解决cannot create PGC_POSTMASTER variables after startup问题

最近，在Ubuntu18下搭建PostgreSQL10.06测试环境，添加rum插件没有问题，但是在添加pg_jieba的时候碰到了cannot create PGC_POSTMASTER variables after startup问题。

在postgres的源码guc.c里头，有这么一句：

/*
* Only allow custom PGC_POSTMASTER variables to be created during shared
* library preload; any later than that, we can't ensure that the value
* doesn't change after startup. This is a fatal elog if it happens; just
* erroring out isn't safe because we don't know what the calling loadable
* module might already have hooked into.
*/
if (context == PGC_POSTMASTER &&
!process_shared_preload_libraries_in_progress)
elog(FATAL, "cannot create PGC_POSTMASTER variables after startup");

说是需要在共享库预加载的时候才能够创建PGC_POSTMASTER变量，但是尝试了重启系统并不能解决任何问题。

最后在/etc/postgresql/10/main/postgresql.conf文件中，找到如下一行，添加pg_jieba：

shared_preload_libraries = ‘pg_jieba’

重新启动postgres服务。这时候再使用命令查看动态库状态：

# SHOW shared_preload_libraries;
 shared_preload_libraries
--------------------------
 pg_jieba
(1 row)

SHOW

jieba插件已经显示预加载了。这时候再创建扩展：

# CREATE EXTENSION "pg_jieba";
CREATE EXTENSION

成功启动_(:з」∠)_