Greater than or equal to ALL() and equal to MAX() speed

Question

I have a relation in PostgreSQL named product which contains 2 fields: id and quantity, and I want to find the id of the

Answer 1

there is the 3rd variant

SELECT id FROM product 
WHERE quantity = (SELECT quantity FROM product ORDER BY quantity DESC NULLS LAST LIMIT 1)

if the table has btree index as (quantity DESC NULLS LAST) this variant will be super-fast