CREATE SCHEMA myschema;
SET search_path = 'myschema';

CREATE TABLE items_with_colors(
item_id serial PRIMARY KEY,
color text NOT NULL
);

CREATE OR REPLACE FUNCTION check_too_many_blue() RETURNS TRIGGER
AS $func$
DECLARE
cnt bigint;
BEGIN
IF (pg_catalog.current_setting('transaction_isolation') <> 'serializable') THEN
   RAISE EXCEPTION 'This database must be used in transaction isolation level SERIALIZABLE!';
END IF;
SELECT COUNT(*) INTO cnt
  FROM items_with_colors
 WHERE color = 'blue';
IF cnt > 7 THEN
   RAISE EXCEPTION 'Too many blue records!';
END IF;
RETURN NULL;
END;
$func$ LANGUAGE plpgsql SET search_path FROM CURRENT;

CREATE TRIGGER check_too_many_blue
 AFTER INSERT OR UPDATE ON items_with_colors
   FOR EACH STATEMENT EXECUTE PROCEDURE check_too_many_blue();