一、MySQL的dd表介紹

二、代碼跟蹤

三、知識應用

四、總結

一、MySQL的dd表介紹
MySQL的dd表是用來存放表結構和各種建表信息的，客戶端建的表都存在mysql.table和mysql.columns表里，還有一個表mysql.column_type_elements比較特殊，用來存放SET和ENUM類型的字段集合值信息。看一下下面這張表的mysql.columns表和mysql.column_type_elements信息。為了縮短顯示長度，這里只展示幾個重要的值。

#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表說明如下：
ordinal_position 是該字段在表里的偏移量，這里多了3個字段， DB_ROLL_PTR 、 DB_TRX_ID 、 id 是用來執行undo的，記錄了字段的版本信息。

default_value_utf8 是用來保存默認值的。options里面有interval_count用來保存集合類型的數值數的。columns表的options的key一共有如下幾種：

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"}; 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)
#這里的column_id=4286是col4的id值，x和y分別對應了set定義時候的2個集合值。
二、代碼跟蹤
現在重新啟動數據庫，跟蹤一下這個columns表怎么轉為代碼里面的TABLE的field對象。首先找到表的dd信息然后打開表獲取field信息。

mysql> select * from t1;

輸入該命令后找到columns表轉為field的代碼：

#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 #fill_column_from_dd函數里面最重要的是make_field函數，把字段從dd::Column轉為table的field然后賦值給TABLE_SHARE。reg_field = make_field(*col_obj, charset, share, rec_pos, null_pos, null_bit_pos);

三、知識應用
session每次獲取表的信息都是在第一次打開表的時候就做好了，下次如果表沒有變化就從Table_cache直接獲取表信息?，F在假設我們要改col4字段的集合值又不想通過alter語句來執行，那就可以直接對dd表進行操作。注意，該操作對生產環境有很大風險，這里只用來進行知識實踐，不能用來在生產環境實際操作。

把col4的x,y值改為a,b：首先試著插入col4=x的記錄，現在還沒更改dd表所以插入成功。

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

接著開始改col4的集合值：

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: 0mysql> 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: 0mysql> 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: 0mysql> 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)

現在再插入一條col4=x的記錄發現還是成功的，這是因為t1沒有重新從dd表轉為TABLE信息，需要重啟后再試。

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

重啟數據庫，然后登錄。再次插入col4=x發現報錯了，因為這時候的TABLE信息是重新從dd表轉化的。

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的記錄成功，說明更改成功。

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,發現已經成功更改。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 */ +-------+-------------------------+

四、總結
實際上更改表結構如果通過alter命令來改流程跟上面也是一樣的，也是通過更新dd表來實現表結構的變更，這里只是從更底層來介紹。以上的操作在實際生產中不能直接操作，風險很大，會影響現有的記錄和相關的功能。這里只是作為一個案例來更好的說明dd的工作流程，幫助大家遇到問題知道怎么從底層排查。

總結

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