Tried to query a data dictionary, but it failed with ORA-01219.
SQL> select * from dba_tablespaces;
select * from dba_tablespaces
ERROR at line 1:
ORA-01219: database or pluggable database not open: queries allowed on fixed tables or views only
ORA-01219 means that the data dictionary is not allowable to be queried at MOUNT state. To solve the error, you should do either of the following ways: