No, you do not have to go to the end of chain.

When you issue the DBFIND on the master, the beginning of chain, end of
chain and chain count is returned.  If you issue a DBGET mode 6, you will
start reading from the end of the chain.  I can't see why a backward
chained read is inefficient as there is no reason in IMAGE for it to be.

The only real inefficiency might come from the following scenario.  Let us
say your dataset is freshly re-organized (you just did a detpack) and you
have long chains, say over 100 entries and the entry is wide.  In other
words, your chain of records occupies several IMAGE blocks.  Ordinarily,
IMAGE benefits from this as the system brings in multiple pages when a read
is issued, thus the subsequent blocks are already in memory when the
records are requested while following the chain.  I am not sure if the
system will bring pages which are before the requested record or only pages
which are after the requested record.  If the system does not bring back
pages which are before the requested record, then the system will have to
access the disk drive for each block as it reads in the records in backward
chain order.  If this is the case, reading in backward order will be less
efficient than reading forward.

Kind regards,

Denys. . .

Denys Beauchemin
HICOMP America, Inc.
(800) 323-8863  (281) 288-7438         Fax: (281) 355-6879
[log in to unmask]                             www.hicomp.com



-----Original Message-----
From:   Michael A. Dobies [SMTP:[log in to unmask]]
Sent:   Friday, September 05, 1997 10:35 AM
To:     [log in to unmask]
Subject:        Re: Image Inefficient?

Don't you have to go to the endof chain, before you can go backwards?

Michael Dobies

-----Original Message-----
From:   Gary Groves [SMTP:[log in to unmask]]
Sent:   Friday, September 05, 1997 7:49 AM
To:     [log in to unmask]
Subject:        Image Inefficient?

I need to create a new Image dataset. I want to retrieve the records in
reverse date order (LIFO). I was going to do a backwards chain read.
Someone told me that a reverse chain read was not efficient.

For other design reasons, I've decided to use a composite Omnidex key,
but I'm curious if the above is true. If so, Why?

Gary Groves

Kwestions, Kwalms, Kweries, Komments....Kall!


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