A redundancy in a conceptual schema corresponds to a piece of information that can be derived (that is, obtained through a series of retrieval operations) from other data in the database.
The presence of a redundancy in a database may be decided upon the following factors:
An advantage: a reduction in the number of accesses necessary to obtain the derived information;
A disadvantage: because of larger storage requirements, (but, usually at negligible cost) and the necessity to carry out additional operations in order to keep the derived data consistent.
The decision to maintain or delete a redundancy is made by comparing the cost of operations that involve the redundant information and the storage needed, in the case of presence or absence of redundancy.