Compact是如何存儲(chǔ)大數(shù)據(jù)的？

更新時(shí)間:2018-11-16 來源:黑馬程序員技術(shù)社區(qū) 瀏覽量:

最近，在工作中遇到了MySQL中如何存儲(chǔ)長(zhǎng)度較長(zhǎng)的字段類型問題，于是花了一周多的時(shí)間抽空學(xué)習(xí)了一下，并且記錄下來。

MySQL大致的邏輯存儲(chǔ)結(jié)構(gòu)在這篇文章中有介紹，做為基本概念：InnoDB 邏輯存儲(chǔ)結(jié)構(gòu)

注：文中所指的大數(shù)據(jù)指的是長(zhǎng)度較長(zhǎng)的數(shù)據(jù)字段，包括varchar/varbinay/text/blob。

Compact行格式

我們首先來看一下行格式為Compact是如何存儲(chǔ)大數(shù)據(jù)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: 81920Max_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)

我們建立一張測(cè)試表，插入數(shù)據(jù)：

CREATE TABLE `row` ( `content` varchar(65532) NOT NULL DEFAULT '') ENGINE=InnoDB DEFAULT CHARSET=latin1mysql> insert into row(content) select repeat('a',65532);Query OK, 1 row affected (0.03 sec)Records: 1 Duplicates: 0 Warnings: 0

我們使用py_innodb_page_info.py工具來查看表中的頁分布：

[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: 1Uncompressed BLOB Page: 4File Space Header: 1B-tree Node: 1File Segment inode: 1

可以看出，第4頁的<B-tree Node>, page level <0000>格式為數(shù)據(jù)頁，存放著MySQL的行數(shù)據(jù)。<Uncompressed BLOB Page>可以理解為MySQL存放大數(shù)據(jù)的地方，暫且叫作外部存儲(chǔ)頁。Compact格式?jīng)]有將大數(shù)據(jù)全部放在數(shù)據(jù)頁中，而是將一部分?jǐn)?shù)據(jù)放在了外部存儲(chǔ)頁中。那么，是全部數(shù)據(jù)在外部存儲(chǔ)頁中，還是一部分?jǐn)?shù)據(jù)。假如是一部分?jǐn)?shù)據(jù)，這一部分是多少呢？

我們使用hexdump -Cv row.ibd查看一下數(shù)據(jù)頁<B-tree Node>, page level <0000>，也就是第4頁：

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+..:?|

我們可以看出，數(shù)據(jù)頁中存儲(chǔ)了一部分?jǐn)?shù)據(jù)，算下來一共是768字節(jié)，然后剩余部分存儲(chǔ)在外部存儲(chǔ)頁中。那么數(shù)據(jù)頁與外部存儲(chǔ)頁、外部存儲(chǔ)頁與外部存儲(chǔ)頁是如何連接在一起的呢？

我們觀察這一行：

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 |................|

這一行是前綴768字節(jié)的結(jié)尾。注意最后的20個(gè)字節(jié)：

00 00 00 02：4字節(jié)，代表外部存儲(chǔ)頁所在的space id00 00 00 04：4字節(jié)，代表第一個(gè)外部頁的Page no00 00 00 26：4字節(jié)，值為38，指向blob頁的header00 00 00 00 00 00 fc fc：8字節(jié)，代表該列存在外部存儲(chǔ)頁的總長(zhǎng)度。此處的值為64764，加上前綴768正好是65532。（注意一點(diǎn)，雖然表示BLOB長(zhǎng)度的是8字節(jié)，實(shí)際只有4個(gè)字節(jié)能使用，所有對(duì)于BLOB字段，存儲(chǔ)數(shù)據(jù)的最大長(zhǎng)度為4GB。）

驗(yàn)證下第一個(gè)外部存儲(chǔ)頁的頭部信息：

4097 00010000 cd c3 b6 8e 00 00 00 04 00 00 00 00 00 00 00 00 |??.............|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個(gè)字節(jié)為File Header(關(guān)于InnoDB數(shù)據(jù)頁的詳細(xì)結(jié)構(gòu)請(qǐng)參見《MySQL技術(shù)內(nèi)幕 InnoDB存儲(chǔ)引擎》4.4)，這個(gè)簡(jiǎn)單提一下：

cd c3 b6 8e：4字節(jié)，該頁的checksum。00 00 00 04：4字節(jié)，頁偏移，此頁為表空間中的第5個(gè)頁。00 00 00 00：4字節(jié)，當(dāng)前頁的上一個(gè)頁。此頁為<Uncompressed BLOB Page>，所以沒有上一頁。00 00 00 00：4字節(jié)，當(dāng)前頁的下一個(gè)頁。此頁為<Uncompressed BLOB Page>，所以沒有下一頁。00 00 00 00 00 06 b8 a2：8字節(jié)，該頁最后被修改的日志序列位置LSN。00 0a：2字節(jié)，頁類型，0x000A代表BLOB頁。00 00 00 00 00 00 00 00：8字節(jié)，略過。00 00 00 02：頁屬于哪個(gè)表空間，此處指表空間的ID為2。

