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背景：
用innodb_table_monitor来查看表内部的存储信息和索引结构是一个好的办法。再之前的MySQL 字符串主键和整型主键分析中提到了一些内容，但没有细讲，现在来好好的分析下。
使用方法：
建立一张 innodb_table_monitor的表：
create table innodb_table_monitor(a int)engine=innodb;
表建立之后，会每隔1m20s间隔把监控到的信息写到error日志中。要是停止监控的话，只需要删除表就可以了。注意：要是一直开启的话，错误日志会变的非常大。
日志的具体信息：
===========================================
121103 10:25:57 INNODB TABLE MONITOR OUTPUT
===========================================
--------------------------------------
TABLE: name SYS_FOREIGN, id 0 11, flags 0, columns 7, indexes 3, appr.rows 8
COLUMNS: ID: DATA_VARCHAR prtype 524292 len 0; FOR_NAME: DATA_VARCHAR prtype 524292 len 0; REF_NAME: DATA_VARCHAR prtype 524292 len 0; N_COLS: DATA_INT len 4; DB_ROW_ID: DATA_SYS prtype 256 len 6; DB_TRX_ID: DATA_SYS prtype 257 len 6; DB_ROLL_PTR: DATA_SYS prtype 258 len 7;
INDEX: name ID_IND, id 0 11, fields 1/6, uniq 1, type 3
root page 46, appr.key vals 8, leaf pages 1, size pages 1
FIELDS: ID DB_TRX_ID DB_ROLL_PTR FOR_NAME REF_NAME N_COLS
INDEX: name FOR_IND, id 0 12, fields 1/2, uniq 2, type 0
root page 47, appr.key vals 4, leaf pages 1, size pages 1
FIELDS: FOR_NAME ID
INDEX: name REF_IND, id 0 13, fields 1/2, uniq 2, type 0
root page 48, appr.key vals 3, leaf pages 1, size pages 1
FIELDS: REF_NAME ID
--------------------------------------
TABLE: name SYS_FOREIGN_COLS, id 0 12, flags 0, columns 7, indexes 1, appr.rows 8
COLUMNS: ID: DATA_VARCHAR prtype 524292 len 0; POS: DATA_INT len 4; FOR_COL_NAME: DATA_VARCHAR prtype 524292 len 0; REF_COL_NAME: DATA_VARCHAR prtype 524292 len 0; DB_ROW_ID: DATA_SYS prtype 256 len 6; DB_TRX_ID: DATA_SYS prtype 257 len 6; DB_ROLL_PTR: DATA_SYS prtype 258 len 7;
INDEX: name ID_IND, id 0 14, fields 2/6, uniq 2, type 3
root page 49, appr.key vals 8, leaf pages 1, size pages 1
FIELDS: ID POS DB_TRX_ID DB_ROLL_PTR FOR_COL_NAME REF_COL_NAME
--------------------------------------
上面显示，其中输出结构的开始部分会包含2张内部数据字典表用于维护外键的信息，表名为SYS_FOREIGN、SYS_FOREIGN_COLS。看看用户创建表的信息：
1，有主键的表（有数据）：
CREATE TABLE `test` (
`uid` char(36) NOT NULL DEFAULT ‘‘,
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
`status` tinyint(4) DEFAULT NULL,
PRIMARY KEY (`uid`),
KEY `idx_id` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 TABLE: name test/test, id 0 718, flags 1, columns 7, indexes 2, appr.rows 635787
2 COLUMNS: uid: DATA_MYSQL DATA_NOT_NULL len 108; id: DATA_INT DATA_BINARY_TYPE DATA_NOT_NULL len 4; name: DATA_VARMYSQL len 765; status: DATA_INT DATA_BINARY_TYPE len 1; DB_ROW_ID: DATA_SYS prtype 256 len 6; DB_TRX_ID: DATA_SYS prtype 257 len 6; DB_ROLL_PTR: DATA_SYS prtype 258 len 7;
3 INDEX: name PRIMARY, id 0 1387, fields 1/6, uniq 1, type 3
4 root page 3, appr.key vals 635787, leaf pages 4056, size pages 4078
5 FIELDS: uid DB_TRX_ID DB_ROLL_PTR id name status /*主键包含所有列*/
6 INDEX: name idx_id, id 0 1388, fields 1/2, uniq 2, type 0
7 root page 4, appr.key vals 638241, leaf pages 1834, size pages 1896
8 FIELDS: id uid /*表中定义的二级索引包含主键uid列*/
第一部分（1）：表信息：表名（test），表ID（718），列数（7），索引数（2），索性的基数<Cardinality>（625787，会变动）；
第二部分（2）：列信息：包括列名、列类型（DATA_XXXX）、是否NULL、以及列字段的长度。后面还有额外三列（主键ID，事务ID，回滚子指针） 总共7列；
• DATA_xxx(列类型)： These symbols indicate the data type. There may be multiple DATA_xxx symbols for a given column.
• prtype（列的数据类型的字符集编码，空性，符号性，以及是否它是一个二进制字符串）： The column‘s “precise” type. This field includes information such as the column data type, character set code, nullability,signedness, and whether it is a binary string. This field is described in the innobase/include/data0type.h source file.
• len（列字段的长度）：The column length in bytes.
• prec（类型的精确值）：The precision of the type.
第三部分（3～5）：索引信息：索引名、索引ID，fields m/n (m代表用户定义索引中的列数/n代表总的索引列数，其中包含附加的internal columns)，由于没有显示的定义主键或者非空的唯一索引，InnoDB会建表的时候自动的创建名字为GEN_CLUST_INDEX的Clustered Index。如果显示的定义一个主键的话，这个时候INDEX中name的值是：PRIMARY。以及 一些page信息和索引的基数信息。最后则是被索引的列，和 fields m/n 里的n对应。
• type（索引类型：聚集索引1，唯一索引2，普通索引0，既聚集又唯一3）： The index type. This is a bit field. For example, 1 indicates a clustered index and 2 indicates a unique index, so a clustered index (which always contains unique values), will have a type value of 3. An index with a type value of 0 is neither clustered nor unique. The flag values are defined in the innobase/include/dict0mem.h source file.
• root page（索引节点：根节点）： The index root page number.
• appr. key vals（索引的基数）： The approximate index cardinality.
• leaf page（页节点大小）： number of leaf pages in the index.
• size pages（总页大小）： The approximages: The approate total number of pages in the index.
• FIELDS（被索引的列，主键包含所有列以及隐藏列，二级索引包含主键）： The names of the fields in the index. For a clustered index that was generated automatically, the field list begins with the internal DB_ROW_ID (row ID) field. DB_TRX_ID and DB_ROLL_PTR are always added internally to the clustered index, following the fields that comprise the primary key. For a secondary index, the final fields are those from the primary key that are not part of the secondary index.
2，上面的表是有主键的，看看无主键的表（没有数据）：
CREATE TABLE `test1` (
`uid` char(36) NOT NULL DEFAULT ‘‘,
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
`status` tinyint(4) DEFAULT NULL,
KEY `idx_id` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 TABLE: name test/test1, id 0 820, flags 1, columns 7, indexes 2, appr.rows 0
2 COLUMNS: uid: DATA_MYSQL DATA_NOT_NULL len 108; id: DATA_INT DATA_BINARY_TYPE DATA_NOT_NULL len 4; name: DATA_VARMYSQL len 765; status: DATA_INT DATA_BINARY_TYPE len 1; DB_ROW_ID: DATA_SYS prtype 256 len 6; DB_TRX_ID: DATA_SYS prtype 257 len 6; DB_ROLL_PTR: DATA_SYS prtype 258 len 7;
3 INDEX: name GEN_CLUST_INDEX, id 0 1524, fields 0/7, uniq 1, type 1 /*1 说明只是主键，不是唯一*/
4 root page 3, appr.key vals 0, leaf pages 1, size pages 1
5 FIELDS: DB_ROW_ID DB_TRX_ID DB_ROLL_PTR uid id name status
6 INDEX: name idx_id, id 0 1525, fields 1/2, uniq 2, type 0
7 root page 4, appr.key vals 0, leaf pages 1, size pages 1
8 FIELDS: id DB_ROW_ID
