Question

I have a JPQL like this one:

select distinct d 
from Department d
left join fetch d.employees

When I want to fetch one of the lazy property of my Department entity, the distinct is not working any more.

select distinct d, substring(d.htmlDescription, 1,400)
from Department d
left join fetch d.employees

The query returns as much Department as the number of employees in it.

The substring(d.htmlDescription) is important because the property is defined as a CLOB (type TEXT under postgresql):

@Column(columnDefinition = "TEXT")
@Basic(fetch = FetchType.LAZY)
String htmlBody;

The substring function is translated in sql thus limiting the amount of data transfered beetween the database and the web server.

As a workaround, I tried to break the query in two parts : 

select d, substring(d.htmlDescription, 1,400)
from Department d where d in (
    select distinct d1
    from Department d1  left join fetch d1.employees
)

This doestn’t work because the JOIN FETCH must not be used in the FROM clause of a subquery.

Answer 1

Finally I found a solution to my problem by :

1. modifying my mapping
2. cutting the request in 2 calls.

The htmlBody field is now in another entity. Thus the departement entity is lighter.

class Department{
...
@OneToOne (fetch = FetchType.LAZY, 
    cascade = {CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REMOVE})
Content content = new Content();
...
}


class Content{
...
@Column(columnDefinition = "TEXT")
@Basic(fetch = FetchType.LAZY)
String htmlBody;
...
}

I can then use the following requests :

    List<Department> deps = em.get().createQuery(
            "select distinct d " +
                    "from Department d " +
                    "order by d.id desc ", Department.class)
            .setFirstResult(first)
            .setMaxResults(count)
            .getResultList();

    List<Object[]> tuple = em.get().createQuery(
            "select d, substring(d.content.htmlBody, 1,400)" +
                    "from Department d " +
                    "left join fetch d.employees" +
                    "where d in (:deps) order by d.id desc")
            .setParameter("deps", deps)
            .getResultList();

    ... //Filter the duplicates due to the fetching

That way, I have 2 sql queries. The fetching of employees is done in the second query witch occurs on a small amount of datas. The substring is realized in SQL. Perfect!