Oracle database views

v$database - displays information about the database from the control file.
v$instance - displays the state of the current instance.
v$controlfile - lists the names of the control files.
v$logfile - contains information about redo log files.
v$log - displays log file information from the control file.
v$datafile - contains datafile information from the control file.
v$recover_file - display the status of files needing media recovery
v$tempfile - displays tempfile information.
v$tablespace- displays tablespace information from the control file.
v$dispatcher - displays information about the dispatcher processes.
v$shared_server - contains information on the shared server processes.
v$circuit - contains information about virtual circuits, which are user connections to the database through dispatchers and servers.
v$session - lists session information for each current session.
v$transaction - list active transactions
v$process - contains information about the currently active processes.
v$lock - lists the locks currently held by the Oracle Database and outstanding requests for a lock or latch.
v$services - displays the services in the database.
v$sga - displays summary information about the system global area (SGA).
v$pgastat - provides PGA memory usage statistics as well as statistics about the automatic PGA memory manager when it is enabled
v$sql - lists statistics on shared SQL area without the GROUP BY clause and contains one row for each child of the original SQL text entered.
v$sqlarea - lists statistics on shared SQL area and contains one row per SQL string. It provides statistics on SQL statements that are in memory, parsed, and ready for execution.
v$sqltext - view contains the text of SQL statements belonging to shared SQL cursors in the SGA.

v$parameter - displays information about the initialization parameters that are currently in effect for the session.
v$parameter2 - displays information about the initialization parameters that are currently in effect for the session, with each list parameter value appearing as a row in the view.
v$spparameter - displays information about the contents of the server parameter file. If a server parameter file was not used to start the instance, then each row of the view will contain FALSE in the ISSPECIFIED column.
v$system_parameter - displays information about the initialization parameters that are currently in effect for the instance.
v$system_parameter2 - displays information about the initialization parameters that are currently in effect for the instance, with each list parameter value appearing as a row in the view.

dba_tablespaces - describes all tablespaces in the database.
dba_data_files - describes data files
dba_segments - describes the storage allocated for all segments in the database.
dba_rollback_segs - describes rollback segments
dba_free_space - describes the free extents in all tablespaces in the database.
dba_ts_quotas - describes tablespace quotas for the current user

v$pwfile_users - lists users who have been granted SYSDBA and SYSOPER privileges as derived from the password file
dba_profiles -  displays all profiles and their limits
dba_users - Information about all users of the database
dba_role_privs - describes the roles granted to all users and roles in the database
dba_tab_privs - describes all object grants in the database
dba_sys_privs - describes system privileges granted to users and roles

dba_tables - description of all relational tables in database
dba_indexes - description of all indexes in database
dba_tab_columns - describe all columns for tables, views and clusters in database
dba_tab_cols - describe all columns for tables, views and clusters in database (including hiden columns)
index_stats - stores information from the last analyze index ... validate structure

dba_hist_snapshot - display info about snapshots in Workload Repository
dba_hist_baseline - -display info about baselines
dba_outstanding_alerts - describe outstanding alerts
dba_alert_history - describe time-limited alerts that are no longer outstanding

v$sesstat - user session statistics. To view name of statistics of STATISTIC#, query v$statname
v$statname - decoded statistics for staistics number stored in v$sesstat si v$sysstat
v$sysstat - display system statistics. To view name of statistics of STATISTIC#, query v$statname
v$sgastat - display detailed info about sga
v$sgainfo - display brief info about sga
v$sga_dynamic_components - display info about sga dynamic components
v$pgastat - display pga memory usage statistics

v$sga_target_advice - info about SGA_TARGET
v$pga_target_advice - info about PGA_TARGET
v$memory_target_advice - info about MEMORY_TARGET

dba_objects - describes all objects in db
all_objects - describe all objects in db available to user
user_objects - describe all user objects
dba_dependencies - describe all dependencies in db for packages, procesdures, functions, views, pachage bodies and triggesr
all_dependencies
user_dependencies
dba_indexes - describe all indexes in db
all_indexes
user_indexes

v$instance_recovery - monitors the mechanisms available to users to limit recovery I/O
v$recovery_file_dest - display info about disk quota and currect disk ussage in flash recovery area
v$flash_recovery_area_usage - usage info for flash recovery areas
v$diag_info - lists all important ADR (Automaic Diagnostic Repository) locations for current Oracle database instance

v$recover_file - display the status of files needing media recovery (e.g. missing datafiles)