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Oracle DBA · Lesson 1

The Instance and the Database

Oracle has two parts: what is running, and the files that keep saved data. Split those, and “the database is down” stops being a vague ticket.

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Oracle DBA Lesson 1 — The Instance and the Database

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Instance vs database

Instance = memory and processes. That is the part of Oracle that is running.

Database = the collection of files on disk. Table data is saved in those files.

Stop the instance and you release memory and end the processes. The files stay. A clean shutdown does not delete saved table data.

If the instance crashes and the files survive, crash recovery uses redo to make the files consistent at open. If a data file is missing, restarting the instance does not replace it — restore from backup, then recover.

Start, mount, open

1. NOMOUNT — start the instance. Memory and background processes come up. The database is not mounted yet.
2. MOUNT — identify the database. The instance reads the control file and learns which data files belong. Table data is still not available to applications.
3. OPEN — open the data files so applications can read and write.

STARTUP NOMOUNT
STARTUP
SHUTDOWN

As shown in the lesson. Full STARTUP takes you through to open when the files are healthy.

Check where you are

SELECT instance_name, status, database_status
FROM   v$instance;

SELECT name, open_mode, database_role
FROM   v$database;

Typical STATUS values: STARTED (roughly NOMOUNT), MOUNTED, OPEN.

Gotchas

• Clean shutdown stops the instance; rows in data files remain.
• Instance crash ≠ missing data file.
• Mounted ≠ open for applications.

Quick quiz

Choose one answer per question, then submit. You’ll see the correct answer and a short why.

1. What is the Oracle instance?

The data files on disk where table rows are stored Memory and processes — the part of Oracle that is running Only the control file The listener alone

2. After STARTUP NOMOUNT, can applications read table data?

Yes — NOMOUNT opens all data files Yes — if the control file is present No — the database is not mounted or open yet Only SYS can read tables in NOMOUNT

3. SALES has 1,200 rows. You do a clean shutdown, then start up again. How many rows are in SALES?

1,200 — clean shutdown does not delete saved table data 0 — shutdown clears all tables Unknown until you rebuild indexes Only half, until redo applies

4. Instance crashes but data files are intact. What if a data file is missing instead?

Restart always recreates missing data files Redo alone recreates a missing data file Mount automatically restores from the FRA Intact files: redo at open. Missing file: restore from backup, then recover

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