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Oracle instance/database states

An Oracle instance is/has either
  • closed consistently
  • crash closed
  • started (but not mounted)
  • mounted a database
  • open(ed) a database

Started

When the instance is started, it has read the init.ora (or spfile) and allocated the SGA.

Mounted database

The instance has opened the control files, but not yet opened the database. In order to find the control files, the instance uses the init parameter control_files which it has read when it started.
A database can be mounted by any number of instances at the same time. If it is mounted by two more instances, this is a Oracle Parallel Server environment.

Open database

An opened database is available for normal operations. When the database is opened, the instance opens the redo log files and the online database files.
A database cannot be opened if any of the online datafiles need media recovery.

Restricted mode

When the instance is in restricted mode, only users with the restricted session system privilege can connect to the database.