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9.2. 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 transaction (that committed since the initial read).

phantom 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 uncommitted Possible Possible Possible
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.

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