Oracle 表连接之Hints

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hash_join可以通过swap_join_inputs来强制指定build表(或者驱动表)，no_swap_join_inputs来强制
指定prob表(探查表,或者说被驱动表)
配合leading或者ordered可以控制多表之前的连接顺序
----------------创建4个测试表
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;
/*
Leading 控制表的关联顺序为T2,T3,T1,T4.先t2,t3做jion,USE_HASH指定表的连接方式,
然后结果集与T1进行关联，通过SWAP_JOIN_INPUTS来指定T1表作为(T2,T3 Hash Join结果集)关联的驱动表。
然后结果集与T4进行关联，通过NO_SWAP_JOIN_INPUTS来指定T4作为探查表(或者说被驱动表)
*/

SELECT
/*+                             
 Leading(T2,T3,T1,T4)
 USE_HASH(T3)                    
 USE_HASH(T1)                    
 SWAP_JOIN_INPUTS(T1)            
 USE_HASH(T4)                    
 NO_SWAP_JOIN_INPUTS(T4)         
 */
 *
  FROM T1,T2,T3,T4
 WHERE T1.OBJECT_ID = T2.OBJECT_ID
   AND T2.OBJECT_NAME = T3.OBJECT_NAME
   AND T3.OWNER = T4.OWNER
   AND T4.OWNER = 'MYDB';
执行计划
----------------------------------------------------------
Plan hash value: 3177457315

-------------------------------------------------------------------------------------
| Id  | Operation            | Name | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |      |  2284 |  1846K|       |  1616   (1)| 00:00:20 |
|*  1 |  HASH JOIN           |      |  2284 |  1846K|       |  1616   (1)| 00:00:20 |
|*  2 |   HASH JOIN          |      |   299 |   181K|  7656K|  1424   (1)| 00:00:18 |
|   3 |    TABLE ACCESS FULL | T1   | 35780 |  7232K|       |   192   (2)| 00:00:03 |
|*  4 |    HASH JOIN         |      |   375 |   151K|  9584K|   851   (1)| 00:00:11 |
|   5 |     TABLE ACCESS FULL| T2   | 44802 |  9056K|       |   193   (2)| 00:00:03 |
|*  6 |     TABLE ACCESS FULL| T3   |     8 |  1656 |       |   191   (1)| 00:00:03 |
|*  7 |   TABLE ACCESS FULL  | T4   |     8 |  1656 |       |   192   (2)| 00:00:03 |
-------------------------------------------------------------------------------------

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

   1 - access("T3"."OWNER"="T4"."OWNER")
   2 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")
   4 - access("T2"."OBJECT_NAME"="T3"."OBJECT_NAME")
   6 - filter("T3"."OWNER"='MYDB')
   7 - filter("T4"."OWNER"='MYDB')