Re: Logical file access paths

[Jim Langston <jlangston@xxxxxxxxxxxxxxxx>]

When you originally create the Logical file the OS will read every
record in the file and put them into the logical file sorted by the
key(s) you selected.  It can take quite a while to read 10M records,
figure out where each record goes in the index, then write it to the
index (the logical file).  It also has to build something called an
"access path", which I am not really sure what this is, but it can
take a little while to build this also (I've watched when copying
files, first it will reindex, then it will build the access path).

Now, you seem to be stating that if you update the key in every
record of the file, it does not take as long as building the logical
in the first place.  Two reasons I could think of this are.  1. You
do not have to rebuild the access path, that is built once and
reused for the life of the logical (or so it seems to me).  2. Are
you certain you are changing the key for every record so that
it actually moves within the logical?

Regards,

Jim Langston

A man who knows, and knows that he knows is a wise man.  Follow him.


Colin Thorpe wrote:

> Good morning/afternoon,
>
> I have a question regarding access paths.
>
> When I create a logical file over a physical file containing 10,000,000
> records, it takes quite some time.
>
> This is presumably building some kind of Matrix/lookup table?
>
> This I can understand.
>
> So then,
> Why, when I update the key to this logical in a Do loop (so attempting to
> re-read the file) , does it not have to update the matrix/lookup table
> (before the re-read) and therefore take a long time to complete.
>
> ie.
> KEY     SETLL           FILE1
>         DOW             *IN31=*OFF
> KEY     READE   FILE1
>         (CHANGE KEY)
>         UPDATE  FILE1   (move this records position within the logical
> order)
>         ENDDO
>
> Thanks in advance
>
> Colin
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