What is SCN in Oracle dB

SCN stands for System Change Number in the Oracle database. It is a logical, internal timestamp used to order events within the database and ensure the ACID properties of transactions. SCNs help mark the point in time before which all changes are known to be on disk, allowing for efficient recovery and avoiding unnecessary redo operations. Essentially, SCNs act as a clock for the database, incrementing every time a COMMIT is made. They are also used in FLASHBACK queries and to restore data to a specific point in time. In the Oracle database architecture, the System Change Number (SCN) plays a crucial role in maintaining data consistency, concurrency, and recoverability. Here are some key details on the Oracle architecture and SCN: *Oracle Architecture*: The Oracle database system is divided into the database (physical storage) and the instance (memory and processes). Multiple instances can access the same database, but an instance can only access one database at a time. The database contains datafiles, redo log files, and control files, while the instance includes the System Global Area (SGA) and background processes. *SCN*: SCN is a logical timestamp used to track changes in the database and maintain the order of events. It helps in achieving the ACID properties of transactions. SCN is incremented each time a COMMIT occurs and is used in various scenarios, such as: - *Consistency*: SCN helps maintain data consistency by ensuring that a read operation retrieves data from a consistent point in time, using the SCN to reference the required version of data blocks. - *Recovery*: During database recovery, SCN helps identify the point to which the database needs to be restored, minimizing data loss and ensuring database consistency. - *Flashback Queries*: SCN is used to view data as it appeared at a specific point in time, enabling users to retrieve data that may have been accidentally deleted or modified. In summary, SCN is a critical component of the Oracle database architecture, ensuring the reliability and consistency of the database through its role in transaction management, recovery, and data consistency.