JPA eager fetch does not join

Question

What exactly does JPA s fetch strategy control  I can t detect any difference between eager and lazy  In both cases JPA Hibernate does not automatically join many-to-one relationships   Example  Person has a single address  An address can belong to many people  The JPA annotated entity classes look like    Entity public class Person        Id     public Integer id       public String name        ManyToOne fetch FetchType LAZY or EAGER      public Address address      Entity public class Address        Id     public Integer id       public String name      If I use the JPA query   select p from Person p where       JPA Hibernate generates one SQL query to select from Person table  and then a distinct address query for each person   select     from Person where     select     from Address where id 1 select     from Address where id 2 select     from Address where id 3   This is very bad for large result sets  If there are 1000 people it generates 1001 queries  1 from Person and 1000 distinct from Address   I know this because I m looking at MySQL s query log  It was my understanding that setting address s fetch type to eager will cause JPA Hibernate to automatically query with a join  However  regardless of the fetch type  it still generates distinct queries for relationships   Only when I explicitly tell it to join does it actually join   select p  a from Person p left join p address a where       Am I missing something here  I now have to hand code every query so that it left joins the many-to-one relationships  I m using Hibernate s JPA implementation with MySQL   Edit  It appears  see Hibernate FAQ here and here  that FetchType does not impact JPA queries  So in my case I have explicitly tell it to join

User · Answer

The fetchType attribute controls whether the annotated field is fetched immediately when the primary entity is fetched. It does not necessarily dictate how the fetch statement is constructed, the actual sql implementation depends on the provider you are using toplink/hibernate etc.

If you set fetchType=EAGER This means that the annotated field is populated with its values at the same time as the other fields in the entity. So if you open an entitymanager retrieve your person objects and then close the entitymanager, subsequently doing a person.address will not result in a lazy load exception being thrown.

If you set fetchType=LAZY the field is only populated when it is accessed. If you have closed the entitymanager by then a lazy load exception will be thrown if you do a person.address. To load the field you need to put the entity back into an entitymangers context with em.merge(), then do the field access and then close the entitymanager.

You might want lazy loading when constructing a customer class with a collection for customer orders. If you retrieved every order for a customer when you wanted to get a customer list this may be a expensive database operation when you only looking for customer name and contact details. Best to leave the db access till later.

For the second part of the question - how to get hibernate to generate optimised SQL?

Hibernate should allow you to provide hints as to how to construct the most efficient query but I suspect there is something wrong with your table construction. Is the relationship established in the tables? Hibernate may have decided that a simple query will be quicker than a join especially if indexes etc are missing.

User · Answer

JPA doesn t provide any specification on mapping annotations to select fetch strategy  In general  related entities can be fetched in any one of the ways given below   SELECT    one query for root entities   one query for related mapped entity collection of each root entity    n 1  queries SUBSELECT    one query for root entities   second query for related mapped entity collection of all root entities retrieved in first query   2 queries JOIN    one query to fetch both root entities and all of their mapped entity collection   1 query   So SELECT and JOIN are two extremes and SUBSELECT falls in between  One can choose suitable strategy based on her his domain model   By default SELECT is used by both JPA EclipseLink and Hibernate  This can be overridden by using    Fetch FetchMode JOIN    Fetch FetchMode SUBSELECT    in Hibernate  It also allows to set SELECT mode explicitly using  Fetch FetchMode SELECT  which can be tuned by using batch size e g   BatchSize size 10    Corresponding annotations in EclipseLink are    JoinFetch  BatchFetch

User · Answer

Two things occur to me   First  are you sure you mean ManyToOne for address    That means multiple people will have the same address   If it s edited for one of them  it ll be edited for all of them   Is that your intent   99  of the time addresses are  private   in the sense that they belong to only one person    Secondly  do you have any other eager relationships on the Person entity   If I recall correctly  Hibernate can only handle one eager relationship on an entity but that is possibly outdated information   I say that because your understanding of how this should work is essentially correct from where I m sitting

User · Answer

I had exactly this problem with the exception that the Person class had a embedded key class  My own solution was to join them in the query AND remove   Fetch FetchMode JOIN   My embedded id class    Embeddable public class MessageRecipientId implements Serializable         ManyToOne targetEntity   Message class  fetch   FetchType LAZY       JoinColumn name  messageId       private Message message      private String governmentId       public MessageRecipientId                public Message getMessage             return message             public void setMessage Message message            this message   message             public String getGovernmentId             return governmentId             public void setGovernmentId String governmentId            this governmentId   governmentId             public MessageRecipientId Message message  GovernmentId governmentId            this message   message          this governmentId   governmentId getValue

