Minimal contraction of preference relations

Changing preferences is very common in real life. The expressive power of the operations of preference change introduced so far in the literature is limited to adding new information about preference and equivalence. Here, we discuss the operation of discarding preferences: preference contraction. We argue that the property of miniimality and the preservation of strict partial orders are crucial for contractions. Contractions can be further constrained by specifying which preferences should not be contracted. We provide algorithms for computing minimal and minimal preference-protecting contraction. We also show some preference query optimization techniques which can be used in the presence of contraction.