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CS240A: Databases and Knowledge Bases From Deductive Rules to Active Rules Carlo Zaniolo Department of Computer Science University of California, Los Angeles

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Maintenance Every time the database is changed re-compute the concrete view, or Perform delta maintenance using techniques similar to the differential fixpoint of deductive databases Things are more complex here because you can have both additions ( + ) an subtractions ( - )

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Positive Delta Rules prnt(X, Y) :- father(X, Y). prnt(X, Y) :- mother(X, Y). gprnt(X, Y) : prnt(X, Z), prnt(Z, Y). Assume that some new tuples are added to mother: 1. + prnt(X,Y) : + mother(X,Y). 2. + gprnt(X,Y) : + prnt(X,Z), prnt(Z,Y). 3. + gprnt(X,Y) : prnt(X,Z), + prnt(Z,Y). Finite differentiation w.r.t. all changing predicates

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Closure is the TC of base, and new records are inserted into base(fr, to) Deductive rules for transitive closure of base(fr, to): closure(X,Y) : base(X,Y) closure(X,Y) : closure(X,Z), base(Z,Y). 1. + closure(X,Y) : + base(X,Y). 2. + closure(X,Y) : + closure(X,Z), base(Z,Y). …a careful analysis shows that we need a third rule: 3. + closure(X,Y) : closure(X,Z), + base(Z,Y). Example: assume that at time t1 we have the following facts: base(a,b). closure(a,b). Then insertion of + base(b,c) yields + closure(b,c) by rule 1. But rule 2 produces nothing: we need rule 3 to get: + closure(a,c)

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Rule 2 and 1. 1. + closure(X,Y) : + base(X,Y). 2. + closure(X,Y) : + closure(X,Z), base(Z,Y). 3. + closure(X,Y) : closure(X,Z), + base(Z,Y). We can emulate rules 1 and 3 create rule transclosure1 on base when inserted then insert into closure (select * from inserted ) union select closure.fr, inserted.to from inserted, base where closure.to=inserted.fr Then use triggers to finish the closure computation: create rule transclosure2 on closure when inserted then insert into closure (select inserted.fr, closure.to from inserted, base where inserted.to=base.from) This second rule will trigger itself recursively

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Computing Transitive Closure: non-recursive trigger! Deductive rules for transitive closure of base(fr, to): closure(X,Y) : base(X,Y) closure(X,Y) : closure(X,Z), closure(Z,Y). One new tuple at the time. + base only contains one tuple 1. + closure(X,Y) : + base(X,Y). 2. + closure(X,Y) : closure(X,Z), + base(Z,Y). 3. + closure(X,Y) : + base(X,Z), closure(Z,Y). 4. + closure(X,Y) : closure(X,Z), + base(Z,W), closure(W,Y). Proposition: If + base only contains one tuple, these rules only need to be execute once! Or, + contains many rows but we use the “for each row” semantics. … in fact if they are executed sequentially we no longer need rule 4.

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Delta Maintenance In general, in addition to addition We need to consider deletions and updates. Deletions are mor

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Delta Maintenance after Deletion Discount concrete view defined as follows: discount(X)<- member(X). discount(X) <- senior(X). Let us add new members: + discount(X)<- + member(X).... Done! Remove old members: - discount(X)<- - member(X). Not Done... + discount(X)<- senior(X). Is also needed! or + discount(X)<- - member(X), senior(X). Better! These are called re-insert rules

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More general programs Database: Station(city, state). Train(city1, city2). Deductive rules: r1. Route(c1,c2) :-Train(c1,c2). r2. Route(c1,c2) : Route(c1,c3), Route(c3,c2). r3. ReachCal(c) : Station(c,s), s = ''California'‘. r4. ReachCal(c) : Route(c,c2), ReachCal(c2).

