前言

事務(wù)性數(shù)據(jù)字典與原子DDL，是MySQL 8.0推出的兩個(gè)非常重要的新特性，之所以將這兩個(gè)新特性放在一起，是因?yàn)閮烧呙芮邢嚓P(guān)，事務(wù)性數(shù)據(jù)字典是前提，原子DDL是一個(gè)重要應(yīng)用場(chǎng)景。

MySQL 8.0之前的數(shù)據(jù)字典

MySQL 8.0之前的數(shù)據(jù)字典，主要由以下三部分組成：

（1）操作系統(tǒng)文件

db.opt：數(shù)據(jù)庫元數(shù)據(jù)信息frm：表元數(shù)據(jù)信息par：表分區(qū)元數(shù)據(jù)信息TRN/TRG：觸發(fā)器元數(shù)據(jù)信息ddl_log.log：DDL過程中產(chǎn)生的元數(shù)據(jù)信息

（2）mysql庫下的非InnoDB系統(tǒng)表

mysql> select table_schema,table_name,table_type,engine from information_schema.tables where table_schema=’mysql’ and engine<>’InnoDB’;+--------------+------------------+------------+--------+| table_schema | table_name | table_type | engine |+--------------+------------------+------------+--------+| mysql | columns_priv | BASE TABLE | MyISAM || mysql | db| BASE TABLE | MyISAM || mysql | event | BASE TABLE | MyISAM || mysql | func | BASE TABLE | MyISAM || mysql | general_log | BASE TABLE | CSV || mysql | ndb_binlog_index | BASE TABLE | MyISAM || mysql | proc | BASE TABLE | MyISAM || mysql | procs_priv | BASE TABLE | MyISAM || mysql | proxies_priv | BASE TABLE | MyISAM || mysql | slow_log | BASE TABLE | CSV || mysql | tables_priv | BASE TABLE | MyISAM || mysql | user | BASE TABLE | MyISAM |+--------------+------------------+------------+--------+12 rows in set (0.00 sec)

（3）mysql庫下的InnoDB系統(tǒng)表

mysql> select table_schema,table_name,table_type,engine from information_schema.tables where table_schema=’mysql’ and engine=’InnoDB’;+--------------+---------------------------+------------+--------+| table_schema | table_name| table_type | engine |+--------------+---------------------------+------------+--------+| mysql | engine_cost| BASE TABLE | InnoDB || mysql | gtid_executed | BASE TABLE | InnoDB || mysql | help_category | BASE TABLE | InnoDB || mysql | help_keyword | BASE TABLE | InnoDB || mysql | help_relation | BASE TABLE | InnoDB || mysql | help_topic| BASE TABLE | InnoDB || mysql | innodb_index_stats | BASE TABLE | InnoDB || mysql | innodb_table_stats | BASE TABLE | InnoDB || mysql | plugin | BASE TABLE | InnoDB || mysql | server_cost| BASE TABLE | InnoDB || mysql | servers | BASE TABLE | InnoDB || mysql | slave_master_info | BASE TABLE | InnoDB || mysql | slave_relay_log_info | BASE TABLE | InnoDB || mysql | slave_worker_info | BASE TABLE | InnoDB || mysql | time_zone | BASE TABLE | InnoDB || mysql | time_zone_leap_second | BASE TABLE | InnoDB || mysql | time_zone_name | BASE TABLE | InnoDB || mysql | time_zone_transition | BASE TABLE | InnoDB || mysql | time_zone_transition_type | BASE TABLE | InnoDB |+--------------+---------------------------+------------+--------+19 rows in set (0.00 sec)

我們可以看到，數(shù)據(jù)字典被分布到多個(gè)地方，一方面不利于元數(shù)據(jù)統(tǒng)一管理，另一方面容易造成數(shù)據(jù)的不一致(由于操作系統(tǒng)文件、非InnoDB系統(tǒng)表均不支持事務(wù)，執(zhí)行DDL操作無法保證ACID)。

MySQL 8.0的數(shù)據(jù)字典

為了解決上述問題，MySQL 8.0將數(shù)據(jù)字典統(tǒng)一改進(jìn)為InnoDB存儲(chǔ)引擎存儲(chǔ)，具體分為兩部分：

（1）數(shù)據(jù)字典表：存放最重要的元數(shù)據(jù)信息，位于mysql庫下，存儲(chǔ)在mysql共享表空間(mysql.ibd)

（2）其他系統(tǒng)表：存放輔助的元數(shù)據(jù)信息，位于mysql庫下，存儲(chǔ)在mysql共享表空間(mysql.ibd)

數(shù)據(jù)字典表

數(shù)據(jù)字典表是不可見，既不能通過select訪問，也不會(huì)出現(xiàn)在show tables或information.schema.tables結(jié)果里；嘗試訪問會(huì)報(bào)以下錯(cuò)誤：

mysql> select * from mysql.tables limit 10;ERROR 3554 (HY000): Access to data dictionary table ’mysql.tables’ is rejected.

