Changes in DQL (April 2003)

This new version of DQL might appropriately be characterized as a "minor revision" of the first version.

• The DQL specification is abstract in the sense that it does not describe a normative external syntax for the language. However, the language has a simple structure for which external syntaxes for various usage environments can easily be constructed. Specifically, the language is a collection of object types (e.g., query, query answer, etc.) such that each instance of an object type consists of a set of property-value pairs. The new specification explicitly points out that structure of DQL and provides a pointer to a proposed XML syntax for the language.
• A don't-bind variables list was added to a query so that all query variables are explicitly listed in either the must-bind variables list, the may-bind variables list, or the don't-bind variables list. That change alleviates the need for variables to be syntactically distinguishable in a query pattern.
• The term "answer set" was incorrectly used in the initial DQL specification to refer to both the collection of answers in an answer bundle and to the largest set of answers to a query that are entailed by the answer KB such that no answer in the set is entailed by any other answer in the set. In the new specification, the former has been renamed "answer collection".
• As is common practice in data base query languages, a DQL query can now optionally contain an "answer pattern" containing all of the variables that the server is being asked to provide bindings for, and each query answer contains an "answer pattern instance" that implicitly specifies the bindings to those variables.
• Minor changes were made in the protocol for a query-answering dialogue to strengthen its extensibility and to simplify its structure. In particular, a "server continuation" was changed from being a component of an answer bundle consisting of either a process handle or one or more termination tokens to being an object containing a process handle and optionally other property-value pairs that a client sends to a server. Thus, now a server sends a client an answer bundle containing an answer collection and either a process handle or a non-empty set of termination tokens, and a client sends a server a server continuation in order to request that the server produce additional answers. Also, a "server termination" object type was added to the protocol enabling a client to explicitly inform a server that no more requests for answers will be made in a given query-answering dialogue.