Re: "extract" command enhancement (subfiles and BIFs)

Jim,

I recently spent some time trying to optimize a page by page subfile 
positioning 
routine.  I started by reading every 4096th record until I hit a greater key 
and then 
went back/forth by half until I hit 1 or the key.  Worked pretty good, but it 
was slower 
than my sequential search which just read every 64th record and then backed up 
when it found a key greater.  I found out that with a scrollable cursor, to get 
the 
record n records away the system actually performed I/O for all of the inner 
records.  Seems odd to that the system performs I/O when simply counting index 
entries would do. 

David Morris

>>> Jim Langston <jlangston@conexfreight.com> 09/23/99 01:24PM >>>
Incidentally, this "halving" technique is one I learned years ago in school 
from a
math teacher.  By using the halving technique, you can pick any number from 1
to 100 within 7 "guesses", with the average being 4 or 5 (not 3.5 because of the
refinement on each consecutive pick)..  On a simple sequential search, 1, 2, 3,
4, etc... it could take 100 picks, with the average being 50.  With 1000 records
the halving technique would take at most 10 guess, and I'm not even going to
try to figure out the average (7 maybe?).  With sequential search, max 1000 and
average of 500.

It's a really good technique, and I have used it in searches before, but there 
is
the necessity that your data is sorted on what you're looking for.  And this is
basically the exact same way the OS does it when it does a SETLL.

Regards,

Jim Langston


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