Indexed Subfiles and QTP

[Chris Sharman]

>Bear in mind however that sequential access, on an indexed file, still means
>reading the file in primary key order (using RMS anyway).  This by it's
>nature incurs overhead not associated with reading a true sequential file.

>I like (and am interested) Allison's test as well.  It would be interesting
>to see if an indexed subfile has better or worse performance than an indexed
>file created through the dictionary.  I'd still bet that a true sequential
>file will blow away your best, most optimized index file, even on a pure
>sequential read...

In RMS, a well organised index file (ie one with no bucket splits) read
sequentially would provide performance virtually identical to sequential, as
data is stored in primary key order, and sequential reads can read only the
data buckets, completely ignoring all index buckets.

PHD CREATE creates a very poorly tuned RMS indexed file. I've not checked out
indexed subfiles, but I've no reason to imagine they'd be any better.
For populating large indexed files, I would normally write a sequential file,
then use the standard VMS tools (CONVERT, ANALYZE/RMS, EDIT/FDL) to convert &
tune it. Even if you could create a file with well-chosen bucket sizes etc, any
index not populated in order would still potentially finish up with bucket
splits & in need of a VMS convert. We re-tune & convert any files that need it
every Sunday, via an automated procedure.

It's a good thing to do to your Powerhouse dictionary too (it's just another
indexed file, heavily used).

Chris
_______________________________________________________________________
Chris.Sharman@CCAgroup.co.uk		http://www.ccagroup.co.uk/
CCA Stationery Ltd, Eastway, Fulwood, Preston, Lancashire, PR2 9WS.


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