哈希连接(hash join) 原理

哈希连接(hashjoin)
   访问次数：驱动表和被驱动表都只会访问0次或1次。
   驱动表是否有顺序：有。
   是否要排序：否。
   应用场景： 1. 一个大表，一个小表的关联；
                         2. 表上没有索引；
                         3. 返回结果集比较大。

原理我们说的简单一点，先把驱动表的关联字段hash到PGA中（当然rowid也在PGA中），然后扫描被驱动表，取第一条数据，将关联的字段hash 一下探测PGA中的小表，如果匹配则关联，再取第二条........。

 下面我们来做个试验：

SQL> create table test1 as select * from dba_objects where rownum <=100;
SQL> create table test2 as select * from dba_objects where rownum <=1000;
SQL> exec dbms_stats.gather_table_stats(user,'test1');
SQL> exec dbms_stats.gather_table_stats(user,'test2');
SQL> alter session set statistics_level=all;

SQL> select /*+leading(t1) use_hash(t2)*/count(*)
      from test1 t1, test2 t2
     where t1.object_id = t2.object_id;
  COUNT(*)
----------
       100

SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  3f2mts0kt82u2, child number 0
-------------------------------------
select /*+leading(t1) use_hash(t2)*/count(*)   from test1 t1, test2 t2  where t1.object_id = t2.object_id
Plan hash value: 2544416891

----解释一下：

Starts为该sql执行的次数。
E-Rows为执行计划预计的行数。
A-Rows为实际返回的行数。A-Rows跟E-Rows做比较，就可以确定哪一步执行计划出了问题。
A-Time为每一步实际执行的时间（HH：MM：SS.FF），根据这一行可以知道该sql耗时在了哪个地方。
Buffers为每一步实际执行的逻辑读或一致性读。
Reads为物理读。
OMem、1Mem为执行所需的内存评估值，0Mem为最优执行模式所需内存的评估值，1Mem为one-pass模式所需内存的评估值。
0/1/M 为最优/one-pass/multipass执行的次数。
Used-Mem耗的内存

------------------------------------------------------------------------------------------------------------------
| Id  | Operation           | Name  | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
------------------------------------------------------------------------------------------------------------------
|   1 |  SORT AGGREGATE     |       |      1 |      1 |      1 |00:00:00.01 |      19 |       |       |          |
|*  2 |   HASH JOIN         |       |      1 |    100 |    100 |00:00:00.01 |      19 |  1066K|  1066K| 1162K (0)|
|   3 |    TABLE ACCESS FULL| TEST1 |       |   1000 |   1000 |00:00:00.01 |      15 |       |       |          |
------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")

SQL> select /*+leading(t1) use_hash (t2)*/count(*)
      from test1 t1, test2 t2
     where t1.object_id = t2.object_id
       and t1.object_id = 99999;
  COUNT(*)
----------
         0

SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  f9zwsrs05kg0n, child number 0
-------------------------------------
select /*+leading(t1) use_hash (t2)*/count(*)   from test1 t1, test2 t2  where t1.object_id =
t2.object_id    and t1.object_id = 99999
Plan hash value: 2544416891
------------------------------------------------------------------------------------------------------------------
| Id  | Operation           | Name  | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
------------------------------------------------------------------------------------------------------------------
|   1 |  SORT AGGREGATE     |       |      1 |      1 |      1 |00:00:00.01 |       4 |       |       |          |
|*  2 |   HASH JOIN         |       |      1 |      1 |      0 |00:00:00.01 |       4 |   921K|   921K|  176K (0)|
|*  3 |    TABLE ACCESS FULL| TEST1 |       |      1 |      0 |00:00:00.01 |       4 |       |       |          |
|*  4 |    TABLE ACCESS FULL| TEST2 |       |      1 |      0 |00:00:00.01 |       0 |       |       |          |
------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")
   3 - filter("T1"."OBJECT_ID"=99999)
   4 - filter("T2"."OBJECT_ID"=99999)

SQL> select /*+leading(t1) use_hash (t2)*/count(*)
  2    from test1 t1, test2 t2
  3   where t1.object_id = t2.object_id
  4     and 1=2;
  COUNT(*)
----------
         0
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  bnrfbt4ybxnnp, child number 0
-------------------------------------
select /*+leading(t1) use_hash (t2)*/count(*)   from test1 t1, test2 t2  where t1.object_id =
t2.object_id    and 1=2
Plan hash value: 1013001923
---------------------------------------------------------------------------------------------------------
| Id  | Operation            | Name  | Starts | E-Rows | A-Rows |   A-Time   |  OMem |  1Mem | Used-Mem |
---------------------------------------------------------------------------------------------------------
|   1 |  SORT AGGREGATE      |       |      1 |      1 |      1 |00:00:00.01 |       |       |       |
|*  2 |   FILTER             |       |      1 |        |      0 |00:00:00.01 |       |       |       |
|*  3 |    HASH JOIN         |       |      0 |    100 |      0 |00:00:00.01 |   921K|   921K|       |
|   4 |     TABLE ACCESS FULL| TEST1 |       |    100 |      0 |00:00:00.01 |       |       |       |
|   5 |     TABLE ACCESS FULL| TEST2 |       |   1000 |      0 |00:00:00.01 |       |       |       |
---------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - filter(NULL IS NOT NULL)
   3 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")