.ibd文件存储问题关于MySQL中overflow page和int的问题

Question

我在使用

hexdump -C demo.ibd

学习mysql存储数据的过程中遇到了一些问题。这是我的演示表。

CREATE TABLE `demo` (
  `id`   int NOT NULL,
  `age`  int NOT NULL,
  `name` varchar(20000) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=ascii

INSERT INTO demo(id, age, name) 
VALUES
(1, 10, 'joker'),
(2, 11, 'tom'),
(3, 12, 'jerry'),
(4, 12, 'aaaa...20000 in total...a'),
(5, 16, NULL);

MySQL 8.0.32，行格式动态。

以下是我根据

hexdump

看到的，我拆分了二进制数据进行分析

05                          # lengths of variable-length columns
00                          # a bit vector for indicating NULL columns
00 00 10 00 21              # 5-byte record header
80 00 00 01                 # id: 1 [1]
00 00 00 00 05 1c           # transaction_id
82 00 00 01 0a 01 10        # roll_pointer
80 00 00 0a                 # age: 10
6a 6f 6b 65 72              # name: "joker"

03                          # lengths of variable-length columns
00                          # a bit vector for indicating NULL columns
00 00 18 00 1f              # 5-byte record header
80 00 00 02                 # id: 2
00 00 00 00 05 1e           # transaction_id
82 00 00 01 17 01 10        # roll_pointer
80 00 00 0b                 # age: 11
74 6f 6d                    # name: "tom"

05                          # lengths of variable-length columns
00                          # a bit vector for indicating NULL columns
00 00 20 00 20              # 5-byte record header
80 00 00 03                 # id: 3
00 00 00 00 05 23           # transaction_id
81 00 00 01 10 01 10        # roll_pointer
80 00 00 0c                 # age: 12
6a 65 72 72 79              # name: jerry

                            # lengths of variable-length columns [2]
01                          # a bit vector for indicating NULL columns
00 00 28 ff 91              # 5-byte record header
80 00 00 04                 # id: 4
00 00 00 00 05 25           # transaction_id
81 00 00 01 11 01 10        # roll_pointer
80 00 00 0c                 # age: 12
                            # name: NULL

14 c0                       # lengths of variable-length columns [3]
00                          # a bit vector for indicating NULL columns
00 00 30 ff 74              # 5-byte record header
80 00 00 05                 # id: 5
00 00 00 00 05 2d           # transaction_id
82 00 00 01 10 01 10        # roll_pointer
80 00 00 10                 # age: 16
00 00 00 02 00 00 00 05     # This is a 20-byte pointer which point to overflow page
00 00 00 01 00 00 00 00     # name: "aaa..." which length is 24000 Bytes
00 00 5d c0                 # 0x5dc0 = 24000

我有三个问题，在第 4 行，第 28 行和第 37 行。

[1] 为什么MySQL不使用二进制补码来存储有符号整数？

[2] 当行的变长字段都为NULL时，长度是否没有额外的字节？

[3] 当变长字段过长导致页面溢出时，多出的字节表示什么？

如果您能为我解释一下，我将不胜感激