Mount, Nomount, Restricted and Open

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Mount, Nomount, Restricted and Open 
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NOMOUNT
Oracle reads initialization files in this order:
1. spfile.
2. If not found read from pfile.
3. If not found read from init.ora file.

According to the settings database instance is started:
- Memory structures (SGA) allocated.
- Background processes started.
- Alert<SID>.log and trace file are opened for writing.
- Control is not loaded.

MOUNT
Control Files are loaded.
Datafiles and Online Redo Log files are not opened yet.

OPEN
Datafiles are opened.
Online Redo Log file are opened.


Privilege needed for Oracle Startup:
To startup Oracle instance one need to login as sysdba

How to switch between MOUNT/NOMOUNT modes:
Switch to NOMOUNT mode:
STARTUP NOMOUNT;

Switch to MOUNT mode:
STARTUP MOUNT;
ALTER DATABASE MOUNT; (from NOMOUNT mode)

Switch to OPEN mode:
STARTUP;
ALTER DATABASE OPEN; (from MOUNT mode)

Restricted Mode
In Restricted mode - the database is open, but only privileged user can connect.

Switch to RESTRICT mode:
STARTUP RESTRICT;
or
ALTER SYSTEM ENABLE RESTRICTED SESSION;

To go out from the restricted mode:
ALTER SYSTEM DISABLE RESTRICTED SESSION;