implementing (something like) UNIQUE constraint using PL/pgSQL

Hello,

in our application we need to implement a constraint that enforces 'at
most N rows with this value', that is we have a table with 'flag' column
and for each value there should be at most 10 rows (for example, the
exact number does not matter).

I'm trying to implement a PL/pgSQL trigger to enforce this constraint,
but I'm not sure my solution is 'the best one possible'. The first
solution I came up with is this:

=====================================================================

CREATE OR REPLACE FUNCTION at_most() RETURNS trigger AS $$
DECLARE
v_cnt INTEGER;
p_cnt INTEGER;
BEGIN

IF TG_NARGS >= 1 THEN
p_cnt := TG_ARGV[0]::integer;
ELSE
p_cnt := 1;
END IF;

SELECT COUNT(*) INTO v_cnt FROM my_table WHERE flag = NEW.flag;
IF v_cnt > p_cnt THEN
RAISE EXCEPTION 'Too many rows with this flag!'
END IF;

RETURN NEW;

END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER at_most AFTER INSERT OR UPDATE ON my_table FOR EACH ROW
EXECUTE PROCEDURE at_most(10);

=====================================================================

But that obviously does not work as two sessions can reach the SELECT
INTO statement at the same time (or until one of them commits). Thus
there could be more than 'cnt' rows with the same value.

Then I used a 'FOR UPDATE' lock on a separate 'lock table' that already
contains all possible values of 'flag' (in reality the trigger tries to
insert that value and catches the 'duplicate key' exception but that's
not important here). The trigger is then

=====================================================================

CREATE OR REPLACE FUNCTION at_most() RETURNS trigger AS $$
DECLARE
v_cnt INTEGER;
p_cnt INTEGER;
BEGIN

IF TG_NARGS >= 1 THEN
p_cnt := TG_ARGV[0]::integer;
ELSE
p_cnt := 1;
END IF;

PERFORM flag FROM lock_table WHERE flag = NEW.flag FOR UPDATE;

SELECT COUNT(*) INTO v_cnt FROM my_table WHERE flag = NEW.flag;
IF v_cnt > p_cnt THEN
RAISE EXCEPTION 'Too many rows with this flag!';
END IF;

RETURN NEW;

END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER at_most AFTER INSERT OR UPDATE ON my_table FOR EACH ROW
EXECUTE PROCEDURE at_most(10);

=====================================================================

This works (af far as I know), but I'm not sure it's the best possible
solution - for example I'd like to remove the lock table. Is there some
'more elegant' way to do this?

Tomas

PS: Is there some up to date 'best practices' book related to PL/pgSQL?
All books I've found on Amazon are pretty old (about 5 years) or are
related to 'beginners' or different areas of development (PHP, etc.)