| In this step, the tree is mapped onto equivalent trees by using a set of formal rules inherited from relational algebra The trees are numbered based on the heuristics to filter candidates that are obviously no recommended. The general rule in this case is "move the nonspatial operators SELECT and PROJECT down in the tree". For each alternative is possible to define the rank Rank = selectivity - 1 / differential cost selectivity(p): cardinality (output(p)) / cardinality (input(p)) The space of alternatives is generated with rules of relational algebra based on notions of commutativity, associativity and distributivity |
Última modificación: Thursday, 24 de November de 2005, 15:08