Re: AW: AW: Ideas for logicals (index). -- MIDRANGE-L

One last question,

I've gotten the go ahead to use EVI. And we've created one by company and
meal period. My last question (hopefully), I think it would be worthwhile
to do one by company and item and then by company and restaurant. The one
by item would end up being a 2-btye vector and the unit one will be a 2
byte vector too. But should I just do an EVI with company only, EVI by
meal period only, EVI by unit only, EVI by item only. Maybe even do one
by date because it'll be around hundred years before it'll reach the 4-byte
correct?

I'm not sure if company should be included because unit is shared between
the two companies. Item numbers technically could and obviously meal period
is shared between the two.

If I do it just on the single fields, then the query engine could combine
multiple EVIs together and find matches. correct?

--

Michael Schutte
Admin Professional

Bob Evans Holiday Farmhouse Feast, Serves 6-8 l $74.99
A complete homestyle meal TO GO, ready to heat at home, serve & enjoy!
Perfect for Thanksgiving, Christmas or holiday entertaining.
For more information, visit www.FarmhouseFeast.com

Charles Wilt
<charles.wilt@gma
il.com> To
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11/20/2009 10:46 Subject
AM Re: AW: AW: Ideas for logicals
(index).

Please respond to
Midrange Systems
Technical
Discussion
<midrange-l@midra
nge.com>

Mark,

That's a new one to me. I always thought SQL indexes could share
another SQL index.

A quick test seems to confirm it, but where is this documented?

The "conformance to ANSI SQL standards" seems a bit fishy to me. As
I'd expect the sharing to be an implimentation detail tucked deep
inside the OS. I wonder if Chuck or somebody has more information.

Charles

On Fri, Nov 20, 2009 at 10:08 AM, Mark S. Waterbury
<mark.s.waterbury@xxxxxxx> wrote:

Do not forget about Access Path Sharing.

DDS logical files can "share" access paths (including with pre-existing
SQL indexes), while SQL indexes never share access paths (due to
conformance with ANSI SQL standards, apparently.)

So it is probably still a "good idea" to create SQL indexes first, then
create any DDS Logical Files, so that those LFs can potentially share
access paths with some of your SQL indexes.

Mark S. Waterbury

> Birgitta Hauser wrote:

Instead of creating the SQL index first, can I just specify 64k into

the

PAGESIZE keyword?

Yep.
BTW that's why I wrote "per Default".
Sometime ago I asked Mike Cain something about indexes and page sizes.

suggested not to talk about the different page sizes that can be

specified

since V5R4 for logical files and SQL indexes. The IBMers will not speak
about this subject either.

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: midrange-l-bounces@xxxxxxxxxxxx
[mailto:midrange-l-bounces@xxxxxxxxxxxx] Im Auftrag von
Michael_Schutte@xxxxxxxxxxxx
Gesendet: Friday, 20. November 2009 15:05
An: Midrange Systems Technical Discussion
Betreff: Re: AW: Ideas for logicals (index).

Instead of creating the SQL index first, can I just specify 64k into the
PAGESIZE keyword? FYI our default in *KEYLEN not 8k.

--

Michael Schutte
Admin Professional

Bob Evans Holiday Farmhouse Feast, Serves 6-8 l $74.99 A complete

homestyle

meal TO GO, ready to heat at home, serve & enjoy!
Perfect for Thanksgiving, Christmas or holiday entertaining.
For more information, visit www.FarmhouseFeast.com

"Birgitta Hauser"
<Hauser@sss-softw
are.de>

Sent by: "'Midrange Systems Technical
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11/20/2009 01:26

Subject

AM AW: Ideas for logicals (index).

Please respond to
Midrange Systems
Technical
Discussion
<midrange-l@midra
nge.com>

... yet another comment about indexes and keyed logical files:
An SQL index has per default a page size of 64K, while a DDS described
logical file only has per default a page size of 8K.
Because of the larger page size an SQL index cannot share access path

with a

DDS described logical file.
In this way, if you first create an index and after a logical file with

the

same key fields, you'll create only a single access path, which must be
actualize with every modification of the base dates.
If you first create a logical file and after an SQL index with the same

key

fields access path cannot be shared, that means 2 access paths are

created

which must be both actualized.

SQL statements that can use SQL indexes will perform better than if they
have to use DDS described logical files with the same key fields

instead.

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

worse than training your staff and losing them? Not training them and
keeping them!"

