Memory Structure and Processes Overview

constraints. Keys describe the relationships between the different tables and columns of a relational database. Individual values in a key are called key values.

The different types of keys include:

Primary key: The column or set of columns included in the definition of a table’s PRIMARY KEY constraint. A primary key’s values uniquely identify the rows in a table. Only one primary key can be defined for each table.

Unique key: The column or set of columns included in the definition of a UNIQUE constraint.

Foreign key: The column or set of columns included in the definition of a referential integrity constraint.

Referenced key: The unique key or primary key of the same or a different table referenced by a foreign key.

SQL*Plus Overview

SQL*Plus is a tool for entering and running ad-hoc database statements. It lets you run SQL statements and PL/SQL blocks, and perform many additional tasks as well. Through SQL*Plus, you can:

Enter, edit, store, retrieve, and run SQL statements and PL/SQL blocks

Format, perform calculations on, store, print, and create Web output of query results

List column definitions for any table access, and copy data between SQL databases

Send messages to, and accept responses from, an end user

Perform database administration

See Also: SQL*Plus User’s Guide and Reference

Memory Structure and Processes Overview

An Oracle server uses memory structures and processes to manage and access the database. All memory structures exist in the main memory of the computers that constitute the database system. Processes are jobs that work in the memory of these computers.

The architectural features discussed in this section enable the Oracle server to support:

Introduction to the Oracle Server 1-21

Memory Structure and Processes Overview

Many users concurrently accessing a single database

The high performance required by concurrent multiuser, multiapplication database systems

Figure 1–3 shows a typical variation of the Oracle server memory and process structures.

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Memory Structure and Processes Overview

Figure 1–3 Memory Structures and Processes of Oracle

System Global Area

Datafiles

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Memory Structure and Processes Overview

Note: In a UNIX environment, most Oracle processes are part of one master Oracle process, rather than being individual processes. On Windows NT, all processes consist of at least one thread. A thread is an individual execution within a process. Threads enable concurrent operations within a process so that a process can run different parts of its program simultaneously on different processors. A thread is the most fundamental component that can be scheduled on Windows NT. In UNIX documentation, such as this book, whenever the word "process" is mentioned, it is considered a "thread" on Windows NT.

An Oracle Instance

An Oracle server consists of an Oracle database and an Oracle server instance. Every time a database is started, a system global area (SGA) is allocated and Oracle background processes are started. The combination of the background processes and memory buffers is called an Oracle instance.

Real Application Clusters: Multiple Instance Systems

Some hardware architectures (for example, shared disk systems) enable multiple computers to share access to data, software, or peripheral devices. Real Application Clusters take advantage of such architecture by running multiple instances that share a single physical database. In most applications, Real Application Clusters enable access to a single database by users on multiple machines with increased performance.

Real Application Clusters are inherently high availability systems. The clusters that are typical of Real Application Clusters environments can provide continuous service for both planned and unplanned outages.

An Oracle server uses memory structures and processes to manage and access the database. All memory structures exist in the main memory of the computers that constitute the database system. Processes are jobs that work in the memory of these computers.

Note: Real Application Clusters are available only with Oracle9i

Enterprise Edition.

See Also: Oracle9i Real Application Clusters Concepts

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