SWAP_JOIN_INPUTS Oracle Hint

swap_join_inputs是針對哈希連接的hint，它的含義是讓優(yōu)化器交換原哈希連接的驅(qū)動表和被驅(qū)動表的順序，即在依然走哈希連接的情況下讓原哈希連接的驅(qū)動表變被驅(qū)動表，讓原哈希連接的被驅(qū)動表變?yōu)轵?qū)動表。

注意，在swap_join_inputs hint中指定的目標(biāo)表應(yīng)該是原哈希連接中的被驅(qū)動表，否則oracle會忽略該hint。

/*+ swap_join_inputs(原哈希連接的被驅(qū)動表) */

其使用范例如下：

select /*+ leading(dept) use_hash(emp) swap_join_intputs(emp) */ * from emp,dept where
emp.deptno=dept.deptno

測試案例：

SCOTT@ORA12C> create table t1 as select * from dba_objects where rownum<2;
Table created.
SCOTT@ORA12C> create table t2 as select * from dba_objects where rownum<12;
Table created.
SCOTT@ORA12C> create table t3 as select * from dba_objects where rownum<22;
Table created.

收集統(tǒng)計信息：

SCOTT@ORA12C> exec dbms_stats.gather_table_stats(ownname => 'SCOTT',tabname => 'T1',estimate_percent => 100,cascade => true,method_opt => 'for all columns size 1',no_invalidate => false);
PL/SQL procedure successfully completed.
SCOTT@ORA12C> exec dbms_stats.gather_table_stats(ownname => 'SCOTT',tabname => 'T2',estimate_percent => 100,cascade => true,method_opt => 'for all columns size 1',no_invalidate => false);
PL/SQL procedure successfully completed.
SCOTT@ORA12C> exec dbms_stats.gather_table_stats(ownname => 'SCOTT',tabname => 'T3',estimate_percent => 100,cascade => true,method_opt => 'for all columns size 1',no_invalidate => false);
PL/SQL procedure successfully completed.

3個表的記錄如下：

SCOTT@ORA12C> select count(*) from t1;
 COUNT(*)
-----------------
1
1 row selected.
SCOTT@ORA12C> select count(*) from t2;
 COUNT(*)
-----------------
       11
1 row selected.
SCOTT@ORA12C> select count(*) from t3;
 COUNT(*)
-----------------
       21
1 row selected.

現(xiàn)在我們來讓表T2和T3做哈希連接，由于T3表的記錄數(shù)比T2表的記錄數(shù)多，所以這里指定T3為哈希連接的被驅(qū)動表：

select /*+ ordered use_hash(t3) */ t2.object_name,t3.object_type
  2  from t2,t3 where t2.object_id=t3.object_id;
Execution Plan
----------------------------------------------------------
Plan hash value: 1730954469
---------------------------------------------------------------------------
| Id  | Operation   | Name | Rows  | Bytes | Cost (%CPU)| Time  |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |  |    11 |   220 |6   (0)| 00:00:01 |
|*  1 |  HASH JOIN   |  |    11 |   220 |6   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T2   |    11 |   110 |3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| T3   |    21 |   210 |3   (0)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - access("T2"."OBJECT_ID"="T3"."OBJECT_ID")

可以看到，上述SQL的執(zhí)行計劃現(xiàn)在走的是哈希連接，并且被驅(qū)動表示表T3.

如果我們想讓哈希連接的被驅(qū)動表由T3變成T2，可以在上述sql加入swap_join_inputs hint：

select /*+ ordered use_hash(t3) swap_join_inputs(t3) */ t2.object_name,t3.object_type
  2  from t2,t3 where t2.object_id=t3.object_id;
Execution Plan
----------------------------------------------------------
Plan hash value: 1723280936
---------------------------------------------------------------------------
| Id  | Operation   | Name | Rows  | Bytes | Cost (%CPU)| Time  |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |  |    11 |   220 |6   (0)| 00:00:01 |
|*  1 |  HASH JOIN   |  |    11 |   220 |6   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T3   |    21 |   210 |3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| T2   |    11 |   110 |3   (0)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - access("T2"."OBJECT_ID"="T3"."OBJECT_ID")

用leading(t3) use_hash(t2)也可以同樣達到目的：

select /*+ leading(t3) use_hash(t2) */ t2.object_name,t3.object_type
  2  from t2,t3 where t2.object_id=t3.object_id;
Execution Plan
----------------------------------------------------------
Plan hash value: 1723280936
---------------------------------------------------------------------------
| Id  | Operation   | Name | Rows  | Bytes | Cost (%CPU)| Time  |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |  |    11 |   220 |6   (0)| 00:00:01 |
|*  1 |  HASH JOIN   |  |    11 |   220 |6   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T3   |    21 |   210 |3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| T2   |    11 |   110 |3   (0)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - access("T2"."OBJECT_ID"="T3"."OBJECT_ID")

