Re: 'locking' the SELECTs based on indices...

> I lock just that particular row, which is no good. I need to have all
> the codes for the service 1 locked, so if it happens that two users send
> the very same code, one has to fail. Therefore, from within plpgsql I
> first do:

I'm a bit tired tonight so I'll simplify your example :

CREATE TABLE stuff ( a INT, b INT );

Basically you want to lock ALL rows with a certain value of a, in order
to perform an operation on only one of them.
You could do this :

CREATE TABLE all_as ( a INT PRIMARY KEY )
CREATE TABLE stuff ( a INT REFERENCES all_as(a), b INT );

Now all the rows in "stuff" that have the same value of "a" reference the
same row in "all_as".
All you have to do is

SELECT * FROM all_as WHERE a=the value FOR UPDATE

and you lock all rows having that particular value of a in the big table.