今日总结 – MySQL 8.0新特性 — 事务性数据字典与原子DDL

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-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。

正文完