| 10G Fast Dual特性 | DBA之路：OoNiceDream |

10G Fast Dual特性

作者：OoNiceDream【转载时请务必以超链接形式标明文章原始出处和作者信息】
链接：http://www.dbaroad.me/archives/2008/12/10g-fast-dual.html

从10G开始引入了FAST DUAL的概念，一些通过dual表计算，如select 1 from dual; select sysdate from dual; 已经不再需要访问data block了：

sys@ORCL>select * from v$version;
 
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Prod
PL/SQL Release 10.2.0.1.0 - Production
CORE    10.2.0.1.0      Production
TNS for Linux: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production
 
sys@ORCL>
sys@ORCL>select 1 from dual;
 
         1
----------
         1
 
 
Execution Plan
----------------------------------------------------------
Plan hash value: 1388734953
 
-----------------------------------------------------------------
| Id  | Operation        | Name | Rows  | Cost (%CPU)| Time     |
-----------------------------------------------------------------
|   0 | SELECT STATEMENT |      |     1 |     2   (0)| 00:00:01 |
|   1 |  FAST DUAL       |      |     1 |     2   (0)| 00:00:01 |
-----------------------------------------------------------------
 
 
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          0  consistent gets
          0  physical reads
          0  redo size
        404  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

这一特性是通过隐含参数_fast_dual_enabled进行控制的：

sys@ORCL>@getpar
Enter value for par: fast_dual
 
NAME                  VALUE       ISDEFAULT ISMOD      ISADJ
--------------------- ----------- --------- ---------- -----
_fast_dual_enabled    TRUE        TRUE      FALSE      FALSE

另外，如果是使用select * from dual;仍然会去访问data block,产生逻辑读：

sys@ORCL>select * from dual;
 
D
-
X
 
 
Execution Plan
----------------------------------------------------------
Plan hash value: 272002086
 
--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     1 |     2 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| DUAL |     1 |     2 |     2   (0)| 00:00:01 |
--------------------------------------------------------------------------
 
 
Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          3  consistent gets
          0  physical reads
          0  redo size
        407  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed