How to handle a large set of data using Spring Data Repositories?

Question

I have a large table that I'd like to access via a Spring Data Repository.

Currently, I'm trying to extend the PagingAndSortingRepository interface but it seems I can only define methods that return lists, eg.:

public interface MyRepository extends 
        PagingAndSortingRepository<MyEntity, Integer>
{
  @Query(value="SELECT * ...")
  List<MyEntity> myQuery(Pageable p);
}

On the other hand, the findAll() method that comes with PagingAndSortingRepository returns an Iterable (and I suppose that the data is not loaded into memory).

Is it possible to define custom queries that also return Iterable and/or don't load all the data into memory at once?

Are there any alternatives for handling large tables?

Answer 1

We have the classical consulting answer here: it depends. As the implementation of the method is store specific, we depend on the underlying store API. In case of JPA there's no chance to provide streaming access as ….getResultList() returns a List. Hence we also expose the List to the client as especially JPA developers might be used to working with lists. So for JPA the only option is using the pagination API.

For a store like Neo4j we support the streaming access as the repositories return Iterable on CRUD methods as well as on the execution of finder methods.

Answer 2

The implementation of findAll() simply loads the entire list of all entities into memory. Its Iterable return type doesn't imply that it implements some sort of database level cursor handling.

On the other hand your custom myQuery(Pageable) method will only load one page worth of entities, because the generated implementation honours its Pageable parameter. You can declare its return type either as Page or List. In the latter case you still receive the same (restricted) number of entities, but not the metadata that a Page would additionally carry.

So you basically did the right thing to avoid loading all entities into memory in your custom query.

Please review the related documentation here.