Abstract
The power of classical query languages is linked to the fact that they express a restricted class of declarative programs. The class of semantic objects expressible through queries in the relational calculus, for example, is limited in a number of ways: They have PTIME data complexity, and their expressiveness is well understood. Although relational calculus queries may not return finite results, a natural subclass of the relational calculus does; namely the class of range-restricted queries. This class gives guarantees of finite output, and is complete in this respect, capturing all relational calculus queries whose outputs are always finite, the safe queries.
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