不過，在debug模式下，是可以訪問這些隱藏的數(shù)據(jù)字典表的；我們重新編譯安裝(過程略)，并以debug模式啟動(dòng)進(jìn)程，再次嘗試訪問，結(jié)果如下：

mysql> SET SESSION debug=’+d,skip_dd_table_access_check’;mysql> SELECT name, schema_id, hidden, type FROM mysql.tables where schema_id=1 AND hidden=’System’;+------------------------------+-----------+--------+------------+| name | schema_id | hidden | type |+------------------------------+-----------+--------+------------+| catalogs | 1 | System | BASE TABLE || character_sets| 1 | System | BASE TABLE || check_constraints | 1 | System | BASE TABLE || collations | 1 | System | BASE TABLE || column_statistics | 1 | System | BASE TABLE || column_type_elements | 1 | System | BASE TABLE || columns | 1 | System | BASE TABLE || dd_properties| 1 | System | BASE TABLE || events | 1 | System | BASE TABLE || foreign_key_column_usage | 1 | System | BASE TABLE || foreign_keys | 1 | System | BASE TABLE || index_column_usage | 1 | System | BASE TABLE || index_partitions | 1 | System | BASE TABLE || index_stats | 1 | System | BASE TABLE || indexes | 1 | System | BASE TABLE || innodb_ddl_log| 1 | System | BASE TABLE || innodb_dynamic_metadata | 1 | System | BASE TABLE || parameter_type_elements | 1 | System | BASE TABLE || parameters | 1 | System | BASE TABLE || resource_groups | 1 | System | BASE TABLE || routines | 1 | System | BASE TABLE || schemata | 1 | System | BASE TABLE || st_spatial_reference_systems | 1 | System | BASE TABLE || table_partition_values | 1 | System | BASE TABLE || table_partitions | 1 | System | BASE TABLE || table_stats | 1 | System | BASE TABLE || tables | 1 | System | BASE TABLE || tablespace_files | 1 | System | BASE TABLE || tablespaces | 1 | System | BASE TABLE || triggers | 1 | System | BASE TABLE || view_routine_usage | 1 | System | BASE TABLE || view_table_usage | 1 | System | BASE TABLE |+------------------------------+-----------+--------+------------+32 rows in set (0.01 sec)

其他系統(tǒng)表

其他系統(tǒng)表，可以通過show tables或information_schema.tables查看，均以改進(jìn)為InnoDB存儲(chǔ)引擎(general_log、slow_log例外，這兩張表并未記錄元數(shù)據(jù)信息，只是用于記錄日志)：

mysql> select table_schema,table_name,engine from information_schema.tables where table_schema=’mysql’;+--------------+---------------------------+--------+| TABLE_SCHEMA | TABLE_NAME| ENGINE |+--------------+---------------------------+--------+| mysql | columns_priv | InnoDB || mysql | component | InnoDB || mysql | db | InnoDB || mysql | default_roles | InnoDB || mysql | engine_cost| InnoDB || mysql | func | InnoDB || mysql | general_log| CSV || mysql | global_grants | InnoDB || mysql | gtid_executed | InnoDB || mysql | help_category | InnoDB || mysql | help_keyword | InnoDB || mysql | help_relation | InnoDB || mysql | help_topic| InnoDB || mysql | innodb_index_stats | InnoDB || mysql | innodb_table_stats | InnoDB || mysql | password_history | InnoDB || mysql | plugin | InnoDB || mysql | procs_priv| InnoDB || mysql | proxies_priv | InnoDB || mysql | role_edges| InnoDB || mysql | server_cost| InnoDB || mysql | servers | InnoDB || mysql | slave_master_info | InnoDB || mysql | slave_relay_log_info | InnoDB || mysql | slave_worker_info | InnoDB || mysql | slow_log | CSV || mysql | tables_priv| InnoDB || mysql | time_zone | InnoDB || mysql | time_zone_leap_second | InnoDB || mysql | time_zone_name | InnoDB || mysql | time_zone_transition | InnoDB || mysql | time_zone_transition_type | InnoDB || mysql | user | InnoDB |+--------------+---------------------------+--------+33 rows in set (0.00 sec)

