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What file is mysql frm?

青灯夜游

Feb 17, 2022 pm 03:08 PM

mysql

在mysql中，frm的意思为“表定义”，是描述数据表结构的文件。frm文件是用来保存每个数据表的元数据信息，包括表结构的定义等。frm文件跟数据库存储引擎无关，也就是任何存储引擎的数据表都必须有frm文件，命名方式为“数据表名.frm”。

What file is mysql frm?

本教程操作环境：windows7系统、mysql8版本、Dell G3电脑。

在mysql中，frm的意思为“表定义”，是描述数据表结构的文件。

在MYSQL中建立任何一张数据表，在其数据目录对应的数据库目录下都有对应表的.frm文件,.frm文件是用来保存每个数据表的元数据(meta)信息，包括表结构的定义等。

.frm文件跟数据库存储引擎无关，也就是任何存储引擎的数据表都必须有.frm文件，命名方式为数据表名.frm，如user.frm. .frm文件可以用来在数据库崩溃时恢复表结构。

通常frm文件是不会损坏的，但是如果出现特殊情况出现frm文件损坏也不要放弃希望，例如下面报错：

1	`150821 16:31:27 [ERROR] /usr/local/mysql51/libexec/mysqld: Incorrect information in file: './t/test1.frm'`

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当修复MyISAM和InnoDB表时，MySQL服务会首先去调用frm文件，所以我们只能通过修复frm文件进行后面的数据恢复。

MySQL通过sql/table.cc的create_frm()函数创建frm文件，创建出来的frm文件是二进制文件，需要通过hexdump解析成16进制来分析。

create_frm()函数对frm文件头部定义的代码

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/* Create a .frm file */
 
File create_frm(THD *thd, const char *name, const char *db,
                const char *table, uint reclength, uchar *fileinfo,
          HA_CREATE_INFO *create_info, uint keys, KEY *key_info)
{
  register File file;
  ulong length;
  uchar fill[IO_SIZE];
  int create_flags= O_RDWR | O_TRUNC;
  ulong key_comment_total_bytes= 0;
  uint i;
 
  if (create_info->options & HA_LEX_CREATE_TMP_TABLE)
    create_flags|= O_EXCL | O_NOFOLLOW;
 
  /* Fix this when we have new .frm files;  Current limit is 4G rows (QQ) */
  if (create_info->max_rows > UINT_MAX32)
    create_info->max_rows= UINT_MAX32;
  if (create_info->min_rows > UINT_MAX32)
    create_info->min_rows= UINT_MAX32;
 
  if ((file= mysql_file_create(key_file_frm,
                               name, CREATE_MODE, create_flags, MYF(0))) >= 0)
  {
    uint key_length, tmp_key_length, tmp, csid;
    bzero((char*) fileinfo,64);
    /* header */
    fileinfo[0]=(uchar) 254;
    fileinfo[1]= 1;
    fileinfo[2]= FRM_VER+3+ test(create_info->varchar);
 
    fileinfo[3]= (uchar) ha_legacy_type(
          ha_checktype(thd,ha_legacy_type(create_info->db_type),0,0));
    fileinfo[4]=1;
    int2store(fileinfo+6,IO_SIZE);        /* Next block starts here */
    /*
      Keep in sync with pack_keys() in unireg.cc
      For each key:
      8 bytes for the key header
      9 bytes for each key-part (MAX_REF_PARTS)
      NAME_LEN bytes for the name
      1 byte for the NAMES_SEP_CHAR (before the name)
      For all keys:
      6 bytes for the header
      1 byte for the NAMES_SEP_CHAR (after the last name)
      9 extra bytes (padding for safety? alignment?)
    */
    for (i= 0; i < keys; i++)
    {
      DBUG_ASSERT(test(key_info[i].flags & HA_USES_COMMENT) == 
                 (key_info[i].comment.length > 0));
      if (key_info[i].flags & HA_USES_COMMENT)
        key_comment_total_bytes += 2 + key_info[i].comment.length;
    }
 
    key_length= keys * (8 + MAX_REF_PARTS * 9 + NAME_LEN + 1) + 16
                + key_comment_total_bytes;
 
