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IBM recommends DYNSLT if the number of records selected is over 80%. Here's
the scoop from InfoCenter. Note the bit on access path sharing, but also
note the use of "can" in many places - classic "it depends" stuff here.
Dynamic select/omit
With dynamic select/omit, when a program reads records from the file, the
system only returns those records that meet the select/omit values. That
is, the actual select/omit processing is done when records are read by a
program, rather than when the records are added or changed. However, the
keyed sequence access path contains all the keys, not just keys from
selected records. Access paths using dynamic select/omit allow more access
path sharing, which can improve performance. For more information about
access path sharing, see Using existing access paths.
To specify dynamic select/omit, use the dynamic selection (DYNSLT) keyword.
With dynamic select/omit, key fields are not required.
If you have a file that is updated frequently and read infrequently, you
may not need to update the access path for select/omit purposes until your
program reads the file. In this case, dynamic select/omit might be the
correct choice. The following example helps describe this.
You use a code field (A=active, I=inactive), which is changed infrequently,
to select/omit records. Your program processes the active records and the
majority (over 80%) of the records are active. It can be more efficient to
use DYNSLT to dynamically select records at processing time rather than
perform access path maintenance when the code field is changed.
HTH
Vern
At 08:05 PM 2/6/2003 +0000, you wrote:
Correct.
This is really one of those "it all depends". By using the READE approach
you are tackling the requirement through an application program - which
means that the same solution has to be "coded" in every program that would
require that selection path.
By using the DYNSLT approach, it is being done by the database manager ans
all an application (RPG, SQL, Java etc.) has to do is open the file and
process it.
As Steve pointed out, the use of DYNSLT can be quiet an overhead depending
on the number of row returned.
FWIW, I would try the DYNSLT approach first ('cos I always prefer to do as
much as I can in the database as opposed to within a program) and then, if
I wasn't happy with the performance, go for the READE.
HTH
Paul
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