About SCN's Understanding System Checkpoint SCN (V $ Database (Checkpoint_Change #)) Data File Checkpoint (V $ datafile (checkpoint_change #)) Data File Termination SCN (V $ DataFile (Last_change #)) Checkpoints stored in data files Start SCN (V $ dataFile_Header (CheckPoint_Change #) 1. System Checkpoint SCN When a checkpoint action is complete, Oracle stores the SCN of the system checkpoint into the control file .Select Checkpoint_Change # from v $ database2, data file checkpoint SCN When a checkpoint is complete, Oracle stores the SCN of each data file in the control file. SELECT NAME, CheckPoint_change # from v $ datafile 3, start SCNORACLE to store this checkpoint SCN in each data file In the file header, this value is called start SCN because it is used to check if the database instance is started, check whether the database is restored .Select Name, checkpoint_change # from v $ datafile_header4, terminating SCN Each data file termination SCN is stored In the control file .Select Name, Last_Change # from v $ datafile During the normal database operation, all the termination SCNs of the data file in the online reading and write mode are NULL.5, and the SCN value during the database is running After the database opens and runs, the system checkpoint in the control file is controlled, and the data file checkpoint SCN in the control file is the same. The startup SCN in each data file header is the same. The termination SCN of each data file in the control file For null. In the process of safely shutting down the database, a checkpoint action will be executed. At this time, the termination SCN of all data files will set the value of the SCN in the data file header. When the database is restarted, Oracle will The one in the file header is compared to the database file checkpoint SCN. If the two values match each other, Oracle will then compare the termination SCN and control file in the data file in the data file. If these two The value is also consistent, it means that all data blocks have been submitted, all of which do not lose in the process of shutting down the database, so the process of starting the database does not require any recovery operation, at which time the database can be opened. After all the databases are opened, the data file stored in the control file terminates the value of the SCN again. Change to NULL, which means that the data file has been opened and can be used normally.
