The most useful view that ORACLE DBAs must be familiar with

DBA View

Serial number
1	Dba_data_files	Information about the database file
2	Dba_db_links	All database links in the database
3	Dba_extents	Include all partitions in the database
4	Dba_free_space	Free partitions in all table spaces
5	Dba_indexes	Description of all indexes in the database
6	Dba_ind_columns	Columns that compress indexes on all tables and on the cluster
7	Dba_objects	All objects in the database
8	Dba_rollback_segs	Description of the rollback segment
9	Dba_segments	Storage space allocated for all database segments
10	Dba_sequences	Description of all order books in the database
11	Dba_synonyms	All synonyms in the database
12	Dba_tables	Description of all tables in the database
13	Dba_tablespaces	Description of all table spaces in the database
14	Dba_tab_columns	All table descriptions, views, and columns of the cluster
15	Dba_tab_grants	Permissions granted by objects in the database
16	Dba_tab_privs	Permissions granted by objects in the database
17	Dba_ts_quotas	All user table space limits
18	Dba_users	About all user information about the database
19	Dba_views	All views in the database

v$ View

Serial number
1	V$datafile	Data file information used by the database
2	V$librarycache	Governance information for SQL statements in a shared pool
3	V$lock	Information about the Lock
4	V$log	To extract information about the Redo log group from the control file
5	V$logfile	Log Group files related information
6	V$parameter	Parameters
7	V$process	Current process Information
8	V$rollname	Rollback Segment Information
9	V$rollstat	Continuous machine rollback segment statistic information
10	V$rowcache	In-memory data dictionary activity/performance information
11	V$session	Information about an active session
12	V$sesstat	Report statistics for the current session in V$session
13	V$sqlarea	Information using the current cursor in a shared pool
14	V$statname	The meaning of the statistics reported in the V$sesstat
15	V$sysstat	System statistics based on current operation session
16	V$waitstat	Wait For information

    1, view database name and archive status   sql> select Name,log_mode from V$database;   2,   View the instance name and status of the database   sql> select Instance_name,status from V$instance;    3, view the table space name, status, and management method   Sql> select Tablespace_name,status,extent_management from Dba_tablespaces;   4, view the location and name of the control file   sql> select name from V$controlfile;   5, view the group name, number of members, status and size of the log file   sql> select Group#,members,bytes,status from V$log;   6, view the location of the log file   sql> select Group#,member from V$logfile;   7, view the location and size of the data file   sql> select name,bytes from V$datafile;   8, view user-owned objects and types   sql> select Object_name,object_type from dba_objects where owner= ' SCOTT ';   9, view temporary file information   sql> select name,bytes from V$tempfile;    10, view archived log information   sql> select name, first_change#,next_change# from V$archived_log;