5.2. TRANSACTION CONTROL: ISOLATION LEVELS

5.2.1. Defining Isolation Levels

Four isolation levels are defined by ANSI SQL to permit you to choose the level of data protection you need for a given transaction. Isolation Levels let you balance acceptable correctness with performance (concurrency). Isolation Levels are implemented through locking of data. The four isolation levels are as follows.

  • READ UNCOMMITTED— least strict, data inconsistency possible

  • READ COMMITTED— default for Oracle and MSS

  • REPEATABLE READ— fewer errors possible

  • SERIALIZABLE— most strict, result will be correct

Three inconsistency problems that you may experience due to lack of isolation are listed below.

  • Dirty Read = read data which is not committed (and might be rolled ...

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