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什么是网站建设与优化软文营销的写作技巧有哪些

news 2025/7/19 9:47:45
什么是网站建设与优化,软文营销的写作技巧有哪些,wordpress内存缓存无插件,黄山自驾游攻略前言 在OB中，drop表可以通过 回收站 或者 以往的备份恢复来还原单表。当delete数据时，由于delete操作的对象不会进入回收站，此时需要通过闪回查询功能查看delete的数据，以便后续恢复 本次实验版本为 OceanBase 4.2.1.8&#xff0…

前言

在OB中，drop表可以通过 回收站 或者 以往的备份恢复来还原单表。当delete数据时，由于delete操作的对象不会进入回收站，此时需要通过闪回查询功能查看delete的数据，以便后续恢复

本次实验版本为 OceanBase 4.2.1.8，MySQL租户。参考OB社区 banjin 老师文章进行测试验证

OceanBase 社区

undo_retention

该参数作用为系统应保留的多版本数据范围，单位为秒。属于租户级别变量，默认为1800秒。当出现delete误删除数据时，可以通过该参数进行闪回查询，恢复误删除数据，可以根据实际业务需求适当更改该参数值

obclient [tpcc]> ALTER SYSTEM SET undo_retention=1800;
Query OK, 0 rows affected (0.129 sec)obclient [tpcc]> SHOW PARAMETERS LIKE 'undo_retention';
+-------+----------+-----------+----------+----------------+-----------+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------+--------+---------+-------------------+
| zone  | svr_type | svr_ip    | svr_port | name           | data_type | value | info                                                                                                                                                                           | section | scope  | source  | edit_level        |
+-------+----------+-----------+----------+----------------+-----------+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------+--------+---------+-------------------+
| zone3 | observer | 10.0.0.63 |     2882 | undo_retention | NULL      | 1800  | the low threshold value of undo retention. The system retains undo for at least the time specified in this config when active txn protection is banned. Range: [0, 4294967295] | TENANT  | TENANT | DEFAULT | DYNAMIC_EFFECTIVE |
| zone2 | observer | 10.0.0.62 |     2882 | undo_retention | NULL      | 1800  | the low threshold value of undo retention. The system retains undo for at least the time specified in this config when active txn protection is banned. Range: [0, 4294967295] | TENANT  | TENANT | DEFAULT | DYNAMIC_EFFECTIVE |
+-------+----------+-----------+----------+----------------+-----------+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------+--------+---------+-------------------+
2 rows in set (0.009 sec)

官方解释如下

DML闪回查询

准备测试数据
obclient [tpcc]> create table banjin_flash (id int ,name varchar(10),dizhi varchar(10),primary key (id));
Query OK, 0 rows affected (0.568 sec)obclient [tpcc]> insert into banjin_flash values (1,'zhangsan','北京');
Query OK, 1 row affected (0.135 sec)obclient [tpcc]> insert into banjin_flash values (2,'lisi','上海');
Query OK, 1 row affected (0.002 sec)obclient [tpcc]> insert into banjin_flash values (3,'wangwu','天津');
Query OK, 1 row affected (0.002 sec)obclient [tpcc]> insert into banjin_flash values (4,'zhaoliu','河北');
Query OK, 1 row affected (0.002 sec)obclient [tpcc]>

更改数据

记录更改数据时的时间，便于后续闪回恢复

obclient [tpcc]> select now();
+---------------------+
| now()               |
+---------------------+
| 2024-11-16 14:25:10 |
+---------------------+
1 row in set (0.002 sec)obclient [tpcc]> update banjin_flash set dizhi = '湖南' where name='lisi';
Query OK, 1 row affected (0.010 sec)
Rows matched: 1  Changed: 1  Warnings: 0obclient [tpcc]> select now();
+---------------------+
| now()               |
+---------------------+
| 2024-11-16 14:25:30 |
+---------------------+
1 row in set (0.001 sec)obclient [tpcc]> delete from banjin_flash;
Query OK, 4 rows affected (0.004 sec)obclient [tpcc]> select now();
+---------------------+
| now()               |
+---------------------+
| 2024-11-16 14:25:50 |
+---------------------+
1 row in set (0.001 sec)obclient [tpcc]>

