Hmm.... that's a really intesting idea, Tino. Since we're probably
talking about 1000000 numbers max, a query on this table would work
fairly fast, and operationally simple. I'll think about that.
Thanks,
Eric
Tino Wildenhain wrote:
Hi,
Am Mi, den 20.10.2004 schrieb Eric E um 19:52:
Hi Tino,
Many thanks for helping me.
I know that the sequence issue is a troubling one for many on the list.
Perhaps if I explain the need for a continuous sequence I can circumvent
some of that:
This database is for a laboratory, and the numbers in sequence
determine storage locations for a sample. Having a physical space in
our storage boxes tells us something has happened - the sample was used
up, broken, in use, etc - and account for that missing sample. If the
generated sequence has holes in it, we cannot tell if a sample is
properly not in the rack, or if that hole was simply generated by the
database. Allowing empties would also fill up limited box space with
spaces generated by the database.
If anyone has a brilliant idea for how a non-continuous sequence could
address the needs, I'd be delighted to hear it, but short of that I
think I have to keep this requirement.
Maybe you skip the sequence thingy alltogether in this case and
use an approach like this:
initialize a table with all possible locations and mark them
as empty.
CREATE TABLE locations (location_id int2,taken bool);
(you might want to have a timestamp for changes too)
Whenever you change state of a location, do it like this
(perhaps in a function)
SELECT INTO loc_id location_id FROM locations WHERE taken
FOR UPDATE;
IF FOUND THEN
UPDATE location SET taken=true WHERE location_id=loc_id;
ELSE
RAISE EXCEPTION 'no free location anymore';
...
AND the other way round for freeing a location.
The SELECT ... FOR UPDATE should lock the candidate
position in the table so concurrent
transactions have to wait then then find another
free cell when they wake up.
Advantage: not a full table scan. Only the first
matching row should be used and locked.
Not this is only a rough sketch and you should
look for the actual syntax and more flesh for
the function.
Regards
Tino
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