that's just the way that it is displayed on the screen, not the actual
size of the data/variable. it's the same with date and timestamps, DSPPFM
"converts" the data to be "easier on the eye" so to speak.
Thanks,
Tommy Holden
From:
"Don Cavaiani" <dcavaiani@xxxxxxxxxxxxx>
To:
"Midrange Systems Technical Discussion" <midrange-l@xxxxxxxxxxxx>
Date:
08/14/2008 12:19 PM
Subject:
RE: Database character fields
What is saved with VARLEN defined ??? I tried it, did a dsppfm, and it
looks like all the max. bytes are used anyway - even on records NOT
exceeding the average length defined?? Or, is this something "hidden"
by OS400?
-----Original Message-----
From: midrange-l-bounces@xxxxxxxxxxxx
[mailto:midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of Mike Cunningham
Sent: Thursday, August 14, 2008 11:54 AM
To: 'Midrange Systems Technical Discussion'
Subject: RE: Database character fields
Yes, the DB2/400 parts all make sense and I was aware of the
rules-of-thumb on how to setup variable length fields and the average
actual use to reduced hits on overflow access. To me (and from the
perspective of DB2/400) fixed length fields are better than variable
length fields if you have a very good idea on the data that will be
going in the field. To me a last name field which on average might hold
10 characters but might hold 30 is not a candidate for variable length.
A field that holds a description for a class (I work at a college) that
might be 100 characters for most but could be 1,000 for a few, is a good
candidate for variable length. Yet when I look at MS SQL columns just
about all character fields are defined as variable length. Got my
curiosity up to want to understand why.
-----Original Message-----
From: midrange-l-bounces@xxxxxxxxxxxx
[mailto:midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of Mark S. Waterbury
Sent: Thursday, August 14, 2008 10:31 AM
To: Midrange Systems Technical Discussion
Subject: Re: Database character fields
Hi, Mike:
I don't know about those other databases, but with DB2/400, I suggest
you define your VARCHAR fields with a minimum length that should be
large enough to contain the "average" or most frequently occuring
strings. That way, most of the time, the data will just fit right there,
within the fixed portion of the record, where that space for that field
is allocated. Only when the data does not fit within this space, will
DB2/400 be forced to use the "overflow" technique. Note that, in this
case, the entire "fixed" area is unused (blank or nulls) and the entire
varchar string data is stored in the overflow area for that record.
So by carefully choosing the minimum or default size to allocate for
those fields, you can ensure that, 80% or 90% of the time, you will not
incur any additional overhead.
Does that "make sense"?
Regards,
Mark S. Waterbury
Mike Cunningham wrote:
Not looking to start a war over which is better, just looking to
update my knowledge.
When we create character fields in DB2/400 we usually setup fixed
length, no null support fields. If the character fields is long (e.g.
100+ bytes) and we suspect the data will vary greatly in actually used
length, we will make it a variable length field. The only time we use
null fields is for data we are importing in from other systems where the
file can have null values. In those cases the file with null field
support is usually a work file used for the import and they we move the
data into non-null fields in the production files. In other databases
(like MS SQL) the standard looks like it is just the opposite. All
character fields are variable length with null support unless you take
extra steps to not do that. My training (and it has been some years)
said that variable length fields are good for saving storage space but
bad for overhead. That the database had to do extra work to manage the
variable length, tracking the actually number of bytes in use and manage
the !
ov!
erflow areas when the data in the fields changed from 10 characters
to 1,000 and back to 10. Is it still true that variable length fields
are less efficient and if so why do other databases have that as the
default? Or is this something specific to the implementation of the
database? Is DB2/400 move efficient with fixed length but MS SQL more
efficient with variable length?
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