ORACLE Architecture

Basic Knowledge on Oracle Database

Today I wanted to publish the overview of an Oracle database.  By the end of this post, You will came to know the following information.

·         Definition of an Oracle, Oracle Server and Oracle Database.

      ·         Physical and Logical structure of the database.

Oracle is the name of database management system and is developed by Oracle Corporation.

Oracle Server manages the data in the database. Users will access the oracle server using the SQL commands. So Oracle server receives SQL commands from users and executes them on the database.

An Oracle database is a logical collection of inter related data or is a collection of data, treated as a single unit. The purpose of a database is to store and retrieve related information. A database server is the key to solving the problems of information management. In general, a server reliably manages a large amount of 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.

Oracle Database is the first database designed for enterprise grid computing, the most flexible and cost effective way to manage information and applications. Enterprise grid computing creates large pools of industry-standard, modular storage and servers.

The database has Logical structures and the Physical Structures.

Oracle Database Physical Structure:

The Physical structure of the database is placed in Physical level. It is physically a set of three operating system files.

o   Data Files : It contains the data of the database.  Every table that is stored in the database is a part of these files. Only Oracle Server can interpret these datafiles.

o   Redo log files : Every database has a set of two or more Redo Log files. The set of redo log files is known as databases redo log. Redo Log files are used in the failure recovery. All the changes made to the database are written to redo log files.

o   Control files : This can be treated as a heart of a database. Contain information required to verify the integrity of the database.   The control file will record the following information

·         Database name

·         Name and location of data files and redo log files and ect.  

I will publish the detailed information on these Physical structure of the database in the further posts.

Oracle Database Logical Structure: 

Oracle database is divided into logical storage units known as Tablespaces, which can be used to group related logical structures together. Logical structure mainly consists of following structures starting with the bigger unit and going down to the smallest. 

• Tablespace
• Segments
• Extents
• Data blocks

Tablespace:  Each database is logically divided into one or more tablespaces. One or more datafiles are explicitly created for each tablespace to physically store the data of all logical structures in a tablespace. The combined size of the datafiles in a tablespace is the total storage capacity of the tablespace. 

Segment :  Tablespace consists of one or more segments. A segment is a set of extents allocated for a certain logical structure stored in the same tablespace. The segments can be of one of following types:

o   Data segment  (stores user data within the database)

o   Index Segment (store Indexes)

o   Temporary Segment ( stores rollback segment)

o   Rollback/Undo segment (created when sql statement needs a temporary work area)

Extent : An extent is a specific number of contiguous data blocks, obtained in a single allocation, used to store a specific type of information in one data file. 

Data Blocks : Oracle database data is stored in data blocks(at the lowest logical level).  One data block corresponds to a specific number of bytes of physical database space on disk(default 8K). The standard block size is specified by the DB_BLOCK_SIZE initialization parameter.

Oracle Data blocks are the smallest unit of I/O in the database and they are mapped to OS block on the storage device.