Question

In my web application I have an Object with a OneToMany relationship with Child.
When selecting my Objects I execute the following query:

from Object o

and then i should print for every object how many children it has with

// Foreach
object.children.size()

Assuming the object has a lot of children (let’s say 30’000);
Is it a waste of resources calling size() or the ORM framework (in my case Hibernate) will take care of that without loading all the children?

Answer 1

Using JPA (the standard):

• Your @OneToMany relationship is by default lazy-loaded (i.e. default value for fetch=FetchType.LAZY). But calling Entity.getCollection().size() would trigger lazy loading to retrieve all of the child collection – so yes, it would be fairly slow, unless you needed to operate on all/most elements anyway. Note: for all (sane) implementations of JPA, this will NOT issue 30,000 seperate queries – it will issue one query that returns 30,000 rows in the result set.
• If you need most elements or you wanted to cache in advance change to @OneToMany(fetch=FetchType.EAGER)
• The common way do obtain data statistics without retrieving every single object is simply to use JPQL via EntityManager.getQuery()/getTypedQuery()/getNamedQuery() (or even SQL via getNativeQUery()). This is quite simple and highly performant:

int empSize = em.createQuery("SELECT SIZE(d.employees) FROM Department d")     
                 .getSingleResult();    

OR ALTERNATIVELY

// Pre-compiled query placed against entity class for highest performance   
@NamedQueries({
    @NamedQuery(name="Department.EmployeeSize",
                query="SELECT SIZE(d.employees) FROM Department"),
    ... // other named queries
})
@Entity
public class Department {

    ...

}

// Then to use the query:    
int empSize = em.createNamedQuery("Department.EmployeeSize", Integer.class)     
                 .getSingleResult();    

• You can enable level 2 caching. JPA Caching Summary

  To statically configure level 2 caching:

Map propertiesMap = new HashMap();
// Valid values are ALL, NONE, ENABLE_SELECTIVE, DISABLE_SELECTIVE
propertiesMap.add("javax.persistence.sharedCache.mode", "ENABLE_SELECTIVE");
EntityManagerFactory = Persistence.createEntityManagerFactory("myPUName", propertiesMap);

ALTERNATIVELY use persistence.xml:

<persistence-unit name="EmployeeService">
    ...
    <shared-cache-mode>ENABLE_SELECTIVE</shared-cache-mode>

</persistence-unit>

Then mark which entities should be automatically cached in the level 2 cache:

@Cacheable(true)
@Entity
public class Employee {
    ...
}

• Can also configure caching dynamically, as part of a particular query

Using Proprietary approach (e.g. Hibernate “Extra-Lazy” collections):

• Same performance as simply issuing JPQL/SQL query
• Save a couple of lines of code (@org.hibernate.annotations.LazyCollection( EXTRA ) annotation v @NamedQuery annotation and execution)
• Not standard – JPA trained developers won’t know about it
• Not portable – can only be used with that vendor. There is an industry trend towards standard JPA, away from proprietary features. Many different JPA implementations are out there.