Query question

Hello,

Thanks to the replier (Martijn -- thank you very much!) to an earlier
question I had about MVCC, I've decided to re-think entirely my use of the
status column in a table. What I've decided to do is to put all of my
new or changed records into a "holding" table, then after they are
indexed, commit the changes to their final location. This has worked
extremely well, except when I am querying the holding table.

This is not the actual table, but my problem can be demonstrated by this
(mode can be "U" for an update/insert or "D" for a delete):

CREATE TABLE listings (
trans_id SERIAL,
mode CHAR(1),
listing_id INT,
region_id INT,
category INT
);

.. so, my process goes along and inserts all these rows into the table,
about 2,000,000 a day. Then it comes time to query the data, I do a query
like this:

"SELECT * FROM listings ORDER BY region_id, category, listing_id,
trans_id" -- this is *very* expensive obviously, but since multiple rows
can be inserted for the same listing_id I have to get the data into some
deterministic order.

There can be multiple writers adding to this listings table, when it comes
time to process it, what I want to do is get only the last transaction for
a given listing_id, because the earlier ones don't matter. On top of
that, each region_id and category_id has its own index. I need to be able
to process the indexes in-full, one-at-a-time because there are too many
to hold that many open filehandles/processes at one time.

So, my question is, is there some way to return the rows in a
deterministic order, without actually having to do an explicit sort on the
data? What I mean is, I don't care if category_id 4 / region_id 10 /
listing_id 10000 comes before category_id 1 / region_id 1 / lisitng_id 1
-- I just need them returned to me in that sort of grouped order (although
sorted by trans_id). And would this even be more efficient in the first
place or am I barking up the wrong tree?

I hope this makes sense, I've been up all night so not thinking too
clearly....

Thanks!

- Greg