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	Java Programming with Oracle SQLJ
		Copyright
		Table of Contents
		Preface
		1. Introduction
		2. Relational Databases, SQL, and PL/SQL
		3. Fundamental SQLJ Programming
		4. Database Objects
		5. Collections
		6. Deploying SQLJ in the JServer
		7. Large Objects
		8. Contexts and Multithreading
		9. Advanced Transaction Control
			9.1 Transaction Processing Problems
			9.2 Transaction Isolation Levels
			9.3 The SET TRANSACTION Statement
			9.4 A Transaction Isolation Level Example
		10. Performance Tuning
		11. Combining JDBC, SQLJ, and Dynamic SQL
		A. Java and Oracle Type Mappings
		B. Oracle Java Utilities Reference
		C. SQLJ in Applets, Servlets, and JavaServer Pages
		Colophon
		Index

Database > Java Programming with Oracle SQLJ > 9. Advanced Transaction Control > 9.2 Transaction Isolation Levels

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9.2 Transaction Isolation Levels

The degree to which the results of one transaction are visible to other transactions is known as the transaction isolation level. You can set the isolation level on a per-transaction basis. The following four levels are defined in the SQL standard:

SERIALIZABLE

This is the default specified by the SQL standard. Dirty reads, non-repeatable reads, and phantom reads are not permitted in this isolation level. SERIALIZABLE is the most "strict" of the isolation levels.

REPEATABLE READ

Dirty reads and non-repeatable reads are not permitted, but phantom reads are permitted. This isolation level is not supported by Oracle.

READ COMMITTED

This is the default used by Oracle. Dirty reads are not permitted, but non-repeatable reads and phantom reads are permitted.

READ UNCOMMITTED

Dirty reads, non-repeatable reads, and phantom reads are permitted. This isolation level is not supported by Oracle.

Oracle's default isolation level of READ COMMITTED is acceptable for most programs. Your other choice is SERIALIZABLE, but you should use it only if you have an extremely good reason, as it may slow the performance of your SQL statements. You may change the transaction isolation level using the SQL SET TRANSACTION statement.

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Index terms contained in this section

READ COMMITTED, transaction isolation level
READ UNCOMMITTED, transaction isolation level
REPEATABLE READ, transaction isolation level
SERIALIZABLE, transaction isolation level
transaction isolation level
      READ COMMITTED
      READ UNCOMMITTED
      REPEATABLE READ
      SERIALIZABLE