RE: Database character fields -- MIDRANGE-L

Mike

I'm working some with MySQL - so I googled for "variable length column mysql storage" and got to the v5 manual, which says this--

Variable-length string types are stored using a length prefix plus data.

So it is likely that this is all stored together - not in separate places.

Now I believe that only the iSeries has the ability to set an allocated length, which is the amount set aside in the fixed-length space for these fields. This can have some advantages, while other databases use the length, probably, to know where everything ends and starts.

Just guessing on some of this - heh!

Vern

-------------- Original message --------------
From: Mike Cunningham <mcunning@xxxxxxx>

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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