MIDRANGE dot COM Mailing List Archive



Home » MIDRANGE-L » August 2014

Re: Variable length fields question



fixed

The term /always/ is an absolute and thus surely innaccurate, as absolutes /always/ are ;-) And the definition of "when needed" is just as problematic. The scope of the topic probably needs to be narrowed to better discuss.

Intuitively, the vlen segment need not be visited by the DB, whenever the length of a value for the currently-accessed row does not require any overflow. For that reason, the /always/ seems excessive for the implication by Alan with regard to accessing the vlen segment, and seems likely to be, to what Jon alludes for the /when needed/ comment. That merely reflects my understanding of the quoted replies, but I offer also, FWiW:

In RLA the /always/ is probably fairly accurate. Any row in which the Record Format includes the VarLen field(s), the full data will necessarily be retrieved [from the fixed portion and vlen segment] for a read; effectively two reads, *whenever* data for the row had been [or for a write must be] stored in that vlen overflow area. The SQL works pretty much the same, but the statement defines the record format for the data accessed, rather than the declared\accessed /file/ defining the record format. Thus for example the RLA program that accesses a record format HasVCflds will for each row that has overflowed data must access the vlen\aux segment whereas a SQL program with a request to "SELECT nonVCfld01, nonVCfld02 FROM FileWithRcdFmtHasVCflds" will never /need/ to visit the vlen segment. But of course similar to how an SQL statement can limit the data accessed, a RLA program can refer to a Logical File (LF) that precludes references to any VarLen fields, thus similarly eliminating any access of the vlen segment by the underlying database support.

Thus for (2.) [i.e. example assumes most values fit within the fixed-portion rather than overflowing to the aux SID], I believe a more accurate description is that both the storage and performance characteristics are likely to be generally favorable, rather than "only" the former. Or as I stated in my reply to the OP, I would expect "the effect could be noticeably positive due to reduced overall storage in the dataspace and minimal secondary read\write activity required [to access the data in the variable-len segment]."

Regards, Chuck

On 29-Jul-2014 11:33 -0500, Jon Paris wrote:
Be interested in whether Chuck or anyone else know differently Alan -
but my understanding has always been that the overflow is only
accessed as and when needed.

On 2014-07-29, at 12:16 PM, Alan Campin wrote:

Based on what has been said on the forum before,

1. Whenever you display anything from the database, the database
will always present it as if there was one field. The sticking
together occurs at the lowest level of the DB.
2. According to what I know, it does not matter. IBM always go to
the overflow and reads it in no matter what so I am not sure the
VARLEN does you any good. I guess the only thing it would do for
you is reduce the amount of disk storage required.

On Tue, Jul 29, 2014 at 10:09 AM, John R. Smith, Jr. wrote:

I have some questions about variable length fields in files
created using DDS.

When I learned about them, I was told that if there is a field
with a max of 100 bytes with a VARLEN(20), the system allocates
20 bytes in the record and if the value is longer than 20 bytes,
it stores the additionally needed bytes elsewhere. When the
record is read, the OS knows to retrieve the 20 bytes from the
record and concatenates the additional bytes from the elsewhere
storage (if any exist) and it returns a 100 byte field. The net
result is less DASD tied up with extraneous spaces in the file
with no impact to the developer. However, when I look at the
file via DSPPFM or WRKLNK, I see all 100 bytes even if all 100
are spaces.

My questions are:
1) Is the OS smart enough to pad out the 21-100 bytes when
displayed via WRKLNK and DSPPFM so I don't see the difference or
is my understanding of how the DDS VARLEN works flawed?
2) Assuming my VARLEN is guessed correctly and most records fit
into the allocated bytes, is there any noticeable impact with
using VARLEN fields?
3) If my guess is incorrect and a big chunk of records require
the additional 21-100 bytes, does this have a huge performance
impact?






Return to Archive home page | Return to MIDRANGE.COM home page

This mailing list archive is Copyright 1997-2014 by MIDRANGE dot COM and David Gibbs as a compilation work. Use of the archive is restricted to research of a business or technical nature. Any other uses are prohibited. Full details are available here. If you have questions about this, please contact