참조 : http://en.wikipedia.org/wiki/Multiversion_concurrency_control
> multiversion concurrency control (abbreviated MCC or MVCC) is a concurrency
control [http://en.wikipedia.org/wiki/Concurrency_control] method commonly used
by database management systems
[http://en.wikipedia.org/wiki/Database_management_system] to provide concurrent
access to the database.
MVCC는