數(shù)據(jù)字典視圖

剛剛提到，數(shù)據(jù)字典表只能在debug模式下訪問，那么在生產(chǎn)環(huán)境中，我們應(yīng)該怎么去獲取元數(shù)據(jù)信息呢？答案是通過information_schema庫下的數(shù)據(jù)字典視圖。和Oracle數(shù)據(jù)庫的設(shè)計(jì)理念一樣，將元數(shù)據(jù)信息存放在基表中(x$、$)，然后通過視圖(v$、dba_/all_/user_)的方式提供給用戶查詢；MySQL數(shù)據(jù)庫也是如此，將元數(shù)據(jù)信息存放在mysql庫的數(shù)據(jù)字典表中隱藏起來，然后提供information_schema庫視圖給用戶查詢：

mysql> select table_schema,table_name,table_type,engine from information_schema.tables where table_schema=’information_schema’; +--------------------+---------------------------------------+-------------+--------+| TABLE_SCHEMA | TABLE_NAME | TABLE_TYPE | ENGINE |+--------------------+---------------------------------------+-------------+--------+| information_schema | ADMINISTRABLE_ROLE_AUTHORIZATIONS | SYSTEM VIEW | NULL || information_schema | APPLICABLE_ROLES | SYSTEM VIEW | NULL || information_schema | CHARACTER_SETS | SYSTEM VIEW | NULL || information_schema | CHECK_CONSTRAINTS | SYSTEM VIEW | NULL || information_schema | COLLATION_CHARACTER_SET_APPLICABILITY | SYSTEM VIEW | NULL || information_schema | COLLATIONS | SYSTEM VIEW | NULL || information_schema | COLUMN_PRIVILEGES | SYSTEM VIEW | NULL || information_schema | COLUMN_STATISTICS | SYSTEM VIEW | NULL || information_schema | COLUMNS| SYSTEM VIEW | NULL || information_schema | ENABLED_ROLES | SYSTEM VIEW | NULL || information_schema | ENGINES| SYSTEM VIEW | NULL || information_schema | EVENTS| SYSTEM VIEW | NULL || information_schema | FILES | SYSTEM VIEW | NULL || information_schema | INNODB_BUFFER_PAGE | SYSTEM VIEW | NULL || information_schema | INNODB_BUFFER_PAGE_LRU| SYSTEM VIEW | NULL || information_schema | INNODB_BUFFER_POOL_STATS | SYSTEM VIEW | NULL || information_schema | INNODB_CACHED_INDEXES | SYSTEM VIEW | NULL || information_schema | INNODB_CMP | SYSTEM VIEW | NULL || information_schema | INNODB_CMP_PER_INDEX | SYSTEM VIEW | NULL || information_schema | INNODB_CMP_PER_INDEX_RESET | SYSTEM VIEW | NULL || information_schema | INNODB_CMP_RESET | SYSTEM VIEW | NULL || information_schema | INNODB_CMPMEM | SYSTEM VIEW | NULL || information_schema | INNODB_CMPMEM_RESET | SYSTEM VIEW | NULL || information_schema | INNODB_COLUMNS | SYSTEM VIEW | NULL || information_schema | INNODB_DATAFILES | SYSTEM VIEW | NULL || information_schema | INNODB_FIELDS | SYSTEM VIEW | NULL || information_schema | INNODB_FOREIGN | SYSTEM VIEW | NULL || information_schema | INNODB_FOREIGN_COLS | SYSTEM VIEW | NULL || information_schema | INNODB_FT_BEING_DELETED| SYSTEM VIEW | NULL || information_schema | INNODB_FT_CONFIG | SYSTEM VIEW | NULL || information_schema | INNODB_FT_DEFAULT_STOPWORD | SYSTEM VIEW | NULL || information_schema | INNODB_FT_DELETED | SYSTEM VIEW | NULL || information_schema | INNODB_FT_INDEX_CACHE | SYSTEM VIEW | NULL || information_schema | INNODB_FT_INDEX_TABLE | SYSTEM VIEW | NULL || information_schema | INNODB_INDEXES | SYSTEM VIEW | NULL || information_schema | INNODB_METRICS | SYSTEM VIEW | NULL || information_schema | INNODB_SESSION_TEMP_TABLESPACES | SYSTEM VIEW | NULL || information_schema | INNODB_TABLES | SYSTEM VIEW | NULL || information_schema | INNODB_TABLESPACES | SYSTEM VIEW | NULL || information_schema | INNODB_TABLESPACES_BRIEF | SYSTEM VIEW | NULL || information_schema | INNODB_TABLESTATS | SYSTEM VIEW | NULL || information_schema | INNODB_TEMP_TABLE_INFO| SYSTEM VIEW | NULL || information_schema | INNODB_TRX | SYSTEM VIEW | NULL || information_schema | INNODB_VIRTUAL | SYSTEM VIEW | NULL || information_schema | KEY_COLUMN_USAGE | SYSTEM VIEW | NULL || information_schema | KEYWORDS | SYSTEM VIEW | NULL || information_schema | OPTIMIZER_TRACE | SYSTEM VIEW | NULL || information_schema | PARAMETERS | SYSTEM VIEW | NULL || information_schema | PARTITIONS | SYSTEM VIEW | NULL || information_schema | PLUGINS| SYSTEM VIEW | NULL || information_schema | PROCESSLIST | SYSTEM VIEW | NULL || information_schema | PROFILING | SYSTEM VIEW | NULL || information_schema | REFERENTIAL_CONSTRAINTS| SYSTEM VIEW | NULL || information_schema | RESOURCE_GROUPS | SYSTEM VIEW | NULL || information_schema | ROLE_COLUMN_GRANTS | SYSTEM VIEW | NULL || information_schema | ROLE_ROUTINE_GRANTS | SYSTEM VIEW | NULL || information_schema | ROLE_TABLE_GRANTS | SYSTEM VIEW | NULL || information_schema | ROUTINES | SYSTEM VIEW | NULL || information_schema | SCHEMA_PRIVILEGES | SYSTEM VIEW | NULL || information_schema | SCHEMATA | SYSTEM VIEW | NULL || information_schema | ST_GEOMETRY_COLUMNS | SYSTEM VIEW | NULL || information_schema | ST_SPATIAL_REFERENCE_SYSTEMS | SYSTEM VIEW | NULL || information_schema | ST_UNITS_OF_MEASURE | SYSTEM VIEW | NULL || information_schema | STATISTICS | SYSTEM VIEW | NULL || information_schema | TABLE_CONSTRAINTS | SYSTEM VIEW | NULL || information_schema | TABLE_PRIVILEGES | SYSTEM VIEW | NULL || information_schema | TABLES| SYSTEM VIEW | NULL || information_schema | TABLESPACES | SYSTEM VIEW | NULL || information_schema | TRIGGERS | SYSTEM VIEW | NULL || information_schema | USER_PRIVILEGES | SYSTEM VIEW | NULL || information_schema | VIEW_ROUTINE_USAGE | SYSTEM VIEW | NULL || information_schema | VIEW_TABLE_USAGE | SYSTEM VIEW | NULL || information_schema | VIEWS | SYSTEM VIEW | NULL |+--------------------+---------------------------------------+-------------+--------+73 rows in set (0.00 sec)mysql> show create table information_schema.tablesG*************************** 1. row ***************************View: TABLES Create View: CREATE ALGORITHM=UNDEFINED DEFINER=`mysql.infoschema`@`localhost` SQL SECURITY DEFINER VIEW `information_schema`.`TABLES` AS select (`cat`.`name` collate utf8_tolower_ci) AS `TABLE_CATALOG`,(`sch`.`name` collate utf8_tolower_ci) AS `TABLE_SCHEMA`,(`tbl`.`name` collate utf8_tolower_ci) AS `TABLE_NAME`,`tbl`.`type` AS `TABLE_TYPE`,if((`tbl`.`type` = ’BASE TABLE’),`tbl`.`engine`,NULL) AS `ENGINE`,if((`tbl`.`type` = ’VIEW’),NULL,10) AS `VERSION`,`tbl`.`row_format` AS `ROW_FORMAT`,if((`tbl`.`type` = ’VIEW’),NULL,internal_table_rows(`sch`.`name`,`tbl`.`name`,if((`tbl`.`partition_type` is null),`tbl`.`engine`,’’),`tbl`.`se_private_id`,(`tbl`.`hidden` <> ’Visible’),`ts`.`se_private_data`,coalesce(`stat`.`table_rows`,0),coalesce(cast(`stat`.`cached_time` as unsigned),0))) AS `TABLE_ROWS`,if((`tbl`.`type` = ’VIEW’),NULL,internal_avg_row_length(`sch`.`name`,`tbl`.`name`,if((`tbl`.`partition_type` is null),`tbl`.`engine`,’’),`tbl`.`se_private_id`,(`tbl`.`hidden` <> ’Visible’),`ts`.`se_private_data`,coalesce(`stat`.`avg_row_length`,0),coalesce(cast(`stat`.`cached_time` as unsigned),0))) AS `AVG_ROW_LENGTH`,if((`tbl`.`type` = ’VIEW’),NULL,internal_data_length(`sch`.`name`,`tbl`.`name`,if((`tbl`.`partition_type` is null),`tbl`.`engine`,’’),`tbl`.`se_private_id`,(`tbl`.`hidden` <> ’Visible’),`ts`.`se_private_data`,coalesce(`stat`.