Estimator
The estimator determines the overall cost of a given execution plan. The estimator generates three different types of measures to achieve this goal:
-
Selectivity
This measure represents a fraction of rows from a row set. The selectivity is tied to a query predicate, such as
last_name='Smith', or a combination of predicates. -
Cardinality
-
Cost
This measure represents units of work or resource used. The query optimizer uses disk I/O, CPU usage, and memory usage as units of work.
If statistics are available, then the estimator uses them to compute the measures. The statistics improve the degree of accuracy of the measures.
最重要的就是上面三個方面:
1.選擇度
2.基數
3.成本