ORACLE CBO 的 SQL 自动转换（Cost Based Transformations）之六

同之前聊过的 Group By 配置最优机能同等的，还有 Distinct 配置最优机能（Distinct Placement）

Distinct 配置最优机能（Distinct Placement）

还是用下面的 Test case 进行简单的演示，说明一下 Distinct Placement 动作时执行计划的样子。

drop table t1 purge;
drop table t2 purge;
create table t1(c1 number, c2 number not null);
create table t2(c1 number, c2 number not null);
insert into t1 values (1,1);
insert into t1 values (1,2);
insert into t1 values (2,2);
insert into t2 values (1,2);
commit;

SQL> SQL> select /*+ place_distinct(t2) */
      distinct t1.c2, t2.c2
from t1, t2
where t1.c1 = t2.c1;  2    3    4

        C2         C2
---------- ----------
         1          2
         2          2


Execution Plan
----------------------------------------------------------
Plan hash value: 107983552

-----------------------------------------------------------------------------------------
| Id  | Operation             | Name            | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |                 |     2 |   104 |     8  (25)| 00:00:01 |
|   1 |  HASH UNIQUE          |                 |     2 |   104 |     8  (25)| 00:00:01 |
|*  2 |   HASH JOIN           |                 |     2 |   104 |     7  (15)| 00:00:01 |
|   3 |    VIEW               | VW_DTP_AE9E49E8 |     1 |    26 |     4  (25)| 00:00:01 |
|   4 |     HASH UNIQUE       |                 |     1 |    26 |     4  (25)| 00:00:01 |
|   5 |      TABLE ACCESS FULL| T2              |     1 |    26 |     3   (0)| 00:00:01 |
|   6 |    TABLE ACCESS FULL  | T1              |     3 |    78 |     3   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("T1"."C1"="ITEM_1")

Note
-----
   - dynamic sampling used for this statement (level=2)

特征是使用了内部转换的试图 VW_DTP_*。

关闭此功能的方法是 "_optimizer_distinct_placement" = false。