Controlled and Uncontrolled Redundancy

Redundancy is

• Direct if a value is a copy of another


• Indirect if the value can be derived from other values:
  
  
  • Simplifies retrieval but complicates update
  
  
  • Conversely integration makes retrieval slow and updates easier
  
  
  
  


• Data redundancy can lead to inconsistency in the database unless
  controlled.


• The system should be aware of any data duplication - the system is
  responsible for ensuring updates are carried out correctly.


• A DB with uncontrolled redundancy can be in an inconsistent state - it can
  supply incorrect or conflicting information


• A given fact represented by a single entry cannot result in inconsistency
  - few systems are capable of propagating updates i.e. most systems do not
  support controlled redundancy.