Re: Help - "Business Day" Problem

On Fri, 27 Apr 2007 18:30:27 +0200, "Michel Cadot"
<micadot{at}altern{dot}org> wrote:


"sybrandb" <sybrandb@xxxxxxxxx> a écrit dans le message de news: 1177681440.943124.58320@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
| On Apr 27, 3:23 pm, "pankaj_wolfhun...@xxxxxxxxxxx"
|
| Exactly.
| In fact the most efficient way to check for existence of a record is
| select 1
| from dual
| where exists
| (select 1
| from emp where empno = :1)
|
| Subqueries over DUAL: I love them!
|
| --
| Sybrand Bakker
| Senior Oracle DBA
|

Is this faster than:

select 1 from emp where empno = :1 and rownum = 1;

-----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 4 | 0 (0)| 00:00:01 |
|* 1 | COUNT STOPKEY | | | | | |
|* 2 | INDEX UNIQUE SCAN| PK_EMP | 1 | 4 | 0 (0)| 00:00:01 |
-----------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

1 - filter(ROWNUM=1)
2 - access("EMPNO"=TO_NUMBER(:1))

Your query:

-----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 2 (0)| 00:00:01 |
|* 1 | FILTER | | | | | |
| 2 | FAST DUAL | | 1 | | 2 (0)| 00:00:01 |
|* 3 | INDEX UNIQUE SCAN| PK_EMP | 1 | 4 | 0 (0)| 00:00:01 |
-----------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

1 - filter( EXISTS (SELECT /*+ */ 0 FROM "EMP" "EMP" WHERE
"EMPNO"=TO_NUMBER(:1)))
3 - access("EMPNO"=TO_NUMBER(:1))


Regards
Michel Cadot


I don't see much difference, I must say.
But I was implicitly referring to the approach I see many times:
select count(*)
into dummy
from emp
where empno = :emp;
if dummy > 0 then
select ename into
....
from emp
where empno = : emp;

I even saw this when the predicate wasn't a PK but a FK.

--
Sybrand Bakker
Senior Oracle DBA
.

Relevant Pages