[Info-vax] Help with DECBasic: reading a record in an INDEX SEQ file using a compound key

[Hein RMS van den Heuvel]

On Jan 6, 3:16 pm, Paul Sture <paul.nos... at sture.ch> wrote:
> In article
> <768fc126-2441-4edc-83b9-6e71e2de0... at m3g2000yqf.googlegroups.com>,
>  Hein RMS van den Heuvel <heinvandenheu... at gmail.com> wrote:
>
> > Notice how  I replaces FIND + GET with a direct call to GET.
> > That saves a trip into RMS.
> > There are  times and places where FIND should be used, but they are
> > relatively rare.
>
> FIND + GET is something that I remember a RSTS BASIC programmer banging
> on about many years ago for performance reasons, and I never understood
> why he got so heated about it.

Ok, if you really want to know...

The $FIND call has two potential purposes:

- The $FIND call does NOT copy the record data to the user provided
record buffer, and thus saves on memory transfers and decompression.
Not a big deal really. It helps just a tiny bit (1 % CPU ? ;-) for
example when expunging a bunch of records.

- More importanly (IMHO) It establishes a CURRENT record without
changing the NEXT record.  Thus you can be reading a file record after
record by ordered by primary or alternate key, and still make a side
trip to see whether an other records exists and update or delete if it
does exist. The update is of course a little scary, as per above you
did not get the old data, yet you will 'blindly' provide new data.
Hmmm... ample opportunity for 'hidden' updates.

The $find has two missing features IMHO
1) It does not return the record length.. for quick counts or to grab
the right size buffer.
2) It does not have an option to just return an RFA for alternate
keys. It will always go to the data record to make sure it was not
deleted, but causing what may be an expensive IO + potential RRV IO.
3) It does not have an option to just return the KEY value. Again for
alternate keys without visiting the data: Find persons with lastname
which starts with V and ends in L

The reason I want the second feature is because I'd lover to do a
multi-key lookup by doing a  FINDs and establishing arrays of
candidates, then visit the data records based on matching, or non-
matching, RFA. For example: find all 'SMITH' records, find all
'NASHUA' records, retrieve data for records in both collections. Since
this feature is not there, and RMS goes 'down' to the data record
anyway, you might as well get all 'SMITH's records and look in the
data for the other match criteria.

Cheers,
Hein



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