PostgreSQL: latest price per product with DISTINCT ON (greatest-n-per-group)

-- ======================================================================
-- Q: How do you get the latest price per product (greatest-n-per-group) in
-- PostgreSQL?
-- ======================================================================

SELECT DISTINCT ON (product_id) product_id, amount, created_at
FROM price_history
ORDER BY product_id, created_at DESC;
-- index: (product_id, created_at DESC)