What is Controlled redundancy in a database?

Answer 1

In non-database systems each application has its own private files. This can often lead to redundancy in stored data, with resultant waste in storage space. In a database the data is integrated. The database may be thought of as a unification of several otherwise distinct data files, with any redundancy among those files partially or wholly eliminated. Data integration is generally regarded as an important characteristic of a database. The avoidance of redundancy should be an aim, however, the vigour with which this aim should be pursued is open to question. 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.