Estimator
Theestimatordetermines 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 aslast_name=’Smith’, or a combination of predicates.
Cardinality
This measure represents the number of rows in a row set.
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个方面:
1.选择度
2.基数
3.成本
，“人无完人金无足赤”，只要是人就不会是完美的，
