Oracle DBA FAQ - Oracle Basic Concepts

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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

What Is the Relation of a User Account and a Schema?

User accounts and schemas have a one-to-one relation. When you create a user, you are also implicitly creating a schema for that user. A schema is a logical container for the database objects (such as tables, views, triggers, and so on) that the user creates. The schema name is the same as the user name, and can be used to unambiguously refer to objects owned by the user.

What Is a User Role?

A user role is a group of privileges. Privileges are assigned to users through user roles. You create new roles, grant privileges to the roles, and then grant roles to users.

What is a Database Schema?

A schema is a collection of logical structures of data, or schema objects. A schema is owned by a database user and has the same name as that user. Each user owns a single schema. Schema objects can be created and manipulated with SQL and include: tables, views, and other types of data objects.

What Is a Database Table?

A database table is a basic unit of data logical storage in an Oracle database. Data is stored in rows and columns. You define a table with a table name, such as employees, and a set of columns. You give each column a column name, such as employee_id, last_name, and job_id; a datatype, such as VARCHAR2, DATE, or NUMBER; and a width. The width can be predetermined by the datatype, as in DATE. If columns are of the NUMBER datatype, define precision and scale instead of width. A row is a collection of column information corresponding to a single record.

What Is a Table Index?

Index is an optional structure associated with a table that allow SQL statements to execute more quickly against a table. Just as the index in this manual helps you locate information faster than if there were no index, an Oracle Database index provides a faster access path to table data. You can use indexes without rewriting any queries. Your results are the same, but you see them more quickly.

What Is an Oracle Tablespace?

An Oracle tablespace is a big unit of logical storage in an Oracle database. It is managed and used by the Oracle server to store structures data objects, like tables and indexes.

Each tablespace in an Oracle database consists of one or more files called datafiles, which are physical structures that conform to the operating system in which Oracle is running.

What Is an Oracle Data File?

An Oracle data file is a big unit of physical storage in the OS file system. One or many Oracle data files are organized together to provide physical storage to a single Oracle tablespace.

What Is a Static Data Dictionary?

Data dictionary tables are not directly accessible, but you can access information in them through data dictionary views. To list the data dictionary views available to you, query the view DICTIONARY. Many data dictionary tables have three corresponding views:

An ALL_ view displays all the information accessible to the current user, including information from the current user's schema as well as information from objects in other schemas, if the current user has access to those objects by way of grants of privileges or roles.
A DBA_ view displays all relevant information in the entire database. DBA_ views are intended only for administrators. They can be accessed only by users with the SELECT ANY TABLE privilege. This privilege is assigned to the DBA role when the system is initially installed.
A USER_ view displays all the information from the schema of the current user. No special privileges are required to query these views.

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