闪回数据

1.可以看到由于之前删除了表中的数据，在当前状态查看不到表中任何数据

2.在 2024-11-16 14:25:30 时间段 dizhi 为上海的数据已经被修改为 湖南

3.在 2024-11-16 14:25:10 时间段 表中的数据为初始时候的状态

obclient [tpcc]> select * from banjin_flash;
Empty set (0.002 sec)obclient [tpcc]>
obclient [tpcc]>
----  在 2024-11-16 14:25:30 时间段 dizhi 为上海的数据已经被修改为 湖南
obclient [tpcc]> SELECT * FROM banjin_flash AS OF SNAPSHOT time_to_usec('2024-11-16 14:25:30') * 1000;
+------+----------+--------+
| id   | name     | dizhi  |
+------+----------+--------+
|    1 | zhangsan | 北京   |
|    2 | lisi     | 湖南   |
|    3 | wangwu   | 天津   |
|    4 | zhaoliu  | 河北   |
+------+----------+--------+
4 rows in set (0.002 sec)
---  在 2024-11-16 14:25:10 时间段 表中的数据为初始时候的状态
obclient [tpcc]>
obclient [tpcc]> SELECT * FROM banjin_flash AS OF SNAPSHOT time_to_usec('2024-11-16 14:25:10') * 1000;
+------+----------+--------+
| id   | name     | dizhi  |
+------+----------+--------+
|    1 | zhangsan | 北京   |
|    2 | lisi     | 上海   |
|    3 | wangwu   | 天津   |
|    4 | zhaoliu  | 河北   |
+------+----------+--------+
4 rows in set (0.001 sec)obclient [tpcc]>
obclient [tpcc]>

过程中有DDL的闪回

表增字段后的闪回效果

可以看到如果后续对表进行了alter操作，闪回查询时新加字段被Default 值补齐，如果没有default值会用null补齐

obclient [tpcc]> alter table banjin_flash add column dianhua decimal(11) default 1;
Query OK, 0 rows affected (0.178 sec)obclient [tpcc]>
obclient [tpcc]> SELECT * FROM banjin_flash AS OF SNAPSHOT time_to_usec('2024-11-16 14:25:30') * 1000;
+------+----------+--------+---------+
| id   | name     | dizhi  | dianhua |
+------+----------+--------+---------+
|    1 | zhangsan | 北京   |       1 |
|    2 | lisi     | 湖南   |       1 |
|    3 | wangwu   | 天津   |       1 |
|    4 | zhaoliu  | 河北   |       1 |
+------+----------+--------+---------+
4 rows in set (0.015 sec)obclient [tpcc]> SELECT * FROM banjin_flash AS OF SNAPSHOT time_to_usec('2024-11-16 14:25:10') * 1000;
+------+----------+--------+---------+
| id   | name     | dizhi  | dianhua |
+------+----------+--------+---------+
|    1 | zhangsan | 北京   |       1 |
|    2 | lisi     | 上海   |       1 |
|    3 | wangwu   | 天津   |       1 |
|    4 | zhaoliu  | 河北   |       1 |
+------+----------+--------+---------+
4 rows in set (0.001 sec)obclient [tpcc]> alter table banjin_flash add column dianhua1 decimal(11) ;
Query OK, 0 rows affected (0.083 sec)obclient [tpcc]> SELECT * FROM banjin_flash AS OF SNAPSHOT time_to_usec('2024-11-16 14:25:30') * 1000;
+------+----------+--------+---------+----------+
| id   | name     | dizhi  | dianhua | dianhua1 |
+------+----------+--------+---------+----------+
|    1 | zhangsan | 北京   |       1 |     NULL |
|    2 | lisi     | 湖南   |       1 |     NULL |
|    3 | wangwu   | 天津   |       1 |     NULL |
|    4 | zhaoliu  | 河北   |       1 |     NULL |
+------+----------+--------+---------+----------+
4 rows in set (0.015 sec)obclient [tpcc]> SELECT * FROM banjin_flash AS OF SNAPSHOT time_to_usec('2024-11-16 14:25:10') * 1000;
+------+----------+--------+---------+----------+
| id   | name     | dizhi  | dianhua | dianhua1 |
+------+----------+--------+---------+----------+
|    1 | zhangsan | 北京   |       1 |     NULL |
|    2 | lisi     | 上海   |       1 |     NULL |
|    3 | wangwu   | 天津   |       1 |     NULL |
|    4 | zhaoliu  | 河北   |       1 |     NULL |
+------+----------+--------+---------+----------+
4 rows in set (0.001 sec)obclient [tpcc]>