之后是4字節(jié)的00 00 3f ca，這里的值為16330，代表此BLOB頁的有效數(shù)據(jù)的字節(jié)數(shù)。00 00 00 05代表下一個(gè)BLOB頁的page number。

我們看最后一個(gè)<Uncompressed BLOB Page>，第8個(gè)頁：

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|......

最后一頁的有效數(shù)據(jù)大小為0x00003d9e=15774，768+16330*3+15774 = 65532字節(jié)，符合初始插入數(shù)據(jù)的大小。
由于這是最后一個(gè)<Uncompressed BLOB Page>，所以指向下一個(gè)<Uncompressed BLOB Page>的指針為ff ff ff ff。

由此我們可以很清晰的看出數(shù)據(jù)頁與BLOB頁的連接關(guān)系(引用淘寶數(shù)據(jù)庫月報(bào)上的一張圖)：

我們來再看一個(gè)比較有意思的例子。：

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

我們建立一張新表，有11個(gè)blob字段。然后向每個(gè)字段插入1000字節(jié)的數(shù)據(jù)，MySQL會(huì)提示ERROR 1030 (HY000): Got error 139 from storage engine，什么意思呢？

InnoDB是以B+樹來組織數(shù)據(jù)的，假如每一行數(shù)據(jù)都占據(jù)一整個(gè)Page頁，那么B+樹將退化為單鏈表，所以InnoDB規(guī)定了一個(gè)Page必須包含兩行數(shù)據(jù)。也就是一行數(shù)據(jù)存儲(chǔ)在Page上的大小大概為8000字節(jié)。
而上面的例子，一行數(shù)據(jù)有11個(gè)1000字節(jié)的數(shù)據(jù)，Page層肯定放不下，所以在Page層留下768*11=8448字節(jié)，已經(jīng)超過了8000字節(jié)，所以MySQL會(huì)提示ERROR 1030 (HY000): Got error 139 from storage engine。我們很輕松的定義一個(gè)字段，來存儲(chǔ)11000個(gè)字節(jié)，但是卻無法將他們分成11個(gè)字段來存儲(chǔ)，有點(diǎn)意思！

那么如何解決上面的問題呢？

將行格式轉(zhuǎn)為接下來要說的Dynamic格式。此種格式只用20字節(jié)指向外部存儲(chǔ)空間。將多個(gè)blob字段轉(zhuǎn)為一個(gè)blob字段。多個(gè)字段可以用數(shù)組存儲(chǔ)，然后json_encode打包進(jìn)blob。

我們向表中插入一條有效記錄：

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.ibdpage 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: 1Uncompressed BLOB Page: 9File Space Header: 1B-tree Node: 1File Segment inode: 1

我們可以看出這一行數(shù)據(jù)有9個(gè)外部存儲(chǔ)頁，而我們一共就插入了9列數(shù)據(jù)，是不是當(dāng)每一列的數(shù)據(jù)在page頁放不下，都單獨(dú)申請(qǐng)一個(gè)外部存儲(chǔ)頁，而互相之前不共享外部存儲(chǔ)頁。我們看一下page頁的結(jié)構(gòu)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 |.......&.......@|......

根據(jù)前面的分析，我們現(xiàn)在可以看出，外部存儲(chǔ)頁是不共享的，即使一個(gè)列的數(shù)據(jù)多出一個(gè)字節(jié)，這一個(gè)字節(jié)也是獨(dú)占一個(gè)16KB空間的大小，這很浪費(fèi)存儲(chǔ)空間。（當(dāng)然，這對(duì)現(xiàn)代計(jì)算機(jī)可能不是問題，呵呵）。

說了這么多，總結(jié)下Compact格式存儲(chǔ)大數(shù)據(jù)的缺點(diǎn)：

由于存在768字節(jié)的前綴在Page頁，所以會(huì)存在能定義一個(gè)字段，存儲(chǔ)11000字節(jié)，但是不能定義11個(gè)字段，每個(gè)字段存儲(chǔ)1000字節(jié)的"bug"。外部存儲(chǔ)頁不共享，即使多余一個(gè)字節(jié)也是獨(dú)享16KB的頁面。
Dynamic行格式

