Multiversion Concurrency Control
PostgreSQL makes use of Multiversion Concurrency Control (MVCC) to maintain data consistency. Understanding, at least in concept, how MVCC works can be beneficial to a PostgreSQL administrator or developer.
Most popular RDBMSs make use of table or row locks to maintain database consistency. Typically, these locks occur at the physical level of the file. These locks are used to prohibit two or more instances from writing to the same row (or table) concurrently.
PostgreSQL uses a more advanced method for ensuring database integrity. In MVCC, each transaction sees a version of the database as it existed at some near point in the past. It is import to distinguish that the transaction is not seeing the actual data, ...
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