RE: Better way (more efficient) to find existance of row

  • From: "Powell, Mark" <mark.powell2@xxxxxx>
  • To: "oracle-l@xxxxxxxxxxxxx" <oracle-l@xxxxxxxxxxxxx>
  • Date: Wed, 22 May 2013 20:05:02 +0000

Or just eliminate the count.  Perform the update, check number of rows updated, 
if zero then insert.

-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On 
Behalf Of Norman Dunbar
Sent: Wednesday, May 22, 2013 7:06 AM
To: oracle-l@xxxxxxxxxxxxx
Subject: Re: Better way (more efficient) to find existance of row

Morning Paresh,

On 21/05/13 18:23, Paresh Yadav wrote:
> Using exceptions to handle if-else is a bad idea. The performance will 
> be slow.
Have you tried this and run timings on it? One of my firct contract jobs years 
ago was to go through many millions of lines of PL/SQL, written by a third 
party, and find places to improve it.

There were numerous occurrences of this:

select count(*)
into whatever
from table
where some_condition;

if whatever = 0 then
   insert into table values ....;
else
   update table set .......;
end if;

We did some timings and found that, on average, using exceptions was faster. 
The problem being that the where clause will run through the entire table, 
unless restricted by a rownum = 1, unless there's an index (there invariably 
wasn't!)

That was in the days before the MERGE statement as mentioned by Niall, and 
although I have not yet done any timings myself, I suspect (!) that MERGE will 
be even faster than using exceptions.
When I get a few mins to set up a table with a few million rows, I'll try it 
out and report back, unless someone else already has the info.


> Better to use If else and try to put most probable outcome of the 
> comparison in the if part.
Well, any good developer knows to put the most likely outcome first in an 
IF/CASE/whatever statement - don't they? ;-)


Cheers,
Norm.

-- 
Norman Dunbar
Dunbar IT Consultants Ltd

Registered address:
Thorpe House
61 Richardshaw Lane
Pudsey
West Yorkshire
United Kingdom
LS28 7EL

Company Number: 05132767
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