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.

Table 10-1. Postgres Isolation Levels

Dirty Read Non-Repeatable Read Phantom Read  
Read uncommitted Possible Possible Possible
Read committed Not possible Possible Possible
Repeatable read Not possible Not possible Possible
Serializable Not possible Not possible Not possible
Postgres offers the read committed and serializable isolation levels.