Relation Instances

Relation Instances

A relation about movies is not stationary; rather, relations change over time. We expect that these changes include the tuples of the relation, such as insertion of new tuples as movies are added to the database, changes to existing tuples if we get revised or corrected information about a movie, and perhaps deletion of tuples for movies that are expelled from the database for some reason.

It is less common for the schema of a relation to change. However, there are situations where we might want to add or delete attributes. Schema changes, while possible in commercial database systems, are very expensive, because each of perhaps millions of tuples needs to be rewritten to add or delete elements. If we add an attribute, it may be difficult or even impossible to find the correct values for the new element in the existing tuples.

We shall call a set of tuples for a given relation an instance of that relation. For instance, the three tuples shown in the following figure form an instance of relation Movies. Most probably, the relation Movies has changed over time and will continue to change over time. For example, in 1980, Movies did not contain the tuples for Mighty Ducks or Wayne's World. However, a conventional database system maintains only one version of any relation:  the set of tuples that are in the relation "now". This instance of the relation is called the current instance.

The relation Movies




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