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  3. June 2004

Re: Variable length fields

When you use variable-length fields, there is always an additional auxiliary data space associated with the physical file. This space will have segment header, etc., which is part of the overhead. The main data space will have an index or pointer of some type to a location in the auxiliary space.

BTW, there are some gotchas. For one, when you change a variable-length field that has data longer than the allocated length, the location in the auxiliary space is not reused. There's no easy way it could and have acceptable performance. Maybe no way at all. At any rate, the aux space will keep growing. It seems I remember that one of RGZPFM and CPYF COMPRESS(*YES) does NOT reclaim the no-longer-used parts of the auxiliary space. It takes digging down in SST to see this, but it's been at least 2 years since I did this.

IBM has recommendations that depend on your goals - performance vs. space usage - somewhere on one of the sites.

Vern

At 04:54 PM 6/30/2004, you wrote:

> That did not match what we were expecting to see at all.  We thought
> that using variable length fields would take up only as much storage as
> they really need.  Why then would a variable length file be *longer*
> than a fixed length file.

I would think that, given that variable length fields have a certain
amount of overhead, you probably wouldn't realize any space saving until
you have an awful lot more records in the file. Maybe as few as a few
dozen, perhaps a few hundred, possibly a few thousand.

But that's just a guess.

--
JHHL


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