Re: Variable length fields question

[Charles Wilt <charles.wilt@xxxxxxxxx>]

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?

Probably, you are basically doubling the disk I/O.

2)  Assuming my VARLEN is guessed correctly and most records fit into the
allocated bytes, is there any noticeable impact with using VARLEN fields?

Possibly an improvement depending on how you're using them.  For instance,
if you store the data in VARLEN fields and remove a bunch of %TRIMR() from
code that was concatenate multiple fields together.  You can
see considerable improvement without the negative impact you'd see with
with respect to question 3.

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?

Yes, the OS is smart enough to pad out the field for display.  It doesn't
read the overflow area unless there's data there.  (ie length is > 20 in
your example)

Charles


On Tue, Jul 29, 2014 at 12:09 PM, John R. Smith, Jr. <smith5646@xxxxxxxxx>
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?
>
> Thanks.
> John
>
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