接著我們首先看一下行格式為Dynamic是如何存儲(chǔ)大數(shù)據(jù)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: 16384Max_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)

創(chuàng)建和compact格式一樣的表：

CREATE TABLE `row` ( `content` varchar(65532) NOT NULL DEFAULT '') ENGINE=InnoDB DEFAULT CHARSET=latin1insert into row(content) select repeat('a',65532);Query OK, 1 row affected (0.03 sec)Records: 1 Duplicates: 0 Warnings: 0

看下頁分布：

[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: 1Uncompressed BLOB Page: 5File Space Header: 1B-tree Node: 1File Segment inode: 1

第4頁是數(shù)據(jù)頁，第5-9頁是二進(jìn)制頁。我們直接看磁盤中第4頁的數(shù)據(jù)：

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格式有著明顯的不同，當(dāng)大數(shù)據(jù)在Page頁存放不下時(shí)，Dynamic行格式不會(huì)留768字節(jié)在Page頁，并且將全部大數(shù)據(jù)都放在外部存儲(chǔ)頁。具體的數(shù)據(jù)頁和外部存儲(chǔ)頁的連接關(guān)系同Compact格式一樣。

我們?cè)倏纯碊ynamic格式的外部存儲(chǔ)頁是不是每一個(gè)列獨(dú)享外部存儲(chǔ)空間，還是同Compact格式實(shí)驗(yàn)過程一樣：

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

看一下外部存儲(chǔ)頁數(shù)據(jù)：

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的外部存儲(chǔ)頁也不是共享的。

但是MySQL為什么要這么設(shè)計(jì)呢？可能是為了實(shí)現(xiàn)簡(jiǎn)單吧，沿著鏈表通過有效數(shù)據(jù)大小就能讀取blob的全部數(shù)據(jù)。假如多個(gè)字段的blob混在一起，可能設(shè)計(jì)更復(fù)雜，要更新每個(gè)字段的偏移量之類的，更新的話頁數(shù)據(jù)管理也比較麻煩。我的個(gè)人猜測(cè)，呵呵。

總結(jié)下Dynamic格式存儲(chǔ)大數(shù)據(jù)的特點(diǎn)：

當(dāng)數(shù)據(jù)頁放不下時(shí)，MySQL會(huì)將大數(shù)據(jù)全部放在外部存儲(chǔ)頁，數(shù)據(jù)頁只留指向外部存儲(chǔ)頁的指針。外部存儲(chǔ)頁不共享，即使多余一個(gè)字節(jié)也是獨(dú)享16KB的頁面。
將列放入外部存儲(chǔ)頁的標(biāo)準(zhǔn)

當(dāng)一行中的數(shù)據(jù)不能在數(shù)據(jù)頁中放下，需要申請(qǐng)外部存儲(chǔ)頁時(shí)，MySQL需要決定將哪一列的數(shù)據(jù)放到外部存儲(chǔ)頁，遵循的規(guī)則如下：

長(zhǎng)度固定的字段不會(huì)被放到外部存儲(chǔ)頁（int、char(N)等）長(zhǎng)度小于20字節(jié)的字段不會(huì)被放到外部存儲(chǔ)頁。（假如放到外部存儲(chǔ)頁，不僅會(huì)單獨(dú)占據(jù)16KB，還要額外的20字節(jié)指針，沒有必要）對(duì)于Compact和REDUNDANT格式的行數(shù)據(jù)，長(zhǎng)度小于768字節(jié)的字段不會(huì)被放到外部存儲(chǔ)頁。（這個(gè)原因很顯然，本來就不夠768字節(jié)的前綴，總不能生搬硬湊吧）。

當(dāng)有多個(gè)大數(shù)據(jù)字段滿足上面條件，需要被放到外部存儲(chǔ)頁時(shí)，MySQL會(huì)優(yōu)先選擇大的字段放到外部存儲(chǔ)頁，因?yàn)檫@樣可以最大限度的省下數(shù)據(jù)頁的空間，使得更多的字段能夠被放到數(shù)據(jù)頁。

由于有較多的實(shí)驗(yàn)過程，所以顯得比較亂，建議看到這篇文章人自己實(shí)踐一遍，畢竟自己動(dòng)手會(huì)思考更多的問題與細(xì)節(jié)，理解的也比較深刻，哈哈哈。

作者：黑馬程序員大數(shù)據(jù)培訓(xùn)學(xué)院
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