命令格式：

切换数据库：\c [database_name]

切换用户：\c - [user_name]

先连上用户postgres、数据库postgres

$su - postgres

Last login: Wed Mar 1 13:16:48 CST 2017 on pts/1
-bash-4.2$psql

psql (9.2.18)
Type "help" for help.

postgres=#

切换数据库

postgres=# \c zzj_db;

You are now connected to database "zzj_db" as user "postgres".

zzj_db=#

切换用户

zzj_db=# \c - zzj_user;

Password for user zzj_user:

Youare now connected to database "zzj_db" as user "zzj_user".

zzj_db=>