),repeat('e',1000),repeat('f',1000),repeat('g',1000),repeat('h',1000),repeat('i',1000),repeat('j',1000),repeat('k',1000);
ERROR 1030 (HY000): Got error 139 from storage engine

我们建立一张新表，有11个blob字段。然后向每个字段插入1000字节的数据，MySQL会提示ERROR 1030 (HY000): Got error 139 from storage engine，什么意思呢？

InnoDB是以B+树来组织数据的，假如每一行数据都占据一整个Page页，那么B+树将退化为单链表，所以InnoDB规定了一个Page必须包含两行数据。也就是一行数据存储在Page上的大小大概为8000字节。
而上面的例子，一行数据有11个1000字节的数据，Page层肯定放不下，所以在Page层留下768*11=8448字节，已经超过了8000字节，所以MySQL会提示ERROR 1030 (HY000): Got error 139 from storage engine。我们很轻松的定义一个字段，来存储11000个字节，但是却无法将他们分成11个字段来存储，有点意思！

那么如何解决上面的问题呢？

将行格式转为接下来要说的Dynamic格式。此种格式只用20字节指向外部存储空间。
将多个blob字段转为一个blob字段。多个字段可以用数组存储，然后json_encode打包进blob。

我们向表中插入一条有效记录：

mysql>  insert into testblob(blob1,blob2,blob3,blob4,blob5,blob6,blob7,blob8,blob9) select repeat('a',8000),repeat('b',8000),repeat('c',8000),repeat('d',8000),repeat('e',8000),repeat('f',8000),repeat('g',8000),repeat('h',8000),repeat('i',8000);
Query OK, 1 row affected (0.12 sec)
Records: 1  Duplicates: 0  Warnings: 0
mysql>  insert into testblob(blob1,blob2,blob3,blob4,blob5,blob6,blob7,blob8,blob9) select repeat('a',8000),repeat('b',8000),repeat('c',8000),repeat('d',8000),repeat('e',8000),repeat('f',8000),repeat('g',8000),repeat('h',8000),repeat('i',8000);
Query OK, 1 row affected (0.12 sec)
Records: 1  Duplicates: 0  Warnings: 0

[root@localhost mysql]# python py_innodb_page_info.py -v com/testblob.ibd
page offset 00000000, page type <File Space Header>
page offset 00000001, page type <Insert Buffer Bitmap>
page offset 00000002, page type <File Segment inode>
page offset 00000003, page type <B-tree Node>, page level <0000>
page offset 00000004, page type <Uncompressed BLOB Page>
page offset 00000005, page type <Uncompressed BLOB Page>
page offset 00000006, page type <Uncompressed BLOB Page>
page offset 00000007, page type <Uncompressed BLOB Page>
page offset 00000008, page type <Uncompressed BLOB Page>
page offset 00000009, page type <Uncompressed BLOB Page>
page offset 0000000a, page type <Uncompressed BLOB Page>
page offset 0000000b, page type <Uncompressed BLOB Page>
page offset 0000000c, page type <Uncompressed BLOB Page>
Total number of page: 13:
Insert Buffer Bitmap: 1
Uncompressed BLOB Page: 9
File Space Header: 1
B-tree Node: 1
File Segment inode: 1

我们可以看出这一行数据有9个外部存储页，而我们一共就插入了9列数据，是不是当每一列的数据在page页放不下，都单独申请一个外部存储页，而互相之前不共享外部存储页。我们看一下page页的结构就知道了：

3130 0000c390  61 61 61 61 61 61 61 61  61 61 61 61 61 61 61 61  |aaaaaaaaaaaaaaaa|
 3131 0000c3a0  61 61 61 61 00 00 00 05  00 00 00 04 00 00 00 26  |aaaa...........&|
...
...
 3180 0000c6b0  62 62 62 62 62 62 62 62  00 00 00 05 00 00 00 05  |bbbbbbbb........|
 3181 0000c6c0  00 00 00 26 00 00 00 00  00 00 1c 40 63 63 63 63  |...&.......@cccc|
...
...
 3229 0000c9c0  63 63 63 63 63 63 63 63  63 63 63 63 00 00 00 05  |cccccccccccc....|
 3230 0000c9d0  00 00 00 06 00 00 00 26  00 00 00 00 00 00 1c 40  |.......&.......@|
...
...