    length= next_io_size((ulong) (IO_SIZE+key_length+reclength+
                                  create_info->extra_size));
    int4store(fileinfo+10,length);
    tmp_key_length= (key_length < 0xffff) ? key_length : 0xffff;
    int2store(fileinfo+14,tmp_key_length);
    int2store(fileinfo+16,reclength);
    int4store(fileinfo+18,create_info->max_rows);
    int4store(fileinfo+22,create_info->min_rows);
    /* fileinfo[26] is set in mysql_create_frm() */
    fileinfo[27]=2;                // Use long pack-fields
    /* fileinfo[28 & 29] is set to key_info_length in mysql_create_frm() */
    create_info->table_options|=HA_OPTION_LONG_BLOB_PTR; // Use portable blob pointers
    int2store(fileinfo+30,create_info->table_options);
    fileinfo[32]=0;                // No filename anymore
    fileinfo[33]=5;                             // Mark for 5.0 frm file
    int4store(fileinfo+34,create_info->avg_row_length);
    csid= (create_info->default_table_charset ?
           create_info->default_table_charset->number : 0);
    fileinfo[38]= (uchar) csid;
    /*
      In future versions, we will store in fileinfo[39] the values of the
      TRANSACTIONAL and PAGE_CHECKSUM clauses of CREATE TABLE.
    */
    fileinfo[39]= 0;
    fileinfo[40]= (uchar) create_info->row_type;
    /* Next few bytes where for RAID support */
    fileinfo[41]= (uchar) (csid >> 8);
    fileinfo[42]= 0;
    fileinfo[43]= 0;
    fileinfo[44]= 0;
    fileinfo[45]= 0;
    fileinfo[46]= 0;
    int4store(fileinfo+47, key_length);
    tmp= MYSQL_VERSION_ID;          // Store to avoid warning from int4store
    int4store(fileinfo+51, tmp);
    int4store(fileinfo+55, create_info->extra_size);
    /*
      59-60 is reserved for extra_rec_buf_length,
      61 for default_part_db_type
    */
    int2store(fileinfo+62, create_info->key_block_size);
    bzero(fill,IO_SIZE);
    for (; length > IO_SIZE ; length-= IO_SIZE)
    {
      if (mysql_file_write(file, fill, IO_SIZE, MYF(MY_WME | MY_NABP)))
      {
        (void) mysql_file_close(file, MYF(0));
        (void) mysql_file_delete(key_file_frm, name, MYF(0));
    return(-1);
      }
    }
  }
  else
  {
    if (my_errno == ENOENT)
      my_error(ER_BAD_DB_ERROR,MYF(0),db);
    else
      my_error(ER_CANT_CREATE_TABLE,MYF(0),table,my_errno);
  }
  return (file);
} /* create_frm */

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open_binary_frm()函数对对frm索引部分定义的代码

for (i=0 ; i < keys ; i++, keyinfo++)
  {
    keyinfo->table= 0;                           // Updated in open_frm
    if (new_frm_ver >= 3)
    {
      keyinfo->flags=       (uint) uint2korr(strpos) ^ HA_NOSAME;
      keyinfo->key_length= (uint) uint2korr(strpos+2);
      keyinfo->key_parts=  (uint) strpos[4];
      keyinfo->algorithm=  (enum ha_key_alg) strpos[5];
      keyinfo->block_size= uint2korr(strpos+6);
      strpos+=8;
    }
    else
    {
      keyinfo->flags=     ((uint) strpos[0]) ^ HA_NOSAME;
      keyinfo->key_length= (uint) uint2korr(strpos+1);
      keyinfo->key_parts=  (uint) strpos[3];
      keyinfo->algorithm= HA_KEY_ALG_UNDEF;
      strpos+=4;
    }
 
    keyinfo->key_part=     key_part;
    keyinfo->rec_per_key= rec_per_key;
    for (j=keyinfo->key_parts ; j-- ; key_part++)
    {
      *rec_per_key++=0;
      key_part->fieldnr=    (uint16) (uint2korr(strpos) & FIELD_NR_MASK);
      key_part->offset= (uint) uint2korr(strpos+2)-1;
      key_part->key_type=    (uint) uint2korr(strpos+5);
      // key_part->field=    (Field*) 0;    // Will be fixed later
      if (new_frm_ver >= 1)
      {
    key_part->key_part_flag= *(strpos+4);
    key_part->length=    (uint) uint2korr(strpos+7);
    strpos+=9;
      }
      else
      {
    key_part->length=    *(strpos+4);
    key_part->key_part_flag=0;
    if (key_part->length > 128)
    {
      key_part->length&=127;        /* purecov: inspected */
      key_part->key_part_flag=HA_REVERSE_SORT; /* purecov: inspected */
    }
    strpos+=7;
      }
      key_part->store_length=key_part->length;
    }
  }
  keynames=(char*) key_part;
  strpos+= (strmov(keynames, (char *) strpos) - keynames)+1;
 
  //reading index comments
  for (keyinfo= share->key_info, i=0; i < keys; i++, keyinfo++)
  {
    if (keyinfo->flags & HA_USES_COMMENT)
    {
      keyinfo->comment.length= uint2korr(strpos);
      keyinfo->comment.str= strmake_root(&share->mem_root, (char*) strpos+2,
                                         keyinfo->comment.length);
      strpos+= 2 + keyinfo->comment.length;
    } 
    DBUG_ASSERT(test(keyinfo->flags & HA_USES_COMMENT) == 
               (keyinfo->comment.length > 0));
  }

