Refreshing a warehouse consists in propagating updates on
source data to correspondingly update the base data and
derived data stored in the warehouse. There are two sets of
issues to consider: when to refresh, and how to refresh.
Usually, the warehouse is refreshed periodically (e.g., daily or
weekly). Only if some OLAP queries need current data (e.g.,
up to the minute stock quotes), is it necessary to propagate
every update. The refresh policy is set by the warehouse
administrator, depending on user needs and traffic, and may
be different for different sources.