B.Sc. CSIT 7th Semester

Understanding Oracle Instance

oracle instance


  • Oracle is relational database management system which uses relational data model.
  • In the relational data model, data is seen by the users in the form of tables.

Oracle Server

  • Oracle server consist of an oracle instance and a oracle database.

Oracle Instance

Oracle instance consist of memory and process.

Physical and Logical Database Structures

Oracle database architecture depends upon two types of storage:

  • Physical database structure
  • Logical database structure

Physical Database Structure

Physical (disk) storage contains all the files in the database. They are:

  • Data Files
  • Redo-Log Files
  • Control Files
oracle instance

The above figure represents entity relationship diagram for physical and logical storage. The crow’s foot notation represents one to many relationship.

1. Data files

  • Data files contain database’s data.
  • The data of logical data structures such as tables and indexes is stored in data files of the database.
  • One or more data files form a logical unit of database storage called a tablespace.

2. Redo log files

The purpose of these files is to record all changes made to data. These files protect database against failures.

3. Control files

  • They are the binary files that record the physical structure of the database.
  • They include:
  •  Database name 
  • Name and location of data files and redo log files
  • Time stamp of database creation

Logical Database Structure

  • Logical storage structures appear on the disk but are not part of the dataset.  
  • They helps users locate specific data and improve the efficiency of retrieval process.
  • They include:
  • Tablespaces
  • Segments
  • Extents
  • Data Blocks
oracle instance

Fig: Data files in database

oracle instance

Fig: Data files in Table Space

oracle instance

Fig: Segments in Tablespace

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Fig: Extents in a segments

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Fig: Block in extent


Database is logically divided into one or more tablespaces. Each tablespace creates one or more data files to physically store data.

Data blocks

Data block represents specific number of bytes of physical database space on disk.


An extent represents continuous data blocks that are used to store specific data information.


A segment is a set of extents allocated for a certain logical structure.

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Karina Shakya