Isolation level

There are four levels of transaction isolation that are defined by the ANSI/ISO SQL. IBM® Netezza® SQL supports the SQL grammar for defining all four isolation levels:

• Read committed
• Read uncommitted
• Repeatable read
• Serializable

These isolation levels prevent the following occurrences between concurrent transactions:

Dirty reads

A transaction reads data that is written by concurrent uncommitted transactions.

Nonrepeatable reads

A transaction rereads data that it previously read and finds that the data is modified by another transaction (that committed since the initial read).

Phantom read

A transaction reruns a query that returns a set of rows that satisfy a search condition and finds that the set of rows changed because of another recently committed transaction.

The following table describes the four isolation levels.