FAST DUAL 引入
在10G之前，每次都要调用dual都会产生逻辑IO，如果一个系统大量的调用DUAL，会产生的逻辑IO会非常惊人。
到了10G，对于DUAL有优化。
SQL> select * from v$version;

BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bi
PL/SQL Release 10.2.0.4.0 - Production
CORE 10.2.0.4.0 Production
TNS for IBM/AIX RISC System/6000: Version 10.2.0.4.0 - Productio
NLSRTL Version 10.2.0.4.0 - Production

SQL> set autotrace traceonly
--注意优化仅仅是针对特定的DUAL语句，如果是查看dual的数据，那是没有变化的。
SQL> select * from dual;

Execution Plan
----------------------------------------------------------
Plan hash value: 3543395131

--------------------------------------------------------------------------
| 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
----------------------------------------------------------
         24 recursive calls
          0 db block gets
          7 consistent gets
          3 physical reads
          0 redo size
        511 bytes sent via SQL*Net to client
        487 bytes received via SQL*Net from client
          2 SQL*Net roundtrips to/from client
          0 sorts (memory)
          0 sorts (disk)
          1 rows processed

SQL> select sysdate from dual;

Execution Plan
----------------------------------------------------------
Plan hash value: 1546270724

-----------------------------------------------------------------
| 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
        519 bytes sent via SQL*Net to client
        487 bytes received via SQL*Net from client
          2 SQL*Net roundtrips to/from client
          0 sorts (memory)
          0 sorts (disk)
          1 rows processed

这是9I的TRACE 结果：
SQL> select * from v$version where rownum < 2;

BANNER
----------------------------------------------------------------
Oracle9i Release 9.2.0.4.0 - Production
SQL> set autotrace traceonly
SQL> select sysdate from dual;

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT ptimizer=CHOOSE
1 0 TABLE ACCESS (FULL) OF 'DUAL'

Statistics
----------------------------------------------------------
          0 recursive calls
          0 db block gets
          3 consistent gets
          0 physical reads
          0 redo size
        383 bytes sent via SQL*Net to client
        499 bytes received via SQL*Net from client
          2 SQL*Net roundtrips to/from client
          0 sorts (memory)
          0 sorts (disk)
          1 rows processed

10G FAST DUAL 的优化