CentOS7安装完Oracle11gR2+创建完成用户与表后，无法执行sql语句，也sys查询不到任何表，上网搜不到答案，求助

Question

$ sqlplus /nolog
SQL> conn sys/oracle as sysdba
SQL> select table_name from dba_tables;
select table_name from dba_tables

                   *

ERROR at line 1:
ORA-00942: table or view does not exist
SQL> select username,default_tablespace from dba_users;
select username,default_tablespace from dba_users

                                          *

ERROR at line 1:
ORA-00942: table or view does not exist
SQL> select * from session_privs;
select * from session_privs

          *

ERROR at line 1:
ORA-00942: table or view does not exist
SQL> select user from dual;

USER

SYS
SQL> select * from user_role_privs;
select * from user_role_privs

          *

ERROR at line 1:
ORA-00942: table or view does not exist