check the general health of the database

SELECT file_id,
       tablespace_name,
       file_name,
       status
FROM   sys.dba_data_files;
SELECT file#,
       name,
       status,
       enabled
FROM   v$datafile;
SELECT *
FROM   v$backup;
SELECT *
FROM   v$recovery_status;
SELECT *
FROM   v$recover_file;
SELECT *
FROM   v$recovery_file_status;
SELECT *
FROM   v$recovery_log;
SELECT username,
       command,
       status,
       module
FROM   v$session;