oracle12c approx_count_distinct

SQL> alter session set container=pdb1;会话已更改。SQL> create table t1 as select level as data from dual connect by level<=10000;表已创建。SQL> insert /*+ APPEND */ into t1 select a.data from t1 a cross join t1 b;已创建 100000000 行。SQL> commit;提交完成。SQL> exec dbms_stats.gather_table_stats(user,'T1');PL/SQL 过程已成功完成。SQL> set timing onSQL> select ocunt(distinct data) as data_c from t1;select ocunt(distinct data) as data_c from t1       *第 1 行出现错误:ORA-00904: "OCUNT": 标识符无效已用时间:  00: 00: 00.00SQL> select count(distinct data) as data_c from t1;    DATA_C----------     10000已用时间:  00: 00: 09.40SQL> select approx_count_distinct(data) as data_c from t1;    DATA_C----------     10050已用时间:  00: 00: 04.57