Docker Postgres “PANIC: could not locate a valid checkpoint record”

It seemed that my Postgres database was not properly shut down when rebooting and when I tried to use docker-compose to start it again, the following message was shown in docker logs:

To fix this, first shut down this container (docker-compose down), then start the container in interactive mode:

After the transaction log is reset, everything should be fine. Now you can start your containers again (docker-compose up -d).

Migrating JComments to WordPress

I switched back to WordPress from Joomla! after I registered this new domain. After migrated all the posts to WordPress using FG Joomla to WordPress, I noticed the comments have not been transferred. After some digging I didn’t find satisfactory plugin to do that, so I wrote my own PHP script to fulfill this task. The code is also available from GitHub.

每隔一定时间间隔查询是否有新消息

Oracle VPD策略实现行级安全性

VPD 即Virtual Private Databases,提供了对数据库的精密访问控制 (graind access control (FGAC) ),使用VPD,可以在数据记录集定义用户的访问权限。

通过VPD 策略,相当于用户操作数据库中的数据时隐式添加一些条件。比如我们要实现特定角色只能操作数据库表中的特定行的数据,这就可以用 VPD 策略实现。现在以车队长只能操作员工表中的属于他的车队的员工的数据信息为例具体说明:

1. 构造策略函数:

2. 授予执行策略函数的权限:

3. 向数据库表添加策略:

最开始不明白策略函数的作用,花了好几天上网查资料才知道原来函数的返回值便是你想往表上加的隐式约束。
比如,你向EMP表添加了下面的策略函数:

现在,如果你以admin身份登录执行 ‘SELECT * FROM EMP’ , 则实际执行的是 ‘SELECT * FROM EMP WHERE 1 = 1’ , 其中 ‘1 = 1’ 这个条件是策略函数的返回值;如果你以其他身份登录执行 ‘SELECT * FROM EMP’ , 则实际执行的是 ‘SELECT * FROM EMP WHERE 1 = 2’ 。