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hexdump是Linux下的一个二进制文件查看工具，可以将二进制文件转换为ASCII、10进制、16进制或8进制进行查看。

hexdump 参数
-C 每一字节以16进制显示，一行共16个字节，显示十六进制存储的文本内容
-b 每一字节以八进制显示，一行共16个字节，一行开始以十六进制显示偏移值；
  0000000 177 105 114 106 002 001 001 000 000 000 000 000 000 000 000 000
-c 每一字节以ASCII字符显示，其余同上；
  0000000 177 E L F 002 001 001 \0 \0 \0 \0 \0 \0 \0 \0 \0
-n 只解释指定长度字节
  单位：默认十进制，0x或0X开头则为16进制，0开头则为8进制。默认为字节，b则为512字节，k则为1024字节，m则为1048576字节
-d 双字节十进制显示
-o 双字节八进制显示
-v 去除中间显示的“*”字符
-x 双字节十六进制显示
-e 格式化参数

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实例版本与表字符集：

参考：https://www.percona.com/blog/2015/07/09/obtain-mysql-version-frm-file/

建表的实例版本0x033
语句hexdump -s 0x33 -n 2 -v -d table.frm 
[root@test1 ~]# hexdump -s 0x33 -n 2 -v -d /data/3308/test/test1.frm
0000033   50153
0000035
所以版本为5.1.53，因为5.1/5.5和5.6在字段类型定义上有不同，所以确定好建表实例版本很重要，字段类型定义见下面
 
表字符集0x026 
21=utf8
08=latin1
1c=GBK
语句hexdump -s 0x26 -n 1 table.frm

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frm列属性：

、列序号（初始列序号为4）
、字段长度，整形长度
、字段长度，latin1字符集字符类型长度，GBK字符集字符类型varchar长度*2，varchar(30)相当于就是60字节长度，换成16进制是3c，utf8字符集字符类型varchar长度*3，varchar(30)相当于就是90字节长度，换成16进制是5a
、
、
、
、
、Flags for zerofill, unsigned, etc.(int 1b)
、Additional flags,and scale if decimal/numeric(DEFAULT NULL 80,NOT NULL 40,DEFAULT &#39;VALUE&#39; 00)
、代码定义unireg_type，AUTO_INCREMENT of
、
、代码定义interval_nr
、字段类型
、字符集
、备注长度
、备注长度

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字段类型（注意5.6版本字段类型有不同，会影响数据恢复）：

Data type for v5.1&v5.5 (v5.6)
fe=char
fa=mediumtext
f6=decimal
fc=text
of=varchar
01=tinyint
02=smallint
03=int
04=float
05=real
07=timestamp (v5.6 11=timestamp)
08=bigint
09=mediumint
10=bit
ob=time (v5.6 13=time)
oc=datetime (v5.6 12=datetime)
0d=year
0e=date

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表中所含索引：

偏移量在0x1000之后的一段是frm索引部分，用hexdump -C打开后很容易找到
0x1000：有几个索引
0x1001：全部索引包含几个字段
索引名是明文，具体索引结构见示例。

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表：

CREATE TABLE `test3` (
  `a` int(11) NOT NULL,
  `b` varchar(10) DEFAULT NULL,
  `c` int(11) NOT NULL,
  PRIMARY KEY (`a`),
  UNIQUE KEY `uniq_1` (`b`,`c`),
  KEY `idx_1` (`c`,`b`),
  KEY `idx_2` (`c`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

