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Storage concepts in Oracle

Introduction to Physical Storage Structures

One characteristic of an RDBMS is the independence of logical data structures such as tables, views, and indexes from physical storage structures. Because physical and logical structures are separate, you can manage physical storage of data without affecting access to logical structures. For example, renaming a database file does not rename the tables stored in it.
An Oracle database is a set of files that store Oracle data in persistent disk storage. This section discusses the database files generated when you issue a CREATE DATABASE statement:
  1. Data files and temp files: A data file is a physical file on disk that was created by Oracle Database and contains data structures such as tables and indexes. A temp file is a data file that belongs to a temporary tablespace. The data is written to these files in an Oracle proprietary format that cannot be read by other programs.
  2. Control files: A control file is a root file that tracks the physical components of the database.
  3. Online redo log files: The online redo log is a set of files containing records of changes made to data.
A database instance is a set of memory structures that manage database files. Figure 11-1 shows the relationship between the instance and the files that it manages.


Diagram consisting of Tablespace, Extents, Data blocks and Segments
Diagram consisting of Tablespace, Extents, Data blocks and Segments
  1. Data blocks: Data blocks are the smallest level of storage management. They map to operating system blocks.
  2. Extents: Extents are composed of contiguous blocks on a disk.
  3. Segments: A segment is typically associated with a single table or index. A segment can contain one or more extents.
  4. Tablespace: The tablespace is at the top of the storage hierarchy. A tablespace is directly connected with one or more datafiles, which are managed by the underlying operating system.