AW: SQL and host indicator

["BirgittaHauser" <Hauser@xxxxxxxxxxxxxxx>]

Charles,

Dan and Elvis both talked about the complete table:
"If no filtering is provided then the count can be obtained from the table
object itself in a single IO."
"In fact, SQE has a fast path for COUNT usage with no selection criteria
(i.e. no WHERE clause)"

With a where clause SQE may get the number of rows from the statistics or if
there is an appropriate Binary Tree Index or an EVI an Index Access or an
symbol table scan can be performed. 

In either way, to avoid any trouble, an SQL statement should be coded to
help the optimizer to find the best access path. 

Mit freundlichen Grüßen / Best regards

Birgitta Hauser

"Shoot for the moon, even if you miss, you'll land among the stars." (Les
Brown)
"If you think education is expensive, try ignorance." (Derek Bok)
"What is worse than training your staff and losing them?  Not training them
and keeping them!"

-----Ursprüngliche Nachricht-----
Von: rpg400-l-bounces@xxxxxxxxxxxx [mailto:rpg400-l-bounces@xxxxxxxxxxxx] Im
Auftrag von Wilt, Charles
Gesendet: Monday, July 21, 2008 20:00
An: RPG programming on the AS400 / iSeries
Betreff: RE: SQL and host indicator


Birgitta,

I don't believe this is the case anymore.

> From the "High Performance Data Access Methods" presentation Q&A by Dan 
> Cruikshank, IBM and Elvis
Budimlic, CTI.

"Dan - No, the count(*) returns a derived column representing the total
number of rows in the result set based on row selection (ie WHERE or ON
clause settings). If no filtering is provided then the count can be obtained
from the table object itself in a single IO.

Elvis - COUNT(*) and COUNT(specificColumn) are functionally the same, so
there should be no performance difference, nor have we ever observed a
contrary case. In fact, SQE has a fast path for COUNT usage with no
selection criteria (i.e. no WHERE clause). If you do have selection
criteria, it is far more important to ensure you have proper indexes to help
query optimizer satisfy the selection criteria.

Dan - When and where to create an EVI depends mainly on your application
requirements. In my presentation I created an EVI to obtain a fast count."



Charles Wilt 
--
Software Engineer
CINTAS Corporation - IT 92B
513.701.1307
wiltc@xxxxxxxxxx


> -----Original Message-----
> From: rpg400-l-bounces@xxxxxxxxxxxx 
> [mailto:rpg400-l-bounces@xxxxxxxxxxxx]
> On Behalf Of BirgittaHauser
> Sent: Monday, July 21, 2008 1:33 PM
> To: 'RPG programming on the AS400 / iSeries'
> Subject: AW: SQL and host indicator
>
> Hi,
>
> If not necessary, I'd not use a count(*), because all rows must be 
> read. If I only want to check if a row exists, I'll use the following 
> SELECT
> statement:
>
> Clear wPrvFL;
> Exec SQL Select '1' into :wPrvFl
>            From  ....
>            Where ...
>            Fetch First Row Only;
>
> --> If a row is found wPrvFl is set to *On, otherwise it will not be
> changed.
> --> The executions stops after the first row is found.
> --> An index only access might be performed.
>
> Mit freundlichen Grüßen / Best regards
>
> Birgitta Hauser
>
> "Shoot for the moon, even if you miss, you'll land among the stars." 
> (Les
> Brown)
> "If you think education is expensive, try ignorance." (Derek Bok)
> "What is worse than training your staff and losing them?  Not training
> them
> and keeping them!"
>
> -----Ursprüngliche Nachricht-----
> Von: rpg400-l-bounces@xxxxxxxxxxxx 
> [mailto:rpg400-l-bounces@xxxxxxxxxxxx]
> Im
> Auftrag von David FOXWELL
> Gesendet: Monday, July 21, 2008 17:56
> An: RPG programming on the AS400 / iSeries
> Betreff: SQL and host indicator
>
>
> Hi all,
>
> I just compiled this. I just wanted to ask if this a perfectly normal 
> way to set an indicator with SQL.
>
> Thanks
>
>
>
> D wPrvFl          S                        N
>
> EXEC SQL
>
> SELECT
>
> CASE WHEN COUNT(*) > 0 THEN '1' ELSE '0' END INTO : wPrvFl
>
> FROM etc
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