-----Ursprüngliche Nachricht-----
Von: midrange-l-bounces@xxxxxxxxxxxx
[mailto:midrange-l-bounces@xxxxxxxxxxxx] Im Auftrag von Birgitta Hauser
Gesendet: Friday, 20. November 2009 07:07
An: 'Midrange Systems Technical Discussion'
Betreff: AW: Ideas for logicals (index).

Hi,

to get a little more theoretically:

There are two kinds of indexing strategies:
1. Proactive Approach - Create the indexes you already know you'll need
You need indexes created over the fields/columns needed to join with
other files/tables.
You also need indexes over fields/columns which will used in the

where

conditions with comparison = or IN-Predicate or Like without leading %.
Other access path can be determined with the reactive indexing

strategy.

You may also create indexes over the fields/columns you'll use in

Group

By and Order By.
But Group By and Order By are not as important as Join and Where
conditions.
The main goal is to get access to the requested data as fast as

possible.

In this way the optimizer may use an index with other key fields than
specified in the order by clause.
Because it is sometimes faster to store the data in a temporary

object

and sort the information stored in the temporary object,
than using an index with the requested order by columns.
Example: You specified 3 fields in the where clause (for example

delivery

date and special delivery terms),
but want to have your result ordered after Order No.
In this situation it may be faster to use an index with

delivery

date and delivery terms instead of using an index with OrderNo.
When Creating Indexes specify the most selective columns first.

2. Reactive Approach - Analyze your SQL environments
2.1. Look at the system wide index advisor, which is a physical
file/table SYSIXADV which exists since V5R4.
In the SYSIXADV all index advices made by both query engines

(CQE

and SQE) are stored.
You'll also get information how often the indexes are advised

and

when the last advice occurred.
You simply can use SQL or query to access this file, better is

use the tools integrated into the iSeries Navigator
2.2. Run your SQL Statement with iSeries Navigator's Run an SQL

Script

and use Visual Explain for explanation and to look for advises.
You may also use a STRDBG (without any parameters) in Green

Screen

before executing STRSQL or calling a program with embedded SQL.
With STRDBG information about your SQL statements are written

into

the joblog, which can be analyzed.
... but STRDBG is no longer a strategic product for SQL

information

and does not contain all information.
2.3. Use STRDBMON or iSeries Navigator's Performance Monitors to

collect

information about SQL statements
To analyze these information use the tools integrated into the
iSeries Navigator database.

There is a very good white paper about indexing strategies written by

Mike

Cain. Even though it is written several years ago (and may not have the
newest features) it excellently covers the basics.
http://www-03.ibm.com/servers/enable/site/bi/strategy/strategy.pdf

DDS described logical files and SQL-Indexes:
Forget about logical files, only create SQL Indexes. Even though an SQL
index cannot be specified directly in an SQL statement it can be used

with

native I/O like any keyed logical file. Beginning with release 6.1 it is
also possible to create additional columns (for example quantity*price)

and

use them as key fields. Further it is also possible to specify where
conditions in an index (which is even more powerful then

SELECT/OMIT-Clauses

in DDS).
You only need DDS described logical files if you need to access a keyed

join

file with native I/O.

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

worse than training your staff and losing them? Not training them and
keeping them!"

-----Ursprüngliche Nachricht-----
Von: midrange-l-bounces@xxxxxxxxxxxx
[mailto:midrange-l-bounces@xxxxxxxxxxxx] Im Auftrag von
Michael_Schutte@xxxxxxxxxxxx
Gesendet: Thursday, 19. November 2009 22:34
An: Midrange Systems Technical Discussion
Betreff: Ideas for logicals (index).

I'm looking for some suggestions. We're going to be creating a large
summary file and I'd like to get some ideas on what logical files I

should

create. Normally, we fly by the seat of our pants and create them when
needed. But since this file is going to be large, I'd like to get them
created beforehand.

Anyway, the file unique key is by Company, Restaurant, Date, Item, Meal
Period. I believe SQL will mainly be used to retrieve the data from

this

file.

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.

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

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.

Finally, I think a locigal 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

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.

(Please no DDL solutions, boss doesn't want to start using them yet

because

not too many of us have experience in them).

--

Michael Schutte
Admin Professional

Bob Evans Holiday Farmhouse Feast, Serves 6-8 l $74.99 A complete

homestyle

meal TO GO, ready to heat at home, serve & enjoy!
Perfect for Thanksgiving, Christmas or holiday entertaining.
For more information, visit www.FarmhouseFeast.com

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