听了猫大师的课，对Oracle CBO又加深了一步理解：
单表选择率：
selectivity=1/ndv*a4nulls
ocard来自dba_oracle账号tables.num_rows
ndv 来自dba_tab_col_statistics中的num_distinct where 条件为 owner=” AND table_name=” and column_name=”
a4nulls 来自dba_tab_col_statistics中的num_nulls where 条件为 owner=” AND table_name=” and column_name=”