Query Optimization Using Multiple SIP Strategies

The magic sets rewriting technique (cf. [Ull89]) seems to be the most promising approach to evaluating queries bottom-up for database systems with a powerful view concept. This is in particular the case for systems which will implement the new SQL3 standard and hence will allow the de nition of recursive views. It has been shown that bottom-up query evaluation via magic sets is basically equivalent to top-down approaches and for readability reasons we use Datalog expressions throughout the paper. The attractiveness of magic sets lies in its generality and e ciency. Several approaches to improve the magic set method have been proposed which are often applicable in special cases only, e.g. [SSES90] and [Sagiv90]. Another approach called envelopes [Sagiv90] is as general as the magic set method and can be better than magic sets in many cases. However, given a query execution plan obtained by a single magic set transformation, this plan is normally not reoptimized dynamically, i.e. the actual number of facts generated during query processing cannot be considered.