Fwd: Stock update like application

This message was sended to pgsql-performance first but I can't see much
activity there, so I'm trying it here :-) Sorry for those in both lists !

Hi!
  I'm developing a small application which I'd like to be as fast as
 possible. The program simply receives an order by modem and has to give an
 answer with the products my enterprise will be able to send them. The number
 of products could be as much as 300-400 and I don't want to make my clients
 put I high time-out before the answer is sended.

I do also need to use transactions as I start calculating before the whole
order has been received and if an error occurs everything has to be rolled
back.

Under this circumstances which way do you think it would be faster?

- Make a sequence for each product (we're talking about 20000 available
products so I think it is very big but it might give a really fast answer).

- Using standard SQL queries: SELECT the product, and if there are enough
units UPDATE to decrease the number of available ones. (This one I suppose
it's not very fast as two queries need to be processed for each product).

- Using a CURSOR or something like this which I'm not used to but I've seen
 in the examples.

Should I have the queries saved in the database to encrease performance?

I hope I explained well enough :-) Thanks in advance!

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