1和2对比异同：
第一行和第二行他们的信息是一样的。
第三行出现不一致，因为1的表是有主键的，而2的表没有主键，2会自动生成一个Row_ID来代替，而导致第五行也不一致，因为2比1多了一个Row_ID(6个字节）。所以2的表中被索引的列有7个，而1只有6个，2没有定义索引列，而1定义了一个（fields）。因为二级索引包含主键列（第8行），主键不一样了，导致二级索引的列也会不一样。
3，测试有没有主键表大小问题：
CREATE TABLE `test2` (
`uid` char(36) NOT NULL DEFAULT ‘‘,
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
`status` tinyint(4) DEFAULT NULL,
PRIMARY KEY `idx_id` (`id`)
) ENGINE=InnoDB; /*有自增主键*/
CREATE TABLE `test3` (
`uid` char(36) NOT NULL DEFAULT ‘‘,
`id` int(11) NOT NULL,
`name` varchar(255) DEFAULT NULL,
`status` tinyint(4) DEFAULT NULL
) ENGINE=InnoDB; /*无主键*/
CREATE TABLE `test4` (
`uid` char(36) NOT NULL DEFAULT ‘‘,
`id` int(11) NOT NULL,
`name` varchar(255) DEFAULT NULL,
`status` tinyint(4) DEFAULT NULL,
PRIMARY KEY (`uid`)
) ENGINE=InnoDB; /*有字符串主键*/
--------------------------------------
TABLE: name test/test2, id 0 824, flags 1, columns 7, indexes 1, appr.rows 625647
COLUMNS: uid: DATA_MYSQL DATA_NOT_NULL len 108; id: DATA_INT DATA_BINARY_TYPE DATA_NOT_NULL len 4; name: DATA_VARMYSQL len 765; status: DATA_INT DATA_BINARY_TYPE len 1; DB_ROW_ID: DATA_SYS prtype 256 len 6; DB_TRX_ID: DATA_SYS prtype 257 len 6; DB_ROLL_PTR: DATA_SYS prtype 258 len 7;
INDEX: name PRIMARY, id 0 1532, fields 1/6, uniq 1, type 3
root page 3, appr.key vals 625647, leaf pages 4056, size pages 4070
FIELDS: id DB_TRX_ID DB_ROLL_PTR uid name status
--------------------------------------
TABLE: name test/test3, id 0 825, flags 1, columns 7, indexes 1, appr.rows 617020
COLUMNS: uid: DATA_MYSQL DATA_NOT_NULL len 108; id: DATA_INT DATA_BINARY_TYPE DATA_NOT_NULL len 4; name: DATA_VARMYSQL len 765; status: DATA_INT DATA_BINARY_TYPE len 1; DB_ROW_ID: DATA_SYS prtype 256 len 6; DB_TRX_ID: DATA_SYS prtype 257 len 6; DB_ROLL_PTR: DATA_SYS prtype 258 len 7;
INDEX: name GEN_CLUST_INDEX, id 0 1533, fields 0/7, uniq 1, type 1
root page 3, appr.key vals 617020, leaf pages 4311, size pages 4326
FIELDS: DB_ROW_ID DB_TRX_ID DB_ROLL_PTR uid id name status
--------------------------------------
TABLE: name test/test4, id 0 826, flags 1, columns 7, indexes 1, appr.rows 655560
COLUMNS: uid: DATA_MYSQL DATA_NOT_NULL len 108; id: DATA_INT DATA_BINARY_TYPE DATA_NOT_NULL len 4; name: DATA_VARMYSQL len 765; status: DATA_INT DATA_BINARY_TYPE len 1; DB_ROW_ID: DATA_SYS prtype 256 len 6; DB_TRX_ID: DATA_SYS prtype 257 len 6; DB_ROLL_PTR: DATA_SYS prtype 258 len 7;
INDEX: name PRIMARY, id 0 1534, fields 1/6, uniq 1, type 3
root page 3, appr.key vals 655560, leaf pages 4056, size pages 4078
FIELDS: uid DB_TRX_ID DB_ROLL_PTR id name status
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原文地址：http://www.cnblogs.com/conanwang/p/5946250.html