由此可見在兩個表關(guān)聯(lián)的時候，可以用其他hint代替swap_join_inputs來達到相同的目的：

那么多表關(guān)聯(lián)呢：

select /*+ ordered use_hash(t3) */ t1.owner,t2.object_name,t3.object_type
  2  from t2,t3,t1 where t2.object_id=t3.object_id and t1.object_type=t3.object_type;
Execution Plan
----------------------------------------------------------
Plan hash value: 98820498
----------------------------------------------------------------------------
| Id  | Operation    | Name | Rows  | Bytes | Cost (%CPU)| Time   |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |   | 4 |   120 | 9   (0)| 00:00:01 |
|*  1 |  HASH JOIN    |   | 4 |   120 | 9   (0)| 00:00:01 |
|*  2 |   HASH JOIN    |   |11 |   220 | 6   (0)| 00:00:01 |
|   3 |    TABLE ACCESS FULL| T2   |11 |   110 | 3   (0)| 00:00:01 |
|   4 |    TABLE ACCESS FULL| T3   |21 |   210 | 3   (0)| 00:00:01 |
|   5 |   TABLE ACCESS FULL | T1   | 1 |10 | 3   (0)| 00:00:01 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - access("T1"."OBJECT_TYPE"="T3"."OBJECT_TYPE")
   2 - access("T2"."OBJECT_ID"="T3"."OBJECT_ID"

可以看到，現(xiàn)在上述sql的執(zhí)行計劃是先由表T2和表T3做哈希連接，然后將他們做哈希連接的連接結(jié)果集再和表T1做一次哈希連接。

表T1的記錄數(shù)為1，表T2的記錄數(shù)為11，表T3的記錄數(shù)為21，所以當(dāng)表的T2和T3做哈希連接時，記錄數(shù)多的表T3應(yīng)該是被驅(qū)動表，這是因為我們在上述sql中使用了ordered hint和use_hash HINT指定表T3作為表T2和T3連接的時的被驅(qū)動表，所以oracle這里選擇了表T2和T3做哈希連接，并且選擇了表T3作為該哈希連接的被驅(qū)動表，這是沒有問題的，現(xiàn)在問題在于表T1的記錄數(shù)僅為1，所以當(dāng)表T2和T3做哈希連接的結(jié)果再和表T1做哈希連接時，表T1應(yīng)該是驅(qū)動表，而不是在上述執(zhí)行計劃里顯示的那樣作為第二個哈希連接的被驅(qū)動表。

使用下面HINT:

select /*+ ordered use_hash(t3) */ t1.owner,t2.object_name,t3.object_type
  2  from t1,t2,t3 where t2.object_id=t3.object_id and t1.object_type=t3.object_type;
Execution Plan
----------------------------------------------------------
Plan hash value: 38266800
------------------------------------------------------------------------------
| Id  | Operation      | Name | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |      |   4 | 120 |   9   (0)| 00:00:01 |
|*  1 |  HASH JOIN      |      |   4 | 120 |   9   (0)| 00:00:01 |
|   2 |   MERGE JOIN CARTESIAN|      |  11 | 220 |   6   (0)| 00:00:01 |
|   3 |    TABLE ACCESS FULL  | T1   |   1 |  10 |   3   (0)| 00:00:01 |
|   4 |    BUFFER SORT      |      |  11 | 110 |   3   (0)| 00:00:01 |
|   5 |     TABLE ACCESS FULL | T2   |  11 | 110 |   3   (0)| 00:00:01 |
|   6 |   TABLE ACCESS FULL   | T3   |  21 | 210 |   3   (0)| 00:00:01 |
------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - access("T2"."OBJECT_ID"="T3"."OBJECT_ID" AND
      "T1"."OBJECT_TYPE"="T3"."OBJECT_TYPE")

select /*+ leading(t1) use_hash(t3) */ t1.owner,t2.object_name,t3.object_type
  2  from t1,t2,t3 where t2.object_id=t3.object_id and t1.object_type=t3.object_type;
Execution Plan
----------------------------------------------------------
Plan hash value: 2308542799
----------------------------------------------------------------------------
| Id  | Operation    | Name | Rows  | Bytes | Cost (%CPU)| Time   |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |   | 7 |   210 | 9   (0)| 00:00:01 |
|*  1 |  HASH JOIN    |   | 7 |   210 | 9   (0)| 00:00:01 |
|*  2 |   HASH JOIN    |   | 7 |   140 | 6   (0)| 00:00:01 |
|   3 |    TABLE ACCESS FULL| T1   | 1 |10 | 3   (0)| 00:00:01 |
|   4 |    TABLE ACCESS FULL| T3   |21 |   210 | 3   (0)| 00:00:01 |
|   5 |   TABLE ACCESS FULL | T2   |11 |   110 | 3   (0)| 00:00:01 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - access("T2"."OBJECT_ID"="T3"."OBJECT_ID")
   2 - access("T1"."OBJECT_TYPE"="T3"."OBJECT_TYPE")

加入以下hint，就解決:

SELECT /*+ ordered use_hash(t3) swap_join_inputs(t1) */
 t1.owner, t2.object_name, t3.object_type
  FROM t2, t3, t1
 WHERE t2.object_id = t3.object_id
  5     AND t1.object_type = t3.object_type;
Execution Plan
----------------------------------------------------------
Plan hash value: 3071514789
----------------------------------------------------------------------------
| Id  | Operation    | Name | Rows  | Bytes | Cost (%CPU)| Time   |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |   | 4 |   120 | 9   (0)| 00:00:01 |
|*  1 |  HASH JOIN    |   | 4 |   120 | 9   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL | T1   | 1 |10 | 3   (0)| 00:00:01 |
|*  3 |   HASH JOIN    |   |11 |   220 | 6   (0)| 00:00:01 |
|   4 |    TABLE ACCESS FULL| T2   |11 |   110 | 3   (0)| 00:00:01 |
|   5 |    TABLE ACCESS FULL| T3   |21 |   210 | 3   (0)| 00:00:01 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - access("T1"."OBJECT_TYPE"="T3"."OBJECT_TYPE")
   3 - access("T2"."OBJECT_ID"="T3"."OBJECT_ID")