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From: Dennis on 24 Feb 2010 23:20

From: Allen Browne on 25 Feb 2010 00:37
The mantra in the subject line is purely a rule-of-thumb for beginning
database designers. It has no bearing on your question about data access.

Your question revolves around how the data is physically stored on the disk.
You're saying that there is a delay to move the disk heads to different
physical locations to retrieve records, and that delay represents degraded
performance.

While there's some truth in that simple view, the reality is much more
complex than that. Access uses pointers to store the data. If you define a
Text field as 255 characters but enter 2 characters in it, it does not use
up 510 bytes (unicode) of disk space to store those 2 characters.
Internally, the database keeps track of where each field starts in each
record. Some fields (memo, OLE, attachments, MVFs) are not actually stored
in-line with the other data in the record either. So it's not a matter of a
single record being faster to retrieve. There are many other factors at work
here, including whether the database has been compacted recently, whether
you are using page- or record-locking, whether the disk is badly fragmented,
how intelligent are the various layers of caching (physical on the drive, or
the controller, or the operating system, or in Access), the Rushmore
algorithms in JET, and so on. Then we may have to start another mindset to
handle SSD storage issues as well.

In the end, you don't really need to consider these as factors when
designing JET tables. It makes no sense to break a 60-field table down into
one main table with several one-to-one relationships just for performance
reasons. The management of those relationships alone represents unnecessary
complexity for the database and for developer. You may be forced to go that
route if you are subclassing, or if your record is too wide to fit into the
4K page size; otherwise don't even consider it (which is probably what you
were thinking in posting.)

Having said that, having 60 fields in one table is unusual. The database I
happen to be working on right now has 93 tables, and none of them has more
than 32 fields (including 6 for tracking when and by whom the record was
created, most recently edited, and deactivated.)

--
Allen Browne - Microsoft MVP. Perth, Western Australia
Tips for Access users - http://allenbrowne.com/tips.html
Reply to group, rather than allenbrowne at mvps dot org.


"Dennis" <Dennis(a)discussions.microsoft.com> wrote in message
news:BC1859BB-D279-4F37-AAE7-9BA2948A8EE6(a)microsoft.com...
> Hi,
>
> First let me apolozie for the empty question below. I hit the Post button
> my mistake and it posted a blank question. Sorry.
>
> Hi,
>
> I am restating my question because based upon the responses I receive I
> obviously stated my question incorrectly. So please let me try again.
> Hopefully I will be a bit more sucessful this time around.
>
>
> In a previous question, I stated that I added 30 fields the membership
> table
> (one of many in the system). The initial implementation was SO
> successful,
> that the user requested quite a few enhancements resulting in the in a
> HUGE
> increase in the scope of the original project. These changes resulted in
> the addition of 30 new fields to the membership table.
>
> The last time people responded to my question, they were concerned about
> the
> whether or not these were normalized fields. Please let me state that the
> row has been normalized to 3rd normal form and these fields are NOT
> re-occurring fields.
>
> One MVP who responded to my original question stated "Fields are
> expensive,
> records are cheap".
>
> Maybe I am misinterpreting his comment. But to me this means that it is
> better to have smaller row in multiple linked tables than it is to have
> one
> large row that contains all of the normalized data. (IE – it is better to
> split a large normalize row in a single table into multiple rows in
> multiple
> tables). I hope I explained that the right way.
>
>
> My question pertains to the difference in the disk access time it takes to
> retrieve one record from the disk over a network versus retrieving
> multiple
> records versus the overhead involved in Access parsing a lot of fields out
> of
> a large row or parsing the same number of fields from multiple smaller
> rows.
>
>
> I've always been taught the exact opposite - that "Fields are cheap,
> records
> are expensive" since going to disk is so SLOW versus accessing data in
> memory.
>
> Is there something different about Access where the statement "Fields are
> expense, records are cheap" is true? I'm using Access on local machine
> where
> the front and backs end reside on the same machine as well as having
> multiple
> front ends on each client's machine tied into the back end which resides
> on a
> file server over a cat 5 hardwired Ethernet network.
>
> My question is strictly concerning the data access time of multiple row
> over
> the network versus Access' overhead of extracting data from multiple small
> rows versus one large row. And we are assuming a 3rd normal form database
> design.
>
> And it may well be that I am totally misinterpreting the “Fields are
> expensive, records are cheap” comment.
>
> Thank you for your comments.
>
>
>
> Dennis
>
From: John W. Vinson on 25 Feb 2010 01:22
On Wed, 24 Feb 2010 20:51:01 -0800, Dennis <Dennis(a)discussions.microsoft.com>
wrote:

>The last time people responded to my question, they were concerned about the
>whether or not these were normalized fields. Please let me state that the
>row has been normalized to 3rd normal form and these fields are NOT
>re-occurring fields.
>
>One MVP who responded to my original question stated "Fields are expensive,
>records are cheap".

That was me, and that (non-normal structure) was my concern. It is impossible
to tell from a brief post how knowledgable the poster might be about
relational design. As you well know, wide-flat, spreadsheetish, non-normalized
designs are a very common trap for beginners.

