Re: Ideas for logicals (index).

[Charles Wilt <charles.wilt@xxxxxxxxx>]

On Thu, Nov 19, 2009 at 4:34 PM,  <Michael_Schutte@xxxxxxxxxxxx> wrote:
> I believe one logical should be created by Company and Meal Period, then
> just throw in restaurant, date and item behind it.   I say Company and meal
> period first because currently there are only 2 possible values for company
> and 4 possible values for Meal Period.

Company and Meal Period sound like possible candidates for an EVI
index.  EVI indexes ofter considerable help in ad-hoc queries.

> But then my concern comes in when join in other files.
>
> For example, I may want to get region 1 restaurants only.  So I'm going to
> need a logical by company and restaurant, but should I throw in the other
> fields too?
>
> Then we might need to join in the Item Master file to select only
> beverages. So a logical by company and item would be needed.

You certainly want indexes on the foreign key fields.  The white paper
by Mike Cain that Birgitta gave you a link to goes over that.

> Finally, I think a logical by  Company, date would be good to have.
>
> I guess I'm all over the place here, not really sure how to explain myself.
> Does anyone have any suggestions?  I've read for SQL indexes that you don't
> want to create indexes with the same fields (but in different order)
> because it doesn't really buy you anything (take longer for record to be
> inserted).  Would this be true for logicals too?  What's the harm if we add
> the other fields anyway, just in case someone in the future wants to do I/O
> reads on the file or for the group by clause?
>
> I think I've told before that you want to try to create you logicals where
> the key fields with the least amount of unique values are first.

Actually, it's the other way around.  You want the most unique, or
selective, columns first.  The white paper by Mike Cain that Birgitta
gave you a link to is specific in that.

> (Please no DDL solutions, boss doesn't want to start using them yet because
> not too many of us have experience in them).

You need to get this changed, TODAY!

Especially if you're planning on using SQL to query the file.  Two main reasons:
SQL tables and indexes perform better than DDS logical and physical
files.  (Even with RPG RLA access)
EVI indexes can only be created with SQL DDL

Read this:
http://www-03.ibm.com/systems/resources/systems_i_software_db2_pdf_Performance_DDS_SQL.pdf

Also this related Redbook is good to have:
http://www.redbooks.ibm.com/abstracts/sg246393.html


Good luck!
Charles