Raccolta domande teoriche aa 201819

Pagina 1

Politecnico di Milano Technologies for Information Systems Questions a.a. 2018-2019 Author Tommaso Scarlatti January 15, 2019 1 Data Integration Describe the main differences between materialized and virtual data integration, explaining which of these two approaches is used in the case of Data Warehousing and why. Data integration process aims at combining data coming from different data sources, providing the user with a unified vision of the data. Data has four interesting dimensions to be considered: Volume, Variety, Velocity and Veracity. The are two relevant ways of integrating Database Systems: • Materialized Integration: data are merged in a new database. This is the approach adopted in data warehouses, and the software to access, scrape, transform, and load data into warehouses, became known as extract, transform, and load (ETL)systems. ETLisperformedperiodically, buildingan historyofthe enterprise, allowing systematic or ad-hoc data analysis and mining. This approach is not indicated when data needs to be kept up-to-date. • Virtual Integration:this approach leaves the information requested in the local sources. The virtual approach will always return a fresh answer to the query. The query posted to the global schema is reformulated into the formats of the local information system. The information retrieved needs to be combined to answer the query. Describe the conflict analysis step in data integration; describe the most important classes of conflicts and provide a set of examples. Once the identification of the sources and the reverse engineering of their schemata have been completed, the next step in the data integration process is conflict resolution and reconstructing. It starts with the identification of related concepts across sources: a simple process if manual…