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:
You are now connected to database "zzj_db" as user "zzj_user".
zzj_db=>