Slide 24 of 41
Notes:
Query unnesting is very important: canonical forms have been shown to be useful in other domains (i.e. disjunctive normal forms in logic). Normalization eliminates intermediate data structures created when two operations are composed. It often improves performance as well. The reason is that query unnesting promotes the operators of the inner query to the same level as the operators of the outer query, thus allowing operator mix-up. Query unnesting also makes physical algorithms simpler because their predicates do not contain complex computations, such as other queries.
