Select unlocked row in Postgresql

Select... order by id desc limit 1 for update

Users

-----------------------------------------
ID        | Name       |      flags
-----------------------------------------
1         |  bob       |        0
2         |  fred      |        1
3         |  tom       |        0
4         |  ed        |        0

select * from queue where flag=0 order by id desc for update;
update queue set flag=1 where id=:id;
--if you really want the lock:
select * from queue where id=:id for update;
...

Select... order by id desc offset THREAD_NUMBER limit 1 for update

UPDATE users SET flags = 1 WHERE id = ( SELECT id FROM users WHERE flags = 0 ORDER BY id DESC LIMIT 1 ) RETURNING ...;

SELECT ... WHERE FLAG=0 ... FOR UPDATE

UPDATE ... SET FLAG=1 WHERE ID=:id

select  *
into l_sms
from sms
where prefix_id = l_prefix_id
    and invoice_id is null
    and pg_try_advisory_lock(sms_id)
order by suffix
limit 1;

Users

-----------------------------------
ID        | Name       |      flags
-----------------------------------
1         |  bob       |        0
2         |  fred      |        1
3         |  tom       |        0
4         |  ed        |        0

BEGIN;

UPDATE "Users"
SET ...
WHERE ...;
RETURNING ( column list );

COMMIT;

FOR messages in select * from public.messages where sendTime >= CURRENT_TIMESTAMP - '1 SECOND'::INTERVAL AND sendTime <= CURRENT_TIMESTAMP + '1 SECOND'::INTERVAL AND sent is FALSE FOR UPDATE LOOP
-- DO SMTH
END LOOP;

FOR messages in select * from public.messages where sendTime >= CURRENT_TIMESTAMP - '1 SECOND'::INTERVAL AND sendTime <= CURRENT_TIMESTAMP + '1 SECOND'::INTERVAL AND sent is FALSE AND is_locked(msg_id) IS FALSE FOR SHARE LOOP
-- DO SMTH
END LOOP;

CREATE OR REPLACE FUNCTION is_locked(integer) RETURNS BOOLEAN AS $$
DECLARE
    id integer;
    checkout_id integer;
    is_it boolean;
BEGIN
    checkout_id := $1;
    is_it := FALSE;

    BEGIN
        -- we use FOR UPDATE to attempt a lock and NOWAIT to get the error immediately
        id := msg_id FROM public.messages WHERE msg_id = checkout_id FOR UPDATE NOWAIT;
        EXCEPTION
            WHEN lock_not_available THEN
                is_it := TRUE;
    END;

    RETURN is_it;

END;
$$ LANGUAGE 'plpgsql' VOLATILE COST 100;