ts_Cardinality_selectivy

先收集统计信息
 BEGIN
   DBMS_STATS.GATHER_TABLE_STATS(ownname          => 'SCOTT',
                                 tabname          => 'T1',
                                 estimate_percent => 100,
                                 method_opt       => 'for all columns size 1',
                                 no_invalidate    => FALSE,
                                 degree           => 1,
                                 cascade          => TRUE);
 END;
 /


然后跑以下脚本，查看表选择性、基数、直方图（查看表所有统计信息）
select a.column_name,
       b.num_rows,
       a.num_distinct Cardinality,
       round(a.num_distinct / b.num_rows * 100, 2) selectivity,
       a.histogram,
       a.num_buckets
  from dba_tab_col_statistics a, dba_tables b
 where a.owner = b.owner
   and a.table_name = b.table_name
   and a.owner = 'SCOTT'
   and a.table_name = 'TEST';
若选择率低于20%；并且列出现在where中，则必须收集直方图