编者按：
本文作者系杨昱明，现就职于甲骨文公司，从事数据库方面的技术支持。希望能通过发表文章，把一些零散的知识再整理整理。个人主页:https://blog.csdn.net/weixin_50513167，经其本人授权发布。
Hash Join 是不使用索引等价结合时，最有效的结合方式，所以，使用的机会非常之多。
Hash Join 是在内存中作出 Hash Table 用来存放结合数据。通常，会先访问数据量少的表，之后再访问数据量多的表，这样能保证性能。
先访问的表叫做 Build表，第二个访问的表叫做 Probe表。那么假如 3个以上的表进行 Hash Join 时，各表之间的结合顺序，以及那个表作为 Build表，那个表作为 Probe表，如何来控制呢。
下面我们介绍一下相关的 Hint 来进行有效的控制。
代码语言：javascript复制LEADING Hint （指定 Hash Join 顺序）
SWAP_JOIN_INPUTS Hint（指定 Build 表）
NO_SWAP_JOIN_INPUTS Hint（指定 Probe 表）
USE_HASH (指定使用 Hash Join)下面用几个例子来做一下展示。
准备：
代码语言：javascript复制drop table t1 purge;
drop table t2 purge;
drop table t3 purge;
create table t1(c1 number, c2 number);
create table t2(c1 number, c2 number);
create table t3(c1 number, c2 number);
insert into t1 values (1,1);
insert into t2 values (1,2);
insert into t3 values (1,3);
commit;Case 1：
结合顺序是 A->B(Build 表)->C
代码语言：javascript复制SQL> select /* leading(a b c) use_hash(b c) swap_join_inputs(b) */ * from t1 a, t2 b, t3 c where a.c1=b.c1 and a.c1=c.c1;
C1 C2 C1 C2 C1 C2
---------- ---------- ---------- ---------- ---------- ----------
1 1 1 2 1 3
Execution Plan
----------------------------------------------------------
Plan hash value: 1184213596
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 78 | 9 (0)| 00:00:01 |
|* 1 | HASH JOIN | | 1 | 78 | 9 (0)| 00:00:01 |
|* 2 | HASH JOIN | | 1 | 52 | 6 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL| T2 | 1 | 26 | 3 (0)| 00:00:01 |
| 4 | TABLE ACCESS FULL| T1 | 1 | 26 | 3 (0)| 00:00:01 |
| 5 | TABLE ACCESS FULL | T3 | 1 | 26 | 3 (0)| 00:00:01 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("A"."C1"="C"."C1")
2 - access("A"."C1"="B"."C1")Case 2：
结合顺序是 A->B(Build表)->C(Build表)
代码语言：javascript复制SQL> select /* leading(a b c) swap_join_inputs(b) swap_join_inputs(c) */ * from t1 a, t2 b, t3 c where a.c1=b.c1 and a.c1=c.c1;
C1 C2 C1 C2 C1 C2
---------- ---------- ---------- ---------- ---------- ----------
1 1 1 2 1 3
Execution Plan
----------------------------------------------------------
Plan hash value: 1487401159
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 78 | 9 (0)| 00:00:01 |
|* 1 | HASH JOIN | | 1 | 78 | 9 (0)| 00:00:01 |
| 2 | TABLE ACCESS FULL | T3 | 1 | 26 | 3 (0)| 00:00:01 |
|* 3 | HASH JOIN | | 1 | 52 | 6 (0)| 00:00:01 |
| 4 | TABLE ACCESS FULL| T2 | 1 | 26 | 3 (0)| 00:00:01 |
| 5 | TABLE ACCESS FULL| T1 | 1 | 26 | 3 (0)| 00:00:01 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("A"."C1"="C"."C1")
3 - access("A"."C1"="B"."C1")Case 3：
结合顺序是 A->C(Build表)->B
代码语言：javascript复制SQL> select /* leading(a c b) USE_HASH(c b) swap_join_inputs(c) */ * from t1 a, t2 b, t3 c where a.c1=b.c1 and a.c1=c.c1;
C1 C2 C1 C2 C1 C2
---------- ---------- ---------- ---------- ---------- ----------
1 1 1 2 1 3
Execution Plan
----------------------------------------------------------
Plan hash value: 2467348796
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 78 | 9 (0)| 00:00:01 |
|* 1 | HASH JOIN | | 1 | 78 | 9 (0)| 00:00:01 |
|* 2 | HASH JOIN | | 1 | 52 | 6 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL| T3 | 1 | 26 | 3 (0)| 00:00:01 |
| 4 | TABLE ACCESS FULL| T1 | 1 | 26 | 3 (0)| 00:00:01 |
| 5 | TABLE ACCESS FULL | T2 | 1 | 26 | 3 (0)| 00:00:01 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("A"."C1"="B"."C1")
2 - access("A"."C1"="C"."C1")另外，以上面的例子来说，可以用 C->B->A 的顺序进行 Hash Join 吗。不可以哈，因为 B表和 C表之间没有结合键，这样会产生笛卡尔积。
代码语言：javascript复制SQL> select /* leading(c b a) USE_HASH(c b) */ * from t1 a, t2 b, t3 c where a.c1=b.c1 and a.c1=c.c1;
C1 C2 C1 C2 C1 C2
---------- ---------- ---------- ---------- ---------- ----------
1 1 1 2 1 3
Execution Plan
----------------------------------------------------------
Plan hash value: 2210115829
------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 78 | 9 (0)| 00:00:01 |
|* 1 | HASH JOIN | | 1 | 78 | 9 (0)| 00:00:01 |
| 2 | MERGE JOIN CARTESIAN| | 1 | 52 | 6 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL | T3 | 1 | 26 | 3 (0)| 00:00:01 |
| 4 | BUFFER SORT | | 1 | 26 | 3 (0)| 00:00:01 |
| 5 | TABLE ACCESS FULL | T2 | 1 | 26 | 3 (0)| 00:00:01 |
| 6 | TABLE ACCESS FULL | T1 | 1 | 26 | 3 (0)| 00:00:01 |
------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("A"."C1"="B"."C1" AND "A"."C1"="C"."C1")
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