Re: lookups and performance

[Hans Boldt <boldt@xxxxxxxxxx>]

Richard B Baird wrote:
> hey all,
>
> i'm trying to trim a few microseconds per record off of my weekly 2 million
> record update.
>
> I've loaded a conversion table into two field array / data structure - the
> table is several hundred records.
>
> this is how I coded the array:
>
> D                 DS
> D $items                              dim(9999) ascend
> D $aItem                              like(AITEM) overlay($items:1) inz(0)
> D $pItem                              like(PITEM) overlay($items:*next)
> inz(0)
>
> and my lookup looks like this:
>
> C                   eval      $dx = %lookup(CITEMNUM:$aItem:$look1)
>
> C                   if        $dx > 0
> C                   eval      CITEMNUM = $pItem($dx)
> C                   end
>
> ... 
>
> here are my questions:
>
> 1.  would the lookup be faster if I changed it to a table %tlookup rather
> than a paired field array %lookup?

Probably not.

> 2.  does my example above still do a binary search (which i assume is
> faster) even though the array argument of my lookup bif is only part of the
> array ds which doesn't have the ascend keyword, while the DIMmed field has
> the ascend keyword?

The OVERLAY subfields inherit the sequence attribute of the parent. 
So yes, a %LOOKUP on $AITEM will use a binary search. (Fortunately, 
$AITEM is at the beginning of the array element. A %LOOKUP on $PITEM 
would prpbably fail since the elements are probably not in ordered 
sequence.)

If shaving a few microseconds off your job is important, you could 
try profiling a search using the C run-time library function 
"bsearch()". There are three possibilities: It could be faster, it 
oculd be slower, or perhaps there's no measurable difference. Only 
your performance testing and analysis can give you a definite answer.

Cheers! Hans