前言
事务性数据字典与原子DDL，是MySQL 8.0推出的两个非常重要的新特性，之所以将这两个新特性放在一起，是因为两者密切相关，事务性数据字典是前提，原子DDL是一个重要应用场景。
MySQL 8.0之前的数据字典
MySQL 8.0之前的数据字典，主要由以下三部分组成：
（1）操作系统文件
db.opt：数据库元数据信息
frm：表元数据信息
par：表分区元数据信息
TRN/TRG：触发器元数据信息
ddl_log.log：DDL过程中产生的元数据信息（2）mysql库下的非InnoDB系统表
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库下的InnoDB系统表
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)我们可以看到，数据字典被分布到多个地方，一方面不利于元数据统一管理，另一方面容易造成数据的不一致(由于操作系统文件、非InnoDB系统表均不支持事务，执行DDL操作无法保证ACID)。
MySQL 8.0的数据字典
为了解决上述问题，MySQL 8.0将数据字典统一改进为InnoDB存储引擎存储，具体分为两部分：
（1）数据字典表：存放最重要的元数据信息，位于mysql库下，存储在mysql共享表空间(mysql.ibd)
（2）其他系统表：存放辅助的元数据信息，位于mysql库下，存储在mysql共享表空间(mysql.ibd)
数据字典表
数据字典表是不可见，既不能通过select访问，也不会出现在show tables或information.schema.tables结果里；尝试访问会报以下错误：
mysql> select * from mysql.tables limit 10;
ERROR 3554 (HY000): Access to data dictionary table 'mysql.tables' is rejected.不过，在debug模式下，是可以访问这些隐藏的数据字典表的；我们重新编译安装(过程略)，并以debug模式启动进程，再次尝试访问，结果如下：
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)其他系统表
其他系统表，可以通过show tables或information_schema.tables查看，均以改进为InnoDB存储引擎(general_log、slow_log例外，这两张表并未记录元数据信息，只是用于记录日志)：
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)数据字典视图
刚刚提到，数据字典表只能在debug模式下访问，那么在生产环境中，我们应该怎么去获取元数据信息呢？答案是通过information_schema库下的数据字典视图。和Oracle数据库的设计理念一样，将元数据信息存放在基表中(x$、$)，然后通过视图(v$、dba_/all_/user_)的方式提供给用户查询；MySQL数据库也是如此，将元数据信息存放在mysql库的数据字典表中隐藏起来，然后提供information_schema库视图给用户查询：
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.tables\G
*************************** 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: utf8
collation_connection: utf8_general_ci
1 row in set (0.00 sec)数据字典缓存
为了减少磁盘IO，提高访问效率，MySQL 8.0引入了数据字典缓存。数据字典缓存是一块全局共享区域，通过LRU算法进行内存管理，具体包括：
tablespace definition cache partition：用于缓存表空间定义对象；大小限制由参数tablespace_definition_cache决定。
schema definition cache partition：用于缓存模式定义对象；大小限制由参数schema_definition_cache决定。
table definition cache partition：用于缓存表定义对象；大小限制由参数max_connections决定。
stored program definition cache partition：用于缓存存储过程定义对象；大小限制由参数stored_program_definition_cache决定。
character set definition cache partition：用于缓存字符集定义对象；硬编码限制256个。
collation definition cache partition：用于缓存排序规则定义对象；硬编码限制256个。原子DDL
首先，了解一下什么是原子性？原子性是指，一个事务执行要么全部成功，要么全部失败。
在MySQL 8.0之前，由于不支持原子DDL，在服务进程异常挂掉或服务器异常宕机的情况下，有可能会导致数据字典、存储引擎结构、二进制日志之间的不一致。
在MySQL 8.0中，数据字典均被改造成InnoDB存储引擎表，原子DDL也被引入进来。原子DDL是将数据字典更新、存储引擎操作、二进制日志写入放到同一个事务里执行，要么全部成功提交，要么全部失败回滚。
接下来，我们还是先通过一个例子，来了解一下原子DDL。在这个例子中，DROP TABLE t1, t2属于同一个事务；在5.7版本中，出现了一个事务部分、成功部分失败的情况，即DROP TABLE t1成功、DROP TABLE t2失败；但在8.0版本中，因为DROP TABLE t2失败，导致整个事务全部失败回滚；这个例子就很好地体现了原子性和非原子性的区别。
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 |
+----------------+在对原子DDL有初步了解后，接下来介绍一下具体过程：
（1）prepare：创建需要的对象，并将ddl日志写入到mysql.innodb_ddl_log；ddl日志记录了如何前滚和回滚ddl操作。
（2）perform：执行ddl操作。
（3）commit：更新数据字典并提交。
（4）post-ddl：重放和删除ddl日志。只有在实例异常宕机情况下，ddl日志才会继续保存在mysql.innodb_ddl_log；在在实例重启后，进行实例恢复阶段，ddl日志会重放和删除；如果第3步-数据字典更新已经成功提交，并写入redo log和binlog，那么ddl操作成功；否则，ddl操作失败，并根据ddl日志进行回滚最后，再介绍一下，怎么查看DDL日志？
其中一个方法，是在debug级别下，访问表mysql.innodb_ddl_log进行查看(不推荐)
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)
);另一个办法，是可以将DDL日志打印到error log进行查看(推荐)
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.log
2020-08-17T19:55:09.804345+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 : 57
2020-08-17T19:55:09.816850+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 : 58
2020-08-17T19:55:09.820623+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 : 59
2020-08-17T19:55:09.837695+08:00 73 [Note] [MY-012485] [InnoDB] DDL log post ddl : begin for thread id : 73
2020-08-17T19:55:09.837721+08:00 73 [Note] [MY-012486] [InnoDB] DDL log post ddl : end for thread id : 73总结
MySQL 8.0对于数据字典的改进，带来了很多好处，包括元数据统一管理、数据字典缓存、information_schema性能提升、原子DDL等等。
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