Re: Varchar and numerical sort sequence

Craig,

MySQL has a generated column functionality (without the need for a
trigger).
Another column can be added like follows:

Create table test_table (
Text1 varchar(100),
Text2 varchar(100) as (LPAD(TRIM(Text1),100," "))
)

Then an index can be placed on the Text2 column for ordering.


Michael Salsman


-----Original Message-----
From: RPG400-L [mailto:rpg400-l-bounces@xxxxxxxxxxxxxxxxxx] On Behalf Of
Craig Richards
Sent: Thursday, April 11, 2019 05:51
To: RPG programming on IBM i <rpg400-l@xxxxxxxxxxxxxxxxxx>
Subject: Re: Varchar and numerical sort sequence

Hi Birgitta,

Thanks for your reply.

I think I understand what you are saying.
I believe that an index is needed, I think that applying a scalar function
to the ORDER BY column won't be fast enough.

If I understand correctly, your idea is to have a view with the
Right-Adjusted column in it.
And build a Derived Index on the main table which uses the same expression
as the View Column.

I haven''t done anything with derived indices before.

I'm getting an error trying to create one with the nested functions:
e.g:

create index vi on glevarc00
(lpad(trim(evttype), 30) as RAdj)
===Index expression 0 not valid===

I can get it to work for a single function:
create index vi on glevarc00
(trim(evttype) as RAdj) S

I think my syntax is correct in the failing attempt?

thanks and regards,
Craig

On Thu, 11 Apr 2019 at 11:52, Birgitta Hauser <Hauser@xxxxxxxxxxxxxxx>
wrote:

What about using LPAD for right adjusting the content (you don't have
to care where the column includes invalid numeric values or not) and
order the result?

ORDER BY LPAD(Trim(YourCharCol), 15) Desc Fetch 10 Rows only

In either way I'd define a view which includes the an additional
column with the LPAD function.
... you easily can add an ORDER BY over the additional column in your
program.

Additionally I'd create a derived index containing a Key Column (with
the LPAD function).
If the syntax of the view column and the syntax of the Key Column in
the index match, the optimizer can use the index. ...and return the
requested very quick.

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!"
„Train people well enough so they can leave, treat them well enough so
they don't want to.“ (Richard Branson)


-----Original Message-----
From: RPG400-L <rpg400-l-bounces@xxxxxxxxxxxxxxxxxx> On Behalf Of
Craig Richards
Sent: Donnerstag, 11. April 2019 09:58
To: RPG programming on the IBM i (AS/400 and iSeries)
<rpg400-l@xxxxxxxxxxxxxxxxxx>
Subject: Varchar and numerical sort sequence

Someone was asking a question on a python board that I subscribe to.

He has a table in a MySQL database which contains a VarChar which
represents a Confirmation Number.

It may be NULL.
And I assume it may contain invalid numerics.

He wants to be able to (for example) retrieve the last 10 Confirmation
Numbers based on a numerical sort sequence.

Obviously he can't order by the existing VarChar Confirmation Number
because it's alpha and he will get results like:
901
900
90
899
898

So he has been using CAST to convert the NOT NULL values to INT and
then sorting on that but his response time for the query is very slow
due to the volume of data (and also I think it's part of a larger
query)

I don't think views will help his cause as you can't use ORDER BY.

My first thought was - can he add another column to the table (over
which he could add a non-unique index) - make it NOT NULLABLE and
default to 0.
Then in the application, modify it when a valid numeric value is
entered in the Confirmation Numer column.

If it's not viable to modify the application - would it make sense to
use an UPDATE TRIGGER to keep the new column maintained.

I don't actually recall having modified the after image in a trigger
program
- is that a reasonable thing to do?

Any advice/comments gratefully accepted.
thanks,
Craig
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