More Warehouse Design Issues Data cleansing E.g.,correct mistakes in addresses(misspellings,zip code errors) Merge address lists from different sources and purge duplicates How to propagate updates Warehouse schema may be a(materialized)view of schema from data sources What data to summarize Raw data may be too large to store on-line Aggregate values(totals/subtotals)often suffice Queries on raw data can often be transformed by query optimizer to use aggregate values Database System Concepts-6th Edition 20.8 @Silberschatz,Korth and SudarshanDatabase System Concepts - 6 20.8 ©Silberschatz, Korth and Sudarshan th Edition More Warehouse Design Issues Data cleansing E.g., correct mistakes in addresses (misspellings, zip code errors) Merge address lists from different sources and purge duplicates How to propagate updates Warehouse schema may be a (materialized) view of schema from data sources What data to summarize Raw data may be too large to store on-line Aggregate values (totals/subtotals) often suffice Queries on raw data can often be transformed by query optimizer to use aggregate values