A materialized view is said to be refreshed when we make it consistent with changes to its underlying tables. The process of refreshing a view to keep it consisten with changes to the underlying table is often referred to as view maintenance. Two questions to consider are

$1.$ How do we refresh a view when an underlying table is modified? Tow issues of particular interest are how to maintain views incrementally, that is$,$ without recomputing from scratch when there is a change to an underlying table' and how to mantian views in a distributed envrionments such as a data warehouse.

$2.$ When should we refresh a view in response to a change to an underlying table.