I 0 = (ffx 0 A; Y 0 Bg =: 0 G; ;)
نویسندگان
چکیده
0 = fx 0 a; y 0 bg I 1 = (ffx x 0 ; z z 0 g 0 =: 1 g; ;) 1 = fx a; z z 0 g Hence, the original constraint can be simplied to C 1 = f:s(a; z 0);r(a;z 0)g, which is easily refuted by looking up s(a,c 50) and disproving r(a,c 50). It should be quite obvious that the advantage of the additional exibility provided by our mechanism heavily depends on a good procedure for deciding whether to take an exact, unexact, or even intermediate step. This decision could be based on statistical knowledge about the database, such as the number of stored facts or the selectivity of predicate arguments. In the variant of the rst example there is a small number (4) of explicit facts for predicate p 2 and a rather large number (100) for predicate r. Let us further assume the system has gained some statistical knowledge about the low selectivity of the rst argument of r. Then, the decision for rst taking an unexact step, and proceeding with exact steps only can be motivated as follows. The deletion of p 2 (b;a) yields 0 = fx 0 b; y 0 ag and the upper rule in g. 1 is simplied to f:p 2 (b;a);:r(a;z 1);q(b;z 1)g. Now, one may proceed with either q(b; z 1) or all of its implicitly deleted instantiations. The latter, however, are likely to be numerous since the large number of r-facts and the low selectivity of the rst argument will probably result in a large number of instantiations of r(a; z 1). Hence, we choose to reason unexactly which { using the other deductive rule { yields the head literal r(b; b). Since that is already ground it can easily be checked for being really deleted (exact step). Proceeding from r(b; b) we again reach the upper rule which is now simplied to f:p 2 (x 3 ; b); :r(b; b); q(x 3 ; b)g. Opposite to the rst case, the number of really deleted instantiations of the head literal will be small, namely at most the number of facts for p 2 (= 4). Hence, we take another exact step nding only one instantiation of p 2 (x 3 ; b) stored in the database which results in the substitutions 3 and 4 suitable for disproving consistency. 5 Conclusion A unifying approach …
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