MySQL中dd::columns表结构转table过程及应用详解

吾爱主题阅读：149 2022-11-15 17:03:00 评论：0

一、MySQL的dd表介绍

MySQL的dd表是用来存放表结构和各种建表信息的，客户端建的表都存在mysql.table和mysql.columns表里，还有一个表mysql.column_type_elements比较特殊，用来存放SET和ENUM类型的字段集合值信息。看一下下面这张表的mysql.columns表和mysql.column_type_elements信息。为了缩短显示长度，这里只展示几个重要的值。

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 #建表： CREATE TABLE t1(id int not null auto_increment primary key ,col1 number,col2 VARCHAR (100),col3 pls_integer, col4 enum( 'x' , 'y' ) default 'x' ,col5 set ( 'x1' , 'y1' )) partition by hash(id) partitions 3; SET SESSION debug= '+d,skip_dd_table_access_check' ; mysql> select name ,ordinal_position,type,default_value_utf8,options,column_type_utf8 from mysql.columns where table_id=383; + -------------+------------------+-----------------------+--------------------+-------------------+------------------+ | name | ordinal_position | type | default_value_utf8 | options | column_type_utf8 | + -------------+------------------+-----------------------+--------------------+-------------------+------------------+ | col1 | 2 | MYSQL_TYPE_NEWDECIMAL | NULL | interval_count=0; | decimal (65,0) | | col2 | 3 | MYSQL_TYPE_VARCHAR | NULL | interval_count=0; | varchar (100) | | col3 | 4 | MYSQL_TYPE_LONG | NULL | interval_count=0; | int | | col4 | 5 | MYSQL_TYPE_ENUM | x | interval_count=2; | enum( 'x' , 'y' ) | | col5 | 6 | MYSQL_TYPE_SET | NULL | interval_count=2; | set ( 'x1' , 'y1' ) | | DB_ROLL_PTR | 8 | MYSQL_TYPE_LONGLONG | NULL | NULL | | | DB_TRX_ID | 7 | MYSQL_TYPE_INT24 | NULL | NULL | | | id | 1 | MYSQL_TYPE_LONG | NULL | interval_count=0; | int | + -------------+------------------+-----------------------+--------------------+-------------------+------------------+ 8 rows in set (0.00 sec)

mysql.columns表说明如下：

ordinal_position是该字段在表里的偏移量，这里多了3个字段，DB_ROLL_PTR、DB_TRX_ID、id是用来执行undo的，记录了字段的版本信息。

default_value_utf8是用来保存默认值的。options里面有interval_count用来保存集合类型的数值数的。columns表的options的key一共有如下几种：

1 2 3 static const std:: set <String_type> default_valid_option_keys = { "column_format" , "geom_type" , "interval_count" , "not_secondary" , "storage" , "treat_bit_as_char" , "zip_dict_id" , "is_array" };

1 2 3 4 5 6 7 8 9 mysql> select * from mysql.column_type_elements where column_id=4286; + -----------+---------------+------+ | column_id | element_index | name | + -----------+---------------+------+ | 4286 | 1 | x | | 4286 | 2 | y | + -----------+---------------+------+ 2 rows in set (0.01 sec) #这里的column_id=4286是col4的id值，x和y分别对应了 set 定义时候的2个集合值。

