Call now: 252-767-6166  
Oracle Training Oracle Support Development Oracle Apps

 E-mail Us
 Oracle Articles
New Oracle Articles

 Oracle Training
 Oracle Tips

 Oracle Forum
 Class Catalog

 Remote DBA
 Oracle Tuning
 Emergency 911
 RAC Support
 Apps Support
 Oracle Support

 SQL Tuning

 Oracle UNIX
 Oracle Linux
 Remote s
 Remote plans
 Application Server

 Oracle Forms
 Oracle Portal
 App Upgrades
 SQL Server
 Oracle Concepts
 Software Support

 Remote S


 Consulting Staff
 Consulting Prices
 Help Wanted!


 Oracle Posters
 Oracle Books

 Oracle Scripts

Don Burleson Blog 







Oracle Dirty Reads

Oracle Database Tips by Donald BurlesonApril 26, 2015

Oracle Dirty Reads

There are a few basics on how transactions operate within the relational database model.   In terms of how database isolation levels function, the ANSI/ISO SQL standard (SQL92) breaks these down into four levels of transaction isolation. Each isolation level contains various degrees of impact on transaction processing. The fourth level of transaction isolation, read uncommitted, is not supported by Oracle.  Dirty reads, fuzzy or non-repeatable reads and phantom reads are three phenomena of concern to Oracle database performance.

The isolation levels are explained in terms of dirty reads, fuzzy or non-repeatable reads, or phantom reads, that must be prevented between concurrently executing transactions.  

  • Dirty reads:  Dirty reads occur when a transaction reads data that has been written by another transaction that has not yet been committed.
  • Fuzzy or non-repeatable reads:  Fuzzy reads occur when a database transaction re-reads data it has already read and then finds that another committed transaction has modified or deleted the same data.
  • Phantom reads:  Phantom reads occur when a transaction in Oracle re-executes a query that returns the rows that satisfy a particular search and discovers that another committed transaction has already inserted additional rows that meet the condition.

The database standard for SQL92 expounds these four levels of isolation and whether or not dirty reads, non-repeatable reads or phantom reads are possible within the level of isolation, as shown in the following table: 


Isolation Level

Dirty Read

Nonrepeatable Read

Phantom Read

Read uncommitted




Read committed

Not possible



Repeatable read

Not possible

Not possible



Not possible

Not possible

Not possible

The Oracle database uses the read committed and serializable isolation levels in addition to a read-only mode. By default, Oracle uses the read committed isolation level for database transactions.

Thus, according to the information in the table, dirty reads are only possible at the read uncommitted isolation level; however, the serializable isolation level is sufficient to prevent dirty reads as well as non-repeatable and phantom reads.

At the Oracle default level of read committed, dirty reads are not possible, but fuzzy or phantom reads are.

Portions of this article were excerpted from Exploring Oracle Internals: Tips and Tricks for the Oracle DBA by Ben Prusinski.

Get the Complete
Oracle SQL Tuning Information 

The landmark book "Advanced Oracle SQL Tuning  The Definitive Reference"  is filled with valuable information on Oracle SQL Tuning. This book includes scripts and tools to hypercharge Oracle 11g performance and you can buy it for 30% off directly from the publisher.




Burleson is the American Team

Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

Verify experience! Anyone considering using the services of an Oracle support expert should independently investigate their credentials and experience, and not rely on advertisements and self-proclaimed expertise. All legitimate Oracle experts publish their Oracle qualifications.

Errata?  Oracle technology is changing and we strive to update our BC Oracle support information.  If you find an error or have a suggestion for improving our content, we would appreciate your feedback.  Just  e-mail:  

and include the URL for the page.


Burleson Consulting

The Oracle of Database Support

Oracle Performance Tuning

Remote DBA Services


Copyright © 1996 -  2017

All rights reserved by Burleson

Oracle ® is the registered trademark of Oracle Corporation.

Remote Emergency Support provided by Conversational



Oracle Training at Sea
oracle dba poster

Follow us on Twitter 
Oracle performance tuning software 
Oracle Linux poster