John McKown wrote:
On Fri, 24 Apr 2009 07:15:15 -0500, Chase, John <[email protected]> wrote:
-----Original Message-----
From: IBM Mainframe Discussion List On Behalf Of John McKown
[ snip ]
. . .. Take a case in point. A programmer has two VSAM KSDS
files. He is reading one file sequentially and wants to see in the
second
file has as associated record in it (both files are keyed
identically). How
to do that? I will grant that in the past, this was a "no brainer".
However,
this "normal" (perhaps sub-normal?) programmer did it the "easy" way.
He
read the first file sequentially. Then, for each record read, he did a
keyed
read of the second file. The second file contained about 5% of the
number of
records in the first file. So 95% of the time, the keyed read got a
"record
not found" and, as a plus, ended up with its buffers flushed and
positioning
lost.
Of course the correct way to do this is to read each file sequentially
in
parallel (they are in the same order due to using the "same" key
values),
doing a "match merge" type operation. But that mindset seems, at least
around here, to have disappeared. I did the "match merge" code to show
how
to do it (with a dramatic reduction in CPU and elapsed time). The
programmer
considered __that__ to be "too advanced" and "obscure".
IBM already provides a program to do exactly that: ICETOOL.
Syncsort provides a similar program. Don't know about CA-Sort.
Why bother to reinvent that wheel?
-jc-
I simplified too much. The code was more like (pseudo-COBOL):
read master-file
perform until eof-on-master-file
move master-key to second-key
read second-file key second-key
if record-not-found-second then
.... do default processing
else
.... do processing based on data in second-file
end-if
read mater-file
end-perform
it is still "better" (personal opinion) to read both files sequential,
matching the key values and doing the required processing.
--
Interesting. In our intro to COBOL course, we actually use
match merge as a way to approach complex logic structures.
Not everything has to be online, keyed access.
Kind regards,
-Steve Comstock
The Trainer's Friend, Inc.
303-393-8716
http://www.trainersfriend.com
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