Locking in Oracle

A lock is a mechanism Oracle uses to prevent destructive interaction between transactions accessing the same resource. Here, the resource could be user objects, such as tables; or user data, such as table rows; or internal objects, such as shared data structures in memory. A detailed discussion of locking and concurrency concepts in Oracle is beyond the scope of this book. For that, I refer you to Chapter 13 of Oracle Database Concepts Guide (10g Release 1) and Chapter 3 of Tom Kyte's Expert One-on-One Oracle (Apress, ISBN: 1-59059-243-3).

In this chapter, I will define what lost updates are and how to deal with them when programming in Oracle. Throughout the chapter, unless otherwise stated explicitly, assume a default transaction isolation level of READ COMMITTED (see Chapter 4 for a discussion of transaction isolation levels).