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.
Udemy Course of Code Decode on Microservice k8s AWS CICD link:
openinapp.co/u...
Course Description Video :
yt.openinapp.c...
Referral Code : Cod3095
Playstore link nxtlvl.in/2xa
App Store Link nxtlvl.in/xft
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
Using HQL Query - "from Department d join fetch d.listOfEmployees Employee e”
Criteria criteria = session.createCriteria(Department.class);
criteria.setFetchMode("listOfEmployees", FetchMode.EAGER);
.
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 HQL Query - "from Department d join fetch d.listOfEmployees Employee e”
Using Criteria
Criteria criteria = session.createCriteria(Department.class);
criteria.setFetchMode("listOfEmployees", FetchMode.EAGER);
Most Asked Core Java Interview Questions and Answers: • Core Java frequently a...
Advance Java Interview Questions and Answers: • Advance Java Interview...
Java 8 Interview Questions and Answers: • Java 8 Interview Quest...
Hibernate Interview Questions and Answers:
• Hibernate Interview Qu...
Spring Boot Interview Questions and Answers:
• Advance Java Interview...
Angular Playlist: • Angular Course Introdu...
SQL Playlist: • SQL Interview Question...
GIT: • GIT
Subscriber and Follow Code Decode
Subscriber Code Decode: www.youtube.co...
LinkedIn : / codedecodeyoutube
Instagram: / codedecode25
#N+1problem #hibernate #codedecode