select dummy 与 select sysdate的区别

select  dummy from dual ，select  sysdate from dual之间的区别在哪里呢；

下面来看一下：
SQL> set autotra traceonly exp
SQL> select dummy from dual;

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 |
--------------------------------------------------------------------------

SQL> select sysdate from dual;
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 |
-----------------------------------------------------------------
SQL> 

从实行计划，可以看出前者是一个 逻辑读操作；而后者就好比是一个对 函数的调用，是不需要逻辑读的；