[hibernate-issues] [Hibernate-JIRA] Commented: (HHH-3930) one-to-one causes redundant select query

[Stuart Douglas (JIRA)]

    [ http://opensource.atlassian.com/projects/hibernate/browse/HHH-3930?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=33303#action_33303 ] 

Stuart Douglas commented on HHH-3930:
-------------------------------------

I have run into this issue, and it is really hurting performance. I think the way to solve it is to add a new Map to PersistenceContext than maps foreign key values to the owner entity, with the key being a tupple of (EntityType,PropertyName,foreignKeyValue). Then when a lazy fetch is triggered from the non owning side of a relationship this map can be checked first to see if the object is already in the PeristenceContext thus avoiding the extra SQL.

> one-to-one causes redundant select query
> ----------------------------------------
>
>                 Key: HHH-3930
>                 URL: http://opensource.atlassian.com/projects/hibernate/browse/HHH-3930
>             Project: Hibernate Core
>          Issue Type: Bug
>          Components: core
>    Affects Versions: 3.3.1
>            Reporter: Martijn Dashorst
>
> @Entity
> public class Address {
>     @Id
>     @GeneratedValue(strategy = GenerationType.AUTO)
>     private Long id;
>     @OneToOne(mappedBy = "address")
>     private Customer customer;
> }
> @Entity
> public class Customer {
>     @Id
>     @GeneratedValue(strategy = GenerationType.AUTO)
>     private Long id;
>     @OneToOne(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
>     private Address address = new Address();
>     public Long getId() {
> 	return id;
>     }
>     public Address getAddress() {
> 	return address;
>     }
> }
> This mapping causes 2 instead of the expected 1 query to retrieve a Customer and its Address from the db:
> select * from Customer customer0_ left outer join Address address1_ on customer0_.address_id=address1_.id where customer0_.id=?
> select * from Customer customer0_ left outer join Address address1_ on customer0_.address_id=address1_.id where customer0_.address_id=?
> Changing the mapping to a LAZY fetch type:
> @OneToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
> private Address address = new Address();
> Causes 3 select queries instead of the expected 2 queries to retrieve a Customer (and its Address) from the db:
> select * from Customer customer0_ where customer0_.id=?
> select * from Address address0_ left outer join Customer customer1_ on address0_.id=customer1_.address_id where address0_.id=?
> select * from Customer customer0_ where customer0_.address_id=?
> The third select is superfluous because the relationship is already completely known: you already have the customer, so why not just set it on the address entity?
> Making the address field in Customer a @ManyToOne doesn't make a difference.
> Making the customer field in Address a @OneToMany does remove the extra select, but forces our model to change the relationship from Customer to List<Customer> where we *know* there'll be only 1 element.
> Apparently Hibernate can figure out the reverse relationship with a @ManyToOne - @OneToMany without the need for additional queries, can't this be extended to @OneToOne bidirectional relationships as well?

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