-- apply appropriate locks to ...

create or replace function
   transfer(N integer, Src text, Dest text)
   returns integer
declare
   sID integer; dID integer; avail integer;
begin
   --
   LOCK TABLE Accounts IN SHARE UPDATE EXCLUSIVE MODE;
   --
   select id,balance into sID,avail
   from   Accounts where name=Src for update;
   if (sID is null) then
      raise exception 'Invalid source account %',Src;
   end if;
   select id into dID
   from Accounts where name=Dest for update;
   if (dID is null) then
      raise exception 'Invalid dest account %',Dest;
   end if;
   if (avail < N) then
      raise exception 'Insufficient funds in %',Src;
   end if;
   -- total funds in system = NNNN
   update Accounts set balance = balance-N
   where  id = sID;
   -- funds temporarily "lost" from system
   update Accounts set balance = balance+N
   where  id = dID;
   -- funds restored to system; total funds = NNNN
   --
   COMMIT;
   --
   return nextval('tx_id_seq');
end;