In Oracle database technology, an Oracle System Change Number SCN is best described as which of the following with respect to tracking changes in the database?

Introduction / Context:
This question focuses on the Oracle System Change Number SCN, an important internal concept in Oracle databases. SCN values are used by Oracle to mark the commit points of transactions and to coordinate recovery, backup, and read consistency. Understanding SCN helps explain how Oracle can restore data to a consistent point in time and manage multi version read consistency across sessions.

Given Data / Assumptions:

We are dealing with Oracle specific terminology, namely the System Change Number SCN.
SCN is related to tracking changes and supporting recovery mechanisms.
Options mention dirty reads, deadlocks, explicit locks, and committed changes.
One option describes SCN as a value incremented when database changes are made.
We need to choose the statement that best matches Oracle documentation on SCN.

Concept / Approach:
In Oracle, the SCN is a logical timestamp that advances as transactions commit. It does not advance solely due to locks or anomalies such as dirty reads. SCN values are recorded in redo logs and data file headers, allowing Oracle to know the state of the database at particular points in time. During recovery, Oracle uses SCN values to apply redo logs up to a target SCN. Read consistency is maintained by ensuring that queries see data as of a particular SCN, which corresponds to a consistent snapshot.

Step-by-Step Solution:
Step 1: Recall that SCN is associated with transaction commits rather than with specific anomalies or lock tracking.Step 2: Identify the option that mentions SCN being incremented when database changes are made and used for recovery and consistency.Step 3: Compare this with options that focus only on dirty reads, deadlocks, or locks, which do not accurately describe SCN behavior.Step 4: Eliminate the option that treats SCN as a permanent identifier for tables, as that is not its purpose.Step 5: Select the statement that aligns with SCN being a logical timestamp for committed changes and a key part of Oracle recovery mechanisms.

Verification / Alternative check:
Oracle documentation describes SCN as a monotonically increasing number that Oracle uses to order events and to ensure data consistency. It is associated with redo entries and commit points. Recovery operations often specify a target SCN to which the database should be recovered. This description matches the option that refers to incrementing SCN when changes are made and its use for recovery and consistency. No documentation states that SCN is incremented only on dirty reads or deadlocks, confirming that those options are incorrect.

Why Other Options Are Wrong:
The option about dirty reads is incorrect because SCN does not track isolation anomalies directly. The statement that SCN is incremented only when deadlocks occur is clearly wrong, since deadlocks are rare and SCN advances constantly during normal operation. The option that SCN keeps track exclusively of explicit locks misunderstands its role, as lock management is handled separately. Describing SCN as a permanent identifier for tables is also wrong; table identifiers are different internal structures and do not change with each commit.

Common Pitfalls:
Students sometimes confuse SCN with row level version numbers or with lock identifiers. Another pitfall is assuming that SCN is tied to error conditions like deadlocks rather than routine commits. To avoid these misunderstandings, remember that SCN is a logical clock for the database, marking when changes are committed and supporting consistent views and recovery. Keeping this mental model helps accurately interpret questions about Oracle internal mechanisms.

Final Answer:
An Oracle System Change Number SCN is a value that is incremented whenever committed changes are made to the database and is used for recovery and consistency.