1. What is scn?
SCN to identify the database at a precise moment commit. When a transaction is committed, it is given a unique identifier transaction SCN. SCN at the same time as the internal clock mechanism for Oracle database, can be seen as logical clocks, each database has a global SCN generator.
SCN (System Change Number), also known as the system change number, is a very important database data structures.
As a database internal logic clock, according to SCN and sorting database transactions, Oracle has also relied SCN to achieve consistency reading (Read Consistency) and other important database functionality. In addition to the distributed transaction (Distributed Transactions), SCN is also very important.
SCN is the only database, and increase over time, but may not be consistent. Unless rebuild the database, the value of SCN will never be reset to zero.
Has been a lot of controversy for SCN, many people think SCN refers System Commit Number, and usually SCN change when submitting, so a lot of the time, these two terms are often repeated in the document. Even in the official documents of Oracle, SCN also often System Change/Commit Number two forms.
Which word in the end is really not very important, it is important to know SCN is Oracle internal clock mechanism, Oracle database through SCN to maintain consistency and recovery mechanisms through the implementation of Oracle SCN crucial.
SCN is ubiquitous in the database, a common transaction table, control files, data file header, log files, and so on recorded data block header SCN value.
Labeled different prefixes, SCN also have different names, such as checkpoint SCN (Checkpint SCN), Resetlogs SCN and the like.