User · Answer

Try with   select p from Person p left join FETCH p address a where      It works for me in a similar with JPA2 EclipseLink  but it seems this feature is present in JPA1 too

User · Answer

to join you can do multiple things  using eclipselink   in jpql you can do left join fetch  in named query you can specify query hint  in TypedQuery you can say something like query setHint  quot eclipselink join-fetch quot    quot e projects milestones quot     there is also batch fetch hint query setHint  quot eclipselink batch quot    quot e address quot      see http   java-persistence-performance blogspot com 2010 08 batch-fetching-optimizing-object-graph html

User · Answer

The fetchType attribute controls whether the annotated field is fetched immediately when the primary entity is fetched. It does not necessarily dictate how the fetch statement is constructed, the actual sql implementation depends on the provider you are using toplink/hibernate etc.

If you set fetchType=EAGER This means that the annotated field is populated with its values at the same time as the other fields in the entity. So if you open an entitymanager retrieve your person objects and then close the entitymanager, subsequently doing a person.address will not result in a lazy load exception being thrown.

If you set fetchType=LAZY the field is only populated when it is accessed. If you have closed the entitymanager by then a lazy load exception will be thrown if you do a person.address. To load the field you need to put the entity back into an entitymangers context with em.merge(), then do the field access and then close the entitymanager.

You might want lazy loading when constructing a customer class with a collection for customer orders. If you retrieved every order for a customer when you wanted to get a customer list this may be a expensive database operation when you only looking for customer name and contact details. Best to leave the db access till later.

For the second part of the question - how to get hibernate to generate optimised SQL?

Hibernate should allow you to provide hints as to how to construct the most efficient query but I suspect there is something wrong with your table construction. Is the relationship established in the tables? Hibernate may have decided that a simple query will be quicker than a join especially if indexes etc are missing.

User · Answer

Two things occur to me   First  are you sure you mean ManyToOne for address    That means multiple people will have the same address   If it s edited for one of them  it ll be edited for all of them   Is that your intent   99  of the time addresses are  private   in the sense that they belong to only one person    Secondly  do you have any other eager relationships on the Person entity   If I recall correctly  Hibernate can only handle one eager relationship on an entity but that is possibly outdated information   I say that because your understanding of how this should work is essentially correct from where I m sitting

User · Answer

If you use EclipseLink instead of Hibernate you can optimize your queries by  query hints   See this article from the Eclipse Wiki  EclipseLink Examples JPA QueryOptimization   There is a chapter about  Joined Reading

User · Answer

mxc  is right  fetchType just specifies when the relation should be resolved   To optimize eager loading by using an outer join you have to add   Fetch FetchMode JOIN    to your field  This is a hibernate specific annotation

User · Answer

I had exactly this problem with the exception that the Person class had a embedded key class  My own solution was to join them in the query AND remove   Fetch FetchMode JOIN   My embedded id class    Embeddable public class MessageRecipientId implements Serializable         ManyToOne targetEntity   Message class  fetch   FetchType LAZY       JoinColumn name  messageId       private Message message      private String governmentId       public MessageRecipientId                public Message getMessage             return message             public void setMessage Message message            this message   message             public String getGovernmentId             return governmentId             public void setGovernmentId String governmentId            this governmentId   governmentId             public MessageRecipientId Message message  GovernmentId governmentId            this message   message          this governmentId   governmentId getValue

User · Answer

to join you can do multiple things  using eclipselink   in jpql you can do left join fetch  in named query you can specify query hint  in TypedQuery you can say something like query setHint  quot eclipselink join-fetch quot    quot e projects milestones quot     there is also batch fetch hint query setHint  quot eclipselink batch quot    quot e address quot      see http   java-persistence-performance blogspot com 2010 08 batch-fetching-optimizing-object-graph html

User · Answer

If you use EclipseLink instead of Hibernate you can optimize your queries by  query hints   See this article from the Eclipse Wiki  EclipseLink Examples JPA QueryOptimization   There is a chapter about  Joined Reading

