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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 q1q’
ComputationComplexity
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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)
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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
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
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
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)