Re: CHAIN Versus SETLL and READ When Data Needed -- RPG400-L

Lim -

You're correct about the pointer being lost, and I just wrote a quick testprogram to verify that when the CHAIN is NOT successful, the file fieldsretain whatever value was previously in them, so RPG doesn't clear them onan unsuccessful CHAIN.

However, as far as SETLL and performance, the following is straight from theV5R3 RPG Reference:

SETLL does not cause the system to access a data record. If you are onlyinterested in verifying that a key actually exists, SETLL with an equalindicator (positions 75-76) or the %EQUAL built-in function is a betterperforming solution than the CHAIN operation in most cases. Under specialcases of a multiple format logical file with sparse keys, CHAIN can be afaster solution than SETLL.


- sjl

----- Original Message -----

From: "Lim Hock-Chai" <Lim.Hock-Chai@xxxxxxxxxxxxxxx>
To: "RPG programming on the AS400 / iSeries" <rpg400-l@xxxxxxxxxxxx>
Sent: Wednesday, February 28, 2007 3:57 PM
Subject: RE: CHAIN Versus SETLL and READ When Data Needed

Yes.  If chain failed, the file pointer will be lost.
I was replying about Steve's email regarding it clears the buffer if
chain failed.  I don't believe that is true.  If chain failed, all data
in the program remain the same, which is why I think using setll and
then read will not have any performance gain.  I think it will make it
run slower (Not to mention ugly coding :)).



-----Original Message-----
From: rpg400-l-bounces@xxxxxxxxxxxx
[mailto:rpg400-l-bounces@xxxxxxxxxxxx] On Behalf Of Marvin Radding
Sent: Wednesday, February 28, 2007 3:44 PM
To: rpg400-l@xxxxxxxxxxxx
Subject: RE: CHAIN Versus SETLL and READ When Data Needed


message: 1
date: Wed, 28 Feb 2007 15:04:47 -0600
from: "Lim Hock-Chai" <Lim.Hock-Chai@xxxxxxxxxxxxxxx>
subject: RE: CHAIN Versus SETLL and READ When Data Needed

AFAIK, chain will not clear any data in the program when not found.
With that say, I think chain is faster.  Since setll and read require
two trips.  If chain is not found, it would be like setll.

From the manual:


When the CHAIN operation is successful, the file specified in name is
positioned such that a subsequent read operation retrieves the record
logically following or preceding the retrieved record. When the CHAIN
operation is not completed successfully (for example, an error occurs or
no record is found), the file specified in name must be repositioned
(for example, by a CHAIN or SETLL operation) before a subsequent read
operation can be done on that file.


Just so no one is lead astray, if a CHAIN fails, the pointer is
undefined and any read operation will results in an error.

Marvin


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