Re: Tables with VARCHAR

Thanks, Jon and Charles.

Suspected that there is no simple answer and I need to make content
analysis.
So first of all I've identified the 3 fields as potential candidates for
switching to VARCHAR.
They are of 80, 255 and 140 length.
Then I applied a query that Charles advised to smaller pieces of data -
just one month instead of 12 years.
Selected 2 one-month chunks, in the beginning and in the end of the period,
as data usage could change also.
Got numbers, sorted them and now I can easily find, that, for example for
CHAR(80) field, in 99% of records actual field length is 30 or less. This
should make sense to switch to VARCHAR2(80) ALLOCATE(30) I suppose?
By the way - are there any "rules of thumb" for this %? Should it be 80% or
99% for example?

I thought that I've indicated it in my question, however probably forgot -
to write that I'm using record-level access (_Rreadx) functions.
Do I understand correctly that for these functions ALLOCATE number has no
effect? I mean that if record has one field VARCHAR2(80), its size will be
ALWAYS 82 bytes in the returned buffer?

And I have not fully understood if this 16-bytes pointer to "overflow" area
is allocated to every record or only to those, where this "overflow"
happened in reality...
Will now make some experiments with the information obtained.

Thanks again,
J.A.




On Wed, Apr 4, 2018 at 7:12 PM, Charles Wilt <[email protected]> wrote:

> Yep...it depends...
>
> what's "pretty large char fields"?  The entire record is only 650
> bytes...so they can't be all that large to my way of thinking..
>
> What's "almost empty"?
>
> run a
> select length(trim(MYFLD)), count(*) as cnt
> from myfile
> group by  length(trim(MYFLD))
> order by 2 desc
>
> If the field is CHAR(100) and most of the records have 20 chars or less...
>
> Then you might be better off with VARCHAR(100) ALLOCATE (20)
>
> That'd save 78 bytes for every record with less than 20 chars.
>
> However, one additional point to consider.  Are you using SQL or record
> level access (RLA)?
>
> With RLA, the system will do two disk I/Os for every record with more than
> 20 chars.  With SQL, those 2 I/Os will only be done if you ask for MYFLD to
> be returned.
>
> (Thus the reason ALLOCATE(0) is not recommended, unless most records are
> empty)
>
> Charles
>
>
> On Wed, Apr 4, 2018 at 8:30 AM, Jon Paris <[email protected]>
> wrote:
>
> > As is often the case - it depends.
> >
> > When you define a field as Varchar you can specify both a "normal" length
> > and a maximum length.  Only the "normal" part is held with the main
> record
> > and therefore if most of your records are within that length then the
> file
> > size will reduce.
> >
> > There is however an overhead for linking to the extended space - can't
> > recall how long it is 20 bytes is ringing a faint bell.  This topic has
> > been discussed in the past on the Midrange list - I'd check it for you
> but
> > have to run out.
> >
> >
> > Jon Paris
> >
> > www.partner400.com
> > www.SystemiDeveloper.com
> >
> > > On Apr 4, 2018, at 10:05 AM, Jevgeni Astanovski <[email protected]>
> > wrote:
> > >
> > > Hi,
> > > Does anyone here know if switching from CHAR to VARCHAR gives any
> > positive
> > > effect in terms of table size?
> > > Explain the situation briefly.
> > > I've got an application written on ILE/C that works with rather big
> table
> > > (historical data since 1996). Currently the table size is 260M records
> > and
> > > PF size 170GB.
> > > The record size is app. 650 bytes and it has some pretty large char
> > fields,
> > > that can be either empty or almost empty :-)
> > >
> > > Technically I can make a new table with VARCHARs and rewrite my program
> > to
> > > use VARCHAR instead of CHAR, but shall it reduce the table size?
> > >
> > > Somehow I suspect that it will only increase the table...
> > >
> > > TIA and regards,
> > > Jevgeni.
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