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;
select dummy 与 select sysdate的区别
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转载自blog.csdn.net/w892824196/article/details/84707076
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