* Description: Knowledge of Oracle database SCN
* Compatiablity: RDBMS 11g, 12c
* Date: 05:33 PM EST, 04/24/2017

<1> SCN and CheckPoint:
     |__ System Change Number(SCN) is an Oracle Clock, that keeps on ticking. This number is utilized by Oracle to log the changes made to the database. 
     |   This number is quite useful while recovering the database from any failure.
     |   SCN is 8Byte (48bit) number represented as 2 parts - SCN_BASE and SCN_WRAP. An SCN_BASE is a 4Byte(32 bits) number and SCN_WRAP is a 2Byte(16 bits) number. 
     |   Whenever SCN_BASE reaches its maximum (2 power 32 = 4294967296 ) , SCN_WRAP goes up by one and SCN_BASE will be reset to 0. 
     |   This continues till SCN_WRAP reaches it’s maximum(i.e. 2 power 16 = 65536).
     |                       SCN = (SCN_WRAP * 4294967296) + SCN_BASE.
     |   Now the question arises in our mind – what happens when SCN reaches it’s maximum? and how the Oracle handles that situation?
     |   Let's say you have fresh database with both SCN_WRAP and SCN_BASE at value 0. As the database is up and running, SCN value keeps on increasing for every second. 
     |   For SCN_BASE to reach it's maximum, it will take around 136 years( 4294967296/(365*24*60*60) – Assuming one transaction per second.). 
     |   This is just for one Wrap and imagine you have still 65535 wraps to complete. So, no need to fear about reaching maximum.
     |   Even if it does reach maximum, then SCN will be reseted to 0, thus causing a new incarnated database. So, all your old backups and Archived logs becomes useless. 
     |   Need to take fresh backup's.
     |__ SQL> select name,checkpoint_change# from v$datafile_header;
                     NAME                                                 CHECKPOINT_CHANGE#
                     ---------------------------------------------------- ------------------
                     /u02/app/oracle/oradata/mask11g/system01.dbf                 1685610
                     /u02/app/oracle/oradata/mask11g/sysaux01.dbf                 1685610
                     /u02/app/oracle/oradata/mask11g/undotbs01.dbf                1685610
                     /u02/app/oracle/oradata/mask11g/users01.dbf                  1685610
<2> CheckPoint:
     |__ Checkpoint is an event that happens at a particular time(=SCN). When this happens, all datafiles and control files are updated with SCN at that time. 
         A successful checkpoint guarantees that all database changes up to the checkpoint SCN have been recorded in the datafiles. 
         As a result, only those changes made after the checkpoint need to be applied during recovery.

<4> Checking Data File Checkpoint SCN, Start SCN, and Stop SCN:	
     |__ SQL> select file#,name,checkpoint_change#, last_change# from v$datafile;  -- Data File CheckPoint SCN, and Stop SCN in control file
              select file#,name,checkpoint_change# from v$datafile_header;         -- Start SCN in each data file header

<3> Reference:
     |__ https://saruamit4.wordpress.com/2014/11/01/checkpoint-scn/