Useful Query for DBA

Database default information:
Select username,profile,default_tablespace,temporary_tablespace from dba_users;

Database Structure information:
SELECT /*+ ordered */ d.tablespace_name tablespace, d.file_name filename
, d.bytes filesize, d.autoextensible autoextensible, d.increment_by * e.value increment_by, d.maxbytes maxbytes
FROM sys.dba_data_files d, v$datafile v
, (SELECT value FROM v$parameter WHERE name = 'db_block_size') e
WHERE (d.file_name = v.name)
UNION
SELECT d.tablespace_name tablespace, d.file_name filename, d.bytes filesize, d.autoextensible autoextensible
, d.increment_by * e.value increment_by, d.maxbytes maxbytes
FROM sys.dba_temp_files d, (SELECT value
FROM v$parameter
WHERE name = 'db_block_size') e
UNION
SELECT '[ ONLINE REDO LOG ]', a.member, b.bytes, null, TO_NUMBER(null), TO_NUMBER(null)
FROM v$logfile a, v$log b
WHERE a.group# = b.group#
UNION
SELECT '[ CONTROL FILE ]', a.name, TO_NUMBER(null), null, TO_NUMBER(null), TO_NUMBER(null)
FROM v$controlfile a
ORDER BY 1,2;

Database Character Set Informations:
Select * from nls_database_parameters;

Database Segment Managment  Informations:
Select TABLESPACE_NAME, BLOCK_SIZE, EXTENT_MANAGEMENT, SEGMENT_SPACE_MANAGEMENT from dba_tablespaces;

Database Object Information:
Select owner,object_type,count(*) from dba_objects Where owner not IN ('SYS','MDSYS','CTXSYS','HR','ORDSYS','OE','ODM_MTR','WMSYS','XDB','QS_WS',
'RMAN','SCOTT','QS_ADM','QS_CBADM', 'ORDSYS', 'OUTLN', 'PM', 'QS_OS', 'QS_ES', 'ODM', 'OLAPSYS','WKSYS','SH','SYSTEM','ORDPLUGINS','QS','QS_CS')
Group by owner,object_type order by owner;