`data_length`,0),coalesce(cast(`stat`.`cached_time` as unsigned),0))) AS `DATA_LENGTH`,if((`tbl`.`type` = ’VIEW’),NULL,internal_max_data_length(`sch`.`name`,`tbl`.`name`,if((`tbl`.`partition_type` is null),`tbl`.`engine`,’’),`tbl`.`se_private_id`,(`tbl`.`hidden` <> ’Visible’),`ts`.`se_private_data`,coalesce(`stat`.`max_data_length`,0),coalesce(cast(`stat`.`cached_time` as unsigned),0))) AS `MAX_DATA_LENGTH`,if((`tbl`.`type` = ’VIEW’),NULL,internal_index_length(`sch`.`name`,`tbl`.`name`,if((`tbl`.`partition_type` is null),`tbl`.`engine`,’’),`tbl`.`se_private_id`,(`tbl`.`hidden` <> ’Visible’),`ts`.`se_private_data`,coalesce(`stat`.`index_length`,0),coalesce(cast(`stat`.`cached_time` as unsigned),0))) AS `INDEX_LENGTH`,if((`tbl`.`type` = ’VIEW’),NULL,internal_data_free(`sch`.`name`,`tbl`.`name`,if((`tbl`.`partition_type` is null),`tbl`.`engine`,’’),`tbl`.`se_private_id`,(`tbl`.`hidden` <> ’Visible’),`ts`.`se_private_data`,coalesce(`stat`.`data_free`,0),coalesce(cast(`stat`.`cached_time` as unsigned),0))) AS `DATA_FREE`,if((`tbl`.`type` = ’VIEW’),NULL,internal_auto_increment(`sch`.`name`,`tbl`.`name`,if((`tbl`.`partition_type` is null),`tbl`.`engine`,’’),`tbl`.`se_private_id`,(`tbl`.`hidden` <> ’Visible’),`ts`.`se_private_data`,coalesce(`stat`.`auto_increment`,0),coalesce(cast(`stat`.`cached_time` as unsigned),0),`tbl`.`se_private_data`)) AS `AUTO_INCREMENT`,`tbl`.`created` AS `CREATE_TIME`,if((`tbl`.`type` = ’VIEW’),NULL,internal_update_time(`sch`.`name`,`tbl`.`name`,if((`tbl`.`partition_type` is null),`tbl`.`engine`,’’),`tbl`.`se_private_id`,(`tbl`.`hidden` <> ’Visible’),`ts`.`se_private_data`,coalesce(cast(`stat`.`update_time` as unsigned),0),coalesce(cast(`stat`.`cached_time` as unsigned),0))) AS `UPDATE_TIME`,if((`tbl`.`type` = ’VIEW’),NULL,internal_check_time(`sch`.`name`,`tbl`.`name`,if((`tbl`.`partition_type` is null),`tbl`.`engine`,’’),`tbl`.`se_private_id`,(`tbl`.`hidden` <> ’Visible’),`ts`.`se_private_data`,coalesce(cast(`stat`.`check_time` as unsigned),0),coalesce(cast(`stat`.`cached_time` as unsigned),0))) AS `CHECK_TIME`,`col`.`name` AS `TABLE_COLLATION`,if((`tbl`.`type` = ’VIEW’),NULL,internal_checksum(`sch`.`name`,`tbl`.`name`,if((`tbl`.`partition_type` is null),`tbl`.`engine`,’’),`tbl`.`se_private_id`,(`tbl`.`hidden` <> ’Visible’),`ts`.`se_private_data`,coalesce(`stat`.`checksum`,0),coalesce(cast(`stat`.`cached_time` as unsigned),0))) AS `CHECKSUM`,if((`tbl`.`type` = ’VIEW’),NULL,get_dd_create_options(`tbl`.`options`,if((ifnull(`tbl`.`partition_expression`,’NOT_PART_TBL’) = ’NOT_PART_TBL’),0,1),if((`sch`.`default_encryption` = ’YES’),1,0))) AS `CREATE_OPTIONS`,internal_get_comment_or_error(`sch`.`name`,`tbl`.`name`,`tbl`.`type`,`tbl`.`options`,`tbl`.`comment`) AS `TABLE_COMMENT` from (((((`mysql`.`tables` `tbl` join `mysql`.`schemata` `sch` on((`tbl`.`schema_id` = `sch`.`id`))) join `mysql`.`catalogs` `cat` on((`cat`.`id` = `sch`.`catalog_id`))) left join `mysql`.`collations` `col` on((`tbl`.`collation_id` = `col`.`id`))) left join `mysql`.`tablespaces` `ts` on((`tbl`.`tablespace_id` = `ts`.`id`))) left join `mysql`.`table_stats` `stat` on(((`tbl`.`name` = `stat`.`table_name`) and (`sch`.`name` = `stat`.`schema_name`)))) where ((0 <> can_access_table(`sch`.`name`,`tbl`.`name`)) and (0 <> is_visible_dd_object(`tbl`.`hidden`)))character_set_client: utf8collation_connection: utf8_general_ci1 row in set (0.00 sec)

