How to do fast query without inner join

refer
https://medium.com/squad-engineering/blazingly-fast-querying-on-huge-tables-by-avoiding-joins-5be0fca2f523

WITH user_ids AS
  (SELECT id
   FROM user
   WHERE account_id IN
       (SELECT generate_series(1,1000)))
SELECT purchase.id
FROM purchase
WHERE user_id IN
    (SELECT id
     FROM user_ids);

JOIN:

SELECT "purchase"."id"
FROM "purchase"
INNER JOIN "user" ON ("purchase"."user_id" = "user"."id")
WHERE "user"."account_id" IN
    (SELECT generate_series(1,1000));