FAST DUAL执行计划是Oracle10g的新特性。对于利用DUAL进行的计算，可以不用真正的访问表，从而快速的得到结果。
在实际系统中，由于中间件采用了WEBLOGIC，中间件为了确保数据库连接没有问题，需要在每个SQL前面执行一个SELECT 1 FROM DUAL语句，通过这个执行过程来检测数据库连接是否正常。
SQL> SET AUTOT ON
SQL> SELECT 1 FROM DUAL;
1
----------
1
已选择 1 行。
执行计划
----------------------------------------------------------
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 |
-----------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
0 consistent gets
0 physical reads
0 redo size
509 bytes sent via SQL*Net to client
492 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 * FROM DUAL;
D
-
X
已选择 1 行。
执行计划
----------------------------------------------------------
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 |
--------------------------------------------------------------------------
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
512 bytes sent via SQL*Net to client
492 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
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