How do I implement FIFO?

I want to be able to have multiple clients pulling single, mutually
exclusive records from a table in the same order that they were inserted
into the table.  Imagine one group of people placing objects on a conveyor
belt, while at the other end another group of people are grabbing the first
item to come off the belt.

I think I know how to lay out the table:   create table XYZ       (       insert_seq_num serial,       is_processed
booleandefault FALSE,       some_data      varchar(10)       );
 

Inserts are obvious.  What I'm stumped by is how to write a SELECT that will
return the next available row in a way that two clients querying at the same
time can't grab the same one.

I think I can do it with temporary tables.  I.e.   select * into temp ABC from XYZ       where is_processed = FALSE and
         insert_seq_num = (select MIN(insert_seq_num)                               from XYZ where is_processed =
FALSE);
   update XYZ set XYZ.is_processed = TRUE where exists       (select * from temp where ABC.insert_seq_num =
XYZ.insert_seq_num);
   select * from temp;   commit;

But even if that would work, temp tables aren't supported in 6.4.  Is there
another way to do this without using temp tables?

Mark
---
Mark Wright
mwright@pro-ns.net
mark_wright@datacard.com