表删除字段后的闪回效果

可以看到如果对表进行了 alter 删除字段操作后，无法使用闪回还原数据

obclient [tpcc]> alter table banjin_flash drop column dianhua1;
Query OK, 0 rows affected (0.835 sec)obclient [tpcc]> SELECT * FROM banjin_flash AS OF SNAPSHOT time_to_usec('2024-11-16 14:25:10') * 1000;
ERROR 1412 (HY000): Unable to read data -- Table definition has changed
obclient [tpcc]>
obclient [tpcc]> SELECT * FROM banjin_flash AS OF SNAPSHOT time_to_usec('2024-11-16 14:25:30') * 1000;
ERROR 1412 (HY000): Unable to read data -- Table definition has changed
obclient [tpcc]>
obclient [tpcc]>

truncate 闪回效果

重新初始化数据，发现truncate操作后也无法进行闪回数据

obclient [tpcc]> create table banjin_flash (id int ,name varchar(10),dizhi varchar(10),primary key (id));
Query OK, 0 rows affected (0.106 sec)obclient [tpcc]> insert into banjin_flash values (1,'zhangsan','北京');
Query OK, 1 row affected (0.025 sec)obclient [tpcc]> insert into banjin_flash values (2,'lisi','上海');
Query OK, 1 row affected (0.003 sec)obclient [tpcc]> insert into banjin_flash values (3,'wangwu','天津');
Query OK, 1 row affected (0.053 sec)obclient [tpcc]> insert into banjin_flash values (4,'zhaoliu','河北');
Query OK, 1 row affected (0.002 sec)obclient [tpcc]> select now();
+---------------------+
| now()               |
+---------------------+
| 2024-11-16 14:44:19 |
+---------------------+
1 row in set (0.001 sec)obclient [tpcc]> update banjin_flash set dizhi = '湖南' where name='lisi';
Query OK, 1 row affected (0.014 sec)
Rows matched: 1  Changed: 1  Warnings: 0obclient [tpcc]> select now();
+---------------------+
| now()               |
+---------------------+
| 2024-11-16 14:44:37 |
+---------------------+
1 row in set (0.001 sec)obclient [tpcc]> truncate table banjin_flash;
Query OK, 0 rows affected (0.099 sec)obclient [tpcc]> SELECT * FROM banjin_flash AS OF SNAPSHOT time_to_usec('2024-11-16 14:44:19') * 1000;
ERROR 1412 (HY000): Unable to read data -- Table definition has changed
obclient [tpcc]>
obclient [tpcc]> SELECT * FROM banjin_flash AS OF SNAPSHOT time_to_usec('2024-11-16 14:44:37') * 1000;
ERROR 1412 (HY000): Unable to read data -- Table definition has changed
obclient [tpcc]>

总结

1.当出现delete操作时，可以根据 OB的undo_retention功能 进行闪回查询查看误删除的数据

2.后续过程中如果出现 删除字段 以及 truncate表操作 无法进行闪回数据

3.truncate 操作的对象既不会进入 回收站 也无法通过闪回查询，生产环境一定要谨慎操作

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