Faster access to DUAL

Since Oracle 10g there is a quicker way to access the DUAL table. Many applications use this table such as a "ping" from an application server. Below demonstrates how to SELECT from it over the "old" DUAL table. The difference is 0 consistent gets over 3 consistent gets using the old method. This may not sound like much but adds up for an application executing thousands of these requests in a short span of time.

In order to use FAST DUAL avoid using "SELECT * FROM DUAL", if possible use "SELECT 1 FROM DUAL".

Example:

SQL> set autotrace traceonly timing on
SQL> select * from dual;

Elapsed: 00:00:00.00

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

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
407 bytes sent via SQL*Net to client
380 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 1 from dual;

Elapsed: 00:00:00.01

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


Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
0 consistent gets
0 physical reads
0 redo size
404 bytes sent via SQL*Net to client
380 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;

Elapsed: 00:00:00.00

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


Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
407 bytes sent via SQL*Net to client
380 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 1 from dual;

Elapsed: 00:00:00.00

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


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

Some more information on DUAL:

http://www.orafaq.com/wiki/Dual