Oracle Internals: An Introduction

Process Memory

In addition to the SGA, or System Global Area, each Oracle process uses three similar global areas as well:

• The Process Global Area (PGA)

• The User Global Area (UGA)

• The Call Global Area (CGA)

Many DBAs are unclear about the distinction between the PGA and the UGA. The distinction is as simple as that between a process and a session. Although there is commonly a one-to-one relationship between processes and sessions, it can be more complex than that. The most obvious case is a Multi-Threaded Server configuration, in which there can be many more sessions than processes. In such configurations there is one PGA for each process, and one UGA for each session. The PGA contains information that is independent of the session that the process may be serving at any one time, whereas the UGA contains information that is specific to a particular session.

The PGA

The Process Global Area, often known as the Program Global Area, resides in process private memory, rather than in shared memory. It is a global area in the sense that it contains global variables and data structures that must be accessible to all modules of the Oracle server code. However, it is not shared between processes. Each Oracle server process has its own PGA, which contains only process-specific information. Structures in the PGA do not need to be protected by latches because no other process can access them.

The PGA contains information about the operating system resources that the process is using, and some ...