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十六进制文件打开：

[root@test1 ~]# hexdump -C /data/3308/test/test3.frm 
00000000  fe 01 0a 0c 03 00 00 10  01 00 00 30 00 00 74 05  |...........0..t.|
00000010  28 00 00 00 00 00 00 00  00 00 00 02 79 00 09 00  |(...........y...|
00000020  00 05 00 00 00 00 21 00  00 00 00 00 00 00 00 74  |......!........t| #表字符集
00000030  05 00 00 e9 c3 00 00 10  00 00 00 00 00 00 00 00  |................| #标红的是建表实例版本号
00000040  2f 2f 00 00 20 00 00 00  00 00 00 00 00 00 00 00  |//.. ...........|
00000050  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00001000  04 06 00 00 1d 00 00 00  04 00 01 00 00 00 01 80  |................|
00001010  02 00 00 1b 40 04 00 68  00 22 00 02 00 00 00 02  |....@..h."......|
00001020  80 06 00 00 00 80 1e 00  03 80 25 00 00 1b 40 04  |..........%...@.|
00001030  00 69 00 22 00 02 00 00  00 03 80 25 00 00 1b 40  |.i.".......%...@|
00001040  04 00 02 80 06 00 00 00  80 1e 00 01 00 04 00 01  |................|
00001050  00 00 00 03 80 25 00 00  1b 40 04 00 ff 50 52 49  |.....%...@...PRI|
00001060  4d 41 52 59 ff 75 6e 69  71 5f 31 ff 69 64 78 5f  |MARY.uniq_1.idx_|
00001070  31 ff 69 64 78 5f 32 ff  00 00 00 00 00 00 00 00  |1.idx_2.........|
00001080  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00001570  00 00 00 00 ff 00 00 00  00 00 00 00 00 00 00 00  |................|
00001580  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00001590  00 00 00 00 00 00 00 00  00 00 00 00 00 00 06 00  |................|
000015a0  49 6e 6e 6f 44 42 00 00  00 00 00 00 00 00 00 00  |InnoDB..........|
000015b0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00002000  9a 01 00 10 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00002010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00002100  01 00 03 00 3f 00 34 00  00 00 28 00 08 00 00 00  |....?.4...(.....|
00002110  00 00 00 00 00 00 50 00  16 00 01 00 00 00 00 00  |......P.........|
00002120  3f 00 04 03 02 14 29 20  20 20 20 20 20 20 20 20  |?.....)         |
00002130  20 20 20 20 20 20 20 20  20 20 20 20 20 20 20 20  |                |
00002140  20 20 20 20 20 20 20 20  20 20 20 20 20 20 20 00  |               .|
00002150  04 00 02 61 00 05 00 02  62 00 06 00 02 63 00 04  |...a....b....c..|
00002160  02 0b 0b 00 02 00 00 1b  40 00 00 00 03 3f 00 00  |........@....?..|
00002170  05 02 1e 1e 00 06 00 00  00 80 00 00 00 0f 21 00  |..............!.|
00002180  00 06 02 0b 0b 00 25 00  00 1b 40 00 00 00 03 3f  |......%...@....?|
00002190  00 00 ff 61 ff 62 ff 63  ff 00                    |...a.b.c..|

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通过上面的颜色区分，圈出的黄色部分是索引属性，下面红蓝绿三色是三列属性。

列属性结构：

What file is mysql frm?

红色部分：字段序号（4开始，4、5、6就是字段第一第二第三）
蓝色部分：字段长度
棕色部分：是否为空
绿色部分：字段类型
黄色部分：字符集

索引属性结构：

What file is mysql frm?

索引头部：

淡蓝色部分：索引统计数
粉色部分：索引总共有多少列

索引主体：

棕色部分：是否唯一索引
红色部分：表中列的序号
绿色部分：表中对应列的属性

字段默认值：

字段默认值不保存在字段属性中，而是保存在描述表引擎的那段中
int类型默认值保存为十六进制需转换十进制，char类型默认值保存为十六进制文本可通过hexdump -C直接看到
如果没有索引段则默认值在，0x1011后，如果有索引段，则位置顺延
例如表
CREATE TABLE `test1` (
  `a` int(11) NOT NULL DEFAULT &#39;2010&#39;,
  `b` varchar(10) NOT NULL DEFAULT &#39;2011&#39; ,
  `c` int(11) default &#39;30&#39;,
  `d` varchar(10) NOT NULL DEFAULT &#39;Yes&#39; 
)engine=innodb default charset=utf8;
 
*
00001000  00 00 00 00 02 00 ff 00  00 00 00 00 00 00 00 00  |................|
00001010  fe da 07 00 00 04 32 30  31 31 00 00 00 00 00 00  |......2011......|
00001020  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00001030  00 00 00 00 1e 00 00 00  03 59 65 73 00 00 00 00  |.........Yes....|
00001040  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00001050  00 00 00 00 00 00 00 00  00 06 00 49 6e 6e 6f 44  |...........InnoD|
00001060  42 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |B...............|
00001070  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
column a:da 07 00 00
column b:04 32 30 31 31 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
column c:1e 00 00 00 
column d:03 59 65 73 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00需要注意char字段的默认值是根据字段长度和字符集相关的，如上表varchar(10)，utf8是3bit，就是30个十六进制长度。

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The process of starting MySQL in Docker consists of the following steps: Pull the MySQL image to create and start the container, set the root user password, and map the port verification connection Create the database and the user grants all permissions to the database

phpMyAdmin comprehensive use guide Apr 10, 2025 pm 10:42 PM

phpMyAdmin is not just a database management tool, it can give you a deep understanding of MySQL and improve programming skills. Core functions include CRUD and SQL query execution, and it is crucial to understand the principles of SQL statements. Advanced tips include exporting/importing data and permission management, requiring a deep security understanding. Potential issues include SQL injection, and the solution is parameterized queries and backups. Performance optimization involves SQL statement optimization and index usage. Best practices emphasize code specifications, security practices, and regular backups.

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