oracle 散列联结 hash join

用sys登录。

有关hash join的原理见http://blog.csdn.net/feier7501/article/details/22695117

测试：

SQL> conn /as sysdba
已连接。
SQL> create table segs as select * from dba_segments where owner='SYS';

表已创建。

SQL> create table objts as select * from dba_objects where owner='SYS';

表已创建。

SQL> select count(*) from segs;

  COUNT(*)
----------
      1655

SQL> select count(*) from objts;

  COUNT(*)
----------
     22994

SQL> create index idx_segs_name on segs(segment_name);

索引已创建。

SQL> create index idx_objts_name on objts(object_name);

索引已创建。

SQL> exec dbms_stats.gather_table_stats(user,'SEGS',cascade => true);

PL/SQL 过程已成功完成。

SQL> exec dbms_stats.gather_table_stats(user,'OBJTS',cascade => true);

PL/SQL 过程已成功完成。

SQL> set autotrace traceonly
SQL> alter system flush buffer_cache;

系统已更改。

SQL> alter system flush shared_pool;

系统已更改。

SQL> select * from segs, objts where segs.segment_name = objts.object_name;

已选择2175行。


执行计划
----------------------------------------------------------
Plan hash value: 779051904

----------------------------------------------------------------------------
| Id  | Operation          | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |       |  1674 |   297K|    80   (4)| 00:00:01 |
|*  1 |  HASH JOIN         |       |  1674 |   297K|    80   (4)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| SEGS  |  1655 |   145K|     7   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| OBJTS | 22994 |  2065K|    72   (3)| 00:00:01 |
----------------------------------------------------------------------------

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

   1 - access("SEGS"."SEGMENT_NAME"="OBJTS"."OBJECT_NAME")


统计信息
----------------------------------------------------------
        972  recursive calls
          0  db block gets
        631  consistent gets
        346  physical reads
          0  redo size
     173106  bytes sent via SQL*Net to client
       1969  bytes received via SQL*Net from client
        146  SQL*Net roundtrips to/from client
          6  sorts (memory)
          0  sorts (disk)
       2175  rows processed

SQL>

测试例子转自： http://lizhen3708693.iteye.com/blog/1631360