Re: Usage of Indexes and its Effects on an RPG Program

I have written many data warehouse routines, that churn through record
counts in the hundreds of millions.  And because speed has been the
principal goal (other than accuracy), I have conducted many tests using READ
vs. READE.  I have found that the real difference between the two is that
READE is doing a LOOKUP each time it runs, while READ is obviously only
grabbing the next record.  If you use READE and specify BLOCK(*NO), versus
using READ and specifying BLOCK(*YES), and overriding the block size to an
I/O block of 128k, the differences are very real.  In smaller cases (reading
a few hundred to a few thousand records), the differences are smaller.

One of the key things to watch out for though, is if you use READE, and
specify BLOCK(*YES), as Scott suggested for the test.  In this case, you are
doing a DB LOOKUP, reading a block-size buffer of records, and tossing the
entire block out at the start of each READE operation -- now that is the
worst case, and will result in staggering performance differences.

So I don't believe I have ever 'bashed' READE, it is good for certain
operations, and for smaller volumes.  You just have to consider what the
goal of the program is.  I have gone so far as to study the file being read,
and figure out what the average/max/min records per account is, and set the
block size accordingly, to read all of account's records in one READ.

Bottom line -- If you are asking about performance, you are going to get
advice from people like me that are willing to go the extra mile for those
few seconds/minutes.

On 12/14/05, Rich Duzenbury <rduz-midrange@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, 2005-12-14 at 16:49 -0500, rob@xxxxxxxxx wrote:
> > "In my opinion, there should be..."
>
> What I've seen is many folks bash READE, and recommend replacing it with
> a READ + test, with only anecdotal evidence to back up the claim that it
> runs faster.  I've at least bothered to write test code that shows that
> in this one circumstance, they operate in very close to the same amount
> of time.
>
> Since I have no access to the source code for OS/400, I can only do my
> best to devise a fair test program, run it, and interpret the results.
> I also try to use my experience as a developer to hypothesize about how
> another programmer might write the function(s) in question.
>
> Regards,
> Rich
>
>
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