Temporary Segments used during Table & Index creation & sorting - Oracle Datafiles and Tempfiles

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• Nirja Shah   -Posted on 25 Sep 15
  - In addition to the physical files used by the database, Oracle maintains many types of logical structures.
  
  - Some of them are:
  
  - Schemas
  - In Oracle, a schema is essentially the same as an account or a username.
  - Each object in the database is owned by a schema.
  - Every Oracle database is created with two initial schemas:SYS, which is used to store the data dictionary and SYSTEM which often stores some data dictionary extensions as well as critical tables for other tools.
  - Other schemas are created by the DBA.
  - Each schema can be granted quotas in any tablespace.
  - There is no necessary relationship between a schema and a tablespace.
  
  - Temporary segments
  - They are used by Oracle during table and index creation and for sorting as well as for temporary storage required for other operations such as hash joins.
  
  - Tables
  - All data in the database is stored in tables.
  - Data includes not only user data but also contents of the data dictionary.
  
  - Indexes
  - Indexes are used both to facilitate the quick retrieval of data from the table and to enforce the uniqueness of column values.
  - Indexes are stored in separate segments from the table data, except for index- organized tables,which are actually indexes that contain all the data.

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