數(shù)據(jù)字典緩存

為了減少磁盤IO，提高訪問效率，MySQL 8.0引入了數(shù)據(jù)字典緩存。數(shù)據(jù)字典緩存是一塊全局共享區(qū)域，通過LRU算法進(jìn)行內(nèi)存管理，具體包括：

tablespace definition cache partition：用于緩存表空間定義對(duì)象；大小限制由參數(shù)tablespace_definition_cache決定。schema definition cache partition：用于緩存模式定義對(duì)象；大小限制由參數(shù)schema_definition_cache決定。table definition cache partition：用于緩存表定義對(duì)象；大小限制由參數(shù)max_connections決定。stored program definition cache partition：用于緩存存儲(chǔ)過程定義對(duì)象；大小限制由參數(shù)stored_program_definition_cache決定。character set definition cache partition：用于緩存字符集定義對(duì)象；硬編碼限制256個(gè)。collation definition cache partition：用于緩存排序規(guī)則定義對(duì)象；硬編碼限制256個(gè)。

原子DDL

首先，了解一下什么是原子性？原子性是指，一個(gè)事務(wù)執(zhí)行要么全部成功，要么全部失敗。

在MySQL 8.0之前，由于不支持原子DDL，在服務(wù)進(jìn)程異常掛掉或服務(wù)器異常宕機(jī)的情況下，有可能會(huì)導(dǎo)致數(shù)據(jù)字典、存儲(chǔ)引擎結(jié)構(gòu)、二進(jìn)制日志之間的不一致。

在MySQL 8.0中，數(shù)據(jù)字典均被改造成InnoDB存儲(chǔ)引擎表，原子DDL也被引入進(jìn)來。原子DDL是將數(shù)據(jù)字典更新、存儲(chǔ)引擎操作、二進(jìn)制日志寫入放到同一個(gè)事務(wù)里執(zhí)行，要么全部成功提交，要么全部失敗回滾。

接下來，我們還是先通過一個(gè)例子，來了解一下原子DDL。在這個(gè)例子中，DROP TABLE t1, t2屬于同一個(gè)事務(wù)；在5.7版本中，出現(xiàn)了一個(gè)事務(wù)部分、成功部分失敗的情況，即DROP TABLE t1成功、DROP TABLE t2失??；但在8.0版本中，因?yàn)镈ROP TABLE t2失敗，導(dǎo)致整個(gè)事務(wù)全部失敗回滾；這個(gè)例子就很好地體現(xiàn)了原子性和非原子性的區(qū)別。

