PostgreSQL script gives wrong output - postgresql

Question

Im trying to make a script that will assign a ticket to the agent from specific group and with the lowest amount of tickets. I've already made a loop that can assign ticket to the agent with lowest amount of tickets but when I add the lines where it needs to assign to the specific group the script gives output for only one and the same agent.
This is the output:
NOTICE: Agent ID: 2
NOTICE: Ticket amount: 3
NOTICE: Agent ID: 2
NOTICE: Ticket amount: 3
NOTICE: Assign the ticket to agent 2
DO
Query returned successfully in 506 msec.
do $$
declare
counter int := 1;
zgloszenia int := 0;
agenci int;
agent int :=0;
o int := 1000;
x int;
zgloszenie int;
begin
select count(usr_id) from vuserssupportgroups into agenci;
while agenci > counter loop
select usr_id from vuserssupportgroups where (supportgroupid = '1') order by usr_id limit 1 into agent;
select count(1) from vincidents where serviceuserid = agent and (statusname = 'Nowy' or statusname = 'Otwarty' or statusname = 'W realizacji') into zgloszenia; --policzenie incydentów
select * from incidents where serviceuserid is null and (statusid = '1') into zgloszenie;
raise notice 'Agent ID: %', agent;
raise notice 'Ilość zgłoszeń: %', zgloszenia;
if zgloszenia<o then
o := zgloszenia;
x := agent;
end if;
counter = counter + 1;
end loop;
update incidents
set serviceuserid = x
where incidentid = zgloszenie and serviceuserid is null and supportgroupid = '1';
raise notice 'Przypisz zgłoszenie do agenta %', x;
end
$$;

Answer 1

you wrote nice race condition. When you use UPDATE based on some selected data, then you should to use FOR UPDATE clause ever (you don't need it if you use isolation level SERIALIZABLE, but this can have some performance or operations impacts)