Materialized views--- materialilzed views are available in some very large capacity type relational datablases. A materialized view materializes underlying physical data by making a physical copy ofdata from tables. So, unlike a view as described previously,when a query is executed against a materilalized view, the materialized view is physically accessed rather than the underlyilng tables. The objective is to free the underlying tables for other uses, effectively creating two separate physical copies. Caterialized views are often used to aggregate large data sets down to smaller sized data sets, in data warehouses and data marts. The biggest potential problem with materialized views is how often they are refreshed and brought up to date wuth any changes to theru underlying tables. Another attribute of materialized views is the ability of some database engines to allow a query directed at an underlyilng table to be automatically redirected to a physically much smaller materialized view, sometimes called automated query rewrite. Queries can be automatically rewritten by the query optimizer if the query rewrite can help to increase query performance.
Chapter 4
Table
Figer 4-24: A many-to-many relationship finds duplicate records when unique records are sought.
Another way to look at 3NF is as displayed iln figure 4-25, where fields common to more than one table can be moved to a new table, as shown by the creation of the FOREIGN-EXCHANGE table. At first, this looks like a 2NF transformation because fields not dependent on the primary key are removed to the new table, however, currencies should be conceived as being dependent upon location. Both customer and supplier have addresses and, thus there are transitive dependencies between currencies, through addresses (location), ultimately to customers and suppliers
