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  2. RPG400-L
  3. December 2012

RE: SQL Question

Even without indexes, performance *might* be good (not very often though).

If Fetch First Row Only is specified and the the engine chooses to read sequentially and a row is found in the first few rows read, then performance will seem good. No more reads will occur.

However, the solution below would continue until all 10 million records are read.

Best, as you know, to have indexes and to write the SQL so that it stops after finding a matching record.

________________________________________
From: rpg400-l-bounces@xxxxxxxxxxxx [rpg400-l-bounces@xxxxxxxxxxxx] on behalf of Charles Wilt [charles.wilt@xxxxxxxxx]
Sent: 21 December 2012 13:03
To: RPG programming on the IBM i / System i
Subject: Re: SQL Question

Without an index...any solution will perform poorly...

The recommended why to check for existence is by using the EXISTS clause

// important to init found
// as if there are no records existing
// it is left unchanged
found = *OFF;
exec SQL
select '1' into :found
from sysibm/sysdummy1
where exists (select * from myfile
where key = :mykey);

HTH,
Charles


On Fri, Dec 21, 2012 at 3:41 AM, McGovern, Sean
<Sean.McGovern@xxxxxxxxxxxx>wrote:

This is not recommended as performance may be poor.

Imagine if there are 10 million rows on the table and no suitable index
for the WHERE clause.

This solution may result in all 10 million records being read just to
determine if 1 row exists (which may be the first row read) !

________________________________________
From: rpg400-l-bounces@xxxxxxxxxxxx [rpg400-l-bounces@xxxxxxxxxxxx] on
behalf of Alan Shore [ashore@xxxxxxxx]
Sent: 19 December 2012 20:39
To: RPG programming on the IBM i / System i
Subject: RE: SQL Question

Another way is
Select count(*) from file where value = whatever

Alan Shore
Programmer/Analyst, Direct Response
E:AShore@xxxxxxxx
P:(631) 200-5019
C:(631) 880-8640
"If you're going through Hell, keep going" - Winston Churchill


-----Original Message-----
From: Alan Shore
Sent: Wednesday, December 19, 2012 3:38 PM
To: RPG programming on the IBM i / System i
Subject: RE: SQL Question


Select * from file where value = whatever

Alan Shore
Programmer/Analyst, Direct Response
E:AShore@xxxxxxxx
P:(631) 200-5019
C:(631) 880-8640
"If you're going through Hell, keep going" - Winston Churchill

-----Original Message-----
From: rpg400-l-bounces@xxxxxxxxxxxx [mailto:rpg400-l-bounces@xxxxxxxxxxxx]
On Behalf Of Jeff Young
Sent: Wednesday, December 19, 2012 3:29 PM
To: RPG programming on the IBM i / System i
Subject: SQL Question

All,
I am having a brain freeze (aka senior moment).
I want to use SQL to determine if a record exists in a file for a given
value.
There will be either only 1 record in the file for the value, or it will
not exist.

Thanks,

--
Jeff Young
Sr. Programmer Analyst
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