根据前面的分析，我们现在可以看出，外部存储页是不共享的，即使一个列的数据多出一个字节，这一个字节也是独占一个16KB空间的大小，这很浪费存储空间。（当然，这对现代计算机可能不是问题，呵呵）。

说了这么多，总结下Compact格式存储大数据的缺点：

由于存在768字节的前缀在Page页，所以会存在能定义一个字段，存储11000字节，但是不能定义11个字段，每个字段存储1000字节的"bug"。
外部存储页不共享，即使多余一个字节也是独享16KB的页面。

Dynamic行格式

接着我们首先看一下行格式为Dynamic是如何存储大数据的：

mysql> select version();
+-----------+
| version() |
+-----------+
| 5.7.14    |
+-----------+
1 row in set (0.00 sec)

mysql> show table status like 'row'\G;
*************************** 1. row ***************************
           Name: row
         Engine: InnoDB
        Version: 10
     Row_format: Dynamic
           Rows: 0
 Avg_row_length: 0
    Data_length: 16384
Max_data_length: 0
   Index_length: 0
      Data_free: 0
 Auto_increment: NULL
    Create_time: 2017-01-03 22:45:16
    Update_time: NULL
     Check_time: NULL
      Collation: latin1_swedish_ci
       Checksum: NULL
 Create_options:
        Comment:
1 row in set (0.00 sec)
mysql> select version();
+-----------+
| version() |
+-----------+
| 5.7.14    |
+-----------+
1 row in set (0.00 sec)

mysql> show table status like 'row'\G;
*************************** 1. row ***************************
           Name: row
         Engine: InnoDB
        Version: 10
     Row_format: Dynamic
           Rows: 0
 Avg_row_length: 0
    Data_length: 16384
Max_data_length: 0
   Index_length: 0
      Data_free: 0
 Auto_increment: NULL
    Create_time: 2017-01-03 22:45:16
    Update_time: NULL
     Check_time: NULL
      Collation: latin1_swedish_ci
       Checksum: NULL
 Create_options:
        Comment:
1 row in set (0.00 sec)

创建和compact格式一样的表：

CREATE TABLE `row` (
  `content` varchar(65532) NOT NULL DEFAULT ''
) ENGINE=InnoDB DEFAULT CHARSET=latin1

insert into row(content) select repeat('a',65532);
Query OK, 1 row affected (0.03 sec)
Records: 1  Duplicates: 0  Warnings: 0
CREATE TABLE `row` (
  `content` varchar(65532) NOT NULL DEFAULT ''
) ENGINE=InnoDB DEFAULT CHARSET=latin1

insert into row(content) select repeat('a',65532);
Query OK, 1 row affected (0.03 sec)
Records: 1  Duplicates: 0  Warnings: 0

看下页分布：

[root@localhost mysql]# python py_innodb_page_info.py -v row.ibd 
page offset 00000000, page type <File Space Header>
page offset 00000001, page type <Insert Buffer Bitmap>
page offset 00000002, page type <File Segment inode>
page offset 00000003, page type <B-tree Node>, page level <0000>
page offset 00000004, page type <Uncompressed BLOB Page>
page offset 00000005, page type <Uncompressed BLOB Page>
page offset 00000006, page type <Uncompressed BLOB Page>
page offset 00000007, page type <Uncompressed BLOB Page>
page offset 00000008, page type <Uncompressed BLOB Page>
Total number of page: 9:
Insert Buffer Bitmap: 1
Uncompressed BLOB Page: 5
File Space Header: 1
B-tree Node: 1
File Segment inode: 1

第4页是数据页，第5-9页是二进制页。我们直接看磁盘中第4页的数据：

3073 0000c000  dc 2d b0 f5 00 00 00 03  ff ff ff ff ff ff ff ff  |.-..............|
3074 0000c010  00 00 00 00 00 a3 4b 59  45 bf 00 00 00 00 00 00  |......KYE.......|
3075 0000c020  00 00 00 00 00 36 00 02  00 a6 80 03 00 00 00 00  |.....6..........|
3076 0000c030  00 7f 00 05 00 00 00 01  00 00 00 00 00 00 00 00  |................|
3077 0000c040  00 00 00 00 00 00 00 00  00 64 00 00 00 36 00 00  |.........d...6..|
3078 0000c050  00 02 00 f2 00 00 00 36  00 00 00 02 00 32 01 00  |.......6.....2..|
3079 0000c060  02 00 1c 69 6e 66 69 6d  75 6d 00 02 00 0b 00 00  |...infimum......|
3080 0000c070  73 75 70 72 65 6d 75 6d  14 c0 00 00 10 ff f1 00  |supremum........|
3081 0000c080  00 00 00 02 00 00 00 00  00 07 07 a7 00 00 01 1b  |................|
3082 0000c090  01 10 00 00 00 36 00 00  00 04 00 00 00 26 00 00  |.....6.......&..|
3083 0000c0a0  00 00 00 00 ff fc 00 00  00 00 00 00 00 00 00 00  |................|
3084 0000c0b0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
3085 0000c0c0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
3086 0000c0d0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
3087 0000c0e0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
...
...
...

