Hibernate N+1 problem and solution | Hibernate Interview Questions and Answers | Code Decode

In this video of n+1 problem in code decode we have explained about the what the problem is all about and how to solve this problem.

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What is Hibernate N+1 Select Problem
The N + 1 Select problem is a performance issue in Hibernate. In this problem, a Java application makes N + 1 database calls (N = number of child objects fetched). For example, if N= 2, the application makes 3 (N+1= 3) database calls.

Example - Employees and Departments have Many To one relationship . One Department ( Parent ) can have multiple Employees (Child)

// Unidirectional mapping . By default its lazy
Using
@OneToMany(fetch = FetchType.LAZY)
@JoinColumn(name="Dept_id") // dept_id will be column in Employee table.

Now we need to fetch all departments ,

What all steps will be done now to fetch all departments -

by default its lazy so first a call goes to Department table and fetch all departments (only id and name — No employes list fetched)

Then while returning, for each department now a call goes to fetch list of employees. So N calls goes now, each for 1 department.

What is Hibernate N+1 Select Problem’s Solution
At SQL level, what ORM needs to achieve to avoid N+1 is to fire a query that joins the two tables and get the combined results in single query.

Spring Data JPA Approach-

using left join fetch, we resolve the N+1 problem
using attributePaths, Spring Data JPA avoids N+1 problem

Hibernate Approach
.

What is @EntityGraph(attributePaths = {"listOfEmployees"})
At SQL level, what ORM needs to achieve to avoid N+1 is to fire a query that joins the two tables and get the combined results in single query.

Spring Data JPA Approach-

using left join fetch, - we can use JOIN FETCH. The FETCH keyword of the JOIN FETCH statement is JPA-specific. It instructs the persistence provider to not only join the two database tables contained in the query, but also initialize the association on the returned entity. It works with both JOIN and LEFT JOIN statements.

using attributePaths, - EntityGraphs are introduced in JPA 2.1 and used to allow partial or specified fetching of objects. When an entity has references to other entities we can specify a fetch plan by EntityGraphs in order to determine which fields or properties should be fetched together.

What is @EntityGraph(attributePaths = {"listOfEmployees"})

Hibernate Approach

Using Criteria

Hibernate Interview Questions and Answers:

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[chetankhandave]

You are one of the best teacher I have ever seen. Your explanation in such simple language shows your thorough study. Thanks for the video and Best of luck.

[arunr]

The best explanation I have seen for n+1 problem...

[rajat_singla]

Great explanation. Would be great if you can explain join fetch and entity graphs in detail as you mentioned towards the end of the video. Thanks.

[gopishettymahindra]

Great Explanation . The Best Explanation I have seen for N+1 Problem

[samuelr-td]

thanks for your explanation. This is one of the important interview quuestion

[srisureshshetty]

Very good explanation. Thanks for the tutorial.

[mantusubudhi]

if we are using fetchType as Eager then the N+1 problem should be fixed right . at the end we are dealing with object creation of realted tables .why do we need so much complications then ?

[theprofessor]

Looking forward to the next video on this.

[abhilashchaparala]

Great explanation. If possible could you please explain entity graphs in detail. Thanks for the video

[ayushjain]

Thanks for this explanation
one question if I set fetch type as eager then will this problem arise?

[sureshmanne]

how is the {"listOfEmployees"} referred to Employee table?

[sandysworld]

Great explanation.. pls do explain JOIN FETCH and entity graph as well.

[anurani]

Great explanation..pls do a video on entity graph

[ajai]

Why can't we just use Eager fetch in such scenarios?

[dmytro]

very cool and usefull video, thanks a lot)

[phanimc]

please explain entity graphs in detail. Thanks for the video

[shubhamsamdani]

Thanks for this one.
please prepare one video for entity graph.

[syedfaizan]

where is the transaction video after propogation i mean you told you will cover transaction isolation

[sahilpatil]

Thanks, this issue asked multiple times in interview but i have not answered 😑

Pls help in best way to fetch result in custom dto after join non relationship table

[syedfaizan]

thanks but please transaction isolation