Hypothetical queries to deductive databases
Troels Andreasen, Henning Christiansen
Department of Computer Science
Roskilde University, P.O.Box 260, DK-4000 Roskilde, Denmark
Hypothetical queries are queries embedding
hypotheses about the database,
which must be taken into account during evaluation.
In principle, the answer
to a hypothetical query h>q with a hypothesis
h is the result of
evaluating q against the database revised with h.
In case h is inconsistent with the database,
query evaluation becomes
a special case of counterfactual reasoning.
this paper we discuss and compare different approaches
queries. We pay in this context special attention
to potentials for efficient evaluation.
As a central part of the paper we present and discuss
our own approach "counterfactual exceptions",
which have the important
property, as opposed to the other approaches discussed,
to require only
minor overhead in query evaluation.
This approach is thus realistic for
implementation in environments supporting large databases.
The "price" for efficient evaluation is an
altered semantics, as compared to the other approaches.
However, we will
argue that our semantics is at least as appropriate
for database applications.
Eds. Geske, U., Ruiz, C., Seipel, D.,
Proc. of the 5th International Workshop
on Deductive Databases and Logic Programming.
Workshop in Conjunction with
ICLP'97, Leuven, Belgium, July 11, 1997.
pp. 37-48, 1997.