Oracle7 Server Concepts

The Oracle Server

The Oracle Server is a relational database management system that provides an open, comprehensive, and integrated approach to information management. An Oracle Server consists of an Oracle database and an Oracle instance. The following sections describe the relationship between the database and the instance.

Structured Query Language (SQL)

SQL (pronounced SEQUEL) is the programming language that defines and manipulates the database. SQL databases are relational databases; this means simply that data is stored in a set of simple relations. A database can have one or more tables. And each table has columns and rows. A table that has an employee database, for example, might have a column called employee number and each row in that column would be an employee's employee number.

You can define and manipulate data in a table with SQL commands. You use data definition language (DDL) commands to set up the data. DDL commands include commands to creating and altering databases and tables.

You can update, delete, or retrieve data in a table with data manipulation commands (DML). DML commands include commands to alter and fetch dat. The most common SQL command is the SELECT command, which allows you to retrieve data from the database.

In addition to SQL commands, the Oracle Server has a procedural language called PL/SQL. PL/SQL enables the programmer to program SQL statements. It allows you to control the flow of a SQL program, to use variables, and to write error-handling procedures.

Database Structure

An Oracle database has both a physical and a logical structure. Because the physical and logical server structure are separate, the physical storage of data can be managed without affecting the access to logical storage structures.

Physical Database Structure An Oracle database's physical structure is determined by the operating system files that constitute the database. Each Oracle database is made of three types of files: one or more datafiles, two or more redo log files, and one or more control files. The files of an Oracle database provide the actual physical storage for database information.

Logical Database Structure An Oracle database's logical structure is determined by

one or more tablespaces. (A tablespace is a logical area of storage explained later in this chapter.)

the database's schema objects. A schema is a collection of objects. Schema objects are the logical structures that directly refer to the database's data. Schema objects include such structures as tables, views, sequences, stored procedures, synonyms, indexes, clusters, and database links.

The logical storage structures, including tablespaces, segments, and extents, dictate how the physical space of a database is used. The schema objects and the relationships among them form the relational design of a database.

An Oracle Instance

Every time a database is started, a system global area (SGA) is allocated and Oracle background processes are started. The system global area is a an area of memory used for database information shared by the database users. The combination of the background processes and memory buffers is called an Oracle instance. Figure 1 - 1 illustrates a multiple process Oracle instance.

Figure 1 - 1. An Oracle Instance

An Oracle instance has two types of processes: user processes and Oracle processes.

A user process executes the code of an application program (such as an Oracle Forms application) or an Oracle Tool (such as Server Manager).

Oracle processes are server processes that perform work for user processes and background processes that perform maintenance work for the Oracle Server.

Communications Software and SQL*Net

If the user and server processes are on different computers of a network or if the user processes connect to shared server processes through dispatcher processes, the user process and server process communicate using SQL*Net. Dispatchers are optional background processes, present only when a multi-threaded server configuration is used. SQL*Net is Oracle's interface to standard communications protocols that allows for the proper transmission of data between computers. See "Oracle and SQL*Net"

The Oracle Parallel Server: Multiple Instance Systems

Some hardware architectures (for example, shared disk systems) allow multiple computers to share access to data, software, or peripheral devices. Oracle with the Parallel Server option can take advantage of such architecture by running multiple instances that "share" a single physical database. In appropriate applications, the Oracle Parallel Server allows access to a single database by the users on multiple machines with increased performance.