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:
A transaction reads data written by concurrent uncommitted transaction.
A transaction re-reads data it has previously read and finds that data has been modified by another committed transaction.
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 9-1. Postgres Isolation Levels
|Dirty Read||Non-Repeatable Read||Phantom Read|
|Read committed||Not possible||Possible||Possible|
|Repeatable read||Not possible||Not possible||Possible|
|Serializable||Not possible||Not possible||Not possible|