CREATE TABLE test.rate
(
    date date NOT NULL,
    rate numeric(10,4) NOT NULL,
)

CREATE TABLE test.purchase
(
    prod_id text COLLATE pg_catalog."default",
    import_id text COLLATE pg_catalog."default",
    exw_price_usd numeric(10,2),
    exw_price_eur numeric(10,2),
    import_exp numeric(10,2),
    stock_price numeric(10,2),
    qty numeric(10,0),
    date date NOT NULL
)

BEGIN
SELECT rate
    FROM test.rate
    INNER JOIN test.purchase
	ON purchase.date=rate.date;	
UPDATE test.purchase
SET purchase.exw_price_eur=purchase.exw_price_usd/rate.rate;
RETURN NEW.exw_price_eur;
END;

CREATE TRIGGER t_conv
    AFTER INSERT OR UPDATE OF exw_price_usd, date
    ON test.purchase
    FOR EACH ROW
    EXECUTE PROCEDURE test.f_conv();

INSERT INTO test.purchase(
	exw_price_usd, date)
	VALUES (42, '10-10-2020');

BEGIN
UPDATE test.purchase pr
SET exw_price_eur = exw_price_usd/e.rate
FROM test.eurusd  e
WHERE e.date=pr.date;
RETURN NEW.exw_price_eur;
END