centos7 Authentication failure

root@localhost ~]#su

bash-4.2$ su
Password: 
su: Authentication failure //这里切换的是系统用户,现在还不清楚为什么postgres成了我的系统用户
bash-4.2$ su
Password: 
[root@localhost ~]# 

 //qie huan yong  hu:

[root@localhost data]# psql -h 127.0.0.1 -U postgres -d postgres
Password for user postgres: 
psql (9.5.6)
Type "help" for help.

postgres=# 

 psql -h 127.0.0.1 -U postgres(yonghu) -d test1(shujuku)

[root@localhost data]# psql -h 127.0.0.1 -U postgres -d test1
Password for user postgres: 
psql (9.5.6)
Type "help" for help.

test1=# \q