Transaction Isolation

The ANSI/ISO SQL standard defines four levels of transaction isolation in terms of three phenomena that must be prevented between concurrent transactions. These undesirable phenomena are:

dirty reads

A transaction reads data written by concurrent uncommitted transaction.

non-repeatable reads

A transaction re-reads data it has previously read and finds that data has been modified by another committed transaction.

phantom read

A transaction re-executes a query returning a set of rows that satisfy a search condition and finds that additional rows satisfying the condition has been inserted by another committed transaction.

The four isolation levels and the corresponding behaviors are described below.