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:
- Better performance because of fewer locks
- Greatly reduced deadlocking
- Simplified application design and management
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, xmin and xmax, indicating the identifiers of the two transactions when the version was created and deleted, respectively. The value of xmax is NULL if that version has not been deleted yet.
The general idea is that, instead of...