和Compact格式有着明显的不同，当大数据在Page页存放不下时，Dynamic行格式不会留768字节在Page页，并且将全部大数据都放在外部存储页。具体的数据页和外部存储页的连接关系同Compact格式一样。

我们再看看Dynamic格式的外部存储页是不是每一个列独享外部存储空间，还是同Compact格式实验过程一样：

CREATE TABLE `testblob` (
  `blob1` blob NOT NULL,
  `blob2` blob NOT NULL,
  `blob3` blob NOT NULL,
  `blob4` blob NOT NULL,
  `blob5` blob NOT NULL,
  `blob6` blob NOT NULL,
  `blob7` blob NOT NULL,
  `blob8` blob NOT NULL,
  `blob9` blob NOT NULL,
  `blob10` blob NOT NULL,
  `blob11` blob NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

mysql>   insert into testblob(blob1,blob2,blob3,blob4,blob5,blob6,blob7,blob8,blob9,blob10,blob11) select repeat('a',8000),repeat('b',8000),repeat('c',8000),repeat('d',8000),repeat('e',8000),repeat('f',8000),repeat('g',8000),repeat('h',8000),repeat('i',8000),repeat('j',8000),repeat('k',8000);
Query OK, 1 row affected (0.10 sec)
Records: 1  Duplicates: 0  Warnings: 0
CREATE TABLE `testblob` (
  `blob1` blob NOT NULL,
  `blob2` blob NOT NULL,
  `blob3` blob NOT NULL,
  `blob4` blob NOT NULL,
  `blob5` blob NOT NULL,
  `blob6` blob NOT NULL,
  `blob7` blob NOT NULL,
  `blob8` blob NOT NULL,
  `blob9` blob NOT NULL,
  `blob10` blob NOT NULL,
  `blob11` blob NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

mysql>   insert into testblob(blob1,blob2,blob3,blob4,blob5,blob6,blob7,blob8,blob9,blob10,blob11) select repeat('a',8000),repeat('b',8000),repeat('c',8000),repeat('d',8000),repeat('e',8000),repeat('f',8000),repeat('g',8000),repeat('h',8000),repeat('i',8000),repeat('j',8000),repeat('k',8000);
Query OK, 1 row affected (0.10 sec)
Records: 1  Duplicates: 0  Warnings: 0

看一下外部存储页数据：

4599 00011f60  61 61 61 61 61 61 61 61  61 61 61 61 61 61 00 00  |aaaaaaaaaaaaaa..|
 4600 00011f70  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|

好的，可以不用向下看其他列的了，Dynamic的外部存储页也不是共享的。

但是MySQL为什么要这么设计呢？可能是为了实现简单吧，沿着链表通过有效数据大小就能读取blob的全部数据。假如多个字段的blob混在一起，可能设计更复杂，要更新每个字段的偏移量之类的，更新的话页数据管理也比较麻烦。我的个人猜测，呵呵。

总结下Dynamic格式存储大数据的特点：

当数据页放不下时，MySQL会将大数据全部放在外部存储页，数据页只留指向外部存储页的指针。
外部存储页不共享，即使多余一个字节也是独享16KB的页面。

由于有较多的实验过程，所以显得比较乱，建议看到这篇文章人自己实践一遍，毕竟自己动手会思考更多的问题与细节，理解的也比较深刻，哈哈哈。

参考资料：http://dev.mysql.com/doc/refman/5.7/en/column-count-limit.html
http://mysqlserverteam.com/externally-stored-fields-in-innodb/ https://www.percona.com/blog/2010/02/09/blob-storage-in-innodb/
http://mysql.taobao.org/monthly/2016/02/01/

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