5.7版本：mysql> CREATE TABLE t1 (c1 INT);mysql> DROP TABLE t1, t2;ERROR 1051 (42S02): Unknown table ’test.t2’mysql> SHOW TABLES;Empty set (0.00 sec)8.0版本：mysql> CREATE TABLE t1 (c1 INT);mysql> DROP TABLE t1, t2;ERROR 1051 (42S02): Unknown table ’test.t2’mysql> SHOW TABLES;+----------------+| Tables_in_test |+----------------+| t1 |+----------------+

在對(duì)原子DDL有初步了解后，接下來介紹一下具體過程：

（1）prepare：創(chuàng)建需要的對(duì)象，并將ddl日志寫入到mysql.innodb_ddl_log；ddl日志記錄了如何前滾和回滾ddl操作。（2）perform：執(zhí)行ddl操作。（3）commit：更新數(shù)據(jù)字典并提交。（4）post-ddl：重放和刪除ddl日志。只有在實(shí)例異常宕機(jī)情況下，ddl日志才會(huì)繼續(xù)保存在mysql.innodb_ddl_log；在在實(shí)例重啟后，進(jìn)行實(shí)例恢復(fù)階段，ddl日志會(huì)重放和刪除；如果第3步-數(shù)據(jù)字典更新已經(jīng)成功提交，并寫入redo log和binlog，那么ddl操作成功；否則，ddl操作失敗，并根據(jù)ddl日志進(jìn)行回滾

最后，再介紹一下，怎么查看DDL日志？

其中一個(gè)方法，是在debug級(jí)別下，訪問表mysql.innodb_ddl_log進(jìn)行查看(不推薦)

CREATE TABLE mysql.innodb_ddl_log ( id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY, thread_id BIGINT UNSIGNED NOT NULL, type INT UNSIGNED NOT NULL, space_id INT UNSIGNED, page_no INT UNSIGNED, index_id BIGINT UNSIGNED, table_id BIGINT UNSIGNED, old_file_path VARCHAR(512) COLLATE UTF8_BIN, new_file_path VARCHAR(512) COLLATE UTF8_BIN, KEY(thread_id));

另一個(gè)辦法，是可以將DDL日志打印到error log進(jìn)行查看(推薦)

mysql> set global innodb_print_ddl_logs=on;Query OK, 0 rows affected (0.00 sec)mysql> set global LOG_ERROR_VERBOSITY=3;Query OK, 0 rows affected (0.00 sec)mysql> create table test(id int);Query OK, 0 rows affected (0.04 sec)$ tail -100f mysql-error.log2020-08-17T19:55:09.804345+08:00 73 [Note] [MY-012473] [InnoDB] DDL log insert : [DDL record: DELETE SPACE, id=57, thread_id=73, space_id=12, old_file_path=./test/test.ibd]2020-08-17T19:55:09.804396+08:00 73 [Note] [MY-012478] [InnoDB] DDL log delete : 572020-08-17T19:55:09.816850+08:00 73 [Note] [MY-012477] [InnoDB] DDL log insert : [DDL record: REMOVE CACHE, id=58, thread_id=73, table_id=1069, new_file_path=test/test]2020-08-17T19:55:09.816887+08:00 73 [Note] [MY-012478] [InnoDB] DDL log delete : 582020-08-17T19:55:09.820623+08:00 73 [Note] [MY-012472] [InnoDB] DDL log insert : [DDL record: FREE, id=59, thread_id=73, space_id=12, index_id=160, page_no=4]2020-08-17T19:55:09.820673+08:00 73 [Note] [MY-012478] [InnoDB] DDL log delete : 592020-08-17T19:55:09.837695+08:00 73 [Note] [MY-012485] [InnoDB] DDL log post ddl : begin for thread id : 732020-08-17T19:55:09.837721+08:00 73 [Note] [MY-012486] [InnoDB] DDL log post ddl : end for thread id : 73

總結(jié)

MySQL 8.0對(duì)于數(shù)據(jù)字典的改進(jìn)，帶來了很多好處，包括元數(shù)據(jù)統(tǒng)一管理、數(shù)據(jù)字典緩存、information_schema性能提升、原子DDL等等。

以上就是解析MySQL8.0新特性——事務(wù)性數(shù)據(jù)字典與原子DDL的詳細(xì)內(nèi)容，更多關(guān)于MySQL8.0新特性的資料請(qǐng)關(guān)注好吧啦網(wǎng)其它相關(guān)文章！