Information Integration Using Logical Views Jeffrey D. Ullman

Overview Information Integration Systems Global-as-view (Gav.) vs. Local-as-view (Lav.) Query Reformulation Specification of Source Description Adding new sources

Query Reformulation Problem: rewrite a user query expressed in the mediated schema into a query expressed in the source schema Given a query Q in terms of the mediator schema relations, and descriptions of information sources Find a query Q’ that uses only the source relations, such that – Q’  Q, and – Q’ provides all possible answers to Q given the sources

Solving Queries by Views Mediator Relations Source Relations

Query Rewriting Using Views Query Containment: q’  q   D q’(D)  q(D) Query Equivalence: q’=q  q’  q ^ q  q’ Given query q and view definitions V={v1, …, vn} q’ is an Equivalent Rewriting of q using V if – q’ refers only to views in V, and – q’ = q q’ is an Maximally-Contained Rewriting of q using V if – q’ refers only to views in V and – q’  q, and – There is no rewriting q1, such that q’  q1 and q1  q’

Computation Complexity

Complexity of Query Containment Conjunctive Queries (CQ) (NP-Complete) – Q1: p(X,Z) :- a(X,Y) & a(Y,Z) – Q2: p(X,Z) :- a(X,Y) & a(V,Z) CQ’s With Negation ( -Complete) – Q1: p(X,Z) :- a(X,Y) & a(Y,Z) & NOT a(X,Z) CQ’s With Arithmetic Comparision ( -Complete) – Q1: p(X,Z) :- a(X,Y) & a(Y,Z) & X<Y Datalog Programs – p(A,C) :- a(A,B) & b(B,C)

Specification of Source Description Views: resources that used by integrator to help to answer queries Gav. Mediator relation defined as view over source relations Lav. Source relation defined as view over mediator relations

Information Integration Systems Information Manifold (IM) – AT&T – Local-as-View (Lav) – Description logic – Source relations defined as views of mediator relations ( a collection of global predictions) Tsimmis – Stanford and IBM – Global-as-View (Gav) – Mediator relations defined as views of source relations

IM Example Global Predicates: Mediator relations

IM Example (Cont.) Views: Source Relations Query: “What are Sally’s phone and office?” Mediator Relations

IM Example (Cont.) Answer: Source Relations Query reformulation : Bucket Algorithm (check query containment  NP-Complete (query length) )

Advantages and Disadvantages (IM) Advantage: adding new sources – Mediator (global predicates, source descriptions) – Query processing Disadvantages : query reformulation (Bucket algorithm)

Tsimmis OEM and MSL Mediator Relations

Tsimmis Example Exported OEM Objects Query: “What are Sally’s phone and office?” Mediator Relations Source Relations

Advantage and Disadvantage ( Tsimmis) Advantage – Query reformulation: rule unfolding Disadvantage – Mediation description – Adding, removing, and modifying source description

IM vs. Tsimmis Query Reformulation Adding Sources Levels of Mediation Semistructured Data Constraints Automatic Generation of Components (Wrappers and Mediators)