Introduction

In this chapter we will focus on preventing, detecting, and tuning contention-related issues.

In a database, as in any information system, there is a limited amount of resources to be shared among many users. When more than one user asks for a resource—which can be a file, a row in a table, a block in the buffer cache—there is a need for a synchronization mechanism to avoid the contemporary use of the resource itself by more than one user at a time.

In this chapter we will investigate two different synchronization mechanisms used in Oracle databases: locks and latches.

Locks are used to regulate access to a shared resource, such as a row or a table, while latches protect shared data structures and shared memory allocations in the System Global Area.

The main difference between locks and latches is that latches are held for a short time, while the purpose of locks is to protect resources for a relatively long time. Another difference is that locks involve a queuing mechanism and...