What is the difference between JOIN and JOIN FETCH when using JPA and Hibernate

Question

Please help me understand where to use a regular JOIN and where a JOIN FETCH   For example  if we have these two queries  FROM Employee emp JOIN emp department dep   and  FROM Employee emp JOIN FETCH emp department dep   Is there any difference between them  If yes  which one to use when

User · Answer

Dherik   I m not sure about what you say  when you don t use fetch the result will be of type   List lt Object    gt  which means a list of Object tables and not a list of Employee    Object 0  refers an Employee entity  Object 1  refers a Departement entity    When you use fetch  there is just one select and the result is the list of Employee List lt Employee gt  containing the list of departements  It overrides the lazy declaration of the entity

User · Answer

in this link i mentioned before on the comment  read this part       A  fetch  join allows associations or collections of values to be   initialized along with their parent objects using a single select    This is particularly useful in the case of a collection  It   effectively overrides the outer join and lazy declarations of the   mapping file for associations and collections    this  JOIN FETCH  will have it s effect if you have  fetch   FetchType LAZY  property for a collection inside entity example bellow    And it is only effect the method of  when the query should happen   And you must also know this        hibernate have two orthogonal notions   when is the association fetched and how   is it fetched  It is important that you do not confuse them  We use   fetch to tune performance  We can use lazy to define a contract for   what data is always available in any detached instance of a particular   class    when is the association fetched --  your  FETCH  type  how is it fetched --  Join select Subselect Batch  In your case  FETCH will only have it s effect if you have department as a set inside Employee  something like this in the entity    OneToMany fetch   FetchType LAZY  private Set lt Department gt  department    when you use   FROM Employee emp JOIN FETCH emp department dep   you will get emp and emp dep  when you didnt use fetch you can still get emp dep but hibernate will processing another select to the database to get that set of department   so its just a matter of performance tuning  about you want to get all result you need it or not  in a single query eager fetching   or you want to query it latter when you need it lazy fetching    Use eager fetching when you need to get small data with one select one big query   Or use lazy fetching to query what you need latter many smaller query    use fetch when     no large unneeded collection set inside that entity you about to get communication from application server to database server too far and need long time you may need that collection latter when you don t have the access to it outside of the transactional method class

User · Answer

JOIN  When using JOIN against an entity associations  JPA will generate a JOIN between the parent entity and the child entity tables in the generated SQL statement   So  taking your example  when executing this JPQL query   FROM Employee emp JOIN emp department dep   Hibernate is going to generate the following SQL statement   SELECT emp   FROM employee emp JOIN department dep ON emp department id   dep id      Note that the SQL SELECT clause contains only the employee table columns  and not the department ones  To fetch the department table columns  we need to use JOIN FETCH instead of JOIN    JOIN FETCH  So  compared to JOIN  the JOIN FETCH allows you to project the joining table columns in the SELECT clause of the generated SQL statement   So  in your example  when executing this JPQL query   FROM Employee emp JOIN FETCH emp department dep   Hibernate is going to generate the following SQL statement   SELECT emp    dept   FROM employee emp JOIN department dep ON emp department id   dep id      Note  that  this time  the department table columns are selected as well  not just the ones associated with the entity listed in the FROM JPQL clause    Also  JOIN FETCH is a great way to address the LazyInitializationException when using Hibernate as you can initialize entity associations using the FetchType LAZY fetching strategy along with the main entity you are fetching

User · Answer

If you have  oneToOne mapping set to FetchType LAZY and you use second query  because you need Department objects to be loaded as part of Employee objects  what Hibernate will do is  it will issue queries to fetch Department objects for every individual Employee object it fetches from DB    Later  in the code you might access Department objects via Employee to Department single-valued association and Hibernate will not issue any query to fetch Department object for the given Employee    Remember  Hibernate still issues queries equal to the number of Employees it has fetched  Hibernate will issue same number of queries in both above queries  if you wish to access Department objects of all Employee objects

User · Answer

In this two queries  you are using JOIN to query all employees that have at least one department associated   But  the difference is  in the first query you are returning only the Employes for the Hibernate  In the second query  you are returning the Employes and all Departments associated    So  if you use the second query  you will not need to do a new query to hit the database again to see the Departments of each Employee   You can use the second query when you are sure that you will need the Department of each Employee  If you not need the Department  use the first query   I recomend read this link if you need to apply some WHERE condition  what you probably will need   How to properly express JPQL  quot join fetch quot  with  quot where quot  clause as JPA 2 CriteriaQuery   Update  If you don t use fetch and the Departments continue to be returned  is because your mapping between Employee and Department  a  OneToMany  are setted with FetchType EAGER  In this case  any HQL  with fetch or not  query with FROM Employee will bring all Departments  Remember that all mapping  ToOne   ManyToOne and  OneToOne  are EAGER by default

[java] What is the difference between JOIN and JOIN FETCH when using JPA and Hibernate

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