Oracle DBA FAQ Basic Concepts
What Is Oracle?
Oracle is a company. Oracle is also a database server, which manages
data in a very structured way. It allows users to store and retrieve related
data in a multiuser environment so that many users can concurrently access
the same data. All this is accomplished while delivering high performance.
A database server also prevents unauthorized access and provides efficient
solutions for failure recovery.
What Is an Oracle Database?
An Oracle database is a collection of data treated as a big unit in
the database server.
What Is an Oracle Instance?
Every running Oracle database is associated with an Oracle instance.
When a database is started on a database server (regardless of the type
of computer), Oracle allocates a memory area called the System Global Area
(SGA) and starts one or more Oracle processes. This combination of the
SGA and the Oracle processes is called an Oracle instance. The memory and
processes of an instance manage the associated database's data efficiently
and serve the one or multiple users of the database.
What Is a Parameter File?
A parameter file is a file that contains a list of initialization parameters
and a value for each parameter. You specify initialization parameters in
a parameter file that reflect your particular installation. Oracle supports
the following two types of parameter files:
- Server Parameter Files - Binary version. Persistent.
- Initialization Parameter Files - Text version. Not persistent.
What Is a Server Parameter File?
A server parameter file is a binary file that acts as a repository
for initialization parameters. The server parameter file can reside on
the machine where the Oracle database server executes. Initialization parameters
stored in a server parameter file are persistent, in that any changes made
to the parameters while an instance is running can persist across instance
shutdown and startup.
What Is a Initialization Parameter File?
An initialization parameter file is a text file that contains a list
of initialization parameters. The file should be written in the client's
default character set. Sample initialization parameter files are provided
on the Oracle distribution medium for each operating system. A sample file
is sufficient for initial use, but you will probably want to modify the
file to tune the database for best performance. Any changes will take effect
after you completely shut down and restart the instance.
What is System Global Area (SGA)?
The System Global Area (SGA) is a memory area that contains data shared
between all database users such as buffer cache and a shared pool of SQL
statements. The SGA is allocated in memory when an Oracle database instance
is started, and any change in the value will take effect at the next startup.
What is Program Global Area (PGA)?
A Program Global Area (PGA) is a memory buffer that is allocated for
each individual database session and it contains session specific information
such as SQL statement data or buffers used for sorting. The value specifies
the total memory allocated by all sessions, and changes will take effect
as new sessions are started.
What Is a User Account?
A user account is identified by a user name and defines the user's
attributes, including the following:
• Password for database authentication
- Privileges and roles
- Default tablespace for database objects
- Default temporary tablespace for query processing work space
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