Spring data jpa avoid in memory pagination and n+1 using specification - spring

Question

I am trying to avoid in-memory pagination and N+1 while using Spring Data JPA Specification.
To be specific, I'm using the below method provided by the framework.
Page<T> findAll(#Nullable Specification<T> spec, Pageable pageable);
I tried to avoid N+1 by using #EntityGraph on the method (don't know if it's good or not) and after some research, I still don't know how to work around the in-memory pagination.
The database I'm using is Postgres if it matters
Are there any solutions to this problem?

Answer 1

The problem is that as soon as you fetch some kind of *-to-many association, Hibernate will do in-memory pagination, so #EntityGraph won't help. What you need is a special query that does pagination on the main/root entity and fetches associations in a second query.
I think this is a perfect use case for Blaze-Persistence Entity Views.
I created the library to allow easy mapping between JPA models and custom interface or abstract class defined models, something like Spring Data Projections on steroids. The idea is that you define your target structure(domain model) the way you like and map attributes(getters) via JPQL expressions to the entity model.
A DTO model for your use case could look like the following with Blaze-Persistence Entity-Views:
#EntityView(User.class)
public interface UserDto {
#IdMapping
Long getId();
String getName();
Set<RoleDto> getRoles();
#EntityView(Role.class)
interface RoleDto {
#IdMapping
Long getId();
String getName();
}
}
Querying is a matter of applying the entity view to a query, the simplest being just a query by id.
UserDto a = entityViewManager.find(entityManager, UserDto.class, id);
The Spring Data integration allows you to use it almost like Spring Data Projections: https://persistence.blazebit.com/documentation/entity-view/manual/en_US/index.html#spring-data-features
Page<UserDto> findAll(Specification spec, Pageable pageable);
The best part is, it will only fetch the state that is actually necessary!
But you can even use it with plain entities if you like in which case this will also run more efficient queries as you can read about in the documentation: https://persistence.blazebit.com/documentation/core/manual/en_US/index.html#pagination