1 CIS607, Fall 2005 Semantic Information Integration Presentation by Paea LePendu Week 8 (Nov. 16)

2 Questions from Homework 6 Questions related to some data integration concepts? – The author mentions that most data integration systems use the GAV model. It seems that the LAV model's difficulty in processing queries makes it unappealing. However, it is noted in the paper that it is easier to add sources to the global schema using the LAV model. Is the difficulty in answering queries using the LAV model so prohibitive that the advantages are negligible? -- Shiwoong – What is a conjunctive query? -- Shiwoong – Why is local-as-view expressed in global terms, and GAV in local terms? Doesn’t this seem counterintuitive? -- Enrico – What is an unfolding strategy? – Enrico – What does "arity" mean? The "Reasoning on Queries" section confuses me. What do they mean, "verifying whether one query returns a subset of the result computed by the other query in all databases". All what databases? -- Amanda

3 Questions from Homework 6 (cont ’ d) About data integration concepts? – (This might be a shallow question): The tutorial says that in LAV "adding a new source simply means enriching the mapping with a new assertion, without other changes". I am curious about this: this seems only true if the qG is qualified with source's namespace, otherwise the query result might be influenced by the added source thus contradict to such assertion. - Zebin – The paper talks about LAV/GAV transformation. Is it easy/common in practice? If yes, then the construction and the query for multiple sources are readily solved. Furthermore, is there adaptive transformation for transformation from LAV to GAV? – Zebin – What are the other differences between LAV and GAV? – Is there any methods to judge if a query is good or bad, or say hard or easy ？ -- Jiawei

4 Questions from Homework 6 (cont ’ d) About the approach. – Why processing queries in the LAV approach is a difficult task and why GAV approach provides a specification mechanism that has a more procedural flavor with respect to the LAV approach? -- Donghwi – I cannot understand why the first order logic interpretation of the assertions in the mapping cannot able to cope with inconsistencies between sources.? – Donghwi – The paper compares LAV and GAV. The conclusion is that it is easier to extend LAV with more sources while it is easier to process query with GAV. My understanding is that for GAV, when adding a new source, new mapping and corresponding query processing has to be added between it and the global schema. Isn’t it almost equally hard to add a new source in LAV and GAV? -- Dayi – Is it equally easy to solve data inconsistency in LAV as in GAV? -- Dayi

5 Questions from Homework 6 (cont ’ d) Consider the following relations: – e1: {(1,a), (2,b), (3,c)} e2:{(a,!), – The result for query Q(x,y) is The content of V3 is {1, 2}. The content of V1 is the same as e1. The contents of V2 is the same as e2. Then the query q’(x, y) is actually the following set of pairs: S = {(x, y) | there exist z, such that V3(x) and V1(x, y) and V2(z, y) } However, (1,!) is clearly not a member of this set since V1(1,!) is not true. Similarly, is not a member either. So, S is equal to empty set, which is not the maximally-contained result. Was I making stupid errors here? – -- Xiaofang How exactly are the inverse rules constructed? I didn't understand this section of the paper. -- Julian

6 Questions from Homework 6 (cont ’ d) Other questions. – It seems that this approach can somehow help to solve the problem of semantic gaps of heterogeneous data if we think views as the heterogeneous data sources and queries as on a common schema definition. Then what are the pros and cons of this approach? -- Xiaofang – Would it be possible to build a bridge so that users could work from semantic schemas (such as OWL) instead of the mediated database schema? -- Julian