Re: lookups and performance

["Booth Martin" <Booth@xxxxxxxxxxxx>]

I am curious if removing the K from the F-spec would make a difference i
performance that is more noticeable than a faster %Lookup?


(I won't even mention using Matching records and Primary/secondary.) 
 
 
 
---------------------------------------------------------
Booth Martin   http://www.MartinVT.com
Booth@xxxxxxxxxxxx
---------------------------------------------------------
 
-------Original Message-------
 
From: RPG programming on the AS400 / iSeries
Date: Thursday, June 19, 2003 2:06:54 PM
To: rpg400-l@xxxxxxxxxxxx
Subject: Re: lookups and performance
 
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
 
 
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