User · Answer

JPA doesn t provide any specification on mapping annotations to select fetch strategy  In general  related entities can be fetched in any one of the ways given below   SELECT    one query for root entities   one query for related mapped entity collection of each root entity    n 1  queries SUBSELECT    one query for root entities   second query for related mapped entity collection of all root entities retrieved in first query   2 queries JOIN    one query to fetch both root entities and all of their mapped entity collection   1 query   So SELECT and JOIN are two extremes and SUBSELECT falls in between  One can choose suitable strategy based on her his domain model   By default SELECT is used by both JPA EclipseLink and Hibernate  This can be overridden by using    Fetch FetchMode JOIN    Fetch FetchMode SUBSELECT    in Hibernate  It also allows to set SELECT mode explicitly using  Fetch FetchMode SELECT  which can be tuned by using batch size e g   BatchSize size 10    Corresponding annotations in EclipseLink are    JoinFetch  BatchFetch

User · Answer

Two things occur to me   First  are you sure you mean ManyToOne for address    That means multiple people will have the same address   If it s edited for one of them  it ll be edited for all of them   Is that your intent   99  of the time addresses are  private   in the sense that they belong to only one person    Secondly  do you have any other eager relationships on the Person entity   If I recall correctly  Hibernate can only handle one eager relationship on an entity but that is possibly outdated information   I say that because your understanding of how this should work is essentially correct from where I m sitting

User · Answer

mxc  is right  fetchType just specifies when the relation should be resolved   To optimize eager loading by using an outer join you have to add   Fetch FetchMode JOIN    to your field  This is a hibernate specific annotation

User · Answer

The fetchType attribute controls whether the annotated field is fetched immediately when the primary entity is fetched. It does not necessarily dictate how the fetch statement is constructed, the actual sql implementation depends on the provider you are using toplink/hibernate etc.

If you set fetchType=EAGER This means that the annotated field is populated with its values at the same time as the other fields in the entity. So if you open an entitymanager retrieve your person objects and then close the entitymanager, subsequently doing a person.address will not result in a lazy load exception being thrown.

If you set fetchType=LAZY the field is only populated when it is accessed. If you have closed the entitymanager by then a lazy load exception will be thrown if you do a person.address. To load the field you need to put the entity back into an entitymangers context with em.merge(), then do the field access and then close the entitymanager.

You might want lazy loading when constructing a customer class with a collection for customer orders. If you retrieved every order for a customer when you wanted to get a customer list this may be a expensive database operation when you only looking for customer name and contact details. Best to leave the db access till later.

For the second part of the question - how to get hibernate to generate optimised SQL?

Hibernate should allow you to provide hints as to how to construct the most efficient query but I suspect there is something wrong with your table construction. Is the relationship established in the tables? Hibernate may have decided that a simple query will be quicker than a join especially if indexes etc are missing.

User · Answer

The fetchType attribute controls whether the annotated field is fetched immediately when the primary entity is fetched. It does not necessarily dictate how the fetch statement is constructed, the actual sql implementation depends on the provider you are using toplink/hibernate etc.

If you set fetchType=EAGER This means that the annotated field is populated with its values at the same time as the other fields in the entity. So if you open an entitymanager retrieve your person objects and then close the entitymanager, subsequently doing a person.address will not result in a lazy load exception being thrown.

If you set fetchType=LAZY the field is only populated when it is accessed. If you have closed the entitymanager by then a lazy load exception will be thrown if you do a person.address. To load the field you need to put the entity back into an entitymangers context with em.merge(), then do the field access and then close the entitymanager.

You might want lazy loading when constructing a customer class with a collection for customer orders. If you retrieved every order for a customer when you wanted to get a customer list this may be a expensive database operation when you only looking for customer name and contact details. Best to leave the db access till later.

For the second part of the question - how to get hibernate to generate optimised SQL?

Hibernate should allow you to provide hints as to how to construct the most efficient query but I suspect there is something wrong with your table construction. Is the relationship established in the tables? Hibernate may have decided that a simple query will be quicker than a join especially if indexes etc are missing.

User · Answer

Two things occur to me   First  are you sure you mean ManyToOne for address    That means multiple people will have the same address   If it s edited for one of them  it ll be edited for all of them   Is that your intent   99  of the time addresses are  private   in the sense that they belong to only one person    Secondly  do you have any other eager relationships on the Person entity   If I recall correctly  Hibernate can only handle one eager relationship on an entity but that is possibly outdated information   I say that because your understanding of how this should work is essentially correct from where I m sitting

User · Answer

Try with   select p from Person p left join FETCH p address a where      It works for me in a similar with JPA2 EclipseLink  but it seems this feature is present in JPA1 too

[java] JPA eager fetch does not join

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