JPA + Hibernate-父子表之间的内部联接

Question

EDIT (Completely reformulated approach):

I'm trying to promote the use of JPA in a new project but I'm struggling with a supposedly trivial problem: An INNER JOIN between two tables (parent and child).

I will provide only the essential info and leave all the rest out. Please feel free to ask more info if it's needed. There are two tables LANGUAGE and MESSAGE_RESOURCE, where the parent table is LANGUAGE (Primary Key ID_LANGUAGE) and the child table has a Foreign Key to the parent table also named ID_LANGUAGE.

The Language (parent) class:

@Entity
@Table(name = "PF_LANGUAGE")
public class Language {
    @Id
    @Column(name = "ID_LANGUAGE", nullable = false)
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int idLanguage;

    @OneToMany(mappedBy="language",targetEntity=MessageResource.class, fetch=FetchType.EAGER)
    private Collection<MessageResource> messageResources;
}

The child class:

@Entity
@Table(name = "PF_MESSAGE_RESOURCE")
public class MessageResource {

    @Id
    @Column(name = "ID_MESSAGE_RESOURCE", nullable = false)
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int idMessageResource;

    @ManyToOne(optional=false)
    @JoinColumn(name="ID_LANGUAGE")
    private Language language;
}

I'm fetching the results with a named query:

entityManager.createNamedQuery("select l, r from Language l join l.messageResources r");

This results in a result Object array where each entry contains one Language, MessageResource pair. The problem is that this is getting done in separate queries.

I can see in debug output that the first query is an INNER JOIN between both tables, containing the columns from both tables in the output, so this should be enough. But JPA is doing 2 additional queries (the number of LANGUAGE records) fetching the child table values for each parent table again, which should be not necessary.

First query which is enough to get all data:

select
   language0_.ID_LANGUAGE as ID1_5_0_,
   messageres1_.ID_MESSAGE_RESOURCE as ID1_4_1_,
   language0_.CODE as CODE5_0_,
   language0_.DATE_INS  as DATE3_5_0_,
   language0_.DESCRIPTION  as DESCRIPT4_5_0_,
   messageres1_.DATE_INS  as DATE2_4_1_,
   messageres1_.KEY as KEY4_1_,
   messageres1_.ID_LANGUAGE as ID5_4_1_,
   messageres1_.VALUE as VALUE4_1_ 
from
   PF_LANGUAGE language0_ 
inner join
   PF_MESSAGE_RESOURCE messageres1_ 
on language0_.ID_LANGUAGE=messageres1_.ID_LANGUAGE

Two redundant queries like the following are also run against the database after the first inner join (they are run once for each LANGUAGE table records):

select
    messageres0_.ID_LANGUAGE as ID5_5_1_,
    messageres0_.ID_MESSAGE_RESOURCE as ID1_1_,
    messageres0_.ID_MESSAGE_RESOURCE as ID1_4_0_,
    messageres0_.DATE_INS  as DATE2_4_0_,
    messageres0_.KEY as KEY4_0_,
    messageres0_.ID_LANGUAGE as ID5_4_0_,
    messageres0_.VALUE as VALUE4_0_ 
from
    PF_MESSAGE_RESOURCE messageres0_ 
where
    messageres0_.ID_LANGUAGE=?

I need to eliminate the two redundant additional queries generated by JPA. The first inner join is enough to get all data.

Need help with this one. Any clues?

Answer 1

I am not sure if it is the actual solution, but my experience with eager fetch is bad. It is always fetching in some way that I am not expected.

Your query is weird, it doesn't make sense that you are selecting both Language and MessageResource

You may have a try:

Remove fetch=FetchType.EAGER in Language's relationship to MessageResource, and change the query to something

select l from Language l join fetch l.messageResources where ....

It should give you the Language, and the aggregated message resources of that instance, all in one SQL.

Something off-topic, I wonder why you are having MessageResource.language being insertable=false, updatable=false . It seems contradicting as in Language, you specified the relationship mapped by this field but you are now making it non-insertable/updatable.

Answer 2

I don't quite understand why do you expect inner join in this case.

optional = "false" at @ManyToOne means that every MessageResource must have a Language , but it does not imply that every Language must have at least one MessageResource , therefore left join when loading Language is correct in this case - Language that you load may have no MessageResource s associated with it.

EDIT: The purpose of find() is to load an object with the given id if it exists in the database. If you want something semantically different (for example, to load all Language s that have at least one MessageResource ), you need to write a query for it, such as select l from Language l join l.messageResources .

Answer 3

There does not seem to be a standard way by using standard annotation to force generation of inner query (JPA is a standard and you are using an implementation provider For eg, Hibernate, TopLink, Open JPA etc). Different JPA implementations use different join strategy. For more details please go through this and this .

Answer 4

尝试改变fetch=FetchType.EAGER为的MessageResources来fetch=FetchType.LAZY既然你明确的加入，改变FetchType到LAZY可能的帮助。