Access to a database through a user view can be serviced quickly when the view is
materialized, ie the transformed data is explicitly stored. In the presence of database
updates, however, the materialized view can become costly to maintain; often it must be
completely rederived from the base data using the view definition. Under some conditions
the view can be updated directly given only the view definition, the current contents of the
materialized view, and the update operation (still expressed against the base data), without …