Querying the Integrated Data

The traditional way of query processing involves: getting a declarative query from the user; parsing it; passing it through a query optimizer which produces an efficient query execution plan that describes how to exactly evaluate the query (i.e., apply which operators, in what order using what algorithm); and finally executing the plan.

In data integration systems this is not so straight forward because the query coming from the user is not asked on a single source but rather on an integration of data sources, each having different query formulation and processing mechanisms defined on different data models.