Logical file access paths

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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