1. Lazy Loading을 쓰면 N+1 발생

2. Eager 전략도 N+1 발생

3. join fetch 쓰면 쓸데 없이 많이 조인하게 됨

4. in query

@Query("select distinct ac.user.id from Account ac")
    List<Long> findUserIdDistinct();

    @Query("select ac from Account ac where ac.user.id in :userIds")
    List<Account> findByUserIds(@Param("userIds") List<Long> userIds);

@Test
    public void findAllV3_test(){
        List<Long> userIds = accountRepository.findUserIdDistinct();
        userIds.stream().forEach(id -> {
            System.out.println("디버그 : "+id);
        });
        accountRepository.findByUserIds(userIds);
    }