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.
One thought I had, and I'd love to hear what people think of this, is to build a table of storage location numbers that are available for use. That way the search for new numbers could be pushed off until some convenient moment well after the user requests them.
Thanks again for any ideas.
Cheers,
Eric
Tino Wildenhain wrote:
Hi,
On Tue, 2004-10-19 at 01:16, Eric E wrote:
Hi,
I have a question about sequences. I need a field to have values with no holes in the sequence. However, the values do not need to be in order.
My users will draw a number or numbers from the sequence and write to the field. Sometimes, however, these sequence numbers will be discarded (after a transaction is complete), and thus available for use. During the transaction, however, any drawn numbers need to be unavailable.
I would like the next user who draws a number to draw the lowest number she can, starting with the holes in the sequence.
This continuous sequence is absolutely required by our company, as the fact that the sequence has no holes is used to check for much more serious problems.
I would recheck this requirement. What should actually be achieved with the check for no holes in the numbering? Remember you can always enumerate using a set returning function or by means of a temporary sequence for a query.
So my question is:
what's the most effective way to get the next available number?
There is none.
My present method is to do a query that finds the first and last number in each of the holes, step through those holes, and then start generating new numbers. Unfortunately, this involves doing a table scan each time - before I generate the number, and does not produce the transaction-safety I want.
You cannot eat the cake and keep it - either you have holes or you have transaction security or you have bad performance by locking the whole table on insert.
Regards Tino
---------------------------(end of broadcast)--------------------------- TIP 8: explain analyze is your friend
