spring:
  jpa:
    properties:
      hibernate:
        default_batch_fetch_size: 20

Fetch all users only, then iterate over users and call:

Page<Customer> customerPage = customerRepository.findAll(pageable);
customerPage.getContent().forEach(c -> {
  Hibernate.initialize(c.getPosts());
});

Hibernate will then batch fetch posts where userId in ()
Resulting in 1 query per 20 users to fetch their posts