select dummy 與 select sysdate的區別
select dummy from dual ，select sysdate from dual之間的區別在哪裡呢；
下面來看一下：
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>
從實行計劃，可以看出前者是一個 邏輯讀操作；而後者就好比是一個對 函式的呼叫，是不需要邏輯讀的；