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Addition to DB tables: + Rules Here the delta occur in the base relations: r1i. + Route(c1,c2) :- + Train(c1,c2). r2i1. + Route(c1,c2) :- + Route(c1,c3), Route(c3,c1). r2i2. + Route(c1,c2) :- Route(c1,c3), + Route(c3,c2). r3i. + ReachCal(c) :- + Station(c,s), s = ''California'' r4i1. + ReachCal(c) :- + Route(c,c1), ReachCal(c1). r4i2. + ReachCal(c) :- Route(c,c1), + ReachCal(c1). DB: + Station(city, state). +Train(city1, city2). Rules: r1. Route(c1,c2) :-Train(c1,c2). r2. Route(c1,c2) : Route(c1,c3), Route(c3,c2). r3. ReachCal(c) : Station(c,s), s = ''California'‘. r4. ReachCal(c) : Route(c,c2), ReachCal(c2).

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Delete Rules With concrete views, we need to consider the situation where tuples are deleted from database relations Updates can be treated by combining insert rules and delete rules Delete rules are more complex than insert rules: for insertion. Eg. Add-to vs delete-from b1. + c (X,Y) : + b1(X,Y). - c (X,Y) : - b1(X,Y). c (X,Y) : b2(X,Y). Positive : second rule cannot change the + of the first rule c (X,Y) : b2(X,Y). Negative : The second rule can neutralize the - of the first rule

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Delete Rules: Students taking a Class, dropping the class--Negative Delta in (P, Cno). %P means professor tk(Cno, St). grds(P, St) :- in(P, Cno), tk(Cno, St). %concrete view Assume that the pairs in - are deleted from tk: - grds(P, St) :- in(P, Cno), - tk(Cno, St). (1) These are the tuples that we should consider for elimination from the concrete view. But this is a necessary but not sufficient condition for elimination. Why? … the student could be taking more than one course from the same instructor. Solution: reinsert rules—the original rules constrained by - grds + grds(P, St) : - grads(P, St), in(P, Cno), tk(Cno, St). (2) Thus we first delete and then reinsert. Or we could delete those in (1) who are not in (2): the difference between - and +.

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Delta Maintenance after Deletion Discount concrete view defined as follows: discount(X)<- member(X). discount(X) <- senior(X). Let us add new members: + discount(X)<- + member(X).... Done! Remove old members: - discount(X)<- - member(X). Not Done... + discount(X)<- senior(X). Is also needed! or + discount(X)<- - member(X), senior(X). Better! These are called re-insert rules

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Delete Rules after some trains and stations are eliminated r1d. - Route(c1,c2) :- - Train(c1,c2) r2d1. - Route(c1,c2) :- - Route(c1,c3), OLD_Route(c3,c2) r2d2. - Route(c1,c2) :- OLD_Route(c1,c3), - Route(c3,c2) r3d. - ReachCal(c) :- - Station(c,s), s = ''California'' r4d1. - ReachCal(c) :- - Route(c,c1), OLD_ReachCal(c1) r4d2. - ReachCal(c) :- OLD_Route(c,c1), - ReachCal(c1) DB: - Station(city, state). - Train(city1, city2). Rules: r1. Route(c1,c2) :-Train(c1,c2). r2. Route(c1,c2) : Route(c1,c3), Route(c3,c2). r3. ReachCal(c) : Station(c,s), s = ''California'‘. r4. ReachCal(c) : Route(c,c2), ReachCal(c2).

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Reinsert Rules: using the new_Station and new_Train and the rules with the - magic r1r. + Route(c1,c2) : - Route(c1,c2), Train(c1,c2) r2r. + Route(c1,c2) : - Route(c1,c2), Route(c1,c3), Route(c3,c2). r3r. + ReachCal(c) : - ReachCal(c), Station(c,s), s = ''California'' r4r. + ReachCal(c) : - ReachCal(c), Route(c,c1),ReachCal(c1). new_ DB: Station(city, state). Train(city1, city2). Rules: r1. Route(c1,c2) :-Train(c1,c2). r2. Route(c1,c2) : Route(c1,c3), Route(c3,c2). r3. ReachCal(c) : Station(c,s), s = ''California'‘. r4. ReachCal(c) : Route(c,c2), ReachCal(c2).

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