Querying the deep web

Data stored outside Web pages and accessible from the Web, typically through HTML forms, consitute the so-called Deep Web. Such data are of great value, but difficult to query and search. We survey techniques to optimize query processing on the Deep Web, in a setting where data are represented in the relational model. We illustrate optimizations both at query plan generation time and at runtime, highlighting the role of integrity constraints. We discuss several prototype systems that address the query processing problem