Hibernate HQL join fetch not recursively fetching

Question

I have the following query and method

private static final String FIND = "SELECT DISTINCT domain FROM Domain domain LEFT OUTER JOIN FETCH domain.operators LEFT OUTER JOIN FETCH domain.networkCodes WHERE domain.domainId = :domainId";

@Override
public Domain find(Long domainId) {
    Query query = getCurrentSession().createQuery(FIND);
    query.setLong("domainId", domainId);
    return (Domain) query.uniqueResult();
}

With Domain as

@Entity
@Table
public class Domain {
    @Id
    @GenericGenerator(name = "generator", strategy = "increment")
    @GeneratedValue(generator = "generator")
    @Column(name = "domain_id")
    private Long domainId;

    @Column(nullable = false, unique = true)
    @NotNull
    private String name;

    @Column(nullable = false)
    @NotNull
    @Enumerated(EnumType.STRING)
    private DomainType type;

    @OneToMany(cascade = {
            CascadeType.PERSIST,
            CascadeType.MERGE
    }, fetch = FetchType.EAGER)
    @JoinTable(joinColumns = {
            @JoinColumn(name = "domain_id")
    }, inverseJoinColumns = {
            @JoinColumn(name = "code")
    })
    @NotEmpty
    @Valid // needed to recur because we specify network codes when creating the domain
    private Set<NetworkCode> networkCodes = new HashSet<>();

    @ManyToMany(fetch = FetchType.EAGER)
    @JoinTable(joinColumns = {
            @JoinColumn(name = "parent", referencedColumnName = "domain_id")
    }, inverseJoinColumns = {
            @JoinColumn(name = "child", referencedColumnName = "domain_id")
    })
    private Set<Domain> operators = new HashSet<>();
    // more
}

I would expect this single query to fetch the Set<NetworkCode> and Set<Domain> relations, but it doesn't. Say that the Domain I query has two operators, Hibernate would perform 1 + 2 * 2 = 5 queries

Hibernate: select distinct domain0_.domain_id as domain1_1_0_, domain2_.domain_id as domain1_1_1_, networkcod4_.code as code2_2_, domain0_.name as name1_0_, domain0_.type as type1_0_, domain2_.name as name1_1_, domain2_.type as type1_1_, operators1_.parent as parent1_0__, operators1_.child as child4_0__, networkcod3_.domain_id as domain1_1_1__, networkcod3_.code as code5_1__ from domain domain0_ left outer join domain_operators operators1_ on domain0_.domain_id=operators1_.parent left outer join domain domain2_ on operators1_.child=domain2_.domain_id inner join domain_network_codes networkcod3_ on domain0_.domain_id=networkcod3_.domain_id inner join network_code networkcod4_ on networkcod3_.code=networkcod4_.code where domain0_.domain_id=?
Hibernate: select operators0_.parent as parent1_1_, operators0_.child as child4_1_, domain1_.domain_id as domain1_1_0_, domain1_.name as name1_0_, domain1_.type as type1_0_ from domain_operators operators0_ inner join domain domain1_ on operators0_.child=domain1_.domain_id where operators0_.parent=?
Hibernate: select networkcod0_.domain_id as domain1_1_1_, networkcod0_.code as code5_1_, networkcod1_.code as code2_0_ from domain_network_codes networkcod0_ inner join network_code networkcod1_ on networkcod0_.code=networkcod1_.code where networkcod0_.domain_id=?
Hibernate: select operators0_.parent as parent1_1_, operators0_.child as child4_1_, domain1_.domain_id as domain1_1_0_, domain1_.name as name1_0_, domain1_.type as type1_0_ from domain_operators operators0_ inner join domain domain1_ on operators0_.child=domain1_.domain_id where operators0_.parent=?
Hibernate: select networkcod0_.domain_id as domain1_1_1_, networkcod0_.code as code5_1_, networkcod1_.code as code2_0_ from domain_network_codes networkcod0_ inner join network_code networkcod1_ on networkcod0_.code=networkcod1_.code where networkcod0_.domain_id=?

I'm guessing this is because I'm joining the operators Domain elements but they have to join themselves.

Is there an HQL query I can execute that would do both?

Answer 1

The Hibernate Relations Works with different Fetch Strategies..!!

Hibernate provides 4 strategies for retrieving data:

SELECT

@OneToMany(mappedBy="tableName", cascade=CascadeType.ALL)
@Column(name="id") 
@Fetch(FetchMode.SELECT)

In this Method there are Multiple SQLs fired. This first one is fired for retrieving all the records in the Parent table. The remaining are fired for retrieving records for each Parent Record. This is basically the N+1 problem. The first query retrieves N records from database, in this case N Parent records. For each Parent a new query retrieves Child. Therefore for N Parent, N queries retrieve information from Child table.

JOIN

@OneToMany(mappedBy="tableName", cascade=CascadeType.ALL)
@Column(name="id")
@Fetch(FetchMode.JOIN) 

This is similar to the SELECT fetch strategy except that fact that all database retrieval take place upfront in JOIN fetch unlike in SELECT where it happens on a need basis. This can become an important performance consideration.

SUBSELECT

 @OneToMany(mappedBy="tableName", cascade=CascadeType.ALL)
 @Column(name="id")
 @Fetch(FetchMode.SUBSELECT)

Two SQLs are fired. One to retrieve all Parent and the second uses a SUBSELECT query in the WHERE clause to retrieve all child that has matching parent ids.

BATCH

@OneToMany(mappedBy="tableName", cascade=CascadeType.ALL)
@Column(name="id")
@@BatchSize(size=2)

The batch size maps to the number of Parent whose child are retrieved. So we can specify the number of records to be fetched at a time.But Multiple queries will be executed.!!

one-to-many & many-to-many allows - join, Select and SubSelect

many-to-one & one-to-one allows - Join and Select

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Hibernate also distinguishes between (when is the associations are fetched)

1.Immediate fetching -

an association, collection or attribute is fetched immediately, when the Parent is loaded. (lazy=“false”)

2.Lazy collection fetching -

a collection is fetched when the application invokes an operation upon that collection. (This is the default for collections.(lazy=“true”)

3."Extra-lazy" collection fetching -

individual elements of the collection are accessed from the database as needed. Hibernate tries not to fetch the whole collection into memory unless absolutely needed (suitable for very large collections) (lazy=“extra”)

4.Proxy fetching -

a single-valued association is fetched when a method other than the identifier getter is invoked upon the associated object. (lazy=“proxy”)

5."No-proxy" fetching -

a single-valued association is fetched when the instance variable is accessed. Compared to proxy fetching, this approach is less lazy.(lazy=“no-proxy”)

6.Lazy attribute fetching -

an attribute or single valued association is fetched when the instance variable is accessed. (lazy=“true”)

one-to-many & many-to-many allows Immediate, Layzy, Extra Lazy

many-to-one & one-to-one allows Immediate Proxy, No Proxy