这三类表连接方式是Oracle最基本的连接方式：
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 create table test2 as select * from dba_objects where rownum 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
------------------------------------------------------------------------------------------------------------------
| 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 | | 100 | 100 |00:00:00.01 | 4 | | | |
| 4 | TABLE ACCESS FULL| TEST2 | | 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
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