You're obviously not a beginner (now that I know more about your background!)
so my reply was out of line. My only excuse is that at the time I did NOT know
your level of skill. I apologize for jumping the gun.

What I consider "expensive" is that an improperly normalized table structure
will require far more work in getarounds, contorted queries, form and report
maintenance, and code than would a properly normalized design. Given that you
have (it seems) valid Entities with sixty or more discrete, atomic,
non-interdependent Attributes, I'll just express my mixed admiration and
sympathy and bow out.
--

John W. Vinson [MVP]
From: Dennis on 25 Feb 2010 02:25
Allen,

Thank you very much for the answering my question.

I understand the issue of disk access quite complex given all those issues
and SCSI versus SATA, non-raid vs different level of raids, speed of the
controller, network switches, routers, and all of the other issues that you
brought up just causes my head to hurt when I even try to think about them.
So you are correct, I did take a VERY simplistic approach in my question.

I did not realize that database actually kept track of where each field
starts and stop. The other variable length record databases I have worked
with just buried a start and stop marker in the record. So while the
"presentation" layer of the database that knew where each field started and
ended, the actual engine had no idea.

I will have to go back and check if it would be possible for all of the
fields to be filled out and possible exceeds the 4k record / row limit. I
did not know about the 4 k limit. Thank you for bringing that up!!!!! You
may have saved me again.

You are correct when you say I was thinking about breaking my 60 field
record into multiple tables. I interpreted the comment from the MVP as
suggesting I should break up 60 field record up into multiple tables. When
an MVP takes time to comment on something I doing, I do try to follow their
advice. It has not yet led me astray.

I have since been informed that the comment "Fields are expensive...” refers
to the dollar cost of added the field to the forms, reports, and the future
maintenance cost associated with that field. But that is another story.

My background is with large commercial insurance companies. All of the
companies have had relational databases designed to the 3rd normal form.
Between the policy information, reinsurance tracking, state, federal, rate
development information we have to maintain for each policy, the typical
policy master table had well over 200 fields in it. The policy master table
just had fields that were common to all policies! Any data that applied to a
specified line (auto, homeowners, etc) is stored in line specific master
policy field. Our coverage tables have over 100 fields. Our Claim master
table had over 100 fields in it.

So for me, 60 fields are actually pretty small. However, I will go back and
re-examine my design. There are things that I could put in a separate table,
but they really are properties of the member.

Thank you for responding and supplying all that wonderful information. It
gives me a better understanding of how Access works internally.


Thanks once again,

Dennis



From: Dennis on 25 Feb 2010 03:24


John,

I know it was you, but I did not feel it was appropriate for me to use your
name in my questions, hence “an MVP commented”.

John, no offence was taken by your comment. I did go back and re-read my
original posting and realized that I did not provide anywhere near enough
information for you to think of anything else. That was entirely my fault
for not being more detailed. I'm still having a problem drawing the line
between not enough detail and too much detail. Sorry about that. I agree
that when you read a brief posting, it is very difficult to tell what a
person's background is. In trying to payback you MVP's for answering my
questions, I have been trying to answer forum questions that I have
previously asked and received answers. I know it is impossible to tell from
a brief posting how knowledgeable a poster might be. Fully agree there.

I also agree with your comment about wide-flat spreadsheetish non-normalized
designs are. I responded to someone tonight about the training database.
They had quite a few issues with the tables. I fully understand the issue.

I had NO problem what so ever with your response and you have not reason at
all to apologize. Personally, I feel bad that you feel the way you do. I
never took offence at your comments.

While I have a lot of db experience, I am still a newbie when it comes to
Access. I thought that Access was like other database engines. While that
is somewhat true, it is mostly a false statement because the VBA code and
events work so differently from anything I've used. I'm still beating my
head against that learning cliff.

I interpreted your comment as implying that it was better to have a couple
of small master tables that one large master table. Since it was coming from
an MVP, I figured that I had better ask some more questions. Since I did not
know how to ask you directly, I just put it on the forum.

I agree with comment about a field being expensive in an improperly
normalized table structure resulting in all sorts of workarounds. 10 of the
30 new fields had to do with funeral information for a deceased member.
Since all of the information is specific to the individual member, I included
on the member table. You might be able to argue that it should go in it own
table, but for data entry and reporting simplicity I included it my member
table.

Actually, your comment did help me because I was reminded about Access 4k
record size limitation. I'm not sure if a completely filled out record will
excess 4k or not, I will have to check. So that alone will prevent a problem
from occurring.

If the potential record size will exceed 4k, I will break on the funeral
information into a separate table as that information is quite lengthy.

I do have one question for you about your “non-interdependent” field. What
do you mean by that? To me, a policy effective date and expiration date are
interdependent because the effective date has to be before the expiration
date. Also City, St, and zip seem to be interdependent. So, what do you
mean by non-interdependent?

Please, never bow out of one my questions. I have learned so much from you
and the other MVP that I would be on the loosing end of that deal.

Thank you for all of your assistance. I have no way to express my
appreciation except to say if you are ever in the Orlando, Fl area I would be
glad to give you and airboat ride in the whiles of the Florida swamps. I'll
show you the real Florida.

Thanks again for all of your help.

Dennis

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