MySQL中dd::columns表結(jié)構(gòu)轉(zhuǎn)table過程及應用詳解

更新時間：2022年09月21日 10:40:35 作者：GreatSQL社區(qū)

MySQL的dd表是用來存放表結(jié)構(gòu)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，對mysql dd::columns表結(jié)構(gòu)相關(guān)知識感興趣的朋友一起看看吧

一、MySQL的dd表介紹

MySQL的dd表是用來存放表結(jié)構(gòu)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?？匆幌孪旅孢@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(zhí)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(shù)值數(shù)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)
<strong>#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。</strong>

二、代碼跟蹤

現(xiàn)在重新啟動數(shù)據(jù)庫，跟蹤一下這個columns表怎么轉(zhuǎn)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(zhuǎn)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

<strong>#fill_column_from_dd函數(shù)里面最重要的是make_field函數(shù)，把字段從dd::Column轉(zhuǎn)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);</strong>

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(xiàn)在假設(shè)我們要改col4字段的集合值又不想通過alter語句來執(zhí)行，那就可以直接對dd表進行操作。注意，該操作對生產(chǎn)環(huán)境有很大風險，這里只用來進行知識實踐，不能用來在生產(chǎn)環(huán)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(xiàn)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: 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    |<strong>集合值已改</strong>
|      4286 |             2 | b    |<strong>集合值已改</strong>
+-----------+---------------+------+
2 rows in set (0.00 sec)

現(xiàn)在再插入一條col4=x的記錄發(fā)現(xiàn)還是成功的，這是因為t1沒有重新從dd表轉(zhuǎn)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(shù)據(jù)庫，然后登錄。再次插入col4=x發(fā)現(xiàn)報錯了，因為這時候的TABLE信息是重新從dd表轉(zhuǎn)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(fā)現(xiàn)已經(jīng)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(jié)

實際上更改表結(jié)構(gòu)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(xiàn)表結(jié)構(gòu)的變更，這里只是從更底層來介紹。以上的操作在實際生產(chǎn)中不能直接操作，風險很大，會影響現(xiàn)有的記錄和相關(guān)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。

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