PostgreSQL implements Multiversion Concurrency Control (MVCC), which allows users to read data at the same time as writers make changes. This is an important feature for concurrency in database applications, as it can allow the following:
MVCC is a core part of PostgreSQL and cannot be turned off; nor would you really want it to be. The internals of MVCC have some implications for the DBA that need to be understood: each row represents a row version, and therefore it has two system columns—
xmax—indicating the identifiers of the two transactions when the version was created and ...