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Information Integration Using Logical Views
Jeffrey D. Ullman
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Overview
Information Integration Systems
Global-as-view (Gav.) vs. Local-as-view (Lav.)
Query Reformulation Specification of Source
Description Adding new sources
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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
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Solving Queries by Views
Mediator Relations
Source Relations
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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 q1q’
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ComputationComplexity
p
k
p
k
pk
p
k
p
k 1
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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)
p
2
p
2
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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
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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
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IM Example
Global Predicates: Mediator relations
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IM Example (Cont.)
Views: Source Relations
Query: “What are Sally’s phone and office?”
Mediator Relations
Mediator Relations
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IM Example (Cont.)
Answer: Source Relations
Query reformulation : Bucket Algorithm (check query containment NP-Complete (query length) )
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Advantages and Disadvantages (IM)
Advantage: adding new sources– Mediator (global predicates, source descriptions)– Query processing
Disadvantages : query reformulation (Bucket algorithm)
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Tsimmis
OEM and MSL Mediator Relations
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Tsimmis Example
Exported OEM Objects
Query: “What are Sally’s phone and office?”
Mediator Relations
Source Relations
Source Relations
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Advantage and Disadvantage ( Tsimmis)
Advantage– Query reformulation: rule unfolding
Disadvantage– Mediation description– Adding, removing, and modifying source description
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IM vs. Tsimmis
Query Reformulation Adding Sources Levels of Mediation Semistructured Data Constraints Automatic Generation of Components
(Wrappers and Mediators)