二、代码跟踪

现在重新启动数据库，跟踪一下这个columns表怎么转为代码里面的TABLE的field对象。首先找到表的dd信息然后打开表获取field信息。

1	`mysql>` `select` `*` `from` `t1;`

输入该命令后找到columns表转为field的代码：

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 #0 fill_column_from_dd ( thd=0x555558b35a06 <std::char_traits<char>::compare(char const*, char const*, unsigned long)+61>, share=0x7fffe83f1b60, col_obj=0x555558bb0a5e <std::__cxx11::basic_string<char, std::char_traits<char>, Stateless_allocator<char, dd ::String_type_alloc, My_free_functor> >::compare(std::__cxx11::basic_string<char, std::char_traits<char>, Stateless_allocator<char, dd ::String_type_alloc, My_free_functor> > const&) const+142>, null_pos=0x7fffe83f1880 "\005" , null_bit_pos=32767, rec_pos=0x7fff2c05ac10 "explicit_encryption" , field_nr=0) at /mysql/sql/dd_table_share .cc:955 #1 0x00005555593c4c17 in fill_columns_from_dd (thd=0x7fff2c006890, share=0x7fff2cbf19e8, tab_obj=0x7fff2cbb9b38) at /mysql/sql/dd_table_share .cc:1235 #2 0x00005555593c9e54 in open_table_def (thd=0x7fff2c006890, share=0x7fff2cbf19e8, table_def=...) at /mysql/sql/dd_table_share .cc:2408 #3 0x0000555558e76a13 in get_table_share (thd=0x7fff2c006890, db=0x7fff2cbeeff0 "db1", table_name=0x7fff2cc03210 "t1" , key=0x7fff2cbeed87 "db1" , key_length=7, open_view= true , open_secondary= false ) at /mysql/sql/sql_base .cc:801 #4 0x0000555558e76f08 in get_table_share_with_discover (thd=0x7fff2c006890, table_list=0x7fff2cbee9b8, key=0x7fff2cbeed87 "db1" , key_length=7, open_secondary= false , error=0x7fffe83f1ea4) at /mysql/sql/sql_base .cc:889 #5 0x0000555558e7cd34 in open_table (thd=0x7fff2c006890, table_list=0x7fff2cbee9b8, ot_ctx=0x7fffe83f2380) at /mysql/sql/sql_base .cc:3230 #6 0x0000555558e81769 in open_and_process_table (thd=0x7fff2c006890, lex=0x7fff2c01bdf0, tables=0x7fff2cbee9b8, counter=0x7fff2c01be48, prelocking_strategy=0x7fffe83f2408, has_prelocking_list= false , ot_ctx=0x7fffe83f2380) at /mysql/sql/sql_base .cc:5118 #7 0x0000555558e833bd in open_tables (thd=0x7fff2c006890, start=0x7fffe83f23f0, counter=0x7fff2c01be48, flags=0, prelocking_strategy=0x7fffe83f2408) at /mysql/sql/sql_base .cc:5928 #8 0x0000555558e85626 in open_tables_for_query (thd=0x7fff2c006890, tables=0x7fff2cbee9b8, flags=0) at /mysql/sql/sql_base .cc:6904 #9 0x0000555559075720 in Sql_cmd_dml::prepare (this=0x7fff2cbef400, thd=0x7fff2c006890) at /mysql/sql/sql_select .cc:372 #10 0x00005555590760bf in Sql_cmd_dml::execute (this=0x7fff2cbef400, thd=0x7fff2c006890) at /mysql/sql/sql_select .cc:527 #11 0x0000555558fedc8e in mysql_execute_command (thd=0x7fff2c006890, first_level=true) at /mysql/sql/sql_parse .cc:4794 #12 0x0000555558fefe25 in dispatch_sql_command (thd=0x7fff2c006890, parser_state=0x7fffe83f3990, update_userstat= false ) at /mysql/sql/sql_parse .cc:5399 #13 0x0000555558fe52d3 in dispatch_command (thd=0x7fff2c006890, com_data=0x7fffe83f4b70, command =COM_QUERY) at /mysql/sql/sql_parse .cc:2000 #14 0x0000555558fe3643 in do_command (thd=0x7fff2c006890) at /mysql/sql/sql_parse .cc:1448 #15 0x000055555920e200 in handle_connection (arg=0x555560a65790) at /mysql/sql/conn_handler/connection_handler_per_thread .cc:307 #16 0x000055555ae36375 in pfs_spawn_thread (arg=0x5555608a2e20) at /mysql/storage/perfschema/pfs .cc:2899 #17 0x00007ffff77a6609 in start_thread (arg=<optimized out>) at pthread_create.c:477 #18 0x00007ffff76cb163 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

1 2	`<strong>` `#fill_column_from_dd函数里面最重要的是make_field函数，把字段从dd::Column转为table的field然后赋值给TABLE_SHARE。` `reg_field = make_field(*col_obj, charset, share, rec_pos, null_pos, null_bit_pos);<` `/strong` `>`

三、知识应用

session每次获取表的信息都是在第一次打开表的时候就做好了，下次如果表没有变化就从Table_cache直接获取表信息。现在假设我们要改col4字段的集合值又不想通过alter语句来执行，那就可以直接对dd表进行操作。注意，该操作对生产环境有很大风险，这里只用来进行知识实践，不能用来在生产环境实际操作。

