Transaction isolation and multiversion concurrency control

There is a fundamental concept is called transaction in relational databases. Transaction is a unit of data change. PostgreSQL implements a set of principles related to transaction processing, which is called ACID:

• Atomicity: Every transaction can be fixed (saved or committed) only as a whole. If one part of a transaction fails, the whole transaction is aborted and no changes are saved at all.

• Consistency: All data integrity constraints are checked and all triggers are processed before a transaction is fixed. If any constraint is violated, then the transaction cannot be fixed.

• Isolation: A database system manages concurrent access to the data objects by parallel transactions in a way that guarantees that the results of their execution is the same as if they were executed serially, one after the other.

• Durability: If a transaction is fixed, the changes it has made to the data are written to the non-volatile memory (disk). So, even...