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 transaction (that committed since the initial read).
A transaction re-executes a query returning a set of rows that satisfy a search condition and finds that the set of rows satisfying the condition has changed due to another recently-committed transaction.
The four transaction isolation levels and the corresponding behaviors are described in Table 9-1.
Table 9-1. SQL Transaction Isolation Levels
|Isolation Level||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|
PostgreSQL offers the read committed and serializable isolation levels.