Interview Question 22 : Explain about oracle database each stage in open till open include nomount , mount , open ?

 

Oracle Database Startup Stages

(From SHUTDOWN to OPEN State)

Oracle database startup happens in three logical stages:

NOMOUNT  →  MOUNT  →  OPEN

Each stage gives Oracle access to different components of the database.

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1. NOMOUNT Stage

What happens in NOMOUNT?

At this stage:

• ✅ Instance is started
• ✅ Memory structures and background processes are created
• ❌ Database files are NOT accessed

Components Started

During NOMOUNT:

• SGA is allocated
• Background processes start:
  • SMON
  • PMON
  • DBWn
  • LGWR
  • CKPT
• Initialization parameter file is read:
  • spfile or pfile

What is NOT Available?

• Control files ❌
• Data files ❌
• Redo log files ❌

📌 At NOMOUNT, Oracle knows how to start, but not which database to open.

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Command

STARTUP NOMOUNT;

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When is NOMOUNT Used?

• Creating a new database
• Re‑creating control files
• Troubleshooting severe corruption
• Checking initialization parameters

✅ Example:

CREATE DATABASE testdb;

(Database must be in NOMOUNT state before this command)

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Diagram

NOMOUNT
|
|-- SGA allocated
|-- Background processes started
|-- Init. parameters read

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2. MOUNT Stage

What happens in MOUNT?

At this stage:

• ✅ Control files are opened
• ✅ Oracle knows the database structure
• ❌ Data files and redo logs are still NOT opened

Oracle now knows:

• Database name
• Data file locations
• Redo log file locations
• SCN and checkpoint information

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Command

STARTUP MOUNT;

or

ALTER DATABASE MOUNT;

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What is Accessible?

✅ Control files
❌ Data files
❌ Redo logs
❌ User sessions

Only DBA users can connect:

sqlplus / as sysdba

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When is MOUNT Used?

• Database recovery
• Renaming a database
• Enabling ARCHIVELOG mode
• Restoring datafiles
• Handling missing or corrupt files

✅ Example:

ALTER DATABASE ARCHIVELOG;

✅ Example:

RECOVER DATABASE;

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Diagram

MOUNT
|
|-- Control files opened
|-- Database structure known
|-- No user access

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3. OPEN Stage

What happens in OPEN?

At this stage:

• ✅ Data files are opened
• ✅ Redo log files are opened
• ✅ Database is fully operational

Oracle verifies:

• Data files match control file SCN
• Crash recovery (if needed) is completed
• Redo is applied automatically

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Command

ALTER DATABASE OPEN;

or simply:

STARTUP;

(Default STARTUP = NOMOUNT → MOUNT → OPEN)

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Database in OPEN State

• ✅ Users can connect
• ✅ DML and DDL allowed
• ✅ Transactions possible
• ✅ Applications start working

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OPEN Modes

Oracle can open the database in different modes:

a) READ WRITE (Default)

ALTER DATABASE OPEN;

b) READ ONLY

ALTER DATABASE OPEN READ ONLY;

Used for:

• Reporting
• Data verification
• Standby databases

c) RESTRICTED

ALTER SYSTEM ENABLE RESTRICTED SESSION;

ALTER DATABASE OPEN;

Only users with RESTRICTED SESSION privilege can connect.

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Diagram

OPEN
|
|-- Data files opened
|-- Redo log files opened
|-- Users allowed
|-- Database operational

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Summary Table

Stage	Instance	Control Files	Data Files	Redo Logs	User Access
NOMOUNT	✅ Yes	❌ No	❌ No	❌ No	❌ No
MOUNT	✅ Yes	✅ Yes	❌ No	❌ No	❌ No
OPEN	✅ Yes	✅ Yes	✅ Yes	✅ Yes	✅ Yes

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Real‑World DBA Example

Scenario: Control file corruption
Steps:

SHUTDOWN IMMEDIATE;

STARTUP NOMOUNT;

CREATE CONTROLFILE ...

ALTER DATABASE MOUNT;

ALTER DATABASE OPEN; 

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Interview‑Ready One‑Line Explanation

Oracle database startup occurs in three stages: NOMOUNT starts the instance, MOUNT opens control files, and OPEN makes the database available to users by opening data files and redo logs.