Basic Oracle Concepts and Programming Interview Preparation Guide

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.

An Oracle database is a collection of data treated as a big unit in the database server.

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.

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.

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.

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.

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.

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.

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

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.