把col4的x,y值改为a,b：首先试着插入col4=x的记录，现在还没更改dd表所以插入成功。

1 2	`mysql>` `insert` `into` `t1` `values` `(1,1,` `'aa'` `,1,` `'x'` `,` `'x1'` `);` `Query OK, 1 row affected (0.03 sec)`

接着开始改col4的集合值：

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 mysql> SET SESSION debug= '+d,skip_dd_table_access_check' ; Query OK, 0 rows affected (0.02 sec) mysql> update mysql.columns set default_value_utf8= 'a' ,column_type_utf8= 'enum(\'a\',\'b\'))' where table_id=383 and name = 'col4' ; Query OK, 1 row affected (0.03 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> select name ,ordinal_position,type,default_value_utf8,options,column_type_utf8 from mysql.columns where table_id=383; + -------------+------------------+-----------------------+--------------------+-------------------+------------------+ | name | ordinal_position | type | default_value_utf8 | options | column_type_utf8 | + -------------+------------------+-----------------------+--------------------+-------------------+------------------+ | col1 | 2 | MYSQL_TYPE_NEWDECIMAL | NULL | interval_count=0; | decimal (65,0) | | col2 | 3 | MYSQL_TYPE_VARCHAR | NULL | interval_count=0; | varchar (100) | | col3 | 4 | MYSQL_TYPE_LONG | NULL | interval_count=0; | int | | col4 | 5 | MYSQL_TYPE_ENUM | a | interval_count=2; | enum( 'a' , 'b' )) |集合值已改 | col5 | 6 | MYSQL_TYPE_SET | NULL | interval_count=2; | set ( 'x1' , 'y1' ) | | DB_ROLL_PTR | 8 | MYSQL_TYPE_LONGLONG | NULL | NULL | | | DB_TRX_ID | 7 | MYSQL_TYPE_INT24 | NULL | NULL | | | id | 1 | MYSQL_TYPE_LONG | NULL | interval_count=0; | int | + -------------+------------------+-----------------------+--------------------+-------------------+------------------+ 8 rows in set (0.00 sec) mysql> update mysql.column_type_elements set name = 'a' where column_id=4286 and element_index=1; Query OK, 1 row affected (0.03 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> update mysql.column_type_elements set name = 'b' where column_id=4286 and element_index=2; Query OK, 1 row affected (0.03 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> select * from mysql.column_type_elements where column_id=4286; + -----------+---------------+------+ | column_id | element_index | name | + -----------+---------------+------+ | 4286 | 1 | a |集合值已改 | 4286 | 2 | b |集合值已改 + -----------+---------------+------+ 2 rows in set (0.00 sec)

现在再插入一条col4=x的记录发现还是成功的，这是因为t1没有重新从dd表转为TABLE信息，需要重启后再试。

1 2	`mysql>` `insert` `into` `t1` `values` `(2,1,` `'aa'` `,1,` `'x'` `,` `'x1'` `);` `Query OK, 1 row affected (0.02 sec)`

重启数据库，然后登录。再次插入col4=x发现报错了，因为这时候的TABLE信息是重新从dd表转化的。

1 2	`mysql>` `insert` `into` `t1` `values` `(2,1,` `'aa'` `,1,` `'x'` `,` `'x1'` `);` `ERROR 1265 (01000): Data truncated` `for` `column` `'col4'` `at` `row 1`

插入col4=a的记录成功，说明更改成功。

1 2	`mysql>` `insert` `into` `t1` `values` `(3,1,` `'aa'` `,1,` `'a'` `,` `'x1'` `);` `Query OK, 1 row affected (0.02 sec)`

查看建表信息,发现已经成功更改。

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 mysql> show create table t1; + -------+-------------------------+ | Table | Create Table | + -------+-------------------------+ | t1 | CREATE TABLE `t1` ( `id` int NOT NULL AUTO_INCREMENT, `col1` decimal (65,0) DEFAULT NULL , `col2` varchar (100) DEFAULT NULL , `col3` int DEFAULT NULL , `col4` enum( 'a' , 'b' ) DEFAULT 'a' ,更改成功 `col5` set ( 'x1' , 'y1' ) DEFAULT NULL , PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8mb4 COLLATE =utf8mb4_0900_ai_ci /*!50100 PARTITION BY HASH (`id`) PARTITIONS 3 */ + -------+-------------------------+