Re: Help - "Business Day" Problem

<sybrandb_at_hccnet.nl> a écrit dans le message de news: ndj433pb0t46mim6siqoq1et3cuciai58b_at_4ax.com...
| On Fri, 27 Apr 2007 18:30:27 +0200, "Michel Cadot"
| <micadot{at}altern{dot}org> wrote:
|
| >
| >"sybrandb" <sybrandb_at_gmail.com> a écrit dans le message de news: 1177681440.943124.58320_at_r30g2000prh.googlegroups.com...
| >| On Apr 27, 3:23 pm, "pankaj_wolfhun..._at_yahoo.co.in"
| >|
| >| 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

I didn't have any opinion, that was just an open remark. I pretty understand your point. :)

Regards
Michel Cadot Received on Fri Apr 27 2007 - 14:41:49 CDT