no_swap_join_inputs 与 leading

首先建立测试用表如下

create table t1 as select * from dba_objects;
create table t2 as select * from dba_objects;
create table t3 as select * from dba_objects;
create table t4 as select * from dba_objects;

如下语句用leading改驱动表无效

SELECT /*+ leading(t1) use_hash(t1 t2)  use_hash(t1 t3) */
 t1.object_id,t2.object_name,t3.object_name
  FROM t1
 INNER JOIN t2 ON (t1.object_id = t2.object_id)
 INNER JOIN t3 ON (t1.object_name = t3.object_name)

Execution Plan
----------------------------------------------------------
Plan hash value: 1573120526

------------------------------------------------------------------------------------
| Id  | Operation	    | Name | Rows  | Bytes |TempSpc| Cost (%CPU)| Time	   |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |	   |  1218K|   260M|	   |  2841   (1)| 00:00:35 |
|*  1 |  HASH JOIN	    |	   |  1218K|   260M|  8656K|  2841   (1)| 00:00:35 |
|   2 |   TABLE ACCESS FULL | T3   |   113K|  7320K|	   |   347   (1)| 00:00:05 |
|*  3 |   HASH JOIN	    |	   | 77853 |	11M|  8576K|  1445   (1)| 00:00:18 |
|   4 |    TABLE ACCESS FULL| T1   | 96439 |  7440K|	   |   347   (1)| 00:00:05 |
|   5 |    TABLE ACCESS FULL| T2   | 77853 |  6006K|	   |   347   (1)| 00:00:05 |
------------------------------------------------------------------------------------

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

   1 - access("T1"."OBJECT_NAME"="T3"."OBJECT_NAME")
   3 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")

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

这时可以与no_swap_join_inputs配合

SELECT /*+ no_swap_join_inputs(t3) leading(t1) use_hash(t1 t2)  use_hash(t1 t3) */
 t1.object_id,t2.object_name,t3.object_name
  FROM t1
 INNER JOIN t2 ON (t1.object_id = t2.object_id)
 INNER JOIN t3 ON (t1.object_name = t3.object_name);
Execution Plan
----------------------------------------------------------
Plan hash value: 261998084

------------------------------------------------------------------------------------
| Id  | Operation	    | Name | Rows  | Bytes |TempSpc| Cost (%CPU)| Time	   |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |	   |  1218K|   260M|	   |  2841   (1)| 00:00:35 |
|*  1 |  HASH JOIN	    |	   |  1218K|   260M|	12M|  2841   (1)| 00:00:35 |
|*  2 |   HASH JOIN	    |	   | 77853 |	11M|  8576K|  1445   (1)| 00:00:18 |
|   3 |    TABLE ACCESS FULL| T1   | 96439 |  7440K|	   |   347   (1)| 00:00:05 |
|   4 |    TABLE ACCESS FULL| T2   | 77853 |  6006K|	   |   347   (1)| 00:00:05 |
|   5 |   TABLE ACCESS FULL | T3   |   113K|  7320K|	   |   347   (1)| 00:00:05 |
------------------------------------------------------------------------------------

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

   1 - access("T1"."OBJECT_NAME"="T3"."OBJECT_NAME")
   2 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")

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

SELECT /*+ no_swap_join_inputs(t3) leading(t2) use_hash(t1 t2)  use_hash(t1 t3) */
 t1.object_id,t2.object_name,t3.object_name
  FROM t1
 INNER JOIN t2 ON (t1.object_id = t2.object_id)
 INNER JOIN t3 ON (t1.object_name = t3.object_name);
Execution Plan
----------------------------------------------------------
Plan hash value: 1184213596

------------------------------------------------------------------------------------
| Id  | Operation	    | Name | Rows  | Bytes |TempSpc| Cost (%CPU)| Time	   |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |	   |  1218K|   260M|	   |  2841   (1)| 00:00:35 |
|*  1 |  HASH JOIN	    |	   |  1218K|   260M|	12M|  2841   (1)| 00:00:35 |
|*  2 |   HASH JOIN	    |	   | 77853 |	11M|  6920K|  1445   (1)| 00:00:18 |
|   3 |    TABLE ACCESS FULL| T2   | 77853 |  6006K|	   |   347   (1)| 00:00:05 |
|   4 |    TABLE ACCESS FULL| T1   | 96439 |  7440K|	   |   347   (1)| 00:00:05 |
|   5 |   TABLE ACCESS FULL | T3   |   113K|  7320K|	   |   347   (1)| 00:00:05 |
------------------------------------------------------------------------------------

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

   1 - access("T1"."OBJECT_NAME"="T3"."OBJECT_NAME")
   2 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")

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

记之