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Information Integration Using Logical Views Jeffrey D. Ullman.

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1 Information Integration Using Logical Views Jeffrey D. Ullman

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

3 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

4 Solving Queries by Views Mediator Relations Source Relations

5 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’

6 Computation Complexity

7 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

8 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

9 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

10 IM Example Global Predicates: Mediator relations

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

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

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

14 Tsimmis OEM and MSL Mediator Relations

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

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

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


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